Abstract:
The anthropomorphic force-reflective master arm controls a remote robotic arm and includes a plurality of rotatably joined mechanical links extending from a base to provide up to six degrees of freedom. Three of the link members are rotatably coupled to each other to form a handle, so that axes of rotation of each of the handle link members intersect at a user&#39;s hand position. Cable and pulley assemblies for the link joints are connected to their corresponding backdrive motors, the backdrive motors being disposed on the base to provide efficient transmission of forces experienced by the remote machine to the master arm handle.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to robotic control systems, and particularly to an anthropomorphic force-reflective master arm that allows a human operator to map his hand motion to a remote slave tool in unstructured environments in which autonomous robots cannot be used. 
         [0003]    2. Description of the Related Art 
         [0004]    It is often necessary that a human operator manually control the motion of a remote tool being held by an arbitrary slave device, e.g., a robotic arm manipulating a device outside a satellite in space, an underwater robotic arm, etc. The remote slave device is sometimes located in a hostile or unstructured environment, which justifies the need to keep the human operator in a safe remote location. The interconnection between the human interface system and the slave device is arbitrary, and may use a dedicated or public network. The interface is designed to permit the operator hand-operated translation and rotation of the control, and to transmit such changes to the slave device so that the changes are superimposed to a current tool position and orientation. 
         [0005]    An improvement to this human interface would provide the capability to simultaneously measure all hand changes in position and orientation in order to minimize the number of iterations needed for tool set up in a desired configuration. Forces and torques exerted on the tool by a workpiece would be streamed from the slave device to reflect back on the operator&#39;s hand. The interface must provide force feedback to let the operator feel the forces displayed on its motors. An increased force feedback gain is desired to provide acceptable fidelity and sensitivity to small force/torque feedback magnitudes because the interface inertia felt at the operator hand must be very small. 
         [0006]    Thus, an anthropomorphic force-reflective master arm solving the aforementioned problems is desired. 
       SUMMARY OF THE INVENTION 
       [0007]    The anthropomorphic force-reflective master arm is a lightweight, backdrivable, six degree of freedom robotic arm that can serve as a master arm to control the motion of a remote slave arm. The master arm includes up to six serially connected rotary joints that extend from a grounded base to a handle that can be grasped and manipulated by an operator. The grounded base houses six motors. The position of operator hand origin depends only on the first three rotary joints (nearest to the base). The last three rotary joints (nearest to the handle) have concurrent rotation axes that intersect at the operator hand origin and are used for rendering the rotation of the operator&#39;s hand. 
         [0008]    A lightweight, balanced mechanism is used for the last three rotary joints, which are arranged to directly measure operator forearm rotation, operator horizontal elevation, and operator vertical elevation, respectively. The operator feels the same impedance in all rotational directions due to the balanced mechanism in the last three rotary joints, which improves force feedback fidelity. This arrangement uncouples hand translation from hand orientation. 
         [0009]    Since the motors are grounded at the base, a back drivable transmission uses pre-tensioned cable and lightweight pulleys to connect each motor to its corresponding joint. The fidelity and reversibility of the transmission mechanism facilitates the display of kinesthetic force feedback on the operator hand. The master arm provides a singularity-free mechanism to render the operator hand motion and map it to a remote tool while providing a high fidelity kinesthetic force display. The master arm weighs three kilograms, has more than one cubic meter of work envelope, and has better similarity to the human arm than previous designs. 
         [0010]    Sensors determine movement of the handle and transmit corresponding signals to a control computer. The control computer maps movement of the handle to a remote slave arm. Similarly, sensors at the remote slave arm determine reactive forces resulting from the mapped movement of the slave arm and transmit corresponding signals to the control unit. The control unit sends corresponding signals to activate the motors at the base of the master arm to reflect the forces encountered by the slave arm to the handle, so that the operator senses reaction of the workpiece to movement of the slave arm as though the operator were manipulating the slave arm directly. 
         [0011]    These and other features of the present invention will become readily apparent upon further review of the following specification and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a perspective view of an anthropomorphic force-reflective master arm according to the present invention, the cables being omitted for clarity. 
           [0013]      FIG. 2  is a schematic view of the cable interlink transmission and motor configuration of the anthropomorphic force-reflective master arm according to the present invention. 
           [0014]      FIG. 3  is a perspective view of a cable guide system for degrees of freedom  4 ,  5  and  6  of an anthropomorphic force-reflective master arm according to the present invention. 
       
    
    
       [0015]    Similar reference characters denote corresponding features consistently throughout the attached drawings. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    As shown in  FIG. 1 , the present invention relates to an anthropomorphic force-reflective robotic master arm (AFRMA)  10  that includes a plurality of links serially connected at rotary joints. The arm  10  extends from a base  12  to a handle  14  in a fashion similar to a human arm. A plurality of motors M 1  through M 6  are disposed on the base  12  by mounting blocks  206  to generate force/torque components according to feedback from a slave (remote) arm. Location of the motors M 1  through M 6  on the base  12  instead of at the rotational link joints improves the responsiveness of the arm  10 . 
         [0017]    The master arm  10  is sampled at regular time intervals by sensors connected to a control computer. Cartesian changes in operator hand position and orientation are transmitted to the control computer to map movement of a slave arm that may be kinematically different from the master arm  10 . All six rotatable joints are mechanically decoupled from each other and have no backlash due to the pre-tensioned transmission cables. A remote slave arm can respond by a motion that is a replica of operator hand motion driving the master arm  10 . 
