IRJET- Signal Conditioning Card for Load Cell

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 05 Issue: 03 | Mar-2018 www.irjet.net p-ISSN: 2395-0072
© 2018, IRJET | Impact Factor value: 6.171 | ISO 9001:2008 Certified Journal | Page 2727
SIGNAL CONDITIONING CARD FOR LOAD CELL
Sowmya T S1, C Prabhavathi 2, Navayashree K3
1MTech Digital communication department, Siddaganga institute of technology Tumakuru
2 Associate professor Telecommunication department, Siddaganga institute of technology Tumakuru
3 SaiTektronix Pvt. Ltd Bengaluru
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Abstract - In industry environment the most commonly
measured parameters are strain, force and pressure.Themost
commonly used sensing devices in order to measure above
parameter are strain gauge load cell. There are differenttypes
of strain gauges are available resistive, capacitive,
piezoelectric transducer type and semiconductor type strain
gauges are there. Strain gauge sensor is a passive transducer
that converts the mechanical elongation, displacement
produced due to force or load acting on them into change in
resistance, capacitance and inductance value. They are low
impedance devices, they required exact excitation (input)
value because there signal (output) will be in milli volt range,
the typical load cell will be having four strain gauges which
are all arranged in Wheatstone bridge manner with
impedance of 270 ohms or 350 ohms. The load cell will be
having six wires, two of them for “excitation” voltage, two of
them is for giving “signal” and two of them are for “sens”, in
order to maintain the excitation voltage as the output from
load cell are in milli volt range they need to be conditioned for
next stages with low cost, high precision, reliability,
amplification and noise reduction capability. The low output
signal after amplification is to be send to filtering, analog to
digital conversion then the output is programmablecontrolled
by microcontroller ATmega 168p then they againconvertedto
analog signal using digital to analog converter (DAC)because
the next level will be of analog component in industry that is
programmable logic controller (PLC) in order to display the
weight and to send the packed things to suitable place.
Key Words: load cell, sens, PLC, strain gauge, signal
conditioner.
1. INTRODUCTION
Measuring the parameters like strain, force and pressure is
very important in industrial environment because, if it is a
product based industry if the measurement is not correct it
may leads to loss of money they have invested or if any
industry is based on producing the animal feed for example
if it is a poultry based, then for getting a protein rich egg,
food feeder should be in measurable amount if it is less or
more also there will be a loss in production. [1]
There is a need in constantly monitoringtheweight,
here for measuring any load, load cell are used. Load cell are
the type of transducer or a sensor which convertstheloador
any force applied on them into electronic signals. The
electronic signals refer to change of voltage, current or
frequency it depend on the load applied.Themostcommonly
used load cell is resistive strain gauge based load cell.
In industry load cell may be present at some place
and the monitoring of weight will be in some other place
there will be copper wire connectivity between them which
may be of 80-100 meter in length. The applied voltage to
load cell while reaching it there may be a resistive voltage
drop in excitation voltage to load cell hence, there is a need
for conditioning the applied voltage. The signal conditioner
may be used in order to amplify any signal, filtering,
interfering with microcontroller,analogtodigitalconversion
(ADC), protection, linearization and for error compensation.
The signal conditioning is a technique of making any signal
from transducer or sensor to suits for processing by data
acquisition equipment.
2. System flow
In industrial environment weighing of a definite amount of
thing is done automatically by the use of load cell. Fig (1)
shows the product flow in industrial environment. [4]
Fig. 1. Product flow of load cell signal conditioner
When the weight is applied on the load cell, correspondingly
its internal resistance changes and the input excitation of
load cell needs to be maintained using signal conditioning
card and the milli volt output of load cell needs to be
amplified in signal conditioning card, then the signal from
signal conditioning card is given to programmable logic
controller (PLC) in industry.
3. Proposed system development
The proposed system of signal conditioning make easy in
product weight measurement and in packing system. The
signal conditioning card development was having the
purpose ofmaintainingthe excitation voltage tothe loadcell,
improving the resolution of ADC and maintain the output of
DAC8760 of (-10 to +10V) because, its next stage is PLC
which required the same range as mentioned. The below Fig
(2) providesthe theoretical approachfor designingthesignal
conditioning card for the load cell using printed circuit
board (PCB).

