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

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.

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

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.

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.

3

The names of the numbered components are as follows:

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.

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

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.

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.

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

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

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:

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:

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.

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.

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.