System and method for tensioning a robotically actuated tendon

A tendon tensioning system includes a tendon having a proximal end and a distal end, an actuator, and a motor controller. The actuator may include a drive screw and a motor, and may be coupled with the proximal end of the tendon and configured to apply a tension through the tendon in response to an electrical current. The motor controller may be electrically coupled with the actuator, and configured to provide an electrical current having a first amplitude to the actuator until a stall tension is achieved through the tendon; provide a pulse current to the actuator following the achievement of the stall tension, where the amplitude of the pulse current is greater than the first amplitude, and return the motor to a steady state holding current following the conclusion of the pulse current.

TECHNICAL FIELD

The present invention relates generally to systems and methods for maintaining tension on a robotically actuated tendon.

BACKGROUND

Robots are automated devices that may be capable of manipulating objects using a series of links. The links are interconnected by one or more actuator-driven robotic joints. Each joint in a typical robot represents at least one independent control variable, or a degree of freedom. End-effectors are the particular manipulators used to perform a task at hand, such as grasping a work tool. Therefore, precise motion control of the various robotic manipulators helps to achieve the required mobility, dexterity, and work task-related functionality.

Dexterous robots may be used where a direct interaction is required with devices or systems specifically designed for human use, i.e., devices requiring human-like levels of dexterity to properly manipulate. The use of dexterous robots may also be preferred where a direct interaction is required with human operators, as the motion of the robot can be programmed to approximate human motion. Such robots may include a plurality of fingers that can be actuated remotely using tendons, thus reducing the overall size and weight of the robot. Such tendons must be kept taut at all times to within a calibrated tension level, however, to actuate a finger, the tendon must be transitioned to a higher tension level. To maintain, for example, a gripping force using the finger, the tendon must remain at the higher tension level until commanded to relax.

SUMMARY

A tendon tensioning system includes a tendon having a proximal end and a distal end, an actuator, and a motor controller. The actuator may include a drive screw and a motor, and may be coupled with the proximal end of the tendon and configured to apply a tension through the tendon in response to an electrical current.

The motor controller may be electrically coupled with the actuator and may be configured to provide an electrical current with a first amplitude to the actuator until a stall tension is achieved through the tendon. The controller may then provide a pulse current to the actuator following the stall, where the amplitude of the pulse current is greater than the first amplitude, and subsequently return the motor to a steady state holding current following the conclusion of the pulse current. In an embodiment, the motor controller may be configured to allow the tension maintained in the tendon to dwell at the stall tension for a period of time before providing the pulse current to the actuator.

The holding current provided to the actuator may have an amplitude less than a maximum steady state current level of the system, and the amplitude of the pulse current may be less than a maximum instantaneous current level of the system. Additionally, the pulse current provided to the actuator may be operative to increase the tension provided through the tendon to a boosted tension level, where the boosted tension level being greater than the stall tension. The actuator may then be configured to maintain the boosted tension level through the tendon following the conclusion of the pulse current.

In one configuration, the distal end of the tendon may be coupled with a finger of a dexterous robot. As such, the actuator may be a finger actuator. Additionally, the drive screw of the actuator may include a ball screw, with a ball nut disposed about the ball screw and coupled with the tendon. The actuator may be configured to maintain at least a minimal tension on the tendon at all times during operation, the minimal tension being less than the stall tension.

A method for controlling an actuator to tension a tendon may include providing a tendon and an actuator, where the actuator is configured to apply a tension through the tendon in response to an electrical current. The tendon may have a proximal end and a distal end, where the proximal end of the tendon is coupled to the actuator. The method may further include driving the actuator through an initial electrical current that has a first amplitude until a stall tension is achieved through the tendon, delivering a pulse current to the actuator following the stall, with the amplitude of the pulse current being greater than the first amplitude, and returning the electrical current delivered to the motor to a steady state holding amplitude following the conclusion of the pulse current.

