Quarter-Bridge Temperature Compensation for Force / Torque Sensors

Abstract

To allow for compensating for errors due to thermal drift of a force / torque sensor employing strain gauges.SOLUTION: A hardware temperature compensation procedure provided herein comprises substantially eliminating thermal drift of a plurality of load-sensing strain gauges associated with changes in temperature using trimming resistors and a single unstressed strain gauge. The strain gauges are connected in a quarter-bridge configuration in multiple parallel stages. The unstressed strain gauge in the quarter-bridge configuration is connected in parallel. The trimming resistors are added across one or more of the unstressed and load-sensing strain gauges in the compensation procedure that substantially eliminates thermal drift of the load-sensing strain gauges over a predefined temperature range.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates generally to robotic force / torque sensors, and more particularly to a system and method for temperature compensating multiple load-sensing strain gauges in a Wheatstone quarter-bridge circuit configuration with stress-free strain gauges. [Background technology]

[0002] Robots are essential for manufacturing, testing, assembling, and packaging products, assisted and remote surgery, space exploration, operation in hazardous environments, and many other applications. Many robots and robotic applications require quantification of the forces applied or experienced, such as material removal (grinding, polishing, etc.), part assembly, remote excavation, or other manipulation of the environment.

[0003] An industrial robot typically comprises a versatile actuator, or "arm," that is programmed to move through space and act on a workpiece along multiple degrees of freedom. A variety of different tools, also known as end effectors, can be attached to the robot arm to perform different tasks.

[0004] In applications where the amount of force a robot applies to a workpiece must be monitored and controlled, and / or the forces experienced by the robot are fed back to control the robot's motion ("force-controlled" operations), a force / torque (F / T) sensor is interposed between the robot arm and the tool.

[0005] One conventional type of F / T sensor uses strain gauges to measure the deformation of a small beam connecting two mechanical components, one coupled (directly or indirectly) to a robotic arm and the other coupled (directly or indirectly) to a robotic tool. One compact example of such an F / T sensor is described in commonly assigned U.S. Pat. No. 10,422,707 (the "'707 patent"), which is incorporated herein by reference in its entirety.

[0006] Figure 1, reproduced from the '707 patent, shows a central "hub" coupled to the tool, called the tool adapter plate (TAP). Another body, annularly disposed around and spaced from the TAP, referred to in the art as the mounting adapter plate (MAP), is coupled to the robot arm. The MAP and TAP are connected by a plurality of relatively thin, and therefore mechanically deformable, beams arranged radially around the TAP, resembling the spokes of a wheel in one configuration. Relative forces or torques between objects respectively coupled to the TAP and the MAP tend to move the MAP relative to the TAP, resulting in slight deformation or bending of at least some of the beams.

[0007] Typically, strain gauges attached to multiple surfaces of at least some of the beams detect this deformation. A beam undergoing mechanical deformation stretches slightly along one side and compresses along the opposite side. Resistive strain gauges rigidly attached to the sides of such a beam experience a corresponding extension or contraction, respectively, and the resistance of the strain gauge is proportional to its length. Thus, changes in the resistance of the strain gauges can be detected and quantified, such as by some configuration of a Wheatstone bridge circuit, and the signals from multiple gauges can be combined to resolve the forces and torques acting on the F / T sensor.

[0008] The '707 patent describes strain gauges attached to only one side of a deformable beam, as shown in Figure 1. This configuration allows for a very compact sensor body and ease of strain gauge placement during manufacturing, while still resolving all forces and torques. The strain gauges are connected in a quarter-bridge circuit, as shown in Figure 2, which is reproduced from Figure 3 of the '707 patent.

[0009] The primary source of error in robotic force / torque sensors is inaccuracy due to thermal drift. Sources of thermal drift include ambient temperature changes, ambient temperature gradients, and self-heating. For silicon strain gauges, the change in the gauge circuit's output voltage due to temperature changes can be several times larger than the change in output voltage due to induced stress. In fact, silicon strain gauges can be considered better temperature sensors than stress sensors. Connecting strain gauges in a half-bridge topology can compensate for temperature effects only if the gauges are well matched and positioned precisely opposite each other. Furthermore, in addition to affecting strain gauge output, temperature changes in robotic force-torque sensors can induce mechanical stresses due to uneven expansion / compression of structural elements, which the sensor can interpret as an applied load or force.

[0010] One approach to compensating for thermal drift is to capture the effects of temperature changes on the strain gauge and mathematically remove them. Figure 3, which is Figure 8 of the '707 patent, shows a stress-free, temperature-compensated strain gauge attached to an unstressed mechanical member of an F / T sensor. Because this strain gauge does not undergo mechanical deformation to change its resistance, any change in resistance is due only to changes in temperature of the F / T sensor body. The signal from the stress-free strain gauge is mathematically removed from the signal from the load-sensing strain gauge to compensate for thermal effects.

[0011] WO 2018 / 200668, assigned to the assignee of the present disclosure and incorporated herein by reference in its entirety, describes subtracting the voltage output by the quarter bridge circuit of the stress-free strain gauge from the voltage output by the quarter bridge circuit for each load-sensing strain gauge. Figure 4, which is Figure 2 of the '668 PCT publication, shows the stress-free strain gauge SGU signal being subtracted from the signals of strain gauges SG0 and SG1.

[0012] However, due to differences in the temperature coefficients of the individual strain gauges, the load-sensing strain gauge circuit will still exhibit a variation in output with temperature even when the output of the stress-free strain gauge circuit is subtracted. That is, at least some of the load-sensing strain gauges will exhibit a different change in resistance with temperature than the stress-free strain gauges. This is referred to herein as the effective temperature coefficient for the stress-free strain gauges. These effective temperature coefficients result in thermal drift as the temperature changes, introducing errors into force and torque measurements.

