What is Controlling Torque ? - Spring Control and Gravity Control

What is Controlling Torque ?

     The device used for controlling the deflection of the pointer in measuring instruments over the scale is called the control device. The torque which controls the movements of the pointer is called the control torque.

Necessity of Controlling Torque :

     In indicating instruments, if the deflection system alone is present, then the deflecting torque produced due to a small amount of quantity being measured will drive the pointer to its full-scale deflection position. Also, the pointer will not return to its zero position after the removal of supply. Therefore, under the influence of the deflecting torque, the magnitude of the movement of the pointer will be unsteady or unstable. Hence an additional force is required which helps the pointer to maintain a steady position.

     A control system produces a control torque in proportion to the pointer displacement. These controlling torque will be always in opposition to the deflecting torque. When both torques become equal, the pointer will attain a steady position. Also, the pointer will return to its zero position after indicating a reading. Hence, the controlling torque controls the pointer deflection so that, the reading becomes proportional to the quantity being measured.

Methods of Providing Controlling Torque :

The controlling torque can be produced in two ways. They are,
  • Spring control
  • Gravity control

Spring Control :

     In this, the controlling torque is produced by using springs as shown below.

Spring Control and Gravity Control

     Two hairsprings S1 and S2 are wound on the spindle and are coiled in such a way that they are opposite to each other and act against each other. When deflecting torque gets applied on the pointer, the starts moving. At this moment one of the springs unwinds itself and the other gets twisted. The spring that gets twisted or wounded tightly will oppose the deflecting torque by a force called controlling torque.

     The amount of controlling torque produced will be proportional to the angle of deflection (θ) of the pointer, whereas the deflecting torque (Td) depends upon the current flowing through the coil i.e., Td increases with an increase in current and vice-versa.

At steady state,
Td = Tc
KI = Kθ (K = constant)
∴ I ∝ θ

     Since current is directly proportional to the deflection angle, uniform scales can be graduated. The springs used are usually made up of low resistance bronze alloy and consist of a large number of turns to avoid deformation in the spring.

Advantages of Spring Control :

  • A Uniform scale is present since the current flowing is proportional to deflections.
  • Readings with high accuracy can be obtained.
  • This method of providing controlling torque is simple.
  • This system can be used in any position.
  • It is most preferable and is commonly used in many systems.

Disadvantages of Spring Control :

  • The spring control torque depends upon the temperature changes.
  • Spring control system requires high cost.
  • Controlling torque cannot be varied.

Gravity Control :

     Gravity controlled instruments are used for producing controlling torque and will be independent of temperature. In such instruments, the moving system when deflecting through an angle θ, from its position produces controlling torque (Tc). A small weight, W is attached to the moving arm. By adjusting the weight, W on the arm, the controlling torque, Tc can be varied as shown in the figure below.

Spring Control and Gravity Control

     Initially i.e., at zero position, the control weight is vertical at position A, when the pointer deflected at an angle θ the control weight will be in position B as shown in the figure below, producing a component Wsinθ. The component Wsinθ produce's controlling torque.

Spring Control and Gravity Control
∴ T = W sinθ l

Where l is the distance from the axis of rotation. At steady state, when Td = Tc,
Spring Control and Gravity Control

     The disadvantage with this system is that the spindle should always's be placed vertically as the control torque is produced under gravity and leveled properly. But the advantage of gravity control is, it is independent of temperature, cheap, and does not get deteriorated with time.

Advantages of Gravity Control :

  • Gravity control is cheaper than spring control.
  • This method of providing controlling torque is simple.
  • In this method, variable controlling torques are achieved.
  • The controlling torque does not depend upon the temperature changes.

Disadvantages of Gravity Control :

  • Gravity controlled instruments must be kept in a vertical position.
  • The scale is not uniform.
  • The system used in this method is delicate.
  • This method is very rarely used and is only used in some of the indicating and portable instruments.

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