Digital control system block diagram and advantages

A digital control system is a closed-loop system in which a digital computer acts as the controller. The plant is analog, so the system contains A/D and D/A interfaces, and the controller works only at sampling instants $t = kT$.

Block diagram

 r(kT) -->(+)--> Digital  --> D/A --> Hold
           ^ -   computer             (ZOH)
           |                            |
           |                            v
          A/D                       Actuator
           ^                            |
           |                            v
          S/H                         Plant ---> y(t)
           ^                            |
           +-------- Sensor <-----------+

Explanation of blocks

1. Sensor (transducer): measures the controlled variable (speed, temperature) and gives an analog voltage.
2. Sample-and-hold (S/H): samples the sensor signal every $T$ seconds and holds the value steady while it is converted.
3. A/D converter: quantizes the held value into one of $2^n$ levels and codes it as an n-bit binary word.
4. Digital computer: reads the digital value, compares it with the reference $r(kT)$ and computes the control output $u(kT)$ from a difference equation (e.g. PID or lead compensator).
5. D/A converter: converts the binary output into an analog voltage.
6. Hold circuit (ZOH): keeps the D/A output constant between updates, giving a staircase signal $u(t)$.
7. Actuator: power amplifier, motor or valve that applies the control effort to the plant.
8. Plant: the continuous process whose output $y(t)$ is controlled.
9. Clock: generates the sampling instants and keeps S/H, A/D, computer and D/A in step.

Operation

Every sampling period: sample, convert, compute, output, hold. Between samples the plant runs open-loop with the held input. If the sampling period is small compared with the plant's time constants (8 to 10 samples per cycle of oscillation), the system behaves almost like a continuous one.

Signal types in the loop

• Continuous analog: $y(t)$, $u(t)$.
• Sampled (discrete-time) analog: S/H output.
• Digital (quantized and coded): inside the computer.

Advantages of digital control over analog control

• Flexibility: the control law is software; it can be changed (P to PID, new gains) without rewiring hardware.
• Accuracy and repeatability: digital values do not drift with temperature, ageing or component tolerance.
• Noise immunity: binary signals are far less affected by noise and interference than analog voltages.
• Complex control laws: nonlinear, adaptive, optimal and self-tuning control are easy to code but very hard to build with op-amps.
• Time sharing: one computer can control many loops through multiplexing, reducing cost per loop.
• Data handling: easy storage, logging, display, alarms and communication over networks (SCADA, DCS).
• Lower cost and size: cheap microcontrollers replace bulky analog hardware.
• Reliability and diagnostics: self-checking and fault detection are built into software.

Limitations (for balance)

• Sampling and the ZOH add phase lag, which can reduce stability if $T$ is large.
• Quantization adds a small error (noise).
• Needs A/D and D/A interfaces and software design effort.

Example: a CNC machine tool uses a microcontroller to read encoders, run a PID position loop at 1 kHz, and drive the axis motors through PWM amplifiers.