I85PREP
⌘ K
Galicia PAU Technology and Engineering II study materials
Galicia PAU Technology and Engineering II study resources

Galicia PAU Technology and Engineering II: Control systems and automation: sensors, actuators, open/closed loop, PLCs and robotics

Galicia PAU Technology and Engineering II · Work through your exam, one topic at a time.

All guide sections

Control systems and automation: sensors, actuators, open/closed loop, PLCs and robotics

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: PT100, 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 does not measure the output; it is valid only for stable, predictable processes.
  • Closed loop uses feedback: error e = setpoint − measured value. On/off control uses hysteresis; proportional (K_p) acts on the present error, integral removes steady-state error, derivative anticipates and damps oscillation.

PLCs and robotics

  • Scan cycle: read inputs → execute the program → update outputs. Scan time limits the response speed.
  • Digital I/O (24 V DC) and analogue I/O (0–10 V, 4–20 mA); addressing %I, %Q, %M.
  • 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 4 mA live zero is what allows a broken wire (0 mA) to be detected. With a 10-bit ADC over 0–10 V, the resolution is 10/1024 ≈ 9.8 mV.

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.