Title: Temperature Sensors Continued Strain Sensor
1Temperature Sensors ContinuedStrain Sensor
2Dynamic 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
3Temperature Sensor Range
4Strain Gauge Sensor Physical Structure
Bonding pads
Strain sensitivity direction
5Strain 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
6Semi-conductive Strain Element
Both L and ? are function of strain, constant
volume
R?L/A
Non-constant volume
High gauge factors (Piezoresistive factor)
7Example Strain Gauge Factors
8Strain 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
9Strain 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.
10Example
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
11Example 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.
12Homework 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?