Force sensor

Abstract

A method of determining a force applied between a first portion and a second portion, the second portion being connected to the first portion via a shape memory alloy (SMA) element such that the force is transmitted via the SMA element. The force is determined based on a resistance of the SMA element, or an amount of energy applied to the SMA element to maintain the resistance of the SMA element at a predetermined value.

Description

[0001] Field

[0002] This application relates to a shape memory alloy force sensor and a method of using the same to measure force. This application also relates to a tool including the shape memory alloy force sensor.

[0003] Background

[0004] Force sensors are used to facilitate interaction with physical systems. Force sensors can be used to obtain quantitative data by converting an applied force into an electrical output.

[0005] A common type of force sensor is a strain gauge. The strain gauge is configured such that when a force (producing strain) is applied, the resistance of the strain gauge changes. Strain gauges are typically fabricated on a flexible membrane and have resistive elements arranged in a Wheatstone bridge configuration. Once the conversion formula is obtained through calibration, the force can be calculated based on the resistance of the strain gauge. Strain gauges can be made into relatively thin sensors; however, due to the area of the membrane itself and the associated wiring for making electrical connections, the application of strain gauges still has limitations. Disadvantages of strain gauges include that the structural element to which they are attached requires a relatively large deformability so that the strain is large enough to be measured. In addition, since the sensor is attached to the structural element, the strain gauge increases the size and may not be suitable for certain geometries, such as cables or wires. Furthermore, the materials used to form strain gauges are generally not suitable for high-temperature and / or chemically sensitive environments.

[0006] Another type of force sensor is a piezoelectric force sensor. Piezoelectric sensors can be based on polymers or ceramics and typically can provide a measurable signal in response to very small strains. However, piezoelectric sensors provide transient signals and are difficult to use for accurately measuring static or slowly varying forces. Considerably high gain amplification is also typically required, which means that piezoelectric force sensors may be vulnerable to electromagnetic interference.

[0007] "Shape Memory Alloy Wire for Force Sensing" by Josephine Selvarani Ruth D and K. Dhanalakshmi, IEEE SENSORS JOURNAL, Vol. 17, No. 4, February 15, 2017, describes the use of a shape memory alloy (SMA) wire stretched between the free end of a cantilever beam and a structure supporting the fixed beam end. The SMA wire is angled with respect to the cantilever beam and is used to infer the force applied to the end of the cantilever beam.

[0008] Summary

[0009] According to a first aspect of the present invention, there is provided a method of determining a force applied between a first part and a second part. The second part is connected to the first part via a shape memory alloy (SMA) element such that the force is transmitted via the SMA element. The force is determined based on the resistance of the SMA element. Alternatively, the force is determined based on the amount of energy applied to the SMA element to maintain the resistance of the SMA element at a predetermined value.

[0010] The first part and the second part may be spaced apart in a first direction. The first part may include a crimp for fixing the SMA element or take the form of a crimp for fixing the SMA element. The second part may include a crimp for fixing the SMA element or take the form of a crimp for fixing the SMA element. The SMA element may extend in the first direction to connect between the first part and the second part. The resistance of the SMA element is the resistance along the SMA element between the first part and the second part.

[0011] The SMA element may include an SMA wire or a length segment thereof or take the form of an SMA wire or a length segment thereof. The SMA element may include a cylinder or take the form of a cylinder. The SMA element may include a rod or take the form of a rod. When moving along a direction parallel to the applied force, the SMA element may have a constant cross-sectional shape and / or area. When moving along a direction parallel to the applied force, the SMA element may have a variable cross-sectional shape and / or area.

[0012] Most of the force applied between the first part and the second part may be transmitted via the SMA element. The SMA element may provide the only non-frictional force parallel to the first direction opposing the movement of the second part relative to the first part. Thus, the SMA element may act as a force transmission element. One or more bearings, guides or similar structures may provide constraints on degrees of freedom other than parallel to the first direction.

[0013] Most of the force may correspond to 50% or more of the applied force. The SMA element may transmit 60% or more of the applied force. The SMA element may transmit 70% or more of the applied force. The SMA element may transmit 75% or more of the applied force. The SMA element may transmit 80% or more of the applied force. The SMA element may transmit 90% or more of the applied force. The SMA element may transmit 95% or more of the applied force. The SMA element may transmit all or substantially all of the applied force.

[0014] Determining the force may include determining the magnitude of the force.

[0015] The force may be applied along the SMA element parallel to the first direction. The SMA element may be configured such that the applied force corresponding to the upper plateau stress of the material of the SMA element varies with the position along the first direction.

[0016] The configuration of the SMA element such that the force applied corresponding to the plateau stress of the SMA material varies with position along the first direction can correspond to the physical shape of the SMA element. Additionally or alternatively, the configuration of the SMA element such that the force applied corresponding to the plateau stress of the SMA material varies with position along the first direction can correspond to the shape and / or arrangement of one or more structural elements in which the SMA element is embedded or to which the SMA element is attached.

[0017] The SMA element can be configured such that the applied force produces a stress gradient along the SMA element. The stress gradient can be between a first portion and a second portion. The stress gradient can be along the first direction.

[0018] A temperature gradient can be applied or generated along the SMA element. The temperature gradient can be between a first portion and a second portion. The temperature gradient can be along the first direction. The temperature gradient can be generated by resistively heating the SMA element using a drive current.

[0019] The cross-sectional area of the SMA element can gradually decrease along the length of the SMA element. The SMA element can gradually decrease between a first portion and a second portion. The SMA element can gradually decrease in the first direction.

[0020] At least a portion of the length of the SMA element can be embedded in or bonded to a second material. The spring constant of the second material can gradually decrease along the length of the SMA element.

[0021] The spring constant of the second material corresponds to the proportionality constant between the applied force transmitted through the length of the second material and the extent of elongation of that length of the second material. The spring constant of the second material can gradually decrease along the length of the SMA element by making the cross-sectional area of the second material gradually decrease. The spring constant of the second material can gradually decrease along the length of the SMA element by varying the stiffness of the second material. For example, the second material can be a blend / composite of two materials (e.g., a matrix and reinforcing fibers) having different volume fractions, or the second material can have different volume fractions or porosities. The spring constant of the second material can gradually decrease along the length of the SMA element by changing the shape of the second material. For example, the depth and / or spacing of serrations (or the like) in the outer surface of the second material can vary.

[0022] The second material may taper in a first direction. Substantially the entire length of the SMA element may be embedded in or bonded to the second material. At least 95% of the length of the SMA element may be embedded in or bonded to the second material. At least 90% of the length of the SMA element may be embedded in or bonded to the second material. At least 85% of the length of the SMA element may be embedded in or bonded to the second material. At least 80% of the length of the SMA element may be embedded in or bonded to the second material. At least 75% of the length of the SMA element may be embedded in or bonded to the second material.

[0023] The second material may include or take the form of a sheath that surrounds and is bonded to the SMA element. The second material is preferably more deformable than the SMA element. The second material may include a deformable material, such as an elastomeric material, or take the form of a deformable material.

[0024] The SMA element may be directly connected to the first part, and a portion of the length of the SMA element may engage a deformable material block that couples the SMA element to the second part.

[0025] This portion of the length of the SMA element may be embedded in or bonded to the deformable material block. This portion of the length of the SMA element may be at least 50%. This portion of the length of the SMA element may be at least 75%. This portion of the length of the SMA element may be at least 80%. This portion of the length of the SMA element may be at least 90%. This portion of the length of the SMA element may be at least 95%. The second part may include one or more extensions that overlap the SMA element in the first direction. The deformable material block may be received within the second part.

[0026] Determining the force based on the amount of energy applied to the SMA element to maintain the resistance of the SMA element at a predetermined value may include: controlling the power applied to cause resistive heating of the SMA element to maintain the length of the SMA element at a constant value, and determining the applied force based on the applied power.

[0027] The length of the SMA element may be maintained at a constant value by monitoring the resistance of the SMA element (i.e., by controlling the applied power to maintain the resistance of the SMA element at a constant value).

[0028] The method may also include an initial step of controlling the power applied to cause resistive heating of the SMA element such that the power history can determine the position of the SMA element within the hysteresis behavior of the SMA material. For example, during the initial step, the SMA element can be heated to a temperature sufficient to convert all or substantially all of the martensitic phase to the austenitic phase, and subsequently allowed to cool to the initial operating value of the applied power.

[0029] According to a second aspect of the invention, there is provided a force sensor comprising a first part and a second part. The second part is connected to the first part via a shape memory alloy (SMA) element such that in response to a force applied between the first part and the second part, the force is transmitted via the SMA element. The force sensor is configured to determine the force applied between the first part and the second part based on the resistance of the SMA element. Alternatively, the force sensor is configured to determine the force applied between the first part and the second part based on the amount of energy applied to the SMA element to maintain the resistance of the SMA element at a predetermined value.

[0030] The force sensor can be used in the method of the first aspect. The force sensor can include features corresponding to any of the features of the method of the first aspect. Any definition applicable to the method (or its features) of the first aspect can equally apply to the force sensor (or its corresponding features).

[0031] The force sensor can be configured to measure the resistance of the SMA element by further including an electrical connection arranged to measure the resistance of the SMA element between the first part and the second part. Additionally and optionally, the force sensor can include a control circuit configured to measure the resistance of the SMA element. The force sensor can be configured to measure the resistance of the SMA element by further including a bridge circuit comprising the SMA element.

