Direct Amplitude Control for Voice Coil Motor on High Frequency Reciprocating Rig

High-frequency reciprocating rig (HFRR) that utilizes direct drive voice coil motor (VCM) is designed to perform pin-on-flat tribotest such as assessing the lubricity of fuel. In this article, we propose direct amplitude control to maintain the amplitude of high-frequency reciprocating motion of VCM on HFRR subjects to frictional load. First, the mathematical model of HFRR is developed, which takes the effect of frictional load into consideration. Subsequently, direct amplitude control is proposed, which takes the error of amplitude rather than position error as performance index. It consists of an amplitude regulator and an offset compensator. Analysis and simulations of the proposed control method are presented as well. Finally, the proposed control method is deployed to a digital signal processor on an HFRR prototype experimental platform. The experiment results indicate a significant improvement of the performance of VCM-based HFRR in terms of amplitude accuracy in high-frequency band compared with that given by traditional PI control. The results also suggest the instability of PR control in this case.