Surface Preparation and bonding techniques

Surface preparation and bonding techniques have been discussed under the following three topics namely:

1. Backing, base or carrier material.
2. Bonding material or cement.
3. Surface preparation and mounting of strain gauges.

Backing, Base or Carrier Material.

The purpose of providing the carrier/backing material ina strain gauge arrangement has been listed as follows;

1. The backing material provides support to the resistance wire (grid) of the strain gauge arrangement.
2. The backing material provides protection to the sensing resistance wire of the strain gauge arrangement. It also provides dimensional stability for the resistance wire of the strain gauge arrangement.

Characteristics Required for Backing Materials

1. The backing material should be an insulator of electricity.
2. The backing material should not absorb humidity, that is should be non-hygroscopic.
3. The backing material should be very thin.
4. It should go along with the adhesive material used to fix (bond) it on the structure under study.
5. It should not be affected by temperature changes.
6. It should be strong enough to transmit the force from the structure under study to the sensing resistance wire.

Bonding Materials or Cements (Adhesive)

The strain gauge has to be fixed (bonded) on the structure under study using an adhesive or paste. These adhesive are called as bonding material or cements.
The different adhesive, their composition and the temperature for which they can be used are shown in following table.

Adhesive, that is, Bonding Material       Composition                 For Temperature
Thermo-plastic cement                               Celluloid dissolved in acetone    Upto 75’C
Thermo-setting Cement                              Phenol resin             From 150’C to 210’C
Special Ceramic  – cement                                 -                                  Above 175’C

Characteristics Required

1. The characteristics required for a bonding material are listed.
2. The bonding material should be an insulator of electricity.
3. The bonding material should not absorb humidity, that is, it should be non-hygroscopic.
4. It should go along with the backing material so that the backing material is fixed (bonded) rigidly on the structure under study.
5. It should not be affected by temperature changes.
6. It should have good shear strength to transmit the force from the structure under study to the sensing resistive wire.
7. It should be easy to apply and should spread easily and should provide good bonding adhesion.
8. The bonding material should have a high creep resistance.

Surface preparation and mounting of strain gauges

The steps involved in preparing a surface to mount a strain gauge are listed:

1. The structure under study is made even and free from dust and dirt by rubbing with an emery sheet or by sand blasting.
2. The even surface is then cleaned by a volatile solution (acetone) using a cloth to remove oil/grease.
3. The bottom side of the backing (gauge carrier) is also cleaned by a solvent using a cloth.

Bonded Strain Gauges

These gauges are directly bonded (that is pasted) on the surface of the structure under study. Hence they are termed as bonded strain gauges. The three types of bonded strain gauges are

1. Fine wire strain gauge
2. Metal foil strain gauge
3. Semi-conductor gauge

Fine wire strain gauge

This is the first type of Bonded Strain Gauges.

Description

The arrangement consists of following parts,

A fine resistance wire diameter 0.025 mm which is bent again and again as shown in diagram. This is done to increase the length of the wire so that it permits a uniform distribution of stress. This resistance wire is placed between the two carrier bases (paper, Bakelite or Teflon) which are cemented to each other. The carrier base protects the gauge from damages. Leads are provided for electrically connecting the strain gauge to a measuring instrument (Wheatstone bridge).

Operation

With the help of an adhesive material, the strain gauge is pasted/bonded on the structure under study. Now the structure is subjected to a force (tensile or compressive). Due to the force, the structure will change the dimension. As the strain gauge is bonded to the structure, the stain gauge will also undergo change in both in length and cross-section (that is, it strained). This strain (change in dimension) changes the resistance of the strain gauge which can be measured using a wheat stone bridge. This change in resistance of the strain gauge becomes a measure of the extent to which the structure is strained and a measure of the applied force when calibrated.

Fine Wire strain gauge Materials

Material Composition
Nichrome Ni - 80% ; Cr – 20%
Constantan Ni – 45%; Cu – 55%
Nickel ----
Platinum ----
Isoelastic Ni – 36%; Cr – 8%; Mo – 0.5%

Advantages of Fine Wire Strain Gauge

The range of this gauge is +/- 0.3% of strain.
This gauge has a high accuracy.
Has a linearity of +/- 1%.

