7R22. Precision Motion Control: Design and Implementation. - Tan Kok Kiong, Lee Tong Heng, Dou Huifang, Huang Sunan (Dept of Elec Eng, Natl Univ, 4 Engineering Dr 3, Singapore, 117576, Singapore). Springer-Verlag London Ltd, Surrey, UK. 2001. 232 pp. ISBN 1-85233-328-6. $89.95.
Reviewed by PH Meckl (Sch of Mech Eng, Purdue Univ, 1288 Mech Eng Bldg, W Lafayette IN 47907-1288).
As both mechanical and electronic components decrease in size, the need for precision manufacturing continues to grow. The authors of this book have skillfully brought together a variety of essential ingredients for success in achieving precision motion control. Their objective is to focus on “enabling technologies in the realization of precision motion positioning systems.” The book presents concepts in “a manner amenable to a broad base of readers, ranging from the academics to the practitioners, by providing detailed experimental verifications of the developed materials.” The book serves as a useful reference for anyone interested in applying the latest technologies to precision motion control.
The authors set the stage by describing a variety of processes in precision manufacturing, including nanofabrication techniques such as lithography, ultra-precision machining, and laser micromachining. They also briefly describe precision metrology devices, including the Scanning Tunneling Microscope (STM) and the Atomic Force Microscope (AFM).
After this short introduction, the book focuses on the development of motion control techniques for precision tracking. The primary actuator considered during controller development is the permanent magnet linear motor (PMLM). Its dynamic model includes the standard electrical and mechanical properties as well as force ripple and nonlinear friction effects as described by the so-called Tustin model. A composite motion control scheme is proposed, which includes feedforward compensation, proportional-integral-derivative (PID) feedback, and nonlinear compensation using Radial Basis Functions. Acceleration feedback is suggested to improve tracking, and a disturbance observer is developed to cancel unknown load disturbances. For more challenging control applications, the book proposes a robust adaptive controller using sliding modes and parameter adaptation to compensate for friction and ripple effects. A feedforward iterative learning controller is also developed for those applications that are repetitive. For each control development, the authors present stability proofs as well as experimental results to demonstrate the effectiveness of each technique.
Since performance of the PID feedback controller depends heavily on the validity of the parameters used in its development, the authors devote an entire chapter to auto-tuning techniques for determining model parameters. They propose a novel two-channel relay auto-tuning technique that is based on a describing function analysis. This approach uses a second parallel relay signal with an integrator so that the relay auto-tuning approach can be applied to servo-mechanical systems that typically do not have the time delays found in chemical process control systems.
Another chapter is devoted to coordinated motion control of several axes. After describing classical master-slave and set-point coordinated control, they propose a fully coordinated control approach that includes inter-axis offsets deduced from a disturbance observer. Again, a complete set of experimental results is provided to compare the effectiveness of each of these approaches.
Another crucial ingredient in any precision motion control scheme is precise measurement of position. A laser measurement system, which uses a linear interferometer and retro-reflector, is described in detail. Then the principles behind geometric error calculation are presented. Measurements from the laser system are compared to those obtained from optical linear encoders, and linear, angular, straightness, and squareness errors are computed. Finally, several approaches to geometric error compensation are described, including look-up tables and a nonlinear parametric approximation that uses Radial Basis Functions. The authors also provide a clever technique to remove bias errors when averaging several measurements that are corrupted by random errors. Since electronic interpolation is required to extract sufficient resolution out of optical encoders, another chapter provides details of several approaches to reduce interpolation errors.
The last important topic for achieving precision motion control is vibration attenuation. The authors focus on external vibration sources and propose two approaches to reduce errors caused by vibrations. One approach uses an adaptive digital notch filter “to identify the resonant frequencies and to subsequently terminate any signal transmission at these frequencies.” The second approach uses an accelerometer mounted on the machine to monitor vibration levels, analyzes their frequency content, and determines when these vibrations exceed the threshold of acceptable performance.
Since all of these control algorithms are implemented in software, another chapter is devoted to details of the dSPACE control platform, which includes automatic code generation using MATLAB Simulink, hardware-in-the-loop testing capability, and a virtual instrument panel as user interface.
Unfortunately, the book does not mention the concept of input shaping for vibration control. In many cases, the motions themselves can cause detrimental vibrations that cannot be tolerated by precision machines. Input shaping modifies the motor commands so that these vibrations are greatly reduced.
However, besides that minor omission, Precision Motion Control: Design and Implementation nicely integrates a number of important topics in precision motion control. It also comes with a complete set of references for further information. All told, it represents a useful reference and an excellent single source of essential topics related to precision motion control.