A reconfigurable rotary series elastic actuator

Abstract

A reconfigurable rotating series elastic element (RSEE) is disclosed that includes an inner tension spring mount, an outer tension spring mount, and a plurality of tension springs connected between the inner tension spring mount and the outer tension spring mount, the location where each spring connects to one or both of the inner tension spring mount and the outer tension spring mount can be changed to adjust the relationship between the output torque and the deflection angle of the RSEE; During relative rotation between the inner and outer tension spring mounts, a magnitude of tension in at least one of the tension springs is set to be different from a magnitude of tension in at least one other of the tension springs.

Description

[Technical field]

[0001] The present invention generally relates to reconfigurable rotational series elastic elements and series elastic actuators comprising such RSEEs. [Background technology]

[0002] In recent years, various assistive robots have been developed to physically assist or augment the human body of people with disabilities. These robotic devices, such as powered exoskeletons for walking assistance, significantly improve the mobility and quality of life of people with disabilities. In such applications, assistive robots necessarily have direct physical interaction with the humans who use them.

[0003] To improve the performance and safety of physical human-robot interaction (pHRI), the design of actuators and corresponding controllers is of great importance. The use of adaptability, including active adaptability and passive physical adaptability provided by control, is considered essential for assistive robots, improving their dynamic adaptability and robustness to the environment, and achieving safe pHRI. Although there have been many new developments in the design and control of various adaptable actuators, it is still difficult to obtain satisfactory pHRI performance in practical applications.

[0004] Series elastic actuators (SEAs) are one such compliant actuator, where a physical elastic element is intentionally introduced in series between a rigid actuator and an external load. Various SEAs have been developed for assistive robots to take advantage of the advantages of SEAs, including lower output impedance, good backdrivability, shock resistance, energy efficiency, smooth and accurate force transmission, and pHRI safety. However, SEAs generally use springs with a given stiffness as the elastic element in force transmission, which is a fundamental limitation of conventional SEAs since the performance of SEA is highly dependent on the spring constant. On the one hand, flexible springs produce high force control fidelity, low output impedance, and reduce static friction, but also limit the force range and force bandwidth. On the other hand, stiff springs increase the force bandwidth but reduce the force fidelity. To achieve the desired force output and sufficient force bandwidth, most existing SEAs use springs with high stiffness, which results in poor force control performance, low inherent compliance and backdrivability.

[0005] To overcome the fundamental limitations of conventional SEAs, a number of novel adaptive actuators have been proposed. Variable stiffness actuators (VSAs) are one of the most studied examples. VSAs can adjust their stiffness based on various operating principles. Among these operating principles, adjusting elastic elements by secondary motors and complex stiffness adjustment mechanisms is the most common approach to achieve stiffness variation. As a result, these actuators are generally complex and heavy, which increases the control complexity and makes them difficult to deploy in assistive robots, especially wearable assistive robots.

[0006] Apart from VSA, introducing nonlinear stiffness into SEA also offers a promising solution to the limitations of traditional SEA. However, existing designs are still limited in achieving nonlinear stiffness to improve the adaptability of assistive robots to various applications. In some cases, nonlinear stiffness behavior was achieved with specially designed cam shapes, resulting in a lack of adaptability to different applications. Several novel and reconfigurable designs are available that can result in nonlinear and tunable stiffness behavior. However, the reconfigurability and tunable stiffness of such devices are achieved using various complicated winding methods on the pulley blocks, which results in limited model accuracy due to friction and makes it difficult to achieve satisfactory control performance in pHRI.

[0007] In order to improve the performance and adaptability of the pHRI to different applications, or at least to provide a useful alternative, it is desirable to provide a novel device design to overcome the aforementioned limitations of existing nonlinear SEAs. Summary of the Invention

[0008] This paper describes an adaptive actuator design, more specifically, a reconfigurable rotary series elastic actuator (SEA) with nonlinear stiffness for assistive robots. The described device features nonlinear stiffness and adjustable stiffness profile provided by a novel and reconfigurable rotary series elastic element (RSEE) with a normal tension spring. The nonlinear stiffness can overcome the limitations of conventional SEAs caused by constant stiffness and improve the performance of human-robot interaction. Different stiffness profiles can be obtained by changing different configurations of the reconfigurable RSEE, thereby making this modular actuator applicable to different assistive robots and tasks.

[0009] A reconfigurable rotating series elastic element (RSEE), comprising: an inner tension spring mount; an outer tension spring mount; a plurality of tension springs connected between the inner tension spring mount and the outer tension spring mount; Equipped with The location at which each spring connects to one or both of the inner and outer tension spring mounts is may be varied to adjust the relationship between the output torque and the deflection angle of the RSEE; and During relative rotation between the inner tension spring mount and the outer tension spring mount, a magnitude of tension in at least one of the tension springs is set to be different from a magnitude of tension in at least one other of the tension springs. RSEE is disclosed.

