Control Systems, Automation and Robotics: Sensors, Actuators, PLCs and Loop Behaviour
The control domain tests whether you can describe a system as blocks, predict its steady-state response, and specify the sensor, actuator and program that make it work. Numerical items usually involve a closed-loop gain or a PLC/sensor resolution calculation; descriptive items ask you to classify a loop or choose an industrial sensor.
Open loop versus closed loop
- Open loop: no measurement of the output; cheap, but any disturbance appears directly in the result.
- Closed loop: a sensor feeds the output back, the controller acts on error e = r − b, and disturbances are attenuated.
- Block algebra with negative feedback: Y/R = G/(1 + G·H), where G is the forward path and H the feedback (sensor) gain.
Controllers and actuators
- On/off (two-position) control oscillates around the setpoint; the hysteresis band limits switching.
- PID: proportional acts on present error, integral removes steady-state offset, derivative damps overshoot.
- Actuators: DC and stepper motors (a stepper of 1.8° per step gives 200 steps per revolution), servomotors with encoders, solenoid valves, and pneumatic cylinders.
Sensors
- Temperature: NTC thermistor (resistance falls with temperature), Pt100, thermocouple.
- Position: inductive proximity for metal, capacitive for any dielectric, limit switches, incremental encoders.
- Force: strain-gauge load cell; light: LDR or photodiode; pressure: piezoelectric or piezoresistive.
- Industrial analogue signal is 4–20 mA, where 4 mA is the zero of the range and also distinguishes a broken wire from a genuine zero.
PLC operation
- Cyclic scan: read inputs into memory, execute the program top to bottom, update outputs, repeat. The scan time limits the fastest detectable event.
- Ladder logic expresses series as AND, parallel as OR, and normally-closed contacts as negation.
- Analogue input resolution: with a 10-bit module over 0–10 V, resolution is 10/1023 ≈ 9.8 mV per count.
Worked example
A proportional controller has G = 5 and a sensor with H = 1 under negative feedback, with a 10 V reference. Y = G/(1 + G·H) · R = 5/6 · 10 = 8.33 V, so the steady-state error is 1.67 V (16.7 %). Raising G to 50 gives 50/51 · 10 = 9.80 V, an error below 2 %: increasing gain reduces offset, but pure proportional control never eliminates it.
Traps that cost marks
- Forgetting the sensor gain inside H, which changes the closed-loop result.
- Claiming proportional control gives zero steady-state error.
- Reversing the PLC scan order (outputs are written last, not first).
- Treating 0 mA as the zero of a 4–20 mA loop.