- Studied the wiring diagram of PI and ON-OFF controller implemented in LabVIEW and analysed its effects - Interfaced strain gauge sensor with LabVIEW software via NI ELVIS for weight measurement

1. A report on
LabVIEW Applications for
QNET – Heat Ventilation and Air
Conditioning (HVAC) module
and
Strain Gauges
Submitted by: Rishikesh Bagwe (2012A8PS401G)
Project Guide: Gautam Bacher
AUG 2015 – DEC 2015

2. i
Abstract
This report has 2 parts first a study of the QNET Ventilation and Air Conditioning (HVAC)
module and second a study of how a strain gauge sensor works and how can we use it to
calculate the weight of an object both of which involves LabVIEW.
The first section tells you how the temperature control of a chamber happens using a ON-OFF
controller, how the response varies by changing the parameters of the controller. It also gives
some insights into the wiring diagram level understanding of the ON-OFF controller already
implemented in LabVIEW.
The second section mentions various circuit configurations to effectively measure the value
from the sensor and also how to amplify it using proper amplifier circuitry. The elimination of
noise is also done in this project. The experiment was constructed in the lab and the results,
plots, diagrams of the same are documented in this report.

3. ii
Table of Contents
Abstract.......................................................................................................................................i
Table of Contents.......................................................................................................................ii
1 QNET – HVAC (Heat Ventilation and Air Conditioning).................................................1
1.1 Introduction.................................................................................................................1
1.2 Simulation Results.......................................................................................................4
2 STRAIN GAUGES.............................................................................................................7
2.1 Introduction – How does strain gauge works?............................................................7
2.2 Full bridge circuit........................................................................................................8
2.3 Weight Measurement using Strain Gauge...................................................................9
2.4 Experiment ................................................................................................................10
2.4.1 Readings without amplifier................................................................................10
2.4.2 Circuit Construction...........................................................................................10
2.4.3 Experimental Setup............................................................................................12
2.4.4 Readings with amplifier and RC filter...............................................................12
2.4.5 Measuring the weight of the object....................................................................13
3 Conclusion .......................................................................................................................14

4. 1
1 QNET – HVAC (Heat Ventilation and Air
Conditioning)
1.1 Introduction
QNET HVAC is a trainer module build to give a practical experience of how a temperature
control happens with a use of an ON-OFF and a PI controller. The module consisted of a
horizontal cylindrical glass chamber. Inside the chamber it has a BJT temperature sensor to
sense the chamber temperature and a heater to increase it. The chamber has an exhaust fan
attached to prevent it from over-heating. There is also a BJT temperature sensor outside the
chamber to measure the temperature of the environment.

5. 2
The names of the numbered components in the above figures.
The temperatures measured are sent to the LabVIEW controller VI (Virtual Instruments)
written for the controllers via a DAQ which is present in NI ELVIS II+
. There is a panel window
in LabVIEW which shows the Real-time graphical representation of the measured temperature.
So the heater goes on and off according to the controller logic. The controller VIs are already
provided by the QNET.
The following is the layout of the front panel of the controller VIs made in LabVIEW.

6. 3
The names of the numbered components are as follows:

7. 4
1.2 Simulation Results
a. Varying the offset of the signal generator
The signal generator generates the set-point of the controller. This time the frequency
of the signal is 0 and the offset the offset from the ambient measured temperature and
not the chamber temperature. Here is the graph of how the chamber temperature varies
when we change the set point.
As you can see the frequency of oscillating chamber temperature (red line) changes as
we change the set-point (blue line). So the heater stops and starts and different
frequencies according to the chamber temperature crossing the set-point.

8. 5
The cooling of chamber at higher temperature values is faster and it rate of cooling
gradually decreases as we move down to lower temperatures, that is why due to faster
cooling rates we get high frequency of oscillating chamber temperature.
b. Varying the relay amplitude (Vh_amp)
The relay is the voltage given to the heater. The amplitude is the amount of voltage
above the offset value (Vh_offset) given to the heater. Here is the graph how the
controller controls the chamber temperature for different values of Vh_amp.
The offset of the heater/relay voltage is kept at 4.00 V. During the first four iterations
the of amplitude Vh_amp is kept at 4 V and then it is decreased to 3.5 and then to 2 V
and 1 V. That is for amplitude 3.5 the voltage oscillates between 0.5 and 7.5 V and
similarly you can see for others As it can be noticed from the graph that for amplitude
2 V the voltage can go lower upto 2 V and not lower than that therefore the heater is

9. 6
always on at 2 V and the controller cannot control the temperature, the chamber
temperature keeps on increasing. Also when you decrease the amplitude you a get a
relatively sluggish response.
c. Varying the relay offset voltage Vh_off
The relay/heater offset voltage Vh_off is the mean voltage of the heater about which
the voltage will oscillate according to Vh_amp. Here is the graph how the controller
controls the chamber temperature for different values of Vh_off.
Now at 2 V the lower part of the heater voltage can go up to 0 V even if the amplitude is
4 V. it cannot go to -2V. So the heater voltage oscillates between 6V and 0V for offset 2V
and amplitude 4V. We decrease the relay offset voltage the overshot of chamber
temperature decreases. Also the frequency of oscillation also increases gradually.

10. 7
2 STRAIN GAUGES
2.1 Introduction – How does strain gauge works?
To understand how the strain gauge works we first have to describe what is strain.
Strain is defined as deformation of a solid due to the force applied in the direction of the
deformation and can be expressed as ε.
ε = dL / Lo
where, dL is the change of length
Lo is the initial length
So the strain is a dimensionless quantity.
This change of length has an effect on the resistance of the material. Now the resistance(R)
depends on the dimensions of the material and is given by
R=ρL/A
where ρ is the resistivity, L is the length and A is the cross sectional area.
Therefore the strain gauge gives the output in terms of changes in the resistance of the material
which we measure by detecting the voltage change accordingly using bridge circuits.

