Standard Industrial Sensors

Question 1

Potentiometer

Answer 1

a resistance element with a sliding contact that can be moved over the length of the element
wire-wound or conductive plastic
rotary (single turn), rotary (multi-turn), or linear

Question 2

Wire wound potentiometer advantages / disadvantages

Answer 2

advantages: not temperature sensitive, linearity between 0.1% and 1%
disadvantages: limited resolution (due to wire diameter)

Question 3

Plastic potentiometer advantages / disadvantages

Answer 3

advantages: unlimited resolution, linearity of 0.05%
disadvantages: temperature sensitive

Question 4

Potentiometer advantages / disadvantages

Answer 4

advantages: low cost, large range
disadvantages: output affected by load resistance, analog output, and sensor is affected by dirt and wear

Question 5

Strain gauge

Answer 5

a wire, metal foil, or semiconductor strip that can be stuck onto a surface to measure its strain
the change in resistance due to the strain can be converted into a voltage
can be attached to flexible elements to increase range - cantilever, ring shaped, U shaped

Question 6

Strain gauge advantages / disadvantages

Answer 6

advantages: no wear
disadvantages: small range unless mechanical element is added, analog output, and temperature sensitive

Question 7

Optical encoder

Answer 7

produce a digital output for an angular (or linear) displacement

incremental: output is a measure of relative displacement
absolute: output is a measure of absolute displacement

Question 8

Optical encoder advantages / disadvantages

Answer 8

advantages: no wear, digital output is relatively insensitive to noise and easy to interface
disadvantages: limited resolution, susceptibility to large vibrations

Question 9

Velocity sensors

Answer 9

used for motion applications, such as robots and machine tools. This is done by

A) differentiating displacement or position
Adv: Low cost
Dis Adv: - Amplifies high frequency noise.
- Adds delay

B) Tachogenerator or Tachometer: This sensor produces an analog voltage that is proportional to the velocity.
Adv:
Dis Adv: Brushes create noise and are subject to wear. (Solution is brushless uses an electric circuit to communicate the signal)

Question 10

Acceleration Sensors

Answer 10

Used in: Airbags, Anti-lock breaking system for detecting skids and for sensing vibration.

Two designs: Mass on a piezoelectric crystal, mass attached to a cantilever beam with strain gauge.
Fn = (1/2pi) * (sqrt(k/m)) while range is dependent on (m/k) t be used for velocity or displacement because it will accumulate error when you integrate

Question 11

Force Sensors (Load cells)

Answer 11

A) Strain gauge type
Adv: Low cost
Dis Adv: worse linearity and greater temperature sensitivity that the piezoelectric type

B) Piezoelectric type:
Adv: Better linearity and temperature sensitivity than the strain gauge type
Dis Adv: Higher cost, especially for constant loads. The charge amplifier must be carefully designed to prevent drift.

Question 12

Temperature sensors

Answer 12

Bimetallic strips: two metallic plates with different temp expansion coefficients
Resistance temperature detectors: by measuring the resistance of a coil of wire
- Adv: simplicity and linearity
- Dis Adv: Small sensitivity

Thermistors: mixture of metal oxides that are very sensitive to temp changes

- Adv: could be easily molded different shapes for the application
- Large sensitivity
- Sensor can be very small (1mm)
- Dis Adv: Very non-linear response so special calibration circuitry is required

Thermocouples:

- Adv: Large range, excellent linearity
- Dis Adv: Very small sensitivity. very susceptible to noise.