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Position/Force Control of Delta Manipulator with Voice Coil Actuator

Academic Project

Project Guide - Dr. Arun Dayal Udai, Mechanical Dept. BIT Mesra & Dr. Sudipto Mukherjee IIT Delhi

Project Members - Durgesh Salunkhe, Anirvan Dutta, Ravi Ranjan, Aaquib Reza Khan

This was an academic project undertaken during my final year. In the project, design and force control of a 3DOF parallel manipulator was implemented. The proposed manipulator used a direct drive voice coil arc actuators to achieve compliance required for human-robot interaction or soft mechanical manipulations. We were successful in implementing a unique method
of controlling position as well as the force at the end-effector of the delta manipulator. The method used in making the manipulator compliant did not need an explicit force sensor.

The parallel manipulators are widely used in industrial applications due to its advantage in load supporting capability. A parallel manipulator can be identified as a closed loop kinematic chain whose end-effector is connected with the base through several kinematic chains that are independent from each other. A Delta manipulator is a one such parallel mechanism with 3 DOF (Degree of Freedom) with all the linear degrees of freedom.

It consisted of 3 Voice Coil Actuators with position encoders, from each voice coil actuators two links used to connect the top platform with a revolute joint attached with the voice coil and two spherical joints joining the 1st link to 2nd link and 2nd link to the top platform. Dual parallelogram mechanism was used to increase the stability of the design. Linear springs were attached to the links of the manipulator which performed dual task of maintaining any intermediate position of the manipulator as well as contributing to the compliance of the manipulator.

Delta Manipultor

The proposed controller scheme provides a method to control the position as well as torque of the delta manipulator. To achieve accurate position, PID (Proportional, Integral, Derivative) Controller was implemented with certain modification. Current limiting based approach was applied to control the torque applied by the Delta Manipulator. The proposed control scheme takes care of non-linearity of the system satisfactorily and also provides a passive compliance. In the proposed design for attaining any given equilibrium position the torque provided by the Voice Coil Actuator balanced the resisting torque provided by the spring. As the spring was kept vertical initially, the linear springs provided a torque proportional to angle traversed i.e more torque has to be provided by the Voice Coil Actuator to traverse a greater angle.

When the enjoined motion of the robot was hindered, or cramped by the surrounding, the robot forcefully tries to overcome the restriction in order to attain the requisite position. Under these conditions the actuator draws large amount of current which fails the purpose of compliance. Thus, an external current loop is employed to control the amount of torque provided by each link of the voice coil actuator. The amount of current drawn at each position in the workspace was calculated using static analysis of the delta.

The driving system with the controller proposed has an added advantage of compliance in the manipulator. We compared the desired
and actual torque required to move the links and the results were quite promising. The proposed manipulator can be used in applications where human robot interaction is involved and is not controlled solely by a stiff position-time controller. As passive compliance does not require a sophisticated control algorithm. Such robots are inexpensive and are equally safe as a active force compliant system and do not affect the precision of end operation. For high torque applications, active force compliance should be preferred over passive compliance.

Due to our novel design and control scheme, we published the result of our project at Advances in Robotics 2017, 3rd International Conference of Robotics Society of India, IIT Delhi.

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