Compliant Payload Rendering Using a Robot System with Coordinated Serial and Parallel Robots

Abstract

The present invention relates to compliant payload presentation using a robotic system having coordinated serial and parallel robots. A robotic system for presenting a payload within a work space includes a pair of serial robots configured to be connected to the payload; a parallel robot coupled to a distal end of one of the serial robots such that the parallel robot is disposed between the distal end and the payload; a sensor located within a kinematic chain extending between the distal end and the payload; and a robotic control system (RCS). The sensor outputs a sensor signal indicative of a measured property of the payload. The RCS includes a coordinated motion controller configured to control the serial robots and a corrective motion controller configured to control the parallel robot. The parallel robot control occurs in response to the sensor signal and concurrently with the control of the serial robots so as to thereby modify the property of the payload in real time.

Description

Technical Field

[0001] Introduction

[0002] The present disclosure relates to robotic systems for compliant presentation of a relatively rigid / non-compliant payload within a specified workspace, as well as associated control architectures and methods. Background Art

[0003] Multi-arm robotic systems are commonly used during manufacturing and assembly in many industries to manipulate heavy or otherwise cumbersome payloads. When two or more robots operate simultaneously while presenting a payload (e.g., by firmly grasping, lifting / raising, lowering, and orienting the payload within a workspace), the robots are considered to be cooperating or collaborating in the execution of a work task. Accordingly, the associated control strategies for governing the operation of the robots in such a work environment are referred to in the art as cooperative payload control. Summary of the Invention

[0004] Described herein are robotic systems and related cooperative control methods for presenting a payload within a three-dimensional workspace using multiple robot types, where the robot types include at least two serial robots and at least one parallel robot. The payloads contemplated herein may be embodied as relatively large, rigid, and bulky objects, such as a partially assembled or fully assembled vehicle chassis. The contemplated type of rigid payload is more susceptible to strain-related damage as compared to a compliant or elastic payload having a structure capable of bending, flexing, or otherwise absorbing the forces imparted during robotic payload presentation. Slight or severe position errors encountered during positioning / presentation control maneuvers, as well as during subsequent execution of work tasks on the presented payload, may sometimes result in excessive strain.

[0005] As is understood in the art, serial robots (such as six-degree-of-freedom (“6-DoF”) articulated industrial robots) use an open kinematic chain, where the six individual joints and various arm segments or links of the robot are connected in series with one another. Accordingly, the term “open chain” is typically used to refer to a particular kinematic chain where the distal link is connected to a single revolute joint. In contrast, parallel robots typically employ a closed-chain kinematic configuration, where the constituent joints and links of the parallel robot are connected in parallel. Accordingly, the distal end of a given link of a parallel robot may be connected to multiple revolute joints. Although parallel robots tend to be smaller and more responsive than serial robots, the closed-chain motion of parallel robots typically results in a reduced range of motion and increased operational stiffness as compared to an open kinematic chain.

[0006] Accordingly, the technical solutions described herein are intended to enable a rigid payload to be gently moved and accurately positioned within a workspace, i.e., in an optimally compliant manner to protect the payload. A scalable control architecture is used to achieve the desired movement, wherein the collective movement of three or more robots (i.e., at least two serial robots and at least one parallel robot as described above) is controlled by the operation of a distributed control system. As part of this strategy, the robot-specific movements and force actions are closely monitored and adjusted in real time by associated control units ("controllers") to impart gentle movement to the payload within a defined workspace.

[0007] Regarding the control system, a first electronic control unit (which, for clarity, is hereinafter referred to as the "coordinated motion controller" within the architecture of the robot control system (RCS)) coordinates the gross and fine movements of the constituent joints of the serial robots. Serial robots are relatively large and heavy devices and thus tend to have greater inertia and correspondingly slower response times compared to the (plural) parallel robots used herein. The RCS also includes a second electronic control unit (i.e., the "correction motion controller" of the RCS framework), wherein, while the coordinated motion controller is controlling the ongoing operation of the serial robots, this additional controller operates on the joints of the smaller / lower inertia parallel robots. These robot-specific controllers together ensure real-time compliant coordinated control of the different robots while protecting the structural integrity of the payload from excessive strain caused by transient or persistent position errors.

[0008] In a non-limiting exemplary configuration, the robotic system includes a pair of serial robots, a parallel robot, a force sensor, and an RCS having a constituent coordinated motion controller and correction motion controller. The serial robots are configured to engage and present the payload in cooperation within the workspace. The parallel robot is connected to the distal end of one of the serial robots (e.g., via a fixture or other suitable end effector) such that the parallel robot is disposed between the distal end and the payload. The force sensor is located within the kinematic chain extending between the distal end and the payload, and the force sensor is configured to output a force signal indicative of the strain on the payload.

[0009] In this embodiment, the coordinated motion controller is configured to control the multi-axis movement of the serial robots within the workspace. This occurs via a first set of actuator control signals. The correction motion controller is configured to respond to the force signal from the force sensor and, concurrently with the control of the multi-axis movement of the pair of serial robots, control the multi-axis movement of the parallel robot via a second set of actuator control signals to thereby reduce in real time the strain on the payload.

[0010] In a non-limiting exemplary configuration, the parallel robot may optionally be implemented as a Stewart platform. In other embodiments, a Delta robot or other suitable parallel robot mechanism may be used.

