Weighing Balance

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DESIGN OF AN ELECTRONIC WEIGHING BALANCE

By Name

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Date

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Abstract

Electronic weighing is more convenient than use of spring balances when determining

weights of large things like vehicles. The weight of the vehicle is given instantly in the digital

readout. Weighing of heavy trucks and cars happens daily and thus it is important to come up

with better ways of determining the weights instantly to reduce congestion in the weigh bridges.

Some of the weighing balances just measure the vehicle’s weight but do not show the vehicle

weight distribution which is very important.

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Contents

Abstract ........................................................................................................................................... 2

Background ..................................................................................................................................... 4

Problem statement ........................................................................................................................... 4

Objectives ....................................................................................................................................... 5

Literature Review............................................................................................................................ 6

Methodology ................................................................................................................................... 9

Program of Work .......................................................................................................................... 16

The Gantt chart ............................................................................................................................. 17

Conclusion .................................................................................................................................... 17

References ..................................................................................................................................... 18

Appendix ....................................................................................................................................... 20

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Background

Most of the electronic weighing balances use load cells which are calibrated according to

the environmental conditions of the place. The longitudinal and lateral distribution of forces is

not of interest in most weigh bridges. They measure the absolute weight of the vehicle only.

When a car is loaded, it is good to determine the weight distribution taking into account the

safety factor of the vehicle structural parts. A car can be loaded with a weight that is below the

weight limit, but the distribution of load in some directions surpass the limit of that direction

leading to increased stresses to the structural parts in that direction. The electronic weighing

balances should have transducers for measuring the distribution of forces in X-axis, Y –axis and

Z-axis. The X and Y axes give the weight distribution in the lateral and longitudinal respectively.

The Z-axis gives the weight distribution in the vertical direction. Load cells used to measure the

weight are piezoelectric transducers or sensors. This is a type of crystal that produces an electric

current when compressed. The more the compressive force, the more the electric current

produced. An electronic circuit usually an AC bridge circuit measures the electric current and

converts it into weight measurement. In most cases, the Z-axis measurement is emphasized than

the longitudinal and lateral measurements. Each axis of load distribution has its electronic circuit

for converting the current registered into weight

Problem statement

Measuring the vehicle load distribution in single axis does not primarily mean that the

vehicle structural components are on the safe side of the stresses and strains exerted by the

weight. All the three axes of load distribution should be measured, specifically the longitudinal

and lateral distribution.

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If the load distribution in the longitudinal and lateral directions is not measured, then the

components which carry the loads will not always be safe. It is important to ensure that every

structural component of the machine is working below the factor of safety limits to avoid failure

due to excessive stresses in a particular part.

This problem occurs in most weigh bridges and garages. In most weigh bridges they

measure the vertical load distribution forgetting the axle load of the vehicle. If the axle load

distribution is high, then it means that then rear wheels are overloaded, and this can result in

damage to the roads.

The load distribution problem needs to be addressed particularly when the vehicle is

loaded. This is done to ensure that all the axes are loaded below the threshold load or else some

components of the car will be under excessive stress.The lateral and longitudinal weight

distribution should be determined to avoid failure of the vehicle. There should be no any axes

that should be overlooked.

Objectives

The main objective of this proposal is to design an electronic device that weighs the

vehicle load distribution in the longitudinal and lateral axes.

The secondary objectives of the desertion are as follows:

 To design re-programmable electronic weighing balance

 To program an electronic weighing balance to measure load distribution in three axes

 To calibrate electronic weighing balance according to the environmental conditions

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Literature Review

The existing electronic weighing balances use different principle in their working

mechanisms. All types must use a sensor to detect weight and convert into change into

capacitance, resistance or inductance. Other sensors produce are excited by the weight exerted

and produce an electric current which is proportional to the weight. The change in the electrical

parameter is converted into readable weight by use of an alternating current bridge circuit and

amplifiers. The secondary devices (bridge circuit and amplifier) process, condition and filter the

output signal from the sensor thus converting it into weight. The various types of the electronic

balances are as described below.

Strain gauge type electronic weighing balance

This kind of weighing balance uses a strain gauge load cell as the transducer. Strain

gauge deformation detects the weight. The strain gauge sensor measures the strain and converts

this into a change in electrical resistance. The change in electrical resistance is directly

proportional to the deformation of the load cell. The load cell consists of two or more strain

gauges. The load cell is connected to Wheatstone bridge circuit. The electrical output signal is

small thus it requires amplification. An instrumentation amplifier is used for that purpose. The

output is scaled to determine the force applied to the force applied to the strain gauge.

