LOAD CELL (STRAIN GAUGE) - WHEATSTONE BRIDGE

What is a Load cell?

• A Load cell is a transducer that is used to convert a force into an electrical signal.

• The most common type of load cell is a strain gauge

Strain gauge

• A strain gauge is a passive type resistance pressure transducer whose electrical resistance changes when it is stretched or compressed. It can be attached to a pressure-sensing diaphragm.

• A strain gauge is a thin, wafer-like device that can be attached to a variety of materials to measure applied strain.

• A strain gauge is a device used to measure the strain of an object.

• The most common type of strain gauge consists of an insulating flexible backing that supports a metallic foil pattern.

• The strain gauge resistance changes when any physical effort or force is applied.

• If the strain gauge is under tension, resistance goes up and strain gauge is under compression, resistance goes down.

• The resistance change of strain gauge is usually converted into voltage by connecting one, two, or four similar gauges, as a Wheatstone bridge, and applying excitation to the bridge. The bridge output voltage is then measured of the pressure sensed by the strain gauges.

Wheatstone bridge

• Wheatstone bridge, also known as the resistance bridge.

• It is used to calculate the unknown resistance by balancing two legs of the bridge circuit, of which one leg includes the component of unknown resistance.

• The circuit is composed of two known resistors P and Q, one unknown resistor S, and one variable resistor R connected in the form of a bridge. This bridge is very reliable as it gives accurate measurements.

• The circuit also consists of a galvanometer and an electromotive force source. The emf source is attached between points a and b while the galvanometer is connected between points c and d. The current that flows through the galvanometer depends on the potential difference across it.

• Current I pass through point a. according to Kirchhoff's law current I divided into two I1 and I2 and pass through resister P and resister R.

• At the second point b current passing through galvanometer G and resister Q as I3 and I4

Here clearly say that
I4 = I1 - I3…………………………(1)

• At the third point d current I5 passing through resister S

Here clearly say that
I5 = I2 + I3…...……………………(2)

• Here resisters P, Q, and S have some fixed-resistance values. R is a variable resistor

• To make null deflection, no current passes through galvanometer G.

• Thus vary the value of resister R until galvanometer G becomes zero. As the G becomes zero it would say that I3 = 0. no current passing through b to d.

If I3 = 0 then
I4 = I1………...……….......…………….(3) (as per equation 1)
I5 = I2 ……...……………………………(4) (as per equation 2)

From the Ohm’s low

VAB = VA – VB = I1P …………………………(5)

VBC = VB – VC = I1Q ………………………..(6)

VAD = VA – VD = I2R .………………………..(7)

VDC = VD – VC = I2S ………………………..(8)

In Balanced condition, galvanometer becomes zero I3 = 0

So, VBD = VB – VD

0 = VB – VD

VB = VD ................................……………….(9)

Now, replace VB to VD in eq (5) and eq (6)

VAB = VA – VD = I1P ..……………………..(10)

VBC = VD – VC = I1Q .……………………..(11)

VAD = VA – VD = I2R ..……………………..(12)

VDC = VD – VC = I2S …………………….. (13)

Now compare eq (10) and (12)

I1P = I2R...………….........………………….(14)

Now compare eq (11) and (13)

I1Q = I2S.…………………………………….(15)

After Dividing eq (15) and eq (14)

S = Q / P * R

By this, we can find the unknown resister value.