Huang Sunan

Precision Motion Control: Design and Implementation

Precision Tracking Motion Control.- Automatic Tuning of Control Parameters.- Co-ordinated Motion Control of Gantry Systems.- Geometrical Error Compensation.- Electronic Interpolation Errors.- Vibration Monitoring and Control.- Other Engineering Aspects.

Precision Motion Control

PI offers the world’s largest portfolio of precision motion technologies for positioning in the accuracy range from one micrometer down to below one nanometer. Fast settling or extremely smooth low speed motion, high positional stability, high resolution and high dynamics – the requirements placed on piezo mechanisms vary greatly and need drivers and controllers with a high degree of flexibility. PI provides a broad spectrum of piezo electronics from versatile general purpose controllers to highly specialized devices. Units come in different levels of integration from customized OEM boards and a plug & play bench-top devices to modular controllers to scalable to almost any number of axes. Piezo mechanisms directly respond to the smallest change in the drive voltage with a change in displacement. Response times of a few microseconds are possible, depending on the mechanical design and the performance of the piezo controller.

Dead Time Compensators

Dead time compensators were among the first predictive controllers formulated. This is not surprising since for time-delay systems, prediction of the behaviour of the system in the future is necessary to formulate the appropriate control action. For a non-predictive controller, the control action or the effect of control action cannot be realised until some time in the future when the system variables may have changed considerably such that the control action is no longer appropriate. While the proportional-integralderivative (PID) controller is widely used in industry, its performance when used in time-delay systems is known to be limited. If a time-delay system is controlled by a PID controller, the derivative action is often disabled to reduce excessive overshoot and oscillations, due largely to misinterpretation of the non-responsiveness of the system (Hagglund and Astrom, 1991). On the other hand, with no derivative action, there is no predictive capability in the remaining PI control action, which is precisely what is needed for these systems. Dead time compensators are employed when better performance is demanded.

Overview of Nanotechnology

This introductory chapter will briefly explain what nanotechnology is, briefly trace its origins and recent historical developments and then to describe representative nanotechnology processes and applications.

Optimal PID Control Based on a Predictive Control Approach

Proportional-integral-(derivative) (PI(D)) controllers have remained the most commonly used controllers in industrial process control for more than fifty years, despite the advance in mathematical control theory. The main reason is that they have a simple structure, which is easily to be understood by engineers, and under practical conditions it performs more reliably than more advanced and complex controllers.

Precision Tracking Motion Control

Among the electric motor drives, permanent magnet linear motors (PMLM) are probably the most naturally akin to applications involving high speed and high precision motion control. The increasingly widespread industrial applications of PMLM in various semiconductor processes, precision metrology and miniature system assembly are self-evident testimonies of the effectiveness of PMLM in addressing the high requirements associated with these application areas. The main benefits of a PMLM include the high force density achievable, low thermal losses and, most importantly, the high precision and accuracy associated with the simplicity in mechanical structure. PMLM is designed by cutting and unrolling their rotary counterparts, literally similar to the imaginary process of cutting a conventional motor rotary armature and rotary stator along a radial plane and unroll to lay it out flat, as shown in Fig. 2.1. The result is a flat linear motor that produces linear force, as opposed to torque, because the axis of rotation no longer exists. The same forces of electromagnetism that produce torque in a rotary motor are used to produce direct linear force in linear motors.

Bipedal locomotion modeled as the central pattern generator (CPG) and regulated by self organizing map for model of cortex

This paper describes a biologically inspired algorithm mimicking locomotion, and associated brain inspired software architecture to model bipedal gait. The central pattern generator (CPG) neural network is first modeled and next, self-organizing maps of gait for running and walking are created. This biological or brain inspired neural network model is finally assimilated to account for cortical control or modulation of gait. This work demonstrates the utility of using the CPG neural networks and self-organizing maps as the controller to mimic normal and abnormal gait patterns. The work presented here is our first step towards developing a biologically inspired neuroprosthetic device to help stroke patients regain normal gait at a faster pace.

Development for precise positioning of air bearing stages

In this paper, we propose control approaches for air bearing stages. First, we consider the case in which air bearing system is a linear stage. A control scheme incoporated with eddy current braking is proposed, where two control inputs are introduced to handle the nonlinear air bearing system. Secondly, we consider the case in which air bearing system is a spherical stage. The control scheme involves Kalman filter and optimal control designs. Finally, experimental results are given to further verify the effectiveness of the proposed methods.

Vibration Monitoring and Control

Mechanical vibration in machines and equipment can occur due to many factors, such as unbalanced inertia, bearing failures on turbines, motors, generators, pumps, drives, turbofans, etc., poor kinematic design resulting in a non-rigid support structure, component failure and/or operations outside prescribed load ratings. The machine vibration signal can be typically characterised as a narrow-band interference signal anywhere in the range from 1 Hz to 500kHz. To prevent equipment damage from the severe shaking that occurs when machines malfunction or vibrate at resonant frequencies, a real-time monitoring or control device will be very useful. When the machine is used to perform highly precise positioning functions, undue vibrations can lead to poor repeatibility properties, impeding any systematic error compensation effort. This results directly in a loss of precision and accuracy achievable.

Co-Ordinated Motion Control of Gantry Systems

Among the various configurations of long travel and high precision cartesian robotic systems, one of the most popular is the H-type which is more commonly known as the moving gantry system. In this configuration, two motors which are mounted on two parallel slides move a gantry simultaneously in tandem. This gantry configuration has been in use for large overhead travelling cranes in ports, rolling mills and flying shear. When positioning precision is of the primary concern, direct drive linear motors are usually used and fitted with aerostatic bearings for optimum performance. The moving gantry stage is usually designed to provide a high-speed, high-accuracy X, Y and Z motion to facilitate automated processes in flat panel display, printed circuit board manufacturing, precision metrology, and circuit assembly where high part placement accuracy for overhead access is necessary. The stage is equipped with a high power density due to the dual drives, and it can yield high speed motion with no significant lateral offset when the two drives are appropriately co-ordinated and synchronised in motion. In certain applications such as in wafer steppers, the dual drives can also be used to produce a small “theta” rotary motion, without any additional rotary actuators. The application domain of the moving gantry stage is rapidly expanding along with the increasingly stringent requirements arising from developments in precision engineering and nanotechnology.