Chapter 10: Displacement and Motion Measurement

1
Chapter 10 Displacement and Motion Measurement

• Overview
• Displacement Measures
• Measurement of Acceleration
• Velocity Measurements

2
Translational Displacement

• Resistive potentiometer
• Wire-wound track or carbon film and plastic film
  tracks
• Problem dirt under slider ?resistance?
  inaccuracy ? 1 f.s.
• Measurement ranges ? 2mm - ? 1m
• Linear variable differential transformer (LVDT)
  measured object is attached to the central iron
  core of the transformer, inaccuracy ? 0.5 f.s.,
  infinite resolution

3
Translational Displacement
4
Rotational Displacement

• Circular potentiometer translational
  displacement with circular bent track, ? 1 f.s
• Incremental shaft encoders count the pulses of
  the shaft. Pulse generator optical (cheapest) or
  magnetic means, resolution ? number of windows on
  disc, give digital output
• Coded disk shaft encoders binary output of an
  absolute measurement of shaft position, Three
  forms optical (cheapest), electrical, magnetic
  energy system

5
Rotational Displacement
6
Measurement of Acceleration

• Seismic accelerometers
• a dK/m
• where
• a acceleration (m2)
• d displacement of the mass relative to the
  frame (m)
• K stiffness of the spring (Nm-1)
• m weigh of the mass (kg)
• Piezoelectric accelerometer
• Output 4-20mA
• Measurement range 50g peak
• Sensitivity 0- 10mm.sec-1
• Electrical noise 0.3mV max
• Supply voltage 10-32V
• Weight 150 gms

7
Acceleration Measurements
8
Velocity Measurements

• Translational velocity
• Differentiation of translation displacement
  noise
• Integration of translation acceleration
• Moving coil transducer
• Rotational velocity
• Differentiation of an angular displacement
• Integration of an angular acceleration
• Mechanical measurement technique
• Electromagnetic techniques

9
Velocity Measurements
Moving coil transducer
Mechanical angular velocity sensor
Optical rotational speed transducer
Electromagnetic angular velocity sensor