Sometimes, a 24-bit resolution ADC is used for scaling.

Strain gauge electronic weighing balances are commonly used in the industries and weigh

bridges. The load cells are particularly stiff and have long life cycles. The strain gauge load cells

work with the principle explained above. They have good resonance values. And planar resistor

deforms linearly. The sensitivity of the load cell is small, but it is magnified. The calibration of

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the load cell is done with known values to ensure accuracy within a given range of

measurements.

The gauges themselves change the deformation into an electrical signal. The gauges are

bonded into the beam or the member in which the weight is to be measured. Four strain gauges

can be connected in series to increase the sensitivity and temperature compensation. Two of the

gauges are in compression while the other two are in tension. They are then wired with

compensation adjustments. When force is exerted to the load cell, the gauges under compression

decrease their resistance while the gauges under tension increase the resistance. Less current

flows through the gauges under tension and more current flows through the gauges under

tension. A potential difference is thus created for the in the output signal of the load cell. The

output signal is thus fed to differential bridge circuit to enhance the measurement precision.

Piezoelectric type electronic weighing balances

This kind of weighing balances uses the piezoelectric crystal transducer. When the crystal

is under pressure, it produces electric current proportional to the pressure. The more the pressure

the more the electric current generated. When weight is applied to the load cell, the crystal

becomes excited and produces current. This type of weighing balance is useful in the

measurement of the dynamic or frequent measurement of weight. The load cell is connected to

the Wheatstone bridge circuit whereby the signal is converted into a significant change in voltage

then the signal is fed to electronic counters to give readable output in the form of weight.

Piezoelectric load cells find applications in places the loading conditions are dynamic.

The strain gauges can fail in such conditions. The electrical output from the load cell is an

impulse output not static like the strain gauge output which is a step function. The voltage signal

is only useful when the strain of the gauge is changing and is not measuring static values. The

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load cell is programmed to detect forces from three principal axes. The lateral and longitudinal

components of the forces can also be registered.

The improvements which can be done to the two types of weighing are:

• Increase the cross-sensitivity of the load cell so that the force exerted can be displayed in

its respective components i.e. X, Y, and Z axes.

• Calibrate the load cells in both lateral and longitudinal directions.

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Methodology

The following steps are followed in the design and programming of an electronic

weighing balance.

• Carrying out a comprehensive literature review both online and in the library

• The design considerations

• Developing the principle of operation

• Developing multiple solutions to the design.

• Analyzing the various solutions and settle the best one.

• Design calculation and programming of the parts involved.

• Sensor selection

• Development of the working programming codes.

• Executing the codes in computers.

• Calibration of the load cell in longitudinal and lateral directions

• Testing of the electric weighing machine.

The electric weighing balance will have the following main parts

 Load cell (sensor)

 Processor (micro-controller)

 Data presentation unit

 Power supply

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The load cell

This is the part that will be converting the force input into a change in resistance. The

load cell will be calibrated in three axes i.e. in vertical, horizontal and longitudinal axes. The

machine will use strain gauge type of load cell. This is because it is easier to re-programme the

sensor and the cross sensitivity can be increased by connecting several cells in series.

The Processor

This is the part that processes the output signal from the sensor. It consists of a

microprocessor which is memory linked to the input-output device. It has the program that

processes the signals and converters. Microprocessors require digital inputs hence where a

microprocessor chip is applied data has to be converted into a digital form before it can be used

as an input to the microprocessor. The output from a microprocessor is digital. Most control

elements require an analog input and so the digital output from a microprocessor has to be

changed into an analog form before they can use it. Microcontrollers are microprocessors with

input and output signal processing incorporated on the same chip and often incorporates

analogue-to-digital and digital-to-analogue converters.

Data presentation unit

This is the unit for displaying the weight measurements. These are usually multi-channel,

available from most of the suppliers. Typically, a 12-bit A-D converter is used, which gives a

0.1%measurement resolution. Alternatively, a 16-bit converter gives 0.015% resolution.