[0032] The force sensor can be configured such that most of the force applied between the first part and the second part is transmitted via the SMA element.

[0033] The force sensor can take the form of a load cell.

[0034] A force can be applied along the SMA element parallel to a first direction. The SMA element can be configured such that the minimum applied force required for superelastic deformation of the SMA element varies with position along the first direction.

[0035] The SMA element can be configured such that the applied force creates a stress gradient along the SMA element.

[0036] The force sensor can be configured to apply or generate a temperature gradient along the SMA element.

[0037] The cross-sectional area of the SMA element may gradually decrease along the length of the SMA element.

[0038] At least a portion of the length of the SMA element may be embedded in or bonded to a second material. The spring constant of the second material may gradually decrease along the length of the SMA element.

[0039] The SMA element may be directly connected to the first part. A portion of the length of the SMA element may be engaged with a deformable material block that couples the SMA element to the second part.

[0040] According to a third aspect of the present invention, there is provided a controller configured to determine a force applied between a first part and a second part of a force sensor according to the second aspect. The force is determined based on the resistance of the SMA element. Alternatively, the force is determined based on the amount of energy applied to the SMA element to maintain the resistance of the SMA element at a predetermined value.

[0041] The controller may be configured to implement the method of the first aspect. The controller may include features corresponding to any features of the method of the first aspect and / or the force sensor of the second aspect. Any definition applicable to the method of the first aspect (or its features) and / or the force sensor of the second aspect (or its features) may equally apply to the controller (or its corresponding features).

[0042] The controller may also be configured to provide power to cause resistive heating of the SMA element.

[0043] The controller may also be configured to control the power applied to cause resistive heating of the SMA element to maintain the length of the SMA element at a constant value and determine the applied force based on the applied power.

[0044] According to a third aspect of the present invention, there is provided a computer program stored on a non-transitory computer-readable medium. When executed by a digital electronic processor, the computer program causes the digital electronic processor to execute the method according to the first aspect.

[0045] The computer program may include features corresponding to any features of the method of the first aspect, the force sensor of the second aspect, and / or the controller of the third aspect. Any definition applicable to the method of the first aspect (or its features), the force sensor of the second aspect (or its features), and / or the controller of the third aspect (or its features) may equally apply to the computer program (or its corresponding features).

[0046] The device may include a force sensor according to the first aspect and a controller according to the third aspect, the controller being connected to the force sensor.

[0047] According to a fifth aspect of the present invention, there is provided a tool comprising an actuator mechanically coupled to an operating element by a force transmission mechanism. The force transmission mechanism includes a first SMA element configured to transmit a force between the actuator and the operating element.

[0048] The tool may include features corresponding to any of the features of the method of the first aspect, the force measuring element of the second aspect, the controller of the third aspect, and / or the computer program of the fourth aspect. Any definition applicable to the method of the first aspect and / or its features, the force measuring element applicable to the second aspect and / or its features, the controller applicable to the third aspect and / or its features, and / or the computer program applicable to the fourth aspect and / or its features may be equally applicable to the tool.

[0049] The first SMA element may include an SMA wire or a length segment thereof or take the form of an SMA wire or a length segment thereof. The first SMA element may include a cylinder or take the form of a cylinder. The first SMA element may include a rod or take the form of a rod. When moving along a direction parallel to the applied force, the first SMA element may have a constant cross-sectional shape and / or area. When moving along a direction parallel to the transmitted force, the first SMA element may have a variable cross-sectional shape and / or area.

[0050] The force transmission mechanism may include one or more elements selected from the following: connectors, machines, cables, chains, belts, pulleys, wheels, hydraulic elements, pneumatic elements, pivots, joints, gears, etc.

[0051] The actuator may be manually driven. The actuator may include a manual force input or take the form of an input for manual force, such as a lever, a wheel, a cable, etc. The actuator may be electrically driven. The actuator may be hydraulically driven. The actuator may be pneumatically driven. The actuator may include a motor, a generator, a valve, a shape memory alloy actuator, etc. or take the form of a motor, a generator, a valve, a shape memory alloy actuator, etc.

[0052] The first SMA element may be the last element or the penultimate element of the force transmission mechanism.

[0053] The first SMA element may be configured to undergo superelastic deformation in response to a predetermined threshold transmission force. In this way, the first SMA element can be used to prevent the operating element from outputting excessive force. In other words, the first SMA element can be used as a "physical fuse".

[0054] The predetermined threshold transmission force being predetermined means that it is set before obtaining one or more measured values. The predetermined threshold transmission force can be set by controlling the shape and / or material of the first SMA element.

[0055] The tool can be configured to apply resistive heating to the first SMA element to control the temperature of the SMA element so as to restore the first SMA element to its original shape after superelastic deformation, and / or to change the temperature of the first SMA element to adjust a predetermined threshold transfer force.

[0056] The tool can also be configured to determine the force transmitted by the first SMA element.

[0057] The force transmitted by the first SMA element can be determined using the method according to the first aspect.

[0058] The tool can also be configured to obtain or measure the force output by the actuator, measure the force transmitted by the first SMA element, and in response to the difference between the force output by the actuator and the force transmitted by the first SMA element, control the temperature of the first SMA element to compensate for the difference.

[0059] The measurement of the force transmitted by the SMA element is preferably but not necessarily carried out according to the method of the first aspect.

[0060] The force output by the actuator can be obtained or determined using any suitable force sensor (such as but not limited to a strain gauge, a piezoelectric sensor), the method of the first aspect, and / or a force sensor of the second aspect, etc.

[0061] The temperature control of the first SMA element can be provided by driving a current through the first SMA element to cause joule heating. The temperature control of the first SMA element can be provided by a separate heating element in thermal contact with the first SMA element.

[0062] The tool can also operate in a stable mode, in which the tool is configured to measure the force transmitted by the first SMA element, and in response to the difference between the force transmitted by the first SMA element and the target force, control the temperature of the first SMA element to compensate for the difference.

[0063] The target force can be predetermined, user-determined, or automatically determined. The target force can be determined based on a moving window average of recently measured force values. For example, the user can adjust the tool to a desired force, and then actuate a switch or a toggle element provided on the tool to activate the stable mode, thereby locking the transmitted force to the target force. This can be particularly advantageous when the actuator is manually driven.

[0064] When the actuator is not manually driven, the target force can correspond to the expected actuator output force. For example, in order to set the expected target force, the tool can query the corresponding drive current for a motor, etc. Then, the target force can be used to perform corrections for any fluctuations in the transmitted force using the first SMA element.

[0065] The tool can also operate in a stabilization mode, in which the tool is configured to determine the length of the first SMA element based on the resistance of the first SMA element, and in response to a difference between the length of the first SMA element and a target length, control the temperature of the first SMA element to compensate for the difference.

[0066] The target length can be predefined, user-determined, or automatically determined. The target length can be determined based on a moving window average of recently measured length values. For example, the user can adjust the tool to a desired position and then actuate a switch or toggle element provided on the tool to activate the stabilization mode, thereby locking the corresponding length of the first SMA element to the target length. This can be particularly advantageous when the actuator is manually driven.

[0067] When the actuator is not manually driven, the target length can correspond to the expected position and state of the operating element. For example, to set the expected target length, the tool can query a corresponding drive current for a motor or the like. The target length can then be used to correct any fluctuations in the positioning provided by using the first SMA element.

[0068] The force transmission mechanism can also include a second SMA element that is spaced apart from the first SMA element via at least one other element of the transmission mechanism. The second SMA element can be connected in series with the first SMA element.

[0069] Any function and / or feature described with respect to the SMA element can additionally or alternatively be implemented using the second SMA element.

[0070] The tool can also be configured to cyclically heat the first SMA element, and optionally also the second SMA element, to cause vibration of the force transmission mechanism.

[0071] The vibration generated by the force transmission mechanism preferably has a low amplitude and / or a high frequency. Preferably, closed-loop control is implemented to ensure that the contraction of the first SMA element is equal to the extension of another opposing second SMA element. The frequency of the vibration can be greater than or equal to 10 Hz. The frequency of the vibration can be greater than or equal to 20 Hz. The frequency of the vibration can be greater than or equal to 50 Hz.

[0072] The vibration generated by the force transmission mechanism can help reduce stickiness in the force transmission mechanism caused by one or more elements transitioning between static and dynamic friction. When there are two SMA elements and they are connected in series with each other, the first SMA element and the second SMA element can be controlled such that the contraction of the first SMA element causes the expansion of the second SMA element, and vice versa.

[0073] The operating element can be configured for grasping. The operating element can be configured for cutting. The tool can be a surgical tool. The surgical tool can be used for minimally invasive surgery.

[0074] The force transmission mechanism can include one or more cables or take the form of one or more cables. The first SMA element can be connected in series with at least one of the one or more cables.

[0075] The force transmission mechanism can include a cable-pulley drive or take the form of a cable-pulley drive. The first SMA element can be connected in series with at least one cable of the cable-pulley drive.

[0076] When a second SMA element is included, the second SMA element can be provided at an end of the cable-pulley drive opposite to the first SMA element. For example, the first SMA element can connect the first end of the cable-pulley drive to the operating element, while the second SMA element connects the second end of the cable-pulley drive to the actuator.

