I want to know the different techniques that can be used for the measurement of resistive sensors.
Looking forward for your answers!
you can use voltage divider, wheatstone bridge and DMM resistance circuit etc.,
Potential divider circuit: it will not give accurate results due to the noise problems (or) wheatstone bridge: which can measure accurate resistance with noise cancellation. WO3 sensor (is resistance sensor) resistance can be measured using Weinstone bridge. The two terminals of a sensor should be connected in the weinstone bridge like a resistor. Rx=(R2.R3)/R1, Rx= is sensor resistance
(or)
Digital multimeter DMM circuit: in this method for a particular range of resistance we will send a known current in to the resistor, and we calculate the voltage across it.
from ohm's law R=V/I, V=known and I=known
As Anand said, a current source, sourcing current to the resistive load (ground related) is the best choice because the voltage drop across the resistor is proportional to the actual resistance. In this case a simple amplifier may be needed to ensure full dynamic range. Unless, in case a voltage divider one needs a linearization algorithm, maybe an amplifier and an offset subtractor. Meanwhile, Wheatstone bridge can render really sensitive measurements when the resistance changes very little.
Thankyou so much both of you :)
Actually i want to measure data from aprox 30 strain guage sensors which are fabricated on a surface. I want fast data extraction from them.
Any suggestion?
Hello Muhammad Khan,
Usually strain gauges in bridge configuration have a standard output range or +/-2mV or +/-20mV rated for a +/-5VDC supply excitation.
If you are testing single (isolated) strain-gauge elements you can either use bridge using passive resistive elements (commonly used resistors) in your circuit to complete the bridge formation and then use a Diff-amp or In-Amp to amplify the signals. As the number of input channels is large (approx. 30 as you said) you will have to use multiplexer. But instead of passive resistive element, you can use identical strain-gauges in the bridge to compensate any temperature drifts etc.
Again there will be a cost trade-off between individual channel amplifiers and then a MUX or a MUX first and then the amplifier.
And finally, your data-acquisition system will come into picture - with more than 30 channels to be sampled - it will invite really high speed scanning and sampling.
You can do 30 conditioning circuits (current source - the measured resistance goes in the feedback branch+amplifier+ floating ground - one op amp at least dual in package is enough) but eventually you either use 30 analog inputs to one or two a/d converters or you use a MUX and a simple multimeter. If you don't need much of a resolution a PIC uC comes in hand since you may find one with 30 analog inputs and 12 bits for the a/d converter. You can try to look for an already-made board with such hardware on it.
Now one problems is that i need to place strain guages on different sides of the metallic bar due to that the wire resistance will also increases so one method could be to use a 3 wire system.
Also what about active bridge circuits?
Does it work correctly?
Any suggestions?
I would recommend a standard bridge of 4 gauges connected to either a differential amplifier of an instrumentation amplifier. Keep the external connection leads (wires) from all 4 of the gauges to same or equal length and then connect to the amplifier circuit.
You can use SMD components such as 0603 or 0805 size resistors and SOT-23-5 5-pin op-amp packages to make your amplifier circuit in less than 10mm X 10mm area (a but critical task) or at-least 1 sq. inch (25mm X 25mm) PCB size. Place it closer to your metallic bar and then take a shielded cable to both power this circuit and take the signal to your next processing stage.
Hope this works fine for you!
Best Luck!
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