Limitation of Fine Wire strain gauge

• These gauges cannot be detached and used again (because the gauges are bonded to the structure).
• These gauges are costly.

Metal Foil Strain Gauge

Description of Metal Foil Strain Gauge

The arrangement consists of the following;

The metal foil of 0.02mm thick is produced using the printed circuit technique. This metal foil is produced on one side of the plastic backing. Leads are soldered to the metal foil for electrically connecting the strain gauge to a measuring instrument (wheat stone bridge).

Operations of Metal foil Strain gauge

With the help of an adhesive material, the strain gauge is pasted/bonded on the structure under study. Now the structure is subjected to a force (tensile or compressive). Due to the force, the structure will change the dimension. As the strain gauge is bonded to the structure, the stain gauge will also undergo change in both in length and cross-section (that is, it strained). This strain (change in dimension) changes the resistance of the strain gauge which can be measured using a wheat stone bridge. This change in resistance of the strain gauge becomes a measure of the extent to which the structure is strained and a measure of the applied force when calibrated. Same as Fine Wire strain gauge operation.

Advantages of Metal foil Strain gauge

• These strain gauges can be manufactured in any shape.
• Perfect bonding of the strain gauge is possible with structure under study.
• The backing can be peeled off and the metal foil with leads can be used directly on the structure under study. In such cases, a ceramic adhesive is to be used.
• These gauges have a better fatigue life.
• Has good sensitivity and have stability even at high temperatures.

Semi – conductor or Piezo Resistive Strain Gauge

Description of Piezo Resistive Strain Gauge.

The arrangement of a semi-conductor strain gauge is as follows:

The sensing element is rectangular filament made as a wafer from silicon or geranium crystals. To these crystals, boron is added to get some desired properties and this process is called doping and the crystals are called doped crystals. This sensing element is attched to a plastics or stainless steel backing. Leads made of gold are drawn out from the sensing element for electrically connecting the strain gauge to a measuring instrument (wheat stone bridge).

There are two types of sensing element namely:

• Negative or n-type (resistance decrease with respect to tensile strain).
• Positive or P-type ( resistance increase with respect to tensile strain).

Operation

With the help of an adhesive material, the strain gauge is pasted/bonded on the structure under study. Now the structure is subjected to a force (tensile or compressive). Due to the force, the structure will change the dimension. As the strain gauge is bonded to the structure, the stain gauge will also undergo change in both in length and cross-section (that is, it strained). When the sensing element (crystal) of the semiconductor strain gauge is strained, its resistivity changes contributing to a change in the resistance of the strain gauge. The change in the resistance of the strain gauge is measured using a wheat stone bridge. . This change in resistance of the strain gauge becomes a measure of the extent to which the structure is strained and a measure of the applied force when calibrated.

Advantages of semi-conductor Strain gauges

• These gauges have high gauge factor and hence they can measure very small strains.
• They can be manufactured to very small sizes.
• They have an accuracy of 2.3%
• They have excellent hysteresis characteristics.
• They have a good frequency of response.
• They have good fatigue life.

Limitation of semi-conductor Strain gauges

• These gauges are brittle and hence they cannot be used for measuring large strain.
• The gauge factor is not constant.
• These gauges have poor linearity.
• These gauges are very costly and are difficult to be bonded onto the structure under study.
• These gauges are sensitive to change in temperature.

Installation of Strain Gauge Video Post

Hello Reader time for real watch about strain gauge and how to install them, this video is Presented by a YouTube user Binsfledengineering

Post you comments below.

Unbonded Strain Gauges

These strain gauges are not directly bonded (that is, pasted) onto the surface of the structure under study. Hence they are termed as unbounded strain gauges.


Description of the Unbonded Strain gauges:
     The arrangement of an unbonded strain gauges consists of the following. Two frames P and Q carrying rigidly fixed insulated pins as shown in diagram. these two frames can move relative with respect to each other and they are held together by a spring loaded mechanism. A fine wire resistance strain gauge is stretched around the insulated pins. The strain gauge is connected to a wheat stone bridge.

Operation of Unbonded strain gauges:

    When a force is applied on the structure under study (frames P & Q), frames P moves relative to frame Q, and due to this strain gauge will change in length and cross section. That is, the strain gauge is strained. This strain changes the resistance of the strain gauge and this change in resistance of the strain gauge is measured using a wheat stone bridge. This change in resistance when calibrated becomes a measure of the applied force and change in dimensions of the structure under study.

