DC motor characterisation and the automatic barrier

Objectives

1. Control the speed of a DC motor by pulse-width modulation (PWM) from the PLC.
2. Measure the motor speed with the proximity sensor and plot the speed–duty-cycle characteristic.
3. Program an automatic barrier driven by a crank, with position switches, vehicle detection and a movement time-out.
4. Measure the duty cycle actually imposed on the motor and classify it by the IEC 60034-1 duty types.

Background

A DC motor supplied by PWM sees an average voltage $D\,V_\text{dc}$, where $D$ is the duty cycle, and a permanent-magnet motor turns at a speed roughly proportional to its armature voltage (Chapter 1). The trainer's motor drives a pointer through a reduction gear; the manual warns that the gear makes the speed–duty relation non-linear, and part of this TP is to measure by how much.

Many real barriers are driven by a crank: the motor always turns the same way, one half-turn raises the arm and the next lowers it. On the trainer the DC motor's pointer plays the crank: the proximity sensor marks arm down, the micro-switch arm up.

Bench set-up

With the trainer OFF, connect as in manual Exercise 10-2: TRANSISTOR OUTPUT Q0.0 → M+; M− → 24 V−; micro-switch NO → I1.0 and COM → 24 V+; proximity sensor PS → I1.1. Turn SPEED ADJ fully clockwise so that the PLC alone sets the speed.

Preparation

1. Using manual Table 10-1.1, write the control byte SMB67 for PWM on Q0.0 with a 1 ms time base, and the values of SMW68 and SMW70 for a 200 ms period and a 40 % duty cycle.
2. The pointer passes the proximity sensor once per revolution. If $\Delta t$ (ms) separates two detections, give the speed in rev/min.
3. Draw the GRAFCET of the barrier of Part B, with at least the steps closed, opening, open, closing and fault.

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)

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

Part A: speed–duty-cycle characteristic

1. Program PWM on Q0.0 with a 200 ms period. The pulse width is held in a V-word that you change from the status chart. RUN_A and STOP_NC enable the output.
2. Measure the time between two detections of the proximity sensor with the 1 ms timer T32: on each rising edge of I1.1, copy the timer value into a V-word and restart the timer. Convert the result to rev/min in the program.
3. Fill in the table for duty cycles from 0 to 100 %. Note the smallest duty cycle at which the motor starts.

4. Repeat three points with a 20 ms period (same duty cycles). Listen to the motor and compare the speeds.

Duty cycle (%)	0	10	20	30	40	50	60	70	80	90	100
$\Delta t$ (ms)
Speed (rev/min)

Part B: automatic barrier

Specification.

1. At rest the arm is down (ARM_DOWN detected) and the motor is stopped.
2. When a vehicle is present (LOOP_ENTRY) and a valid ticket is presented (TICKET), the motor turns until ARM_UP. It runs at full speed and changes to 30 % duty cycle for the last part of the movement; use a time measured in Part A, not a guess.
3. The arm stays up while the vehicle is under it (LOOP_EXIT ON), then closes 2 s after the vehicle has left: the motor turns until ARM_DOWN.
4. Safety. If a vehicle appears under the arm (LOOP_EXIT ON) while it is closing, the arm must not come down on it: the motor continues to the up position, and the closing cycle restarts once the vehicle has gone.
5. Time-out. If the expected position is not reached within 1.5 times the normal travel time, the motor stops and FAULT lights. Only RESET clears it.
6. Count the complete barrier cycles in a counter.

Work.

1. Program the GRAFCET of your preparation with SCR steps.

2. Accumulate the time during which the motor is energised with a retentive timer (TONR), and the total elapsed time. Run ten barrier cycles with realistic pauses between vehicles and calculate the cyclic duration factor.

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. Plot speed against duty cycle. Where does the curve leave a straight line, and what causes the dead zone at low duty cycle? Relate it to the friction and load torque of Chapter 1.
2. Compare the 200 ms and 20 ms PWM periods. Why do industrial drives switch at several kilohertz?
3. From your measured on-time and cycle time, what is the cyclic duration factor of the barrier motor? Which IEC 60034-1 duty type describes it, and what would you write on the motor order?
4. Why is a movement time-out indispensable when a limit switch is the only proof that a movement has finished?