Data acquisition, conversion and distribution

A discrete time (digital) control system takes analog signals from the plant, converts them to numbers (data acquisition and conversion), processes them in a computer, and sends the result back to the plant as analog signals (data distribution).

Block diagram

 DATA ACQUISITION
 Sensor -> Amp -> LPF --+
 Sensor -> Amp -> LPF --+--> Analog --> S/H --> A/D
 Sensor -> Amp -> LPF --+     MUX                |
                                                 v
                                              Digital
                                              computer
 DATA DISTRIBUTION                               |
 Actuator <- Hold <--+                           v
 Actuator <- Hold <--+-- Demux <-- D/A <-- Register

Data acquisition

The data acquisition system collects analog signals from many points of the plant and prepares them for conversion.

1. Transducer/sensor: converts the physical variable (temperature, speed, pressure, position) into a voltage or current.
2. Amplifier: raises the low-level sensor signal to the range of the A/D converter (e.g. 0 to 10 V) and gives isolation.
3. Low-pass (anti-aliasing) filter: removes high-frequency noise so that the signal is band-limited below half the sampling frequency, avoiding aliasing.
4. Analog multiplexer: a set of electronic switches that connects one of many analog channels at a time to the single S/H and A/D, so one converter serves many inputs (time sharing).

Data conversion

5. Sample-and-hold (S/H): takes the sample of the selected signal at the sampling instant and holds it constant while conversion takes place.
6. A/D converter: quantizes the held voltage and codes it into an n-bit binary number. The computer reads this number.
7. Digital computer/controller: compares the measured values with the set points and computes the control action $u(kT)$ using the control algorithm (PID, lead/lag, state feedback).

Data distribution

8. Register/D/A converter: the output word is latched in a register and converted into an analog voltage by the D/A converter.
9. Demultiplexer: routes the D/A output to the correct output channel; a single D/A can serve many actuators.
10. Hold circuit (ZOH): keeps each channel's output constant between updates, producing a staircase signal.
11. Actuator: (motor, valve, heater driver) applies the control action to the plant.

Timing

A clock (timer) gives the sampling period $T$. In each period the computer selects a channel, samples, converts, computes, and updates the outputs. The whole cycle must finish within $T$.

Example: in a boiler, temperature, pressure and level sensors feed one multiplexer and one 12-bit ADC; the computer computes valve openings and sends them through a D/A converter and demultiplexer to the fuel and feed-water valves.