[0013] The Background of the Invention is provided to place embodiments of the present invention in a technical and operational context and to assist those skilled in the art in understanding their scope and usefulness. The approaches described in the Background are approaches that could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Unless expressly identified as such, nothing in this specification is admitted to be prior art merely by virtue of its inclusion in the Background of the Invention section. Summary of the Invention

[0014] The following presents a simplified summary of the present disclosure in order to provide a basic understanding to those skilled in the art. This summary is not an extensive overview of the disclosure, and it is not intended to identify key / critical elements of embodiments of the present invention or to delineate the scope of the present invention. Its sole purpose is to present some concepts disclosed herein in a simplified form as a prelude to the more detailed description that is presented later.

[0015] According to one or more embodiments described and claimed herein, a hardware temperature compensation procedure uses trimming resistors to substantially eliminate any effective temperature coefficient of a load-sensing strain gauge relative to a stress-free strain gauge. The strain gauges are connected in multiple parallel stages in a quarter-bridge configuration. The stress-free strain gauges in the quarter-bridge configuration are connected in parallel. Trimming resistors are added across one or more of the stress-free and load-sensing strain gauges in a compensation procedure that substantially eliminates any effective temperature coefficient of the load-sensing strain gauge relative to the stress-free strain gauge over a predetermined temperature range.

[0016] One embodiment relates to a temperature compensation circuit for a force / torque sensor. The circuit includes a first plurality of first stages connected in parallel. Each first stage is configured to output a respective voltage at a node connecting a fixed resistor and a load-sensing strain gauge in series between a positive power supply voltage and a negative power supply voltage. The circuit also includes a second stage connected in parallel with the first stage. The second stage is configured to output a voltage at a node connecting a fixed resistor and a stress-free strain gauge in series between a positive power supply voltage and a negative power supply voltage. The second stage voltage is subtracted from each first stage voltage. The circuit further includes a stress-free trimming resistor connected in parallel across the stress-free strain gauges. The value of the stress-free trimming resistor is selected to substantially eliminate the effective temperature coefficient of all load-sensing strain gauges, relative to the stress-free strain gauges, of a polarity opposite to that of the power supply connected to the fixed resistor, over a predetermined temperature range.

[0017] Another embodiment relates to a temperature compensation circuit for a force / torque sensor. The circuit includes a first plurality of first stages connected in parallel. Each first stage is configured to output a respective voltage at a node connecting a fixed resistor and a load-sensing strain gauge in series between a positive power supply voltage and a negative power supply voltage. The circuit also includes a second stage connected in parallel with the first stage. The second stage is configured to output a voltage at a node connecting a fixed resistor and a stress-free strain gauge in series between the positive power supply voltage and the negative power supply voltage. The second stage voltage is subtracted from each first stage voltage. The circuit further includes a second plurality of load trimming resistors. Each load trimming resistor is connected in parallel across a load-sensing strain gauge. The value of each load trimming resistor is selected to substantially eliminate the effective temperature coefficient of the polarity of the power supply connected to the fixed resistor of the associated load-sensing strain gauge relative to the stress-free strain gauge over the predetermined temperature range.

[0018] Yet another embodiment relates to a method for compensating multiple load-sensing strain gauges in a force / torque sensor for thermal drift. The circuit includes multiple first stages connected in parallel. Each first stage is configured to output a respective voltage at a node connecting a fixed resistor and a load-sensing strain gauge in series between a positive power supply voltage and a negative power supply voltage. The circuit also includes a second stage connected in parallel with the first stage. The second stage is configured to output a voltage at a node connecting a fixed resistor and a stress-free strain gauge in series between a positive power supply voltage and a negative power supply voltage. The output voltages of the second stage and each first stage are measured, and the output voltage of the second stage is subtracted from the output voltage of each first stage over a predetermined temperature range. In response to at least one load-sensing strain gauge exhibiting an effective temperature coefficient for the stress-free strain gauge of opposite polarity to the polarity of the power supply connected to the fixed resistor, a stress-free trimming resistor is added in parallel across the stress-free strain gauge. The stress-free trimming resistor values ​​are selected to substantially eliminate, over the predetermined temperature range, the effective temperature coefficients of the opposite polarity of the power supply connected to the fixed resistor relative to the stress-free strain gauge for all load-sensing strain gauges. For each load-sensing strain gauge, a load-sensing trimming resistor is added in parallel across the load-sensing strain gauge in response to the load-sensing strain gauge exhibiting an effective temperature coefficient of the polarity of the power supply connected to the fixed resistor relative to the stress-free strain gauge. The load-sensing trimming resistor values ​​are selected to substantially eliminate, over the predetermined temperature range, the effective temperature coefficients of the polarity of the power supply connected to the fixed resistor relative to the stress-free strain gauge for the load-sensing strain gauge. [Brief explanation of the drawings]

[0019] The present invention will now be described in more detail with reference to the accompanying drawings, which show embodiments of the invention. However, the invention should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference characters refer to like elements throughout.

[0020] [Figure 1] FIG. 1 is a cross-sectional view of a prior art force / torque sensor showing a load-sensing strain gauge.

[0021] [Figure 2] FIG. 2 is a diagram of a strain gauge in a prior art quarter bridge circuit.

[0022] [Figure 3] FIG. 3 is a perspective cross-sectional view of a prior art force / torque sensor showing a stress-free strain gauge.

[0023] [Figure 4] FIG. 4 is a prior art circuit diagram illustrating the subtraction of a stress-free strain gauge signal from two load-sensing strain gauge signals.