Application of Unbonded strain gauge:

     Unbonded strain gauge is usedin places where the gauge is to be detached and used again and again.
unbonded strain gauges are used in force, pressure and acceleration measurement.

Advantages of Unbonded strain gauge:

• The range of this gauge is +/- 0.15% strain.
• This gauge has a very high accuracy.

Limitation of unbonded strain gauges

It occupies more space.

Load cell and Load cell Types

If need to read by post on Load Cell, if you are new to my site and read about strain gauge load cell to have a better idea about load cells

There are two types of Load Cells, they are

1. Hydraulic load cells
2. Pneumatic load cells

Hydraulic Load Cell

Basic Priniple of Hydraulic Load cell

When a force is applied on a liquid medium contained in a confined space, the pressure of the liquid increases. This increase in pressure of the liquid is proportional to the appilied force. Hence a measure of the increase in pressure of the liquid becomes a measure of the appilied force when calibrated.

Description of Hydraulic Load Cell

The main parts of a hydraulic load cell are as follows

A dirphragm
A piston with a loading platform (as shown in figure) placed on top of the diaphragm.
A liquid medium which is under a pre-loaded pressure is on the other side of the diaphragm.
A pressure gauge (bourdon tube type) connected to the liquid medium.

Operation of Hydraulic Load Cell

The force to be measured is applied to the piston.
The appilied force moves the piston downwards and deflects the diaphragm and this deflection of the diaphragm increases the pressure in the liquid medium (oil).
This increase in pressure of the liquid medium is proportional to the applied force. The increase in pressure is measured by the pressure gauge which is connected to the liquid meduim.
The pressure is calibrated in force units and hence the indication in the pressure gauge becomes a measure of the force applied on the piston.

Note about Hydraulic Load cell:

As the hydraulic load cell is sensitive to pressure changes, the load cell should be adjusted to zero setting before using it to measure force.
This hydraulic load cell have an accuracy of the order of 0.1 percent of its scale and can measure loads upto upto 2.5*10^5 Kgf
The resolution is about 0.02 prcent.

Pneumatic Load Cell

Basic Principle of Pneumatic Load Cell

If a force is applied to one side of a diaphragm and an air pressure is applied to the other side, some particular value of pressure will be necessary to exactly balance the force. This pressure is proportional to the applied force.

Description of pneumatic Load cell

The main parts of a pneumatic load cell are as follows:

A corrugated diaphragm with its top surface attached with arrangements to apply force.
An air supply regulator, nozzle and a pressure gauge arranged as shown in figure.
A flapper arranged above the nozzle as shown in figure.

Operation of Pneumatic Load cell

The force to be measured is applied to the top side of the diaphragm. Due to this force, the diaphragm deflects and causes the flapper to shut-off the nozzle opening.Now an air supply is provided at the bottom of the diaphragm. As the flapper closes the nozzle opening, a back pressure results underneath the diagram. This back presssure acts on the diaphragm producing an upward force. Air pressure is regulated until the diaphragm returns to the pre-loaded position which is indicated by air which comes out of the nozzle. At this stage, the corresponding pressure indicated by the pressure gauge becomes a measure of the appilied force when calibrated.

Note:

The pneumatic load cell can measure loads upto 2.5*10^3 Kgf.
The accuracy of this system is 0.5 percent of the full scale.

Effects of Feedback

The effects of feedback in systems on other working parameters of the system has been analysed. The parameter considered for analysis are gain, sensitivity, distortion, impedance and bandwidth. A feedback system has been shown in the figure.

Effect of feedback on overall Gain:

From figure it is seen that the transfer function is given by the equation:

M= A/(1- bA)

Hence the feedback reduces the overall gain of the system by a fator of (1-bA).
The quantities A and B are function of frequency and can be adjusted to make the denominator greater than unity.
Hence the gain increases for a particular frequency range and decreases for another frequency range.

Effect of Feedback on Sensitivity:

Sensitivity is the extent to which the system responds to changes in parameters like gain,impedance,etc. Sensitivity is also said to be the ratio of the extent of change of one of the above mentioned parameter to a small change of the determining parameters.