[0010] The inner tension spring mount may be an inner plate, in which case the outer tension spring mount comprises two outer plates, with the inner plate being disposed between the outer plates.

[0011] The outer tension spring mount may include a plurality of spaced apart hitch holes that define locations at which tension springs can be selected to connect to the outer tension spring mount.

[0012] Each tension spring may be connected to an outer tension spring mount by a connecting shaft. Each tension spring may define an angle between the inner tension spring mount and the outer tension spring mount, where the relationship may be adjusted by offsetting the angle of one or more of the tension springs. The relationship may be adjusted by changing the pretension length of one or more of the tension springs.

[0013] The relationship can be adjusted by varying the addition or removal of tension springs between the inner and outer tension spring mounts.

[0014] The RSEE may further comprise two bearings disposed between the inner plate and the outer plate, on either side of the inner plate.

[0015] The aforementioned RSEE and a drive assembly for driving one of the inner tension spring mount and the outer tension spring mount; A series elastic actuator (SEA) comprising:

[0016] The drive assembly may drive an inner tension spring mount.

[0017] The SEA may further comprise a housing for containing the RSEE.

[0018] The SEA may further comprise a first angle gauge for measuring the change in angle of the inner tension spring mount and a second angle gauge for measuring the change in angle of the outer tension spring mount. The drive assembly may comprise a motor for providing a drive force, an embedded gear reducer, and an output shaft connected to the inner tension spring mount and the first angle gauge. The SEA may further comprise a shaft sleeve for fixing the position of the output shaft along the axis of rotation of the inner tension spring mount.

[0019] Each of the angle measuring devices may be an encoder. Each of the encoders may be a rotary encoder.

[0020] Preferably, the RSEE has a nonlinear stiffness, which can overcome the limitations of conventional SEAs caused by constant stiffness. Such an embodiment can utilize normal tension springs rather than specialized torsion springs with low-cost and accurate nonlinear stiffness.

[0021] The configuration described herein provides a large deflection range and high torque resolution compared to existing torsion springs used in rotary series elastic actuators.

[0022] Preferably, the RSEE design is reconfigurable. Thus, by changing the configuration of the reconfigurable RSEE, different stiffness profiles can be provided. Furthermore, the stiffness profile can be tuned, thereby allowing the actuator (or multiple actuators in a modular design) to be used for different assistive robots and tasks. [Brief description of the drawings]

[0023] Embodiments of the invention will now be described, by way of non-limiting examples, with reference to the following drawings, in which: [Figure 1] FIG. 2 is a perspective view of a reconfigurable RSEE within a device in accordance with the present disclosure. [Diagram 2] FIG. 2 is an exploded perspective view of the reconfigurable RSEE actuator of FIG. 1. [Diagram 3] FIG. 1 is a perspective view of a reconfigurable RSEE according to the present disclosure. [Figure 4] FIG. 4 is an exploded perspective view of the reconfigurable RSEE of FIG. [Diagram 5] FIG. 13 is a schematic diagram of an exemplary embodiment of an RSEE configured by varying the pretension length Δl of the tension spring. [Figure 6] FIG. 5 shows the relationship between the output torque and the deflection angle of the RSEE. [Figure 7] FIG. 13 is a schematic diagram of an exemplary embodiment of an RSEE configured by varying the offset angle φ of the tension spring. [Figure 8] The relationship between the output torque and the deflection angle of the RSEE in FIG. [Figure 9] FIG. 13 is a schematic diagram of an exemplary embodiment of an RSEE configured by varying the offset angle φ of the tension spring. [Figure 10] FIG. 9 shows the relationship between the output torque and the deflection angle of the RSEE. [Figure 11]1 shows the mechanical design and prototype of the RRSEAns according to the present teachings, where image (a) is a full view of the computer-aided design (CAD) model, image (b) is an exploded view of the CAD model, image (c) is a first configuration with variable spring pretension length (up to 6 spring pairs), image (d) is a second configuration with variable offset angle (up to 6 spring pairs), image (e) is a 1 / 3 configuration with variable offset angle (up to 4 spring pairs), and image (f) is the RRSEAns prototype used for testing. [Figure 12] Schematic of a reconfigurable RSEE. Image (a) shows the configuration with spring pretension and image (b) shows the configuration with an offset angle. [Figure 13] Image (a) shows the simulation results of the influence of two adjustable variables on the output torque and stiffness of the RRSEAn, and image (b) shows the model validation with experimental results. [Figure 14] 1 is a schematic linearized model of RRSEAn according to the present teachings. [Figure 15] 1 illustrates an embodiment of a cascade PI controller. [Figure 16] 13 shows the frequency response of RRSEAn with different levels of nonlinearity, where image (a) shows the open-loop response with low output torque, image (b) shows the closed-loop response with low output torque, image (c) shows the open-loop response with high output torque, and image (d) shows the closed-loop response with high output torque. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] A reconfigurable rotary series elastic actuator (RRSEAns) with nonlinear stiffness for assistive robots is described. The nonlinear and tunable stiffness profile is generated by a novel reconfigurable rotary series elastic element (RSEE), which can help achieve a good balance between low output impedance, high fidelity of force control, large force bandwidth, and output force range. A linear tension spring is used as the basic elastic element in the reconfigurable RSEE, and the stiffness profile can be tuned by changing the settings of the RSEE. Furthermore, a kinematic model based on two different tuning principles is established, which reveals the effects of tunable parameters on stiffness characteristics and also provides guidance for the design and stiffness tuning.

