exercise 6 open-loop speed · pdf file · 2016-04-18exercise 6 open-loop speed...

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Exercise 6

Open-Loop Speed Control

EXERCISE OBJECTIVE

To understand what is open-loop speed control;To learn how to sense the speed of the trainer Bidirectional Motor;To control the speed of the trainer Bidirectional Motor in the open-loop mode;To describe the effect of a load variation on the motor speed in the open-loopmode.

DISCUSSION

Open-Loop Speed Control Systems

An open-loop speed control system is a system in which the speed of the actuatoris controlled by a setpoint only, and the actual actuator speed is not taken intoaccount.

Figure 6-1 shows the block diagram of an open-loop speed control system, using avalve of the flow type:

The "setpoint" corresponds to the desired actuator speed;

The "valve" provides air flow to the actuator to bring its speed to the setpoint;

"Disturbances" are varying conditions that cause the actuator speed to differ fromthe setpoint;

The "actuator speed" is a function of the flow rate allowed by the valve and thedisturbance(s).

Figure 6-1. Open-loop speed control system.

The setpoint is initially set so that the flow rate allowed by the valve maintains acertain actuator speed. This speed will be maintained under constant actuator load


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and air supply. If, however, one of these parameters varies, the actuator speed couldchange.

Because the system is operating in the open-loop condition, it does not receivefeedback to correct for changes in actuator speed. The operator may have toreadjust the setpoint continuously to maintain the actuator speed at a desired value,which is a time-consuming task and may provide imprecise results.

Open-Loop Control of Motor Speed

In applications where the air supply and the load on the motor remain fairly constantafter the motor has been adjusted to the desired speed, an open-loop speed controlsystem will provide satisfactory results at low cost. If, however, the motor load variesfrom time to time and good speed regulation is of great importance, most likely theopen-loop speed control system will be inappropriate.

Sensing the Speed of the Trainer Bidirectional Motor

As Figure 6-2 shows, the frequency-to-voltage converter of the Signal Conditionersmodule can be used together with the trainer Diffuse Reflective Photoelectric Switchto sense the speed of the pneumatic motor. To do so, the photoelectric switch mustbe positioned perpendicularly to the motor at a distance of 10 cm (4 in). The beamof the photoelectric switch must be pointing in the direction of the white sticker on themotor shaft, and its electrical contact must be connected to the input of thefrequency-to-voltage converter.


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Figure 6-2. The trainer Signal Conditioners.

As the motor rotates, the electrical contact of the photoelectric switch alternatelyopens and closes, thereby producing a 0-24 V pulsed signal whose frequency isproportional to the motor speed. This frequency is converted into a proportionalDC voltage by the frequency-to-voltage converter. The higher the motor speed, thehigher the voltage produced by the frequency-to-voltage converter.

The frequency-to-voltage converter has three different outputs, labeled “N”, “0-5 V”,and “0-10 V”:

The output N has a fixed calibration. It provides a voltage of 1.0 V per 1000 r/minof the motor;

The two other outputs, labeled “0-5 V” and “0-10 V”, can be user-calibrated usingthe potentiometer S (span). This potentiometer sets the converter inputfrequency, and therefore the motor speed, for which the voltage will be maximumat the 0-5 V or 0-10 V output of the converter.


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Figure 6-3. Sensing the speed of the trainer Bidirectional Motor.

If, for example, the span potentiometer is adjusted to obtain 10.0 V at the 0-10 Voutput of the converter when the input frequency is 33 Hz (2000 r/min), the outputvoltage will be 7.5 V if the input frequency is reduced to 24 Hz (1500 r/min). Thespan setting also determines the required setpoint voltage for a particular point ofoperation. In this example, the setpoint voltage must be set at 7.5 V to call for aspeed of 1500 r/min (24 Hz).


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Controlling the Motor Speed Using a Servo Control Valve of the Pressure Type

As seen previously, servo control valves of the pressure type are designed to controlthe pressure in a pneumatic circuit, not the flow. Unfortunately, this is the flow ratewhich must be set to control the speed of a pneumatic motor.

