Gravity and Inertia Compensation for Force / Torque Sensors

Abstract

Gravity and robot motion compensation of force and torque measurements from the robot's F / T sensors. Attachment tool weight W is obtained from user input or through parameter identification. tool and the vector from the origin of the F / T sensor body CF to the center of gravity of the tool. During robot operation, the rotation matrix from the F / T sensor body CF to the inertial reference frame is obtained from the internal inertial measurement unit or from the forward kinematics data of the robot based on W tool The instantaneous values ​​of and are used to compensate for gravity, using the force and torque measurements resolved from the transducer output by the F / T sensor. Additional information is obtained for inertia compensation, including: the attached tool mass m; the angular velocity of the F / T sensor body CF; the angular acceleration of the F / T sensor body CF; and the inertia tensor I defined within the F / T sensor body CF, which includes all moments of inertia and products of inertia. Based on m, and I, the force and torque measurements are compensated for inertial effects.

Description

Technical Field

[0001] The present invention relates generally to robotics, and in particular to compensating robotic force / torque sensors for gravity and inertial forces. Background Art

[0002] Robots are integral to product manufacturing, testing, assembly and packaging, assisted and remote surgery, space exploration, operating in hazardous environments, and many other applications. Many robots and robotic applications require quantifying the forces applied or experienced, such as material removal (grinding, sanding, etc.), remote excavation, or other manipulation of the environment.

[0003] As a representative example, consider a robot performing some tasks on a workpiece. A robot typically includes a general-purpose actuator or "arm" that is programmed to move in space and manipulate the workpiece along multiple degrees of freedom. A robotic tool changer is an electromechanical device that allows a robot to perform many different tasks using many different tools (also called end effectors). A robotic tool changer includes a "master" assembly attached to the robot arm, and multiple similar or identical "tool" assemblies, each of which is attached to a tool that the robot can use. The master assembly and tool assembly are selectively coupled together under the control of the robot control system. The master assembly and tool assembly may also include features for transferring utilities between them - such as AC or DC current, pneumatic fluids, data, etc. - for use by the tool, and providing a path for the tool to relay the data back to the control system. With the tool changer, a robot can perform a task using a first tool, park the first tool in a tool rack, retrieve a second tool, and perform another task using the second tool.

[0004] In applications where it is necessary to monitor and control the degree of force applied by the robot to the workpiece and / or to provide feedback on the forces experienced by the robot to control the robot's motion ("force control" operation), a force / torque (F / T) sensor is attached between the robot arm and the main assembly of the tool changer or between the tool assembly and the tool (or the F / T sensor functionality can be built into the tool changer). Figure 3As shown, one configuration of the F / T sensor 12 includes a component, referred to as a mounting adapter plate (MAP) 14, which is mounted (directly or indirectly) to the robot, and another component, referred to as a tool adapter plate (TAP) 16, to which the tool is connected (directly or indirectly). The MAP 14 and TAP 16 are connected to each other by a plurality of relatively thin (and therefore mechanically deformable) beams 18. Relative forces or torques between the robot and the tool, which are attached to the MAP 14 and TAP 16, respectively, attempt to move the MAP 14 relative to the TAP 16, resulting in slight deformation or bending of at least some of the beams 18. Strain gauges (not shown) attached to the surfaces of the beams 18 detect this deformation and output proportional electrical signals. The outputs of many such sensors can be processed in combination to resolve the forces and torques applied to the F / T sensor 12 along a defined axis. An example of a compact F / T sensor is described in U.S. Patent No. 10,422,707, which is assigned to the assignee of the present disclosure and is incorporated herein by reference in its entirety. Note that Figure 3 The configuration depicted in FIG, in which TAPs 16 are annularly arranged around MAP 14 and beams 18 are arranged like spokes on a wheel, is representative only and is not limiting. Various configurations of F / T sensors are known in the art.

[0005] Thermal drift is a known source of error in the output of force and torque resolution from F / T sensors. One example of temperature compensation for robotic F / T sensors is described in International Patent Publication No. WO 2018 / 200668, which is assigned to the assignee of the present disclosure and is incorporated herein by reference in its entirety.

[0006] For static orientation basis functions, thermal compensation is sufficient for accurate force and torque monitoring. The robot moves the tool to the work position and orientation, and the sensor data is zeroed. As long as the robot maintains the same orientation and moves at a speed low enough to make inertial effects negligible, the F / T sensor provides accurate readings of force and torque as the robot tool completes its task. However, tasks such as contour tracking and 3D part assembly require changes in the orientation and / or position of the robot tool. Orientation changes alter the distribution of the tool's weight along the axes of the F / T sensor's reference frame, while position changes cause inertial forces and / or torques to be introduced from the robot's motion. Both phenomena can adversely affect the accuracy of force and torque measurements. In fact, in many cases, gravitational and inertial forces and torques exceed the required contact forces and torques, making force-controlled robotic operation difficult or impossible.

