Lab 20: Capstone — Complete Conveyor Station
Application Scenario
A distribution center is installing a new outbound conveyor that carries packed cartons to the shipping dock. You are the controls technician. The customer has written a specification. Your job is to design, document, build, test, and demonstrate the control circuit, using only relay logic and the devices on your trainer.
This lab does not give you a step-by-step drawing guide. Every requirement below is something you have already built in an earlier lab. The challenge is to combine them into one working, safe machine.
Learning Outcomes
- Translate a written specification into a complete ladder diagram.
- Choose and combine circuit patterns from earlier labs.
- Plan the use of limited relay contacts and timer functions.
- Produce professional documentation: ladder diagram, contact allocation, node table, and wiring table.
- Test a machine systematically against its specification and demonstrate it to a customer.
Device Assignment
You must use these assignments so that everyone's machine can be tested consistently.
| Device | Function |
|---|---|
| PB4 (N.C.) | EMERGENCY STOP (hold to simulate a latched E-stop) |
| PB3 (N.O.) | RESET |
| PB1 (N.O.) | CONVEYOR START |
| PB2 (N.C.) | CONVEYOR STOP |
| CR1, CR2 | Control relays — your choice of use |
| TR1, TR2, TR3, TR4 | Timers — your choice of use (TR4 may be set to any mode) |
| R1 | START-UP WARNING (horn and beacon) |
| G1 | CONVEYOR MOTOR running |
| G2 | BELT-CLEANING SPRAY (Level 3 only) |
| L3 | Switched control bus |
Specification
The specification is divided into three levels. Level 1 is required to pass. Levels 2 and 3 raise your grade. Complete and test each level before starting the next.
Level 1 — Safe start with warning (required)
- Emergency stop: an E-stop circuit with a master control relay feeds the switched bus L3. Every rung that can start or run the conveyor, warning, or spray must be on L3. After power-up or an E-stop, nothing can run until RESET is pressed (with the E-stop released). RESET must not start anything.
- Start request: PB1 starts, and PB2 stops, as a three-wire circuit with a seal-in.
- Start-up warning: when START is pressed, R1 turns ON for 5 s. The conveyor must not run during the warning.
- Conveyor: at the end of the warning, R1 turns OFF, and G1 turns ON.
- E-stop response: Pressing PB4 at any time immediately turns all outputs OFF.
Level 2 — Belt-clearing run-on
- When STOP (PB2) is pressed, the conveyor runs for 5 s to clear the belt, then stops.
- An E-stop during the run-on immediately stops the conveyor.
Level 3 — Belt-cleaning spray
- Each time the conveyor motor starts, G2 (spray) turns ON for 3 s, then OFF. The spray must not repeat while the conveyor keeps running, and it must not come on during the run-on.
- An E-stop turns G2 OFF immediately.
Required Behavior (Timing Diagram)
Your finished machine must behave as shown. The second cycle shows an E-stop during the start-up warning.
Design Constraints
- Use only the devices on the trainer. Check each relay's and timer's number of contact sets before you commit to a design.
- No more than two wires under any terminal screw.
- The only connection from L1 to L3 is through the master control relay's N.O. contact.
- A timer trigger (TR2 or TR4, off-delay mode) is a dry-contact input: wire a contact directly across pins 5 and 6; never connect 5 or 6 to L1, L2, or L3.
- A full Level 3 design needs about 13 grid lines and about 30 wires. Use two Ladder Diagram Worksheets (the second for wiring rows 24 and up).
Design Process
Step 1 — Choose the pattern for each requirement
Fill in the right-hand column before you draw anything.
| Requirement | An earlier lab that solved it | Timer/relay you will use |
|---|---|---|
| E-stop, MCR, L3, RESET | Lab 17 | |
| Three-wire START/STOP | Lab 02 | |
| 5 s warning before running | Lab 08 | |
| Run-on after STOP | Lab 10 | |
| Fixed-length pulse at motor start | Lab 11 |
Step 2 — Write each output as a sentence
For example: "R1 is ON when the start request is ON and the warning time is not over." Write one sentence for each of: the MCR, the start request, R1, G1, and G2. Underline every and (series) and or (parallel).