Fig. 2. Theoretical approach for accurate weight.
The main purpose of signal conditioning unit is to get the
proper excitation voltage to the load cell with no resistive
voltage drop in copper wire which is connected between
load cell and weight monitoring side, to improve the
resolution from 16bit to 18 bits in ADC so that we can weigh
in terms of 1grams and maintain the output of DAC of (-10V
to +10V). Before input giving to the load cell it is voltage
regulated and given as“excitation” to load cell. Theoutputof
load cell is in (0-30) milli volt range that is called ‘’signal’’
which is amplified in analog to digital converter and the
digitized output is given to microcontroller (ATmega168p)
in which the obtained data is programed. The output of
microcontroller is given to digital to analog converter (DAC)
and its output needs to maintained in the range of (-10V to
+10 V) and its next stage will be programmable logic
controller (PLC). Then the analog counts can be seen in PLC
in an industrial environment.
4. System analysis and design
To design and develop the PCB, hardwareandfirmwareused
in this project are:
A. Hardware design and implementation:
The hardware used are load cell, dual power operational
amplifier, analog to digital converter (ADC),microcontroller,
digital to analog converter (DAC).[5]
1). Load cell: A load cell is a transducer that converts force
into electrical signal. The most of the weighing equipment
uses the weight stone bridge type load cell sensor, whose
output voltage is in proportional to applied weight over it,
the load cell consists four strain gauges which are resistive
type when the load applied on the load cell correspondingly
there resistivity changes which leads to change in output
voltage . The typical load cell diagram is as shown in fig (3a)
under resistive load cell shear beam load cell are used fig
(3b). [2]
Fig. 3(a). Typical load cell diagram.
Fig. 3(b). Single point load cell.
Fig.3(c). Specification of single point load cell.
2). Dual power operational amplifier: They are used to
maintain the input voltage as needed to the load cell; the
operational amplifier used having the features like:
• Drawn able output current of 1A
• With a slew rate of 1.3V/s
• With a band width of 1.1MHz
• It has a capability of internal thermal shutdown
when Temperature increases.

3). Analog to digital converter (ADC): it is an analog front
end module for measuring pressure and in weighing
machines.[1] It can accept the low voltage signal from the
transducer and provide the serial digitized output. They are
used as external ADC in order to increase the resolution to
18bits so load cell can measure in terms of 1grams. It is
having the features like,
• An output count up to 2, 30,000.
• It is having a gain of 2ppm/C.
• It is having a frequency rejection below 150dB.
• It is having an internal offset/tare DAC of 6 bits.
• It is having the reference voltage from 1V to 5V.
4). Microcontroller: Microcontroller is one of the main
functional elements in this piece of work. Microcontroller
used here is ATmega 168p which belongs to Atmel family.
They are used in order to reduce the countfromADCwhichis
24 bits to 16 bits so that it can be given to next stage which is
DAC and for communicating between ADC and DAC usingSPI
protocol. [6]
5). Digital to analog converter (DAC) : The DAC used here is
of 16 bit which is of high precision and low cost they are
designed in order to meet the industrial processor control
systems.
Implementationofhard ware is doneonprintedcircuitboard
(PCB) as shown in fig (5).
Fig. 5. Hardware module on PCB.
The required hardware components for signal conditioning
card are designed. Both the analog and digital sections are
present on same hardware module; each components are
designed andthere 2D and 3D modelswheregenerated,after
arranged them in a précised way the 3D model of the card
look like as shown in above figure , this designed card is
inserted between load cell and PLC.
B. Firmware frame work
The software used here is ATmel studio. It is the
integrated development platform (IDP) for developing and
debugging of all types AVR and SAM type of microcontroller.
The Atmel studio version 7 provides the programmer an
easily handle able environment so that it can write, debug
and compile with the basic language like C, C++ or assembly
language and it is easy to overcome the error if presented.
Fig. 6. The overall program flow of the project.
First we have to initialize SPI and GPIO hardware, and then
we have to check whether ADCoutputis voltageorcurrent.It
can be seen by glowing of their corresponding led’s.
Initializing AD7730 and DAC8760 and check anyinterrupt is
coming from ADC, if it is coming then process the value of
ADC from 24-bits to 16 bit so, that the value can be given to
DAC. If there is no interrupt occurrence then wait until it
obtains from ADC. The overall flow is as shown in fig (6).
5. Experimental Result and its Analysis
Fig. 7. This is the hardware signal conditional model.

