234 Chapter 11 Following on from Chapter 8, we will develop further the topic of driving small motors, since this is a significant application area and illustrates some important real-time control principles. Printers, DVD players, computer hard drives, robots, motor vehicles and many other consumer and industrial products contain microprocessor-controlled motors. The various types of motors have their own drive requirements and dynamic characteristics that must be taken into account in the program design, and can cause complications in practice. The simple permanent magnet brushed direct current (dc) motor is taken as a starting point. 11.1. Motor Control There are two main types of control system, open loop and closed loop. An open loop system is essentially manually controlled or involves operating a load under fixed conditions. For example, a cooling fan will usually not need precise speed control, and might simply be switched on and off from a fixed voltage supply. A closed loop system uses sensors to monitor the system outputs and control them automatically, so, in a motor, the output speed or position is more precisely controlled. The dynamic response (i.e. when there is a change in speed or position) should then be more predictable, particularly when starting or stopping. Position control in a robot arm (Figure 11.1) is a good example of a motor application using digital