PLC applications: conveyor, bottling and traffic control

Why it matters

Conveyors, bottling lines and traffic signals are the classic PLC case studies in university labs and interviews because together they use every basic tool: start/stop logic, sensors, timers, counters, sequences, interlocks and fault handling. Being able to turn a plain-English description of such a system into an I/O list, a sequence and timing calculations is the core practical skill of a PLC engineer.

Key ideas

A design method that works for any application.

1. Write the I/O list: every sensor (inputs) and actuator (outputs) with addresses.
2. Describe the sequence (steps and transitions — SFC or a state table) and the interlocks that must always hold.
3. Decide timing: delays from distances and speeds, fill times, signal phases.
4. Add fault handling: emergency stop, jam/timeout detection, sensor plausibility, safe state on power loss.
5. Write and simulate the program, then commission on the machine.

Conveyor control.

• Inputs: start, stop (NC), emergency stop, motor overload, photo-eyes or proximity sensors, speed sensor or encoder. Outputs: motor contactor or VFD run/speed, diverter/ejector solenoids, stack lights.
• Start/stop with seal-in; several conveyors in a line are started downstream first and stopped upstream first so material never piles onto a stopped belt.
• Jam detection: if a product blocks (or never reaches) a sensor for longer than the expected transit time plus margin, a TON trips the line.
• Tracking and rejecting: when a product is detected (or found faulty) at one point, the ejector further down must fire when it arrives. Delay = distance/speed for a constant-speed belt; for variable speed, count encoder pulses or use a shift register clocked by the encoder.
• Counting and sorting: a CTU counts products into a batch; sensors at different heights distinguish sizes and set diverter gates.

Bottling line.

• Stations: infeed, filling, capping, labelling, inspection/reject, packing. An indexing line moves all bottles one pitch, stops, and all stations work at once on different bottles; a continuous line uses rotary machines synchronised to the conveyor.
• Filling by time (fixed flow), by level probe, or by flow meter/weight (more accurate). Filling must be interlocked with "bottle present" — no bottle, no fill.
• Cycle time of an indexing line = slowest station time + index time; the slowest station is the bottleneck and the place to add parallel heads.
• Counters track bottles per crate and production totals; reject counts feed quality reports.

Traffic-light control.

• Phases for a simple two-road junction: NS green → NS amber → all red → EW green → EW amber → all red → repeat. The all-red clearance interval lets vehicles clear the junction.
• Implementation: a step sequencer with one TON per phase, or a single cycle timer whose elapsed time is compared with phase boundaries.
• Safety interlock: conflicting greens must be impossible — each green output is blocked by the other's green and amber, independently of the sequence.
• Extensions: pedestrian push-buttons (demand phases), vehicle detectors (actuated control extending green up to a maximum), night mode (flashing amber), and a fail-safe flashing mode on detected faults.

Formulas

t = d / v

• Transit time (s) for distance d (m) at belt speed v (m/s).

t_fire = d / v − t_act

• Ejector delay (s) after detection; t_act = actuator response time (s).

n_counts = d × PPR / (π D)

• Encoder counts for a belt travel d (m); PPR = encoder pulses per revolution; D = drive-roller diameter (m).

t_fill = V / Q

• Fill time (s) for volume V (L) at flow Q (L/s).

T_cycle = max(t_station) + t_index

• Cycle time (s) of an indexing line.

Rate = 60 / T_cycle (per minute) and 3600 / T_cycle (per hour)

C = Σ (G_i + A_i + R_i)

• Traffic signal cycle length (s): green, amber and all-red times of each phase. Green split of phase i = G_i / C.

Worked examples

Example 1 (standard). A photo-eye detects faulty bottles on a belt moving at 0.6 m/s. The reject pusher is 0.9 m downstream and needs 50 ms to extend. (a) After what delay should the PLC fire it? (b) If the drive roller is 0.1 m in diameter with a 500 PPR encoder, how many counts correspond to 0.9 m?

1. t = d / v = 0.9 / 0.6 = 1.5 s.
2. t_fire = 1.5 − 0.05 = 1.45 s (TON preset 1.45 s, started on detection).
3. n_counts = d × PPR / (π D) = 0.9 × 500 / (π × 0.1) = 1432 counts.

Answers: fire after 1.45 s; or after ≈ 1432 encoder counts (the encoder method stays correct if the belt speed changes).

Example 2 (GATE level). An indexing bottling line fills 500 mL bottles at 0.25 L/s, caps in 1.5 s and labels in 1.2 s; indexing takes 0.8 s. (a) Find the cycle time and output per hour. (b) A two-road junction uses NS green 30 s, EW green 20 s, amber 4 s and all-red 2 s after each green. Find the signal cycle length, the number of cycles per hour and the NS green split.

1. t_fill = V / Q = 0.5 L / 0.25 L/s = 2.0 s — the slowest station (capping 1.5 s, labelling 1.2 s).
2. T_cycle = 2.0 + 0.8 = 2.8 s.
3. Rate = 3600 / 2.8 = 1285.7 → 1285 bottles per hour (21.4 per minute).
4. C = (30 + 4 + 2) + (20 + 4 + 2) = 62 s.
5. Cycles per hour = 3600 / 62 = 58.1.
6. NS green split = 30 / 62 = 0.484 (48.4 %).

Answers: 2.8 s and ≈ 1285 bottles/h; C = 62 s, ≈ 58 cycles/h, NS split ≈ 48 %. Adding a second filling head (2 s for two bottles, indexing two pitches) would make capping the bottleneck.

Common mistakes

• Adding all station times for an indexing line — stations work in parallel, so only the slowest one counts.
• Starting upstream conveyors first, causing pile-ups.
• Ejector timing from distance/speed when the belt speed varies — use encoder counts.
• Filling without a "bottle present" interlock.
• Traffic programs where a fault or mis-sequence could show two conflicting greens; the interlock must be independent of the sequence logic.
• Forgetting the all-red clearance interval and amber in cycle-time calculations.
• No timeout on transitions, so a failed sensor stops the line silently.

For GATE ME

Application questions are usually small numericals — transit and ejector delays, fill times, cycle times and throughput, counts per batch, traffic cycle length — plus logic questions asking for the ladder/SFC sequence or the interlock for a described system. Practise writing the I/O list and sequence from a short description, and identifying the bottleneck station.

Quick check

1. A belt moves at 0.8 m/s. How long does an item take to travel 16 m between sensors?
2. An indexing line has stations of 3 s and 2 s and a 1 s index. What is its cycle time?
3. Why are conveyors in a line stopped upstream first?
4. A traffic cycle has greens of 25 s and 25 s with 3 s amber and 2 s all-red after each. What is C?
5. What prevents two conflicting greens if the sequence logic fails?

Answers: 1. 20 s 2. 4 s 3. So downstream belts keep clearing material and nothing piles onto a stopped belt 4. 60 s 5. A hard interlock: each green is blocked by the other road's green and amber outputs