In this lecture, we are going to learn about the damping system in measuring instruments. we will discuss the damping torque and different types of damping torques used in measuring instruments. So let’s start with the basic knowledge of the damping system and then we will discuss their types in detail.
The deflecting torque provides some deflection and controlling torque acts in the opposite direction to that of deflecting torque. So before coming to the rest, the pointer always oscillates due to inertia, about the equilibrium position. Unless the pointer rests, the final reading can not be obtained. So to bring the pointer to rest within a short time, a damping system is required.
The system should prove a damping torque only when the moving system is in motion. Damping torque is proportional to the velocity of the moving system but it does no depends on the operating current. It must not affect controlling torque or increase friction.
The quickness with which the moving system settles to the final steady position depends on relative damping. If the moving system reaches its final position rapidly but smoothly without oscillations, the instrument is said to be critically damped.
If the instrument is underdamped, the moving system will oscillate about the final steady position with decreasing amplitude and will take some time to come to rest. While the instrument is said to be overdamped if the moving system moves slowly to its final steady position.
In an 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 the damping system for various types of damping conditions is shown in the figure.
Types of Damping Torque in Measuring Instruments
The following methods are used to produce damping torque.
|1. Air friction damping
|2. Fluid friction damping
|3. Eddy current damping
1. Air Friction Damping:
This arrangement consists of a light aluminum piston which is attached to the moving system as shown in the figure below:
The piston moves in a fixed air chamber. It is close to one end. The clearance between the piston and wall chambers is uniform and small. The piston reciprocates in the chamber when there are oscillations. When the piston moves into the chamber, the air inside is compressed and the pressure of air developed due to friction opposes the motion of the pointer.
There is also opposition to the motion of the moving system when the 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 settling down the pointer to its final steady position very quickly.
2. Fluid Friction Damping
Fluid friction damping may be used in some instruments. The method is similar to ar friction damping, only air is replaced by working fluid. The friction between the disc and fluid is used for opposing the motion. Damping force due to fluid is greater than that of air due to viscosity. The disc is also called a vane.
The arrangement is shown in the figure. It consists of a vane attached to the spindle which is completely dipped in the oil. The frictional force between the oil and the vane is used to produce the damping torque, which opposes the oscillating behavior of the pointer.
Advantages of the Fluid Friction damping Method:
- Due to the more viscosity of the fluid, more damping is provided.
- The oil can also be used for insulation purposes.
- Due to the upthrust of oil, the load on the bearings is reduced, thus reducing the frictional errors.
Disadvantages of the Fluid Friction damping Method:
- This can be only used for instruments that are in a vertical position.
- Due to oil leakage, the instruments can not be kept clean.
3. Eddy Current Damping
When a conductor moves in a magnetic field cutting the flux, e.m.f. gets induced in it. And the direction of this e.m.f. is so as to oppose the cause producing it.
In this method, an aluminum disc is connected to the spindle. The arrangement of the disc is such that when it rotates, it cuts the magnetic flux lines of a permanent magnet. The arrangement is shown in the figure.
When the pointer oscillates, the aluminum disc rotates under the influence of the magnetic field of the damping magnet. So discs cut the flux which causes an induced e.m.f in the disc. The disc is a closed path hence induced e.m.f. circulated current through the disc called eddy current.
The direction of such eddy current is so as opposed to the cause producing it. The cause is relative motion between the disc and the field. Thus it produces an opposing torque so as to reduce the oscillations of the pointer. This brings the pointer to rest quickly. This is the most effective and efficient method of damping.