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Temperature Sensors Continued Strain Sensor

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A railroad track support column has four solid-state strain gauges cemented to it. ... is to be able to measure the strain on the column as trains pass over it. ... – PowerPoint PPT presentation

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Title: Temperature Sensors Continued Strain Sensor


1
Temperature Sensors ContinuedStrain Sensor
2
Dynamic Response of Temperature Sensor
When a thermal mass is moved from a medium at
temperature T1 to another medium at temperature
T2 The thermal mass temperature T during the
transition is
? is a time constant determined by the thermal
mass material
3
Temperature Sensor Range
4
Strain Gauge Sensor Physical Structure
Bonding pads
Strain sensitivity direction
5
Strain Sensors
Strain ?L/L
L
?L
Strain gauge sensors R?L/A For material with
constant volume R ?L2/V ?R2 ?L ?L
/V ?R/R 2 ?L /L For material with
non-constant volume ?R/R Gs ?L /L
Strain gauge sensors converts strain to
electrical signals
6
Semi-conductive Strain Element
Both L and ? are function of strain, constant
volume
R?L/A
Non-constant volume
High gauge factors (Piezoresistive factor)
7
Example Strain Gauge Factors
8
Strain Measurement Using Single Strain Sensor
Balance the bridge before applying strain to the
strain sensor, RsR
Apply strain RsR?R
RS
R
-
Vb
V0-
R
R
For a given strain sensor (L, Gs), by measuring
V0 and Vb, we can find ?L
9
Strain Measurement Using Four-Strain Gauge
Sensors
Tensile strain
t1
t2
b1
b2
b2
Compressive strain
t2
-
Vb
V0
t1
b1
4-strain sensor system is more accurate, and is
not temperature sensitive.
10
Example
A railroad track support column has four
solid-state strain gauges cemented to it. The
force on the column is straight down (no bending,
only compression). Your goal is to be able to
measure the strain on the column as trains pass
over it. Two of the strain gauges resistances are
the p-type gauges, the other two strain gauges
are the n-type gauges.
n-type
p-type
11
Example continued
  • Assume that all gauges have the same unstrained
    resistance R.
  • Draw a bridge circuit to measure the compressive
    strain on the beam. Indicate the positions of
    the P-type and N-type strain gauges.
  • Derive an expression relating the bridge output
    to the compressive strain on the column.

12
Homework and Reading
  • Handout5
  • Reading P243-250
  • Homework 14 Due Wed. 4/6
  • Redo the example problem if the n and p type
    train have the following characteristics
  • n-type
    p-type
  • What is the sensitivity of the bridge at very
    small strain?
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