Exercises: 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).

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.

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 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).

Explain what a limit switch is, where it is physically installed, and what information it provides to the PLC. Give an example of its use in an automatic garage door.

Every automation system documents its input/output (I/O) list (which sensor connects to each terminal and which actuator comes out of each one). Explain why this documentation is essential years later when a different person needs to repair the machine.

The diagram shows a PLC controlling a conveyor belt: two components send signals to the central unit, which controls two components of the conveyor, as shown in the diagram.

State which device occupies the central position, describe the path of the signals from the push-button and sensor to the motor and piston, and explain where the system's "intelligence" resides (what it decides and by what criteria).

A parking lot 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.

A factory changes its product twice a year and consequently alters the sequence of operations on its packaging line. Explain why with PLCs these changes take hours whereas with hard-wired relays they took days, and what other costs (production downtime, wiring errors, documentation) are reduced with programmed automation.

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).

The timing diagram shows the operation of an automatic parking lot barrier over ten seconds: the upper signal indicates when the barrier is open and the lower signal indicates when the sensor detects a car, as shown in the diagram.

(a) State the moment the car arrives. (b) Calculate how many seconds the barrier remains open. (c) In light of the timing diagram, analyze what risk arises if the car stops under the barrier for longer than expected.

An interlock prevents a parking lot 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.

To automate a machine, one can choose between hard-wired relays, an industrial PLC, or an educational microcontroller. Compare the three (robustness in an industrial environment, ease of changing the process, cost, required training) and reason which one you would choose for a school workshop and which for a food manufacturing factory.

Due to a logic error, a program simultaneously activates the "raise" and "lower" outputs of a motor. Explain what could happen (short circuit or destructive mechanical stress), why in addition to preventing it in software, hardware protection is installed (electrical interlock between the two contactors), and what this protection consists of.

In a bottling plant, it is required to automate a conveyor belt that feeds two consecutive stations separated by a fixed distance: a liquid filling station and a capping station.

1. a) An operator proposes programming the PLC so that a bottle enters, is filled (duration 3.0 s), moves forward to the capper, has the cap placed (duration 1.5 s), and is evacuated before letting the next one in. Calculate the theoretical maximum production (bottles/hour) if each movement of the belt (entry, between stations and evacuation) takes 1.2 s.
2. b) Design the state diagram (GRAFCET) of an efficient concurrent solution, where both stations work at the same time on different bottles once they are in position.
3. c) Define the PLC input/output (I/O) table required for this concurrent solution, including position sensors and an optical quality control sensor that prevents filling if the bottle arrives knocked over.

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.

A plastics injection company is considering replacing manual part removal with an automated system using a PLC-controlled pneumatic pick-and-place manipulator.

• Initial investment in equipment and installation: 32,000 pesos.

• Annual maintenance and consumption of the system: 2,400 pesos/year.

• Two operators rotated on this task (cost of 21,000 pesos/year per operator).

• With the PLC, the cycle time per part drops from 14.0 s to 9.5 s and the defect rate due to handling falls from 3.2 % to 0.4 % over an output of 150,000 parts/year (the cost of reworking a defective part is 0.80 pesos).

1. a) Calculate the net annual economic savings generated by the automation and the payback period of the investment (payback in years).
2. b) In light of the data, analyse the technical transformation of employment in the plant: which routine tasks disappear, which new technical profiles (programming, electromechanical maintenance) become essential, and which ergonomic risks are eliminated for the staff.

Design the PLC control for a pushbutton-operated traffic light for a pedestrian crossing on a busy avenue. The system must prevent lockouts and ensure smooth traffic flow:

• The vehicle traffic light can only turn red if a pedestrian has pressed the pushbutton, but a minimum green time for vehicles of 25.0 s must be guaranteed between cycles, regardless of when it is pressed.

• If a passerby presses repeatedly or keeps the pushbutton jammed with an object, the system must not queue requests or restart timings erroneously.

1. a) Draw up the allocation table for inputs (indicating how the pushbutton edge is handled), outputs (green, amber, and red lights for cars; green and red for pedestrians), and timers.
2. b) Design the complete state diagram (GRAFCET) of the cycle, including the amber warning phase for cars (3.0 s) and the pedestrian flashing green phase prior to closing (4.0 s).
3. c) Predict the safety failure that would occur if the red lamp for vehicles burns out, and propose a design modification (current sensor or feedback) so that the PLC switches to emergency flashing amber mode.

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 centimeters 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.

A chemical reactor is equipped with digital high-level sensors to prevent spills of corrosive substances. Two safety architectures in the PLC are evaluated:

1. a) With two sensors ($S_1$ and $S_2$), formulate the truth table and the simplified Boolean logic function for two distinct outputs: 1) reagent inlet shut-off command (activated if at least one indicates a high level) and 2) discrepancy alarm for a faulty sensor (activated if only one of the two indicates a high level for more than 3.0 s). Explain why this design can cause unnecessary false shutdowns if a sensor fails short-circuited.
2. b) To solve the previous problem, a 2oo3 (two out of three) majority voting system is implemented with three independent sensors ($A$, $B$, and $C$). Obtain the simplified Boolean logic function for the shutdown command and justify how the system reacts to an isolated failure of any one of the three sensors.
3. c) If the probability of a sensor failing with a false positive over one month is $p = 0.05$, calculate with reasoning the probability that the 2oo3 system shuts down the reactor by mistake (at least two sensors failing at the same time) compared to a single-sensor simple system.

On a conveyor line transporting boxes to a palletiser, a photoelectric sensor detects the passage of packages (at a rate of one every 2.0 to 4.0 s when the upstream line is producing normally).

1. a) Design in ladder logic (Ladder) or using a state diagram a "watchdog" type timer that stops the conveyor and activates an alarm signal if the sensor does not detect any box for 25.0 uninterrupted seconds while the drive motor is running.
2. b) Explain two causes of completely different nature that can trigger this condition (a mechanical/operational breakdown and an electrical failure in the sensor loop) and deduce what diagnostic test the PLC or the operator should perform to distinguish between them.
3. c) Justify the economic, energy, and operator safety impact of omitting this timer control and leaving the conveyor running indefinitely with no load.

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 behavior: 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.