Patent ID: 12214488

DETAILED DESCRIPTION

The disclosure will now be described in detail with reference to the accompanying drawings and examples. As will be apparent to one skilled in the art, the embodiments described in the present disclosure are merely exemplary and represent only a subset of all such embodiments. In particular, all other embodiments obtained by a person of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of the present disclosure.

Most conventional, advanced robotic arms have a single-DOF torque sensor in each joint to measure the torque each corresponding joint generates for joint torque control. Such a torque-control based robotic arm has the following disadvantages.

First, it is difficult to prevent the torque sensor from being affected by force and torque applied in other directions (e.g., different from the torque dimension the torque sensor is designed to sense), which is called sensor crosstalk. Thus, the sensor can deviate from a true torque value under different loading conditions (e.g., joint torque coupling). There are usually mechanical structures designed to reduce this effect, such as using bearings to constrain the force and torque that can transmit through the torque sensor. However, a mechanical structure may not always be able to fully reduce this effect. For example, a bearing can still deform under a bending moment perpendicular to the rotation axis. Therefore, the above-mentioned effect can be reduced but not eliminated. There are sensor design techniques to reduce torque sensor crosstalk effect, such as using multiple transducers (e.g., a strain gauge) in different positions to compensate for the effect. However, the effectiveness of such a technique is limited by design complexity, compactness requirements and manufacturing accuracy.

Second, in conventional robots, the torque sensor must be protected by a set of bearings to reduce the joint torque coupling effect. Therefore, the controlled torque delivered by the joint will be reduced by the friction from bearings, which impairs the force control accuracy. Third, the torque sensor is usually placed close to a gear drive (e.g., a harmonic drive). The gear drive can apply torque and force to the sensor in other directions when being actuated, which also impairs the sensing accuracy. One phenomenon impairing sensing accuracy is torque ripple appearing in the sensing signals. Fourth, due to manufacturing limitations such as non-ideal part tolerance and concentricity existing in the joint mechanism, the sensor will experience different micro deformations when the joint output position is different, which also impairs the sensing accuracy.

Accordingly, the present disclosure provides a robotic arm which has a multi-DOF force and/or torque sensor in at least some of the joints to sense more force and/or torque information transmitted through the joint and the link than conventional robotic arms.

FIG.1illustrates a structural diagram of a robotic arm100according to an embodiment of the present disclosure. The robotic arm100may include multiple links131-137and multiple joints121-127. The links131-137are successively connected by the joints121-127. The joints121-127may be of two basic types, pitch joints and roll joints. The roll joints (e.g., the joints121,123,125and127as shown inFIG.1) may provide rotation about the longitudinal axis of adjacent links and the pitch joints (e.g., the joints122,124and126as shown inFIG.1) may provide rotation about axes substantially perpendicular to the roll joint axes. In some examples, an end effector140may be connected to the last joint (e.g., the joint127). In the embodiment shown inFIG.1, the robotic arm100is a 7-axis robotic arm. It should be appreciated that the below disclosed technical scheme may also be implemented for other types of robotic arms with more axes or less axes.

At least two of the joints121-127may each include a sensor which is configured to measure force and torque information (including three-direction force and three-direction torque information) of more than one of the six DOF of its respective joint. For example, the sensor may be a multi-DOF force and/or torque sensor. For instance, the joints126and127may be equipped with the multi-DOF force and/or torque sensor, or the joints124-127may all be equipped with the multi-DOF force and/or torque sensor. Alternatively, in some embodiments, all the joints121-127may each include the multi-DOF force and/or torque sensor.

