SERVO SYSTEM

BYNITISH SINHA

M.TECH(CAD-CAM)

DEFINITION

Servo is an automatic device that uses error-sensing feedback to correct the performance of a mechanism.

HISTORY

In 1886 used feedback device by SS Great Eastern.

In 1888 Electrical Servomechanism in Elisha Gray’s

Telautograph.

In 1909 Ragonnet power reverse mechanism was

steam powered servo amplifier for linear.

In 1930 using PRC to control searchlight.

Classification of servo system

• Closed loop system

• Open loop system

a) Automatic washing machine b) traffic signal systemc) home heating system

Closed loop system

System where out put has an effect upon the input quantity in such a manner as to maintain the desired output value.

Open loop system

Closed loop system

Advantage Disadvantage

Accuracy: They are more accurate than open loop system due to their complex construction.

Noise reduction ability: they are composed of a feedback mechanism, so they clear out the errors between input and output signals.

Construction: They are relatively more complex in construction and hence it adds up to the cost making.

Stability: It is less stable than open loop system .

it also reduces the overall gain of the system.

Open loop system

Advantage Disadvantage

Simplicity and stability:they are economical and

stable too due to their simplicity.

Construction: Since these are having a simple layout so are easier to construct.

Accuracy and Reliability:they are very inaccurate in terms of result output and hence they are unreliable too.

Due to the absence of a feedback mechanism, they are unable to remove the disturbances occurring from external sources.

FUNCTIONAL SERVO LOOPS

Position Servo Loop

Velocity Servo Loop

Acceleration Servo

Position Servo Loop

Control the position of the load it is driving.. Position a great number of devices. For ac position servo system, amplitude and

phase of the ac error signal determine amount & direction the load will be driven.

Velocity Servo Loop

VELOCITY of the output is sensed. When the velocity loop is at null position, an error

signal is still present and the load is moving. speed is governed by the level of the error.

SERVO CHARACTERISTICS Time lag.Over damped : This is an excessive time requirement

for the system to reach synchronization.Under damped : The unfavourable trait is an erratic

operation around the point of synchronization.

Servo feed control system in micro- EDM

To Keep The Gap Distance Constant, Adaptive Servo

Feed Control Is Used In Conventional EDM.The gap distance in micro-EDM is much shorter

than that in conventional EDM due to the small pulse energy.

In conventional EDM, to realize servo feed control, it is necessary to connect a pair of electric wires to monitor the average working voltage.

Servo feed control systems were therefore developed for the RC pulse generator and transistor type isopulse generator.

Servo feed control system for RC pulse generator

It consists of an average working voltage detection circuit, comparator, servo feed control circuit to calculate the feed rate, and machine drive system.

The average working voltage is monitored as a feedback signal to control the feeding speed.

The average working voltage is monitored as a feedback signal to control the feeding speed.

Two large resistors of 47 kΩ are inserted between one wire and the tool electrode, and the other wire and the work piece.

Machined surface in micro-EDM

Machined surface was smoother use transistor type isopulse generator.

Discharge energy from the transistor type isopulse generator is constant for each discharge.

Removal rate in Semifinishing

Three times higher than that of the RC pulse generator with servo feed control.

Improve removal rate by about 24 times higher than that of the conventional RC pulse generator with constant feed rate control.

Removal rate in finishing

Removal rate of RC(SFC) was about 15 times higher than that of RC (CFRC).

Removal rateof the transistor type iso pulse generator with the servo feed control was about 1.8 times higher than that of the RC pulse generator with servo feed control.

Effects of servo feed control Machined surface

Effects of servo feed control in finishing are

greater compared to semifinishing.

I. the pulse energy in finishing is smaller.

II. short circuiting occurs more easily.

III. unstable machining during finishing.