Introduction
Strain is the relative change in length of material when subjected to stress; bonded wire strain gauge is a device that is used to measure the strain. The device measures strain in terms of resistance which is affected by a metallic foil design within the device. Whenever the foil is distorted, electrical resistance changes. This resistance is related to strain directly. This paper will discuss bonded wire strain gauge with regards to its discovery, evolution and applications.
Operation of a strain gauge
When an object is subjected to a stretching force it stretches. When a strain gauge is attached to that object, the foil stretches too. The effect of this is that the electrical resistance is more along the length of the object. However if it is compressed to a point that it does not change its original form permanently, then the object appears broad and short in length. Thus the electrical resistance is less (James 2). With this kind of resistance it is possible to measure the strain applied to the object.
The architects behind this device were known as Edward Simmons together with Arthur C. Ruge. They came up with this device in 1938. The marvel of the idea was that they could introduce the gauge factor to be able to show the relation between the electrical resistance of an object to its strain.
The gauge factor can be achieved by having the change in resistance divided by the resistance of the undeformed gauge. The obtained result is then divided by the strain. In other words;
Gauge factor = (∆R/Rg)/e
Where : ∆R is change in resistance due to strain.
:Rg is the resistance of undeformed gauge and the e is the strain.
Figure 1.0: Strain gauge (National Instruments 2)
APPLICATION
This device is used in engineering applications. One of the fields in which it is used is mechanical engineering. It is used to measure the stress generated by machinery.
Another application includes the aviation industry. The aircrafts are designed in a manner that they are delicate to any kind of malfunction. The component is used to measure stress in various parts of the airplane.
It is used in the medical sector too. It is used in DNA synthesis, robotic sensing, medical patient lift as well as medical behavioral testing i.e. lab rats. All these are some fields in which the sensor comes in handy and helps facilitate development.
These gauges usually errors like any other device but to each error there is a way to counter it and ensure efficiency is maintained. Some of the errors include:
- Zero offset – this kind of error is usually rectified by introducing parallel resistors to each of the arms of the gauge.
- Linearity – this occurs when the sensitivity of the device fluctuates with the pressure range around it.
- Temperature coefficient of the gauge factor also results in some errors. This error is usually overcome by having a fixed resistance introduced to the input. This will compensate any kind of decrease in sensitivity with temperature.
- In some cases the strain gauges usually add more weight to what they are intended to measure. This may be disadvantageous especially when dealing with vibration profiles of hardware they are supposed to measure.
- Despite all these drawbacks there are some advantages that come with. In some cases maximum sensitivity was required. The full bridge circuit was thus introduced within the gauges.
Figure 2.0: full-bridge gauge circuit
Process of evolution of strain gauges
Evolution shows how the gauge was introduced to mankind and the developments it has undergone with time (Efunda Inc 1). In order to best understand this topic of evolution we shall discuss the various types of gauges present and see why some are in use while others were discarded.
The initial gauges introduced were based on inductors and capacitors. They were vulnerable to vibrations as well as having a lot of complication in their circuitry. They also had some requirements when it came to installing them. This made them really hard to use and as a result were discarded in the industry.
Later on in 1938 there came the metallic wire type strain gauge which could be fastened onto the strained surface by some kind of adhesive. It was possible to have a change in length conveyed to the resistor which in turn could invert this to strain.
The semiconductor strain gauges were also introduced in the 1970 and were majorly being used within the automotive industry. This gauge however had some modification done to it. This is that it used stress to measure resistance as opposed to strain.
Later on with advancement in technology there was the introduction of the thin film strain gauges. These gauges did not require to be fastened onto the surface which was being measured its strain (RDP Group 1). It was bonded with the surfaces via vacuum deposition as well as sputtering method.
Further advancement was the diffused semiconductor strain gauges. They did not require one to bond them with the surface. They eliminated any errors that arise due to hysteresis. The temperature related errors were then compensated by the introduction of advanced transmitter designs.
Works Cited
James, B. Strain Gauge Measurements; a tutorial. Accessed 29-April-2013, from
http://www.ing.unp.edu.ar/electronica/asignaturas/ee016/anexo/r-an078.pdf 2008.
. National Instruments Electrical resistance strain gauge circuits. Accessed 29-April-2013, from http://soliton.ae.gatech.edu/people/jcraig/classes/ae3145/Lab2/strain-gages.pdf, 2010 .
Efunda Inc. Electrical strain gauges. Accessed 29-April-2013, from http://www.efunda.com/DesignStandards/sensors/strain_gages/strain_gage_theory.cfm, 2013.
RDP Group. How it works – strain gauge pressure transducer. Accessed 29-April-2013, from http://www.rdpe.com/ex/hiw-sgpt.htm, 2013.
