FORCES & CONTROLS

CLASSIFICATION DIFFERENTFORCES CONTROLS

MEASUREMENT

• Controlling Force

The system producing a controlling force is called a controlling system. This force is required in an indicating instruments in order that the current produces deflection of the pointer proportional to the magnitude.
The functions of the controlling system are :
To produce a force equal and opposite to the deflecting force at the final steady position of pointer in order to make the deflection of the pointer definite for a particular magnitude of current.
To bring the moving system back to zero when the force causing the ionstrument moving system to deflect is removed.

Two types of controlling system.

GRAVITY CONTROL :

In this type of control, a small weight is placed on an arm attached to the moving system. The position of this weight is adjustable. This weight produces a controlling torque due to gravity.
Here T_c = W*sinθ*l = Wlsinθ = K_g

SPRING CONTROL :

A spring is attached to the moving system which exerts a controlling torque.
The essential reqirements for instrument springs are –
(A) They should be non-magnetic,
(B) They should be free from mechanical fatigue,
(C) Where springs are used to lead current into moving system they should have a small resistanc.
Here T_c = Ebt³θ/12l N-m
Where E =Young modulus
t =Thickness of spring
b = Width of spring
θ = Angular deflection

DIFFERENT FORCES

CLASSIFICATION DIFFERENTFORCES

DIFFERENT TYPE OF FORCE

DIFFERENCE BETWEEN ABSOLUTE INSTRUMENTS AND SECONDARY INSTRUMENTS:

➢ Absolute instruments take more time to compute there magnitude of the quantity being measured.
➢ These type of instruments are used in the standard laboratory or institutions.
➢ Secondary instruments take less time to compute the magnitude of the quantity being measured.
➢ These type instruments are used in every sphere of measurement.

TYPES OF ANALOG INSTRUMENTS :  

➢ Permanent magnet moving coil(PMMC)
➢ Moving Iron
➢ Induction
➢ Rectifier
➢ Hot wire
➢ Thermo couple
➢ Electrostatic
➢ Electro-dynamometer

BASIC REQUIREMENTS FOR MEASUREMENTS  

➢ OPERATING FORCE
➢ DEFLECTING FORCE
➢ CONTROLLING FORCE
➢ DAMPING FORCE

• DEFLECTING FORCE

The deflecting force is produced by the diflecting system or moving system. The deflecting system of an instrument converts the electric current or potential into a mechanical force called deflecting force. The deflecting system acts as the prime mover responsible for deflection of the pointer.

CLASSIFICATION

CLASSIFICATION DIFFERENTFORCES

CLASSIFICATION OF INSTRUMENTS  

• ABSOLUTE INSTRUMENTS  

These instruments give the magnitude of the quantity under measurement in terms of physical constants of the instrument.
Example : Tangent Galvanometer, Rayleigh,s current balance.

• SECONDARY INSTRUMENTS  

These instruments are so design that the quantity being measured can only be measured by observing the output indicating by the instrument.
Example : Voltmeter, Ammeter.

ACCORDING TO THE MODES OF OPERATION  

• ANALOG INSTUMENTS : 

The signals that vary in a continous fashion and take on an infinite number of values in agiven range are called analog signals and the devices taht produce analog signals are called analog devices.

• DIGITAL INSTRUMENTS : 

The signals which vary in discrete steps and thus take up only a finite number of values in a given range are called digital signals and the devices that produce digital signals are called digital devices.

ACCORDING TO THE METHOD USED FOR COMPARING THE UNKNOWN QUANTITY WITH THE UNIT OF MEASUREMENT  

Direct measuring instruments (example : ammeter , voltmeter )
Comparision instrument (example : AC and DC briges )

ACCORDING TO THE PRINCIPLE OF OPERATION  

➢ Magnetic effect
➢ Heating effect
➢ Electrostatic effect
➢ Electromagnetic effect
➢ Hall effect
➢ Thermal effect

*The instruments are also classified on the following catagories  

• INDICATING INSTRUMENTS  :

Indicating instruments are those instruments which indicate the magnitude of a quantity being measured.  They generally make use of a dial and pointer for this purpose.
They are classified in two catagories as Electro mechanical instruments and Electronic instruments.

• RECORDING INSTRUMENTS   :

Recording instruments give a continous record of the quantity being measured over a specified period.
The variations of the quantity being measured are recorded on a paper sheet fixed or moving by a pen.

• INTEGRATING INSTRUMENTS  :

Integrating instruments totalize events over a specified period of time.
Example : Ampere-hour and Watt-hour meter.