[0024] [Figure 5] FIG. 5 is a circuit diagram showing a quarter-bridge circuit of six load-sensing strain gauges and a stress-free strain gauge.

[0025] [Figure 6] FIG. 6 is a graph of the measured strain gauge response versus temperature before compensation.

[0026] [Figure 7] FIG. 7 is a graph of the simulated change in measured strain gage response with respect to temperature, the response having been partially compensated to remove the effective negative temperature coefficient.

[0027] [Figure 8]FIG. 8 is a graph of the simulated change in measured strain gauge response with respect to temperature, the response being fully compensated to also eliminate the effective positive temperature coefficient.

[0028] [Figure 9] FIG. 9 is a graph of simulated changes to the measured strain gauge response of FIG. 8 after multiple iterations of the compensation procedure.

[0029] [Figure 10] FIG. 10 is a flow diagram of a method for compensating multiple load-sensing strain gauges in a force / torque sensor for thermal drift.

[0030] [Figure 11] FIG. 11 is a flow diagram of a procedure for determining a trimming resistor.

[0031] [Figure 12A] FIG. 12A is a graph of measured strain gauge response versus temperature before temperature compensation.

[0032] [Figure 12B] FIG. 12B is a graph of the measured strain gauge response versus temperature after temperature compensation. DETAILED DESCRIPTION OF THE INVENTION

[0033] The present invention will be described, for simplicity and exemplary purposes, primarily by reference to exemplary embodiments thereof. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be readily apparent to those skilled in the art that the present invention may be practiced without being limited to these specific details. Well-known methods and structures are not described in detail herein in order to avoid unnecessarily obscuring the present invention.

[0034] Embodiments of the present invention are described herein in the context of the compact force / torque (F / T) sensor described in the above-incorporated U.S. Pat. No. 10,422,707 (the "'707 patent"). However, the present invention is not limited to this application, and those skilled in the art will readily recognize that embodiments may be advantageously applied to a wide variety of F / T sensors. Figures 1-3 herein are reproduced from Figures 1, 3, and 8, respectively, of the '707 patent, and Figure 4 is reproduced from Figure 2 of the '668 PCT publication. For clarity of description, the element numbering in the '707 patent is adopted herein.

[0035] FIG. 1 shows a top view of one embodiment of an F / T sensor 10. The TAP 12 is connected to the MAP 14 by three deformable beams 16a, 16b, and 16c. In the illustrated embodiment, each beam 16 is connected directly to the TAP 12 and to the MAP 14 by a thin flexure 17, which aids in deformation of the beams 16 under mechanical load. The TAP 12 is configured to connect to a first object, such as a robotic tool, via a through-hole 30. The MAP 14 is configured to connect to a second object, such as a robotic arm, via a plurality of mounting holes 32. The TAP 12 and the MAP 14 are connected only by the beams 16 and the flexures 17.

[0036] Affixed (only) to the top surface of each beam 16a-c are load-sensing strain gauges 1-6. As used herein, the term "load-sensing strain gauge" refers to a strain gauge attached to a member of the F / T sensor (such as the deformable beam 16) that is subjected to mechanical stress when the F / T sensor measures applied force and / or torque. In other embodiments, load-sensing strain gauges may be attached to multiple surfaces of the deformable beam 16, for example, on opposing surfaces thereof (e.g., opposite, top, and bottom surfaces). The F / T sensor 10 also includes processing circuitry (not shown) that operates to receive electrical signals from each load-sensing strain gauge 1-6 and process the signals to resolve the magnitude and direction of the force(s) and torque(s) applied between the MAP 14 and the TAP 12. Such processing circuitry may comprise, for example, a microprocessor coupled to a memory operable to store program code and sensor data.

[0037] Figure 2 shows two load-sensing strain gauges wired in a quarter-bridge configuration. As described in the '707 patent, this circuit configuration uses the resistances R of the two load-sensing strain gauges. LSn The change in strain is sufficient to distinguish whether the portion of the surface of the beam 16 to which each load-sensing strain gage is attached is in tension or compression. Comparing this information for all six load-sensing strain gages is sufficient to determine the forces Fx, Fy, Fx and torques Tx, Ty, Tz.

[0038] 3 illustrates a stress-free strain gauge 38. The stress-free strain gauge 38 is attached to an extension 37 formed by forming an air gap 35 within the TAP 12. The extension 37 is not deformed by any force or torque applied to or experienced by the F / T sensor 10. Therefore, changes in the resistance of the stress-free strain gauge 35 are caused by temperature changes. As used herein, the term "stress-free strain gauge" refers to a strain gauge, otherwise substantially similar to a load-sensing strain gauge, attached to a member of the F / T sensor that is not subjected to substantial mechanical stress when the F / T sensor measures applied forces and / or torques.

[0039] Figure 4 shows two load-sensing strain gauges SG0 and SG1 in a quarter-bridge circuit configuration. The load-sensing strain gauge signal is partially compensated for thermal drift by subtracting the signal from the stress-free strain gauge SGU, which is also wired in the quarter-bridge circuit. This subtraction of the stress-free strain gauge SGU signal improves the F / T sensor's performance over temperature; however, the change in resistance of each load-sensing strain gauge SG0, SG1 over temperature does not exactly match that of the stress-free strain gauge SGU. Therefore, some of the load-sensing strain gauges SG0, SG1 exhibit an effective temperature coefficient relative to the stress-free strain gauges when the F / T sensor undergoes temperature changes. This effective temperature coefficient adversely affects force and torque measurements as the F / T sensor temperature changes.