DETAILED DESCRIPTION

With reference to the drawings, wherein like reference numbers refer to the same or similar components throughout the several views, and beginning withFIG. 1, a robotic system10is shown that includes a dexterous robot11and a controller12. The robot11may include various manipulators, including a plurality of tendon-driven fingers14. The controller12may be embodied as a processor and/or related hardware devices, or alternatively as nested software-based control loops that are resident in a single or in a distributed hardware device and automatically executed by one or more processors. Furthermore, the controller12may include or embody a control system25that may perform one or more control routines.

To manipulate the fingers14of the dexterous robot11, such as when grasping an object30, the controller12may be operative to vary the tension applied through one or more tendons50, such as schematically shown inFIG. 3. The control system25may employ, for example and without limitation, force-based control and/or a position-based control to regulate the tension maintained within a tendon. Such control may further include closed-loop force and/or position feedback to refine the controllability. Within the control system25, the particular control law being applied, i.e., force or position, may be selected in a manner that depends upon the number of available tension sensors for a given finger14.

In one embodiment, the robot11shown inFIG. 1may be configured with a human-like appearance, and with human-like levels of dexterity to the extent necessary for completing a given work task. Humanoids and other dexterous robots can be used where a direct interaction is required with devices or systems specifically designed for human use, for example any devices requiring human-like levels of dexterity to properly manipulate an object30. The use of a humanoid such as the robot11depicted inFIG. 1may be preferred where a direct interaction is required between the robot and human operators, as motion of the robot can be programmed to closely approximate human motion. The fingers14of robot11may be directly controlled by hardware components of the controller12, e.g., a host machine, server, or network of such devices, via a set of control signals55during the execution of any maneuver or work task in which the robot acts on the object30.

The robot11shown inFIG. 1may be programmed to perform automated tasks with multiple degrees of freedom (DOF), and to perform other interactive tasks or to control other integrated system components, e.g., clamping, lighting, relays, etc. According to one possible embodiment, the robot11may have a plurality of independently- and interdependently-moveable actuator-driven robotic joints, some of which have overlapping ranges of motion. In addition to the various joints23of the fingers14, which separate and move the various phalanges thereof, the robotic joints of robot11may include a shoulder joint, the position of which is generally indicated inFIG. 1by arrow13, an elbow joint (arrow15), a wrist joint (arrow17), a neck joint (arrow19), and a waist joint (arrow21).

Still referring toFIG. 1, each robotic joint may have one or more DOF. For example, certain compliant joints such as the shoulder joint (arrow13) and the elbow joint (arrow15) may have at least two DOF in the form of pitch and roll. Likewise, the neck joint (arrow19) may have at least three DOF, while the waist and wrist (arrows21and17, respectively) may have one or more DOF. Depending on task complexity, the robot11may move with over 42 DOF. Each robotic joint contains and is internally driven by one or more actuators, for example joint motors, linear actuators, rotary actuators, and the like.

In one embodiment, the robot11may include just the lower arm assembly75shown inFIG. 2. In another embodiment, the robot11may include additional human-like components such as a head16, a torso18, a waist20, arms22, hands24, fingers14, and opposable thumbs26, with the various joints noted above being disposed within or between these components. As with a human, both arms22and other components may have ranges of motion that overlap to some extent. The robot11may also include a task-suitable fixture or base (not shown) such as legs, treads, or another moveable or fixed base depending on the particular application or intended use of the robot. A power supply28may be integrally mounted to the robot11, e.g., a rechargeable battery pack carried or worn on the back of the torso18or another suitable energy supply, or which may be attached remotely through a tethering cable, to provide sufficient electrical energy to the various joints for movement of the same.

The controller12may be embodied, as noted above, as a server or a host machine, i.e., one or multiple digital computers or data processing devices, each having one or more microprocessors or central processing units (CPU), read only memory (ROM), random access memory (RAM), electrically-erasable programmable read only memory (EEPROM), a high-speed clock, analog-to-digital (A/D) circuitry, digital-to-analog (D/A) circuitry, and any required input/output (I/O) circuitry and devices, as well as signal conditioning and buffering electronics.