[0011] Within the scope of the present disclosure, an additional serial robot may communicate with the serial robot and the parallel robot, where the additional serial robot performs a work operation on a payload within the workspace. In the case where the payload is a vehicle chassis, for example, the additional serial robot may optionally be implemented as a welding robot operable to perform a welding operation on the vehicle chassis.

[0012] In another aspect of the present disclosure, the RCS may be configured to determine the weight of the payload based on the actual positions of the serial robot and the parallel robot, and thereafter use the derived weight within an impedance control model or framework to determine a second set of actuator control signals. This action allows for proper compensation for the height of the payload above the ground plane.

[0013] The parallel robot may optionally include two or more parallel robots, where each of the parallel robots is connected to a corresponding distal end of a respective one of the pair of serial robots.

[0014] Embodiments are disclosed herein where a corrective motion controller is configured to control the multi-axis motion of the parallel robot to a default stop position that protects the payload in response to an emergency stop signal from an emergency stop (“e-stop”) device.

[0015] In response to a control mode transition signal, the corrective motion controller may optionally be configured to transition between a position control mode and a force control mode, where in the position control mode, the parallel robot assumes a commanded position relative to the payload, and in the force control mode, the parallel robot applies a commanded force to the payload.

[0016] A robot control system is also described herein for use with a robot system having two serial robots and a parallel robot when a payload is presented within a workspace. In this control context, the parallel robot is disposed between the payload and a distal end of one of the serial robots. The system according to an exemplary embodiment includes a coordinated motion controller and a corrective motion controller. The coordinated motion controller is configured to generate a first set of actuator control signals to control the multi-axis motion of the serial robot when a payload is presented within the workspace. In contrast, the corrective motion controller communicates with the coordinated motion controller and is configured to output a second set of actuator control signals in response to a force signal indicative of strain on the payload, the second set of actuator control signals being configured to control the multi-axis motion of the parallel robot concurrently with the multi-axis motion of the serial robot.

[0017] Also disclosed herein is a related method for presenting a payload within a workspace. Representative embodiments of the method include connecting a parallel robot to a distal end of a first serial robot; and connecting the parallel robot to the payload such that the parallel robot is disposed between the distal end and the payload.

[0018] The method further includes connecting a second serial robot to the payload. Once the robots have been connected in this manner, the method includes collaboratively controlling the movement of the first serial robot, the second serial robot, and the parallel robot via a robot control system. This requires outputting a force signal indicative of an actual strain on the payload via a force sensor, wherein the force sensor is located within a kinematic chain extending between the distal end and the payload.

[0019] The method then includes using a coordinated motion controller of the robot control system to control the multi-axis movement of the first serial robot and the second serial robot via a first set of actuator control signals. Similarly, the method includes controlling the multi-axis movement of the parallel robot via a second set of actuator control signals in response to the force signal and concurrently with the control of the multi-axis movement of the first serial robot and the second serial robot, thereby reducing the strain on the payload in real time.

[0020] The present invention further includes the following technical solutions:

[0021] 1. A robot system for presenting a payload within a workspace, the robot system comprising:

[0022] A pair of serial robots configured to be connected to the payload and collaboratively present the payload within the workspace;

[0023] A parallel robot connected to a distal end of one of the pair of serial robots such that the parallel robot is disposed between the distal end and the payload;

[0024] A force sensor located within a kinematic chain extending between the distal end and the payload, the force sensor being configured to output a force signal indicative of a strain on the payload; and

[0025] A robot control system (RCS) comprising:

[0026] A coordinated motion controller in communication with the pair of serial robots and configured to output a first set of actuator control signals to the pair of serial robots, the first set of actuator control signals coordinating the multi-axis movement of the pair of serial robots when the pair of serial robots simultaneously grasp the payload and present the payload within the workspace; and

[0027] A calibration motion controller, which communicates with the parallel robot and the force sensor, and is configured to, in response to the force signal, simultaneously with the first set of actuator control signals, output a second set of actuator control signals to the parallel robot, thereby imparting a calibration motion to the payload and thereby reducing the strain thereon.

[0028] 2. The robot system according to claim 1, wherein the calibration motion controller is configured to transmit the second set of actuator control signals to one or more motorized joint actuators of the parallel robot.

[0029] 3. The robot system according to claim 1, wherein the force sensor is integrated with the parallel robot.

[0030] 4. The robot system according to claim 1, wherein the parallel robot includes a Stewart platform.

[0031] 5. The robot system according to claim 1, further comprising:

[0032] An additional serial robot that communicates with the pair of serial robots and the parallel robot, wherein the additional serial robot is configured to perform a work operation on the payload when the payload is cooperatively presented in the workspace.

[0033] 6. The robot system according to claim 5, wherein the payload is a vehicle chassis, and wherein the work operation is a welding operation performed by the additional serial robot on the vehicle chassis.

[0034] 7. The robot system according to claim 1, wherein the robot control system is configured to determine the weight of the payload based on the actual positions of the pair of serial robots and the parallel robot; and thereafter use the weight within an impedance control model to determine the second set of actuator control signals.

[0035] 8. The robot system according to claim 1, wherein the parallel robot includes two parallel robots, each of which is respectively connected to a corresponding distal end of a corresponding one of the pair of serial robots.