[0040] FIG. 5 shows a circuit for measuring changes in resistance of load-sensing strain gauges in an F / T sensor, and a possible trimming resistor for compensating for thermal drift. Multiple first stages are connected in parallel, each configured to output a respective voltage at a node connecting a fixed resistor and a load-sensing strain gauge in series between a positive supply voltage and a negative supply voltage. The resistance of the ith load-sensing strain gauge is R LSi (i.e., R LS0 , R LS1 ,...R LS(n-1)) In the illustrated circuit, n=6, but this is not a limitation of the present invention. LSi Although properly refers only to the resistance value of the i-th load-sensing strain gauge, for convenience in the following explanation, the strain gauge itself is referred to as R LSi (and the load detection strain gauges are collectively referred to as R LS n load detection strain gauges R LS0 , R LS1 ,...R LS(n-1) Each of these is connected to a corresponding fixed resistor R F0 , R F1 , . . . , R F(n-1) 5, the fixed resistors are connected to a positive voltage source; in other embodiments, the fixed resistors may be connected to a negative voltage source. In practice, all fixed resistors R F may have the same value, such as 1 kΩ. In one embodiment, the fixed resistor R F The value of can be varied to maintain voltage measurements within a preferred range, as described in more detail herein.

[0041] The first stage is connected in parallel between the applied source voltages V+ and V-. The quarter-bridge circuit is connected to a load-sensing strain gauge R LSi and fixed resistor R Fn The voltage measurement is formed by taking a voltage measurement at the midpoint of each first stage between

[0042] The second stage is connected in parallel with the first stage and is connected between the positive and negative supply voltages by a fixed resistor R FU and stress-free strain gauge R U and a fixed resistor R FU is connected to the positive supply voltage. In practice, the fixed resistor R FU is the fixed resistor R of the first stage F The stress-free strain gauge R U is a load detection strain gauge R LSThe only difference is that it is attached to a member of the F / T sensor that is not subjected to mechanical stress under applied force or torque.

[0043] Each strain gauge R LSi , R U The output of each stage is the voltage measured at the midpoint of the corresponding stage. As shown in Figure 4, the output voltage of the second stage is subtracted from the output voltage of each first stage. This is why the fixed resistor R Fn , R FU Using the known value of LSn The applied forces and torques that cause the change in resistance of each load-sensing strain gauge R can be calculated. LSi About the stress-free strain gauge R U , thereby substantially eliminating any effective temperature coefficient for σ, thereby substantially eliminating thermal drift from the force / torque measurements.

[0044] As mentioned above, the fixed resistor R Fn , R FU is connected to a positive voltage source. This is the load-sensing strain gauge R LSn and stress-free strain gauge R U This relative thermal drift, more precisely, the stress-free strain gauge R, determines the direction of the relative thermal drift between the strain gauge R and the strain-free strain gauge R, i.e., the positive or negative direction, also referred to herein as the "polarity." U Load sensing strain gauge R LSi The difference in the rate of change of resistance with increasing temperature is referred to as the stress-free strain gauge R U Load sensing strain gauge R LSn The direction or polarity of the effective temperature coefficient is called the effective temperature coefficient of the fixed resistor R Fn , R FU depends on whether is connected to a positive or negative supply voltage. For clarity, assume the circuit configuration of Figure 5, with a fixed resistor R Fn , R FU is connected to a positive voltage source.

[0045] Figure 5 shows the load-sensing strain gauges R LS A possible trimming resistor R connected in parallel across LST and stress-free strain gauge R U A possible trimming resistor R connected in parallel across UT The trimming resistor R LST , R UT are shown as dashed lines to indicate that each may or may not be added to any given strain gauge during the temperature compensation procedure. LSn Any trimming resistor across the load sense trimming resistor R LST It is called stress-free strain gauge R U Any trimming resistor over a range is referred to herein as a stress-free trimming resistor R UT (called the load sensing trimming resistor R LST does not sense any load and uses a stress-free trimming resistor R UT (Whether or not the resistor is subjected to mechanical stress is irrelevant - the terminology applied to trimming resistors is for reference only). Any trimming resistor R LST , R UT The value of is determined in an iterative process for varying temperature, which in some embodiments may be simulated.

[0046] According to an embodiment of the present invention, a load-sensing strain gauge R LS is the stress-free strain gauge R over a given temperature range. U In the circuit configuration of Figure 5, the stress-free strain gauge R U Trimming resistor R UT Adding stress-free strain gauge R U All load-sensing strain gauges R LS In this circuit configuration, the effective temperature coefficient of the load-sensing strain gauge R LSi Adding a trimming resistor to the stress-free strain gauge R U Load sensing strain gauge R LSi The effective temperature coefficient of decreases (becomes lower).

[0047] First, with no mechanical load applied, the outputs of each first and second stage are measured as the sensor is heated over a predetermined temperature range, and the output voltage of the second stage is subtracted from the output voltage of each first stage. LS The change in resistance of the stress-free strain gauge R U In most practical cases, there is at least one load-sensing strain gauge R LSi exhibits an effective negative temperature coefficient for a stress-free strain gage over a given temperature range.

[0048] Figure 6 shows the load-sensing strain gauge R LSi 1 plots the response of a load-sensing strain gauge R to an increasing temperature. LSn The response of is the difference between the corresponding first stage output voltage and second stage output voltage normalized by the excitation voltage (the difference between V+ and V-). The response is measured with increasing temperature. For example, R LS0 The graph shows the change in voltage, in mV / V, when the F / T sensor body is heated over a given temperature range.

number

[0049] Next, the stress-free trimming resistor R UT The value of is determined, which is the stress-free strain gauge R U (R LSmax-neg As shown in the general case for the circuit in Figure 5, the load-sensing strain gauge R exhibits the most negative effective temperature coefficient. LS1 In one embodiment, a stress-free trimming resistor R UT Determining the value of R can be done by performing a field search of the resistance value and using a stress-free trimming resistor R UT The new value of R is used to replace the current value and the most negative load sensing strain gauge R is used over the given temperature range. LSmax-neg (In Figure 6, R LS1 ) resulting in a virtually zero effective temperature coefficient of R UT R until the value is found. UT and measuring or simulating the output of the first stage over a predetermined temperature range for each iteration of different values ​​of .

