# Tag Info

9

What you are missing is that the common purpose of using a Wheatstone bridge is to balance two sensors. Ie. when we use a Wheatstone bridge we usually have two variable resistances or current sources that we want to balance like this; simulate this circuit – Schematic created using CircuitLab You are right in concluding that if we only have one ...

6

Strain gauges change their resistance depending on the strain they're subjected to. This means that their current draw will vary depending on the strain. The actual resistance of the gauges will be found on the datasheet, but can vary a lot between different products. For example, this one is 120 ohms, and this one is 10 kohms - clearly a big difference. The ...

6

As said by the others, they are resistors, so they will consume power depending on how much current runs through them. That said, if you do not need continuous measurement, you can only activate the measurement current/voltage when you need it. For example, if you make a battery-powered scale that wakes up when the user stands on it, you would keep the ...

5

As on any instrumentation amplifier (INA), of which the AD620 is an example, you can't just leave the REF pin (pin 5) floating. The output voltage is relative to REF, and you need to measure the difference in voltage between the two pins. Also, although the AD620 can operate at low voltage (5V supply), it is not a rail-to-rail device, either on input or ...

5

In strain gauge bridges that I have used (120 Ω and 350 Ω) I've tended to excite them with 2 to 4 mA rather than with a fixed excitation voltage and so 1 or 2 mA would flow through each gauge. Power = $I^2R$ hence, for a 350 Ω gauge it is up to 1.4 mW.

4

Think of this (bridge) circuit as two simple potential dividers R3 and R4 divide the excitation voltage in the ratio V1 = R4/(R3 +R4) * E R2 an R1 divide the excitation voltage in the ratio V2 = R1/(R2 + R1) *E The output voltage, e, is the difference between V1 and V2 (assuming no current is taken by the measuring equipment)

4

The strain gauge output isn't negative, exactly. Its essentially a wheatstone bridge, and the measurement is called a ratiometric measurement. Essentially, what changes is the resistance of the actual sensing element. The wheatstone bridge allows you to measure small changes in resistance. The bridge consists of 4 equal resistances when theres no strain. ...

4

Firstly, regarding the selection of a sensor amplifier, go straight for an instrumentation amplifier such as the AD8221 (that's what I use for bridge and single sensor amplifiers like strain gauges): - The advantage of an InAmp is that its input impedance is very high so it won't load the bridge and there is just one gain-set resistor needed. Regarding the ...

4

Wheatstone Bridge style strain gauges have 4 wires, as shown in the circuit diagram you posted. Red and Black are for your excitation voltage, white and green is your differential output voltage that's scaled by that excitation voltage you provide.

4

Bridge/Strain Gage Signal Conditioner Omega CAD530.00 DMD4059 Maximum Output: 10 Vdc @ 120 mA You can operate from 1 to 10V and power is increased with sensitivity accordingly.

4

The problem with using a voltage reference version is that the circuit depends on the difference between two accurate voltage sources, V and Vref. In the Wheatstone bridge, there is just one voltage source, and there is some error cancelation. Accurate voltage references are noisy and difficult to filter without losing accuracy. They have good long-term ...

3

By definition, a strain gauge amplifier amplifies the small signal from a strain gauge. A strain gauge is a resistive material that changes resistance when deformed as a result of applied force. This change in resistance is usually measured using a Wheatstone bridge. Note that, depending on the design of the strain gauge, one or two or all four arms of ...

3

A TL082 is somewhat inappropriate for low-level instrumentation- the offset can be as bad as 20mV and the drift is not guaranteed but typically is 10uV/°C. A gain of only 1000 means that a 10mV offset will saturate an amplifier with +/-10V supplies, even with 0V in. If it was 1975 and you really had to use something that bad, you could put an AC voltage ...

3

First of all you should really get better resistors. 5% with a strain gauge is just ridiculous. To "trim and calibrate" your measurement you should just leave your gauges be, i.e. leaving them in a known state, measure the output and save it in a variable in your micro. This value should then be subtracted from all subsequent measurements. If you can you ...

3

As a general guideline, it is preferable to connect any cable shields to the metal chassis (not PCB ground), at just one end. To keep things simple when multiple shielded cables connect out from some central device, the shield connection should be done only at the "hub" device, and left open at the "spoke" devices. Similarly, for a chain of devices ...