[0036] 9. The robot system according to claim 1, wherein the calibration motion controller is configured to control the parallel robot to a default stop position to protect the payload in response to an emergency stop ("e-stop") signal from an emergency stop device.

[0037] 10. The robot system according to Scheme 1, wherein, in response to a control mode change signal, the calibration motion controller is configured to switch between a position control mode and a force control mode. In the position control mode, the parallel robot assumes a commanded position relative to the payload, and in the force control mode, the parallel robot applies a commanded force to the payload.

[0038] 11. A robot control system for use with a robot system having two serial robots and one parallel robot when a payload is presented within a workspace, the parallel robot being disposed between the payload and the distal end of one of the serial robots, the robot control system comprising:

[0039] A coordinated motion controller configured to generate a first set of actuator control signals to control the multi-axis motion of the serial robots when the payload is presented collaboratively within the workspace; and

[0040] A calibration motion controller in communication with the coordinated motion controller and configured to output a second set of actuator control signals in response to a force signal indicating strain on the payload, the second set of actuator control signals being configured to control the multi-axis motion of the parallel robot simultaneous with the multi-axis motion of the serial robots, including imparting a calibration motion to the payload.

[0041] 12. The control system according to Scheme 11, further comprising a force sensor configured to output the force signal and connected to or integrated with the parallel robot.

[0042] 13. The control system according to Scheme 11, wherein the calibration motion controller is configured to determine the weight of the payload based on the actual positions of the two serial robots and the parallel robot, and thereafter use the weight to determine the second set of actuator control signals.

[0043] 14. The control system according to Scheme 11, wherein the calibration motion controller is configured to control the multi-axis motion of the parallel robot to a default stop position to protect the payload in response to an emergency stop ("e-stop") signal from an emergency stop switch.

[0044] 15. The control system according to Scheme 11, wherein, in response to a control mode change signal, the calibration motion controller is configured to switch between a position control mode and a force control mode. In the position control mode, the parallel robot assumes a commanded position relative to the payload, and in the force control mode, the parallel robot applies a commanded force to the payload.

[0045] 16. A method for collaboratively presenting a payload within a workspace, the method comprising:

[0046] Connecting a parallel robot to a distal end of a first serial robot;

[0047] Connecting the parallel robot to the payload such that the parallel robot is disposed between the distal end and the payload;

[0048] Connecting a second serial robot to the payload; and

[0049] Controlling the movement of the first serial robot, the second serial robot, and the parallel robot via a robot control system, comprising:

[0050] Measuring, via a force sensor, a force signal indicative of strain on the payload, the force sensor being located within a kinematic chain extending between the distal end and the payload;

[0051] Controlling the multi-axis movement of the first serial robot and the second serial robot via a coordinated motion controller of the robot control system, comprising transmitting a first set of actuator control signals to the first serial robot and the second serial robot; and

[0052] In response to the force signal, simultaneously with the first set of actuator control signals, controlling the multi-axis movement of the parallel robot via a corrective motion controller of the robot control system to thereby reduce the strain on the payload, comprising transmitting a second set of actuator control signals to the second serial robot.

[0053] 17. The method according to claim 16, further comprising:

[0054] When performing a work operation on the payload, controlling the movement of a third serial robot via the coordinated motion controller.

[0055] 18. The method according to claim 17, wherein controlling the movement of the third serial robot comprises performing a welding operation on a vehicle chassis using the third serial robot.

[0056] 19. The method according to claim 16, further comprising:

[0057] Receiving an emergency stop ("e-stop") signal via the corrective motion controller; and

[0058] In response to the emergency stop signal, controlling the multi-axis movement of the parallel robot to a default stop position to protect the payload.

[0059] The method according to embodiment 16 further includes:

[0060] receiving a control mode transition signal via the calibration motion controller; and

[0061] in response to the control mode transition signal, transitioning between a position control mode and a force control mode via the calibration motion controller, in the position control mode, the parallel robot assumes a commanded position relative to the payload, and in the force control mode, the parallel robot applies a commanded force to the payload.

[0062] The above-described features and advantages of the present disclosure, as well as other possible features and advantages, will be apparent from the following detailed description of the best mode for carrying out the present disclosure when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Figure 1 is a diagram of a robotic system and associated control system according to the present disclosure, the control system being configured to provide compliant coordinated control of a rigid payload using serial and parallel robots.

[0064] Figure 2 is a schematic control diagram that depicts the coordinated control of a coordinated motion controller and a calibration motion controller of the robotic control system shown in Figure 1 the coordinated control of the components of the robotic control system shown therein.

[0065] Figure 3 is a flowchart depicting an exemplary method for controlling the robotic system shown in Figure 1 therein. DETAILED DESCRIPTION

[0066] Embodiments of the present disclosure are described herein. However, it will be understood that the disclosed embodiments are merely examples, and other embodiments can take various and alternative forms. The drawings are not necessarily to scale. Some features may be exaggerated or minimized to show details of particular components. Accordingly, the specific structural and functional details set forth herein are not to be construed as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present disclosure.

[0067] Certain terms may be used in the following description for reference purposes only and are not, therefore, intended to be restrictive. For example, terms such as "above" and "below" refer to the directions referenced in the figures. Terms such as "front", "rear", "head", "tail", "left", "right", "back", and "side" describe the orientation and / or position of parts of a component or element within a consistent but arbitrary frame of reference, which becomes clear by reference to the text describing the component or element in question and the associated figures. In addition, terms such as "first", "second", "third", etc. may be used to describe individual components. Such terms may include the words specifically mentioned above, their derivatives, and words with similar meanings.

