C2_Error Channel Investigation and Simple Position Control System

EEE561_C2_rev02

Last Updated: 23/10/2007

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C2: ERROR CHANNEL INVESTIGATION AND SIMPLE POSITION

CONTROL SYSTEM

A: ERROR CHANNEL INVESTIGATION

OBJECTIVES

After completing this experiment, the students will be able to

1. Understand how an error signal can be produced by using an operational amplifier as

a comparator

2. Implement the ‘error channel’ concept.

3. Use two potentiometers to form an error channel.

PRE-REQUISITE

1. Familiar with the components of the DC Modular Servo.

2. Understand the operation of an electronic amplifier.

EQUIPMENT

1. Op Amp Unit OA150A

2. Power Supply PS150E

3. Input potentiometer IP150H

4. Output potentiometer OP150K

5. Baseplate

6. Multimeter

THEORETICAL BACKGROUND

It is possible to use an amplifier of high gain to produce an output Vo that is the (minus)

sum of the input voltages (V1 +V2). This can be done by connecting across the amplifier a

feedback resistor R2, which then multiplies the output by factor k = -R2/R1 where R1 is the

input resistance.

FACULTY OF ELECTRICAL ENGINEERING UNIVERSITI TEKNOLOGI MARA

ELECTRICAL ENGINEERING LAB

(KJE 591 / EEE 561)

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Note: Set the amplifier output to as near zero as possible.

If V2 in Figure 1 is set to have opposite polarity to V1 the output voltage will be

Vo = k (V1 – V2)

If the inputs V1 and V2 are supplied from circular potentiometers with their slides

coupled to a cursor traversing a dial marked in degrees, the input voltages can be added

together to form a simple ‘error channel’ to represent the difference in angular position of

the two cursors.

PROCEDURES

1. Assemble and make connections for all equipment shown in Figure 2.

2. Set the feedback selector switch to 100 kΩ resistor. Connect the multimeter to the

output of the OA150A, switch on and adjust the zero set to as near zero as possible.

Before connecting the two sliders into the operational amplifier inputs, connect the

rotary potentiometer polarities in opposition to each other from the +15V output of

the PS150E.

3. Measure with the multimeter the potential between each slider and 0V, rotating the

cursor till the reading is zero. If this is not corresponding with zero on the angular

scale, then loosen the dial and make an adjustment.

4. Reconnect the multimeter to the operational amplifier output and note the value.

5. Rotate the two cursors to five equal values on the angular scales and note the reading

on the multimeter in Table 1.

Table 1

Scale reading

(degrees)

Amplifier output, Vo

(volts)

V1

V2

R1

R1

R2

-A VO

Figure 1

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Figure 2

6. Set the output of the potentiometer, OP150K to zero and rotate the cursor of the input

potentiometer, IP150H, over its range for five different angles and tabulate your

results in Table 2 below.

Table 2

7. Now, repeat your readings setting the output potentiometer to -60°.

8. Plot your results from steps 6 and 7. By calculation and from the slopes of the curves,

determine the value of Ke (rees

volts

deg).

QUESTIONS

1. What would happen if V2 had the opposite polarity to V1?

2. Are all the output readings in step 5 the same? If not, what could be the reason?

3. After plotting your result in step 8, comment about the two graphs.

4. Discuss about the error channel concept in these experiments from the results.

Input potentiometer

(degrees)

Amplifier output, Vo

(volts)

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B: POSITION CONTROL SYSTEM

INTRODUCTION

The use of motor drive in an open loop and closed loop positional system will be

investigated. Two rotary potentiometers can be used to generate an error signal to show

the misalignment of the output cursor with that of the input cursor. If the output

potentiometer is mounted on the shaft of a geared motor, we would have the basis of an

automatic position control system where the error signal is used to drive the motor in a

direction such as to reduce the misalignment to zero.

THEORETICAL BACKGROUND

Figure 3 shows the schematic diagram of an armature controlled dc motor with load J

(inertia) and daper D (friction). Find the transfer function of the system Θ(s) where

Ea(s)

θ(t) is the angular displacement at the output and ea (t) is the input voltage. Define on the

constant used in the equations.

Figure 3

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OBJECTIVES

After completing this experiment, the students will be able to

1. Observe the operation of a basis open-loop position control system.

2. Understand the operating characteristics of the pre-amplifier

3. Understand the concept of closed-loop feedback in a position control system and

observe the action of a simple motor driven close-loop position control.