While shown as a single device inFIG. 1for simplicity and clarity the various elements of control system12may be distributed over as many different hardware and software components as are required to optimally control the robot11. The individual control routines/systems25resident in the controller12or readily accessible thereby may be stored in ROM or other suitable tangible memory location and/or memory device, and automatically executed by associated hardware components of the control system to provide the respective control functionality.

Referring toFIG. 2, a lower arm assembly75can be used as part of the robot11shown inFIG. 1. Each lower arm assembly75may include a hand24having a plurality of tendon-driven fingers14and a tendon-driven thumb26. The term “tendon-driven” is explained below with reference toFIG. 3. The lower arm assembly75may include a plurality of finger actuators40that each may be respectively configured to selectively pull on, and release one or more tendons50(seeFIG. 3) in a finger14or in a thumb26. The lower arm assembly75may further include a plurality of wrist actuators38for moving the wrist joint (arrow17). Printed circuit board assemblies (PCBA)39for the finger actuators40and/or the wrist actuators38may be positioned on or within the lower arm assembly75as shown for packing efficiency. The lower arm assembly75may be attached to a load cell32, which is used to connect the lower arm assembly to the rest of the arm22of the robot11shown inFIG. 1.

Multiple finger actuators40may correspond to each finger14and thumb26. In general, one finger actuator40may be used for each DOF available plus one additional finger actuator. Therefore, each finger14having three DOF requires four finger actuators40, while each finger having two DOF requires three finger actuators, etc.

Referring toFIG. 3, a schematic perspective view is provided of a possible embodiment of the finger actuator40. As shown, the finger actuator40may be provided with a tendon50having a proximal end51and a distal end53, where the finger actuator40may be coupled with the proximal end51of the tendon50. Additionally, a tendon terminator52may be coupled to the distal end53of the tendon50, which may be operative to couple the tendon50to a finger14of the dexterous robot11. The finger actuator40may include a motor44, a gear drive46, and a linear actuator48, which may cooperate to apply a tension through the tendon50in response to an electrical current. The tendon50is illustrated in an off-center position within the finger14, as more than one tendon may extend within a given finger. The motor44, gear drive46, and linear actuator48may all be located in the lower arm assembly75in order to minimize the packaging space required within the fingers14and the thumb26, and to allow for the larger components of the finger actuator40, such as the linear actuator48, to be remotely packaged with respect to the fingers and thumb.

The tendon50may be protected by a sheath or conduit liner54positioned within a protective outer conduit56. The tension sensor58measures the force of compression on the conduit56to determine the amount of tension placed on the tendon50. Tension in the tendons50can be used by the controller12shown inFIG. 1to calculate the joint torques generated or experienced at the various joints of a given finger14, which in turn can be used by a motor controller90to control the actuation of the fingers and thumbs26of a given hand24.

As the finger actuator40moves the tendon50, the tendon50slides relative to the tension sensor58. The tendon50terminates within the finger14at the tendon terminator52. Movement of the tendon50may cause a relative movement of the tendon terminator52, which, as described above, may be affixed to a portion of a finger14. Force may be placed on the tendon terminator52either internally, i.e., by movement of the finger actuator40, or externally, i.e., on the finger14by the object30ofFIG. 1, which causes the tendon50to exert force on the linear actuator48.

FIG. 4is a perspective, partially cross-sectional illustration of a finger actuator assembly40, such as provided inFIG. 3. As shown, a motor44is powered to drive a gear drive46which, may in-turn cause rotation of a ball screw60. A ball nut62may be disposed about and/or have a threaded engagement with the ball screw60, and a guide pin64may extend from the ball nut62and prevents rotation of the ball nut62in response to movement of the ball screw60. In other embodiments, other linear actuator technology may be employed, such as for example, roller screws or backdriveable lead screws. In an embodiment, the guide pin64may extend at least partially through a slot66defined by the housing68of the linear actuator48. The interference between the guide pin64and the linear actuator housing68can serve to restrain any rotational movement of the guide pin64, and thus the ball nut62. Therefore, as the gear drive46rotates the ball screw60, the ball nut62may translate axially along the ball screw60. The motor44, gear drive46and ball screw60define a finger actuator axis F along which the ball nut62may be configured to travel.

