Measurement, feedback, PLC and automation
This domain tests whether you can describe a control loop, choose a sensor from its specifications, explain what a PLC does in each scan and justify a robot morphology. Expect questions on sensor characteristics, PID behaviour, PLC I/O and degrees of freedom.
Sensors and actuators
- Key specifications: range, sensitivity (Δoutput/Δinput), resolution (smallest detectable change), precision, accuracy, linearity, hysteresis, response time.
- Typical sensors: PT 100, thermocouple, thermistor; potentiometer, encoder, inductive and limit switches; pressure, flow and level sensors; photoelectric cells.
- Actuators: DC/AC motors, stepper, servo, solenoid valves, cylinders, heaters.
Open and closed loop
- Open-loop action does not use output feedback to correct its command. A system may measure outputs for monitoring while its control remains open loop; disturbance rejection and model error must still be evaluated.
- Closed loop uses feedback: error e = setpoint − measured value. On/off control uses hysteresis; proportional (K_p) acts on the present error, integral action accumulates error and can remove appropriate steady-state errors in a stable, unsaturated loop. Derivative action responds to rate of change and can improve damping in suitable tuning, but amplifies noise and does not guarantee stability.
PLCs and robotics
- Scan cycle: read inputs → execute the program → update outputs. Scan time limits the response speed.
- Common examples include 24 VDC digital signals and 0–10 V or 4–20 mA analog ranges. Actual levels, addresses and scan behavior depend on the specified PLC and module; no live wiring or machine operation is prescribed.
- Robotics: degrees of freedom, prismatic and rotational joints, workspace, end effector, repeatability versus accuracy, and morphologies (Cartesian, SCARA, anthropomorphic).
Worked example
A 0–10 bar pressure transmitter gives 4–20 mA. Sensitivity = 16 mA/10 bar = 1.6 mA/bar. A reading of 12 mA means (12 − 4)/1.6 = 5 bar. If it were a 0–20 mA signal, 12 mA would mean 6 bar: the live-zero range makes a 0 mA reading distinguishable from an ordinary zero measurement. It can indicate a wiring or supply fault, but does not uniquely diagnose one cause. With a 10-bit ADC over 0–10 V, the ideal bin width under a 1024-bin convention is 10/1024≈9.8 mV. Endpoint scaling may instead use 1023; the convention must be stated.
Marks lost
- Confusing resolution with sensitivity, or precision with accuracy.
- Claiming a PID loop is open loop.
- Ignoring the 4 mA offset when converting current to pressure.
- Getting the scan-cycle order wrong.
- Confusing repeatability (spread when repeating) with accuracy (closeness to the true value).
- Quoting a sensitivity without units.