- 2077 Chaitra · 6+2 marks
Sampling operations and DCS advantages
Short answer
Periodic sampling uses constant interval T; multiple-order has repeating pattern; multiple-rate uses different periods for different loops; random sampling uses event-triggered instants. Periodic is easiest to analyze with z-transform. Advantages: flexibility, accuracy, noise immunity, complex algorithms, multi-loop sharing, easy data handling.
Compare different types of sampling operations used for sampling of continuous time signal. Also mention the advantages of digital control system over analog.
Sampling converts a continuous signal into a sequence of values taken at certain instants. Sampling operations differ in how the instants are chosen.
Types of sampling operation
- Periodic (conventional) sampling: sampling instants equally spaced, . Most common; analysed with the z-transform.
- Multiple-order sampling: the pattern of sampling instants is repeated periodically, i.e. is constant for all , though spacing inside the pattern is unequal.
- Multiple-rate sampling: two or more different sampling periods are used in one system, e.g. fast sampling for an inner current loop and slow sampling for an outer temperature loop.
- Random sampling: instants are random, e.g. event-triggered or network-based control.
Comparison
| Feature | Periodic | Multiple-order | Multiple-rate | Random |
|---|---|---|---|---|
| Instants | Repeating pattern | Different | Random | |
| Analysis | z-transform | Special methods | Multirate methods | Statistical |
| Hardware | Single clock | Moderate | Several clocks | Event logic |
| Typical use | Most controllers | Special systems | Cascade loops | Networks, events |
| Design effort | Least | More | More | Most |
By pulse shape, sampling may also be ideal (impulse), natural (pulse top follows the signal) or flat-top (S/H). Practical controllers use periodic flat-top sampling, modelled as an ideal sampler followed by a ZOH.
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.
✓ Answer checked by Claude Opus 5.5 (high effort)
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