It is therefore necessary to add a valve, whose orifice varies depending on apressure level, between the servo control valve of the pressure type and thepneumatic motor. This can be done by means of a proportional control valve asshown in Figure 6-4. Detailed valve operation is as follows:

When the pressure at the pilot port (output pressure of the servo control valve)is zero, the proportional control valve is at rest position and no air flows throughit (the valve is non-passing);

When the pressure at the pilot port (output pressure of the servo control valve)starts to increase, the valve becomes actuated and air starts to flow through itsorifice. The opening of the proportional control valve varies in proportion with thepressure level applied to its pilot;

The opening of the proportional control valve is maximum when the pressureapplied to its pilot is maximum.

Figure 6-4. The trainer Proportional Control Valve, Air-Pilot Operated module.

Procedure summary

In the first part of the exercise, Setting Up the Equipment, you will set up theequipment.

In the second part of the exercise, Positioning the Pilot of the Proportional ControlValve, you will learn how to position the pilot of the proportional control valve. Thepilot must be positioned each time the proportional control valve is used.


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In the third part of the exercise, Sensing the Speed of the Trainer Bidirectional Motor,you will learn how to sense the speed of the Bidirectional Motor using the DiffuseReflective Photoelectric Switch and the frequency-to-voltage converter of the SignalConditioners.

In the fourth part of the exercise, Open-Loop Control of Motor Speed, you willperform open-loop control of the motor speed and see the effect of a load variationon the motor speed.

EQUIPMENT REQUIRED

Refer to the Equipment Utilization Chart, in Appendix A of the manual, to obtain thelist of equipment required to perform this exercise.

PROCEDURE

Setting Up the Equipment

G 1. Get the Bidirectional Motor and the Diffuse Reflective Photoelectric Switchfrom your storage location.

G 2. Referring to Figure 6-5, position the photoelectric switch so that it isperpendicular to the motor shaft at a distance of 10 cm (4 in) (2 rows ofperforation). The beam of the photoelectric switch must be pointing in thedirection of the white sticker on the motor shaft. Clamp the motor into place.

Figure 6-5. Photoelectric Switch Positioning.

G 3. Verify the status of the trainer according to the procedure given inAppendix B.


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Note: In order to obtain a motor speed which is stable, the motorshould run at high speed during 1 minute. To do so, put somepneumatic oil in the motor ports and connect the circuit shown inFigure 6-6.

On the Conditioning Unit, open the main shutoff valve and therequired branch shutoff valve at the manifold. Set the mainpressure regulator to obtain 630 kPa (90 psi) on the regulatedpressure gauge.

After 1 minute approximately, close the shutoff valves and turnthe regulator adjusting knob completely counterclockwise.Proceed with the rest of the exercise.

Figure 6-6. Circuit for lubricating the motor.

G 4. Connect the circuit shown in Figure 6-7.

Note: To minimize the pressure drops, use a tube as short aspossible between the outlet port of the motor and the inlet port offlow control valve FCV1, and between the outlet port of flowcontrol valve FCV1 and the muffler module.


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Figure 6-7. Open-loop speed control system.

G 5. Turn on the DC Power Supply and PID Controller. Do not open the shutoffvalves on the Conditioning Unit at this time.


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G 6. On the PID Controller, set the SETPOINT potentiometers 1 and 2 toobtain 0.0 V and 10.0 V, respectively, at the SETPOINT output 1. Thenselect the SETPOINT potentiometer 1.

G 7. On the Conditioning Unit, open the main shutoff valve and the requiredbranch shutoff valves at the manifold.

Set the main pressure regulator to obtain 630 kPa (90 psi) on the regulatedpressure gauge.

G 8. Open the Flow Control Valve FCV1 completely (fully counterclockwise).

G 9. On the Signal Conditioners module, connect a DC voltmeter at the output Nof the f/E converter.

Positioning the Pilot of the Proportional Control Valve

G 10. Referring to Figure 6-8, position the pilot of the proportional control valve byscrewing, or unscrewing, the pilot to obtain 1.0 V at the output N of the f/Econverter. This corresponds to a speed of 1000 r/min. Do not use any tools.