The gear drive46may be connected to the ball screw60via a coupling70, which may allow the coupling70to transmit the torque of the gear drive46while minimizing transmission of any axial load. Additionally, a bearing72may be located between the coupling70and the ball screw60to reduce friction between the actuator housing68and the ball screw60and to carry the axial load transmitted from the tendon50to the ball screw60.

A position sensor74may be mounted to the finger actuator housing68to sense an axial position of the ball nut62along the ball screw60. As shown, the position sensor74may include Hall Effect sensor, with a magnet76attached to the ball nut62. Alternatively, the position sensor74may include a linear encoder or employ other continuous or discrete forms of position sensing.

The tendon50may be attached to the ball nut62in a suitable manner so that movement of the ball nut62may cause a corresponding movement of the tendon50. The movement of the tendon50may either straighten or bend the finger14, depending on which side of the finger14the tendon is disposed on. An additional finger actuator40may be utilized to perform the opposing straightening or bending motion. Therefore, each finger14(shown inFIG. 3) may have at least one finger actuator40for each degree of freedom. In general each finger14may have one actuator per degree of freedom plus one additional actuator. For example, a three DOF finger may have four actuators and a four DOF finger has five actuators.

Each finger actuator40may be controlled by a respective motor controller90that may be electrically coupled with the motor44. Multiple motor controllers90may be included within a broader controller (e.g., controller12shown inFIG. 1), or may be separate from each other. A motor controller90may be configured to either directly provide a motor44with an operating current91, or it may merely provide a higher level command that may otherwise control the flow of current through the motor. For example, in an embodiment, the motor controller90may provide the motor with a digital value between 0 and 255, which may, in turn, be translated by the motor44into a current or torque.

During operation, a minimal tension may be maintained by each tendon50and/or provided by each respective finger actuator40at all times. Because each tendon50is desirably only configured to pull, the minimal tension may allow a finger actuator40to be maintained in a “ready” state without any slack developing in the tendon50. Movement of the ball nut62in a distal direction80along axis F may lessen the tension applied through the tendon50; and conversely, movement of the ball nut62in a proximal direction82may increase the tension applied through the tendon50. To increase the speed at which the nut62may travel along axis F, and thus the response speed of the finger actuator40, the finger actuator40may have a relatively low gear ratio in the gear drive46and/or a low ratio in the ball screw60. In an embodiment, the gear ratio may be, for example and without limitation, approximately 14:1, however other gear ratios may similarly be used. The tradeoff with a lower torque reduction in the actuator40, however, is an increased likelihood that the tension maintained through the tendon50will back drive the ball nut62along the ball screw60. This contrasts with higher gear ratio systems that may have system dynamics inherently more difficult to back drive. To provide a system that can maintain a high tension through the tendon50, yet reduce the likelihood of the ball nut62back-driving along the ball screw60, the motor44may be configured to augment the system dynamics of the gear drive46and ball screw60.

FIG. 5illustrates a current plot100and tension plot102that may be exhibited by a finger actuator40over the duration of a grasping routine (i.e., where the current plot100illustrates the current101provided to the motor44as a function of time104, and the tension plot102illustrates the tension103provided through a tendon50as a function of time104). As shown, the grasping procedure may be divided into, for example, six distinct periods (i.e., periods106,108,110,112,114, and116) that each may describe a different actuator40and/or controller90behavior within the procedure. These periods are meant to be purely illustrative, however, and should not be construed to limit the invention.