[0068] Reference Figure 1 , the robotic system 10 is configured to present a payload 12 within a workspace 13 with the assistance of a robotic control system (RCS) 50. In Figure 1 a representative embodiment, the payload 12 is a vehicle chassis 120 having longitudinal rails 11, where the longitudinal rails 11 are laterally supported by crossbeams 16, and where the rails 11 and crossbeams 16 are constructed of aluminum, steel, or other rigid / non-compliant materials. Depending on the stage of manufacture, wheels 14 may be attached to the vehicle chassis 120. While the vehicle chassis 120 represents the type of rigid / non-compliant payload structure envisioned herein, other payloads 12 may be presented in other applications of this teaching, and thus the vehicle chassis 120 is merely exemplary and non-limiting.

[0069] The robotic system 10 includes a pair of serial robots 20A and 20B, also labeled R1 and R2 for clarity, which are in turn configured to be connected to the payload 12 either directly or indirectly. As understood in the art, such connection may be achieved via a fixture 15, such as a multi-fingered gripper, a clamp, or other suitable end effector. In addition, a parallel robot 30 (R3) is connected to the distal end of one of the serial robots 20A or 20B, in this case to the distal end E1 of the serial robot 20A, using a similar fixture 15 or other suitable end effector. Other embodiments may be implemented where the parallel robot 30 is connected to the distal end E2 of the serial robot 20B, as well as embodiments where both of the serial robots 20A and 20B are connected to corresponding parallel robots 30, where such options are illustrated as serial robot 300 in Figure 1 . When the parallel robot 30 is connected to the distal end E1 in this manner, the parallel robot 30 is disposed between the distal end E1 and the payload 12, as shown.

[0070] Figure 1The robotic system 10 further includes a force sensor 21 (such as a pressure transducer or a piezoelectric sensor) that is located within a kinematic chain extending between the distal end E1 and the payload 12, e.g., attached to or integral with the linkage of the parallel robot 30. The force sensor 21 is configured to output a force signal (arrow F) indicative of the measured strain on the payload 12. The force signal (arrow F) is ultimately communicated to the RCS 50 via suitable transmission conductors (not shown), wirelessly, or in both ways.

[0071] As depicted, the RCS 50 includes a coordinated motion controller (C1) 50-1 and a corrective motion controller (C2) 50-2. The coordinated motion controller 50-1, which communicates with the pair of serial robots 20A and 20B, controls the multi-axis motion of the pair of serial robots within the workspace 13 via a first set of actuator control signals (arrow CC 20A ). In contrast, the corrective motion controller 50-2 communicates with the parallel robot 30 and the force sensor 21 and is configured to control the multi-axis motion of the parallel robot 30 via a second set of actuator control signals (arrow CC 20B ). This action occurs in response to the force signal (arrow F) and simultaneously with the control of the multi-axis motion of the serial robots 20A and 20B, and it has the effect of reducing excessive strain on the payload 12 in real time.

[0072] Figure 1 The RCS 50 can be implemented as one or more digital computers, where each digital computer has a processor (P) and a memory (M). The memory (M) includes a sufficient amount of tangible, non-transitory memory such as read-only memory, flash memory, optical and / or magnetic memory, electrically programmable read-only memory, etc., i.e., computer-readable media. The memory (M) also includes sufficient transient memory such as random access memory, electronic buffers. The hardware of the robotic control system 50 can include a high-speed clock, analog-to-digital and digital-to-analog circuits, and input / output circuits and devices, as well as appropriate signal conditioning and buffering circuits.

[0073] The memory (M) can be programmed with computer-readable instructions for implementing Method 100, where the execution of the instructions ultimately enables the RCS 50 to control various joints, brakes, and locking mechanisms of the robotic system 10 as needed to execute available control modes and / or switch between available control modes. This can occur in response to the measured or derived weight (arrow W) of the payload 12 (e.g., from a weight observer (w-OBS) 49) as explained below and in response to a possible control mode transition signal (arrow CM). The control modes can include a position control mode and a force control mode. In the position control mode, the parallel robot 30 assumes a commanded position relative to the payload 12, and in the force control mode, the parallel robot 30 applies a commanded force to the payload 12. For this purpose, a human-machine interface (HMI) device 55 (e.g., a touchscreen device or a suitable interface executed in the logic of the RCS 50) can be used to facilitate the determination of the control mode. For example, in some methods, the operator can manually select one of the control modes, or the RCS 50 can autonomously determine and select the optimal control mode in real time.

[0074] Still referring to Figure 1 , a primary objective of the present disclosure is to gently move and accurately position the payload 12 within the workspace 13. This is affected by the cooperative operation of the serial robots 20A and 20B, where each serial robot can in turn optionally be implemented as a 6-axis industrial robot that provides translation in three axes (e.g., the xyz Cartesian coordinate system) and rotation, pitch, and yaw for pose control to achieve a total of six degrees of freedom (“6-DoF”). Thus, the serial robots 20A and 20B can be implemented as 6-DoF robots of the type understood in the art.

