The deflecting torque provides some deflection and controlling torque acts in the opposite direction to that of deflection torque. So before coming to the rest, pointer always oscillates due to inertia, about the equilibrium position. Unless pointer rests, final reading cannot be obtained. So to bring the pointer to rest within short time, damping system is required. The system should provide a damping torque only when the moving system is in motion. Damping torque is proportional to velocity of the moving system but it does not depend on operating current. It must not affect controlling torque or increase the friction.
The quickness with which the moving system settles to the final steady position depends on relative damping. If the moving system reaches to its final position rapidly but smoothly without oscillations, the instrument is said to be critically damped. If the instrument is under damped, the moving system will oscillate about the final steady position with a decreasing amplitude and will take sometime to come to rest. While the instrument is said to be over damped if the moving system moves slowly to its final steady position. In over damped case the response of the system is very slow and sluggish. In practice slightly under damped systems are preferred. The time response of damping system for various types of damping conditions is shown in the Fig.1.
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| Fig1 Air Friction Damping |
The following method are used produce damping torque.
1) Air friction damping 2) Fluid friction damping 3) Eddy current damping
1.1 Air Friction Damping
The advantages of this method are:
1) Due to more viscosity of fluid, more damping is provided.
2) The oil can also be used for insulation purposes.
3) due to up thrust of oil, the load in the bearing is reduced, thus reducing the fractional errors.
The disadvantages of this method are:
1) This can be used only for the instruments which are in vertical position.
2)Due to oil leakage, the instruments cannot be kept clean.
1.3 Eddy Current Damping
This arrangement consists of a light aluminium piston which attached to the moving system, as shown in Fig. 1.
The piston moves in a fixed air chamber. It is close to one end. The clearance between piston and wall chamber is uniform and small. The piston is reciprocates in the chamber when there are oscillations. When piston moves into the chamber, air inside is compressed and pressure of air developed due to friction opposes the motion of pointer. There is also opposition to motion of moving system when piston moves out of the chamber. Thus the oscillations and the overshoot gets reduced due to 'to and fro' motion of the piston in the chamber, providing necessary damping torque. This helps in setting down the pointer to its final steady position very quickly.
1.2 Fluid Friction Damping
Fluid friction damping may be used in some instruments. The method is similar to air friction damping, only air is replaced by working fluid. The friction between the disc and fluid is used for opposing motion. Damping force due to fluid is greater than that of air due to more viscosity. The disc is also called vane.
The arrangement is shown in the Fig. 2. It consists of a vane attached to the spindle which is completely dipped in the oil. The fractional force between oil and the vane is used to produce the damping torque, which opposes the oscillating behaviour of the pointer.
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| Fig.2 Fluid Friction Dmaping |
The advantages of this method are:
1) Due to more viscosity of fluid, more damping is provided.
2) The oil can also be used for insulation purposes.
3) due to up thrust of oil, the load in the bearing is reduced, thus reducing the fractional errors.
The disadvantages of this method are:
1) This can be used only for the instruments which are in vertical position.
2)Due to oil leakage, the instruments cannot be kept clean.
1.3 Eddy Current Damping
This is the most effective way of providing damping. It is based on the Faraday's law and Len's law. When a conductor moves in a magnetic field cutting the flux, e.m.f. gets induced in it. And direction of this e.m.f. is so as to oppose the cause producing it.
In this method an aluminium disc is connected to the spindle. The arrangement of disc is such that when it rotates, it cuts the magnetic flux lines of a permanent magnet. The arrangement is shown in the Fig.3.
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| Fig. 3 Eddy Current Damping |
When the pointer oscillates, aluminium disc rotates under the influence of magnetic field of damping magnet. So disc cuts the flux which causes an induced e.m.f. circulates current through the disc called eddy current. The direction of such eddy current is so as to oppose the cause producing it. The cause is relative motion between disc and field. This brings pointer to rest quickly. This is most effective and efficient method of damping.