Sliding mode control with double boundary layer for robust compensation of payload mass and friction in linear motors

Direct drives with linear motors are recently attracting the attention of both industry and academia. The main peculiarity of these systems is the lack of mechanical reduction and transmission devices, which makes the influence of some uncertain electromechanical phenomena (e.g. friction, cogging forces, etc.) and load disturbances much more significant than in the case of conventional rotary actuators. This paper describes a control system for a tubular synchronous linear motor based on a sliding mode controller (SMC) and a Pi-based equivalent disturbance observer. The distinctive peculiarities of the proposed scheme are the use of a control law that guarantees stability of the system regardless of the payload mass, the adoption of a double boundary layer that efficiently contrasts the harmful effects of static friction, and the introduction of a simple Pi-based equivalent disturbance observer that avoids steady state errors regardless model uncertainties and external disturbances. The reduced computational cost of the control alongside with the introduction of effective design criteria for the SMC and disturbance observer make the implementation of the proposed approach as simple as standard cascaded linear control schemes using industrial microcontrollers. The above considerations are validated by extensive experiments.