Page 798 - Mechatronics with Experiments

784 MECHATRONICS

LM 358 R 2 = 100 kΩ
Strain
120 Ω gauge
–
R 1 = 1 kΩ
+
–
200 Ω (Var) 120 Ω + To ADC
–
LM 358
+
5 V
V i
+– LM 358

FIGURE 11.26: Circuit diagram to amplify the voltage output of the strain gauge sensor.

For a given strain gauge, the gauge factor G is known. Then, if we can measure the
change in resistance (ΔR∕R), we can calculate the strain .

Signal Conditioning The signal conditioner has to convert the variations of resis-
tance of the sensor into a proportional electrical voltage signal that is compatible with the
PIC microcontroller.
The resistance variation is converted to a voltage differential by a Wheatstone bridge.
For the applied circuit, see Figure 11.26, the output voltage is
V ΔR
i
V = (11.128)
o
4 R
o
Notice that the location of the variable resistor for the sensor in the Wheatstone bridge
in the figure shown for the experiment is at location for R , whereas it was at location for R 1
2
in Chapter 6. It can be shown that the same output voltage and resistance change relationship
holds except the polarity of the output voltage between these cases. It is instructive for the
student to show that this is true. In Chapter 6, in Equation 6.20, let R = R = R = R and
1
0
4
3
R = R +ΔR. And substitute these in Equation 6.20 and make the same approximations in
2
0
deriving Equation 6.23. Simply let R = R = R = R and R = R +ΔR in Equation 6.20.
0
3
0
2
1
4
Since V is typically in the order of millivolts, an amplification stage has to be
0
provided. An operational amplifier can be used for this purpose. Resistors R (1 kΩ) and
1
R (100 kΩ) (see Figure 11.26) determine the gain
2
R 2
K =− (11.129)
a
R 1
The low impedance R of the amplification stage will strongly affect the output of
1
the bridge if it is directly connected to it. In order to avoid this effect, a voltage follower
stage is provided (referred to also as voltage buffer stage or impedance isolation buffer,
using two op-amps LM 358).
Application Software Description
The measurement principle here is that given the strain gauge properties, that is G, Wheat-
stone bridge parameters, V , R , and in the signal amplification stage, R , R , we can
1
2
i
0
calculate the strain from the output voltage measurement V ADC . Then, given the beam
material and geometric parameters, E, b, h, l , we can calculate the force, F.
s
The software development task includes setting up the registers to configure the pin
connected to the strain gauge sensor as one of the ADC channels to be used. This can be