Fig. 1: Overview of the arm and static balancing bench.


    A 7-degree-of-freedom (DOF) arm that is partially statically balanced has been developed in the robotics laboratory. This prototype was built in order to test a new static balancing technique and to provide a working platform for research on human-robot collaboration . To assist a human in his tasks, a robot must be able to move loads weighing several kilograms, within the workspace of the human. The power of the robot must be minimized to ensure the safety of the user, without compromising the performance of the task. The robot must also be easy to move (backdrivable), intuitive to operate and agile enough so as not to limit the user in his tasks.

    Figure 2 shows the different components of the prototype. The shoulder and the elbow are provided with a torque limiter to reduce the force of a potential impact. Auxiliary encoders enable the detection of a disengaged motor following a collision and allow the position of the robot to be known at any time. The arm and forearm are statically balanced by diaphragm cylinders connected to the balancing bench. The elbow is provided with two pivots and one single actuator. Connected by cables, these two pivots form a rolling-joint which allows a + - 180 degree rotation of the elbow. The wrist is compact and powerful, with 3 degrees of freedom. A 3-finger underactuated Robotiq hand mounted on an ATI force/torque sensor, completes the robot and allows the handling of loads of 10 kg.


    Fig. 2 : Arm description

    The balancing mechanism used in the arm is shown in Figures 3 and 4. The parallelogram structure maintains the vertical reference for the next links. This structure also renders the mechanism insensitive to forward link configurations. A diaphragm cylinder linked to the balancing bench provides the balancing and actuation force. This cylinder configuration has the advantage of being quite simple. The video segment Arm Animation provides a 3D animation of the mechanism.

    Fig. 3 : Schematic of the arm
    balancing mechanism.
    Fig. 4 : Section view of the arm.

    Figures 5 and 6 illustrate the balancing mechanism used in the forearm. Mechanical interference between the first wrist motor and the cylinder make it difficult to use the same configuration used in the arm. In this new configuration, the cylinder remains parallel to the bottom link, thus making it easier to avoid the mechanical interference. The cylinder pushes on a slider which transmits forces to the base through a tension linkage. This configuration is still quite efficient although it is slightly more complicated than the previous configuration. The free space generated in the upper link allows electronic devices to be inserted so as to control the motors of the elbow and wrist. A 3D animation of the mechanism can be seen in the video segment Forearm Animation.

    Fig. 5 : Schematic of the forarm
    balancing mechanism.
    Fig. 6 : Section view of the forarm.

    The arm itself must be used together with the balancing bench, which provides the pressure to the diaphragm cylinders to passively support and actuate the arm. Figure 7 shows a section view of the balancing bench and the video segment Interaction between the arm and balancing bench illustrates the interaction between the arm and its balancing bench. The balancing bench is built from two balancing benches stacked one over the other. The bottom diaphragm cylinder allows the arm to move and the upper cylinder allows the forearm to move. Counterweights mounted on both benches provide the static forces to balance the weight of the robot. Ball screw actuators allow the movement of the arm and forarm from the balancing bench. A moving counterweight makes it possible to compensate for the weight of a payload up to 10 kg.

    Althouth there are some advantages to stacking the balancing benches, this reduces the overall static balancing quality by coupling two degrees of freedom. Figure 8 illustrates the passive behavior of the robot. The potential energy of the system, the function of the arm and the forarm position have been estimated by measuring the static balancing errors. Since physical systems naturally move from a high to a low potential energy state whenever possible, the robot will naturally move from the red to the blue regions, i.e. towards the horizontal position.

    Fig. 8 : Graph of potential energy estimated by measuring the static balancing errors.

    Figure 9 illustrates the forces needed to backdrive the robot from the horizontal position. Because it is easy to manually move the robot, it is possible to teach it a trajectory. The video segment Programming by demonstration shows the teaching of a simple task while the robot is in the very secure passive mode.

    Fig. 9 : Schema of backdrivable forces.

    Finally, the animation Complete assembly gives an overview of the complete integration of the system. The balancing bench is located outside the robot workspace. Steel pipes filled with water link the robot to the balancing bench. The tanks which fill the system are mounted on top of the structure.