[0050] Figure 7 shows the determined stress-free trimming resistor R UT However, stress-free strain gauge R U After being connected across the load-sensing strain gauge R LS The response of the load-sensing strain gauge R LS1 The response of the stress-free trimming resistor R, which previously exhibited the most negative effective temperature coefficient, is now essentially constant over a given temperature range. UT When you add the strain gauge R LS The effective temperature coefficient of the stress-free strain gauge R U That is, the remaining first stage outputs a larger increase in voltage as temperature increases than the second stage.

[0051] Next, the load-sensing trimming resistor R LST is the stress-free strain gauge R UEach load-sensing strain gauge R exhibits a positive effective temperature coefficient LS For each of the first stages, a load sense trimming resistor R LSTi The value of R is determined, which is the load-sensing strain gauge R LSi , which provides a substantially flat response over a predetermined temperature range. LSTi Substitute the new value of R for the load-sensing strain gauge R over the given temperature range for each iteration. LSi A binary search may be applied to measure or simulate the resistance value of

[0052] Figure 8 shows the temperature compensated load-sensing strain gauge R after one iteration of the compensation procedure is completed. LS The response of all load-sensing strain gauges R LS has a substantially constant response over a given temperature range (in this example, R LS0 and R LS5 (Note that the plots of are overlapping.)

[0053] Figure 9 shows the temperature compensated load-sensing strain gauge R after several iterations of the compensation procedure. LS The response is flatter, i.e., all load-sensing strain gauges R LS is the stress-free strain gauge R U It exhibits an effective temperature coefficient of virtually zero for . Furthermore, the response is centered around zero mV / V.

[0054] Those skilled in the art will note that the temperature compensation procedure described above pertains to the quarter-bridge circuit configuration of Figure 5. In particular, in that circuit configuration, fixed resistor R Fn , R FU is connected to a positive voltage source. The power supply voltage is reversed and the fixed resistor R Fn , R FU If the stress-free strain gauge R is connected to a negative voltage source, the opposite bridge output change occurs. U Stress-free trimming resistor R UT The addition of stress-free strain gauge RU In contrast, the load detection strain gauge R LSn Furthermore, the load-sensing strain gauge R LSi Load-sensing trimming resistor R LSTi The addition of stress-free strain gauge R U The load sensing strain gauge R LSi Increase the effective temperature coefficient of

[0055] Therefore, in general, a stress-free trimming resistor R UT is the stress-free strain gauge R over a given temperature range. U All load-sensing strain gauges R LSn of fixed resistor R Fn , R FU The load trimming resistors R are typically chosen to substantially eliminate the effective temperature coefficient of the load trimming resistors R LSTi The value of is the stress-free strain gauge R over a given temperature range. U the associated load-sensing strain gauge R LSi of fixed resistor R Fn , R FU The polarity of the power supply voltage connected to the power supply is selected to substantially eliminate the effective temperature coefficient.

[0056] Figure 10 shows the load-sensing strain gauges R in the F / T sensor. LS 1 is a flow diagram of steps in a method 100 for compensating for thermal drift in a circuit comprising a plurality of first stages connected in parallel. Each first stage i has a fixed resistor R between a positive supply voltage and a negative supply voltage. Fi and load detection strain gauge R LSi and a second stage connected in parallel with the first stage. The second stage is connected between the power supply voltage and the negative power supply voltage by a fixed resistor R FU and stress-free strain gauge R U and a node connecting them in series to output a positive voltage.

[0057] The output voltages of the second stage and each first stage are measured, and the output voltage of the second stage is subtracted from the output voltage of each first stage over a predetermined temperature range (block 102).

[0058] The need and values ​​of any trimming resistors for the various strain gauges are determined in a trimming resistor determination procedure (block 200) described below. In one embodiment, procedure 200 is performed with a computer simulation of the circuit. In one embodiment, the circuit is modeled in Python language and the operation of the circuit over a predetermined temperature range is simulated, although one skilled in the art can implement the model and simulation in any of a variety of programming languages ​​and / or circuit simulation programs. Alternatively, procedure 200 can be performed using discrete resistors or potentiometers, as described in more detail herein, and iteratively thermally cycling the circuit as necessary to converge on the trimming resistor values.

[0059] Once the need for all trimming resistors, and if so, the values ​​of all trimming resistors, have been determined from the trimming resistor determination procedure (block 200), the determined trimming resistors R UT , R LSTn is installed in the circuit (block 104). The circuit is then cycled through a predetermined temperature range, and the load-sensing strain gauge R LS The responses of are measured to verify that they remain substantially constant (block 106).

[0060] 11 illustrates steps in one embodiment of a trimming resistor determination procedure 200. Note that FIG. 11 and the following description of the trimming resistor determination procedure 200 describe the most general case, and are not intended to be limiting unless otherwise specified in FIG. 5 (fixed resistor R Fn , R FU During the measurement process of block 104 of method 100 (FIG. 10), at least one load-sensing strain gauge R LSHowever, stress-free strain gauge R U , exhibiting an effective temperature coefficient of opposite polarity to that of the power supply voltage connected to the fixed resistor (block 202), UT However, stress-free strain gauge R U A stress-free trimming resistor R is added in parallel across the UT The value of R is the sum of all load-sensing strain gauges R over a given temperature range. LS of stress-free strain gauge R U is selected to substantially eliminate any effective temperature coefficients for , of a polarity opposite to that of the power supply voltage connected to the fixed resistor (block 204).