[0025] In addition to the actuator design, the controller design is also important to achieve a satisfactory performance and guaranteed safety of the pHRI. For the controller design, the nonlinear stiffness usually makes it more difficult to achieve accurate and stable force control. As an easy-to-apply and robust controller, in some embodiments, a cascade PID is selected for the force control of the linear SEA. With the cascade PID controller, the velocity loop bandwidth of the driven motor is much higher than the force loop bandwidth of the SEA, and the motor dynamics are decoupled from the load side, so that an effective and robust force control can be achieved. In the following, a cascade PI controller designed for the torque control of the proposed actuator with nonlinear stiffness is described. Based on the adjustable stiffness profile, human-robot interaction tests with highly nonlinear and linear stiffness are carried out, which clearly demonstrates the advantages of the nonlinear stiffness in the pHRI.

[0026] 1-4 show such a device 100 incorporating a reconfigurable rotational series elastic element (RSEE) 102 that imparts nonlinear stiffness to the system to facilitate use in an assistive robot. In some embodiments, the device 100 with the RSEE 102 is provided as a single unit. In other embodiments, the RSEE is provided by itself for incorporation into another device.

[0027] The device 100 comprises a housing assembly 104. The housing assembly 104 holds the device 100 in a fixed position relative to a system for assisting human locomotion. The device is thus mounted to a joint that provides relative movement between two members (e.g., the knee for movement between the upper leg and the lower leg, or the elbow for movement between the lower arm and the upper arm). The housing may be mounted to one member (e.g., the upper arm) and an external tension spring mount (described below) may be mounted to the other member (e.g., the lower arm). For example, the device may be mounted to the knee joint of a system that comprises an upper leg member for fixing to the human's thigh and a lower leg member for fixing to the human's lower leg. The device can then assist in controlled flexion of the knee and help provide appropriate resistance to assist in controlled movement of the leg. Similar comments are true with respect to a device that is placed in another joint of the human, for example the elbow joint.

[0028] The RSEE 102 comprises an inner tension spring mount 106, an outer tension spring mount, here embodied by plates 108a and 108b, and a plurality of tension springs 110. The tension springs 110 extend or connect between the inner tension spring mount 106 and the outer tension spring mounts (108a, 108b). The tension springs 110 may be any suitable spring, but are generally envisioned to be conventional (e.g., linear spring constant) springs.

[0029] As described with reference to Figures 5, 7, and 9, the location at which each spring 110 connects to one or both of the inner and outer tension spring mounts 106 and 108a, 108b can be changed to adjust the relationship between the output torque and the deflection angle of the RSEE 102. This allows the RSD 102 to be reconfigured to adjust, for example, the amount of suspension required to rotate the inner spring mount 106 relative to the outer spring mounts (108a, 108b). The location at which the springs 110 connect to the spring mounts (106, 108a, 108b) is also set such that the magnitude of tension in at least one of the tension springs 110 is different from the magnitude of tension in at least one of the other tension springs 110 during relative rotation between the inner and outer tension spring mounts 106 and 108a, 108b. Thus, the RSEE 102 has a non-linear force response.

[0030] The inner tension spring mount 106 takes the form of an inner plate with springs 110 attached to the inner plate 106 at locations 112 evenly distributed around the circumference of the inner plate 106. In other embodiments, the locations 112 may be non-uniformly distributed around the circumference of the inner plate 106 or may be located at various radial distances from the axis of rotation of the inner tension spring mount.

[0031] The outer tension spring mount similarly comprises two outer plates 108a and 108b. The inner plate 106 is disposed between the outer plates (108a, 108b). In some embodiments, the outer tension spring mount may comprise only a single plate or other member disposed on one side of the inner tension spring mount. However, for stability against bending under laterally applied loads (i.e., loads having a non-zero component parallel to the axis of rotation of the inner tension spring mount), it is desirable for the inner tension spring mount 106 to be disposed between the members of the outer tension spring mount.

[0032] To facilitate relative rotation between the RSEE 102 and the housing 104, a bearing 126 is provided between the RSEE 102 and a plate 128 of the housing 104, where the housing includes two plates 128, 132 between which the RSEE 102 is sandwiched, the plates 128, 132 being connected by an upper platen 130. The drive assembly 120 is connected to one of the plates 132 in a fixed relationship. The inner tension spring mount is attached to or includes a shaft 134. The shaft 134 rotates within a shaft sleeve 136 to maintain precise alignment of the shaft and the housing.

