How does the wheatstone bridge for strain gauges work. Requires half bridge completion resistors to complete the wheatstone bridge. A wheatstone bridge is a network of four resistive legs. This halfbridge configuration, whose circuit diagram is also illus trated in figure 6, yields an output voltage that is linear and approximately doubles the output of. The sensitivity of the strain gauge measurement is dependent on the number of active arms in the bridge. Strain gauge working principle instrumentation tools.
The wheatstone circuit is also well suited for temperature compensation. The strain gauge element typically forms one leg of a bridge circuit which is used to measure changes in resistance of the device. Chapter 9 bridges, strain gages and some variable impedance transducers many transducers translate a change in the quantity you wish to measure into a change in impedance, i. Strain gauges may be purchased as complete units, with both strain gauge elements and bridge resistors in one housing, sealed and encapsulated for.
This design uses a 2 op amp instrumentation circuit to amplify a differential signal created. Jan 25, 2011 strain gauges need to be part of a bridge circuit. The strain gauge measuring grid is manufactured from a copper nickel alloy which has a low and controllable. Developing strain gauges and instruments strain gauge bridge. Dec 17, 2019 samuel hunter christie invented the wheatstone bridge in the year 1833, which became popular with the works of sir charles wheatstone in 1843 an electrical circuit that is set up to measure the unknown value of a resistor and creates a balance between the two legs of the bridge circuit is called a wheatstone bridge. Singlesupply strain gauge bridge amplifier circuit design notes. Strain gauge theory types, formulas and applications. One active strain gauge is referred to as a quarterbridge, two active strain gauges are a halfbridge, and four active strain gauges are a full bridge. To measure the variation in resistance, the strain gauge is placed in a bridge configuration. Laboratory 3 strain gage sensors national instruments. Figure 11 shows a wheatstone bridge circuit diagram.
The bridge supply voltage is selectable via dipswitches between 5v and 10v. Dc bridge circuit applications figure 5, a typical bridge application, details signal conditioning for a 350. A full bridge strain gage configuration has four active strain gages and is available in three different types. Mar 15, 2020 strain gauge bridge circuit shows the measured stress by the degree of discrepancy, and uses a voltmeter in the center of the bridge to provide an accurate measurement of that imbalance. In general, if we define n as the number of active gauges in the wb, n1 for a quarter bridge, n2 for a half bridge. Constant voltage wheatstone bridge circuits specific application cases. May 15, 2019 therefore, to measure extremely a fraction of resistance changes with high accuracy, wheatstone bridge configuration is used. In the case of a bridge system, the strain gauges can work independently in the number of four, two or one.
The same applies to the use of only one strain gauge, then you should attach from the outside of the system up to three resistors, which together form the quarter bridge wheatstone strain gauge. Dec 30, 2017 a strain gauge is a sensor whose resistance varies with applied force. Therefore, if we replace r4 in figure 3 with an active strain gauge, any changes in the strain gauge resistance will unbalance the bridge and produce a nonzero output voltage. Therefore, the strain has little effect on the second gauge, called the. The below figure shows the wheatstone bridge where the unknown resistor is replaced with a strain gauge. A fullbridge strain gage configuration has four active strain gages and is available in three different types. Electrical resistance strain gages are sensors made of thin foil or wiretype conductors that respond to variations in length with variations in electrical resistance. The wheatstone bridge used in most strain gage measurement circuits usually consists of a the gages for actively measuring the strains and b precision resistors incorporated in the measuring instrument for completing the circuit, as shown below. Strain gauge and wheatstone bridge introduction youtube. The strain gauge resistance changes differ in tension load cells vs. In practice, a half bridge and full bridge circuit has more sensitivity than the quarter bridge circuit.