EQUIPMENT REQUIRED

QTY DESIGNATION DESCRIPTION SERIAL NO

1 OA150A Op Amp Unit

1 AU150B Attenuator Unit

1 PA150C Pre-Amp Unit

1 SA150D Servo Amplifier

1 DCM150F DC Motor

1 IP150H Input Potentiometer

1 OP150K Output Potentiometer

1 GT150X Reduction Gear Tacho Unit

1 Multimeter

B1. Knowledge Level

Before starting this assignment you should:

Be familiar with the components of the DC Modular Servo.

Understand the term ‘error channel’ and understand how two rotary potentiometers

can be used to form an error channel.

Understand the operation of electronic amplifiers and the operation of a simple

position control system.

In the previous system at part A, muscle power is the Controller with finger movement

being used as the Actuator. There is even a stage of pre-amplification in which our eye

observes the meter reading and signals the brain to send message along the nerves to the

muscles in our arm and hand. Draw the block diagram of the system described above to

run the motor and what sort of control system would this be?

Show how the above system can be changed into a closed loop system to implement the

position control system by improving the block diagram.

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B2. Characteristic of the Pre-Amplifier PA150C

1. Set up the connection using equipments shown in Figure 5. On the upper

potentiometer of AU150B, using the multimeter, set the output between terminal 2

and 0V to produce +1V.

Figure 5

2. Connect terminal 3 of the upper potentiometer to +15V and terminal 1 and 4 to 0V.

3. Connect terminal 6 of the lower potentiometer to terminal 2 of the upper

potentiometer. This means that the scale positions 1 to 10 will gave input values in

tenths of a volt.

4. Connect terminal 5 of lower potentiometer to pre-amplifier input.

5. For each scale position on the lower potentiometer, measure the output of the

PA150C. Complete Table 3.

Table 3

Scale

Position

Input Signal Pre-amplifier output

V1 (volt) Vo (3) Vo (4) Vo (4-3)

0

1

2

3

4

5

6

7

8

9

10

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6. Repeat step 1, 2, and 3 for the potentiometer to produce –ve voltages from -15V

supply and tabulate your readings in Table 4.

Table 4

Scale

Position

Input Signal Pre-amplifier output

V1 (volt) Vo (3) Vo (4) Vo (4-3)

0

1

2

3

4

5

6

7

8

9

10

Plot graphs of the input voltage against the output voltage for both Table 3 and Table 4

Two (2) sets of graphs have to plot for both Tables 3 and Table 4.

1. Pre-amplifier outputs Vo(3) and Vo(4) versus input Vi

2. Pre-amplifier output Vo (4-3) versus input Vi

The ratio of the output voltage Vo to the input voltage V1 gives the gain K

QUESTION

1. Using the straight part of the curve find the gain of the Pre-Amplifier.

Gain of Pre Amplifier = ______________________

2. State why the gain has to be measured on the straight part of the curve

_____________________________________________________________________

_____________________________________________________________________

3. Explain the reasons for the no-linear portions of the curve

_____________________________________________________________________

_____________________________________________________________________

4. State the range of signals that the inputs should be kept

Range of signals = _____________________________

5. What input value will gave a nil voltage across the outputs?

Input value for zero output = ______________________

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B3. Closed-Loop Position Control System

1. Connect all the components from previous settings which comprises of the op-amp

(OA150A), potentiometer (AU150B), rotary potentiometers, pre-amplifier (PA150C),

motor and reduction gear tacho unit.

The inputs of PA150C will be provided by the error signal from the OA150A while

the outputs will be used to control the motor rotation. Connect the output of PA150C

to the servo amplifier. Connect output potentiometer to the reduction gear tacho unit

to measure the rotation output. The upper potentiometer on the AU150B can be used

as a gain control and should initially set to zero before switching on the power.

2. Adjust the PA150C using the ‘zero set’ knob until the motor stop rotating.

3. Set the IP150H to some arbitrary angle and increase the gain control setting. The

output potentiometer cursor should rotate to and angle nearly equal to that of the input

potentiometer cursor.

If the input cursor stops before arriving at the set position, one is faced with the fact

that the system is tolerant to an error and the motor will not respond till the error

exceeds a certain value. Increase the gain so that this tolerant is overcome and you get

the right alignment. Figure 6 shown the schematic diagram for the system.

Note the different results obtained and complete Table 5.

Table 5

Output Cursor Position in Degrees

Required Actual Misalignment

DISCUSSION

1. Discuss about your results in Table 5.

2. Describe the differences between an open loop and closed loop position control as

seen in the experiments.

3. Draw the block diagram of the open loop position control system and closed loop

position control of the system. Identify the controller and transducer (if there is any)

in your block diagram.

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Figure 6: Schematic diagram for closed-loop position control system

i/p pot.

o/p pot.

Op-amp

Verror

Pre-amp

Motor

+

tacho

Ra La

+15V

-15V

+15V

-15V

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C2_Error Channel Investigation and Simple Position Control System