[0075] In Figure 1 a simplified depiction, for example, the serial robots 20A and 20B can each include a base 22 and a plurality of serially connected arm segments 24, as well as a three-axis wrist assembly 25 positioned at the distal ends E1 and E2. The base 22 and the arm segments 24 are interconnected via rotational joints 28 that include motorized joint actuators, and as part of Control Method 100, the corresponding angular positions of these rotational joints are individually measured by joint position sensors 40 and reported to the RCS 50 as corresponding robot-specific joint positions (θ R1 、θ R2 、θ R3 ).

[0076] During the motion control of serial robots 20A and 20B, even the slightest positional error and mechanical misalignment will tend to impose strain on payload 12. The impact of such strain on payload 12 depends to a large extent on the construction of payload 12, as will be understood by those skilled in the art. However, it is desirable to minimize the magnitude of the strain during the presentation of payload 12 in order to protect payload 12 from damage, whether such damage is caused by the motion of payload 12 during its presentation or when operations are performed on the presented payload 12.

[0077] In a non-limiting illustrative embodiment, for example, an additional serial robot 20C (R4) can be configured to perform a work operation on payload 12. For example, as shown, when the additional serial robot 20C is configured as a welding robot, a welding operation is performed on payload 12. Forces can be imparted to payload 12 due to other events, such as an emergency stop (e-stop) event, where the motion of serial robots 20A and 20B is suddenly stopped by the actuation of emergency stop device 52. Figure 1 Two emergency stop devices 52 are shown to illustrate multiple possible positions within the workspace 13. As part of method 100, a corresponding emergency stop signal (arrow 152) can be communicated by emergency stop device 52 to RCS 50.

[0078] Similarly, one of the serial robots 20A or 20B can experience an automatically generated emergency stop event at any time due to an internal fault. Therefore, RCS 50 is configured to perform a controlled stop, where serial robots 20A and 20B and parallel robot 30 are commanded to stop, stopping with a calibrated maximum allowable deceleration when on path (e.g., the maximum deceleration that protects the structural integrity of robots 20A, 20B, and 30 and payload 12). Although the motion is controlled, such a fault-based automatic emergency stop can occur at any time during the execution of a given motion sequence. Therefore, the compensation of transient errors by parallel robot 30 can be used to help address transient forces during an exemplary controlled stop scenario.

[0079] Also as part of method 100, RCS 50 can process force signals (arrow F) from force sensor 21 to sense or estimate the strain on payload 12. Relative to serial robots 20A and 20B, parallel robot 30 is capable of responding with a far higher bandwidth to mitigate the strain. That is, the smaller and lower inertia parallel robot 30 will tend to have higher dynamic performance and faster control loops than the larger, bulkier serial robots 20A and 20B. Therefore, Figure 1Multiple robots R1, R2, and R3 work together to gently grasp, position, and ultimately present payload 12 within workspace 13 while maintaining rigid body constraints in the resulting motion.

[0080] To ensure precise control of the corrective actions of parallel robot 30 within the intended scope of method 100, parallel robot 30 may optionally be implemented as a Stewart platform as shown or another hexapod robot, or as a Delta robot or other parallel mechanism suitable for the application. As understood in the art, a hexapod robot (such as the illustrated Stewart platform embodiment (see also Figure 2 )) is supported by six telescoping legs or struts 36 (also referred to as prismatic actuators) that are individually mounted to end plates 32 and 34. The independently controllable lengths of struts 36 provide 6-DoF of motion control with an accuracy level of micrometers or micro-radians.

[0081] Although omitted for simplicity from Figure 1 control of struts 36 can be accomplished using servo systems, proportional valves, encoders, etc., as is understood in the art. Relative to serial robots 20A and 20B, parallel robot 30 enjoys higher stiffness and load-bearing capacity due to the arrangement of the many struts 36. The parallel arrangement of struts 36 also causes only the topmost end plate 34 to move, and thus parallel robot 30 has lower inertia compared to the inertia of serial robots 20A and 20B. This enables the use of parallel robot 30 to dynamically control payload 12, as detailed herein. Optional observers can be used to assist or optimize control, where the observers include a gravity observer ("g-OBS") 51 and / or an inertia observer ("i-OBS") 53, both of which are described in further detail below.

[0082] Now referring to Figure 2 , control architecture 60 schematically depicts the operation of coordinated motion controller 50-1 (C1) and corrective motion controller 50-2 (C2) for multi-axis control of serial robots 20A (R1) and 20B (R2) as described above with reference to Figure 1 . Serial robots 20A and 20B are rigidly mounted to the ground either directly or via intervening vertical or horizontal beams, bases, overhead gantries, or other support structures. In contrast, parallel robot 30 is positioned in series with serial robot 20B (see Figure 1 ), and / or in other embodiments in series with serial robot 20A. Force sensor 21 is positioned in series with serial robot 20B (R2) and parallel robot 30 (R3) and is configured to output a force signal (arrow F) indicative of the strain measured on payload 12 as described above.

[0083] When Figure 1 the payload 12 is firmly grasped and supported by the serial robots 20A and 20B with the assistance of at least one parallel robot 30, the coordinated motion controller 50-1 receives the desired position (POS Des ), as an electronic input signal. In a possible embodiment, the desired position (POS Des ) may be generated by the control logic of the RCS 50 of Figure 1 according to a programmed or operator-selected presentation sequence.