[0061] In one embodiment, a stress-free trimming resistor R UT The value of is determined by the stress-free strain gauge R U The load-sensing strain gauge R exhibits the maximum effective temperature coefficient of the opposite polarity to the supply voltage connected to the fixed resistor. LSmax This is done by first identifying the first stress-free trimming resistor R UT1 However, stress-free strain gauge R U Load-sensing strain gauges R are added in parallel across the LSmax The response of this load-sensing strain gauge R is measured again over a range of temperatures. LSmax If the resistor still exhibits an effective temperature coefficient of opposite polarity to that of the supply voltage connected to the fixed resistor, a different stress-free trim resistor R can be used instead of the current stress-free trim resistor. UT2 In one embodiment, j consecutive stress-free trimming resistors R UTj The value of R is selected via a binary search by measuring the response over a given temperature range and comparing it with different stress-free trim resistors R UTj The step of using (substituting) the identified load-sensing strain gauge R LSmax This is repeated iteratively until the resistor R exhibits an effective temperature coefficient of essentially zero (or a small temperature coefficient of the polarity of the supply voltage connected to the fixed resistor). UT The value of R is the sum of all load-sensing strain gauges R during the last measurement iteration.LS The response is stored in the same way as the response of

[0062] Next, the trimming resistor determination procedure 200 determines whether any of the first stages of the circuit is configured to determine the load sense trimming resistor R LST Determine whether stress-free trimming resistors R are required and, if so, their values. UT is the identified load-sensing strain gauge R LSmax was chosen to make the effective temperature coefficient of the remaining load-sensing strain gauge R LS Most, if not all, of the effective temperature coefficients of are now likely to be non-zero and are likely to be of the polarity of the power supply connected to the fixed resistor.

[0063] During the last measurement iteration in block 204, any load-sensing strain gauge R LSi However, stress-free strain gauge R U , the effective temperature coefficient of the polarity of the power supply voltage connected to the fixed resistor (block 206), LSi across the load-sensing trimming resistor R LSTi is added in parallel. Load detection trimming resistor R LSTi The value of is determined by the associated load-sensing strain gauge R over a given temperature range. LSi of stress-free strain gauge R U is selected to substantially eliminate any effective temperature coefficient for .times. ...

[0064] Stress-free trimming resistor R UT Similar to the determination of , in one embodiment, each load sense trimming resistor R LSTi The determination of the value of is carried out by repeated measurements / substitution procedure. Stress-free strain gauge R U The i-th load-sensing strain gauge R that shows the effective temperature coefficient of the polarity of the power supply voltage connected to the fixed resistor LSi For each load, the first load-sensing trimming resistor R LST1 However, the associated load-sensing strain gauge R LSi At least the load sensing strain gauge RLSi The response of the load-sensing strain gauge R is measured again over a range of temperatures. LSi If still exhibits a valid temperature coefficient, the load-sensing strain gauge R LSi Instead of the current load sense trimming resistor, a different load sense trimming resistor R LST2 In one embodiment, j consecutive load sense trimming resistors R LSTj The value of R is selected via a binary search by measuring the response over a given temperature range and using different load-sensing trimming resistors R UTj The step of substituting is the ith load sensing strain gauge R LSi However, stress-free strain gauge R U This process is then repeated for the next first stage until all n load-sensing strain gauges exhibiting effective temperature coefficients of the polarity of the power supply connected to the fixed resistor are compensated.

[0065] Alternatively, all load sense trimming resistors R LSTn The value of the final load-sensing trim resistor R can then be simultaneously changed and the strain gauge response measured over the thermal cycle. LST All values ​​of are stored. Trimming resistors with these values ​​are placed in the circuit (block 104) and verified for temperature stability (block 106), as described above with respect to the method 100 of Figure 10.

[0066] Trimming resistor R LST , R UT In some cases, adding one or more fixed resistors R F , R FU The value(s) of the optional trimming resistor R LSTn , R UT is removed and a measurement cycle is run to measure all load-sensing strain gauges R over a given temperature range. LSThe response of is measured and then the trim resistor determination procedure 200 is restarted at block 202 to obtain an updated fixed resistance value R F , R FU will be executed again.

[0067] Returning to FIG. 5, after performing a compensation procedure, e.g., method 100, trimming resistor R LSTn , R UT As mentioned above, in the first measurement, all the load-sensing strain gauges R LS However, stress-free strain gauge R U If it is found that the trimming resistor R exhibits a zero effective temperature coefficient, LST or R UT is not required.

[0068] Another possibility is to use one or more load-sensing strain gauges R LS In this case, the effective temperature coefficients of all the stress-free strain gauges R U The stress-free trimming resistor R UT is not added, but one or more load-sensing strain gauges R LS Load sense trimming resistor R LST will be added.