The signal conditioning for load cell, for maintaining the
constant excitation voltage to load cell and providing the
output voltage in range of (-10V to +10V) out of DAC8760
which is input to PLC in industry environment is as show in
above fig (7).
Fig.8. Signal from load cell and output voltage of DAC.
After the model is completely set, by providing the voltage
from voltage supply, it will be regulated to provide required
input voltage excitation of 5V to load cell. The output from
load cellwill be in range of (0-30mV), we aregettingis1.2mV
by applying 5Kg of weight on load cell. After checking the
output of load cell which is in the range of (0-30mV) ,that is
given to ADC where any –ve voltage coming from load cell
inside ADC there will be a 6-bit DAC which will add a 1.25mv
to output of load cell so that the load cell voltage remains in
(0-30mV) range then counts from ADC is given to ATmega
168p microcontroller in order to program the ADC and the
DAC toprovide output of (-10to+10V) so that wecandisplay
the counts in voltage and in terms of weight it will be
displayed in industrial used programmable logic controller.
We obtained here is -8.03V for 5Kg weight.
5. CONCLUSION
Signal conditioning card is designed in order to
measure the parameterslikestrain, force and pressureusing
load cell,maintainingthe excitationvoltageof10Vtoloadcell
using dual power operational amplifier, increasing the
external ADC counts with a resolutionof18-bitsforweighing
in terms of 1gram and amplifying the load cell output which
is in milli volt range to (-10V to +10V) in order to give the
input to next stage which is PLC.
ACKNOWLEDGEMENT
The author would like to thank Siddaganga institute of
technology, Associate professor Mrs. C. prabhavathi
department of TCE, Tumkur-572103 for sharing the
knowledge, making me to learn and giving me the best
guidance and sai Tektronix pvt. Ltd, Rajarajeshwari nagara,
Bangalore, for giving the opportunity to work there and to
gain knowledge.
REFERENCES
[1] Tan Jui Ang and Ng Nan Kong,” load cell for dynamic
Force Measurements: an example in thick film Technology,
“electronics Manufacturing technology (IEMT) and 18th
electronics materials and Packing (EMAP) conference 2016
IEEE 37th international pp. 1- 3, 2016.
[2] Anton satire prabuwono, Habibullah Akbar and W.Usino
faculty of Information and comm. technology” Pc based
weight scale system with Load cell for product Inspection,”
international conference on computer Engineering
technology, Singapore, pp. 343-346, 2009.
[3] https://www.allaboutcircuits.com/textbook/direct-
-current /chpt-9/strain-gauges
[4] Ahmad a. zorob, M piriz, c de Jesus Aquino,” building
Signal conditioning for strain gauge sensors,” 2010.
[5] André v.pigatto, Karina o.a. moura,GabrielaFavieiroand
Alexandre balbinot “A new crank arm Based load Cell, with
built-in conditioning circuit and Straingages,Tomeasurethe
components of the force Applied by a Cyclist,”engineeringin
medicine and Biology society (EMBC) 2016 IEEE 38th annual
International conference ofOrlandoFL USA,pp.1983-1986,
2016.
[6] Rakesh Naskar, Partha Das and Ratan mandal,” a unique
Model of Characterization &performance estimation of
various solar photovoltaic cells/modules using
Microcontroller,” 1st international conference on non
Conventional energy (ICONCE), pp.6-9, 2014.

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