DC Position Control

Exp.No: Date : DC POSITION CONTROL SYSTEM (SERVOMOTOR)  AIM: To study the position of DC servomotor s ervomotor control system.  APPARATUS: 1. DC position control system kit. 2. Multimeter. THEORY : Angular position control using a DC motor forms a fine example of feedback controlled system. A DC motor whose speed can be varied by application of various voltages is used through an appropriate gear drive to rotate a shaft. The angular position of the shaft is sensed using an appropriate sensor and depending on the set value and the measured value of the angular position, an error voltage is obtained which is amplified and feedback to control the motor. At equilibrium, the shaft position will correspond to the angular position indicated by the set value.

D.C. POSITION CONTROL SYSTEM It is required to keep the position of the load constant. Such a system where output position is kept constant is called as position control system. The output position is sensed and feedback to the potentiometer used as an error detector. For any change in the output position θ L, the potentiometer generates an error signal proportional to the difference θr and θL. The θ r is the reference position corresponding to the ideal output position. The error signal is given to the amplifier and the output of the amplifier is given to the armature of a d.c. motor. The d.c. motor maintains the output shaft position constant. The entire scheme is called d.c. position control system. Applications of d.c. servomotors are, Air craft control systems Electromechanical Electromechanical actuators Process controllers Robotics Machine Tools     

Precautions: 1. Before switching ON the mains, see that the switches SW3, SW4 are in down ward position i.e. ON position. 2. Keep the input pot P1 in 10 degree position. 3. Adjust the pot P3 (amplifier gain adjustment) in mid position.

4. Now switch ON, the main unit LED ‘R’ and ‘G’ should glow. 5. Do not cross zero degree position by moving pot P1 i.e. between 330 and 10 degree. 6. Do not try to rotate output pot by hand. 7. When system is not in use, keep SW3 andSW4 in off position (upward) to avoid heating. PROCEDURE: I. Without stability feedback (or) Open loop: 1. Adjust the input pot P1 in clockwise direction. 2. When the input is disturbed, the null indicator will be showing some indication. 3. When the input pot is moved in anticlockwise direction, the output pot P2 also moves in the reverse direction. Step change input: 1. Keep the pot P1 at around 180 degree position. 2. Pot P2 also will be in the same position. 3. Change the pot in a step fashion by about 60 to 80 degrees, the output will be observed to change in oscillatory mode before it settles to a final position. 4. The tendency for oscillations is found to be dependent on the amplifier gain setting. For high gain there are too many oscillations where as for low gain, oscillations are reduced but with static error. II. With stability feedback (or) closed loop: 1. Keep the switch SW1 in lower position and SW2 are in down ward position i.e., degenerative mode. 2. Keep the feedback control (P4) in minimum position, output gain indicates oscillations. 3. Take pot P1 to 180 degree position, effect the step input change in one of the direction, output gain indicates oscillations. 4. Now advance the tachogain pot P4 in clockwise direction the output is observed to follow the input in a smooth fashion without oscillation. If the tachogain pot P4 is too much advanced, the output now follows input in a sluggish fashion indicating over damped system. WARNING: Do not operate the DC position control in the regenerative mode for long time, this can damage the potentiometer.

Model Graph:

Open loop Output angle in degree

closed loop

20

40

Output angle in degree

60

80

Input angle in degree

Input angle in degree

TABULAR COLUMN: i) Without Tacho feedback (open loop), without stabilizing feedback. (SW1 upward)

S.No

Input Parameters Angle ( degree)

1

20

2

40

3

60

4

80

5

100

6

120

7

14

8

160

9

180

10

200

11

220

12

240

13

260

14

280

15

300

16

320

Voltage (V)

output Parameters Angle ( degree)

Voltage(V)

% error

ii) With degenerative (-ve) Tacho feedback (closed loop) ,without stabilizing feedback SW1 downward: Input Parameters S.No

Feedback control

Angle ( degree)

1

30

2

90

3

150 20

4

210

5

270

6

320

1

30

2

90

3

150 40

4

210

5

270

6

320

1

30

2

90

3

150 60

4

210

5

270

6

320

1

30

2

90

3

150 80

4

210

5

270

6

320

output Parameters % error

Voltage (V)

Angle ( degree)

Voltage(V)

The other