[0069] In general, load-sensing strain gauge R LSmax shows the largest negative effective temperature coefficient first, and the strain gauge R LSmax the response of the strain gauge R to be substantially constant over a given temperature range (i.e., U (to make its effective temperature coefficient zero for UT The stress-free strain gauge R U This encourages the addition of other load-sensing strain gauges. LS to produce a positive effective temperature coefficient for most or all of the load-sensing trimming resistor R LST Thus, up to (n-1) first stages are connected to the load sense trimming resistor R LSTOne exception is the stress-free trimming resistor R UT The load-sensing strain gauge R is initially zeroed by LSmax However, even in this case, as a practical matter, the stress-free trimming resistor R UT Since the available values ​​of are discrete, this load-sensing strain gauge R LSmax can be chosen to give a slightly positive effective temperature coefficient, other than zero, and similarly the load sense trimming resistor R LST Encourage the addition of

[0070] The data plotted in Figure 6 is measured from a representative F / T sensor. Using a model of the circuit, we can calculate the change in the circuit of Figure 5 (i.e., the trimming resistor R LSTn , R UT The addition and adjustment of trimming resistors (and numerical adjustments) was simulated. Thus, the strain gage responses plotted in Figures 7-9 reflect simulated data. In contrast, Figures 12A and 12B show the strain gage responses measured for the F / T sensor before and after thermal compensation method 100, respectively, where the added and adjusted trimming resistors are actual hardware and the strain gage response was measured over a predetermined temperature range. These graphs demonstrate the dramatic improvement in thermal drift achieved by embodiments of the present invention.

[0071] Embodiments of the present invention offer numerous advantages over the prior art. Over a given temperature range, a single stress-free strain gauge R U For this, multiple load-sensing strain gauges R LSn By substantially eliminating the effective temperature coefficient of , F / T sensor accuracy is improved, rendering other thermal drift mitigation measures ineffective. Strain gage temperature compensation according to embodiments of the present invention results in specific values ​​of individual trimming resistors matched to the specific strain gages installed in the quarter-bridge measurement circuit of the F / T sensor. Compensation may be performed at the time of F / T sensor manufacture, and once the trimming resistors are installed in the circuit, thermal drift is permanently eliminated in hardware for the life of the F / T sensor.

[0072] In general, all terms used herein should be interpreted according to their ordinary meaning in the relevant technical field unless a different meaning is clearly given and / or is implied from the context in which it is used. All references to a / an / the element, apparatus, component, means, step, etc. should be openly interpreted as referring to at least one instance of the element, apparatus, component, means, step, etc. unless explicitly stated otherwise. The steps of any method disclosed herein need not be performed in the exact order disclosed, unless expressly stated and / or implicitly stated, that a step must follow or precede another step. Any feature of any of the embodiments disclosed herein may be applied to any other embodiment, whenever appropriate. Similarly, any advantage of any embodiment may be applied to any other embodiment, and vice versa. Other objects, features, and advantages of the enclosed embodiments will become apparent from the following description. As used herein, the term "configured to" means set up, organized, adapted, or arranged to operate in a particular manner, and is synonymous with the term "designed to." As used herein, the term "substantially" means nearly or essentially, but not necessarily completely. This term encompasses and accounts for mechanical or component value tolerances, measurement errors, random variations, and other sources of inaccuracy.

[0073] Of course, the present invention may be practiced otherwise than as specifically described herein without departing from its essential characteristics. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes which come within the meaning and range of equivalency of the appended claims are intended to be embraced therein.

Claims

1. 1. A temperature compensation circuit for a force / torque sensor, comprising: a first plurality of first stages connected in parallel, each first stage configured to output a respective voltage at a node connecting a fixed resistor and a load-sensing strain gauge in series between a positive power supply voltage and a negative power supply voltage; a second stage connected in parallel with the first stage, the second stage configured to output a voltage at a node connecting a fixed resistor and a stress-free strain gauge in series between a positive power supply voltage and a negative power supply voltage; the second stage voltage is subtracted from each first stage voltage; a stress-free trimming resistor connected in parallel across the stress-free strain gauge; a temperature compensation circuit wherein the value of the stress-free trimming resistor is selected to substantially eliminate the effective temperature coefficient of all load-sensing strain gauges, for the stress-free strain gauge, of a polarity opposite to that of the power supply voltage connected to the fixed resistor, over a predetermined temperature range.

2. further comprising a second plurality of load trimming resistors, each connected in parallel across the load sensing strain gauge; 2. The circuit of claim 1, wherein the value of each load trimming resistor is selected to substantially eliminate the effective temperature coefficient of the polarity of the power supply voltage connected to the fixed resistor of the associated load-sensing strain gauge relative to the stress-free strain gauge over the predetermined temperature range.

3. 3. The circuit of claim 2, wherein the number of the second plurality of load trimming resistors is one less than the number of the first plurality of first stages such that one load sensing strain gage does not have a load trimming resistor connected across it.

4. 4. The circuit of claim 3, wherein the load sensing strain gauge without a load trimming resistor is the load sensing strain gauge that exhibits the largest effective temperature coefficient for the polarity of the power supply voltage connected to the fixed resistor relative to the stress-free strain gauge when no trimming resistor is connected across the stress-free strain gauge.

5. 2. The circuit of claim 1, wherein the stress-free strain gauge is attached to a member of the force / torque sensor that is not subjected to substantial mechanical strain when the force / torque sensor measures applied force and / or torque.

6. 2. The circuit of claim 1, wherein the load-sensing strain gauges are mounted in pairs on the same side of different deformable beams that experience mechanical deformation from the forces and / or torques measured by the force / torque sensors.

7. 7. The circuit of claim 6, wherein the first plurality of first stages is six in number and three deformable beams have load-sensing strain gauges mounted thereon.

8. 1. A temperature compensation circuit for a force / torque sensor, comprising: a first plurality of first stages connected in parallel, each first stage configured to output a respective voltage at a node connecting a fixed resistor and a load-sensing strain gauge in series between a positive power supply voltage and a negative power supply voltage; a second stage connected in parallel with the first stage, the second stage configured to output a voltage at a node connecting a fixed resistor and a stress-free strain gauge in series between a positive power supply voltage and a negative power supply voltage; the voltage of the second stage is subtracted from the voltage of each first stage; a second plurality of load trimming resistors, each connected in parallel across a load sensing strain gauge; a temperature compensation circuit wherein the value of each load trimming resistor is selected to substantially eliminate the effective temperature coefficient of the polarity of the power supply voltage connected to the fixed resistor of the associated load sensing strain gauge relative to the stress-free strain gauge over the predetermined temperature range.

