What is a Damping system or Damping Torque in Measuring Instruments?
The piston moves in a fixed air chamber and is close to one end. The clearance between the piston and the wall of the chamber is uniform and small. When the moving system experiences oscillations, the piston reciprocates in the chamber. As the piston moves into the chamber, it compresses the air inside, and the pressure of the air developed due to friction opposes the motion of the pointer. Similarly, there is opposition to the motion of the moving system when the piston moves out of the chamber.
In a measuring instrument, the deflecting torque provides the initial deflection of the pointer, while the controlling torque acts in the opposite direction to keep the pointer at the correct reading. However, due to the inertia of the moving system, the pointer will oscillate about the equilibrium position before coming to rest. To obtain a final reading, it is necessary to bring the pointer to rest as quickly as possible. This is where the damping system comes in.
The damping system provides a damping torque that is proportional to the velocity of the moving system, but does not depend on the operating current. It is important that the damping torque does not affect the controlling torque or increase the friction in the system.
The quickness with which the moving system settles to its final steady position depends on the relative damping. When the moving system reaches its final position rapidly and 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 some time to come to rest. On the other hand, if the instrument is over damped, the moving system moves slowly to its final steady position and the response of the system is very slow and sluggish. In practice, slightly under damped systems are preferred as they provide a good balance between quickness and stability.
What are over damped, critically and under damped systems?
A damped system is a mechanical or electrical system that experiences a resistive force or torque that opposes its motion. The goal of a damping system is to bring a moving system to its final steady position as quickly and smoothly as possible, without any oscillations or overshooting.
In a critically damped system, the damping force or torque is exactly the right amount to bring the moving system to its final steady position quickly and smoothly without any oscillations. The system reaches the final steady position at the fastest possible rate without overshooting.
In an under-damped system, the damping force or torque is not enough to prevent the moving system from oscillating about the final steady position. The system will take longer to reach the final steady position and will continue to oscillate with decreasing amplitude before coming to rest.
In an over-damped system, the damping force or torque is too strong, causing the moving system to move too slowly and smoothly to the final steady position. The system will not oscillate, but it will take longer to reach the final steady position, and the response of the system will be slow and sluggish.
In practice, slightly under-damped systems are preferred as they provide a good balance between quickness and stability. They reach the final steady position quickly, but still have some damping to prevent excessive oscillations.
Methods of Providing Damping Torque:
- Air friction damping
- Fluid friction damping
- Eddy current damping
- Electromagnetic Damping
1. Air Friction Damping:
Air friction damping is a type of damping system that uses air resistance to slow down the motion of a moving system. The system consists of a light aluminum piston that is attached to the moving system, as shown in Fig.
This "to and fro" motion of the piston in the chamber reduces the oscillations and overshoot of the moving system, providing the necessary damping torque. This helps to quickly bring the pointer to its final steady position. The advantage of air friction damping is that it is relatively simple and low-cost, and it can provide good damping performance without adding much weight to the moving system.
Advantages of air friction damping:
- It is a simple and cheap method for damping oscillations.
- It does not require a permanent magnet and thus does not have problems of field distortion.
- It can be easily maintained and repaired.
- It does not add significant weight to the moving system.
- It does not require frequent adjustments
- Replacement of parts can be done easily
- It provides good damping performance.
Disadvantages of air friction damping:
- The piston must be handled carefully to prevent damage or errors in the readings.
- It may not provide consistent performance over time.
- It may be affected by changes in temperature and humidity.
- It is not as reliable as other types of damping systems.
- Piston can get twisted and cause errors in the readings if mishandled during installation or maintenance.
2. Fluid Friction Damping:
Fluid friction damping is a type of damping system that uses the friction between a fluid and a moving component to slow down the motion of a moving system. It is similar to air friction damping, but instead of using air, a working fluid is used. The friction between the fluid and the moving component is used to produce the damping force. Because the fluid is more viscous than air, the damping force due to fluid is greater than that of air.
The system consists of a disc or vane attached to the spindle, which is completely dipped in the oil. The disc is also called a vane, and it is the component that is in contact with the fluid. The fractional force between the oil and the vane is used to produce the damping torque, which opposes the oscillating behavior of the pointer.
