Types of signals in digital control

In a digital control system the signal changes its form as it goes around the loop. The letters below show the type of signal at the output of each block.

 y(t) -> S/H -> A/D -> CPU -> D/A -> Hold -> Plant
 (a)     (b)    (d)    (d)    (c)    (c)     (a)

Types of signals

1. Continuous-time analog signal (a): defined at every instant and can take any amplitude. Examples: plant output $y(t)$, actuator input.
2. Discrete-time analog (sampled-data) signal (b): defined only at the sampling instants $kT$, but its amplitude is still continuous. Example: output of the sampler/S/H.
3. Continuous-time quantized signal (c): defined at all times but takes only certain amplitude levels. Example: staircase output of a D/A converter with its latch/hold.
4. Digital signal (d): discrete in time, quantized in amplitude and coded in binary. Example: numbers inside the computer and the ADC output.

Signals of type (b) and (d) are both called discrete-time signals; systems containing them are discrete-time control systems.

Advantages of digital control system over analog

• Flexible: control law changed by reprogramming, not rewiring.
• Accurate and drift-free: no change with temperature or ageing of components.
• Better noise immunity of digital signals.
• Complex/adaptive/nonlinear control is easy to implement in software.
• One computer serves many loops (time sharing), so cost per loop is low.
• Easy data storage, display, alarms and networking.
• Smaller, cheaper, more reliable hardware.