9. 9. The circuit of claim 8, further comprising a stress-free trimming resistor connected in parallel across the stress-free strain gauge.

10. 10. The circuit of claim 9, wherein the number of the second plurality of load trimming resistors is one less than the number of the first plurality of first stages such that one load sensing strain gauge does not have a load trimming resistor connected across it.

11. 11. The circuit of claim 10, wherein the load-sensing strain gauge without a load-trimming resistor is the load-sensing strain gauge that exhibited the most negative effective temperature coefficient relative to the unstressed strain gauge prior to connecting the load-trimming resistor.

12. 9. The circuit of claim 8, wherein the stress-free strain gauge is attached to a member of the force / torque sensor that is not subjected to substantial mechanical strain when the force / torque sensor measures applied forces and / or torques.

13. 9. The circuit of claim 8, wherein the load-sensing strain gauges are mounted in pairs on the same side of different deformable beams that experience mechanical deformation from the forces and / or torques measured by the force / torque sensors.

14. 14. The circuit of claim 13, wherein the first plurality of first stages is six in number and three deformable beams have load-sensing strain gauges mounted thereon.

15. 1. A method of compensating multiple load-sensing strain gauges in a force / torque sensor for thermal drift in a circuit comprising: a plurality of parallel-connected first stages, each first stage configured to output a respective voltage at a node connecting a fixed resistor and a load-sensing strain gauge in series between a positive power supply voltage and a negative power supply voltage; and a second stage connected in parallel with the first stages, the second stage configured to output a voltage at a node connecting a fixed resistor and a stress-free strain gauge in series between the positive power supply voltage and the negative power supply voltage, the method comprising: measuring an output voltage of the second stage and each first stage, and subtracting the output voltage of the second stage from the output voltage of each first stage over a predetermined temperature range; adding a stress-free trimming resistor in parallel across the stress-free strain gauges in response to at least one load-sensing strain gauge exhibiting an effective temperature coefficient for the stress-free strain gauge of a polarity opposite to that of the power supply voltage connected to the fixed resistor, the value of the stress-free trimming resistor being selected to substantially eliminate the effective temperature coefficient for all load-sensing strain gauges of the polarity opposite to that of the power supply voltage connected to the fixed resistor over the predetermined temperature range; for each load sensing strain gauge, adding a load sensing trimming resistor in parallel across the load sensing strain gauge in response to the load sensing strain gauge exhibiting an effective temperature coefficient of the polarity of the power supply voltage connected to the fixed resistor relative to the stress-free strain gauge, the value of the load sensing trimming resistor being selected to substantially eliminate the effective temperature coefficient of the polarity of the power supply voltage connected to the fixed resistor for the load sensing strain gauge relative to the stress-free strain gauge over the predetermined temperature range; A method comprising:

16. Adding stress-free trimming resistors identifying a load-sensing strain gauge that exhibits a maximum effective temperature coefficient of said polarity opposite to that of said power supply voltage connected to said fixed resistor relative to said stress-free strain gauge; adding a first stress-free trimming resistor in parallel across the stress-free strain gauge; measuring the response of the identified load-sensing strain gage over the predetermined temperature range; iteratively, in response to the identified load-sensing strain gauge exhibiting an effective temperature coefficient for the stress-free strain gauge of the polarity opposite to the polarity of the power supply voltage connected to the fixed resistor; until the identified load-sensing strain gage exhibits an effective temperature coefficient of substantially zero relative to the stress-free strain gage over the predetermined temperature range. Substituting a different stress-free trim resistor for the current stress-free trim resistor; measuring the response of the identified load-sensing strain gage over the predetermined temperature range; 16. The method of claim 15, comprising:

17. 17. The method of claim 16, wherein repeatedly measuring the response of the identified load-sensing strain gage over the predetermined temperature range comprises simulating a model of the circuit over the predetermined temperature range and recording the simulated response of the identified load-sensing strain gage.

18. Adding a load sense trimming resistor includes, for each load sense strain gage identified as exhibiting a significant temperature coefficient for the polarity of the power supply voltage connected to the fixed resistor relative to the stress-free strain gage: adding a first load sense trimming resistor in parallel across the load sense strain gauge; measuring the response of the load-sensing strain gauge over the predetermined temperature range; repeatedly, in response to the load-sensing strain gauge indicating an effective temperature coefficient of the polarity of the power supply voltage connected to the fixed resistor; until the load-sensing strain gauge exhibits an effective temperature coefficient of substantially zero for the stress-free strain gauge over the predetermined temperature range. Substituting a different load sense trimming resistor for the current load sense trimming resistor; measuring the response of the load-sensing strain gauge over the predetermined temperature range; 16. The method of claim 15, comprising:

19. 20. The method of claim 18, wherein repeatedly measuring the response of the load-sensing strain gauge over the predetermined temperature range comprises simulating a model of the circuit over the predetermined temperature range and recording the simulated response of the load-sensing strain gauge.

20. 16. The method of claim 15, wherein the stress-free strain gauge is attached to a member of the force / torque sensor that is not subjected to substantial mechanical strain when the force / torque sensor measures applied forces and / or torques.

21. 16. The method of claim 15, wherein the force / torque sensor comprises six load-sensing strain gauges, two of which are attached to each of three deformable beams that undergo mechanical deformation from applied forces and / or torques.

22. 22. The method of claim 21, wherein the two load-sensing strain gauges on each deformable beam are mounted on the same side of the deformable beam.

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