Specifications typically for digital recorders are frequency response of 30 kHz, maximum

sampling frequency of 220MHz and data storage up to 3000 data points per channel. Below is an

example of digital data display unit, i.e., digital oscilloscope.

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The electrical diagram representation of the load cell is as follows

The factors to be considered in the design of the electric weighing balance include:

 The maximum amount of the weight to be measured

 The frequency of taking measurements

 The minimum weight detectable

 The desired cross-sensitivity

 The cost of the transducers and microprocessor chips

 The number of axes to be calibrated

 The accuracy, precision and linearity of the results required

The working principle

The electronic weighing machine will have three main elements

 Weight sensor

 Processor

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 Data presentation/ display

The sensor will detect the forces applied and generate electrical signal to the to

the processor. The processor processes, amplifies and filters the signal into signal that can be

displayed. It has pairs of Wheatstone bridges and amplifiers. The digital signal from the

processor will activate the data representation unit to display the results in form of weight.

Calibration of the machine

Calibration is a process of comparing the output of the measuring system or sensor under

against the output of an instrument or sensor of known accuracy when the same input (the

measured quantity) is applied to both devices. This procedure is carried out for a range of data

covering the whole measurement range of the instrument or sensor. Calibration ensures that the

measuring repeatability and accuracy of all measuring systems and transducers used in a device

is known over the whole measurement range, provided that the calibrated instruments and

sensors are used in environmental conditions that are the same as those under which they were

calibrated. For the use of devices and sensors under entirely different environmental conditions,

an appropriate correction has to be made for the ensuring modifying inputs.

In the calibration of the electronic weighing machine, the load cell has placed a

dynamometer. The factors to consider in the calibration process include

• The characteristics and nature of the mechanical interface in which the forces will be

applied

• The direction of transducer excitation. Either tension or compression

• The loading conditions. Whether recurring, waiting times for the sensor to stabilize, the

relaxation time.

• The calibration uncertainty and the precision level wanted.

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• The instruments connected to it including the cables

• Application of increasing loads only or decreasing loads only.

The procedure for calibrating the strain gauge for the electronic weighing balance is as follows

• The strain gage is attached to the beam or the surface by surface degrading, conditioning

and neutralizing.

• Set the beam such that it acts as the cantilever beam. Then measure the dimensions of the

beam i.e. the length, thickness, and breadth.

• Measure the resistance of the strain gage then connect the two ends of the strain gage in

the Wheatstone bridge.

• Depress the gage factor and preset the initial gage factor. The manufacturer indicates the

original gage factor.

• Depress the amplifier to zero and rotate the knob to set the display to zero value.

• Depress the ON button and then see the display. Adjust the display value to zero value. If

the reading is not made zero, then the value selected is made the reference point, and

value will be subtracted from that value

• Measure the value of the hanger weight, then convert into Newton units. Add standard

masses to the hook and then hang them at the free end of the cantilever beam. Record the

strain indicated.

• Repeat the above with five to eight different masses while recording the strains.

• Lastly, repeat the procedure while loading the beam at various direction mutually

perpendicular to the previous one. This is the longitudinal and lateral directions.

• Analyze the results to determine the sensitivity of the strain gage in the longitudinal and

lateral directions.

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The microcontroller program for the machine

The flow chart below shows the general format and the procedure of the working

principle of the machine.

1 ‘SAMPLE PROGRAM: ELECTRONIC WEIGHING SCALE

2 ' 10~299: OPEN PORTS

2 ' 300~899: MAIN MENU

2 ' 900~1299: CHANGE MENU

2 ' 1300~1499: CHANGING INDIVIDUAL MASSES

2 ' 1500~1899: PRINT LABEL MENU

2 ' 1900~4999: PRINT LABELS

2 ' 5000~6999: PRINT FORMATS

2 ' 7000~9999: INPUT FUNCTION AND EXIT CODE

2 ' 10000~12999: ITEM DATABASE

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010 LET ESC$ = CHR$(20) DEFAULT CHARACTER