[0007] The background section of this document is provided to place the embodiments of the present invention in a technical and operational context to help those skilled in the art understand their scope and utility. The approaches described in the background section may be employed, but they are not necessarily approaches that have been previously conceived or employed. Unless expressly indicated otherwise, no statement herein is admitted to be prior art merely by virtue of its inclusion in the background section. References herein Figure 3 To provide an introduction to one configuration of components in an F / T sensor; however, Figure 3 It does not represent prior art. Summary of the Invention

[0008] The following is a simplified summary of the present disclosure to provide a basic understanding for those skilled in the art. This summary is not an extensive overview of the present disclosure and is not intended to identify key / critical elements of embodiments of the present invention or to define the scope of the present invention. The sole purpose of this summary is to present some of the concepts disclosed herein in a simplified form as a prelude to the more detailed description that will be presented later.

[0009] According to one or more embodiments described and claimed herein, force and torque measurements from the robot's F / T sensors are compensated for the effects of gravity and, optionally, also for the effects of robot motion. The weight W of the attached tool is obtained, for example, from user input or through parameter identification. tool and the vector from the origin of the F / T sensor body coordinate system (CF) to the center of gravity of the tool During robot operation, the rotation matrix from the F / T sensor body CF to the inertial reference frame is obtained, for example from an internal inertial measurement unit (IMU) or from the robot's forward kinematics data Based on W tool and and The instantaneous value of is used to compensate for gravity by the force and torque measurements resolved from the transducer output by the F / T sensor. For inertia compensation, additional information is obtained, including: the mass m of the attached tool; the angular velocity of the F / T sensor body CF Angular acceleration of the F / T sensor body CF Linear acceleration of the F / T sensor body CF The inertia tensor I defined in the F / T sensor body CF contains all moments of inertia and products of inertia. The mass m can be obtained from W tool Derived and can be obtained from IMU or forward kinematics and The linear acceleration is obtained by first estimating the gravitational acceleration from the absolute heading data and converting it to an estimated gravity vector This estimated gravity vector is then subtracted from the acceleration vector obtained from the IMU or forward kinematics data. The term I can be obtained from parameter identification or entered by the user if known from analytical determination or CAD software. and I compensate the force and torque measurements resolved from the transducer output by the F / T sensor for inertial effects.

[0010] One embodiment relates to a robotic force / torque (F / T) sensor comprising a transducer configured to generate a signal in response to a force or torque applied to the sensor. The F / T sensor comprises measurement circuitry configured to resolve force and torque measurements from the transducer signal, the force and torque measurements being referenced to a body coordinate system (CF) of the F / T sensor. The F / T sensor further comprises compensation circuitry configured to obtain a weight W of an attached tool. tool ; Get the vector from the origin of the F / T sensor body CF to the center of gravity of the tool Get the rotation matrix from the F / T sensor body CF to the inertial reference frame And according to W tool and Compensate force and torque measurements for the effects of gravity on attached tooling.

[0011] Another embodiment relates to a method for compensating force and torque measurements resolved from transducer output signals of a robotic force / torque (F / T) sensor by measurement circuitry for the effects of gravity caused by the weight of an attached tool. The force and torque measurements are referenced to a body coordinate system (CF) of the F / T sensor. The F / T sensor includes compensation circuitry. The weight W of the tool attached to the F / T sensor is obtained. tool Get the vector from the origin of the F / T sensor body CF to the center of gravity of the tool Get the rotation matrix from the F / T sensor body CF to the inertial reference frame based on W tool and Compensate force and torque measurements for the effects of gravity on attached tooling.

[0012] Yet another embodiment relates to a robotic force / torque (F / T) sensor comprising a transducer configured to generate a signal in response to a force or torque applied to the sensor. The F / T sensor comprises measurement circuitry configured to resolve force and torque measurements from the transducer signal, the force and torque measurements being referenced to a body coordinate system (CF) of the F / T sensor. The F / T sensor comprises compensation circuitry configured to obtain a mass m of an attached tool; an angular velocity m of the F / T sensor body CF; and a compensating circuitry configured to obtain a mass m of an attached tool; and an angular velocity m of the F / T sensor body CF. Get the angular acceleration of the F / T sensor body CF Get the linear acceleration of the F / T sensor body CF Obtain the inertia tensor I defined in the F / T sensor body CF, which contains all moments of inertia and products of inertia; and calculate the inertia tensor I according to m, and I compensate the force and torque measurements for the inertial effects of the robot motion. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. However, the present invention should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided to make this disclosure thorough and complete and to fully convey the scope of the invention to those skilled in the art. Throughout the text, like reference numerals refer to like elements.

[0014] Figure 1 is a diagram of a robot arm with an F / T sensor and tool attached.

[0015] Figure 2 is a block diagram of the attached F / T sensor and tool, with the center of gravity and body coordinate system identified.

[0016] Figure 3 is a perspective cutaway exploded view of an F / T sensor with an IMU.

[0017] Figure 4 is a diagram of a system including an external electronic module that uses robot forward data to obtain parameters for compensation.

[0018] Figure 5 is a flow chart of a method of compensating force and torque measurements resolved from the transducer output signals of an F / T sensor for the effects of gravity caused by the weight of an attached tool.