[0033] The outer tension spring mount includes multiple locations, currently embodied by hitch holes 114, to which the tension spring 110 can be connected. The hitch holes 114 may be evenly distributed around the circumference of the mounted outer tension spring, or, as alternatively shown, the hitch holes 114 may be unevenly distributed around the circumference of the outer tension spring mount, or may be located at various different radial distances from the axis of rotation of the outer tension spring mount. While the tension spring 110 may be directly connected to the outer tension spring mount, the present tension spring 110 is connected to a shaft 113 that is connected at both ends to each of the plates 108a, 108b.

[0034] The inner spring mount (plate 106) rotates relative to the outer spring mount (plates 108a, 108b). To facilitate its rotation, the inner spring mount is connected to the two outer plates 108a and 108b by two bearings 116 on 118. The two bearings 116, 118 are arranged on both sides of the inner plate 106 between the inner plate 106 and the outer plates 108a, 108b. Thus, the inner plate 106 can rotate freely relative to the outer plates 108a and 108b. A tension spring 110 is arranged in the space between the inner plate 106 and the two outer plates 108a, 108b. The tension spring 110 is connected to the inner plate 106 through a hitch point 112 of the inner plate 106, and is connected to the two outer plates 108a, 108b through a connecting shaft 113 arranged in a hitch hole 114 of the two outer plates 108a, 108b.

[0035] 2 and 4, the apparatus 100 includes a drive assembly 120 for driving one of the inner tension spring mounts within the outer tension spring mount. The drive assembly 120 drives the inner tension spring mount.

[0036] The drive assembly 120 includes a motor 122 that drives an output shaft 124. To adjust the force applied by the motor 122, the drive assembly may also include an embedded gear reducer.

[0037] Output shaft 124 is connected to inner plate 106 of reconfigurable RSEE 102 via connector 126. Connector 126 allows force from output shaft 124 to be evenly distributed around the plate or plates whose rotation is imparted by drive assembly 120, here inner plate 106.

[0038] One or more angle measuring devices are provided to control the amount of relative rotation between the attached inner tension spring and the attached outer tension spring, and thereby to control the amount of relative rotation between two members of the exoskeleton, for example for assistive exercises. Each angle measuring device measures the angle between the inner tension spring mount and the outer tension spring mount, or between one of the spring mounts and the housing 104. Here, there are two angle measuring devices. The first angle measuring device 133 measures the change in angle of the output shaft, i.e. the change in angle of the inner plate 106. The first angle measuring device may be a rotary encoder operating in a known manner, fixed to one of the attached inner tension spring and the attached outer tension spring, in order to measure the change in rotation of the other of the inner tension spring mount and the outer tension spring mount. The second angle measuring device 136 measures the change in angle of the outer plates 108a and 108b relative to the shaft 124 or the housing assembly 104, for example measuring the angular rotation of the outer plate or the inner plate with respect to the housing, the motor or some other point. The angle measurer may be any suitable device, such as an encoder or a rotary encoder. Different readings of the angle measurers 133, 136 can be used to estimate the deflection angle of the RSEE 102. In other words, the angle difference between the angles of the inner plate 106 and the outer plates 108a and 108b is the deflection angle of the reconfigurable RSEE 3. The output torque can be accurately calculated based on the kinematic model of the reconfigurable RSEE 3 and Hooke's Law.

[0039] The configuration of the reconfigurable RSEE 3 can be set in various non-linear arrangements as shown in Figures 5, 7, and 9. With reference to Figures 5 and 6, the tension spring 110 extends radially outward from a location 112 to two locations 114. The tension springs 110 are equidistantly spaced around the axis of rotation 138. Despite the equidistant spacing, the relationship between the output torque τ and the deflection angle θ is non-linear, as shown in Figure 6. The output torque increases by an increasing amount with the magnitude of the deflection angle θ. The non-linear relationship can be adjusted by changing the pre-tension length Δl of one or more of the tension springs 110. With reference to Figure 7, another non-linear relationship can be obtained by attaching a first end of multiple springs to location 112 and attaching second ends of the springs to different locations 114. A similar effect can be achieved by connecting multiple springs to location 114, with opposite ends connected to different locations 112, or by a combination of both attachment schemes. As shown in FIG. 8, the relationship between the output torque τ and the deflection angle θ is also nonlinear. In this embodiment, the nonlinear relationship can be adjusted by changing the offset angle (i.e., offset angle) φ of the tension spring 110. The offset angle φ is the angle between the trajectory X of the spring from the position 112 and the two corresponding positions 114 with respect to the radial line Y. For any spring connected to a single position 112, 114, the offset angles may be the same as shown in FIG. 7 or may be different. As shown in FIG. 8, the profile from a deflection angle 0 in the positive direction is equal and opposite to the profile from a deflection angle 0 in the negative direction, but these profiles may no longer be equal if the offset angles between any two springs connected to the same positions 112, 114 are different. FIG. 9 shows an alternative configuration in which changing the offset angle φ of the tension spring 110 changes the nonlinear relationship as shown in FIG. 10.

