Patent ID: 12246435

The figures are not intended to limit the present invention to the specific embodiments they depict. The drawings are not necessarily to scale.

DETAILED DESCRIPTION

The following detailed description of embodiments of the invention references the accompanying figures. The embodiments are intended to describe aspects of the invention in sufficient detail to enable those with ordinary skill in the art to practice the invention. The embodiments of the invention are illustrated by way of example and not by way of limitation. Other embodiments may be utilized and changes may be made without departing from the scope of the claims. The following description is, therefore, not limiting. The scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled.

In this description, references to “one embodiment,” “an embodiment,” or “embodiments” mean that the feature or features referred to are included in at least one embodiment of the invention. Separate references to “one embodiment,” “an embodiment,” or “embodiments” in this description do not necessarily refer to the same embodiment and are not mutually exclusive unless so stated. Specifically, a feature, component, action, step, etc. described in one embodiment may also be included in other embodiments, but is not necessarily included. Thus, particular implementations of the present invention can include a variety of combinations and/or integrations of the embodiments described herein.

Broadly, embodiments of the present invention provide a mobile robot system and method configured to determine the stiffness of a human arm during overground interaction as a human user grasps the robot and exchanges forces with it as if it was a human partner. An embodiment of the mobile robot system may include a mobile base and a robot arm. The mobile base may support the robot arm and adjust its speed to that of the human user. The robot arm may be configured to provide a substantially natural, human-like interaction, and may include substantially low mechanical impedance, such as very low inertia and friction, as well as an ability to sense and apply precise interaction forces to the human user. In particular, the robot arm may have the ability to apply and measure small interaction forces (of, e.g., equal to or less than approximately ten Newtons) as well as inherently low endpoint impedance. In one implementation, the robot arm may include two brushless direct current (DC) motors with no gearheads and a lightweight, symmetric linkage system.

Embodiments have numerous potential applications, including in rehabilitation and physical therapies, living assistance, manufacturing, and entertainment. The ability to determine arm stiffness allows for more effective interaction, including for developing more intuitive and effective walking assistance. Embodiments use a single force perturbation rather than continuous force pulses. This avoids interfering with the continuous overground physical interaction process throughout the entire trial, and eliminates the need for baseline unperturbed trajectory. Further, embodiments use a single trial rather than multiple trials

As referred to herein, “stiffness” is used in its commonly accepted sense, so a human arm is stiffer if the arm muscles are contracted and less stiff if they are not. The opposite of stiff may be referred to as “compliant,” so a stiffer arm is less compliant and a less stiff arm is more compliant. Generally, stiffness can be quantified as a ratio between how much force is applied at the hand (perturbation force, F) and how much the arm moves as a result (displacement, X). The stiffness (K) is then K=F/X.

Referring toFIGS.1-6, an embodiment of the mobile robot system10is shown including a mobile base12, a robot body14coupled with and supported by and above the mobile base12, and a robot arm16coupled with and supported by the robot body14. The mobile base12may be configured to physically support the robot body and to adjust its speed to that of the human user.

In one implementation, the mobile base12may include at least one wheel20, a battery or other power supply22, and a base controller24. For example, the mobile base12may be a four wheel differential drive robot, powered by a twelve Volt DC power supply through two rechargeable batteries, and controlled by a microcontroller which may receive commands from a main controller. In another implementation, the mobile base may include at least one continuous track. The mobile base12may be independently controlled by the base controller22and receive input commands for linear and angular velocity. One or more quadrature encoders (not shown) reading from the motors of the wheels20of the base12may be used for determining the instantaneous position of the mobile robot system10, which can be used for controlling the speed of the system10. The iteration rate for base control may be forty Hertz. Broadly, the design and operational characteristics of the mobile base12may depend on the nature of the interaction (e.g., leading or following) and the needs of the human user.

The robot body14may be configured to physically support the robot arm16. In one implementation, the robot body14may have an adjustable height to allow human users with varying heights to better interact with the mobile robot system10without the discomfort or awkwardness that might otherwise arise from height differences.