011 LET FEED-B = 1028 'FEED BUTTON

021 LET LINE-B = 2044 'ON BUTTON

022 LET ENTER-B = 3070 'ENTER BUTTON

023 LET CANCEL-B = 4094 ' DOWN BUTTON

024 LET RIGHT-B = 5128 FUNC BUTTON

025 LET LEFT-B = 6142 'LEFT BUTTON

026 LET UP-B = 7196 'UP BUTTON

027 LET DOWN-B = 8164 ' CANCEL BUTTON

028 LET FUNC-B = 9232 ''RIGHT BUTTON

321 LET SMENU = 1 Main menu variable

322 LET MMENU = 1 'Change Rate Menu Variable

323 LET MCODE = 1 ' Print Menu Variable

331 LET PMENU = "2080" '8-digit Code

332 LET WEIGHT = "" 'Weight from the display

333 LET MAXRATE = 20 Maximum detectable weight

FLAGS DECLARATION

351 LET EXITFLAG = 00

352 LET TMPFLAG = 00

OPEN PORTS

410 OPEN "LCD" AS #2 'LCD Output

420 OPEN "SPL" AS #3'SBPL Output

430 OPEN "IO" AS #4 'External device input

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Program of Work

Time (weeks)

Activity

Literature review

Design consideration

Developing Working Principle

Sensor selection

Development of working program codes

Assembling of the components

Calibration

Testing of the machine

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The Gantt chart

Weeks

ACTIVITIES

Discussion of the project with

the supervisor

Presentation of the project title

to the projects coordinator

Draft project proposal

Literature review

Weekly progress presentation

Mechanical and electrical

design

Budget preparation and material

requisition

Final project presentation

Conclusion

An electronic weighing device for measuring both longitudinal and lateral force distribution in a

vehicle will assembled. The machine is expected to have very high cross-sensitivity. The

calibration of the machine will be carried out in a dynamometer.

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References

Allen, RG & and Fisher, DK 1990, ‘Low-cost electronic weighing lysimeters’, Transactions of

the ASAE, 33(6), pp.1823-1833.

Angel, S, 1989. Portable electronic scale of minimal thickness and weight. US Patent 4,800,973.

Baumann, A & Bachmann, 1987, Display for an electronic weighing scale. US, Patent D290,

097.

Barani, GA & Khanjani, MJ 2002, ‘A large electronic weighing lysimeter system’, design and

installation1, Vol.12 No.2, pp.89-99.

Childress, JJ & Mickel, TJ 1980, A motion compensated shipboard precision balance

system. Deep Sea Research Part A. Oceanographic Research Papers, 27(11), pp.965-970.

Conti, JA, Pitney-Bowes, Inc 1978, Electronic postage weighing scale, U.S. Patent 4,084,242.

Gard, EA & Bryan JS 1980, Pennsylvania Scale Company: Electronic counting scale. U.S.

Patent 4,219,089.

Germanton, D., Cappiello, M.W., Tasker, R.E. and Petrucelli, S.P., Measurement Specialties,

Inc., 1999. Electrical weighing scale. U.S. Patent 5,886,302.

Gonzalez-Landaeta, R., Casas, O. and Pallas-Areny, R., 2008. Heart rate detection from an

electronic weighing scale. Physiological measurement, 29(8), p.979.

Howell, T.A., McCormick, R.L. and Phene, C.J., 1985. Design and installation of large weighing

lysimeters. Transactions of the ASAE, 28(1), pp.106-0112.

Levy, M., Porter, W.P., Gandhi, K. and McKay, R., Lexicon Corporation, 1983. Electronic scale

for use in a weight control program. U.S. Patent 4,366,873.

Maaz, G, Dardat, K, Klauer, A & Sartorius Ag 1991. Electronic balance with scale on top. US

Patent 4,991,973.

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Neo, B 1992. ‘The implementation of an electronic market for pig trading in Singapore’. The

Journal of Strategic Information Systems, Vol.5, pp.278-288.

Newman, MH &Newman Martin H 1982. Electronic weighing device. US Patent 4,354,562.

Reichmuth, A, Wunderli, S, Weber, M, & Meyer, VR 2004. ‘The uncertainty of weighing data

obtained with electronic analytical balances’. Microchimica Acta, Vol 14, pp133-141.

Zeigner, WL & MacIntosh, JA Inc 1985. Electronic weighing scale for dieters. US Patent D281,

403.

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Appendix

Bridge circuit 5,

Calibration 6,

Crystal 7,

Display 9,

Dynamometer 11,

Gantt chart 14,

Load cell 6,

Piezoelectric sensor 7,

LCD 9,

Sensor 4,

Transducer 4,

Strain gage 4,

Instrumentation 8,

Processor 8,

Micro-chip 6,

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