Types of dynamometer :
1) Mechanical dynamometer
2) strain gauge type dynamometers
3) pneumatic and hydraulic dynamometer
4) piezoelectric dynamometer
5) Electrical dynamometer
Strain gauge type dynamometer.
fig. (a) use of strain gauges for two dimensional force measurement in turning on a lathe
fig. (b) Wheatstone bridge circuit
This type of gauges are widely used because mechanical methods for measurement strain are not very reliable.
It is an electro-mechanical technique and the dynamometer used in process can be termed as electro-mechanical dynamometer.
wheatstone bridge circuit is commonly used in connection with electrical strain gauges.
The force being measured by this method are cutting force ( $f_c$ ) and thrust force ( $f_t$ )
The basic principle involved in this technique is that, the electric resistance of wire changes when it is stretched.
The strain gauges are cemented on four flat structures.
Two sets of strain gauges are cemented, one set on top and bottom and second set on two vertical sides.
These two sets are connected one each, to separate Wheatstone bridge circuits.
During experiment, these two sets of strain gauges are subjected to tension and compression.
Resistance of the stain gauges subjected to tension increases strain gauges subjected to compression decreases.
These changes are measured by Wheatstone bridge.