[0077] The actuator can include a servo actuator or take the form of a servo actuator. Brief Description of the Drawings

[0079] Certain embodiments of the present invention will now be described by way of example only with reference to the drawings, in which:

[0080] Figure 1 is a schematic block diagram of a force sensor;

[0081] Figure 2 is a schematic cross-section of a first exemplary force sensor;

[0082] Figure 3 is a schematic cross-section of a second exemplary force sensor;

[0083] Figure 4 is a schematic stress-strain curve of a shape memory alloy material;

[0084] Figure 5A is a schematic cross-section of a third exemplary force sensor;

[0085] Figure 5B shows in Figure 5A the stress gradient generated in the third exemplary force sensor shown;

[0086] Figure 6 is a schematic cross-section of a fourth exemplary force sensor;

[0087] Figure 7 is a schematic cross-section of a fifth exemplary force sensor;

[0088] Figure 8A Shows the effect of increasing temperature on the stress-strain curve of a shape memory alloy material;

[0089] Figure 8B Shows the effect of decreasing temperature on the stress-strain curve of a shape memory alloy material;

[0090] Figure 9A Is a schematic resistance-strain diagram of a shape memory alloy element;

[0091] Figure 9B Is a schematic plateau stress-temperature diagram of a shape memory alloy material;

[0092] Figure 10 Is a process flow diagram of an active force measurement method;

[0093] Figure 11 Is a block diagram of a tool including a shape memory alloy element;

[0094] Figure 12 Schematically shows a first exemplary surgical tool;

[0095] Figure 13A And Figure 13B Schematically shows a force correction method; and

[0096] Figure 14 Schematically shows a second exemplary surgical tool.

[0097] Detailed description

[0098] In this specification, methods and apparatuses that can use a shape memory alloy (SMA) element as a structural element or a force transmission element and can also measure an applied force will be described. As further described with respect to Figure 4 Using an SMA element to infer an applied force is difficult because the SMA element may experience a significant increase in strain with no or very little change in stress (so-called "superelastic" behavior). However, the inventors of this specification have overcome this difficulty and designed methods that can relate the resistance of an SMA element to the force applied thereto.

[0099] In a first basic method, the SMA element is arranged to have a thermal gradient or a stress gradient along its length such that an increasing stress causes a gradual strain of the SMA element. The strain of the SMA element is measured by monitoring the resistance of the SMA element, which may be related to the length of the SMA element.

[0100] In a second basic method, in which no thermal or stress gradient along the length of the SMA element is required, power is applied to the SMA element to cause joule heating, thereby actively controlling the SMA element to a desired length (resistance) by controlling the relative fractions of the austenite and martensite phases. The power required to achieve this desired length (which is related to the temperature of the SMA element) is used to calculate the force applied to the SMA element.

[0101] Reference Figure 1 shows a schematic block diagram of the force sensor 1.

[0102] The force sensor 1 includes a first part 2 and a second part 3. The first part 2 and the second part 3 are connected together by an SMA element 4 such that the force F applied between the first part 2 and the second part 3 is at least partially transmitted via the SMA element 4. Optionally, the first part 2 and the second part 3 may also be connected by a support structure 5. A fractional part hF of the force F is transmitted via the SMA element 4, and the remaining part (1 - h)F is transmitted via the support structure 5 if it exists (when there is no support structure, h = 1). Generally, if the force F is not coaxially aligned with the force sensor 1, the proportion of each component transmitted via the SMA element 4 may vary, i.e., for the force F = (F x , F y , F z ), the SMA element 4 may transmit a component force h x F x along a first direction x, a component force h y F y along a second direction (y direction), and a component force hzFz along a third axis (z direction).

[0103] The first part 2 and the second part 3 are spaced apart in a first direction (x-axis, as shown). The SMA element 4 extends in the first direction x to connect between the first part 2 and the second part 3. The resistance R of the SMA element 2 is the resistance between the first part 2 and the second part 3 along the length L of the SMA element 4. The SMA element 2 may be formed of nickel-titanium (nitinol), copper-aluminum-nickel, or any other alloy that exhibits a shape memory alloy effect.

[0104] The SMA element 4 is not particularly limited in size or shape, as long as the geometric dimensions of the SMA element 4 meet the characteristic requirements. Examples of the SMA element 4 may include, but are not limited to, an SMA wire or a length segment thereof, a cylinder, a rod, etc. In some examples, the SMA element 4 may have a constant cross-sectional shape and / or area along the first direction x (see, for example, Figure 6 and Figure 7 ). Alternatively, in other examples, the SMA element 4 may have a cross-sectional shape and / or area that varies along the first direction x (see, for example,Figure 5A and Figure 5B )。

[0105] The first part 2 and the second part 3 include fastening / connecting means for mounting and reliably transferring the force F to the SMA element 4. For example, if the SMA element 4 is a wire or a rod, the first part 2 and the second part 3 may each include a crimp connector for fixing the SMA element 4 or take the form of a crimp connector for fixing the SMA element 4.

[0106] The force F is determined based on the resistance R of the SMA element 4 of the force sensor 1. As Figure 1 shown, the controller 6 is connected to the force sensor 1 via an electrical coupling 7 (e.g., two or more wires). The controller 6 is configured to measure the resistance of the SMA element 4 and, based thereon, infer the strain ε of the SMA element 4 along the length of the SMA element 4 (along the first direction x). Based on the strain ε, the component force h of the applied force F along the length of the SMA element 4 can be determined x F x , and the applied force F is determined according to appropriate pre - calibrations. Although any suitable resistance R measurement technique can be used, the Wheatstone bridge resistance measurement method is preferred. The force sensor 1 may include only the connection components for the coupling 7 to the controller 6, or alternatively, other components of the Wheatstone bridge and / or other control / measurement circuits may also be integrated as part of the force sensor 1. In some examples, the controller 6 may be integrated with the force sensor 1 as part of a single package.

[0107] In addition to resistance measurement, the controller 6 can also use the electrical coupling 7 to drive a current I d through the SMA element 4 to utilize Joule heating I d 2 R to heat the SMA element 4 into the transition zone between the martensitic phase and the austenitic phase. As described below, in some examples, the exact amount of the delivered power I d 2 R can be used as part of a method for determining the applied force F.

[0108] Examples of resistance feedback control techniques that can be implemented using the controller 6 are described in WO 2014 / 076463A1, which is incorporated herein by reference. Useful backgrounds for methods of driving SMA wires 19, 22 are also provided in WO 2013 / 175197A1 and WO 2019 / 073212A1, both of which are incorporated herein by reference.

[0109] First Exemplary Force Sensor

[0110] Reference is also made to Figure 2 , which shows a schematic cross-section of a first exemplary force sensor 8 (hereinafter referred to as the "first sensor").

[0111] In the first sensor 8, most of the force F applied between the first part 2 and the second part 3 is transmitted via the SMA element 4. In other words, the fraction h > 0.5, preferably h >> 0.5, for example h > 0.95.

[0112] In the first sensor 8, the support structure 5 takes the form of a hollow cylindrical extension 9 extending from the first part 2 in the first direction x. For example, if the first part 2 is circular in the y-z plane, the extension 9 will take the form of a hollow cylinder and similar shapes. The second part 3 is received within the extension 9 such that the degrees of freedom of the second part 3 relative to the first part 2 are constrained except along the first direction x. In this way, the SMA element 4 coupled between the first part 2 and the second part 3 provides the only non-frictional force in the direction opposite to the movement of the second part 3 relative to the first part 2 parallel to the first direction x. In other words, the SMA element 4 transmits substantially all of the component F of the applied force F in the first direction x x . Preferably, the support structure 5 bears substantially all of any vertical component F y . For example, the hollow extension 9 can be directly configured to have a significantly greater flexural stiffness than the SMA element 4.

[0113] In Figure 2 , the first sensor 8 is shown, in which the second part 3 and the extension 9 are in sliding contact at the interface 10. However, one or more bearings can be provided at the interface to reduce the friction between the extension 9 and the second part 3.

[0114] Second Exemplary Force Sensor

[0115] Reference is also made to Figure 3 , which shows a schematic cross-section of a second exemplary force sensor 11 (hereinafter referred to as the "second sensor").

[0116] The first sensor 8 is easy to explain because substantially all of the component of the force along the first direction is transmitted via the SMA element 4. This configuration may not be suitable for all applications. For example, when the magnitude F of the force F is large, the size of the SMA element 4 required to support the entire load may become too large and thus difficult to heat with Joule heating and / or respond slowly to temperature changes due to the increased thermal mass. Additionally, unless controlled using one of the active configurations described herein, the first sensor 8 will exhibit poor linearity of the SMA response (see Figure 4the σ in UP )。This means that the length L and the resistance R of the SMA element 4 will suddenly change at this stress σ UP while, for a force sensor, it is desirable for the resistance to change proportionally to the applied force. The configurations described herein present various ways for the stress-induced transformation in the SMA element 4 to occur over a greater range of stress σ.

[0117] The second force sensor 11 includes a support structure 5 in the form of a flexure 12 that mechanically connects the second portion 2 and the third portion 3 to the SMA element 4 in parallel. The material, shape, and dimensions of the flexure 12 can be controlled using conventional mechanical design techniques to provide a desired combination of load-bearing capacity, deformability parallel to the first direction x, and deformability in a direction perpendicular to the first direction x.