[0019] Figure 6 is a graph of the original and compensated F / T data. DETAILED DESCRIPTION

[0020] For simplicity and illustrative purposes, the present invention is described primarily by reference to its exemplary embodiments. In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be apparent to one of ordinary skill in the art that the present invention may be implemented without limitation to these specific details. In this description, well-known methods and structures are not described in detail to avoid unnecessarily obscuring the present invention.

[0021] Figure 1A portion of the arm of a robot 10 is depicted, with a force / torque (F / T) sensor 12 attached. As used herein, the term robot 10 refers to both the physical robot arm and the robot control system that controls its operation. Attached to the other side of the F / T sensor 12 is an end effector tool 20. Tool 20, for example, may be a grinder or polisher that contacts a workpiece (not shown) and applies a predetermined force / torque vector (referred to as a "wrench" in the robotics field) to, for example, shape or finish the workpiece's surface. The workpiece may typically have a complex shape, and the robot 10 can move the tool 20 across the workpiece surface by translating and rotating along different degrees of freedom to place the tool 20 in various orientations and spatial locations, thereby enabling it to manipulate the workpiece. Due to the different orientations of the tool 20, the F / T sensor experiences different components of the tool 20's weight along the sensor 12 axis. Furthermore, the movement of the robot 10 induces inertial forces and / or torques on the F / T sensor 12. These effects are indistinguishable at the transducer level from the contact forces and torques between the tool 20 and the workpiece that the F / T sensor 12 is supposed to measure. Unless they can be compensated for, gravity and inertial effects can make force-controlled robotic operations difficult or impossible.

[0022] Motion Compensation

[0023] According to an embodiment of the present invention, one or both of the gravity and inertial effects are dynamically compensated by the F / T sensor 12, either independently (with an integrated inertial measurement unit (IMU)) or in combination with an external compensation unit that receives data from the F / T sensor 12 and the robot 10. Measurement compensation is performed by adding a correction factor to the F / T sensor 12 measurement output, where the correction factor is based on a model of how the F / T measurement is affected by various environmental factors. Note that this disclosure does not consider temperature compensation. As mentioned above, there are known techniques for compensating for temperature drift, and temperature effects are independent of gravity and inertial effects. The basic compensation equation is given by equation (1).

[0024]

[0025] in, is the output of the F / T sensor;

[0026] is the contact force (e.g., between the robotic tool 20 and the workpiece);

[0027] is the force exerted by the weight of the sprung mass of the tool 20 (and sensor 12 ) due to gravity;

[0028] is the force resulting from the linear and / or angular acceleration or velocity of the robotic arm 10;

[0029] is noise from electrical and / or mechanical sources.

[0030] Throughout this disclosure, for convenience, the matrix F is used to refer to the force and torque components. Where the notation F or T refers to a matrix containing only force or torque terms, respectively, this will be explicitly stated or will be obvious to one skilled in the art from the context. Furthermore, this disclosure does not follow a strict Plücker basis, but rather places linear terms above rotational terms. Thus, each "F" term in Equation (1) is a space vector containing the information described in Equation (2):

[0031]

[0032] In force-controlled robotic manipulation, the contact force is the desired quantity to measure. Therefore, equation (1) is rewritten to produce this value. Furthermore, it is assumed that low-pass filtering effectively eliminates item.

[0033]

[0034] Equation (3) represents full compensation—referred to as active dynamic compensation because it is applied continuously—including gravity and inertia compensation components. According to one embodiment, the F / T sensor 12 can be equipped or configured to perform any of three levels of compensation. First, the F / T sensor 12 can perform no compensation, also known in the art as "static compensation." This is the default setting for the F / T sensor 12. Static compensation is characterized by equation (4):

[0035]

[0036] Secondly, the F / T sensor 12 may only perform gravity compensation. This is called active gravity compensation because it occurs continuously and is characterized by equation (5):

[0037]

[0038] Finally, as mentioned above, active dynamic compensation includes gravity compensation and inertia compensation and is characterized by equation (3).

[0039] Gravity compensation

[0040] Figure 1The robotic tool 20 is depicted in a particular orientation. As used herein, the "orientation" of the tool 20 refers to its position and posture in 3-dimensional space, in an inertial reference frame (also known as a gravitational reference frame). In an inertial reference frame, one axis is always vertical (e.g., z), while the other two are always horizontal. The weight of the tool 20 is modeled as a vector pointing from the tool's center of gravity (CG) toward the center of the Earth. In an inertial reference frame, the weight vector of the tool 20 is always parallel to the vertical axis.

[0041] The F / T sensor 12 defines an F / T sensor body coordinate system (CF), which consists of mutually orthogonal x, y, and z axes with their origin in the F / T sensor 12. As the robot arm moves, the F / T sensor body CF translates and rotates in space while remaining fixed from the perspective of the F / T sensor 12. The F / T sensor 12 reports forces and torques referenced to its body CF.