3

I'm going to have a go at answering my own question... The fact I was missing is that load cell output sensitivity is given with respect to load capacity. The load cell's Capacity 40-50 kg. Call it 50kg. The load cell's rated Output Sensitivity 1.0±0.1 mv/v. i.e. At 50kg when 10v excitation is applied the output will be 10 x 1e-3 i.e. 10mV At 25kg and ...

3

INA126 is an InAmp of the two-OpAmp variety. The REF pin, which can be used to provide an ofset for the output. $V_O = G (V_{(+)}-V_{(-)}) + V_{REF}$ This is similar the output offset trimming circuit in fig.2 in the INA126 datasheet.

3

You need to use the sensor in one-half of a wheatstone bridge circuit. The other half can be made from two reasonably identical resistors forming a potential divider like the strain gauge does. +Input of instrumentation amp to centre point on resistors and -Input to centre-point of gauges: - The two 350 ohm resistors that feed the lower input are the two ...

3

Load cells are ratiometric, so the output voltage will be proportional to the excitation voltage. Operating at a low excitation voltage will reduce the fullscale output from the gage, as you have identified. This isn't really a problem other than the fact that the differential output voltage of a load cell is so low to start with. Assuming that you're ...

3

Your bridge connection is most likely rotated by 90 degrees. No bridge would ever have this much offset. If you are sure it is not damaged, first disconnect any circuitry on the output pins and measure again. Then, check the pinouts and make sure they are properly connected. Best bet is to try rotating the strain gauge; that is, apply power to SP and SN ...

3

The way I do it is to use an instrumentation amplifier (TI, Analog devices etc.) that amplifies the difference signal and then apply a variable voltage to the reference input of the InAmp to balance the output to zero. An InAmp basically comprises three op-amps as shown below: - Note the "reference input" bottom right and the potential divider that sets ...

3

There are two ways of using a Wheatstone Bridge. The first classical way is to balance it, that is, adjust the arms until you get a zero output. Then you can assert that the ratios of arm impedances are equal, and calculate your unknowns from your knowns. The second way, which is (I think) a slight abuse of the name Wheatstone Bridge, is to use the bridge ...

3

Since these sensors are essentially resistive networks, the excitation energy could theoretically be arbitrarily small - in practice, noise, interference, leakage and dry contact concerns, and power requirements of the conditioning circuitry, will dictate the lower limit.

2

I've used pressure sensors from Kulite that have been recommended to run at 10V. I contacted them because I wanted to run at 3V with a precision PSU and they said "no problem" but they weren't prepared to "reduce" what the offset accuracy would be. At 10V they said it's offset was +/-5mV of nominal. Full-scale was +/-100mV and they confirmed that running ...

2

If I were brainstorming this, a buckle transducer would certainly come up. This is sometimes use to measure tension in muscles. A muscle would be pulled through some sort of frame, and a pin inserted in such a way as to act like a fulcrum. Strain gages can be placed just about anywhere. Not necessarily an entirely thought out solution. For example, you'd ...

2

That appears to be the specification for two half-bridges used in a full bridge configuration. The input Z is 1k and the output Z is 1K, so that would imply that each arm is 1K. It says 40-50kg for full scale, but it's not clear whether that's the total weight or not. Let's assume it's 45 kg total (22.5 kg for each sensor). Assuming it is the total, ...

2

Mostly, when it comes to full bridge strain gauge circuits we use an instrumentation amplifier with the reference input connected to a DAC output. This is pretty common in industry and allows nulling of offsets via software at any point. Pretty useful for pressure gauge's too.

2

To begin, I don't believe the question is worded properly. A moment around y will not affect B and D differently. I have a feeling they meant to say B and D are used for axial load and A and C are used for measuring the moment. A moment measured at two sides of that object would create equal and opposite forces. In a strain gage, it would come back as equal ...

2

By definition, your bridge is unbalanced. you don't say where the bridge comes from, or what it is, or if all the elements are active, or which ones are simple resistors, but its pretty clear that on one side the resistors have about a 2:1 ratio, and on the other a 1:2 ratio (give or take). Its possible that you've reversed your excitation with your ...

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