[0118] Stress-Strain Characteristics of Shape Memory Alloy

[0119] Assuming axial orientation, the force F on an object (with a constant cross-section) is F = | F |, which is the product of the stress σ and the cross-sectional area A:

[0120] F = σA (1)

[0121] For a regular elastic material, the stress σ is related to the strain ε by Young's modulus E:

[0122] σ = εE (2)

[0123] Therefore:

[0124] F = εEA (3)

[0125] However, while it is relatively straightforward to determine the length L (and thus the strain ε) of the SMA element 4 from the measured change in resistance R, equations (1) to (3) cannot be used to calculate the magnitude F of the applied force F because SMA materials do not exhibit linear elastic behavior above a threshold stress level.

[0126] Also referring to Figure 4 , a schematic stress-strain σ-ε curve for the SMA material is shown.

[0127] In the initial linear elastic region 13, the strain ε increases linearly with the stress σ. As the stress σ increases over the transition region 14, the relationship with the strain ε deviates from linear elasticity because some austenite phase regions begin to undergo a diffusionless (shear) transformation to the martensite phase, eventually reaching the upper plateau region 15, where the stress σ remains substantially constant at the upper plateau stress σ UP, and the strain ε changes adaptively due to the phase transformation of the SMA material from austenite to martensite phase. As the strain ε increases further, another transition region 16 is reached because all available austenite phase has been transformed, thereby entering a second, upper linear elastic region 17. If the stress σ increases further, exceeding the ultimate strength (not shown), permanent plastic deformation may result. Plastic deformation represents an unintended and unwanted behavior in a force sensor and, unless otherwise specified, is not relevant to the presently disclosed force sensor and the tool using the force sensor.

[0128] As the load is reduced, the upper linear elastic region 17 continues to decrease below the upper plateau stress σ as the martensite phase begins to undergo a diffusionless (shear) transformation back to the austenite phase. UP The stress σ is then entered into the transition region 18. This is followed by a lower plateau region 19, where the strain is substantially constant at the lower plateau stress σ LP The lower linear elastic region 13 is reduced as the SMA material transforms from the martensite phase to the austenite phase. When the transformation to the austenite phase is completed, the curved transition section 20 merges back into the lower linear elastic region 13.

[0129] This stress-strain behavior of shape memory alloys is sometimes referred to as "superelasticity", e.g. Figure 4 As shown, significant hysteresis is shown. In addition, the stress-strain curve is a function of the temperature of the SMA material. As the temperature increases, the plateau stress σ UP , σ LP becomes higher, the temperature decreases, and the platform stress σ UP , σ LP Become lower.

[0130] For this reason, although a change δL in the length L of the SMA element 4 can be detected by measuring the resistance R, a simple conversion back to the corresponding stress σ (and therefore force F) is not possible, because:

[0131] - there is no one-to-one mapping on the stress-strain curve; and

[0132] - Single platform stress σ UP , σ LP may correspond to a large range of strain ε.

[0133] The inventors of the present specification have devised two broad approaches to overcome this fundamental problem:

[0134] A. Passive method, in which the force sensor 1 is configured so that the corresponding platform stress (upper platform stress σ UP Or the lower platform stress σ LP One) of the applied force F varies with position along the length of the SMA element 4; and / or

[0135] B. Active method, based on controlling the power supplied to the SMA element 4 for Joule heating in order to control the length of the SMA element 4 (as determined by measuring through the resistor R) to a fixed value.

[0136] The passive and active methods for providing the SMA-based force sensor 1 are described in detail below.

[0137] Passive Configuration

[0138] The first method is to configure the SMA element 4 of the force sensor 1 such that the applied force F corresponding to the plateau stress (upper plateau stress σ UP or lower plateau stress σ LP varies with the position along the length of the SMA element 4.

[0139] This can be achieved in practice by configuring the SMA element 4 to experience a stress and / or temperature gradient along the length L of the SMA element 4 (i.e., between the first part 2 and the second part 3). Generally, this can be achieved by controlling one or more of the physical shape of the SMA element 4 and / or the shape and arrangement of one or more structural elements, where one or more SMA elements 4 are embedded within the one or more structural elements or the SMA element 4 is attached, bonded, or otherwise mechanically coupled to the one or more structural elements. The temperature gradient can be generated by resistively heating the SMA element 4 using a drive current or by an auxiliary heat source. For example, a separate resistive heating coil wound around the SMA element 4 or received / embedded within the SMA element 4.

[0140] Third Exemplary Force Sensor

[0141] Also refer to Figure 5A which shows a schematic cross-section of a third exemplary force sensor 21 (hereinafter referred to as the "third sensor").

[0142] The third sensor 21 is an example of generating a stress gradient in the SMA element 4 and includes a tapered SMA element 22 connecting the first part 2 and the second part 3. The third sensor 21 can include any type of support structure 5 described herein, but for visual clarity, the support structure 5 is omitted from Figure 5A . The SMA element 4 is cylindrically symmetric about the first direction x, and the cross-sectional area of the SMA element 4 gradually decreases along the length of the SMA element 4. Specifically, the radius of the SMA element 4 linearly increases along the length L. In Figure 5A it is shown to gradually decrease from the second part 3 towards the first part 2, but equally, the cross-sectional area of the SMA element 4 can gradually decrease in the opposite direction.

[0143] In the case where the component force F x acts along the length L of the SMA element (parallel to the x-axis as shown in the figure), the stress at a given point is σ = F x / A, such that as the radius r(x) varies:

[0144]

[0145] Also referring to Figure 5B , for a pair of forces and a linearly varying radius r as Figure 5A shown, this relationship between stress σ and position is illustrated.

[0146] The first stress gradient 23 corresponds to the first force F 1 x , for which first force F 1 x , the upper plateau stress σ UP appears at the first position x1 along the SMA element 4. Thus, the narrow regions on both sides of the SMA element 4 will be within the linear elastic regions 13, 17 of the stress-strain curve (see Figure 4 ). If the force is increased to F 2 x > F 1 x , then a second stress gradient 24 is experienced, and the position at which the upper plateau stress σ UP is experienced moves to the second position x2.

[0147] Thus, although the stress-strain curve for each local volume of the SMA material still exhibits plateau segments σ UP , σ LP , the overall force-strain curve (force vs. the engineering strain ΔL / L over the entirety of the SMA element 4) is not, which allows the strain ε of the SMA element (determined by the resistance R) to be correlated with the component force F x applied along the SMA element 4. In use, the thermomechanical history of the SMA element 4 should be tracked so that it can be determined which branch of the stress-strain hysteresis curve the SMA material is undergoing (within the local regions experiencing the plateau stress σ UP or σ LP for a given applied force).

[0148] Although explained in connection with the tapered cylindrically symmetric SMA element 4 scenario, any shape with a cross-sectional area A that varies with position x in a first direction can be used. For example, the SMA element 4 can be wedge-shaped, such as a film or foil with a varying thickness.

[0149] Preferably, the SMA element 4 is also heated, for example by using a drive current I d for Joule heating, and the tapered area A will also change the resistance and thus the local resistance heating I d 2 R, and will also change the rate of heat radiation and thus the rate of heat loss, thereby creating a gradient in temperature and stress σ.

[0150] Fourth Exemplary Force Sensor

[0151] Reference is also made to Figure 6 which shows a schematic cross-section of a fourth exemplary force sensor 25 (hereinafter referred to as the "fourth sensor").

[0152] The fourth sensor 25 is an example of creating a stress gradient in the SMA element 4 and includes an SMA element 4 with a constant cross-section embedded and bonded to a cylindrically symmetric second material 26, the second material 26 including a tapered region 27 that corresponds to the length L of the SMA element 4 between the first part 2 and the second part 3. The proportion of the force component F x supported by the SMA element between the first part 2 and the second part 3 varies and increases as the radius of the tapered region 27 decreases. In this way, the overall force-strain curve (the engineering strain of the SMA element 4 overall) is modified to remove the plateau segments (although these plateau segments are of course still inherent in the stress-strain curve of the actual SMA material at the local level).

[0153] Preferably, as Figure 6 shown, the second material 26 is also fixedly bonded to the second part 3. The second material 26 is preferably more deformable (less rigid) than the material of the SMA element 4. For example, the second material may comprise or take the form of a deformable material, such as an elastomeric material. Preferably, as Figure 6 shown, the entire span of the SMA element 4 between the first part 2 and the second part 3 is embedded (or otherwise joined) in the second material 26. However, in some examples, a smaller portion of the length L of the SMA element 4 may be embedded / joined in the second material 26.

[0154] Importantly, the interfacial adhesion between the SMA element 4 and the second material 26 is strong such that there is no delamination / slipping between the two.

[0155] Preferably, the SMA element 4 is also heated, for example by using a drive current I d for Joule heating, and the tapered region 27 of the second material 26 will also change the heat flux away from the SMA element 4, thereby creating a gradient in temperature and stress σ.

[0156] Despite Figure 6 2 is shown as tapering from the second portion 3 to the first portion 2, but similarly, the tapered region 27 may taper from the first portion 2 to the second portion 3. In addition, the second material 26 need not be cylindrically symmetrical, for example, if the SMA element 4 is in the form of a film or plate, the tapered region 27 may be in the form of a wedge.

