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Control System

wo ypes o con ro sys em are use :

Gravit Control

A small weight is placed on a an arm attached to the moving system This weight produces a controlling torque due to the gravity

Here the com onent of wei ht tr in to restore the ointer back to ZERO position is Wsin .

Thus the controlling torque is, T c= WLsin

The instrument must be mounted in level osition Otherwise ZERO error will occur

Not suited for indicating instrument of portable manner.

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Control System

Spring Control A hair spring is attached to the moving system

It exerts a controlling torque

Essential requirements are Should be non-magnetic Should be proof from mechanical fatigue Tc=K

Comparison between Gravity control and Spring control

Cheaper Independent of temperature variation Does not deteriorate with time

BUT GC gives cramped (crowded) scale at the lower end i.e., does not give accurate result at lower end of scale

More is explained on the board .

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Damping System

A damping force is one which acts on the moving system of the instrument only when it is moving and always oppose its motion

The degree of damping should be adjusted to a value w c s su c ent to ena e t e po nter to r se qu c y to ts e ecte

position without overshooting.

Air friction damping Fluid friction damping

Eddy current damping Electromagnetic damping

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r r c on amp ng Consist of a piston which moves in a fixed air chamber

This chamber is closed at one end

When the piston moves into the chamber, the air inside is compressedand the pressure of air built up oppose the motion

Fluid friction damping Similar to air friction, BUT oil is used in place of air

Viscosity of oil is greater, and so is the damping force

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Edd current dam in When a conductor moves in a magnetic field, an emf is induced in it

and if a closed path is provided, a current (eddy) flows.

electromagnetic torque which oppose the motion This torque is proportional to magnetic field and current

Thus, if the strength of magnetic filed is constant

Torque is proportional to velocity

Electromagnetic damping Movement of a coil in a ma netic field roduces a current in coil,

which interact with magnetic field to produce a torque This torque opposes the movement of coil and slows the response

Used in Galvanometer