In some embodiments, the sensor may be configured to measure a torque applied on its respective joint in the actuation direction of the joint. For example, if a pitch joint122,124or126includes a multi-DOF force and/or torque sensor, the sensor may be utilized to measure a torque in the Y-direction (perpendicular to the X-direction and Z-direction shown inFIG.1). If a roll joint121,123,125or127includes a multi-DOF force and/or torque sensor, the sensor may be utilized to measure a torque in the longitudinal direction between adjacent links. Moreover, the sensor may further be configured to measure force and torque information of at least one of the other five DOF. That is, the sensor may further be configured to measure one or more of the other three directions of forces and/or one or more of the other two directions of torque. For example, the multi-DOF force and/or torque sensor in corresponding joints may be configured to measure the torque in the actuation direction and the force each of the X, Y and Z directions.

The stiffness along each sensing DOF of the multi-DOF force and/or torque sensor can be optimized for better robot dynamics and control performance. In one example, the structure stiffness of the multi-DOF force and/or torque sensor in the actuation direction of the corresponding joint (e.g., around the joint axis) may be lower than the structure stiffness of the multi-DOF force and/or torque sensor in other directions. In such examples, the sensing sensitivity and resolution on the DOF that can be actively adjusted by actuation may be improved. In such examples, the stiffness on other DOFs of the structure may also remain high so as to maintain a high structure stiffness of the entire robotic arm for better control performance and higher mechanical and control bandwidth.

In some embodiments, the multi-DOF force and/or torque sensor may be a six DOF force and torque sensor that is capable of sensing torque and force information for all six DOF transmitting through the corresponding joint and the adjacent link where the joint is located. A six DOF force and torque sensor is designed to sense all of the force and torque experienced at a joint and the adjacent link, and thus may remain accurate under any combination of force and torque. U.S. patent application Ser. No. 16/456,588 discloses one exemplary 6-DOF force and torque sensor. In other examples of the present disclosure, however, other types of six DOF force and torque sensors may also be utilized.

FIGS.2-5show different arrangements of a sensor of a joint. InFIGS.2-5, the joints200a-200deach include an input part201, an output part202, a motor203, a gear drive204, a multi-DOF force and/or torque sensor205and one or more bearing206. The stator of the motor203may be fixed to the input part201and the rotor of the motor203may be fixed to the output part202such that the motor203may drive the output part202to rotate with respect to the input part201. The gear drive204may be connected to the rotor of the motor203to adjust the rotation speed of the output part202and output torque. In some embodiments, the gear drive204may be a harmonic drive. In some examples, the bearing206may be located between the input part201and the output part202to allow relative rotation between these two parts.

In an embodiment as shown inFIG.2, the multi-DOF force and/or torque sensor205of the joint200amay be placed between the input part201and the output part202(e.g., between the gear drive204and the output part202), which is similar to a joint with a single-DOF torque sensor. In this embodiment, since the bearing206is designed to take bending moments from the output part202to the input part201, the multi-DOF force and/or torque sensor205may be configured to only measure a torque in the actuation direction of the joint200aand an axial force transmitted from the output part202to the input part201.

In an embodiment as shown inFIG.3, the multi-DOF force and/or torque sensor205of the joint200bmay be placed between the input part201of the joint200band the previous link301. In this embodiment, the multi-DOF force and/or torque sensor205can be designed to measure force and torque information of any number of the six DOF. For example, the multi-DOF force and/or torque sensor205may be a three DOF force sensor capable of measuring force information in all three force directions, a three DOF torque sensor capable of measuring torque information in all three torque directions, a four DOF force and torque sensor capable of measuring force information in all three force directions and a torque in the actuation direction of the joint200b, etc.

In one embodiment, the multi-DOF force and/or torque sensor205may be a six DOF force and torque sensor that is capable of sensing all the force and torque transmitted between the previous link301and the input part201of the joint200b. The joint200bmay further include a sensor circuit board207communicating with the multi-DOF force and/or torque sensor205. The sensor circuit board207may be located at the input end of the input part201of the joints200band adjacent to the multi-DOF force and/or torque sensor205. This example configuration may largely simplify the wiring configuration of the multi-DOF force and/or torque sensor205and the sensor circuit board207.