[0040] In each of the embodiments shown in Figures 5-10, all of the tension springs can be adjusted to change the non-linear relationship, or a subset of the tension springs can be adjusted. Similarly, in each case, the non-linear relationship can be changed by adding or removing tension strings between the attached inner tension spring and the outer tension spring mount.

[0041] In the embodiments described above and those understood with reference to those embodiments, the relationship between output torque and deflection angle is non-linear, and the stiffness can vary from a minimum value close to 0 to a relatively large value. The non-linear relationship is maintained over a wide range of deflection angles, so that the torque measurement is accurate.

[0042] Changing the configuration of the reconfigurable RSEE 102 can be implemented by adjusting one or more of the pretension length Δl, the offset angle φ of the tension spring 110, and the number of springs. Thus, the nonlinear relationship between the output torque and the deflection angle can be adjusted as shown in Figures 6, 8, and 10. The reconfigurable RSEE 102 and the rotary series elastic actuator are adapted to different usage conditions.

[0043] The criteria can provide a wide range of stiffness including but not limited to 0.095 Nm / ° to 2.330.095 Nm / °.

[0044] Further embodiments will now be described with reference to FIG. 11, which is used for testing. In particular, a disk-shaped motor 1100 with embedded gear reducer and absolute encoder is used to drive the RRSEAn, so that the size and weight of the RRSEAn can be preserved. To further increase the output torque and also reduce the thickness of the RRSEAns, a synchronous belt 1102 was used to transmit the force between the motor 1100 and the RSEE 1104 (i.e., the shaft 1106 driven by the motor 1100 drives the RSEE 1104). The transmission ratio of the synchronous belt can be selected appropriately. For example, the transmission ratio may be 2:1, which can be further increased to meet the requirement of larger output torque, or can be decreased for lower torque. In some embodiments, after the transmission, the RRSEAn can provide a continuous torque of up to 13.2 N·m and a peak torque of more than 30 N·m. This range is adequate for most assistive robots.

[0045] The structural components may be formed from any suitable material, such as an aluminium alloy, which may result in a total weight of the RRSEAn of approximately 1 kg, if desired.

[0046] In the embodiment shown in Figure 11, the series elastic element transmits position control by elasticity and force control partially masking friction and reflected inertia in the motor and transmission mechanism. This allows for precise torque control and low impedance. To eliminate the effect of transmission errors, the deflection angle of the RSEE is measured using two absolute encoders, 1108, 1110. Any suitable encoder, such as a rotary encoder, for example a rotary absolute encoder with 17-bit resolution, can be used.

[0047] In rotary SEAs, elastic elements are used as torque sensors and torque generators. Thus, the performance of a rotary SEA depends heavily on the characteristics of the elastic elements. The present RSEE features a nonlinear stiffness that can better meet the requirements of pHRI control compared to linear stiffness. Unlike other existing rotary SEAs that use custom torsion springs as elastic elements, the main advantage of the RRSEAns is that the desired nonlinear stiffness characteristics are generated by the novel design of the RSEE with inexpensive linear springs. The RSEE is designed based on a coaxial rotation mechanism. In this design, the two coaxial plates (inner plate and outer plate) of the RSEE can rotate relatively and are coupled through a tension spring. The inner plate is driven by a motor through a transmission mechanism, and the outer plate is coupled to an outer load. As a result, compliance from the tension spring is intentionally introduced in series between the input and output sides of the RSEE.

[0048] As shown in FIG. 11 (image (c)), several hitch points 1112, 1114 are evenly arranged on two plates 1116, 1118. A tension spring 1120 is sandwiched between the hitch points of the two plates 1116, 1118 to couple the rotation from the input side to the output side. The stiffness of the coupling is determined by the spring stiffness, the number of springs, the pretension length of the springs, and the offset angle at the initial position. This design allows variable stiffness values ​​and various stiffness profiles to be obtained by selecting different configurations of RSEE, which greatly improves the adaptability of RRSEAns to different applications in assistive robots. Three typical configurations (images (c), (d) and (e)) corresponding to FIG. 5, FIG. 7 and FIG. 9 are shown in FIG. 11.

[0049] For the kinematic design, two basic principles of stiffness adjustment are considered: adjusting the pretension length of the spring and adjusting the offset angle at the initial position, which are described diagrammatically in Fig. 12. A third mechanism for adjustment exists in adding or removing tension springs. The geometric parameters and spring tensions shown in Fig. 12 are expressed as follows: -l0, ΔL are the spring rest length and spring pretension length, respectively; -r1, r2 are the radii of the hitch points of the inner and outer panels, respectively. Specifically, r1 is fixed, and r2 is determined by the spring pretension length ΔL, which can be calculated using formula (1).