The primary benefit of the circuit is its ability to provide extremely accurate measurements in contrast with something like a simple voltage divider. Typical threewire quarter bridge strain gage circuit requires three bridge completion resistors. If the nominal resistance of the strain gauge is designated as rg, then the straininduced change in resistance. The output voltage of the wheatstone bridge is expressed in millivolts output per volt input. A wheatstone bridge is a divided bridge circuit used for the measurement of static or dynamic electrical resistance. Aug 05, 2011 a full bridge circuit is used in applications where complimentary pair of strain gauges is to be bounded to the test specimen. A clever circuit to measure very small changes in resistance is called a wheatstone bridge. In general, if we define n as the number of active gauges in the wb, n1 for a quarter bridge, n2 for a half bridge and n4 for a full bridge. Thus, the strain gauge works to convert an applied pressure into a measurable voltage signal which may be amplified and converted into a 420 ma loop current signal or into a digital fieldbus signal.
A wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. The exact value is specified on each strain gage package. Since the invention of the electrical resistance strain gage more than a half century ago, the wheatstone bridge has become the sensing circuit of choice in most. As this change in resistance is very small in magnitude so its effect can be only sensed by a wheatstone bridge. A strain gauge is a sensor whose resistance varies with applied force. Analog sensor circuit for using a strain gage as a deflection sensor the voltage output from the wheatstone bridge is amplified by the lmc6484 operational amplifier, shown on the right side of the dotted line in figure 3.
Measuring strain with strain gages national instruments. The wheatstone bridge circuit is used in the field of strain gauge measurements to show a nonlinear behaviour between resistance change and output voltage. Quarter bridge with an external dummy strain gauge. Strain gauge is a passive transducer that converts a mechanical elongation or displacement produced due to a force into its corresponding change in resistance r, inductance l, or capacitance c.
For example a strain gage, changes strain ll into a change in relative resistance rr o. Electrical resistance strain gages are sensors fabricated from thin foil or. A strain gauge is basically used to measure the strain in a work piece. Below, the wheatstone bridge circuit will only be considered with respect to its application in. Among them, a universal strain gage has a structure such that a gridshaped sensing element of thin metallic resistive foil 3 to 6m thick is put on a base of thin plastic film 15 to 16 m thick and is laminated with a thin film. Samuel hunter christie invented the wheatstone bridge in the year 1833, which became popular with the works of sir charles wheatstone in 1843 an electrical circuit that is set up to measure the unknown value of a resistor and creates a balance between the two legs of the bridge circuit is called a wheatstone bridge.
Mar 14, 2019 requires halfbridge completion resistors to complete the wheatstone bridge. R, and a second gauge is placed transverse to the applied strain. The threewire quarterbridge circuit tech tip vishay precision. What is a wheatstone bridge and other strain gauges circuits. Resistors r5, r6 and r7 of the wheatstone bridge must match the stain gauge nominal resistance and must be equal to avoid creating a bridge offset voltage. All strain gauge configurations are based on the concept of a wheatstone bridge. Practical strain gage measurements introduction w ith todays emphasis on product liability and energy efficiency, designs must not only be lighter and stronger, but also more thoroughly tested than ever before. The bridge circuit is connected to an analog to digital converter so that the output voltages. Its operation is similar to the original potentiometer. If a metal piece is subjected to a tensile stress, the metal length will increase and thus will increase the electrical.
Requires halfbridge completion resistors to complete the wheatstone bridge. Developing strain gauges and instruments strain gauge. Wheatstone bridge circuit theory, example and applications. Us4911016a semiconductor strain gauge bridge circuit. Because of its outstanding sensitivity, the wheatstone bridge. A strain gauge also spelled strain gage is a device used to measure strain on an object. Strain gauge bridge circuit measuring mode bridge circuit wiring connection to bridge output switching box bridge box 1 quarter bridge with 2wire e. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. The wheatstone bridge is also well suited for the measurement of small changes of a resistance and is, therefore, also suitable for measuring the resistance change in a strain gage sg. The factor k, also known as the gage factor, is a characteristic of the strain gage and has been checked experimentally 2. Bridge measuring circuits in the strain gauge sensor configuration 79 resistors in the bridge, this bridge configuration is called a quarter bridge fig. By the external sense lines of the bridge supply it is guaranteed, even if the lines are long, that a constant voltage is applying to the strain gauge sensor and therefore there is a high measurement accuracy. Wheatstone bridge strain gauge circuits are used extensively inside transducer to convert a mechanical strain into an electrical output signal the basic circuit consists of four resistive elements which are connected together into a diamond shaped configuration. Strain gauges are available with nominal resistances from 30 to 3000 the most common values are 120, 350.
