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RESISTIVE TRANSDUCER
RESISTIVE TRANSDUCER
The resistive transducer element works on the principle that the resistance of the element is directly proportional to the length of the conductor and inversely proportional to the area of the conductor.
R=ρ L/A
‘R’ is the resistance of the conductor.
‘A’ is the side view part of the conductor.
“L’ is the conductor’s length.
‘ρ’ – the resistivity of the conductor.
7.2.1 POTENTIOMETER
Characteristics of Potentiometer
Characteristics of Potentiometer
The potentiometer is very accurate because its works on the comparing method rather than the deflection pointer method for determining the unknown voltages.
It measures the null or balance point which does not require power for the measurement.
The working of the potentiometer is free from the source resistance because no current flows through the potentiometer when it is balanced.
• Consists of a resistance element provided with a sliding contact (called a wiper).
• Motion of the sliding contact may be translatory or rotational or both (resistive elements in the form of helix – helipots).
ROTARY POTENTIOMETER
LINEAR POTENTIOMETER
MORE ABOUT POTENTIOMETER
APPLICATION
APPLICATION
- Potentiometer senses displacement by means of sensing shaft, which is mechanically connected to the point or objects whose displacement, is to be measured.
- Example: Petrol-tank level indicator. In this case, potentiometer is used to indicate/sense the petrol level in a tank as shown in Figure below. The output signal (voltage) is proportional to the petrol level.
EXAMPLES 1
EXAMPLES 1
A resistive transducer with a resistance of 5 kΩ is used in the arrangement in Figure 6.2. Potentiometer 𝑅3 − 𝑅4 is also 5 kΩ and 𝑉𝑡 is 5V. The initial position to be used as a reference point is such that R1=R2 (ie. the shaft is at the center). At the beginning, 𝑅3 and 𝑅4 are adjusted so that the bridge is balanced (Ve = 0). Assuming that the object being monitored moves a maximum resistance of 0.5 in towards A, what will be the new value of 𝑉𝑒? (Shaft distance is 5 in).
SOLUTION;
7.2.2 STRAIN GAUGE
The strain gauge is an example of a passive transducer that senses the strain produced by a force on the wires. Strain can be measured more easily by using variable resistance transducers, it is a common practice to measure strain instead of stress, to serve as an index of pressure.
If a metal conductor is stretched or compressed, its resistance changes since both the length and diameter of the conductor changes.
The resistivity of the conductor also change when subjected to strain, this is called the piezo-resistive effect. Therefore, resistance strain gauges are also known as piezo resistive gauges
R=ρ L/A
ρ =Resistivity
L=length
A=cross sectional area
Types of strain gauge
-Wire strain gauges -Foil strain gauge -Semiconductor strain gauge
GF=K=(ΔR/R ) / (Δl/l )
K = GF = gauge factor
ΔR = the change in the initial resistance in R = the initial resistance (without strain)
l = the change in the length in m
l = the initial length in m (without strain)
This gauge looks like a wire gauge except it uses a metal foil as the sensing element. Foil strain gauge has the same characteristics as the wire strain gauges
The resistive material used to make this gauge is a semiconductor type such as germanium and silicon. Even though semiconductor has a very high gauge factor, it is very sensitive to changes in temperature. Its has poor linearity and more expensive.
EXAMPLES 2
EXAMPLES 2
Given a resistance strain gauge with GF = 2.0. Strain is 1 x 10-6 and R = 120W. Calculate DR.
SOLUTION;
K = ΔR/R
Δl/l
ΔR =KR Δl/l
=2´120Ω ´1´10-6
= 240 μΩ
𝑅 = 120 + 240μ = 120.00024 Ω.
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EXAMPLES
EXAMPLES
1) A displacement transducer with a shaft stroke of 3.0 in. is applied in the circuit of Fig. The total resistance of the potentiometer is 5 k , and the applied voltage VT=5.0V. When the wiper is 0.9 in. from B, what is the value of the output voltage V0?
2) A resistive position transducer with a resistance of 5000 and a shaft stroke of 5.0 in. Potentiometer R3R4 is also 5000 , and VT = 5.0 V. The initial position to be used as a reference point is such that R1=R2 (i.e.. the shaft is at midstroke). At the start of the test, potentiometer R3R4 is adjusted so that the bridge is balanced (VE=0). Assuming that the object being monitored will move a maximum distance of 0.5 in. toward A, what will the new value of VE be?
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