[0084] For example, when presenting the payload 12 in the representative form of a vehicle chassis 120 of Figure 1 , such a sequence may require the serial robots 20A and 20B to use fixtures or other suitable end effectors to grasp opposite ends of the chassis 120, lift the chassis 120 to a predetermined height above the ground surface, and rotate the chassis 120 about its longitudinal axis to expose a particular working surface.

[0085] When the serial robots 20A and 20B stably hold the chassis 120 and support its weight, Figure 1 an additional serial robot 20C (e.g., a welding robot) of

[0086] is then able to operate on the chassis 120. As part of this exemplary sequence, the serial robots 20A and 20B work simultaneously about the tool center point (TCP) on the vehicle chassis 120, doing so according to rigid body constraints.

[0086] As part of this process, the coordinated motion controller 50-1 receives the actual position signals (POS Act ) of the serial robots 20A and 20B (as measured by the joint position sensors 40 as shown in Figure 1 ), as well as a feedback signal (FB Act ) in the form of a measured or calculated position error (i.e., POS Des - POS COORD ). The coordinated motion controller 50-1 also outputs a first set of actuator control signals CC 25A in real time to each of the serial robots 20A and 20B to command the various joint actuators of the serial robots 20A and 20B to move to specific angular positions and / or to maintain a specific posture as needed based on the desired position (POS Des ).

[0087] Simultaneously with the operation of the coordinated motion controller 50-1, the corrective motion controller 50-2 uses the measured force (F) from the force sensor 21 and the feedback signal (FB CORR) to provide slight correction movements or positioning of the parallel robot 30. Other inputs into the coordinated motion controller 50-1 include the actual position (POS Act ). Then, the correction motion controller 50-2 outputs a second set of actuator control signals (CC 25B ) to the parallel robot 30 in real time to command the various joint actuators of the parallel robot 30 to move to specific angular positions and / or to maintain a specific posture as needed to minimize the strain on the presented payload 12.

[0088] Regarding the ongoing operation of the correction motion controller 50-2, its force control logic can be based on an impedance model as described herein, or in another embodiment based on an admittance model. As understood in the art, the general difference between these two control models or modes is that impedance control is used to control the applied force after first detecting a deviation from a calibration setpoint, while admittance control is often used to control motion in response to a force measurement. The RCS 50 can use either model or its implementation logic to compensate for the weight of the payload 12.

[0089] For example, throughout the above exemplary presentation sequence, Figure 2 the correction motion controller 50-2 of can receive information about the specific orientation / motion vector and position of the payload 12 from the coordinated motion controller 50-1. Using this information, the correction motion controller 50-2 can derive the weight ( Figure 1 arrow W of ) and compensate for the weight as part of the method 100.

[0090] The weight-based compensation performed in this way can be enhanced by Figure 1 the gravity observer 51 of, which can continuously evaluate the position of the model of the payload 12 (e.g., one recorded in the memory (M)), and provide a series of weight vectors (arrow 51V), which are subsequently used to subtract the payload weight component of the force (arrow F) measured by the force sensor 21. Similarly, the correction motion controller 50-2 can act on an emergency stop signal ( Figure 1 arrow 152 of ) to maintain a coordinated stop position on the parallel robot 30 during a sudden stop caused by the activation of the emergency stop device 52 to protect the payload 12.

[0091] For a continuous motion control scenario (during which Figure 1 the payload 12 of is repositioned), an optional inertia observer 53 can estimate the dynamic forces (arrow F est), where motion and pose are used to present the payload 12 to a given manufacturing process. Some processes can involve the continuous motion of the serial robots 20A and 20B relative to a process robot (e.g., Figure 1 serial robot 20C), such as when performing arc welding and dispensing (where the payload 12 is continuously repositioned to assist and achieve favorable process conditions), e.g., where "downhand welding" is performed using a robot-mounted arc welding nozzle and torch.

[0092] In an illustrative embodiment, the inertial observer 53 will work with a weight or gravity observer to provide a complete dynamic and static force estimate, which can then be subtracted from the force (arrow F) observed by the force sensor 21. In this way, unexpected forces can be mitigated, compensated for, or canceled out by the parallel robot 30 with a higher motion bandwidth. The resulting control of the "net pose" of the payload 12 will thus result in a far more precise positioning of the payload 12, where the stress induced in the payload 12 is significantly reduced due to the compensation for the dynamic incoordination of the load-carrying serial robots 20A and 20B.

[0093] Referring to Figure 3 , an embodiment of the method 100 for presenting the payload 12 within the Figure 1 workspace 13 is performed using the serial robots 20A and 20B and the parallel robot 30 described above for compliant coordinated payload control. Generally, the method 100 includes firmly connecting the parallel robot 30 to the distal end E1 of a first serial robot (e.g., Figure 1 serial robot 20A). Thereafter, the method 100 includes connecting the parallel robot 30 to the payload 12 such that the parallel robot 30 is disposed between the distal end E2 and the payload 12. The method 100 also includes connecting a second serial robot (e.g., serial robot 20B) to the same payload 12. In an exemplary embodiment of the vehicle chassis 120, for example, the serial robots 20A and 20B can grip opposite ends of the vehicle chassis 120, as shown in Figure 1 .