One or more of these legs can be active sensing elements. In the above circuit, two resistors r 1 and r 2 are equal to each other and r 3 is the variable resistor. In this circuit, r 1 and r 3 are the ratio arms equal to each other, and r 2 is the rheostat arm has a value equal to the strain gage resistance. In this bridge circuit, known today as the wheatstone bridge circuit, unknown resistances are compared with welldefined resistances 1. Tension force causes the strain gauge to get thinner and longer, increasing the resistance.
Strain gages structure of strain gages there are many types of strain gages. Pdf a highprecision ac wheatstone bridge strain gauge. This change in resistance is proportional to the applied strain. The change in resistance of the strain gauge can therefore be utilised to measure strain accurately when connected to an appropriate measuring and indicating circuit e.
This change in resistance imbalances the bridge circuit, causing a voltage vout proportional to the amount of applied pressure. For strain gauge measurement, the wheatstone bridge circuit is used to convert. The strain gauge bonded on the elastic element is a secondary sensor which converts the strain produced by the force applied on the primary element into a resistance change. Strain gaugetransducer,sensor,wheatstone bridge,electrical. Low tolerance resistors must be used to minimize the offset and gain errors due to the bridge resistors. Measurement of strain due to torque with sensitivity of.
In this circuit diagram, a strain gauge is connected into a wheatstone bridge. Such demanding precision calls for a bridge measurement circuit. Unlike the wheatstone bridge shown in the last chapter using a nullbalance detector and a human operator to maintain a state of balance, a strain gauge bridge circuit. The wheatstone bridge has many uses in electronic circuits other than comparing an unknown resistance with a known resistance. The classic configuration associated to the strain gauges is the lowpower, wheatstone bridge 18. Strain gauge bridge circuit measuring mode bridge circuit wiring connection to bridge output switching box bridge box 8 half bridge with 2 active gauges. Typical threewire quarterbridge straingage circuit requires three bridge completion resistors. Again, such a figure is precise if the bridge circuit is fullactive four active strain gauges, one in each arm of the bridge, but only approximate for half bridge and quarter bridge arrangements. Strain gauge working principle your electrical guide.
Read the operation manual carefully before using bonding adhesives and. Therefore, to measure extremely a fraction of resistance changes with high accuracy, wheatstone bridge configuration is used. Explain what would happen to the voltage measured across this bridge circuit v ab if the strain gauge were to be compressed, assuming that the bridge begins in a balanced condition with no strain on the gauge. When used with operational amplifiers, the wheatstone bridge circuit can be used to measure and amplify small changes in resistance, r x due, for example, to changes in light intensity as we have seen above. In general, the gage factor for metal strain gages is about 2. In experimental stress analysis, we apply a given load and then measure the strain on individual. External bridge completion for strain gage circuits.
Stress, strain, and strain gages penn state mechanical engineering. Wheatstone bridge the basic circuit for strain gauge. R3 is an active strain gage measuring the compressive strain. A wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a. Quarter bridge circuit by using two strain gauges in the bridge, the effect of temperature can be avoided. The wheatstone bridge circuit, shown in its basic form in. A semiconductor strain gauge bridge circuit device is disclosed to include a bridge circuit of semiconductor strain gauges, a first zeropoint temperature compensation circuit connected to one of a pair of output terminals of the bridge circuit and including a voltagedividing resistor circuit generating a voltage substantially equal to a potential appearing at the one output terminal at a. One active strain gauge is referred to as a quarter bridge, two active strain gauges are a half bridge, and four active strain gauges are a full bridge. This design uses a 2 op amp instrumentation circuit to amplify a differential signal created by the change in resistance of a strain gauge. The wheatstone bridge is also well suited for the measurement of small changes of a resistance and is, therefore, also suitable for measuring the resistance change in a strain gauge. I show the equation for output voltage and do an example calculation. Bridge measuring circuits in the strain gauge sensor configuration 79 resistors in the bridge, this bridge configuration is called a quarterbridge fig.