Use of 4 Strain Gauge Sensors and LabView DAQ to Measure and Calculate Weigth

We use a weight scale to measure weight but have you wondered how it works?

 This article will explain and help you create your own weight-measuring device.

 Lets learning begin.

Strain Gauge

The resistance strain gauge is a resistive element that changes in length hence resistance, as the force is applied to the base on which it is mounted causing stretching or compression.

Strain gage is low-impedance devices; they require significant excitation power to obtain reasonable levels of output voltages. A strain gauge can be used to measure the force, where four strain gauges are used, two on the top side, and two on the bottom side. The gages are connected in an all-element bridge configuration, which gives maximum sensitivity and is inherently linear.

When the pressure is applied to the plate, two gauge elements of the bridge are subjected to tension; the other two elements are subjected to compression. The corresponding changes in resistance are a measure of the incident pressure. The bridge is excited by a constant voltage or current, producing an electrical signal.

The strain of a body is always caused by an external influence or an internal effect. Strain might be caused by forces, pressures, moments, heat, structural changes of the material, and the like. If certain conditions are fulfilled, the amount or the value of the influencing quantity can be derived from the measured strain value. In experimental stress analysis, this feature is widely used.

Types of Strain Gauge:

•       Linear strain gauges are preferable for measuring strain across a single plane of direction

 

•       Rosette foil strain gauges feature dual measuring grids to offset strain measurements at a 90° angle to measure biaxial stress

 

•       Torsion/shear strain gauges feature uniquely arranged to measure grids for determining the shear stress of torsion bars

 

•       Double linear foil strain gauges feature parallel measuring grids for acquiring perpendicular stress in bending beams

 

•       Full bridge strain gauges feature 4 integrated measuring grids for robust strain measurements of shear stresses in tension and compression bars

 

•       Chain foil strain gauges are equipped with a number of smaller measuring grids to acquire shear force gradients across the measuring bridge.

Working Principle

 

All strain gauge configuration is based on the concept of the Wheatstone bridge. A Wheatstone bridge is a network of four resistive legs. One or more of these legs can be active sensing elements. The figure shows a Wheatstone bridge circuit diagram.

The Wheatstone bridge is the electrical equivalent of two parallel voltage divider circuits. R1 and R2 compose one voltage divider circuit, and R4 and R3 compose the second voltage divider circuit. The output of a Wheatstone bridge is measured between the middle nodes of the two voltage dividers.

A physical phenomenon, such as a change in strain applied to a specimen or a temperature shift changes the resistance of the sensing elements in the Wheatstone bridge. The Wheatstone bridge configuration is used to help measure the small variations in resistance that the sensing elements produce corresponding to a physical change in the specimen.

Strain-gauge configurations are arranged as Wheatstone bridges. The gauge is the collection of all the active elements of the Wheatstone Bridge. There are three types of strain-gauge configurations: quarter-bridge, half-bridge, and full-bridge (Figure 1.2). The number of active element legs in the Wheatstone bridge determines the kind of bridge configuration.

In above figure, we can see the different configurations. Among them, we use the figure “D”, which is all-element varying circuit configuration. As you can as the compression and decompression of the resistance in the respective sides.

 Verifying the Strain Gauge Wiring

 

 

The two wires with the largest resistance with a multimeter. Those are +E and -E. Take the other wires and do the same. We measured the resistance to know a positive strain and a negative strain we got the values around 1k ohm. But how did we know which one is a negative and positive strain? If the resistance increases it's positive and if it decreases, it's negative strain.

Wires are connected to form what is called a Wheatstone bridge arrangement as shown in the figure below.

In the above figure above, 1 and 2 are power supplies where we used 6V feed from the voltage supply and the output is 3 and 4. Four Load cells that are above in figure are specially shaped metal parts that have strain gauges glued to them. So white is positive, red is common, and black is negative.

Measurements

After the circuit was complete, we used different weights to measure the output.

As you can see the increase in output voltage with an increase in weight which gives us nearly a linear graph below. It gives us the equation that can be used in LabView to show the weight based on measured output voltage by DAQ Assistant.

6.   What Is Data Acquisition?

Data acquisition (DAQ) is the process of measuring an electrical or physical phenomenon such as voltage, current, temperature, pressure, or sound with a computer. A DAQ system consists of sensors, DAQ measurement hardware, and a computer with programmable software. Compared to traditional measurement systems, PC-based DAQ systems exploit the processing power, productivity, display, and connectivity capabilities of industry-standard computers providing a more powerful, flexible, and cost-effective measurement solution.

We use LabView to calculate the final weight using the formula that we obtained from the chart. We have created a simple VI. You can find the LabView programming below:

With the completion of the project, we came to acknowledge the working principle stain gauge and we also learned to make the Standalone VI and password protect it. Also, we got the idea of DAQ assistance how it works, and what it can measure. As a group, we were able to tackle the problems and complete the project.

 

References

[Figure 1] China Prices. 2018. strain gauge load cell. [ONLINE] Available at: http://chinaprices.net/search?search=strain+gauge+load+cell. [Accessed 27 April 2019].

 

[Figure 1.2] Intractables. 2018. Strain Gauge. [ONLINE] Available

at: https://www.instructables.com/id/Arduino-Bathroom-Scale-With-50-Kg-Load-Cells-and-H/. [Accessed 28 April 2019].

 

[Figure 1.7 and 1.8] National Instruments. 2010. Measure Voltage Using NI myDAQ. [ONLINE] Available at: http://www.ni.com/example/31436/en/. [Accessed 28 April 2019].

 

[Figure 1.2] AnalogDialogue. 2004. Bridge-Type Sensor Measurements are Enhanced by Autozeroed Instrumentation Amplifiers with Digitally Programmable Gain and Output Offset. [ONLINE] Available

 

at: https://www.analog.com/en/analog-dialogue/articles/bridge-type-sensor-measurements.html. [Accessed 28 April 2019].

 

Jon S. Wilson (2005), Elsevier Inc. Sensor Technology Handbook, Chapter 19 (pp.501-522).

Wikipedia contributors. (2019, April 24). Strain gauge. In Wikipedia, The Free Encyclopedia. 29 April 2019,from: https://en.wikipedia.org/w/index.php?title=Strain_gauge&oldid=893 986619

 

Websites used for research on the topic:

https://www.omega.co.uk/prodinfo/StrainGauges.html https://www.continuummechanics.org/straingauges.html http://www.ni.com/tutorial/7113/en/ http://www.ni.com/example/31436/en/