The robot arm16may be configured to provide a substantially natural, human-like interaction, including having a substantially low mechanical impedance, such as a very low inertia and friction, as well as an ability to sense and apply precise forces to the human user. In one implementation, the robot arm16may include two or more links28driven by two or more servomotors30, and an end effector32supported by the links28. For example, the robot arm16may be a two-dimensional closed loop symmetrical five-link mechanism with two degrees or freedom and include two distal links34A,34B, two proximal links36A,36B, and a ground link38, with the two servomotors30being positioned on either side of the ground link38. For a given offset between the servomotors30, the lengths of the links28may be determined using the method of inequality, sufficiently avoiding singularities while covering a desired work area. For example, the links28may be configured to be symmetrical and have the shortest length while still allowing the end effector32to cover the entire work area, thereby resulting in a low intrinsic mechanical impedance.

The servomotors30may be substantially any suitable electric motors. For example, the servomotors30may be brushless DC motors, configured such that the endpoint speed of the end effector32is approximately between one-half and one and one-half meters-per-second, and configured such that the end effector32is able to generate a force of at least approximately ten Newtons at every point in the work area. Each servomotor30may be associated with a servo drive50, an encoder51configured to measure angular displacement, and a controller, which may be a main real-time controller52. The servo drives50and the force/torque controller54may be alternating current (AC) powered, whereas the main controller52may use a twenty-four Volt DC power supply through an AC-to-DC converter. Two additional twenty-four Volt power supply modules may be used to provide the logic power to the servo drives50. Each encoder51may be a single-ended high resolution optical encoder configured to precisely measure angular displacement up to seven thousand two hundred counts-per-rotation.

The end effector32may be configured to cover the desired range of motion and to follow movement of the human user's hand across the work area. The robot arm16may further include force/torque transducer42and force/torque controller54configured to measure at the end effector32. In one example implementation, the transducer42may be six-axis force transducer configured to precisely measure forces approximately up to seventeen Newtons. The force/torque transducer42may be positioned on the joint of two distal links34A,34B.

As seen inFIG.4, the robot arm16may further include an interaction handle44located on top of the transducer42. The handle44may have substantially any suitable form. In various implementations, the handle may provide a relatively short grip for accommodating one of the human user's hands, two relatively short grips for accommodating both hands, or one relatively long grip. Further, the grip may be shaped or otherwise configured to resemble a human hand.

As seen inFIG.5, the main controller52may be configured as a central processing unit. For example, the main controller52may communicate with the servo drives50through a master-slave EtherCAT protocol, and with the base micro-controller22through an RS-232 serial communication protocol. It may also accept analog voltage data from the force/torque transducer42via the force/torque controller54. A C-series module may be used to input voltage data to the controller52. For monitoring the health of the force/torque controller42, a separate digital input-output module may be used.

The controller52may be configured to provide a background stiffness of between fifty Newton/meters and one hundred Newton/meters to the human user via the end effector. The controller52may be further configured to determine a stiffness of a human arm of the human user at a particular time by generating a single force perturbation at the handle in a direction of the human user, measuring a displacement of the human arm and measuring a peak velocity achieved by the human arm after the single force perturbation, determining the stiffness of the human arm as a function of the single force perturbation and the displacement of the human arm. The controller52may be configured to then control operation of the mobile base and robot arm, including physically supporting the human user, based on the stiffness of the human arm at the particular time.

Data acquisition is of the interaction force at the end effector32by the human arm. The forces at the end effector32may be measured with respect to the force/torque transducer42axis and can be transformed into the robot axis using an appropriate rotation matrix. The positions of the servomotors30may be recorded using the encoder51associated with each servomotor30. The point of interaction, which is the end of the manipulator or the handle44, can be calculated from the positions of the servomotors30using forward kinematics of the robot arm16. The data may be recorded with a sampling rate of, e.g., approximately one kilo-Hertz.