[0042] The magnitude of the tool 20 weight vector and the rotation matrix relating the inertial reference frame to the F / T sensor body CF are the only quantities required to compensate the F / T sensor 12 force measurements for the weight effect of the tool 20. Calculating the gravity torque compensation term requires additional data on the positional offset between the tool 20 center of gravity (CG) and the origin of the F / T sensor body CF. The complete gravity compensation matrix is ​​defined in Equation (6):

[0043]

[0044] in is the rotation matrix from the F / T sensor body CF to the inertial reference frame. This can be obtained from the output of an onboard inertial measurement unit (IMU) (e.g., quaternion orientation output) or from the forward kinematics of the robot 10. This value is updated each time the IMU or forward kinematics are sampled—which occurs at a higher frequency (e.g., up to 400 Hz) relative to the rate of change of the tool 20 orientation during use.

[0045] is the weight vector of the tool 20 in the inertial reference frame. When used without a vector notation, the variable W tool is the scalar weight of the tool 20. Therefore, Where z is the vertical axis in the inertial reference frame. This value can be entered by the user or obtained through parameter identification.

[0046] It is the vector from the origin of the F / T sensor body CF to the tool 20CG. express A skew-symmetric matrix. The value can be entered by the user or obtained through parameter identification.

[0047] Parameter identification refers to a data-driven approach to deriving important physical properties of a system from available measurements. Parameter identification is also known as system identification in the robotics and control systems literature. and measurement.

[0048] Many F / T sensors 12 exhibit reduced sensitivity and / or accuracy in one or more of the six F / T measurements due to the geometry of their construction, the distribution and orientation of the strain gauges, etc. Therefore, in order to obtain and For the most accurate measurement (and therefore the most accurate gravity compensation), it is best to measure these quantities multiple times, each time with the F / T sensor 12 at a different spatial orientation. The resulting data sets are then fitted, for example, using least squares regression, to obtain the final and As in all least squares regressions, maximum accuracy is achieved if the samples are well spaced across its dynamic range. When a usable gravity compensation term is available from only one tool 20 orientation and measurement, accuracy improves (e.g., noise is averaged out) as the number of discrete orientations sampled increases to approximately six. More than six tool 20 orientation measurements increases computational complexity, yet offers little improvement in accuracy.

[0049] Gravity compensation of force

[0050] The force terms are gravity compensated by knowing only the weight of the tool, which obtains a gravity compensation term for each axis referenced to the F / T sensor body CF. The resolved force measurements referenced to the F / T sensor body CF are then compensated for the weight of the tool 20 by subtracting the gravity compensation term.

[0051] To determine the weight of the tool 20 by parameter identification, the robot 10 is commanded to place the tool 20 in j different spatial orientations (j=1, 2, ..., n). The force experienced by the weight of the attached tool 20 in each of these orientations is resolved by the F / T sensor transducer. In addition, a unit gravity vector is obtained for each orientation. The unit gravity vector decomposes the weight vector of magnitude one (always pointing downward along the vertical axis in the inertial reference frame) into its components along the three axes of the F / T sensor body CF in that orientation. The gravity unit vector for the jth orientation is expressed as The resolved force measurement is then described by equation (7) (where F represents the force term only; there is no torque term):

[0052]

[0053] When j sets of samples are stacked together, solve the least squares matrix equation to minimize the squared residual. It is known that if A is an m×n matrix and b is R m dimensional vector, then the least squares solution of Ax=b is the matrix equation A T Ax=A T When applied to equation (7), this yields equation (8):

[0054] W tool =((G T G) -1 (G T F measured )) (8)

[0055] Among them, F measured is the stacked force matrix vector and G is the stacked gravity matrix vector

[0056] Note that the weight of the attached tool 20 may not be a static quantity. For example, a grinder or sander may lose abrasive while in operation. In addition, the robot may change tools 14 during the entire force control operation (e.g., from a grinding tool 20 to a polishing tool 20, where both contact forces are fed back to control the positioning of the robot 10). As another example, the robot tool 20 may include a gripper that grabs an item, moves the item, and places the item. If force control is required throughout, a different value of W may be applied when the tool 20 is holding the item than after the item is placed. tool In one embodiment, multiple tool weight values ​​may be input or derived through the parameter identification process described above. These values ​​may be stored in memory and recalled as needed for use in equation (6).

[0057] Gravity compensation for torque terms

[0058] Vector from the origin of the F / T sensor body CF to the tool CG It can only be measured from the torque value. The torque equation (9) comes from Newtonian physics:

[0059]

[0060] Using the cross product identity and skew-symmetric matrices, this is rearranged into the Ax=b form for least squares regression:

[0061]

[0062] make where j is the measurement / orientation index and A j yes:

[0063]

[0064] Then the least squares formula is:

[0065]

[0066] Using j samples and fitting the data according to Equation 13, we solve for the vector from the origin of the F / T sensor body CF to the center of gravity of the tool 20:

[0067]

[0068] in is a stacked matrix of torque measurements; and

[0069]

[0070] To eliminate noise and obtain a better estimate of the tool's center of gravity, in one embodiment, multiple values ​​of the vector from the F / T sensor CF origin to the tool's center of gravity can be input or derived through the above-described parameter identification process. These values ​​can be stored in memory and recalled as needed for use in equation (6).