For example, if

M= transfer function
K= Determining Parameter

Then the sensitivity (S) is given by:

S= Percentage change in M/ Percentage change in K

Following are the effect of feedback on Sensitivity.



Feedback may reduce sensitivity with respect to certain parameters.
Feedback doesnot affect variations of elements in the feedback path.
Feedback reduces the sensitivity of the system based on variation of parameter in the forward path of the loop. Larger the loop gain Ab, more effective is the feedback in reducing sensitivity.

Effect of Feedback on Distortion

Feedback is used in communication systems to reduce noise and other distortion signals which it might pickup from extraneous sources.
The place of insertion of the extraneous noise to the signal flow is the main factor that determines the extent to which the feedback reduces the effects due to distortion.
Consider the figure which shows a signal flow graph of a system.

A noise signal is inserted at the point shown.

In the absence of the feedback, the output is given by

e0 = A1A2es + A2en
= e0s + e0n

where, e0s = single component of the output.
e0n=The component of the output due to noise.

Output signal to noise ration = output due to signal/output due to noise


Signal to noise ratio = A1A2es/A2en

= A1es/en

Hence to increase the signal to noise ratio, either A1 and /or es is to be increased or en is to be decreased.

If the system is aided by a feedback circuit, the output is given by:


e0 = (A1A2es/1-A1A2b) + (A2en/1-A1A2b)

From the above equation it is clear that the noise component of the output has its gain reduced by a factor 1-A1A2b. Thus the noise is reduced and the overall distortion of the output is reduced.

Effect of Feedback on Impedance

In practice , the system is bound be connected to an external circuit. The working of the system depends on the input and output impedance. Consider the figure shown.


Za = (rpRl)/(rp+Rl+µKRl)

Here the amplifier gain is given by:


A = - µRl/(rp + Rl)

Thus the shunt impedance is reduced by a factor (1-AK). The series impedance is increased by a factor (1- AK).

Effect of Feedback on Bandwidth

In electronic frequency depent circuits, bandwidth is an important characteristic.
Bandwidth is the parameter that measure the ability of the system to reproduce its input signal with high quality and least noise.
It is to be noted that the bandwidth increases with feedback.

Feedback Principle

As I said in the previous post Open loop control system dont give the required level of control as they depend on human judgement.

Example of feedback prnciple:

Home furnace control system must control the temperature in the room and kept it constant. As in open loop system a timer is used to switch on the furnace for some time and then switch it off, accuracy is not obtained. This is because the system doesot act according to the room temperature but according to a preset value of time.

A closed loop control system takes care of this problem. The feedback unit (the main component of the closed loop control system) senses the room temperature and accordingly turns on or off the furnace. The feedback unit feeds the output back to a comparator which is provided with a reference value with which the output is compared to generate an error signal. This error signal generates the required control action. In the home furnace control system, a temperture sensor is used to sense the temperature in the room and this is feedback to an error detecting device. The error detecting device comparres the room temperature with the reference value. If it detects that the room temperature is higher than the reference value, if generates a signal to switch off the furnace. On the other hand, if it detects that the room temperature is lower than the reference value, it generates a signal to switch on the furnace. This has been shown in the figure.

Hence it is clear that a system with feedback (closed loop control system) is efficient than that of a system without feedback (open loop control system).

Another examples of a closed loop control system is the working of a human brain. If a person wants to pick up an article say bag or book, the brain instructs the hand to reach the article and the eyes constantly keep giving the feedback to the brai regrading the posistion of the hand relative to the article. But if the person is asked to reach the article closing his eyes, he can reach it only approximately. Thus the human system is a very accurate feedback control system. The control system taking human as an example is shown in the figure.

While designing a closed loop system, a compromise between stability and accuracy is to be astablished. This is because the gain of the system may exceed over a limit which may cause the system to be over correct, leading the system to become unstable, that is, oscillation of the output without bound.

Note

Gain is the ratio between the amplitude of the output to the amplitude of input. The variation of the gain over a range of the frequencies is called frequency response. Moreover, due to mechanical problems like friction, the system might tend to have a steady state error.