[0157] Furthermore, the basic principle of the fourth sensor 25 is not limited to generating a stress gradient by including a tapered portion 27. Any method that results in a change in the overall spring constant of the second material 26 along the length L of the SMA element 4 may be employed. Here, the spring constant of the second material 26 means a proportional constant between an applied force transmitted through the length of the second material and the extension of the length of the second material 26.

[0158] For example, refer again to Figure 6 , an alternative profile 28 of the second material 26 is shown. The alternative profile 28 includes a cutout 29 (alternatively, a serration or other cutout) that tapers along the length L of the SMA element 4. The free surface of the cutout necessarily has zero stress, which means that the effective load-bearing area of the second material 26 is substantially the same as that using the tapered region 27.

[0159] This is just one example of how to achieve a spatially varying spring constant of the second material 26. Other options include using a second material 26 that is a blend / composite of two materials having different volume proportions (e.g., matrix and reinforcing fibers) along the length L of the SMA element 4. Alternatively, the second material may have a varying void volume fraction along the length L of the SMA element 4.

[0160] Although a constant cross-sectional area A of the SMA element 4 is illustrated and described, this is not required and the SMA element 4 may also have a varying cross-sectional area A in order to increase the stress gradient.

[0161] Fifth Exemplary Force Sensor

[0162] Also refer to Figure 7 , shows a schematic cross-section of a fifth exemplary force sensor 30 (hereinafter referred to as “fifth sensor”).

[0163] The fifth sensor 30 is an example of generating a stress gradient in the SMA element 4 and includes the SMA element 4 directly connected to the first part 2, where a part of the length of the SMA element 4 is joined (embedded in or otherwise bonded to the deformable material block 31) with the deformable material block 31 that couples the SMA element 4 to the second part 3. The part of the length L of the SMA element 4 that is joined with the block 31 can be at least 50%, but preferably greater, and ideally the entire length L (the block 31 can be attached to the first part 2 or can not be attached to the first part 2). The deformable material herein refers to a material that is more deformable (less rigid) than the SMA element 4, such as an elastomeric material (such as silicone rubber).

[0164] The second part 3 includes a hollow cylindrical extension 32 that overlaps the SMA element 4 along the first direction x towards the first part 2. The deformable material block 31 is received within the hollow cylindrical extension 32 of the second part 3 and bonded to the hollow cylindrical extension 32 of the second part 3. Within the part of the SMA element 4 that is not joined with the deformable material block 31, the force F x is all supported by the SMA element 4. On the part joined with the deformable material block 31, the proportion of the force F supported by the SMA element 4 x decreases and necessarily reaches zero at the free end 33 of the SMA element 4. Thus, a stress σ gradient is generated in the SMA element 4 such that the overall force-strain curve (engineering strain) of the SMA element 4 does not include a plateau segment.

[0165] Importantly, the interfacial adhesion between the SMA element 4 and the deformable material block 31 is strong such that there is no delamination / slipping between the two.

[0166] The deformable material block 31 does not have to be cylindrically symmetric. For example, if the SMA element 4 takes the form of a film or a plate, the deformable material block 31 can take the form of a cube, and the extension of the second part 2 can have a shape adapted thereto, such as a rectangular prism or a pair of parallel plates sandwiching the deformable material block 31.

[0167] Active Configuration

[0168] As an alternative to configuring the force sensor 1 to cause a stress and / or temperature gradient in the SMA element 4 in order to eliminate the plateau segment from the overall force-strain characteristics of the force sensor 1, active measurement can be performed based on the power supplied for heating the SMA element 4.

[0169] Supplied to cause resistive heating I of the SMA element 4 d 2The power P of R will quickly reach equilibrium with the heat loss from the SMA element 4 (the sum of radiation, convection, and diffusion), such that a given value of the power P corresponds to the temperature T. The length L of the SMA element 4 is related to the temperature T because the temperature T changes the relative proportion of the martensite phase and the austenite phase (within a certain range).

[0170] Also refer to Figure 8A that increasing the temperature T of the SMA element 4 causes the plateau segments σ UP 、σ LP to move to higher stress σ values.

[0171] The stress-strain curve 33 for the temperature T is shown, and the corresponding stress-strain curve 34 for the elevated temperature T + δT is shown.

[0172] Also refer to Figure 8B that decreasing the temperature T of the SMA element 4 causes the plateau segments σ UP 、σ LP to move to lower stress σ values.

[0173] The stress-strain curve 35 for the decreased temperature T + δT is shown, and the same stress-strain curve 33 shown for the temperature T in Figure 8A is also shown as a reference.

[0174] In the active force sensing configuration, the power P supplied to cause resistive heating I d 2 of the SMA element 4 is controlled to maintain the length L of the SMA element 4 at a fixed value L0. The relationship between the applied power P and the component F x of the force applied to the force sensor 1 can be determined from a calibration experiment for a given fixed length L0, thereby allowing an unknown applied force F to be inferred based on the power P x .

[0175] The length L of the SMA element 4 is maintained at a fixed value L0 by the controller 6 monitoring the resistance R of the SMA element 4. In other words, the controller 6 changes the supplied power P in order to maintain the resistance R of the SMA element 4 at a fixed value R0 (corresponding to the length L0).

[0176] The resistance of the SMA element 4 with a fixed cross-section is given by:[[]]

[0177]

[0178] where ρ is the resistivity, L is the length of the SMA element 4, and A is the cross-sectional area. Thus, the relationship between the resistance R of the SMA element 4 and the strain ε can be calibrated.

[0179] Also refer to Figure 9A, showing a schematic resistance-strain diagram 33.

[0180] For the sake of explanation, the schematic resistance-strain curve 36 is shown as linear. However, the resistance-strain diagram 36 can be non-linear, and for a given force sensor 1 and SMA element 4, the resistance-strain diagram 33 will need to be calibrated before use.

[0181] Also refer to Figure 9B , showing a schematic platform stress-temperature diagram 37.

[0182] The platform stress-temperature diagram 37 corresponds to the variation of the upper platform stress σ UP as a function of temperature T (see Figure 8A and Figure 8B ). As described above, the temperature T of the SMA element is a function of the applied power P. The upper platform stress σ UP is shown, but the lower platform stress σ LP can also be equivalently used, since the behavior with temperature change is the same.

[0183] Specifically refer to Figure 9A and Figure 9B , and the configuration of the force sensor 1 for force measurement for active strain control will be described.

[0184] When the force sensor 1 is not subjected to an external force F, a baseline power P0 is provided (by the controller 6) for Joule heating of the SMA element 4, corresponding to a fixed strain ε0 with a corresponding resistance R0. The power P0 corresponds to a baseline operating temperature T0 = T(P0). The fixed strain ε0 is not necessarily zero, since the strain can be defined relative to the unloaded and unpowered SMA element (i.e., the ambient temperature). The baseline operating temperature T0 should correspond to the point midway between the transformation of the SMA element 4 between the martensitic and austenitic phases.

[0185] When the controller 6 detects a change δR in the resistance R of the SMA element 4, this will correspond to a change δε in the strain (see Figure 9A ).

[0186] The controller will increase or decrease the applied power P as needed to restore the strain ε (and thus the resistance R) to a fixed value ε0 (R0). For example, if the strain ε increases (tension) +δε, the controller 6 detects the corresponding resistance change δR. The controller 6 increases the power P until the resistance R returns to the fixed value R0, corresponding to the power P + δP. However, the increased power will cause the SMA element 4 to equilibrate at an increased temperature T(P + δP), corresponding to an elevated platform stress σ UP . Then this elevated platform stress σ UPConverted to the applied force component F applied to the force sensor 1 x By reducing the power –δP to determine the applied compressive force component –F x The same process can be applied to the applied reduction –δR in the resistance R of the SMA element 4.

[0187] In practice, since the force component F x and the stress σ in the SMA element 4 are related, and since the temperature T and the power P are related, the force sensor 1 configured for active strain control can be directly calibrated to generate the force F along the SMA element by applying a known force (e.g., a standard mass) and measuring the power P that needs to be applied to obtain the desired operating point resistance R0 x versus the applied power P curve provided by the controller 6. In this way, it is not necessary to measure the exact relationship between the resistance R and the strain ε, the power P and the temperature T, or the stress σ in the SMA element 4 and the force F x applied between the first part 2 and the second part 3.

[0188] When the force sensor 1 may need to operate over a wider temperature range, a significant change in the ambient temperature T 环境 may affect the exact relationship between the supplied power P and the corresponding equilibrium temperature T(P) of the SMA element 4. This can be taken into account by performing calibration to determine the force F 环境 applied at ambient temperature ranges T x across the required operating range. These characteristics can be stored in the controller 6, and an ambient temperature sensor (not shown) is coupled to the controller 6 to allow querying of the appropriate characteristics for the measured ambient temperature T 环境 When the measured ambient temperature T 环境 is between multiple values of specifically calibrated values, interpolation can be performed using known techniques (techniques applied to other types of sensors with ambient temperature calibration).

[0189] The active strain control method does indeed require knowing on which branch of the hysteresis loop the SMA element 4 is, in other words, it is necessary to know whether the SMA element 4 is operating at the upper plateau stress σ UP or at the lower plateau stress σ LPWork on this can be done by monitoring the history of the resistance R. Alternatively, when the force sensor 1 is initially turned on, the controller 6 can perform an initial step of controlling the power P to cause resistive heating of the SMA element 4, such that the history of the power P can determine the position of the SMA element 4 in the hysteresis behavior of the SMA material. For example, during the initial step, the controller 6 can heat the SMA element 4 to a temperature sufficient to transform all or substantially all of the martensite phase to the austenite phase, and then allow the SMA element 4 to cool to the initial operating value P0 of the applied power P.

