Hybrid force-position control

Context and objectives

At the microscale, many tasks (micro-assembly, chracterization for example) require either position control, force control or both together. A specific atention has to be paid at this scale for contact/non-contact transitions which often happen. For example, micro-assembling using a microgripper and robot requires to close the microgripper (position control) up to ne finger touching the component to be grasped. once the contact established force has often to be controlled. Such transitions are of great importance at the microscale due to the predominance of surface forces over volume forces, the very high dynamics, the lack of physical model for interaction forces, very important uncertainties and strong non-linearities due to the use of active materials.

To tackle this lock, hybrid force-position is studied especially to efficiently control contact transitions and as a conseqence provide an accurate and stable control of component to be assembled. Several controls cheme are studied and combined :

  • external hybrid force-position that enables to control axis either in position while the others in force (through a selection matrix) ;
  • parallel hybrid force position to control one axis sometimes in position and sometimes in force ;
  • impedance control especially to sucessfully control contact/non-contact transitions in a dynamic way through considering the whole force-position transfer.

Automated micro-assembly based on hybrid force-position control

These scheme are used to control a whole micro-assembly process to suceed an accurate grasping of the component, high speed guiding and accurate releasing tasks. Specific assembly strategies have notably been proposed due to the predominance of pull-off forces (forces acting at contact inducing sticking like behavior). The following control scheme is a typical example applied for achieving automated and complex micro-assembly tasks. Xd input is an array of the desired final position of the component to be assembled while Fr is the desired gripping force and fcd an input to define the assembly strategy despite pull-off forces. FCL here are notably force control algorithm based on sliding mode impedance control to bring online identifiability, dynamic control of contact transisitions and providing robustness.

These works notably conducted to sucessfully achieve automated assembly of micro-optical-benches with cycle times smaller than 1 s including identification of object parameters that are initially unknown.

People involved:

Bhawnath Tiwari, Cédric Clévy, Joel Agnus, Philippe Lutz

Selected publications

B. Komati, C. Clévy and P. Lutz, High Bandwidth Microgripper with Integrated Force Sensors and Position Estimation for the Grasp of Multi-stiffness Microcomponents, IEEE/ASME Transaction on Mechatronics (T-Mech), 21(4), pp. 2039-49, August 2016.

B. Komati, C. Clévy, M. Rakotondrabe and P. Lutz, Dynamic Force/Position Modeling of a one-DOF Smart Piezoelectric Micro-Finger with Sensorized End-Effector, IEEE/ASME AIM International Conference on Advanced Intelligent Mechatronics, Besançon, France, 2014.

B.Komati, C. Clévy and P. Lutz, Force Tracking Impedance Control with Unknown Environment at the Microscale, IEEE ICRA International Conference on Robotics and Automation, Hong Kong, China, June 2014.

B. Komati, K. Rabenorosoa, C. Clévy and P. Lutz, Automated Guiding Task of a Flexible Micropart Using a Two-Sensing-Fingers Microgripper, IEEE Transactions on Automation Science and Engineering (T-ASE), A10(3), pp. 515-524, 2013.

B. Komati, M. R. Pac, I. Ranatunga, C. Clévy, D. Popa, P. Lutz Explicit Force Control v.s. Impedance Control for Micromanipulation ASME-IDETC International Design Engineering Technical Conferences & Computers and Information in Engineering Conference, Portland, USA, August 2013.