Within the first period106, the finger actuator40may be maintained in a “ready” state with a minimal tension118maintained through the tendon50. This minimal tension118may be generated, in part, by a minimal current120passing through the motor44. Upon being provided with an actuation command at time122, the current101may increase, which may cause the finger14to accelerate towards an object30. While the finger14is moving, the tension103through the tendon50may gradually increase, as shown in period108.

At time124, the finger14may engage the object30, wherein the tension103in the tendon50may quickly increase to a steady state level125(i.e., the stall tension125). The ramp-up in tension103can be seen in period110of the tension plot102. At time124, the tension103may become discontinuous due to the external influence of the object30on the finger14. Depending on the compliance of the object30, the tension103through the tendon50may increase at a varying rate. Inevitably, however, the object30that is being grasped will provide enough resistance to cause the motor44to stall, thus providing no further increase in tension (i.e., time126). Once this occurs, the motor controller90may be configured to reduce the current101provided to the motor44down to a lower current level128, also referred to as the steady-state holding current128. In an embodiment, the steady state holding current128may be at or slightly below a maximum steady state current level129. Such a maximum level may represent the greatest amount of current that the motor44and/or controller90can operationally maintain over an indefinite period of time.

Once a stall tension125is reached, the controller90may provide a pulse current130to the motor44, as shown in period114, and initiated at time132. The pulse current130may have a relatively short duration134, though, may have a relatively high amplitude136(as compared with the holding current128). In an embodiment, the duration134of the pulse current130may be, for example and without limitation, approximately 10-50 milliseconds. Additionally, the amplitude136of the pulse current130may be at or slightly less than a maximum allowable instantaneous current137that may be provided by the controller90, or received by the motor44. In embodiment, as shown inFIG. 5, the system may dwell at the stall tension125for a period of time112. In another embodiment, the system need not dwell at the interim stall tension125for any prolonged period of time before providing the pulse current130. As such, the duration of period112may be reduced down to a single instance.

The pulse current130may cause the tension103in the tendon50to increase from the lower, stall tension125to a higher tension level138by forcibly driving the motor44with the high-amplitude surge. Once the pulse130falls back to the holding current128at time142, the higher tension level138may be maintained within the tendon50and finger actuator40, as shown in the tension plot102within period116. In an embodiment, the ability to maintain a higher tension at a lower current may be attributed, in part to frictional dynamics of the ball screw60and/or gear drive46, and, in part to the holding current128applied to the motor44. As such, the holding current128may augment the frictional dynamics of the finger actuator40to discourage the motor44from being back driven. Allowing the reduction to the lower holding current may provide a considerable power savings as compared to relying solely on the motor to maintain a high holding torque/tension.

FIG. 6illustrates a method150of tensioning a robotically actuated finger-tendon50during a grasping routine. As shown upon being provided with a grasping command at160, the finger actuator motor44may be driven by a first current that may cause the finger to move into contact with an object30(Step162). As described above with respect toFIG. 5, upon contact, the tension103provided through the tendon50may increase until the forces generated by the motor44are balanced by the reactionary forces of the object30, at which time the motor44may stall. Once the stall is detected, a “tension boost” command may be provided at Step164. The tension boost command may cause the controller90to provide a pulse current130to the motor44. The pulse current130may forcibly cause the motor44to impart a much higher tension on the tendon50than was initially maintained at the equilibrium state in Step162.

Following the delivery of the pulse current130in Step164, the current level provided to the motor44may fall back down to the initial stall current128reached in Step166. This current128, together with the frictional dynamics of the finger actuator40, may cause the boosted tension (imparted via the current pulse) to be maintained through the tendon50. Step168concludes the routine with a high tension138being maintained by a low current command.

While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims. All directional references (e.g., upper, lower, upward, downward, left, right, leftward, rightward, above, below, vertical, and horizontal) are only used for identification purposes to aid the reader's understanding of the present invention, and do not create limitations, particularly as to the position, orientation, or use of the invention. It is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative only and not as limiting.