[0190] Reference Figure 10 , describes a method for active force measurement. In step 100, power is applied to the SMA element to cause resistive heating in order to maintain the length of the element (which corresponds to the resistance of the element). In step 102, the force applied to the SMA element is determined based on the applied power. Similarly, the force can be determined based on the amount of energy supplied to the SMA element.

[0191] Tool Incorporating SMA

[0192] Also reference Figure 11 , shows a block diagram of a tool 38 including the SMA element 4.

[0193] The tool 38 includes an actuator 39 (including a manually operable mechanism), and the actuator 39 is mechanically coupled to the operating element 40 through a force transmission mechanism 41. The force transmission mechanism 41 includes a first SMA element 4a, which is configured to transmit a force F between the actuator 39 and the operating element 40. The force transmission mechanism 41 further includes one or more linkages 42 for transmitting the force F between the actuator 39 and the operating element 40. Although shown as being mechanically in series with the linkage 42 in Figure 11 , the first SMA element 4a can alternatively be connected in parallel with one or more of the linkages 42.

[0194] The linkage 42 can generally take any form suitable for transmitting the force F, including but not limited to machines, cables, chains, belts, pulleys, wheels, hydraulic components, pneumatic components, pivots, joints, gears, etc.

[0195] The actuator 39 can be electrically driven, hydraulically driven, pneumatically driven or even manually driven. Examples of manually driven actuators 30 include but are not limited to input ends for manual force, such as levers, wheels, cables, etc. Examples of non-manually driven actuators 39 include but are not limited to motors, generators, valves, shape memory alloy actuators, piezoelectric actuators, etc.

[0196] The first SMA element 4a is an example of the SMA element 4 described with respect to the force sensor 1 and can take any form described herein, such as for example a wire, cylinder, rod, plate, foil, etc. The first SMA element 4a can be part of any force sensor 1 described herein, i.e., comprising a first part 2 and a second part 3. Alternatively, the first SMA element 4a can be combined with the first part 2 and the second part 3 to form a force sensor 1 as described herein, the first part 2 and the second part 3 forming part of adjacent (in the force transmission mechanism) connecting elements 42 and / or operating elements 40. In some embodiments, the SMA element 4 may not form part of the force sensor.

[0197] Preferably, but not necessarily, the first SMA element 4a can provide the last or the penultimate element of the force transmission mechanism 41 (or be connected in parallel with the last or the penultimate element of the force transmission mechanism 41). For example, the SMA element 4a is preferably directly coupled to the operating element 40 or to the operating element via an additional connecting element 42.

[0198] The operating element 40 can be configured for gripping, cutting, slicing or any other operation that may be required. For example, the operating element 40 can take the form of a surgical tool, such as a clamp, gripper, scissors, suction nozzle, etc. The tool 38 can be particularly beneficial for implementing surgical tools for minimally invasive surgery (see also Figures 12 to 14 ). The tool 38 or a plurality of tools 38 can form part of a robotic surgical system.

[0199] Alternatively, the tool 38 and the operating element 40 can be configured for an assembly / manufacturing process. For example, the operating element 40 can take the form of a suction nozzle or gripper for a pick and place machine, a soldering iron for soldering components, a sensor that requires contact (such as an eddy current sensor), etc.

[0200] The tool 38 can optionally include a second SMA element 4b connected in series between the force transmission mechanism 41 and the actuator 39, preferably the last or the penultimate element in the direction from the operating element 40 to the actuator 39. Examples of using the second SMA element 4b are described with respect to Figure 14 .

[0201] Overforce Protection

[0202] The first SMA element 4a can be configured to undergo superelastic deformation in response to a transmitted force F of a predetermined threshold magnitude 阈值 In other words, the size, dimensions and / or material of the first SMA element 4a are arranged such that the upper platform stress σ UP corresponds to the desired threshold force F 阈值In this way, the first SMA element 4a can be used to prevent the operating element from outputting excessive force. In other words, the first SMA element can be used as a "physical fuse". This can help protect the operating element 40 and / or the object on which the operating element is used from damage.

[0203] One-time over-force protection may have limited utility, especially if the tool 38 is intended to be reusable. The first SMA element 4a can be heated to restore it to its trained shape, for example by immersion in hot water / oil or application of a hot air gun. However, more conveniently, the tool 38 can include a controller 6 which, after superelastic deformation has occurred, can apply resistive heating to the first SMA element 4a to control the temperature of the first SMA element 4a and reset the first SMA element 4a to its original (trained) shape. Additionally or alternatively, the controller 6 can also modify the supplied power P to change the temperature T of the first SMA element 4a in order to adjust the threshold force F. 阈值 For example, the threshold force F 阈值 can be user-adjustable (within a certain range).

[0204] Force Measurement

[0205] In some examples, the tool 38 can be configured to determine the magnitude of the force F transmitted by the first SMA element 4a. In such examples, the tool 38 also includes a controller 6 and the first SMA element 4a forms part of a force sensor 1. The passive (stress and / or temperature gradient) configuration described above (see Figures 5A to 7 ), the active strain configuration described above (see Figures 8A to 10 ) or a combination of both can be used for force measurement.

[0206] Measurement of the force F transmitted by the first SMA element 4a can be used to provide feedback to the operator, for example using an output display. Additionally, measurement of the force F transmitted by the first SMA element 4a can be incorporated into useful automatic feedback methods, examples of which are described below.

[0207] First Exemplary Surgical Tool

[0208] Surgical tools are allowed to perform specific tasks during operation. These tools can be manually operated or used in a robotic surgical system. The common point between the two applications is the use of a cable to transmit power from an actuation source to the distal end of the instrument, where the operating element 40 performs specific operations such as grasping or cutting. Due to form factor limitations, force and flexibility requirements, etc., the cable usually passes through a series of pulleys, and each pulley results in a loss of transmission efficiency due to friction in the pulley bearings, the varying bending stiffness of the cable, and fatigue. This leads to inaccurate positioning in the distal mechanism. Alternatively, pulleys are not required, and the cable can simply pass through a sheath (the cable is not attached to the sheath).

[0209] The surgical tool implemented according to the tool 38 includes a first SMA element 4a, which, in addition to being used for force F measurement, can itself be used as an actuator to allow changing the tension F in the cable in order to measure and / or compensate for the losses described above.

[0210] Also referring to Figure 12 , a first exemplary surgical tool 43 (hereinafter referred to as the "first tool") is shown.

[0211] The first tool 43 utilizes a cable drive mechanism in series with an SMA element 4 in the form of an SMA wire. The actuator 39 can be a servo actuator for robotic surgery, but in the case of manual surgery, it will be the force applied by the surgeon. The force F is transmitted to the operating element 40, such as the gripper mechanism 44 as shown (but any other operating element 40 useful for surgery can be used). The cable drive mechanism includes a system of a cable 45 and pulleys 46, and for most of the length between the actuator 39 and the gripper 44, the cable 45 is contained within a cable sheath 47, which both protects and constrains the cable 45.

[0212] In the first tool 43, a pair of cables 45 are coupled to each arm of the gripper 44 via respective first SMA elements 4a. The first SMA elements 4a are placed at the distal end of the first tool 43 as the last element of the force transmission mechanism 41. Each first SMA element 4a forms a force sensor 1, where a first portion 2 (such as a crimp joint) is coupled to the respective cable 45, and a second portion 3 is attached to or integrally formed with one of the arms of the gripper 44. In other examples, a single cable 45 that is counteracted by a spring acting on the gripper can be used.

[0213] The controller 6( Figure 12(not shown) is integrated with or coupled to the first tool 43. Each first SMA wire 4a can be coupled to a respective controller 6, or alternatively, a single controller 6 can include multiple channels, each channel for a different force sensor 1. When the first tool 43 is part of a surgical robot, the controller 6 can be an integral part of the surgical robot.

[0214] Using the controller 6 and the force sensor 1, the force applied to the gripper 44 (and applied by the gripper 44) can be accurately and continuously monitored, enabling the surgeon to accurately know the applied force exerted by the operating element 40, such as the gripper 44, on the tissue being manipulated during the surgery using the robotic system and / or the minimally invasive surgical tool. This can help reduce the problem of applying too much or too little force, either of which can potentially cause tissue damage during the surgery.

[0215] When included in the first tool 43, for example, to measure the force F output by the actuator 39 输出 , a pair of second SMA elements 4b can be provided at the end of the cable-pulley drive device opposite to the first SMA element 4a (see Figure 14 ). For example, the first SMA element 4a can connect the first end of the cable-pulley drive device to the operating element, while the second SMA element 4b connects the second end of the cable-pulley drive device to the actuator 39.

[0216] Force Loss Correction

[0217] Also referring to Figure 11 , the tool 38 is not limited to only measuring the magnitude of the force F transmitted by the first SMA element 4a 传递 , and the first SMA element 4a can also be actuated to adjust the transmitted force F 传递 . Generally, the SMA elements 4, 4a, 4b can contract by changing the temperature (usually when heated), or be allowed to expand in response to a force (usually by allowing cooling). This can be controlled by the controller 6 using the power P provided for Joule heating.