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[0050] -θ and q indicate the rotation angles of the inner plate and the outer plate, respectively. -φ1, φ2 indicate the offset angles in the initial position, which in some embodiments are opposite (φ1=-φ2-FIG. 11, image (d)) and in other embodiments are the same (φ1=-φ2-FIG. 11, image (e)). -l1 and l2 indicate the dwell lengths of the two springs in each pair at any deflection angle and can be calculated using equation (2).

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[0051] -F i (i=1,2) represents the tension force of the two springs in each pair, which can be calculated according to Hooke's law by equation (3).

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[0052] By measuring θ and q, the output torque of RRSEAns can be calculated by equation (4).

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[0053] Here, n=4, 6 corresponds to the number of spring pairs. Therefore, the equivalent rotational stiffness of RRSEAns is defined by equation (5).

number

[0054] where β is the deflection angle of the RSEE according to equation (6),

number

[0055] This kinematic model can represent all configurations of the RSEE. For example, when the offset angles of the initial position are set to 0 (φ1 = φ2 = 0), it characterizes the configuration shown in image (c) of Figure 11. Also, when the number of spring pairs is set to 4 and the offset angles of the two springs in each pair are set to be the same (Φ1 = Φ2), it characterizes the configuration shown in image (e) of Figure 11.

[0056] The parameters and number of tension springs are determined by taking into account the limited installation space in the compact RSEE, as well as the requirement of maximum output torque. On the one hand, tension springs with higher stiffness result in higher stiffness and larger output torque of the RSEE at large deflection angles. On the other hand, the stiffness of the RSEE around the initial position remains low due to the design characteristics of the RSEE, which can satisfy the need to establish low impedance in transparent mode. According to the kinematic model and geometric parameters of the RSEE (see Table I), a tension spring with stiffness of 20 kN / m and rest length of 28.5 mm was selected to meet the design criteria of torque exceeding 30 N·m at a deflection of about 30°, and preliminary test results showed that the tension springs conform well to Hooke's law.

[0057] Based on the kinematic model and the tuning principle of the RSEE, a simulation was carried out to clarify the performance limits and characteristics of the RRSEAn. The geometric parameters of the RSEE and the characteristics of the selected springs are shown in Table I. In order to overcome the initial tension of the selected springs and to avoid the completely relaxed situation of the RSEE, which may cause some problems in the use and control of the RRSEAn (K eq=0 - i.e., there is no tension in the spring. In some embodiments, a subset of the springs may be allowed to have zero tension), it is worth noting that the minimum pretension length was set to 0.5 mm for all configurations of the RSEE (including configurations with pretension length and configurations with offset angle). The following specifications were considered in the simulation: - Stiffness range K eq ∈[K min ,K max ] -Maximum allowable output torque τ max -Maximum deflection angle β max [Table 1] Table I - Prototype and controller parameters

[0058] Image (a) of Figure 13 shows the simulation results of the influence of two adjustable variables on the output torque and stiffness of the RRSEAn. In the figure, the dashed and solid lines respectively represent the minimum and maximum pretension lengths that the selected springs can support, which determine the working space of the RRSEAns with any RSEE configuration. In the configuration with the spring pretension length, the maximum output torque was 30.4 N·m and the maximum deflection angle reached 31.4°. The stiffness varied from 0.095 N·m / ° to 2.18 N·m / °. In the configuration with the offset angle, the output torque reached a maximum of 36.5 Nm and the maximum deflection angle was 31.4°. The stiffness could be varied from 0.095 N·m / ° to 2.33 N·m / °. Based on the simulation, the performance limits and characteristics were clearly shown. Finally, as can be seen from the figure, the performance varied for the RSEE configuration.

[0059] To verify the accuracy of the kinematic model, quasi-static tests were performed on a bench test system to compare the experimental torque-deflection characteristics of the RSEE with the theoretical results of equation (4). During the tests, a torque sensor was connected to the RSEE output plate to measure the actual output torque, and the RSEE deflection angle was measured with two absolute encoders. To evaluate the torque-deflection characteristics of the RSEE for different configurations, four separate measurements were performed with pretension lengths of 0.5 mm and 2.0 mm and offset angles of 10° and 20°, as shown in image (b) of Figure 13. The dots, black dashed lines, and solid lines represent the experimental results, fitting curves, and theoretical results, respectively. Comparing the experimental and theoretical results, root mean square (RMS) errors of 0.28, 0.21, 0.24, and 0.22 N·m were found for the four configurations, respectively, which are less than 3.6%, 2.2%, 2.6%, and 2.1% of the peak applied load. The close correlation between the fitting curves of the experimental data and the theoretical predictions indicates that the kinetic model is accurate.