[0094] After the serial robots 20A and 20B have been connected to the payload 12 in this way, the method 100 collaboratively controls the motion of the first serial robot 20A, the second serial robot 20B, and the parallel robot 30 via the RCS 50. This requires outputting a force signal ( Figure 1 and Figure 2 arrow F) indicating the actual or estimated strain on the payload 12 via the force sensor 21, where the force sensor 21 is located within the kinematic chain extending between the distal end E1 and the payload 12. The method 100 also includes controlling the parallel robot 30 via a first set of actuator control signals (arrow CC25A ), a coordinated motion controller 50-1 is used to control the multi-axis motion of the corresponding first serial robot 20A and second serial robot 20B. At the same time, in response to the force signal (arrow F) and simultaneously with the control of the multi-axis motion of the corresponding first serial robot 20A and second serial robot 20B, a second set of actuator control signals (arrow CC 25B ) are used to control the multi-axis motion of the parallel robot 30. The desired end effect is to reduce or eliminate excessive strain on the payload 12 in real time.

[0095] As Figure 3 shown, an exemplary embodiment of the method 100 begins at block B102, where a new position command ("POS Cmd ") is generated and / or received. As described above, as part of a larger control sequence, such a command may originate from other logic in the RCS 50. Then, the method 100 proceeds to block B104.

[0096] Block B104 requires receiving the actual positions of the various robots (in this case, the serial robots 20A and 20B and the parallel robot 30) participating in the collaborative work task. As determined by Figure 1 the joint sensors 40 of, the actual positions are communicated to the coordinated motion controller 50-1 and the corrective motion controller 50-2, i.e., "POS Act → C1, C2". The communication is dynamic according to the calibration loop, such that the RCE 50 continuously knows the positions of the robots R1, R2, and R3 within the workspace 13. The method 100 then proceeds to blocks B105, B106, and B107.

[0097] At block B105 (CC COORD ), the coordinated motion controller 50-1 uses the actual positions from block B104 to generate the necessary first set of actuator control signals ( Figure 1 and Figure 2 the arrow CC of 25A ) in order to present the payload 12 at the desired position in the free space with the desired orientation or pose.

[0098] At block B106 (F→C2), the force sensor 21 outputs a force signal (arrow F) to the corrective motion controller 50-2. The method 100 proceeds to block B108.

[0099] Block B107 (C2 → W) includes using the actual positions from block B104 to derive the weight of the payload 12 ( Figure 1 the arrow W of). Once the force on the payload 12 and the weight of the payload 12 have been determined, the method 100 proceeds to block B108.

[0100] Frame B108 (“ ”) includes determining, via calibration motion controller 50-2, whether the position of parallel robot 30 or the force applied by it needs to be modified. As part of frame B108, calibration motion controller 50-2 may use the measured force from frame B107 as an approximation of the position error between the actual position and the desired position of payload 12 in three-dimensional space. That is, calibration motion controller 50-2 may start from the expectation that once the force sensor 21 is properly calibrated, it should ideally measure negligible force during the control of serial robots 20A and 20B. Therefore, when the magnitude of the measured force increases, this is regarded as indicating an unacceptable increase in the position error. This error state is then communicated to calibration motion controller 50-2. Thereafter, method 100 proceeds to frame B110. Figure 1

[0101] At frame B110, calibration motion controller 50-2 immediately compensates for the position error detected at frame B108 by commanding a rapid actuation of parallel robot 30. Using the weight (W) of payload 12 determined at frame B107, for example, and the force (F) determined at frame B106, calibration motion controller 50-2 may solve a calibration motion equation, for example, . Then, via calibration motion control signal (CC 25B ), the various joints of parallel robot 30 are commanded to corresponding positions to relieve the strain on payload 12.

[0102] Thus, when a rigid part is presented in free space, Figure 1 's RCS 50 can be used to execute Figure 3 the method 100 shown in, where such a part is represented herein as payload 12. This method contemplates connecting multiple serial robots to this part, where at least one of the serial robots makes this connection via an intervening parallel robot connected in series with it. The first controller (C1) (described herein as coordination motion controller 50-1) is configured to control the multi-axis motion of the various serial robots while maintaining the integrity of this part. The second controller (C2) (described herein as calibration motion controller 50-2) is configured to control the multi-axis motion of the smaller, lower inertia, and more responsive parallel robot(s) simultaneously with the control by the first controller (C1) of the multi-axis motion of the serial robots.

[0103] ​Collectively, the serial and parallel robots operating under the coordination and correction control of the RCS 50 achieve the fluid presentation and movement of the payload, more accurate assembly, and an attendant reduction in strain associated with position error on the parts of the collaborative presentation. Given the foregoing disclosure, those skilled in the art will readily appreciate these and other attendant benefits.

[0104] The detailed description and the drawings or figures support and describe the teachings, but the scope of the teachings is defined only by the claims. While the best mode for carrying out the teachings and some of the other embodiments have been described in detail, there are various alternative designs and embodiments for practicing the teachings defined in the appended claims. In addition, the present disclosure expressly includes combinations and sub - combinations of the elements and features presented above and below.

