Semester 1: Sensors & Signal Processing

Question 1

Describe the general structure and function of a strain gauge

Answer 1

A thin resistor with changing resistance depending on length. As the strain gauge extends, the area of the conducting material reduces and the resistance increases, and vice-versa.

Question 2

What is the equation for strain gauge reading from resistance?

Answer 2

Strain = (dR/R) / Kg = dR / (RKg); Kg is the Gauge Constant, R is the normal resistance, dR is the change in resistance.

Question 3

Describe a Wheatstone Bridge Circuit for a single Strain Gauge. For a fixed force, how is the strain found?

Answer 3

A voltage supply connected to a diamond-shaped array of 2 fixed resistors, 1 variable resistor and a Strain Gauge.
Before applying the force, the variable resistor is set such that Vg = 0. The change in resistance required to return Vg to 0 after applying the force is used in the equation:
dR1 = R4(R2 + dR2) / R3 - R1, from which strain is found from the equation:
Strain = (dR/R) / Kg

Question 4

For a time-varying force, how is the strain found?

Answer 4

The variable resistor is not changed, and instead the reading for Vg is used. The equation and derivation are in notes.

Question 5

What is the principle behind Thevenin Equivalent Circuits?

Answer 5

Any linear circuit can be reduced down to a single voltage source and resistor in series.

Question 6

How is Thevenin Equivalent Voltage found across a resistor?

Answer 6

The resistor is removed from the circuit, and the voltage difference between the 2 ports is calculated.

Question 7

How is Thevenin Equivalent Resistance found for a circuit?

Answer 7

Voltage sources are shorted / replaced with wires, and the circuit resistance between the 2 ports is calculated (ignores breaks, open loops).

Question 8

Name the 2 types of Rotary Encoder and briefly describe their differences.

Answer 8

Incremental and Absolute. Incremental Encoders can only measure changes in angle, whereas Absolute Encoders measure angular position directly.

Question 9

What are the advantages of Grey Code over Binary Code for Rotary Encoders?

Answer 9

Grey Code is more reliable as only 1 ‘bit’ is flipped at each increment, whereas with Binary Code multiple bits flip on some increments.

Question 10

How do you determine the number of bits required to store the resolution of data desired?

Answer 10

Delta = (Vmax - Vmin) / 2^n; Delta is the resolution of data desired, n is the number of bits. ‘n’ ROUNDS UP.