Step 3 — Plan your contacts
| Device | Contact sets available | Contacts you will use (and where) |
|---|---|---|
| CR1 | 2 (A, B) | |
| CR2 | 4 (A, B, C, D) | |
| TR1 | 2 (A, B) | |
| TR2 | 2 (A, B) + trigger | |
| TR3 | 2 (A, B) | |
| TR4 | 2 (A, B) + trigger |
Step 4 — Draw the ladder diagram
Draw L1, L2, and the L3 rail. Label every device, pin, and node. Build the diagram level by level, and trace each level against the timing diagram with a pencil before adding the next.
Step 5 — Paper test
Using your drawing only, walk through every row of the Test & Acceptance table below and predict each lamp. Fix the design until every prediction matches the specification.
Step 6 — Documentation and approval
Complete the contact allocation table, node table, and wiring table. Present the complete package to the instructor for approval. Do not wire until it is stamped.
Step 7 — Build, meter check, and test
Wire and meter-check one level at a time. Before the first power-up, confirm with the meter that L1 to L3 read high with the MCR de-energized.
Test & Acceptance (Customer Demonstration)
Demonstrate each test to the instructor, who will initial the last column.
| # | Level | Test | Expected result | Initials |
|---|---|---|---|---|
| 1 | 1 | Power ON; press PB1 | Nothing (RESET required) | |
| 2 | 1 | Press PB3 (RESET) | No output turns ON | |
| 3 | 1 | Press PB1 | R1 ON for about 5 s, then R1 OFF and G1 ON | |
| 4 | 1 | During the warning, press PB4 | R1 OFF immediately; G1 never turns ON | |
| 5 | 1 | Release PB4; press PB1 | Nothing (RESET required) | |
| 6 | 1 | RESET, START, wait for G1; press PB4 | G1 OFF immediately | |
| 7 | 2 | RESET, START, wait for G1; press PB2 | G1 stays ON about 5 s, then OFF | |
| 8 | 2 | START, wait for G1; press PB2, then PB4 within 5 s | G1 OFF immediately at PB4 | |
| 9 | 3 | START and watch G2 | G2 ON for about 3 s when G1 turns ON; never again while G1 stays ON | |
| 10 | 3 | Press PB2 and watch G2 during the run-on | G2 stays OFF | |
| 11 | 3 | START; press PB4 while G2 is ON | G2 OFF immediately | |
| 12 | All | Throughout | R1 and G1 are never ON together (except during a restart in the run-on; explain your design's behavior) |
Deliverables and Grading
| Item | Points |
|---|---|
| Pattern table and output sentences (Steps 1–2) | 10 |
| Contact plan (Step 3) | 5 |
| Ladder diagram: correct, complete, labeled (devices, pins, nodes, L3) | 20 |
| Node table and wiring table: complete, chained, ≤ 2 wires per terminal | 15 |
| Level 1 demonstration (Tests 1–6) — required to pass | 25 |
| Level 2 demonstration (Tests 7–8) | 10 |
| Level 3 demonstration (Tests 9–11) | 10 |
| Workmanship: neat wiring, tight terminals, safe practices | 5 |
| Total | 100 |
Reflection Questions
- Which requirement was the hardest to combine with the others? Why?
- Where did you run short of contacts, and how did you solve it (a lamp across a coil, a different timer, feeding from a node)?
- Explain why the run-on timer must be powered from L3 and not from L1.
- What happens in your design if START is pressed again during the run-on? Is that acceptable for this customer? How could you change it?
- This machine uses 6 relays and timers, and about 30 wires. A small PLC could do the same with a few inputs, outputs, and program lines. List two advantages of the PLC and one thing that must still be hardwired even with a PLC.