[0071] Inertia compensation

[0072] Inertia compensation is much more complex than gravity compensation because it includes inertial, Coriolis, and centripetal coupling terms. The inverse dynamics model of tool 20 relative to the F / T sensor body CF can be derived by applying Newton-Euler mechanics. Assuming a rigid body model of tool 20, the following classical Newton-Euler assumptions yield the following model:

[0073] • According to Newton's third law, the reaction force torque measured by the F / T sensor 12 will be equal and opposite to that experienced by the tool 20 .

[0074] The total force on the system is equal to the time rate of change of linear momentum.

[0075] The total torque on the system is equal to the time rate of change of angular momentum.

[0076] The total force and torque on the F / T sensor body CF—excluding the gravity term—are given by equations (14) and (15), respectively:

[0077]

[0078]

[0079] Where m is the mass of the tool 20. This term can be converted to mass by Obtained from gravity compensation parameter identification.

[0080] is the angular velocity of the F / T sensor body CF. This can be obtained from the data output by the onboard IMU or from the forward kinematics of the robot 10.

[0081] is the angular acceleration of the F / T sensor body CF. This can be distinguished from the IMU data, or from the forward kinematics of the robot 10.

[0082] is the vector from the origin of the F / T sensor body CF to the center of gravity of the tool 20. This value can be input by the user or obtained through the parameter identification described above for gravity compensation.

[0083] is the linear acceleration of the F / T sensor body CF. The onboard IMU outputs acceleration values, including gravitational acceleration and linear acceleration. The linear acceleration can be extracted by estimating the gravitational acceleration from the absolute orientation data converted into an estimated gravity vector. Using this estimated gravity vector, the linear acceleration is calculated as Alternatively, it can be obtained from the robot forward kinematics

[0084] I is the inertia tensor defined in the F / T sensor body CF. The inertia tensor is a matrix containing all moments of inertia and products of inertia. Intuitively, the inertia tensor provides a conversion between angular velocity and angular momentum. This value can be entered by the user or obtained through parameter identification.

[0085] The force and torque vectors of equations (14) and (15) are obtained from measurements performed at j different tool 20 orientations, j = 1, 2, ..., n (although 6 is a reasonable upper limit for n, this is not a limitation of the present invention). Similar to the gravity compensation case, they are then stacked into space vectors according to equation (16) to create the final inertia compensation term (where the superscript 6 refers to the dimensionality of the space vector when used around a traditional coordinate vector):

[0086]

[0087] Identification of inertia compensation terms

[0088] The inertia tensor I is a required term that is difficult to quantify without a solid model containing accurate materials. Therefore, in one embodiment, a parameter estimation method is used to obtain six different parameters of the inertia tensor.

[0089] To establish the relationship between the torque measurement and the inertia tensor, the rotational inertia term is isolated on one side.

[0090]

[0091] in is the torque measured by the 6-axis F / T sensor 12;

[0092] is the torque due to gravity acting through the center of mass (Equation 6, bottom); and

[0093] is the torque produced by the linear acceleration acting through the center of mass (the bottom of Equation 16 This makes the moment of inertia term:

[0094]

[0095] Known

[0096]

[0097] Order I xy =I yx , I xz =I zx and I zy =I yz (20)

[0098] There are now only six unique inertial parameters. If the angular velocity and angular acceleration are given by

[0099]

[0100] as well as

[0101] Multiplying by Equation 18 gives us the pure matrix form.

[0102]

[0103] where θ is the parameter vector of the inertial term, defined as:

[0104]

[0105] as well as,

[0106]

[0107] And the subscript i refers to the sample at time i.

[0108] Equation 23 can be solved for θ in the least squares sense, as

[0109]

[0110] in,

[0111]

[0112] as well as

[0113]

[0114] Each sample in Equations 27 and 28 is generated from a complex trajectory of the robot, which involves simultaneous rotation of the tool mass in all three rotational axes. Each moment generates a data point that can be used for least squares estimation.

[0115] From an accuracy perspective, angular motions that produce a good range of excitation torque ratings for the F / T sensor will provide a better estimate of the inertia tensor; however, angular motion of the tool may produce stresses within the tool that were not anticipated when the tool was designed. For this reason, parameter estimation data collection motions are preferably no more than the speed of the specific application.

[0116] Inertia tensor I, tool weight These items, CG, and tool CG, may not be static. For example, these quantities may vary as the mass of tool 20 changes. In one embodiment, multiple values ​​of I, CG, and tool CG may be input or derived through the above-described parameter identification process. and CG. These values ​​can be stored in memory and recalled as needed for use in equation (6)

[0117] Double mass distribution

[0118] In one embodiment, the sprung mass of the F / T sensor 12 and the mass of the tool are considered separately. This facilitates gravity and inertia compensation for the various tools to which the F / T sensor 12 may be coupled. Figure 2 The F / T sensor 12 is depicted attached to a robotic arm (not shown) and a tool 20 . Figure 2 The center of gravity (CG) of the F / T sensor 12, its body coordinate system (CF), and the tool 20 center of gravity (CG) are depicted.

