Programmable logic controllers in manufacturing

Why it matters

Behind every conveyor, press, packaging machine, robot cell and FMS there is discrete logic: "if the guard is closed and the part is present, start the clamp; after 2 s, start the spindle". Programmable logic controllers (PLCs) replaced cabinets of hard-wired relays for this job in the 1970s and are still the workhorse of factory automation. A production engineer must be able to read a ladder diagram, understand the scan cycle and judge whether a PLC is fast enough for a signal.

Key ideas

What a PLC is. A microprocessor-based controller with ruggedised hardware, designed to read discrete and analog inputs, execute a stored program of logic, timing, counting, sequencing and arithmetic, and drive outputs — in electrically noisy, hot, vibrating environments, continuously.

Why it replaced relays. Relay logic is changed by rewiring; a PLC by editing the program. PLCs are smaller, more reliable (no mechanical contacts in the logic), cheaper for anything beyond a few relays, provide timers and counters without extra hardware, and give diagnostics and network communication.

Components.

• Processor (CPU) and memory — executes the program; memory holds the program, the input/output image tables and data (timers, counters, registers).
• Input modules — discrete (limit switches, push buttons, proximity sensors, photo-eyes) and analog (4–20 mA, 0–10 V from temperature, pressure, level transmitters); they isolate and filter field signals.
• Output modules — relay, transistor or triac outputs driving solenoid valves, contactors, lamps; analog outputs to drives and valves.
• Power supply, programming device (PC software or handheld) and, usually, an HMI and network interfaces. PLCs come as compact (fixed I/O) or modular (rack-mounted cards) units.

The scan cycle. A PLC repeats one cycle continuously:

1. Input scan — read every input into the input image table.
2. Program scan — execute the program rung by rung, top to bottom, left to right, using the image table.
3. Output scan — copy the output image table to the physical outputs.
4. Housekeeping — communications and self-diagnostics. Because inputs are read only once per scan, an input that turns ON and OFF between two input scans can be missed, and the response to an input can take up to about two scans plus the module delays.

Ladder logic. The most common language, drawn like a relay diagram between two vertical power rails.

• Normally open contact ] [ — true when the input bit is 1. Normally closed contact ]/[ — true when the bit is 0.
• Contacts in series make AND; in parallel, OR. The coil ( ) at the right energises when the rung is true.
• Seal-in (latching) circuit: a START push button in parallel with a contact of the motor's own output, in series with a normally closed STOP — the motor stays on after START is released until STOP is pressed. In Boolean form M = (START + M) · STOP′.
• Timers: on-delay (TON) — output turns ON after the input has been true for the preset time; off-delay (TOF); retentive (RTO).
• Counters: up (CTU) and down (CTD) counters with preset values, reset by a separate instruction. IEC 61131-3 defines five languages: ladder diagram (LD), function block diagram (FBD), structured text (ST), instruction list (IL, now deprecated) and sequential function chart (SFC).

Safety. Stop and emergency-stop buttons are wired as normally closed field contacts so that a broken wire stops the machine (fail-safe). Emergency-stop circuits must also act through hard-wired safety relays or a safety-rated PLC, not only through ordinary program logic.

Formulas

T_scan = T_in + T_prog + T_out + T_hk

• Scan time (ms) = input scan + program execution + output update + housekeeping.

T_prog = n_i · t_i

• n_i number of instructions; t_i average execution time per instruction (ms).

T_resp,max ≈ T_filter + 2 · T_scan + T_outdelay

• Worst-case response (ms) from a field input change to the output terminal: the input can change just after an input scan, so it waits almost one scan to be read and one more to be solved and written out.

t_on = L / v ≥ T_filter + T_scan

• For a part of length L (m) passing a sensor at speed v (m/s), the signal must last at least an input filter time plus one scan (s) to be detected reliably.

Worked examples

Example 1 (standard). A PLC program has 4000 instructions at 0.75 µs each. I/O update takes 2 ms and housekeeping 1 ms. The input filter is 10 ms and the output module delay 1 ms. Find the scan time and the worst-case response time.

1. T_prog = n_i · t_i = 4000 × 0.75 µs = 3000 µs = 3 ms.
2. T_scan = 3 + 2 + 1 = 6 ms.
3. T_resp,max ≈ 10 + 2 × 6 + 1 = 23 ms.

Example 2 (GATE level). (a) A rung energises output Y when (A AND B) OR (C AND NOT D). Find Y for A = 1, B = 0, C = 1, D = 0, and for A = 1, B = 0, C = 1, D = 1. (b) Boxes 40 mm long pass a photo-eye on a conveyor. The input filter is 10 ms and the scan time 8 ms. Find the maximum conveyor speed for reliable counting.

1. (a) Y = A·B + C·D′. First case: A·B = 0; C·D′ = 1 · 1 = 1 → Y = 1.
2. Second case: A·B = 0; C·D′ = 1 · 0 = 0 → Y = 0.
3. (b) Required ON time ≥ 10 + 8 = 18 ms = 0.018 s.
4. t_on = L / v ≥ 0.018 → v ≤ 0.040/0.018 = 2.22 m/s.
5. Above this speed some boxes would be missed — use a faster scan, an interrupt or a high-speed counter input.

Common mistakes

• Using a normally closed program contact for a STOP button that is already wired normally closed in the field — the logic inverts and the motor will not start.
• Assuming the PLC reacts instantly; response can be two scans plus module delays.
• Forgetting the seal-in contact, so the motor stops as soon as START is released.
• Writing the same output coil on two rungs — only the last one evaluated wins.
• Relying only on program logic for an emergency stop.

For GATE PI

Expect MCQs on PLC components, the scan cycle, ladder symbols and IEC 61131-3 languages, and on PLC versus relay logic. Short numericals ask for scan time, worst-case response time, or the maximum speed or minimum pulse width for reliable detection; logic questions ask for the output of a ladder rung or the Boolean expression of a circuit such as the seal-in. Practise converting ladder rungs to Boolean expressions and back.

Quick check

1. In which order does a PLC execute the scan cycle?
2. Write the Boolean expression for a start/stop seal-in circuit with motor M.
3. Contacts in series represent which logic function?
4. Scan time 12 ms, input filter 5 ms, output delay 2 ms. Estimate the worst-case response.
5. Why are STOP buttons wired normally closed?

Answers: 1. Input scan, program scan, output scan, housekeeping. 2. M = (START + M)·STOP′. 3. AND. 4. 5 + 2 × 12 + 2 = 31 ms. 5. So that a broken wire or loose terminal stops the machine (fail-safe).