Conveyor line and brick sorting

Objectives

1. Program the start and stop sequence of a line of three conveyors, with pre-start warning and proof of motion before the upstream conveyor may start.
2. Implement the conveyor protections of Chapter 5: zero speed, belt slip, pull cord and belt drift.
3. Track products on a moving belt with a shift register clocked by the belt itself, and reject defective bricks at a downstream station.

Background

In a line of conveyors each belt discharges onto the next. Chapter 5 gives the rules: start against the flow of material (discharge conveyor first), stop with the flow (feeder first, then each belt once it has emptied), prove that a belt really moves before feeding it, and stop the whole line at once on an emergency stop or a pull cord.

The line has three conveyors. C1 (feeder) and C3 (discharge) are relay outputs and lamps. C2, the sorting conveyor, is the DC motor module: one revolution of its pointer is one pitch of the belt, the distance between two brick positions. The proximity sensor on the head pulley gives one pulse per pitch; the micro-switch, struck by the same pointer, plays a speed sensor on the tail pulley.

Bench set-up

With the trainer OFF: Q0.0 → M+, M− → 24 V−; proximity PS → I1.1; micro-switch COM → 24 V+ and NO → I1.0; buzzer BZ+ → RQ0.4, BZ− → 24 V−, common RC2 → 24 V+. SPEED ADJ fully clockwise.

Preparation

1. From your TP 1 measurements, write the nominal time $\Delta t_n$ between two pitch pulses at 100 % duty cycle. Deduce the zero-speed trip threshold $1.5\,\Delta t_n$ and choose a timer of suitable resolution.
2. Draw the GRAFCET of the line with the steps stopped, pre-start warning, C3 starting, C2 starting, C2 proven, running, stopping C1, run-on C2, run-on C3 and tripped.
3. The ejector is 4 pitches downstream of the loading point. Choose the start bit and the length of the SHRB register, and state which bit drives the ejector.

Timer choice

The S7-200 timer number fixes its type and resolution. Enter a duration to see the preset of every group.

Duration (s)

Type	Resolution	Timer numbers	Preset
TON/TOF	1 ms	T32, T96	+1000	exact
TON/TOF	10 ms	T33–T36, T97–T100	+100	exact
TON/TOF	100 ms	T37–T63, T101–T255	+10	exact
TONR	1 ms	T0, T64	+1000	exact
TONR	10 ms	T1–T4, T65–T68	+100	exact
TONR	100 ms	T5–T31, T69–T95	+10	exact

Control byte

Tick the bits you need; the value to move into the control byte appears below. Check each bit against the manual (Table 10-1.1).

Control byteSMB67 · PTO/PWM on Q0.0SMB77 · PTO/PWM on Q0.1SMB37 · HSC0

SM67.0update the cycle timeSM67.1update the pulse widthSM67.2update the pulse countSM67.3time base: 0 = 1 µs, 1 = 1 msSM67.4PWM update: 0 = asynchronous, 1 = synchronousSM67.5PTO: 0 = single segment, 1 = multi-segmentSM67.6mode: 0 = PTO, 1 = PWMSM67.7enable

MOVB 16#00, SMB67 (2#00000000 = 0)

Part A: start and stop sequence

1. START sounds the horn for 5 s, then starts C3. C2 starts 2 s later. C1 starts only when C2 is proven: two pitch pulses received within 3 s of its start. If C2 is not proven, the line trips.
2. STOP_NC stops C1 at once; C2 runs on for 6 pitches (the length of the belt), counted on HEAD_PITCH; C3 stops 5 s after C2.
3. ESTOP_NC or PULL_CORD_NC stops the three conveyors together. After a pull cord, the line restarts only once the cord switch has been reset locally and RESET pressed, followed by a new START.

Event	C1	C2	C3	Time or pitches measured
START pressed
end of warning
C2 proven
STOP pressed
end of C2 run-on
end of C3 run-on

Part B: conveyor protections

1. Zero speed. While C2 runs and is proven, if the time since the last pitch pulse exceeds $1.5\,\Delta t_n$, trip the line (FAULT, horn). Test it by reducing SPEED ADJ, then by pulling the I1.1 lead.
2. Belt slip. Count the pulses of HEAD_PITCH and TAIL_SPEED from the start of C2. If the counts differ by more than 2, the belt is slipping on the drive pulley: trip the line.
3. Belt drift. The first stage lights DRIFT_WARN without stopping the line. The first stage held for more than 10 s, or the second stage (DRIFT_TRIP), stops the line with the normal stop sequence.
4. Keep the cause of the first trip in a V-byte (fault code), so that the maintenance technician knows which protection acted.

Part C: brick sorting

1. A brick placed on C2 is signalled by BRICK; DEFECT held at the same time marks it defective. Latch this information until the next pitch pulse, then shift it into the SHRB register clocked by the rising edge of HEAD_PITCH.
2. When a defective brick reaches the ejector, 4 pitches downstream, pulse EJECTOR for 0.5 s.
3. Count the good bricks delivered to C3 and the rejected bricks. Bricks on the belt are not lost when the line stops: the register shifts only when the belt moves.

Brick	Defective?	Pitch loaded	Pitch ejected / delivered	Correct?
1	no
2	yes
3	yes
4	no
5	yes

Rehearse on the virtual bench

Test your program before the session: paste or open your exported .awl, choose the wiring of this TP, and run the procedure and the fault-injection tests on the simulated trainer.

WiringDC motor module: Q0.0 → M+, PS → I1.1, micro-switch NO → I1.0Stepper: Q0.0 → DIR, Q0.1 → CK, encoder A/B → I0.0/I0.1Hoist: stepper + encoder, micro-switch NC → I1.0 (by hand)Start again

Duty cycle

0 %

Speed

0.0 rpm

C0

0

Pull a lead

M+ lead PS lead (I1.1) micro-switch lead (I1.0)

SPEED ADJ

100 %

Program (STL)Network 1 // Your program: PWM on Q0.0, sensors on I1.0 and I1.1Symbol tablePROX I1.1 proximity sensor (pointer at 0°) MICRO I1.0 micro-switch (pointer at 180°) MOTOR Q0.0 PWM to M+

No errors: the program compiles.

PLC-200 panelTiming diagramProgram status

SIMATIC S7-200 · CPU 224SFRUNSTOPRUNSTOP

I1.0MICROsensor

I1.1PROXsensor

Q0.0MOTOR0

TimePause0.1×0.25×1×4×One scanCold restartt = 0.00 s · 0 scans

The motor turns at about 1 rev/s at full duty, not at all below about 12 % (friction), and ripples with a long PWM period. PS sees the pointer at 0°, the micro-switch at 180°: a crank lifts the barrier arm between them.

Fault-injection tests

Questions

1. Explain why the line starts against the flow and stops with it. What happens to C2 if C1 is started first?
2. Why is the shift register clocked by the belt rather than by a timer? What would go wrong with a timer when the belt speed changes or the line stops?
3. On the trainer the two speed sensors are on the same shaft, so a real slip cannot occur. What did the pulled-lead test really prove, and how would you tell a slipping belt from a failed sensor on a real conveyor?
4. A long belt is often driven by two motors. Using the load-sharing simulation of Chapter 5, explain why the two drives cannot both run as independent speed controllers, and what droop does.