Programmed automation

Explain what a programmable logic controller or PLC is, where it is typically used, and what three operations it performs repeatedly while running (read inputs, execute the program, activate outputs).

Before PLCs, automation was carried out using hard-wired relays: changing the process required rewiring the cabinet. Explain what changed with the arrival of the programmable logic controller and what advantage "changing the program" has over "changing the wiring".

The PLC works by repeating the "scan cycle": reading inputs, executing the program, and updating outputs. Explain this cycle in your own words and why it repeats many times per second.

The inputs of a PLC receive signals and the outputs control devices. Classify these items as a potential input or potential output of a PLC: (a) a start push-button, (b) a motor, (c) a presence sensor, (d) a solenoid valve, (e) a limit switch, and (f) an indicator lamp.

The inputs of a PLC can be digital (on/off) or analogue (a continuous value). Classify these signals as digital or analogue inputs: a start push-button, a presence sensor, a temperature probe from $0$ to $100\ ^\circ\text{C}$, and a float level switch.

The outputs of a PLC control relays or contactors. Explain what an output relay is and why it is required between the PLC electronics and a high-power motor (to isolate the control circuit from the power circuit).

A car park barrier works as follows: a magnetic sensor detects a car, the PLC checks that the barrier is down, activates the motor to raise it, waits a few seconds for the car to pass, and lowers it again. Describe this sequence as an ordered list of steps, indicating at each step whether an input, a program decision, or an output is involved.

The process of a bottling plant follows this timing diagram: when the bottle is detected ($t = 0\ \text{s}$), the conveyor stops; after $0{,}5\ \text{s}$, the filling valve opens for $3\ \text{s}$; after $4\ \text{s}$, the conveyor moves forward again. (a) Describe the complete sequence in ordered steps. (b) Calculate how long the bottle remains stopped under the nozzle. (c) Reason why the valve does not open if no bottle is detected (interlocking).

An interlock prevents a car park barrier from lowering while a car is underneath. Explain what an interlock is in PLC programming, how an output is made dependent on an additional condition (the sensor signal in logical series with the lower command), and why it is considered a safety measure.

In machine safety with PLCs, the principle of "fail-safe" (positive safety) applies: in the event of any power failure or wire break, the system must spontaneously evolve to the state of least risk.

1. a) An installer proposes connecting the emergency stop pushbuttons as normally open (NO) contacts, so that pressing them sends a logical "1" to the PLC input. Technically justify the serious risk of this approach in the event of an accidental disconnection of the conducting wire.
2. b) For each of these three systems, determine the safe state in the event of a total power outage and select the type of actuator or passive mechanism required to guarantee it without CPU intervention:

• 200 kg overhead garage door.

• Inclined conveyor belt lifting ore.

• Hydraulic sheet metal stamping press.

1. c) Explain why the emergency stop must never depend exclusively on the standard PLC program and how it is implemented at the hardware level using dedicated safety relays.

The holding brake of an elevator motor is of the electromagnetic type, actuated by opposing springs: when there is no current in the coil, mechanical springs press the brake shoes against the drum; to release the brake, the coil must be energised.

1. a) Justify why this design complies with the fail-safe principle compared to a brake requiring electric current to apply the shoes, analysing what would happen in the event of a residual current circuit breaker trip or a PLC CPU failure.
2. b) Design the activation/deactivation sequence of the motor and the brake contactor when starting up and when stopping at a floor: why must they not switch at the exact same millisecond if you want to prevent the car from dropping a few centimetres due to gravity or the motor from running while braked?
3. c) Determine how the door opening mechanism and the car actuators must react to a total mains power outage, and describe how a controlled rescue is carried out.

Design the complete PLC automation for a sliding gate for a vehicle car park with the following operating conditions:

• Actuation via reversible motor (signals: Open / Close).

• Limit switch sensors for fully open gate ($FC_A$) and fully closed gate ($FC_C$).

• Infrared photocell in the gate opening ($FT$) and manual opening push button ($P$).

• Open-gate pause timer of 12,0 s before starting the automatic closing.

1. a) Prepare the PLC input and output assignment table, explaining for each sensor whether it should be physically connected as Normally Open (NO) or Normally Closed (NC) based on fail-safe safety against wire breakage.
2. b) Draw the state diagram (GRAFCET) of the system, accurately modelling the safety behaviour: if the photocell beam is interrupted during closing, the gate must stop immediately and reopen completely, resetting the 12,0 s countdown.
3. c) Add to the logic a warning flashing light that activates 2,0 s before the gate starts moving and remains on throughout the entire movement.