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Control systems span four major areas: temperature, pressure and flow, voltage and current, and motion. Motion control is implemented with three major prime movers: hydraulic, pneumatic, and electric motors. This chapter will provide an overview of electric motors and their associated drives.
Motor control requires three basic elements: a motor, a drive, and one or more feedback devices. The drive controls current in order to produce torque; drives also commonly control velocity and sometimes control position. In many control systems, multiple axes of motion must be controlled in synchronization. Usually a separate multiaxis controller provides coordination for the machine while the drives are left to implement the control associated with individual motors. A typical configuration is shown in Figure 15-1. There are many variations of this configuration, including systems that have only one axis and thus have no need of a multiaxis controller and systems where the application is so well defined that the drive and controller are integrated into one device. An example of an integrated system is a computer hard-disk controller.
There are many control functions to be implemented for each axis of motion. First, a position profile, which is a sequence of position commands versus time, must be generated. Second, a position loop must be closed, and usually within that, a velocity loop is closed. The output of the position/velocity loop is a torque command. In brushless motors (including permanent-magnet brushless DC and induction motors), the torque command and the motor position are used to calculate multiple current commands in a process called commutation; for brush motors, commutation is mechanical. Then the current loop(s) must be closed. A power stage delivers power to the motor and returns current feedback. These functions are shown in Figure 15-2.
Figure 15-1. Typical multiaxis motion-control system.
Figure 15-2. Functions of a single axis of motion.
15.1 Definition of a Drive
Defining the term drive can be difficult because most of the functions of an axis of motion can be implemented either in a drive or in a multiaxis controller connected to the drive. At one end of the spectrum, the drive is highly intelligent; a single-axis controller (SAC) performs all the functions in Figure 15-2. At the other end, a power block provides just the power stage; the remaining functions are performed by the multiaxis controller. Other configurations fall within this spectrum. For example, the torque drive, shown in Figure 15-2 in dashed lines, provides all functions after the torque command. In this chapter the term drive will imply torque drive, because the focus here is on the control of servomotors, in which the goal is to produce well-regulated torque; the torque command is a natural dividing point for that discussion.
15.2 Definition of a Servo System
Electronic motion control is a multi-billion-dollar industry. Servo motion is a fraction of that industry, sharing it with non-servo motion, such as stepper motors and variable-frequency systems. A servo system is defined here as the drive, motor, and feedback device that allow precise control of position, velocity, or torque using feed-back loops. Examples of servomotors include motors used in machine tools and automation robots. Stepper motors allow precise control of motion as well, but they are not servos because they are run "open-loop," without tuning and without the need for stability analysis.
The most easily recognized characteristic of servo motion is the ability to control position with rapid response to changing commands and disturbances. Servo applications commonly cycle a motor from one position to another at high rates. However, there are servo applications that do not need fast acceleration. For example, web-handling applications, which process rolled material such as tape, do not command large accelerations during normal operation; usually, they attempt to hold velocity constant in the presence of torque disturbances.
Servo systems must have feedback signals to close control loops. Often, these feedback devices are independent physical components mechanically coupled to the motor; for example, encoders and resolvers are commonly used in this role. However, the lack of a separate feedback device does not mean the system is not a servo. This is because the feedback device may be present but may not be easily identified. For example, head-positioning servos of a hard-disk drive use feedback signals built into the disk platter rather than a separate feedback sensor. Also, some applications use electrical signals from the motor itself to indicate speed. This technology is often called sensorless although the name is misleading; the position is still sensed but using intrinsic motor properties rather than a separate feedback device.