[0119] In this embodiment, Equation 3 is rewritten as:

[0120]

[0121] The subscripts identify the gravitational and inertial forces and torques, respectively, derived from each of the tool 20 and the F / T sensor 12. These are calculated, for example, using the techniques described herein.

[0122] Orientation, speed, and acceleration sensing

[0123] Gravity compensation relies on sensing the instantaneous orientation of the tool 20, and inertia compensation relies on sensing the instantaneous linear and angular velocities of the F / T sensor body CF. These values ​​can be sensed directly by an integrated IMU or obtained from the forward kinematics of the robot 10.

[0124] Inertial Measurement Unit

[0125] Figure 3 The F / T sensor 12 is depicted with an IMU integrated, for example, on a circuit board. The IMU may comprise a 9-axis sensor unit integrating an accelerometer, a gyroscope, a magnetic sensor, and a microcontroller. The IMU is located at the origin of the F / T sensor body CF and is calibrated so that its axes are aligned with the F / T sensor body CF (for clarity, Figure 3 is an exploded parts diagram depicting the IMU as "hovering" above the CF; in the assembled F / T sensor 12, the IMU is mounted to the MAP 14 at the origin of the CF. The IMU then provides absolute orientation and angular rate data at a high frequency (e.g., hundreds of hertz). A suitable IMU is the BN0080 / 85 9-axis SIP IMU, available from CEVA Technologies, Inc. of Rockville, Maryland. https: / / www.ceva-dsp.com / .

[0126] The F / T sensor 12 includes measurement circuitry configured to resolve force and torque measurements from the transducer signals, as is well known in the field of robotics. For example, the measurement circuitry may include processing circuitry configured by appropriate software to calculate force and torque measurements from the transducer signals according to known methods, such as those described in U.S. Patent No. 10,067,019, which is assigned to the assignee of the present disclosure and is incorporated herein by reference in its entirety, or U.S. Patent No. 10,422,707, incorporated above. As described in International Patent Publication WO 2018 / 200668, incorporated above, the measurement circuitry may additionally perform thermal compensation of the force and torque measurements. The processing circuitry may be integrated into the body of the sensor 12, or may be external—e.g., connected via one or more wired or wireless communication links.

[0127] According to an embodiment of the present invention, the F / T sensor 12 further includes compensation circuitry configured to perform gravity and / or inertia compensation of force and torque measurements. The compensation circuitry includes the aforementioned IMU and processing circuitry integrated with or connected to the IMU in a data transmission relationship. The processing circuitry is configured, for example by providing appropriate software, to receive or retrieve data from the IMU and other sources and perform the calculations described herein to achieve gravity and / or inertia compensation of the force and torque measurements.

[0128] In some embodiments, the measurement circuitry and the compensation circuitry may include the same processing circuitry, configured to resolve force and torque measurements from transducer signals by providing appropriate software, or to perform gravity and / or inertia compensation of force and torque measurements. The processing circuitry may include any computing hardware known in the art, such as a dedicated state machine implemented in hardware; programmable logic and appropriate firmware; one or more stored-program processors or digital signal processors (DSPs) and appropriate software; or any combination thereof. The processing circuitry may include peripheral circuitry known in the art, such as memory, coprocessors, data and / or communication interfaces, human / machine interfaces, and the like.

[0129] Forward kinematics data processing

[0130] In some embodiments, due to size and / or cost constraints, it is impractical to integrate an IMU into each F / T sensor 12. In these embodiments, the orientation and velocity data are obtained from the robot's forward kinematics. Forward kinematics refers to the use of the robot's kinematic equations to calculate the position, velocity, or other dynamic characteristics of the tool 20 from the instantaneous values ​​of the robot's parameters (e.g., various joint positions). Figure 4 A block diagram of a system including a robot 10 and an F / T sensor 12 and an external electronics module 22 is depicted. The electronics module 22 receives forward kinematics data from the robot 10 (i.e., the robot control system) and transducer signals from the F / T sensor 12. One or more processors and appropriate software within the electronics module 22 interpret the transducer signals into F / T measurements and calculate and apply gravity and optional inertia compensation terms. The compensated F / T data is fed back to the robot 10 for force control operation. Suitable external electronic modules 22 are available from ATI Industrial Automation, Inc. of Apex, North Carolina. https: / / www.ati-ia.com / .

[0131] method

[0132] Figure 5 The steps of method 100 are depicted for compensating force and torque measurements of a force / torque (F / T) sensor 12 attached to a robot 10 for the effects of gravity due to the weight of an attached tool 20. Force and torque measurements are resolved from transducer output signals by measurement circuitry of the sensor 12. The force and torque measurements are referenced to a body coordinate system (CF) of the F / T sensor 12. The F / T sensor 12 also includes compensation circuitry for executing method 100. The compensation circuitry can be internal to the F / T sensor 12, such as an inertial measurement unit (IMU), or it can be external electronics that receive forward kinematic data from the robot 10.