[0218] The actuation of the first SMA element 4a can be used to compensate for losses caused by friction, mechanical deformability, etc. in the connecting member 42 of the force transmission mechanism 41 between the actuator 39 and the first SMA element 4a.

[0219] Measure or otherwise obtain the force F output by the actuator 39 输出 . For example, any suitable force sensor can be used to measure the force output F 输出 , including but not limited to the combination with the second SMA element 4b described herein (see Figure 14) force sensors 1, strain gauges, piezoelectric force sensors, etc. Alternatively, when the actuator 39 has a controlled force output, the set value of the actuator 39 can be used as F 输出 .

[0220] Measure the force F transmitted by the first SMA element 4a using any combination of the force sensor 1 and methods described herein 传 delivery.

[0221] Determine the force output F 输出 and the transmitted force F 传递 The difference between them, ΔF = F 输出 - F 传递 . If there is a difference ΔF, the controller 6 actuates the first SMA element 4a to compensate for the difference ΔF, i.e., reduce or eliminate (within the measurement accuracy) the difference ΔF. Depending on the sign of the difference ΔF, the actuation can take the form of increasing the temperature of the first SMA element 4a (by increasing the drive current), or it can take the form of allowing the temperature of the first SMA element 4a to decrease (by reducing the drive current, plus natural heat loss).

[0222] This compensation for the transmitted force F 传递 can help minimize the effects of mechanical deformations, clearances, etc. in the force transmission mechanism 41.

[0223] Heating the first SMA element 4a to cause actuation has been described as resulting from Joule heating due to the drive current I d . However, in some examples, the controller 6 can control an alternative heat source external to the SMA element 4 (such as a separate heating element in thermal contact with the first SMA element), such as by generating eddy currents in the SMA element 4, etc.

[0224] Also refer to Figure 13A and Figure 13B , a force correction method will be shown with reference to the first tool 43.

[0225] Specifically refer to Figure 13A , the first SMA element 4a is used to measure the transmitted force F 传递 , and it is found that F 传递 is less than the output force F of the actuator 39 输出 , F 传递 < F 输出 . The difference ΔF = F 输出 - F 传递 manifests as a decrease in the tension in the cable 45 at the distal end (gripper 44), and is caused by a variety of different reasons, including but not limited to the friction between the cable 45 and the cable sheath 47, the mechanical deformability of the cable 45 and / or the pulley 46, etc.

[0226] Specifically refer to Figure 13B, the first SMA element 4a is actuated to reduce its length by an amount δL, thereby increasing the tension at the distal end to compensate for the loss until the transmitted force F 传递 equals the output force F 输出 . In this way, the actuation of the first SMA element 4a can remove the slack caused by the cable-pulley system.

[0227] Although the measurement of the transmitted force F 传递 uses the first SMA element 4a for both measurement and actuation, in other examples, the first SMA element 4a can be used purely for actuation, and alternatively, the transmitted force F can be measured by using a force sensor (any suitable type of force sensor) separate from the first SMA element 4a 传递 .

[0228] Fluctuation / jitter corrections - Stable Mode

[0229] Additionally and / or alternatively, force correction is implemented to match the average output force F 输出 , and the tool 38 can also implement force correction to compensate for the fluctuations in the output force F 输出 .

[0230] For example, when the actuator 39 is manual, such as during a surgical operation using a manually actuated tool 39, this kind of fluctuation may be the result of hand-holding. Even if the actuator 39 is not manual, there may still be fluctuations, such as those generated in the actuator 39 itself and / or propagated by fluctuations in the electrical / hydraulic / pneumatic supply.

[0231] The first SMA element 4a can be used as an actuator to actively stabilize the movement of the tool 38 (especially the operating element 40). This "stabilization mode" can be always active, or it can be selectable by the user when needed. Measure the transmitted force F 传递 and compare it with the target force F 目标 to determine the force difference ΔF 波动 = F 目标 - F 传递 . In response to detecting a non-zero difference ΔF 波动 , the controller 6 actuates the first SMA element 4a (determine heating or cooling according to the positive or negative of the difference ΔF 波动 ) to compensate for and remove the difference ΔF 波动 .

[0232] The target force F 目标 can be predetermined, user-determined, or automatically determined. Taking automatic determination as an example, the target force F can be determined based on the moving window average of the recently measured force values buffered in the controller 6 目标In this way, the fluctuation correction can act as a mechanical low-pass filter, allowing slow and controlled adjustment while smoothing out rapid changes. Even during use, the size of the moving window can be adjustable to allow adjustment of this effect.

[0233] Taking the user determining the target force F 目标 as an example, the user can adjust the tool 38 to the desired force and then activate a switch or a toggle element provided on the tool 38 to activate the stable mode and lock the transmitted force F 传递 to the target force F 目标 , in which case the target force F 目标 can be the transmitted force F when the stable mode is activated 传递 (but for accuracy, preferably it is the average value of the transmitted force F 传递 over a period of time). This can be particularly advantageous when manually driving the actuator 39 when an exact force is desired to be maintained over a period of time.

[0234] Returning to the example of the first tool 43 driven by a cable-pulley, when the actuator 39 is manually driven, the stable mode can be used to remove unwanted movements of the surgeon's hand.

[0235] When the actuator 39 is not manually driven, the target force F 目标 can correspond to the expected output force F of the actuator 输出 . This can allow the combination of force correction for both mechanical losses and fluctuations in the force transmission mechanism 41. For example, to set the expected target force F 目标 , the tool can query the corresponding drive current of a motor or the like.

[0236] Although the measurement of the transmitted force F 传递 has been described as using the first SMA element 4a for both measurement and actuation, in other examples, the first SMA element 4a can be used purely for actuation and the transmitted force F can alternatively be measured by using a force sensor (any suitable type of force sensor) separate from the first SMA element 4a 传递 .

[0237] Friction Reduction

[0238] Mechanical components must overcome friction to move relative to each other, and once the movement starts (dynamic friction), the initial (static) frictional force tends to drop to a lower value. The SMA elements 4a, 4b can be used to keep the force transmission mechanism 41 in motion to avoid "stiction", which may be related to overcoming static friction.

[0239] For this method, the force transmission mechanism 41 of the tool 38 includes a second SMA element 4b. The second SMA element 4b is separated from the first SMA element 4a via one or more connecting members 42 of the force transmission mechanism 41, and the second SMA element 4b is connected in series with the first SMA element 4a. Preferably, the second SMA element 4b is the last or the second last element of the force transmission mechanism 41 in the direction from the operating element 40 to the actuator 39.

[0240] Then, the controller 6 is configured to cyclically heat the first SMA element 4a and the second SMA element 4b to cause vibration of the intermediate connecting member 42 of the force transmission mechanism 41. The vibration generated by the force transmission mechanism 41 is preferably of low amplitude and has a frequency above 10 Hz. Preferably, the frequency of the vibration is greater than or equal to 20 Hz, more preferably greater than or equal to 50 Hz. The vibration should preferably not change the force F transmitted between the first SMA element 4a and the second SMA element 4b (except for the intentional correction as described above). In other words, the contraction of the second SMA element 4b is matched with the expansion of the first SMA element allowed, and vice versa. Preferably, closed-loop control is implemented to ensure that any contraction of the first SMA element 4a is equal to the extension of the other opposing second SMA element 4b, and vice versa.

[0241] The vibration generated by the force transmission mechanism 41 can help reduce the stickiness in the force transmission mechanism 41 caused by the conversion of one or more elements between static friction and dynamic friction.

[0242] In a tool 38 including two (or more) parallel SMA elements 4 that provide actuation (e.g., for manipulating the operating element 40), the phase of the generated vibration can be considered. For example, the vibration in two (or more) parallel SMA elements 4 can be configured such that the phase between the vibrations of the two parallel actuators minimizes any movement of the operating element (at the vibration frequency).

[0243] Also referring Figure 14 , a second exemplary surgical tool 48 (hereinafter referred to as the "second tool") is shown.

[0244] The second tool 48 is the same as the first tool 43, except that the second tool 48 further includes a pair of second SMA elements 4b that connect the input end of the cable pulley system to the actuator 39. The second SMA element 4b can be used for force measurement, such as measuring the output force F 输出 , for use as an input to the correction / stabilization method described above.

[0245] In addition, the first SMA element 4a and the second SMA element 4b can be actuated together to generate vibration 49 for the purpose of reducing friction in the cable pulley system.