11. 8
2.2 Full bridge circuit
Full bridge is one of the arrangements where the 4 strain gauge sensors are connected in a form
of a Wheatstone bridge, and instead of resistances there are these sensors.
In practice, the strain measurements rarely involve quantities larger than a few millistrain (ε ×
10–3). Therefore, to measure the strain requires accurate measurement of very small changes
in resistance. For example, suppose a test specimen undergoes a substantial strain of 500 µε. A
strain gauge with a gauge factor GF = 2 will exhibit a change in electrical resistance of only
2*(500 × 10–6) = 0.1%. For a 120 Ω gauge, this is a change of only 0.12 Ω. To measure such
small changes in resistance, and compensate for the temperature sensitivity (resistance change
due to temperature) strain gauges are almost always used in a bridge configuration with a
voltage source. The general Wheatstone bridge, illustrated below, consists of four resistive
arms with an excitation voltage, VEX, that is applied across the bridge and Vo is measured.
Therefore, Vo = [
𝑅𝑜− ΔR
2𝑅𝑜
−
𝑅𝑜+ ΔR
2𝑅𝑜
] * Vex ,
Vo = -
ΔR
𝑅𝑜
* Vex ,
𝑉𝑜
𝑉𝑒𝑥
= -
ΔR
𝑅𝑜
So the change in resistance is proportional to the voltage measured (Vo).
There is a term Gauge factor(GF). Gauge factor is defined as the ratio of fractional change in
electrical resistance to the fractional change in length (strain). It is a fundamental parameter of
the strain gage describing its sensitivity to strain.
𝑉𝑜
𝑉𝑒𝑥
= - GF * ε

12. 9
2.3 Weight Measurement using Strain Gauge
For a force applied on a cantilever beam, the strain is given by
ε = (6 x F x L) / (W x T2
x Y); F = m*g;
where, F = force (N), m = mass (Kg), g = acceleration due to gravity (9.8m/Sq. sec), L = Length
(m), w = width (m), T = thickness (m), Y = Young's modulus (N/Sq. m) = 200 x 109 N/m2 for
stainless steel.
Therefore we can relate the voltage measured across with mass or weight force applied on the
cantilever beam.
Vo = [
6∗𝑔∗𝐿∗𝑉𝑒𝑥∗𝐺𝐹
𝑊∗𝑇2∗𝑌
] * m
The values of the parameters of the cantilever beam used in the experiment
The cantilever beam used in the
experiment

13. 10
2.4 Experiment
The full bridge circuit is constructed on the cantilever beam itself. Four wire are drawn out 2
for excitation voltage and 2 for measuring the voltage.
2.4.1 Readings without amplifier
First the measurements are taken without the any amplification
Weight (in grams) Voltage (Vo) (in mV)
50 2.02
100 2.39
150 2.75
200 3.15
250 3.52
300 3.90
The Vo values are very low, so these measurements can be affected by noise from fluctuations
in the excitation voltage, interference during transmission of signals through wires etc. So in
order to remove these noise and amplify the values a differential amplifier with buffered inputs
and an RC low pass filter is constructed.
2.4.2 Circuit Construction
Differential amplifier with buffers:

14. 11
The initial buffers cut out some noise because of their high input impedance. The differential
amplifier then amplifies the Vo by 100. Gain =
10 𝐾
100
.
Even after the buffers, some noise is passed through and it get amplified along with the signal
but this noise is a fast acting noise that is it has a higher frequency. In order to filter this noise
we construct a RC low pass filter.
The cutoff frequency for a low filter is given by the formula fc = 1/2πRC. We used a cutoff
frequency of 19 Hz by the use of combination of resistor and capacitor mentioned in the
diagram.
Circuit constructed in the lab:

15. 12
2.4.3 Experimental Setup
We have used NI ELVIS to take in the values and then constructed the amplifier and the filter
on the ELVIS board. To view the final values we used the DAQ card on NI ELVIS to see the
values on LabVIEW DAQ assistance.
2.4.4 Readings with amplifier and RC filter
Weight (in grams) Voltage (Vo) (in V)
50 0.107
100 0.148
150 0.187
200 0.225
250 0.264
300 0.303
350 0.341
The values in the table are the amplified value and these values are stable till the 3rd
decimal
place on the LabVIEW numerical indicator which showed upto 6 decimal places.

16. 13
2.4.5 Measuring the weight of the object
In order to map the amplified voltage values to their corresponding weights we need to know
the slope and y intercept of the line described by the above table.
Graph:
In the graph above the y axis represent the voltage (Vo) measured and x axis represents the
weights So the y intercept of the graph above is 0.694 and its slope is 0.000777.
To calculate the weight (x) from the voltages(y), y = mx +c , (y-c)/m = x. This arithmetic
calculation as done on the values obtained in the LabVIEW and the weight was calculated.

17. 14
3 Conclusion
The simulation results in the first section (QNET - HVAC) comply with the controller logic.
So the ON-OFF controller always tries to maintain the temperature around the set-point. The
PI controller was not simulated. Observing the results in a real-time fashion leads to deeper
understanding of the controller.
In the second section (Strain gauge), with the help of strain gauge we can measure the mass of
the object in real time by using DAQ of NI ELVIS. Also I learned how to measure and calibrate
the measurements of a sensor. Various noise cancellation techniques were used like buffers
and RC filter.