[0133] As indicated by the dashed line, the method 100 can be considered to include two separate phases or modes. In the first phase, parameter identification, the weight W of the tool attached to the F / T sensor 12 is obtained. tool (Block 102). Also in the first stage, the vector from the origin of the F / T sensor body CF to the center of gravity of the tool 20 is obtained. (Block 104). Data W tool and It can be input by the user or obtained during the parametric identification process. In the latter case, the robot 10 positions the tool 20 in one or more different orientations. At each orientation, the force and torque resolved from the transducer and the unit gravity vector are obtained. For example, by formulating and solving a least squares regression model, these data measured from one or more orientations are fitted to obtain the data W. tool and Can be calculated and stored for W tool and to account for changes in the weight or configuration of the tool 20 during robot operation.

[0134] In the second phase of the method 100, the robotic operation (which may include force control operations) obtains the rotation matrix from the F / T sensor body CF to the inertial reference frame (Block 106), and based on W tool and The force and torque measurements are compensated for the effects of gravity on the attached tool (block 108 ). It can be obtained from the internal IMU or external electronics. In the second stage, the An updated gravity compensation term is calculated and applied to the force and torque measurements of sensor 12 based on the updated value of .

[0135] The F / T sensor 12 transducer is an analog device that continuously outputs signals in response to mechanical stress. The rate at which these signals are decomposed into forces and torques is set by the user and can be varied as needed for a particular robotic operation. In one embodiment, the force and torque measurements have a maximum update rate of approximately 8kHz. The frequency of the positioning data depends on the IMU or forward kinematics processing. Although some IMU devices may output data at frequencies up to 1kHz, very high frequency outputs contain a lot of noise. In one embodiment, positioning data is obtained at approximately 400kHz, and gravity and / or inertia compensation is performed using the latest positioning data as each force measurement is made. Therefore, compensation of the measured force and torque terms for gravity and inertial effects occurs at a high frequency relative to changes in the orientation of the robotic tool 20. W is selected from a plurality of stored values ​​based on the tool selected, its estimated mass loss rate, whether it carries known additional mass, etc. tool and The value of .

[0136] In a further refinement of the method 100 (not shown), the force and torque measurements are also compensated for the inertial effects caused by the movement of the robot 10. In the first stage of the method 100, the mass m of the tool is obtained. In the second stage, during the operation of the robot, the following properties of the F / T sensor body CF are also obtained: its angular velocity m angular acceleration Its linear acceleration The inertia tensor I defined in the F / T sensor body CF is also updated. The inertia tensor I includes a matrix containing all moments of inertia and products of inertia. In the second phase, the inertia compensation term is calculated at the same frequency as the gravity compensation term and is also applied to the sensor 12 force and torque measurements. In robotic operations where the effective mass or size of the tool may change, appropriate values ​​for the relevant parameters, such as I, can be selected from multiple stored values. CG etc.

[0137] result

[0138] Figure 6 The significant improvement in force measurement that can be achieved by gravity compensation is depicted. The robot operation depicted is grinding the outer surface of a pipe, and the required contact force F contact is 5 lbs. Both the raw and gravity-compensated force components Fx and Fy are plotted (both the raw and compensated Fz curves are close to zero; these have been omitted from the figures for clarity). In both cases, the gravity-compensated data is swamped by the amplitude of the fluctuations in the raw data—in the case of Fx, by a factor of almost 4! This is an example of why force-controlled robotic operation may not even be possible using uncompensated force measurements. By compensating the Fx and Fy measurements resolved from the transducer output signals to account for the effects due to the tool weight, useful contact force information can be extracted from the data. Note that Figure 6 The compensated data in includes only gravity compensation; applying inertia compensation will additionally remove the effects of robot motion from the measurement data.

[0139] Embodiments of the present invention present numerous advantages over the prior art. By obtaining orientation data from an IMU or forward kinematics, embodiments of the present invention provide continuous, high-speed compensation of measured forces and torques for gravity and inertial effects. By using parameter identification methods, quantities such as tool weight and the vector from the F / T sensor CF origin to the tool's center of gravity can be derived. Figure 6 As shown, the apparatus and methods described and claimed herein significantly improve upon raw force and torque measurements from robotic F / T sensors.

[0140] Generally, all terms used in this article should be interpreted according to their ordinary meaning in the relevant technical field, unless clearly given a different meaning and / or implied from the context in which it is used. Unless clearly stated, all references to elements, devices, assemblies, devices, steps, etc. should be openly interpreted as referring to at least one instance of the elements, devices, assemblies, devices, steps, etc. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless a step is clearly described as being after or before another step and / or it is implied that a step must be after or before another step. As long as it is appropriate, any feature of any embodiment disclosed herein can be applied to any other embodiment. Similarly, any advantage of any embodiment can be applied to any other embodiment, and vice versa. It will be apparent from the description that other purposes, features and advantages of the attached embodiments are provided.