Types of control systems

There are two types of control systems namely:

1. Open loop control systems (non-feedback control systems)
2. Closed loop control systems (feedback control systems)

Open loop control system

If in a physical system there is no automatic correction of the variation in its output, it is called an open loop control system. That is, in this type of system, sensing of the actual output and comparing of this output (through feedback) with the desired input doesnot take place. The system on its own is not in a position to give the desired output and it cannot take into account the disturbances. In these systems, the changes in output can be corrected only by changing the input manually.





These systems are simple in construction, stable and cost cheap. But these systems are inaccurate and unreliable. Moreover these systems donot take account of external disurbances that affect the output and they donot initiate corrective actions automatically.

Examples of open loop control systems:

1. Automatic washing machine
2. traffic signal system
3. home heating system( without sensing, feedback and control)

Any non-feedback control system can be considered as a feedback control system if it is under the supervisio of someone. Although open loop control systems have economical components and are simpe in design, they largley depend on human judgement. As an example, let us consider a home furnace control system. This system must control the temperature in a room, keeping it constant. An open loop system usually has a timer which instructs the system to switch on the furnace for some time and then switch it off. Accuracy cannot be achieved as the system doesnot swith on/off based on the room temperature but it does as per the preset value of time.

Closed loop control system

A closed loop control system is a system where the output has an effect upon the input quantity in such a manner as to maintain the desired output value.

An open loop control system becomes a closed loop control system by including a feedback. This feedback will automatically correct the change in output due to disturbances. This is why a closed loop control system is called as an automatic control system. The block diagram of a closed loop control system is shown in figure.

In a closed loop control system, the controlled variable (output) of the system is sensed at every instant of time, feedback and compared with the desired input resulting in an error signal. This error signal directs the control elements in the system to do the necessary corrective action such that the output of the system is obtained as desired.

The feedback control system takes into account the disturbances also and makes the corrective action. These control systems are accurate, stable and less affected by noise. But these control systems are sophisticated and hence costly. They are also complicated to design for stability, give oscillatory response and feedback brings down the overall gain of the control system.

Measurement of Strain

Strain gauges are devices used to measure the dimensional changes of components under test. Strain gauges are used in a number of applications, some of them have been listed below:

1. Strain gauges are used in force measuring devices such as strain gauge load cell.
2. Strain gauge are used in measurement of vibration / acceleration such as strain gauge accelerometer.
3. Strain gauges along with diaphragm are used in the measurement of pressure.

Some important terms have been explained below:

Strain

Strain is the relative change in dimensions, that is, change in length of given original length.

Strain = change in length/original length = mm/mm (dimensionless).

Strain Gauges 

When a metallic conductor is stretched or compressed, its resistance changes due to a change in the length and diameter (cross section) of the conductor.Hence a strain gauge is a measurement transducer used to measure strain (that is, relative changes in dimension). It is a transducer because it converts information about relative change in dimension to a change in resistance.

Positive Strain

When a strain gauge (metallic conductor) is subjected to tension, it is said to be positively strained. That is, when the strain gauge is subjected to positive strain (tension), its length increases and its area of cross section decreases. As the resistance of a conductor is proportional to its length and inversely proportional to its area of cross section, the resistance of the strain gauge increases with the positive strain.

Negative Strain.

When a strain gauge (metallic Conductor) is subjected to compression , it is said to negatively strained. that is, when the strain gauge is subjected to negative strain (compression), its length decreases and its area of cross section increases. As the resistance of the conductor is proportional to its length and inversely proportional to its area of cross section, the resistance of the strain gauge decreases with negative strain.

Piezoresistivity

There will be a change in resistivity of a conductor when it is strained and this property is called as piezoresistivity.

Poisson's Ratio

Poisson's Ratio = lateral strain, that is, the relative change in dimension in the cross section / Longitudinal strain, that is the relative change in dimension in the length.

                        = (dD/D)/(dL/L)

where, D=Diameter:    L=Length

Gauge Factor (Strain Sensitivity Factor)

The fractional change in resistance due to unit change in length (unit strain) is called as gauge factor.

Gauge Factor, F = (dR/R)/(dL/L)

Where, R = Resistance, L=Length

The magnitude of the strain gauge factor indicates the sensitivity of the strain gauge. the high gauge factor implies that there will be a large change in resistance for a given strain input.