         [0018]    The motors M 1 -M 6  of master arm  10  include threaded rollers  50  and are disposed on the fixed platform  12  to improve the dynamics of master arm  10 . Transmission cables interconnect motors M 1 -M 6  to pulleys at rotational joints DOF 1 -DOF 6 . To the extent practicable, the transmission cables associated with a first link having a specific rotational DOF extend near a rotation axis of a second, interconnected link in order to decouple rotation of the first link from rotation of the second, interconnected link. As shown in  FIG. 1 , the first L 1  and fourth L 4  links exemplify the aforementioned decoupled configuration. 
         [0019]    Again referring to  FIG. 1 , a reducer pulley  202  is mounted on link L 1  and connected to motor M 1  using a flexible steel rope  203 . In the configuration shown, link L 1  has a hollow cylinder  230  extending axially between the link sidearms. Ten cables extend through cylinder  230  to drive the five links L 2  through L 6 .  FIG. 2  shows a cable orientation schematic for an interlink transmission  260  comprising the ten cables ( 2 ,  2 ′,  3 ,  3 ′,  4 ,  4 ′, and  5 ,  5 ′), which are connected to threaded roller group A 2  through A 6  of motor group  270 . Moreover cables  6 ,  6 ′ are associated with DOF 6  and connected to pulley  310 , which is shown in  FIG. 3 . A low friction pulley mechanism is used to guide the cables  2  through  5 ′ from the motor rollers A 4 -A 6  to the interlink transmission, more particularly, to small-dimensioned pulleys at each of the DOF 1 -DOF 6  rotational joints. A configuration similar to cylinder  230  is provided to traverse the fourth link L 4 . The pulley-drive orientation, which includes threaded wheels P 3  and P 4 , ensures the independence between the rotation of link L 1  and the subsequent five links L 2 -L 6 . 
         [0020]    The motor-link transmission  260  is based on a cable-pulley configuration that extends from a motor (one of M 2 -M 6 ) to a link (one of L 2 -L 6 ) through the hollow cylinders  230  and  240 , while uncoupling the transmitted motion from that of the traversed link. The motor-link transmission  260  is based on the cables  2  through  5 ′ being of a multiple, independent closed-loop variety. The connectivity between a motor (one of M 1 -M 6 ) and a link (one of L 1 -L 6 ) is achieved through multiple Cable Pulley Loop (CPL) mechanisms. Each CPL is an independent system. The transmission from motor to link is then achieved using an arbitrary subset of attached (pulley level) CPLs. The first loop L 1  transmits motion from the motor M 1  to the first link L 1  (DOF 1 ). In this and all other links, speed reduction is performed as close as possible to the intended driven link. 
         [0021]    Each loop starts with a threaded roller mounted on the electric motor shaft. The transmission wire is freely wrapped three times around the roller along a machined deep thread. The thread pitch and depth are selected according to the rope diameter. Embedding the wire in the thread will practically eliminate slippage. Both ends of the rope are wrapped around the driven threaded wheel. Each wire is wrapped two times around the wheel (pulley) to provide an acceptable range of motion (ROM) at the end link. In the final wrap, the rope is introduced through a specially designed inclined through-hole to be completely restrained from any slippage by a tightening screw device (not shown). 
         [0022]    The first link L 1  and the second link L 2  are driven by a single loop each. The following links (L 2  through L 6 ) are driven by L 1  cable pulley loop assemblies (CPLs). In this manner, the loops remain independent to reduce physical effort required to maintain the master arm  10  in a localized area, and to improve system reliability. Pre-tensioning the wire is done independently for each loop. The independent pre-tensioned configuration of wires for each loop (CPL) allows a high-speed, low (force) tension cable to be used for the first n−1 CPL&#39;s and, finally, a high (force) tension wire is used for the n th  CPL connected to the corresponding link. 
         [0023]    Due to the aforementioned configuration of drive motors M 1  through M 6  and transmission cables  203 ,  2 ,  2 ′,  3 ,  3 ′,  4 ,  4 ′,  5 ,  5 ′, and  6 , 6 ′, the master arm  10  has low friction, low inertia, and low mass. The motors M 1 -M 6  are disposed on the stable platform  12  to eliminate the potential of damaging the master arm  10  due to excess weight and inertia. Arm fidelity is improved to thereby more accurately transmit a reflected force feedback. Mounting all of the motors M 1  through M 6  on base  12  provides maximum possible force/torque dynamics, as well as enlarging the force transmission bandwidth. The force/torque vector exerted on a slaved tool is sensed by a force sensor, which is generally installed at the wrist of the slave arm. The sensed vector is used to compute the force/torque vector exerted on the slaved tool. The tool force/torque vector is sampled and transmitted at regular time intervals (streamed) to the master arm station, where it is converted into a motor torque vector that reproduces the tool force/torque vector at the operator hand center  14 . This allows the operator to feel the force/torque that is proportional to the one exerted on the remote tool. 
         [0024]    As most clearly shown in  FIG. 3 , the L 5  and L 6  link members have associated pulleys  305  and  310 , respectively. A user&#39;s hand grabs L 6 , which is a vertical member rotatably attached to and extending from L 5 . L 6  is responsive to a twist (yaw) motion of the hand, while pivotal bracket-shaped link L 5  is responsive to a pitch motion of the user&#39;s hand. Cable guides  300  are disposed on L 4  and are threaded onto threaded receivers  312 , making L 4  responsive to a rotation (roll) of the user&#39;s hand. 
         [0025]    It is to be understood that the present invention is not limited to the embodiment described above, but encompasses any and all embodiments within the scope of the following claims.