Digital Position Control

Abstract

In this chapter, position control and its role within motion-control systems is introduced and discussed. Single-axis position controllers are explained and modeled. Analytical design of the position controller structure is given, along with procedures for setting the adjustable feedback parameters. The speed and torque system limits are explained at the end of the chapter, along with the analysis of nonlinear operating modes. Nonlinear control laws, capable of securing a robust large step response, are considered. In the succeeding sections, the structure and parameter setting of linear discrete time position controllers are discussed, analyzed and designed. Within motion-control systems, the position controller may assume different roles and forms. Its basic purpose is to provide for the corrective action that drives the mechanical load, work piece, or tool along a predefined tra-jectory in space. Position trajectories are created by a superior controller, such as the production-cell computer, and they depend on specific operations to be performed within a desired production cycle. At the output, position controllers have to provide the reference values, calculated in such a way that any position error caused by the load disturbance or a reference change, is suppressed and driven to zero. In a number of motion-control applications, position controllers provide the reference of the driving torque and feed this reference to the torque actuator. In other cases, the position controller calculates the speed the system ought to assume in order that the position error is dissipated. In such cases, the output of the position controller is fed to the reference input of the speed controller. Eventually, the torque commands are calculated so as to correct the speed error, assisting the position control task in an indirect way. The physical location of position controllers varies with the application. The host computer in traditional motion control systems, known as the CNC (Computerized Numerical Control), generates the reference profiles for a number of motors that have to effectuate a coordinated motion. In most cases, the CNC comprises individual position controllers. Position control functions are often implemented by relying on dedicated hardware units called the axis cards, located within the CNC. The host computer may control the individual drives by sending them the speed reference. In this case, the speed control function is performed by digital drive controllers,