Claims

1. A robotic system for presenting a payload within a workspace, the robotic system comprising: A pair of serial robots configured to be connected to the payload and cooperatively present the payload within the workspace; A parallel robot connected to the distal end of one of the pair of serial robots such that the parallel robot is disposed between the distal end and the payload; A force sensor located within a kinematic chain extending between the distal end and the payload, the force sensor being configured to output a force signal indicative of strain on the payload; And A robot control system (RCS) comprising: A coordinated motion controller in communication with the pair of serial robots and configured to output a first set of actuator control signals to the pair of serial robots, the first set of actuator control signals coordinating multi-axis motion of the pair of serial robots when the pair of serial robots simultaneously grasp the payload and present the payload within the workspace; And A corrective motion controller in communication with the parallel robot and the force sensor and configured to, in response to the force signal and simultaneously with the first set of actuator control signals, output a second set of actuator control signals to the parallel robot to thereby impart corrective motion to the payload and thereby reduce strain thereon.

2. The robotic system according to claim 1, wherein, The corrective motion controller is configured to transmit the second set of actuator control signals to one or more motorized joint actuators of the parallel robot.

3. The robotic system according to claim 1, wherein, The force sensor is integrated with the parallel robot.

4. The robotic system according to claim 1, wherein, The parallel robot includes a Stewart platform.

5. The robotic system according to claim 1, further comprising: An additional serial robot in communication with the pair of serial robots and the parallel robot, wherein the additional serial robot is configured to perform a work operation on the payload when the payload is cooperatively presented within the workspace.

6. The robotic system according to claim 5, wherein, The payload is a vehicle chassis, and wherein the work operation is a welding operation performed by the additional serial robot on the vehicle chassis.

7. The robotic system according to claim 1, wherein, The robot control system is configured to determine the weight of the payload based on the actual positions of the pair of serial robots and the parallel robot; and thereafter use the weight within an impedance control model to determine the second set of actuator control signals.

8. The robotic system according to claim 1, wherein, The parallel robot includes two parallel robots, wherein each parallel robot is respectively connected to a corresponding distal end of a respective one of the pair of serial robots.

9. The robotic system according to claim 1, wherein, The corrective motion controller is configured to control the parallel robot to a default stop position to protect the payload in response to an emergency stop ("e-stop") signal from an emergency stop device.

10. The robotic system according to claim 1, wherein, In response to a control mode change signal, the corrective motion controller is configured to transition between a position control mode and a force control mode, in the position control mode, the parallel robot assumes a commanded position relative to the payload, and in the force control mode, the parallel robot applies a commanded force to the payload.

11. A robotic control system for use with a robotic system having two serial robots and one parallel robot when presenting a payload within a workspace, the parallel robot being disposed between the payload and a distal end of one of the serial robots, the robotic control system comprising: A coordinated motion controller configured to generate a first set of actuator control signals to control the multi-axis motion of the serial robot when the payload is presented collaboratively within the workspace; and A corrective motion controller in communication with the coordinated motion controller and configured to output a second set of actuator control signals in response to a force signal indicative of strain on the payload, the second set of actuator control signals being configured to control the multi-axis motion of the parallel robot concurrent with the multi-axis motion of the serial robot, including imparting corrective motion to the payload.

12. The control system according to claim 11, further comprising a force sensor configured to output the force signal and connected to or integrated with the parallel robot.

13. The control system according to claim 11, wherein, The corrective motion controller is configured to determine the weight of the payload based on the actual positions of the two serial robots and the parallel robot, and thereafter use the weight to determine the second set of actuator control signals.

14. The control system according to claim 11, wherein, The corrective motion controller is configured to control the multi-axis motion of the parallel robot to a default stop position to protect the payload in response to an emergency stop ("e-stop") signal from an emergency stop switch.

15. The control system according to claim 11, wherein, In response to a control mode transition signal, the corrective motion controller is configured to transition between a position control mode and a force control mode, in the position control mode, the parallel robot assumes a commanded position relative to the payload, and in the force control mode, the parallel robot applies a commanded force to the payload.

16. A method for collaboratively presenting a payload within a workspace, the method comprising: Connect the parallel robot to the distal end of the first serial robot; Connect the parallel robot to the payload such that the parallel robot is disposed between the distal end and the payload; Connect a second serial robot to the payload; and Control the motion of the first serial robot, the second serial robot, and the parallel robot via a robot control system, including: Measure a force signal indicative of strain on the payload via a force sensor, the force sensor being located within a motion chain extending between the distal end and the payload; Control the multi-axis motion of the first serial robot and the second serial robot via the coordinated motion controller of the robot control system, including transmitting a first set of actuator control signals to the first serial robot and the second serial robot; and In response to the force signal, concurrently with the first set of actuator control signals, control the multi-axis motion of the parallel robot via the corrective motion controller of the robot control system to thereby reduce the strain on the payload, including transmitting a second set of actuator control signals to the second serial robot.

17. The method according to claim 16, further comprising: Control the motion of a third serial robot via the coordinated motion controller when performing a work operation on the payload.

18. The method according to claim 17, wherein, Controlling the motion of the third serial robot includes performing a welding operation on a vehicle chassis using the third serial robot.

19. The method according to claim 16, further comprising: Receive an emergency stop ("e-stop") signal via the corrective motion controller; and Control the multi-axis motion of the parallel robot to a default stop position to protect the payload in response to the emergency stop signal.

20. The method according to claim 16, further comprising: Receive a control mode transition signal via the corrective motion controller; and in response to the control mode transition signal, transitioning between a position control mode and a force control mode via the calibration motion controller, in the position control mode, the parallel robot assumes a commanded position relative to the payload, and in the force control mode, the parallel robot applies a commanded force to the payload.

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