Skillful stick-slip motion control of a Cartesian-type robot

Abstract

Friction known as friction heat, friction noise, friction wear and friction force and so on is an undesirable physical phenomenon in almost all machine systems. In this paper, however, a friction force generated by a stick-slip motion is introduced to improve the lapping quality of small LED lens molds. Here, the lapping means the finishing or polishing of metallic molds by using diamond paste. A novel desktop Cartesian-type robot, which has abilities of compliant motion and stick-slip motion, is first presented for lapping small metallic molds with a curved surface such as an LED lens mold. The robot consists of three single-axis devices with a high position resolution of 1 µm built in Cartesian-space. A thin wood stick tool is attached to the tip of the z-axis. The tool tip has a small ball-end shape with a diameter of 1 mm. The control system is composed of a force feedback loop, position feedback loop and position feedforward loop. The force feedback loop controls the polishing force consisting of contact force in normal direction and kinetic friction forces in tangent direction. It is assumed that the kinetic friction forces are generated by Coulomb friction and viscous friction. In order to improve the lapping performance, a stick-slip motion control method is further added to the control system. The small stick-slip motion is orthogonally generated to the tool moving direction, so that the polishing energy can be finely changed to partially improve the lapping quality. The effectiveness is examined through an actual lapping test of an LED lens mould with a diameter of 4 mm.