G 11. On the PID Controller, select the SETPOINT potentiometer 2, and set theflow control valve FCV1 to obtain 2.0 V at the output N of the f/E converter.This setting will limit the motor speed to 2000 r/min.

Select the SETPOINT potentiometer 1, and ensure that the voltage at theoutput N of the f/E converter is still 1.0 V. Readjust the position of the pilotif necessary.

Alternately select SETPOINT potentiometers 1 and 2 to verify the settings.

Note: Depending on your air supply system, the speed of thepneumatic motor may vary significantly. You may need to readjustthe flow control valve.


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Figure 6-8. Positioning of the proportional control valve pilot

Sensing the Speed of the Trainer Bidirectional Motor

G 12. Select the SETPOINT potentiometer 2.

Connect the DC voltmeter to the 0-10 V output of the f/E converter andcalibrate this output using the potentiometer S (span) at 10.0 V.

G 13. On the PID Controller, set the SETPOINT potentiometer 2 to obtain 4.0 Vat the SETPOINT output 1. Measure the voltage now present atthe 0-10 V output of the f/E converter. Is this voltage still 10.0 V? Why?

G 14. Since the span potentiometer is adjusted to deliver 10.0 V at the 0-10 Voutput of the converter when the input frequency is 33 Hz (2000 r/min),calculate the actual motor speed using the voltage you measured in theprevious step.

Actual motor speed: r/min


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Open-Loop Control of Motor Speed

G 15. Open the Flow Control Valve FCV1 completely (fully counterclockwise).

G 16. On the PID Controller, set the SETPOINT potentiometer 2 to obtain a motorspeed of 2000 r/min. This corresponds to a voltage of 2.0 V at the output Nof the f/E converter.

G 17. Record the SETPOINT voltage present at the SETPOINT output 1 in theappropriate cell of Table 6-1. This condition simulates a no-load condition.

LOAD

CONDITION

SETPOINT

VOLTAGE

MOTOR

SPEED

No-load 2000 r/min

Light-load 2000 r/min

Table 6-1. SETPOINT voltages for two load conditions.

G 18. Simulate a speed reduction caused by an increase in motor load (or by anair supply fluctuation) by turning the knob of flow control valve FCV1clockwise until the motor speed becomes 1800 r/min (1.8 V at the output Nof the f/E converter).

G 19. On the PID Controller, readjust the SETPOINT potentiometer 2 to obtain amotor speed of 2000 r/min (2.0 V at the output N of the f/E converter).

Record the SETPOINT voltage present at the SETPOINT output 1 in theappropriate cell of the row Light-load of Table 6-1.

G 20. Referring to the values indicated in Table 6-1, is this open-loop speedcontrol system capable of maintaining the motor speed constant when theload conditions change? Explain.

G 21. What type of control did you achieve when you increased the setpoint tocompensate for the decrease in motor speed? Explain.


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G 22. On the Conditioning Unit, close the shutoff valves, and turn the regulatoradjusting knob completely counterclockwise.

G 23. Turn off the PID Controller and the DC Power Supply.

G 24. Disconnect and store all leads and components.

CONCLUSION

In this exercise, you learned how to measure the speed of the trainer BidirectionalMotor with a photoelectric switch and a frequency-to-voltage converter. You saw thatthe converter output voltage was directly proportional to the motor speed.

You also performed open-loop control of the motor speed. You saw that the motorspeed decreased when you increased the load. Since the system was operating inthe open-loop mode, it did not receive electrical feedback to correct for the decreasein motor speed.

REVIEW QUESTIONS

1. What is an open-loop speed control system?

2. What does “disturbance” mean?

3. Is the open-loop speed control system able to correct for variations in motorspeed? Explain.

4. Should an open-loop speed control system be used in applications where themotor load varies from time to time and good speed regulation is of greatimportance?

exercise 6 open-loop speed · pdf file · 2016-04-18exercise 6 open-loop speed...

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