A sampled data system operates on discrete-time rather than continuous-time signals. A digital computer is used as the controller in such a system. A D/A converter is usually connected to the output of the computer to drive the plant. We will assume that all the signals enter and leave the computer at the same fixed times, known as the sampling times.
A typical sampled data control system is shown in Figure 6.1. The digital computer performs the controller or the compensation function within the system. The A/D converter converts the error signal, which is a continuous signal, into digital form so that it can be processed by the computer. At the computer output the D/A converter converts the digital output of the computer into a form which can be used to drive the plant.
THE SAMPLING PROCESS
A sampler is basically a switch that closes every T seconds, as shown in Figure 6.2. When a continuous signal r (t ) is sampled at regular intervals T , the resulting discrete-time signal is shown in Figure 6.3, where q represents the amount of time the switch is closed.
In practice the closure time q is much smaller than the sampling time T , and the pulses can be approximated by flat-topped rectangles as shown in Figure 6.4.
In control applications the switch closure time q is much smaller than the sampling time T and can be neglected. This leads to the ideal sampler with output as shown in Figure 6.5.
The ideal sampling process can be considered as the multiplication of a pulse train with a continuous signal, i.e.
A sampler and zero-order hold can accurately follow the input signal if the sampling time T is small compared to the transient changes in the signal. The response of a sampler and a zero-order hold to a ramp input is shown in Figure 6.9 for two different values of sampling period.
