Design Example: Turntable Speed Control

Many modern devices use a turntable to rotate a disk at a constant speed. For example, a CD player, a computer disk drive, and a phonograph record player all require a constant speed of rotation in spite of motor wear and variation and other component changes. Our goal is to design a system for turntable speed control that will ensure that the actual speed of rotation is within a specified percentage of the desired speed . We will consider a system without feedback and a system with feedback.

FIGURE 1.23 (a) Open-loop (without feedback) control of the speed of a turntable. (b) Block diagram model.

To obtain disk rotation, we will select a DC motor as the actuator because it provides a speed proportional to the applied motor voltage. For the input voltage to the motor, we will select an amplifier that can provide the required power. The open-loop system (without feedback) is shown in Figure 1.23(a). This system uses a battery source to provide a voltage that is proportional to the desired speed. This voltage is amplified and applied to the motor. The block diagram of the open-loop system identifying control device, actuator, and process is shown in Figure 1.23(b). To obtain a feedback system with the general form of Fig. 1.9, we need to select a sensor. One useful sensor is a tachometer that provides an output voltage proportional to the speed of its shaft. Thus the closed-loop feedback system takes the form shown in Fig. 1.24(a). The block diagram model of the feedback system is shown in Fig. 1.24(b). The error voltage is generated by the difference between the input voltage and the tachometer voltage.

FIGURE 1.24 (a) Closed-loop control of the speed of a turntable. (b) Block diagram model

We expect the feedback system of Figure 1.24 to be superior to the open-loop system of Figure 1.23 because the feedback system will respond to errors and work to reduce them. With precision components, we could expect to reduce the error of the feedback system to one-hundredth of the error of the open-loop system.