[0246] The force sensors and tools described above include at least one SMA element. The term "shape memory alloy (SMA) element" can refer to any element that includes SMA. The SMA element can be described as an SMA wire. The SMA element can have any shape suitable for the purposes described herein. The SMA element can be elongate and can have a circular cross-section or a cross-section of any other shape. The cross-section can vary along the length of the SMA element. The SMA element can have a relatively complex shape, such as a helical spring shape. It is also possible that the length of the SMA element (however defined) can be similar to one or more of the other dimensions of the SMA element. The SMA element can be sheet-like, and such a sheet can be planar or non-planar. The SMA element can be deformable, or in other words, the SMA element can be flexible. In some examples, when connected in a straight line between two components, the SMA element can only apply a tensile force that forces the two components together. In other examples, the SMA element can bend around a component, and when the SMA element tends to straighten in a tensile state, the SMA element can apply a force to the component. The SMA element can be beam-like or rigid and may be capable of applying different forces (e.g., non-tensile forces) to the element. The SMA element can include or not include non-SMA materials and / or components. For example, the SMA element can include a core of SMA and a coating of non-SMA material. Unless the context otherwise requires, the term "SMA element" can refer to any configuration of SMA material that acts as a single actuating element, e.g., the single actuating element can be individually controlled to generate a force acting on the element. For example, the SMA element can include two or more portions of SMA material arranged mechanically in parallel and / or in series. In some arrangements, the SMA element can be part of a larger SMA element. Such a larger SMA element can include two or more components that can be individually controlled, thereby forming two or more SMA elements. The SMA element can include SMA wire, SMA foil, SMA film, or any other configuration of SMA material. The SMA element can be manufactured using any suitable method, e.g., by a method involving drawing, rolling, or deposition and / or other forming processes. The SMA element can exhibit any shape memory effect, e.g., a thermal shape memory effect or a magnetic shape memory effect, and can be controlled in any suitable manner (e.g., by Joule heating, another heating technique, or by applying a magnetic field).

[0247] The force sensor 1 described herein can be any of the following devices or can be disposed in any of the following devices: a smartphone, a protective cover or case for a smartphone, a functional cover or case for a smartphone or an electronic device, a camera, a foldable smartphone, a foldable smartphone camera, a foldable consumer electronic device, a camera with a folding optical device, an image capture device, an array camera, a three-dimensional sensing device or system, a servo motor, a consumer electronic device, a mobile or portable computing device, a mobile or portable electronic device, a laptop computer, a tablet computing device, an e-reader, a computing accessory or computing peripheral, an audio device, a security system, a gaming system, a gaming accessory, a robot or robotic device, a medical device, an augmented reality system, an augmented reality device, a virtual reality system, a virtual reality device, a wearable device, a drone, an aircraft, a spacecraft, a submersible vessel, a vehicle, and an autonomous vehicle, a tool, a surgical tool, a remote control, a piece of clothing, a switch, a dial or button, a display screen, a touch screen, a flexible surface, and a wireless communication device, a tool, a surgical tool, and a manufacturing machine. It should be understood that this is a non-exhaustive listing of exemplary devices. The tool 38 described herein can take the form of a surgical tool, a robotic surgical tool, and a tool for manufacturing. It should be understood that this is a non-exhaustive listing of exemplary devices.

Claims

1. A method for determining a force applied between a first part and a second part, the second part being connected to the first part via a shape memory alloy (SMA) element such that the force is transmitted via the SMA element, wherein, The force is determined based on the resistance of the SMA element, or the amount of energy applied to the SMA element to maintain the resistance of the SMA element at a predetermined value.

2. The method according to claim 1, wherein, Most of the force applied between the first portion and the second portion is transmitted via the SMA element.

3. The method according to claim 1 or claim 2, wherein, The force is applied along the SMA element parallel to a first direction, and wherein the SMA element is configured such that the applied force corresponding to the upper plateau stress of the material of the SMA element varies with position along the first direction.

4. The method according to any one of claims 1 to 3, wherein, The SMA element is configured such that the applied force produces a stress gradient along the SMA element.

5. The method according to any one of claims 1 to 4, wherein A temperature gradient is applied or generated along the SMA element.

6. The method according to any one of claims 1 to 5, wherein The cross-sectional area of the SMA element gradually decreases along the length of the SMA element.

7. The method according to any one of claims 1 to 6, wherein, At least a portion of the length of the SMA element is embedded in or bonded to a second material, and wherein the spring constant of the second material gradually decreases along the length of the SMA element.

8. The method according to any one of claims 1 to 7, wherein The SMA element is directly connected to the first portion, and a portion of the length of the SMA element is engaged with a deformable material block that couples the SMA element to the second portion.

9. The method according to any one of claims 1 to 8, wherein Determining the force based on the amount of energy applied to the SMA element to maintain the resistance of the SMA element at a predetermined value includes: controlling the power applied to cause resistive heating of the SMA element to maintain the length of the SMA element at a constant value; and Determining the applied force based on the applied power.

10. The method according to claim 9, further comprising the initial step of: controlling the power applied to cause resistive heating of the SMA element such that the power history can determine the position of the SMA element in the hysteretic behavior of the SMA material.

11. A force sensor comprising a first portion and a second portion, the second portion being connected to the first portion via a shape memory alloy (SMA) element such that in response to a force applied between the first portion and the second portion, the force is transmitted via the SMA element; Among them, The force sensor is configured to: determine the force applied between the first portion and the second portion based on the resistance of the SMA element, or the amount of energy applied to the SMA element to maintain the resistance of the SMA element at a predetermined value.

12. The force sensor according to claim 11, wherein, The force is applied along the SMA element parallel to a first direction, and wherein the SMA element is configured such that the minimum applied force required for superelastic deformation of the SMA element varies with position along the first direction.

13. The force sensor according to claim 11 or 12, wherein, The SMA element is configured such that the applied force produces a stress gradient along the SMA element.

14. The force sensor according to any one of claims 11 to 13, configured to apply or generate a temperature gradient along the SMA element.

15. The force sensor according to any one of claims 11 to 14, wherein, The cross-sectional area of the SMA element gradually decreases along the length of the SMA element.

16. The force sensor according to any one of claims 11 to 15, wherein At least a portion of the length of the SMA element is embedded in or bonded to a second material, and wherein the spring constant of the second material gradually decreases along the length of the SMA element.

17. The force sensor according to any one of claims 11 to 16, wherein, The SMA element is directly connected to the first portion, and wherein a portion of the length of the SMA element is engaged with a deformable material block that couples the SMA element to the second portion.

18. A controller configured to determine a force between a first part and a second part of a force sensor, the force sensor being the force sensor according to any one of claims 11 to 17, wherein, The force is determined based on the resistance of the SMA element or the amount of energy applied to the SMA element to maintain the resistance of the SMA element at a predetermined value.

19. The controller according to claim 18, further configured to supply power to cause resistive heating of the SMA element.

20. The controller according to claim 19, further configured to: control the power applied to cause resistive heating of the SMA element so as to maintain the length of the SMA element at a constant value; and determine the applied force based on the applied power.

21. A computer program stored on a non-transitory computer-readable medium, wherein, When executed by a digital electronic processor, the computer program causes the digital electronic processor to perform the method according to any one of claims 1 to 10.

22. A device comprising a force sensor and a controller, the controller being connected to the force sensor, the force sensor being the force sensor according to any one of claims 11 to 17, and the controller being the controller according to any one of claims 18 to 20.

23. A tool comprising an actuator mechanically coupled to an operating element via a force transmission mechanism, the force transmission mechanism including a first SMA element configured to transmit a force between the actuator and the operating element.

24. The tool according to claim 23, wherein The first SMA element is the last or the penultimate element of the force transmission mechanism.

25. The tool according to claim 23 or 24, wherein The first SMA element is configured to undergo superelastic deformation in response to a predetermined threshold transmission force.

26. The tool according to claim 25, the tool being configured to apply resistive heating to the first SMA element to control the temperature of the SMA element so as to: restore the first SMA element to its original shape after superelastic deformation; and / or change the temperature of the first SMA element to adjust the predetermined threshold transmission force.

27. The tool according to any one of claims 23 to 26, the tool further being configured to determine the force transmitted by the first SMA element.

28. The tool according to claim 27, wherein, Use the method according to any one of claims 1 to 10 to determine the force transmitted by the first SMA element.

29. The tool according to any one of claims 23 to 28, the tool further being configured to: obtain or measure the force output by the actuator; measure the force transmitted by the first SMA element; control the temperature of the first SMA element to compensate for the difference in response to the difference between the force output by the actuator and the force transmitted by the first SMA element.

30. The tool according to any one of claims 23 to 29, wherein the tool is further capable of operating in a stable mode, in which the tool is configured to: measure the force transmitted by the first SMA element; and control the temperature of the first SMA element to compensate for the difference between the force transmitted by the first SMA element and a target force in response to the difference.

31. The tool according to any one of claims 23 to 29, wherein the tool is further capable of operating in a stable mode, in which the tool is configured to: determine the length of the first SMA element based on the resistance of the first SMA element; and control the temperature of the first SMA element to compensate for the difference between the length of the first SMA element and a target length in response to the difference.

32. The tool according to any one of claims 23 to 31, wherein, The force transmission mechanism further includes a second SMA element, which is spaced apart from the first SMA element via at least one other element of the transmission mechanism, wherein the second SMA element is connected in series with the first SMA element.

33. The tool according to any one of claims 23 to 32, further configured to cyclically heat the first SMA element and optionally cyclically heat the second SMA element to cause vibration of the force transmission mechanism.

34. The tool according to any one of claims 23 to 33, wherein The operating element is configured for grasping.

35. The tool according to any one of claims 23 to 34, wherein The operating element is configured for cutting.

36. The tool according to any one of claims 23 to 35, wherein the tool is a surgical tool.

37. The tool according to any one of claims 23 to 36, wherein The force transmission mechanism includes one or more cables, and the first SMA element is connected in series with at least one of the one or more cables.

38. The tool according to any one of claims 23 to 37, wherein The force transmission mechanism includes a cable-pulley drive, and the first SMA element is connected in series with at least one cable of the cable-pulley drive.

39. The tool according to any one of claims 23 to 38, wherein, The actuator includes a servo actuator.

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