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Experiments on force control of a multi-flexible-link robot

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Title Experiments on force control of a multi-flexible-link robot
Author Malzahn, Jörn
Balachandran, Ribin
Bertram, Torsten
License CC Attribution 3.0 Unported:
You are free to use, adapt and copy, distribute and transmit the work or content in adapted or unchanged form for any legal purpose as long as the work is attributed to the author in the manner specified by the author or licensor.
DOI 10.5446/15412
Publisher TU Dortmund, Lehrstuhl für Regelungssystemtechnik
Release Date 2013
Language Silent film
Production Year 2013
Production Place Dortmund

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Subject Area Engineering
Abstract Structural elasticity represents an undesired effect in a variety of technical systems such as fire rescue turntable ladders, concrete pumps, cherry pickers, cranes and robots. Oscillations prolong settling times and static deflections reduce accuracy. Avoiding structural elasticity therfore most often is a design criterion. However, in this video we intend to show the other side of the coin by exploiting the potential of the elastic properties to sense contact forces. Elasticity is intentionally introduced in an experimental structure and accounted for in the control of the mechanism. The control concept sufficiently mitigates the oscillations as shown in the beginning of the video and position accuracy in the presence of varying payloads can be improved e.g. by means of visual servoing as exemplified in another video (https://www.youtube.com/watch?v=V2NnEU6yGEA). The control concept behind the video follows an independent joint control strategy. The joint angles of each actuator are controlled by a cascaded position controller with an inner velocity and a motor-current loop. The torques acting on the individual joints due to oscillations, gravitational influences and physical interactions of the robot with it's environment are inferred via strain measurements on each link. This information is fed back to each independent joint motion controller to actively influence the reflected joint compliance while simultaneously damping oscillations. Oscillations may occur because of high joint accelerations as well as unforeseen but also planned interactions with the environment. These oscillations are damped regardless of their source. The control concept allows to shape the reflected compliance such that the probability of breaking even fragile objects in case of accidental collisions is significantly reduced. Time-line: 00:12 Oscillation damping during step motion from [0°, 0°, 0°] to [0°, 45°, -45°] 00:27 Damping oscillation due to external impacts 00:37 Passive compliance test at the tip using a soft-ball. With just passive compliance it is clearly visible that the soft-ball gets compressed. 00:58 Active compliance test at the tip using a soft-ball. The compression of the ball is hardly visible. 01:16 Active compliance tests at different points along the structure using a soft-ball. Conventional robots can be equipped with force/torque sensors at the tip. Force control laws enable a user to grab the robot at this sensor and guide it to another desired position. In contrast, the example shows that the flexible links allow the robot to be grabbed along the structure to perform this guidance. 01:34 Pushing the robot at the tip using a feather. 01:45 Accidental collision with a feather in the path and no force control. The robot tries to reach the commanded joint configuration at all cost and breaks the feather. 02:00 Accidental collision with a feather in the path and *activated* force control. The controller limits the force exerted on the feather and stops the robot. Once the feather is removed from the path, the robot approaches the desired joint configuration. 02:12 Accidental collision with a Christmas ball in the path and no force control. Similar to the feather experiment without force control the Christmas ball breaks if the end-effector destination corresponding to the desired joint values lies within the ball. 02:37 Accidental collision with a Christmas ball in the path and *activated* force control. Again, the force controller reduces the exerted forces and saves the Christmas ball.
Keywords oscillation damping
collision detection
flexible robot
elastic link
flexible link
compliance
force control
robotics

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