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# Tag Info

12

The strain gauge elements come with a positively stress-sensitive portion and a negatively stress-sensitive portion. If you wire them up carefully by flipping them around so the stress sensitive portions unbalance the bridge constructively, you can use all four sensors without any extra resistors. jonk's link to the blog post at http://www.nerdkits.com/...

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 ...

7

OK, problem solved. The bridge is connected like this. Only one resistance in the load cells is variable, the other is fixed. Why the confusion above? I was measuring resistance of a load cell which came from a different scale. The cells looked pretty similar, therefore I thought they were the same. But they were not! Eureka! simulate this circuit – ...

7

I'm going to state this up front - a differential amplifier made from an op-amp and four resistors is perfectly fine when (and only when) the input sources are much, much lower impedance than the resistors used. You will not get this when a bridge is attached because, as the bridge is off-balanced by whatever it is you are measuring, its output resistance ...

7

I'm betting a component is quantization noise or other distortion on the AFG3000, which has 14bit output. The harmonic distortion is listed as -60dB in your freq range. You might try using a cleaner sine wave and seeing what you get, or condition the blazes out of the excitation wave you have. In any case, after asking about long cables that may be ...

6

Start with the Wheatstone bridge equation from wikipedia which is the subtraction of two voltage dividers. $$V_G = V_s( \frac{R_x}{R_3+R_x} - \frac{R_2}{R_1+R_2})$$ Sensitivity is the derivative with respect to $R_x$. You'll notice immediately that the voltage divider that doesn't have the element of interest doesn't impact the sensitivity at all. The ...

5

Having a capacitor directly across the op amp inputs always looks iffy to me. It can make the op amp trend to oscillate because high frequency feedback gets attenuated. If you are measuring the 1.5v "latch-up" voltage with a DC volt meter, you might simply be seeing the average voltage of an oscillation. Try placing C1 before R2 and R3 and see if that ...

4

Consider getting rid of the bridge and instrumentation amplifier. Feed the RTD (via 10k) from the same reference voltage that your ADC uses - this is called ratiometric measurements and removes one of the big error sources i.e. because RTD and ADC are both ultimately fed from a common reference voltage, it doesn't matter if that reference voltage drifts. So,...

4

[Disclaimer: I haven't seen such arrangement before. Below is an educated guess.] This looks like a self-test feature, rather than extra excitation. When Q2 is conducting, R11 appears in parallel with R3. That introduces a small imbalance in the bridge. If the measurement circuitry detects this imbalance, then self-test passed successfully.

4

You need to respect the amplifier input common mode range (and output range). If your amplifier (I'll assume it's an AD8221 inamp) had large power supply voltages your bridge configuration would be fine. You're putting about 0.7mA through the RTD element, which is reasonable for a Pt100. Your full-scale span will be 100mV, so you need sufficient gain in the ...

4

Having recommended reading about partition noise I'm finding nothing terribly useful online myself! So I'll try running a few numbers. In the absence of a better reference model, I'll model it simply as 2 shot noise sources, one in each leg of the bridge. (The sum will be simply the shot noise inherent in the same total current from the sig gen). Applying ...

4

I was wondering what the purpose of the Vref is What you have is called a differential amplifier and, the ratio of R3 and R4 should be the same as the ratio of R1 and R2: - When you use a "ref" voltage instead of 0V the output from the differential amplifier is "referenced" to the ref voltage. A typical example might be to make ref = 2.5 volts and, when ...

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

You start with a primitive circuit, and then apply a series of re-configurations, becoming progressively more sophisticated, until you arrive at the final level of sophistication - a single 4 ohm resistor and a source. Along the way, the currents and voltages in/at the various conductors and nodes that you introduce/remove, change. It's not surprising. ...

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

I found that the "A"-resistors are not fixed but show a mirrored resistance change because they undergo a compression instead of an elongation. This is because there is not a simple arc-bending of the metal arm, but, the special rivet fixation of the upper arm part, results in a complex "S"-forming bending .So both sensors can be glued on the same side of ...

3

If all the half-bridge sensors changed their resistance exactly the same when a load was applied, can you see that they could be mounted in parallel - the effective end-to-end resistance would drop from 2 kohm to 1 kohm but that is of no consequence to a bridge measurement circuit. Even if there are disparities in the resistance between two paralleled ...

3

In order for the bridge to produce an output between S- and S+ the bridge must become "unbalanced" when weight is applied. So if all 4 load cells produce equally increasing or decreasing resistances when weight is applied the bridge will remain balanced and the output will not change. If the load cells are arranged such that the left and right halves of ...

3

It isn't a good idea to use resistive temperature sensors this way because of the large change in resistance will delinearize the bridge whether current fed or voltage fed. Devices like strain gauges bearly change their resistance at all and can be ok as a single active element in a bridge but not RTDs. So, use the three wire connection and use a constant ...

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

Here is a 5 second answer to start. First, put a decoupling cap (0.1uF or something like that) between LM324 pins 4 and 11 (its just good practice). Just for testing connect LM324-1 to LM324-2 (unity gain), because an OpAmp is meant to be used with feedback. Of course you know that you will need a differential amp sensing both sides of the bridge to get ...

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

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

There are two key benefits of null-balance measuring methods. The first is that the quantity being measured is not affected in any way by the measurement when the system is in balance; i.e., there are no "loading" effects. The second is that since the measurement is always made at the same point on the indicating device, any nonlinearities in the ...

3

Your first approach is unlikely to work very well because analog multiplexers don't exhibit good resistance matching between channels - in the case of HEF4067B, it's listed as up to 25 ohms mismatch. They also exhibit variable on resistance over voltage: Your second approach should work because current won't be flowing through the MUX, meaning the ...

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

This is a better view of a fairly generic load cell comprising 4 strain gauges: - A simple single-ended power supply can be used (such as 5V) and this is applied between red and black wires with black being connected to 0V. The green and white wires will nominally be centred at about 2.5 volts with no load. One wire's voltage will rise with load whilst the ...

3

With a 24 volt supply, the current thru the limb of the bridge containing the thermistor is: - Nearly 11mA at 0C Nearly 19mA at 50C At 0C this produces a power in the thermistor of 132mW and 63mW at 50C. To me this is a problem. The self-heating of the thermistor will create a significant measurment error at 0C compared to 50C. I'd use an instrumentation ...

3

I found the culprit. Actually it is due to ADC's switching when it takes Sample. I disconnected OPamp's Output from ADC by removing the R from RC and the ringing was gone.

3

The load cell parts you have are most likely arranged as shown in the first diagram below. (This is a typical half bridge.) You should use an ohm meter to confirm the wire colors. In some low cost load cells one resistor (strain gage) will be active (changes with stress), and the other a reference (which can also help temperature compensate the other). ...

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