In an embodiment as shown inFIG.4, the multi-DOF force and/or torque sensor205of the joint200cmay be placed between the output part202of the joint200cand the subsequent link302. In this embodiment, the multi-DOF force and/or torque sensor205can be designed to measure force and torque information of any number of the six DOF. For example, the multi-DOF force and/or torque sensor205may be a three DOF force sensor capable of measuring force information in all three force directions, a three DOF torque sensor capable of measuring torque information in all three torque directions, a four DOF force and torque sensor capable of measuring force information in all three force directions and a torque in the actuation direction of the joint200b, etc.

In one embodiment, the multi-DOF force and/or torque sensor205may be a six DOF force and torque sensor that is capable of sensing all the force and torque transmitted between the output part202of the joint200band the subsequent link302. In this embodiment, there is less compliance between the actuation output and the sensing component (e.g., the multi-DOF force and/or torque sensor205) compared with the above-described embodiment where the multi-DOF force and/or torque sensor205is placed at the input end or inside the corresponding joint (as shown inFIG.2orFIG.3), and therefore the torque sensing accuracy and control performance of the joint200cmay be improved.

In the embodiments shown inFIGS.3and4, since the sensor205is a multi-DOF force and/or torque sensor, it can be placed outside the input part201, the output part202and the bearing206without undermining the sensing accuracy. Thus, the multi-DOF force and/or torque sensor205may be flexibly installed anywhere on the joint for various design benefits such as simplifying the wiring configuration or optimal joint design for better dynamics and control performance.

In an embodiment shown inFIG.5, the joint200dmay include two multi-DOF force and/or torque sensors205and207. The first sensor205may be located between the input part201of the joint200dand the previous link301, while the second sensor207may be located between the output part202of the joint200dand the subsequent link302. Either of the two sensors205and207may be redundant of the other one in order to improve the accuracy of measuring force and torque information. In some examples, additional redundant sensors in the robotic arm100can be used for cross-check for fault detection and better safety. In some embodiments, the sensors205and207may be substantially identical. In other embodiments, the sensor207may be different from the sensor205. For example, the force and torque information the sensor207measures may be different from that the sensor205measures.

FIGS.6A to6Cshow an example scenario where each joint of a robotic arm includes a single-DOF torque sensor. In this example, the robot includes three links411-413, three joints511-513and an end effector414. Each of the joints511-513includes a sensor configured to measure only the torque in the actuation direction of the corresponding joint. When a load610is applied on the end effector414, the sensors in the joints511and513may each sense a one-DOF torque, as shown on the robotic arm10inFIG.6A. When a load620is applied on the link413, the sensors in the joints511and512may each sense a one-DOF torque, as shown on the robotic arm20inFIG.6B. When the load610and the load620are simultaneously applied on the end effector414and the link413respectively, the sensors in the joints511-513may each sense a one-DOF torque, as shown on the robotic arm30inFIG.6C. However, in this example scenario the robot is not able to correctly identify the two loads610and620. Instead, the robot may mistake the loads610and620as a single false load630on the end effector, because the sensing results of the multi-DOF sensors in the situation where one load630is applied is the same as those in the situation where two loads610and620are applied. Accordingly, this may undermine the control performance of the robot and the ability for the robot to function properly in a complex environment.

In comparison,FIGS.7A to7Cshow an example scenario where each joint of a robotic arm includes a multi-DOF force and/or torque sensor. In this example, the robot includes three links411-413, three joints511-513and an end effector414. Each of the joints511-513includes a sensor configured to measure force and/or torque information of multiple, for example, a torque in the actuation direction of the corresponding joint and two forces perpendicular to the actuation direction of the corresponding joint. When a load610is applied on the end effector414of the robotic arm40inFIG.7A, the sensors in the joints511and513may each sense a one-DOF torque and a one-DOF force, and the sensor in the joint512may sense a one-DOF force. When a load620is applied on the link413of the robotic arm50inFIG.7B, the sensors in the joints511and512may each sense a one-DOF torque and a one-DOF force. When the load610and the load620are simultaneously applied on the end effector414and the link413respectively of the robotic arm60inFIG.7C, the sensors in the joints511and512may each sense a one-DOF torque and a two-DOF force, and the sensor in the joint513may sense a one-DOF torque and a one-DOF force. Accordingly, in this implementation, the two loads610and620may be accurately identified since the sensing results of the multi-DOF sensors in the situation where one load630is applied is different from those in the situation where two loads610and620are applied. Thus, with multi-DOF force and/or torque sensors, even with different loads on different locations, the robot can still estimate each load precisely.