If desired, the arm length, from the shoulder to the hand, may be obtained using a motion capture system. For this purpose, reflective markers may be placed on the hand and on the shoulder. This data can then be processed using available software. The sampling rate may be, e.g., approximately two hundred kilo-Hertz. In one implementation, the data from the robot may be decimated to, e.g., approximately two hundred kilo-Hertz before combining the two data. Then, the angular position provided by the servomotor encoders51may be compared with the respective position obtained from motion capture system, which is estimated based on the reflective markers attached to the robot arm16.

Referring also toFIG.7, a method210is shown for determining the stiffness of a human arm at a particular time during overground interaction as the human user grasps a robot and exchanges forces with it as if it was a human partner. In one implementation, the method210may be implemented by and reflect operation of the mobile robot system10described above. As shown in step212, if desired, the height of the robot arm16may be adjusted via the robot body14such that the interaction handle44is aligned with the subject's elbow. As shown in step214, the robot arm16may provide a background stiffness of approximately between fifty Newton/meters and one hundred Newton/meters to the human user via the end effector32.

As shown in step216, the robot arm16may apply a perturbation of force at the handle44in a direction, such as toward the human user, and measure the displacement of the human user's hand. Each determination of stiffness may be based on a single perturbation of force. In one implementation, the robot arm16may apply a force perturbation of approximately between one Newton and five Newtons for approximately one second in the axis perpendicular to the robot's movement direction and toward the human user (the −y direction inFIG.6). During the perturbation, the stiffness control loop may be disabled and replaced by the force controller. The magnitude of the force perturbation may be determined by adding the interaction force due to the background stiffness control when the perturbation was applied and the level of perturbation commanded (for example, approximately between one Newton and five Newtons).

A dynamic model of the human user's arm may be represented as f=m x″+b x′+k×(1), hereafter referred to as “Equation 1,” wherein f is the interaction force and x, x′, and x″ are the resultant displacement, velocity, and acceleration after the perturbation, respectively, and m, b, and k are the endpoint impedance parameters namely inertia, damping and stiffness, respectively. The displacement of the interaction handle44with respect to its position at the onset of the perturbation may be considered to be the resultant displacement due to perturbation. Typically, shortly after the onset of the force perturbation, the hand velocity along the y-direction (seen inFIG.6) decreases as the hand is pushed away from the robot system10until it reaches a negative peak. Then the velocity increases towards the robot system10until it passes zero and reaches a positive peak approximately between two hundred fifty milliseconds and four hundred fifty milliseconds after the onset of the perturbation.

As shown in step218, the robot system10may measure a displacement of the human arm and measure a peak velocity achieved by the human arm after the single force perturbation. As shown in step220, the robot system10may determine the stiffness of the human arm as a function of the single force perturbation and the displacement of the human arm.

The data between the onset of the perturbation to the second or positive peak of the velocity of the human arm contains the passive dynamics and can be used to determine the arm stiffness as described by Equation 1 during the overground pHRI. The positive peak may be obtained by taking the time derivative of the displacement data collected by the encoders. The endpoint position, which is also the human hand position, with respect to the robot system10may be measured through the encoders on the servomotors30. Additionally, the interaction force at the endpoint may be measured through the force/torque transducer42. In one implementation, a second order Butterworth low-pass zero lag filter with forty kilo-Hertz cutoff frequency may be used to filter the recorded force and displacement data in suitable analysis software. The graph of hand velocity versus time may be plotted in analysis software using the filtered data. The second peak on the plot may be identified and the time instant noted. Parameters of Equation 1 may be obtained from the force and displacement data between the onset of the perturbation (time=zero milliseconds) and the time of the second peak (time=between two hundred fifty milliseconds and four hundred fifty milliseconds) using the analysis software function for non-linear regression to estimate m, b, and k. The stiffness value, k, is of primary interest while the inertia and damping values may be used to verify that the regression result is acceptable (for example, both m and b should be positive and realistic).

As shown in step222, operation of the mobile base and robot arm may then be controlled, including physically supporting the human user, based in part on the determined stiffness of the human arm at the particular time, thereby improving control of the system and better managing interaction with the human user during overground movement.

Although the invention has been described with reference to the one or more embodiments illustrated in the figures, it is understood that equivalents may be employed and substitutions made herein without departing from the scope of the invention as recited in the claims.