[0060] In Fig. 13, image (a) shows the simulation results of the influence of two adjustable variables on the output torque and stiffness of the RRSEAn. The solid white line represents the maximum tension that the selected spring can support, showing the working space of the RRSEAn. Top: Output torque τ e Pretension length ΔL and deflection angle β; stiffness K eq Pretension length ΔL and deflection angle β. Bottom: Output torque τ e Offset angle Φ and deflection angle β; rigidity K eq vs. offset angle Φ and deflection angle β. Image (b) is the model validation with experimental results, where the dots, black dashed line and solid line indicate the experimental results, the fitting curve and the simulation results, respectively. The figure on the left shows the experimental results, while the figure on the right shows the comparison of the fitting curve and the simulation results. Top: Configuration with variable pretension length ΔL. Bottom: Configuration with variable offset angle Φ.

[0061] FIG. 13 shows that the stiffness performance of the RRSEAn can be significantly tuned by changing the RSEE configuration. For configuration 1 (FIG. 11, image (c)), the nonlinearity becomes weaker as the pretension length increases, and the output torque and stiffness at the same deflection angle increase. Note that the stiffness profile is still nonlinear even when the pretension length is large. For configuration 2 (FIG. 11, image (d)), the RSEE with offset angle can change its stiffness performance in a wider range compared to configuration 1. Hardening, linear and softening modes can be achieved as the offset angle gradually increases, with the hardening mode being desirable for improving the performance of the pHRI, and the linear mode being achieved at an offset angle of about 20°.

[0062] Apart from the pretension length and offset angle, the number of springs also has a significant effect on the performance of the RSEE. The output torque and stiffness at the same deflection angle are proportional to the number of springs. Therefore, by selecting different numbers of springs, various stiffness and output torque ranges can be achieved. For example, according to the kinematic model, the output torque and stiffness at the same deflection angle of configuration 2 is 1.5 times higher compared to configuration 3 (Figure 11, image (e)) with the same offset angle.

[0063] Next, let us consider the motor control that controls the torque. The equivalent dynamics from the motor to the output of RRSEAns can be explained as shown in Figure 14. In equation (6), the nonlinear stiffness is K eq The dynamic model of RRSEAns from the motor current command to the output torque can be formulated as follows:

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[0064] Here, τ m is the motor output torque, J m ,b m ,k m are the motor's inertia coefficient, damping coefficient, and torque constant, respectively, and i m is the motor current, Js is the inertia of the reducer and rotating parts,

number

[0065] τ m and θ m By canceling out, we obtain the dynamics of the current command with respect to the output torque of RRSEAns as follows:

number

[0066] Here, τ e is formulated as equation (4), and according to equation (4), according to equation (8), the nonlinearity in torque dynamics mainly arises from the nonlinear stiffness.

[0067] According to some embodiments, the SBA includes a controller for controlling the operation of the motor. The controller may be a proportional-integral (PI) controller design as shown in FIG. 15. The PI controller may be a cascade PI controller. The torque control term PI torque is the outer loop controller, and the inner loop controller PI velocity Generate a command from PI velocity is the feedback rate

number

[0068] The outer loop controller is designed as follows.

number

[0069] where s is the Laplace operator. The motor's PI speed loop controller acts as the inner loop with a large bandwidth. The speed controller is designed as follows:

[0070]

number

[0071]

number

[0072] To verify the effectiveness and robustness of the casing PI controller, a simulation was carried out using MAT-LAB / SimuLink software. The simulation results were obtained by controlling the control parameters

number

[0073] In the open loop frequency response test, higher stiffness correlates with higher bandwidth. Referring to image (a) of FIG. 16, as shown in FIG. 13, higher nonlinearity resulted in lower bandwidth because higher nonlinearity corresponds to relatively lower stiffness at the same deflection angle. The higher output torque operation of RRSEAns shown in image (c) of FIG. 16 results in larger bandwidth due to higher stiffness. In comparison, the closed loop bandwidth shown in images (b) and (d) of FIG. 16 is partially reduced. In either case, effective support for typical human movements is within the operating capabilities of the device.

[0074] During the torque tracking and step response tests, the transient process is fast and satisfactory in terms of sinusoidal trajectory and step response. The torque control results demonstrate that with a well-tuned cascade PI controller, the RRSEAns can perform effective and accurate torque tracking in different configurations.

[0075] During shock load testing (eg, accident simulation), sudden changes in the output torque of the RRSEAn quickly recovered to the desired value within a short time interval (about 0.25 seconds) without any tendency for chattering or instability.