In some embodiments, two adjacent joints of a robot may both be equipped with a six DOF force and torque sensor. The two adjacent joints (e.g., the joints125and126inFIG.1) may be denoted by Joint N and Joint (N+1), and the sensor reading on corresponding joints may be {right arrow over (F)}Nand {right arrow over (F)}N+1(converted to the same coordinate), which are both six DOF vectors. {right arrow over (D)} is the total inertia force between the two joints. Assuming a single point contact is applied anywhere on the link (e.g., the link136inFIG.1) between the two joints, then six DOF information of the point contact (e.g., two DOF position on the link, one DOF normal force, two DOF shear forces and one DOF torsional force) can be computed based on T({right arrow over (F)}N, {right arrow over (F)}N+1, {right arrow over (D)}), where T is a transformation function to solve the problem. Such point contact information can be used for multiple purposes, including better human-robot interface and safety.

For example, the robot may know the point contact over its body better so that the robot can react more properly to protect a human operator and differentiate abnormal collision from normal interactive contact. In another example, a human operator can draw certain patterns with certain force profiles on certain links of the robot to give certain commands to the robot. Based on the previous analysis, the effect of the point contact on adjacent joints can be computed by projecting {right arrow over (F)}N, {right arrow over (F)}N+1, {right arrow over (D)} to the corresponding joints so that the local torque controller of each joint can generate additional torque to compensate for this effect. Thus, the whole arm can better resist disturbance over the arm without affecting the operated tasks and the end effector.

In the above-described embodiments, six DOF force and torque sensors are utilized. In certain examples of the present disclosure, sensors capable of measuring force and torque information of less DOFs may be used for detecting simpler contact forces on the arm. For example, four DOF sensors, which are not capable of measuring force and torsion along and around the link axis, can be utilized to perceive contact force on the link in the case that the user only applies normal force with no shear or torsional friction on the arm.

Referring toFIG.1, in some embodiments, each of the joints121-127of the robot100may be equipped with a multi-DOF force and/or torque sensor, which in some aspect may be a six DOF force and torque sensor. In such embodiments, additional redundant sensors in the arm can be fused together to improve the sensing accuracy. For example, averaging the sensors' output on the same force direction from multiple joints of a stationary robotic arm can reduce the overall sensing error in that direction. If a sensor has noise or error standard deviation of σ in one sensing direction, then with a seven DOF arm and a six DOF sensor in each joint, the error standard deviation may become σ/√{square root over (7)}. The force and torque sensors in the joints can be used to accurately estimate external contact force position, orientation and magnitude on each of the robot links131-137, which is useful information for more advanced human-robot interactions and interfaces. Since the torque and force sensors do not have to be placed inside the joints131-137, they can be more flexibly installed anywhere on the corresponding joints for design benefits such as simplifying the wiring configuration or optimal joint design for better dynamics and control performance.

Without further elaboration, it is believed that one skilled in the art can use the preceding description to utilize the claimed inventions to their fullest extent. The examples and embodiments disclosed herein are to be construed as merely illustrative and not a limitation of the scope of the present disclosure in any way. It will be apparent to those having skill in the art that changes may be made to the details of the above-described embodiments without departing from the underlying principles discussed. In other words, various modifications and improvements of the embodiments specifically disclosed in the description above are within the scope of the appended claims. For example, any suitable combination of features of the various embodiments described is contemplated.