Damping System and its types

Damping System:

The damping torque should have a magnitude  that the pointer quickly comes to its final steady position. If the system is underdamped , the moving system will oscillate about the final steady position with a decreasing amplitude. When the moving system moves rapidly but smoothly to its final steady position, the system is said to be critically damped. If the damping torque is more than what is required for critical damping is called over-damped. 

The figure below shows the way an underdamped, an overdamped and critically damped system moves to its final steady position.

Damping System

The methods for producing damping torque are:

1. Air friction damping
2. Fluid friction damping
3. Eddy current damping

1. Air Friction damping:

The air friction damping device is as shown in the figure below. The arrangement consists of a light aluminium piston which is attached to the moving system. This piston moves in a fixed air chamber which is closed at one end. The clearance between piston and the chamber walls is uniform throughout and is very small. When there are oscillations the piston moves into and out of air chamber. When the piston moves inside the chamber, the air inside is compressed and pressure of air, thus builds up, opposes the motion of piston and hence the whole moving system. When the piston moves out of the air chamber, pressure in the closed space falls, and the pressure on the open side of piston is greater than on the other side. Thus there is again an opposition to motion.

Air Friction Damping

2. Fluid Friction damping:

This form of damping is similar to air friction damping. Oil is used in place of air and as the viscosity of oil is greater, the damping force is correspondingly greater.

A disc is attached to the moving system as shown in the figure, this disc dips into an oil pot and is completely submerged in oil. When the moving system moves, the disc moves in oil and a frictional drag is produced. This frictional drag always opposes the motion.

Fluid Friction Damping

 Advantages of fluid friction damping:

• Due to more viscosity of fluid, more damping is provided.
• The oil can also be used for insulation purposes. 
• Due to upthrust of oil, the load on the bearings are reduced, thus reducing the frictional errors.

Disadvantages of fluid friction damping:

• These are used only for instruments in vertical position.
• Due to oil leakage, the instruments cannot be kept clean.

3. Eddy Current damping:

This is the most efficient way of providing damping. It is based on Faraday's law and Lenz's law. When a conductor moves in a magnetic field cutting the flux, the e.m.f gets induced in it. And direction of this e.m.f is so as to oppose the cause producing it.

Eddy current damping

In this method 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 as shown in the figure.

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 in the disc. The disc is a closed path hence induced e.m.f circulate 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. Thus it produces an opposing torque so as to reduce the oscillations of pointer. This brings pointer to rest quickly. This is most effective and efficient method of damping.