Data acquisition block diagram and DCS advantages

Data acquisition system

A data acquisition system collects analog signals from sensors in the plant, conditions them and converts them into digital numbers that the computer can process.

 Sensor1 -> Amp -> LPF --+
 Sensor2 -> Amp -> LPF --+--> Analog -> S/H -> A/D -> CPU
 Sensor3 -> Amp -> LPF --+     MUX      ^      ^
                                |       |      |
                                +-------+------+
                                |
                         Timing and control

1. Transducers: convert physical quantities (temperature, pressure, speed) into electrical signals.
2. Amplifier/signal conditioner: raises millivolt-level signals to the ADC range, provides isolation and rejects common-mode noise.
3. Low-pass (anti-aliasing) filter: removes high-frequency noise and limits the bandwidth below $\omega_s/2$.
4. Analog multiplexer: connects the channels one by one to a single S/H and ADC, so one converter serves many inputs.
5. Sample-and-hold: samples the selected signal at the sampling instant and keeps it steady during conversion.
6. A/D converter: quantizes and codes the value into an n-bit word.
7. Timing and control: selects the channel, gives sample/hold and start-of-conversion commands, and tells the computer when data are ready.

Operation: in every sampling period the channels are scanned in turn; each is sampled, held, converted and stored in memory. The computer then runs the control algorithm.

Example: a water treatment plant scans pH, turbidity and flow sensors through one 12-bit multiplexed ADC every second.

Advantages of digital control system

• 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.