When the moving system experiences oscillations, the vane moves back and forth in the oil, generating friction between the vane and the oil. This friction produces a damping force that slows down the motion of the moving system, helping to bring the pointer to its final steady position quickly and smoothly. The use of a fluid allows for greater damping force than air, which makes the system more effective in damping the oscillations.
Fluid friction damping is commonly used in some instruments, such as meters, gauges, and other measuring devices where precise readings are required. The use of fluid allows for greater damping force than air, which makes the system more effective in damping the oscillations.
Advantages of Fluid friction damping:
- It provides more effective damping torque when it is compared to air friction type damping.
- The oil used serves two purposes, one is for damping, and the other is for heat dissipation, which can prolong the life of the instrument.
- It is the best-suited method for electrostatic type instruments, such as capacitance sensors and accelerometers, as it protects the sensitive electrical components from damage.
- Errors due to friction are reduced to a great extent with oil damping, making it a more reliable method of damping.
- The oil can help to reduce friction and minimize errors, which makes it a more reliable method of damping.
Disadvantages of Fluid friction damping:
- Limited applicability as it can only be used for vertical position instruments.
- Inability to use in portable type instruments due to the added weight and bulk from the fluid container.
- Possibility of leakage of oil in the other parts of the instrument, which can cause damage and make the instrument inoperable, lead to costly repairs and downtime, making the instrument less reliable and less efficient to use. Additionally, leakage of oil can also be a safety hazard as it can cause slippery surfaces, fire risk and environmental pollution.
3. Eddy Current Damping:
Eddy current damping is a method of damping that utilizes the principles of Faraday's law and Lenz's law to provide a highly effective form of damping. It is based on the principle that when a conductor (such as an aluminum disc) moves in a magnetic field, cutting the flux, an e.m.f. (electromotive force) gets induced in it. The direction of this e.m.f. is such that it opposes the cause that produced it.
In this method, an aluminum disc is connected to the spindle of the instrument. The disc is positioned in such a way that when it rotates, it cuts through 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. As the disc cuts through the flux, it induces an e.m.f. in the disc, which creates circulating currents known as eddy currents. The direction of these eddy currents is such that they oppose the cause producing them, which is the relative motion between the disc and the field. This opposing force quickly brings the pointer to rest, making it a highly effective and efficient method of damping.
Eddy current damping is considered one of the most effective ways of providing damping. It is very efficient in reducing the oscillation of the pointer quickly, making the instrument more accurate and reliable. The method is relatively simple, and the setup is easy to install and maintain.
Advantages of Eddy Current Damping:
- Efficiency: Eddy current damping is considered one of the most efficient forms of damping compared to other methods, it quickly brings the pointer to rest, making the instrument more accurate and reliable.
- Instrument type: It is especially used for moving coil and induction-type instruments, it is an effective solution for the damping of these types of instruments.
- Portability: It can be used in portable instruments as well, the method is relatively simple, and the setup is easy to install and maintain, this makes it a good option for portable instruments where weight and size are a constraint.
- Durability: Eddy current damping does not involve any mechanical contact between the damping element and the moving parts of the instrument, this means that it is less likely to wear out or fail over time, making it more durable and reliable over the long term.
- Environmental friendly : Eddy current damping does not require any lubricants or fluids, this makes it environmentally friendly.
Disadvantages of Eddy Current Damping:
- Not applicable for moving iron or dynamo-type instruments.
- Can cause heating and damage the instrument over time, especially at high frequencies.
- Effectiveness is affected by the properties of the material of the conductor.
- Can cause heating.
- Can be affected by the properties of the material of the conductor.
- Not suitable for high-frequency operations.
4. Electromagnetic Damping:
Electromagnetic damping refers to the process by which the movement of a coil in a magnetic field is slowed down due to the interaction between the current produced in the coil and the magnetic field. This interaction generates a torque that opposes the movement of the coil, which results in a slowing of the response.
In a galvanometer, a coil of wire is suspended in a magnetic field, and as the coil moves, it cuts through the lines of magnetic force, inducing a current in the coil. This current then interacts with the magnetic field, generating a torque that opposes the movement of the coil. The magnitude of the current, and hence the damping torque, is dependent on the resistance of the circuit to which the instrument is connected.
The greater the resistance of the circuit, the greater the damping torque, and the slower the response of the coil. This is because a higher resistance in the circuit means that there is more opposition to the flow of current, which in turn leads to a greater interaction between the current and the magnetic field, resulting in a larger damping torque.