Introduction to Transducers and Measurement

1
Introduction to Transducers and Measurement

• Structural Engineering Laboratory
• 560.208
• Spring, 2000

2
Goals

• Introduction to basic measurement equipment
• Exposure to theory of operation of several
  transducers
• Become comfortable with transducers to be used
  later in the semester
• Acquire basic strain-gauging skills

3
Transducer

• A device that converts a physical measurement
  into an electrical signal.
• Types
• passive (need external power supply)
• active (no power required)
• Calibration required
• Reliability, non-interference, etc..

4
Quantities of Interest

• Force/load
• Stress (normal, shear)
• Displacement
• Strain (normal, shear)
• Acceleration/velocity
• Pressure

5
Measuring Strain

• Two major approaches
• electrical resistance gauges
• optical methods
• Required characteristics
• high spatial resolution
• temperature insensitivity
• high frequency response for dynamic measurements

6
Measuring Strain, ctd.

• Resistance gauges
• resistance of a conductor changes with change in
  length (linearly)
• incorporate in a Wheatstone bridge circuit
• supply stable power
• measure resistance change
• calibrate to strain
• one or two active arms

7
Measuring Displacement

• Mechanical
• Surveying e.g., laser
• Dial Gauge
• Electrical/electronic
• Linear variable differential transformer (LVDT)
• Indirect measurement
• Calibrate induced strain in a strain link

8
The LVDT

• Principle of operation
• AC output voltage proportional to displacement of
  (magnetic) core element
• Primary coil provides excitation
• Secondary coils excited proportional to number of
  coils exposed.
• Small displacements (can have big LVDTs)
• Handles and - displacements through winding
  direction of secondary coils

9
Measuring Force and Pressure

• Basic principle
• Deformable element (very stiff)
• Apply load (pressure)
• Induce strains in element
• Calibrate strains with load (pressure)
• Stress?

10
Measuring Motion

• Displacement
• Use e.g., LVDT
• Velocity
• Moving magnet in a coil induces voltage
• Acceleration
• Two components
• Inertial mass
• Mass position measured via displacement (e.g.,
  optical, LVDT) or piezoelectric means.

11
Laboratory Exercise

• Apply strain gauge to test element of known
  dimensions
• Load test specimens incrementally
• Record load, measure strain
• Calculate stress at different load levels
• Plot stress-strain curve for material
• Estimate Youngs modulus

12
Summary Report Requirements

• Schematic of test setup
• Record details of gauges used and equipment
  (available on web site)
• Record dimensions of specimen
• Table of load/strain/stress calculations
• Stress-strain curve
• Youngs modulus estimate
• Make sure all group members names are indicated
  and signed off.