[0076] The pHRI performance was also tested for two configurations (one with low nonlinearity and the other with high nonlinearity) under three conditions: passive mode, transparent (human-in-charge) mode, and assistive (robot-in-charge) mode. In passive mode, the RRSEAns are not powered. In this mode, the reflected torque of the highly nonlinear configuration is lower than that of the less nonlinear configuration due to the lower stiffness around the initial position. This means that the highly nonlinear configuration has a lower mechanical impedance. Furthermore, the low reflected torque of both configurations demonstrates the high backdrivability of the RRSEAn. In transparent mode, the desired torque is set to zero to achieve zero impedance control and minimal human interaction forces. Both configurations showed low interaction torques demonstrating the high compliance of the RRSEAn in transparent mode. It was found that the RRSEAn with high nonlinearity can perform smoother and more comfortable transparent motion due to the lower stiffness around the initial position, which also demonstrates the advantage of nonlinear stiffness in pHRI. Finally, in assistive mode, the basic function of the actuators is activated, i.e., facilitating robotic assistance. In this test, the torque error for both configurations was very small, with the more linear configuration achieving accurate torque tracking with lower deflection angle error.

[0077] The adjustable stiffness profile is a key feature of the present RRSEAn. The adjustment can be achieved by changing the configuration of the RSEE. Task-specific optimization can be achieved based on the adjustable stiffness profile, with different stiffness profiles being suitable for different applications. For example, a configuration with high nonlinearity may be more suitable for robots designed for upper limb rehabilitation or other assistive tasks that require low impedance and precise control of interaction forces. For lower limb exoskeletons intended to correct abnormal gait, a configuration with moderate or low nonlinearity may be a better choice, resulting in less position error and greater torque output and bandwidth.

[0078] It will be understood that many further modifications and permutations of various aspects of the described embodiments are possible, and the described aspects are therefore intended to embrace all such alterations, modifications, and variations that fall within the spirit and scope of the appended claims.

[0079] Throughout this specification and the claims that follow, unless the context requires otherwise, the word "comprises" and variations such as "comprises" and "comprising" are understood to mean the inclusion of a stated integer or step or group of integers or steps but not to the exclusion of any other integer or step or group of integers or steps.

[0080] Reference in this specification to any prior publication (or information derived therefrom) or any known matter is not intended to be, and should not be construed as, an acknowledgement or admission or any form of suggestion that that prior publication (or information derived therefrom) or known matter forms part of the common general knowledge in the field of endeavor to which this specification pertains.

Claims

1. A reconfigurable rotational series elastic element (RSEE) comprising: an inner tension spring mount; an outer tension spring mount; a plurality of tension springs connected between the inner tension spring mount and the outer tension spring mount, wherein the position where each spring connects to one or both of the inner tension spring mount and the outer tension spring mount is changeable to adjust the relationship between the output torque and the deflection angle of the RSEE, and during relative rotation of the inner tension spring mount and the outer tension spring mount, the tension magnitude in at least one of the tension springs is set to be different from the tension magnitude in at least one other of the tension springs. An RSEE.

2. wherein the inner tension spring mount is an inner plate, the outer tension spring mount comprises two outer plates, and the inner plate is disposed between the outer plates. The RSEE according to claim 1.

3. The outer tension spring mount comprises a plurality of spaced hitch holes defining positions where the tension springs can be selected to connect to the outer tension spring mount. The RSEE according to claim 1 or 2.

4. Each tension spring is connected to the outer tension spring mount by a connection shaft. The RSEE according to claim 1 or 2.

5. Each tension spring defines an angle between the inner tension spring mount and the outer tension spring mount, and the relationship can be adjusted by offsetting one or more of the angles of the tension springs. The RSEE according to claim 4.

6. The relationship can be adjusted by changing the pre-tension length of one or more of the tension springs. The RSEE according to claim 4.

7. The RSEE according to claim 1 or 2, wherein the relationship can be adjusted by changing the addition or removal of a tension spring between the inner tension spring mount and the outer tension spring mount.

8. The RSEE according to claim 2, further comprising two bearings arranged on both sides of the inner plate between the inner plate and the outer plate.

9. The RSEE according to claim 1 or 2, and A series elastic actuator (SEA) comprising a drive assembly for driving one of the inner tension spring mount and the outer tension spring mount.

10. The SEA according to claim 9, wherein the drive assembly drives the inner tension spring mount.

11. The SEA according to claim 9, further comprising a housing for accommodating the RSEE.

12. A first angle measuring device for measuring a change in the angle of the inner tension spring mount, and A second angle measuring device for measuring a change in the angle of the outer tension spring mount, the SEA according to claim 9 further comprising the same.

13. The drive assembly is A motor for applying a driving force, An embedded gear reducer, and An output shaft connected to the inner tension spring mount and the first angle measuring device, the SEA according to claim 12 comprising the same.

14. The SEA according to claim 13, further comprising a shaft sleeve for fixing the position of the output shaft along the rotation axis of the inner tension spring mount.

15. The SEA according to claim 12, wherein each angle measuring device is an encoder.

16. The SEA according to claim 15, wherein each encoder is a rotary encoder.

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