Automatic control and robotics: loops, sensors, actuators and PLCs
Questions here test the vocabulary and logic of automation: distinguishing open from closed loop, choosing a sensor, describing how a PLC scans its program, and representing a sequence with GRAFCET.
Open loop versus closed loop
- Open loop: no measurement of the result; cheap and adequate when disturbances are small (stepper positioning, timed irrigation).
- Closed loop: sensor → comparator (error e = setpoint − measurement) → controller → actuator → process, with negative feedback. It corrects disturbances but can oscillate.
- PID: P acts on present error and leaves a steady-state offset; I removes the offset but may overshoot (integral windup); D anticipates the trend but amplifies measurement noise.
Sensors and actuators
- NTC thermistor (resistance falls with temperature), PTC, thermocouple, LDR/photodiode, strain gauges in a Wheatstone bridge, Hall effect, inductive and capacitive proximity detectors, limit switches, incremental encoders with A/B quadrature, absolute encoders, ultrasonic distance sensors.
- Actuators: DC motor, stepper motor (steps per revolution, holding torque), servo, solenoid valve, pneumatic cylinder.
PLC and GRAFCET
- A PLC runs a cyclic scan: read inputs into the input image, execute the program, update outputs, then repeat. Outputs therefore change only at the end of the scan.
- Ladder logic uses contacts in series (AND) and parallel (OR); GRAFCET uses steps, transitions, actions and OR (alternative) or AND (simultaneous) divergence.
Robots
Degrees of freedom, configuration (Cartesian PPP, cylindrical RPP, spherical RRP, SCARA RRP, articulated RRR), workspace, payload, end effector, and the distinction between accuracy (closeness to target) and repeatability (spread of repeated attempts).
Worked example
An incremental encoder gives 500 pulses per revolution and the counter reads 25 000 pulses/s. Speed = 25 000/500 = 50 rev/s = 3000 rpm. A stepper of 1.8° per step has 360/1.8 = 200 steps/rev; for 30 rpm the pulse frequency is 200 × 30/60 = 100 Hz, or 200 Hz in half-step mode.
Traps that cost marks
- Claiming a closed loop is always better; it can be unstable and needs tuning.
- Placing the sensor on the actuator instead of on the controlled variable.
- Confusing accuracy with repeatability.
- Forgetting that a GRAFCET transition fires only when its preceding step is active and the condition is true.
- Updating PLC outputs mid-scan.