[0141] The term unit may have its conventional meaning in the field of electronics, electrical devices and / or electronic equipment, and may include, for example, electrical and / or electronic circuits, devices, modules, processors, memories, logic solid-state and / or discrete devices for performing corresponding tasks, processes, calculations, output and / or display functions, etc., computer programs or instructions, etc., such as those described herein. As used herein, the term "configured to" means set, organized, adjusted or arranged to operate in a specific manner; this term is synonymous with "designed to".

[0142] Of course, without departing from the essential characteristics of the present invention, the present invention may be implemented in a manner different from that specifically set forth herein. The present embodiment is considered in all respects to be illustrative and not restrictive, and all changes within the meaning and equivalent scope of the appended claims are intended to be included therein.

Claims

1. A robotic force / torque (F / T) sensor, comprising a transducer configured to generate a signal in response to a force or torque applied to the F / T sensor, the F / T sensor comprising: measurement circuitry configured to resolve force and torque measurements from the transducer signals, the force and torque measurements being referenced to a body coordinate system CF of the F / T sensor; as well as A compensation circuit configured to obtain the weight W of the attached tool tool ; Get the vector from the origin of the F / T sensor body CF to the center of gravity of the tool Get the rotation matrix from the F / T sensor body CF to the inertial reference frame based on W tool and compensating the force and torque measurements for the effects of gravity on the attached tool; as well as The output is a gravity-compensated force and torque measurement referenced to the body coordinate system CF of the F / T sensor.

2. The F / T sensor according to claim 1, wherein the compensation circuit comprises: an inertial measurement unit integrated with the F / T sensor and calibrated into the F / T sensor body CF; as well as Processing circuitry is coupled in data transmitting relationship to the inertial measurement unit and is configured to perform compensation calculations.

3. The F / T sensor according to claim 1 , wherein the compensation circuit is further configured to: Obtaining the mass m of the attachment tool; Obtain the angular velocity of the F / T sensor body CF Obtain the angular acceleration of the F / T sensor body CF Obtain the linear acceleration of the F / T sensor body CF Obtaining the inertia tensor I defined in the F / T sensor body CF, which includes all moments of inertia and products of inertia; and Also based on m, and I compensate the force and torque measurements for the inertial effects of the robot's motion.

4. A method for compensating force and torque measurements, wherein force and torque measurements resolved from transducer output signals of a force / torque (F / T) sensor by measurement circuitry in a robotic force / torque (F / T) sensor are compensated for gravitational effects caused by the weight of an attached tool, the force and torque measurements being referenced to a body coordinate system (CF) of the F / T sensor, wherein the F / T sensor includes compensation circuitry, the method comprising: Get the weight W of the tool attached to the F / T sensor tool ; Get the vector from the origin of the F / T sensor body CF to the center of gravity of the tool Get the rotation matrix from the F / T sensor body CF to the inertial reference frame based on W tool and compensating the force and torque measurements for the effects of gravity on the attachment tool; as well as The output is a gravity-compensated force and torque measurement referenced to the body coordinate system CF of the F / T sensor.

5. The method according to claim 4, wherein W is obtained in the parameter identification process tool and and obtained during robot operation The force and torque measurements are compensated at high frequency relative to changes in orientation and motion of the robotic tool.

6. The method of claim 4, wherein the compensation circuit comprises an inertial measurement unit (IMU) in the F / T sensor, the IMU being aligned with the F / T sensor body (CF), and wherein W is obtained. tool and including determining W based on data from the IMU tool and 7. The method of claim 4, further comprising compensating the force and torque measurements resolved from transducer output signals for inertial effects caused by the motion of the robot, the method further comprising: Obtaining the mass m of the tool; Obtain the angular velocity of the F / T sensor body CF Obtain the angular acceleration of the F / T sensor body CF Obtain the linear acceleration of the F / T sensor body CF Obtaining an inertia tensor I defined in the F / T sensor body CF, the inertia tensor I including a matrix containing all moments of inertia and products of inertia; as well as Based on m, and I compensate the force and torque measurements for inertial effects caused by the robot's motion.

8. A robotic force / torque (F / T) sensor comprising a transducer configured to generate a signal in response to a force or torque applied to the F / T sensor, the F / T sensor comprising: measurement circuitry configured to resolve force and torque measurements from the transducer signals, the force and torque measurements being referenced to a body coordinate system CF of the F / T sensor; as well as A compensation circuit is configured to: Get the mass m of the attachment tool; Get the angular velocity of the F / T sensor body CF Obtain the angular acceleration of the F / T sensor body CF Get the vector from the origin of the F / T sensor body CF to the center of gravity of the tool Obtain the linear acceleration of the F / T sensor body CF Obtaining an inertia tensor I defined in the F / T sensor body CF, wherein the inertia tensor I includes all moments of inertia and products of inertia; According to m, and I compensating the force and torque measurements for inertial effects of the robot's motion; as well as Outputs inertia-compensated force and torque measurements referenced to the body coordinate system CF of the F / T sensor.

Images