Electronics > Projects, Designs, and Technical Stuff

Precision Load Cell (Wheatstone Bridge) Simulator

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snarlingsnoyl:
First Post on EEVBlog, long time follower...

The goal of this project is to create a precision load cell simulator for testing/debugging load cell frontends/amplifiers/converters/etc. I am no expert in material science, precision resistors, low themal EMF design, etc; so I thought it best to share with everyone and hopefully get some good advice in return.

A load cell simulator is a device that produces a very small ratio-metric output simulating to the output of a strain-gage load cell. It can be used to provide a test signal to an instrument or device that is used to measure load cells.

Typically, the output is referenced to an excitation voltage and specified in terms of mV/V, or milli-volts output per volt of excitation. Most industry load cells are of the full-bridge type with a full scale output range around 2 mV/V; for example, if you excite the load cell with 10 VDC, then the full-scale output of the load cell would be +-20mV ( 2mV/V * 10V excitation).

The device will have a selection of outputs ranging from 0mV/V to 2mV/V in 0.2mV/V steps. Negative outputs will be accomplished via a polarity switch. My initial calculations show that this can be accomplished using a Wheatstone bridge and switching resistors in and out of series/parallel with each bridge leg in order to achieve the desired change in resistance.

Starting with a Wheatstone bridge:


We can then add parallel resistors across R1 & R4. However, in order to keep the total bridge resistance constant, it may be necessary to add resistors in series to R2 & R3. I've been double checking my math with a little LT Spice schematic:


The math (for a full bridge simulator) follows as such:
RB - Bridge Resistance:
     - Typically 350 ohms, also common are 120ohm, 1000ohm
RP - Leg Parallel Resistance:
     - This resistance is added in parallel to the active leg in order to lower its resistance by a calibrated amount
     - RP = RB * ~10 / Sensitivity
     - RP = 350 * 10 / 0.002 (or 2mV/V output) = ~1.75MegOhm
RS - Leg Series Resistance:
     - This resistance must be added to the opposite leg in order to compensate for the lower resistance caused by adding the parallel resistor
     - RS = RB²/(RB + RS)
     - RS = (350²)/(350+1750000) = ~80mOhm

It may be possible to add outputs or switches to reconfigure the simulator into half-bridge or quarter-bridge modes, but that could be a later improvement. Best to start simple.

Another possible improvement is for the simulator to work with both DC and AC excitation. For AC excitation, resistance need to be replaced with impedance. Fortunately, most industrial load cell systems using AC excitation typically only use 100-1000Hz carrier frequencies. This means that parasitic capacitance/inductance would need to be controlled, as well as balancing any resistor inductance, but should be reasonable easy to accomplish at such low frequencies.

As for parts, Caddock's low TC film resistors is the initial choice. However, I have to admit my experience with working with precision equipment is limited, and designing precision equipment is none. So things like temperature coefficients, parasitics, etc are on my radar, but I'm still trying to figure out how much impact on real world design and construction. This things are easy to

Final note, I lacking in the precision test equipment department. This project is going to require some precision resistance testing equipment. I've been looking through eBay and the like for a nice DMM/SMU. Anyone have any suggestions?

Be gentle

RandallMcRee:

You might get more information if you post in Metrology. Everything I am about to say comes from there, so-to-speak.

First, the basics are nicely handled by reading Conrad's DIY Metrology Lab articles. The wheatstone bridge is the basics of diy metrology.
http://conradhoffman.com/mini_metro_lab.html

Attached is a photo of a basic setup using a Kelvin-Varley divider measuring the tempco of a Vishay s102k resistor. It looks crude, but lets examine it. The 720A is a 0.1ppm KVD, so quite good. The aluminum box in front is just used to connect the auxiliary wires needed to create the other half of the bridge. Not very visible is another s102k resistor serving as a reference. (This setup is diagrammed on page 78, figure 9 of Conrad's part 3). The usb stick measures temperature. The gray cable on the left goes to a Fluke 731b to supply 10v to the bridge. The output is measured with an HP3456a.

For a modern DMM folks seem to recommend the DMM6500 or DMM7510, but I've stuck to the tried and true as noted above.

I have not heard anything bad about the Caddock series. For higher precision I have good experiences with the AE z series. e.g.
https://www.newark.com/alpha-electronics/hcz10k000t/resistor-metal-foil-10k-0-01-radial/dp/96Y7830

In looking at what you have written, the major question is the switching. There is a reason that the my bridge is all hard-wired. Its to accurately fix all of the variables having to do with those pesky interconnections. If you need programmable switching--relays,  jfets, analog switches; that's a big step up in complexity and error sources.

Hopefully, this is helpful.

MosherIV:
Hi.

I did not read everything in detail but it seem strang to me that you are trying to use a real bridge to simulate a strain gauge in a bridge.

If you want a simulator, all you need to know is the transfer function.
Knowing that, you know what output you should get for any input.
You then just build a system eg microcontroller with ADC and DAC. The micro then gives the simulated output voltage for a input voltage.

Or am I missing something?

Kleinstein:
For the resistors I would consider the 350 / 120 and 1 K bridges as separate projects.
I would consider some thing like 350 Ohms + a number of around 1 M resistors in parallel  for all the resistors. So to keep the sum constant one could start with 2 x 1 M in parallel and add / remove the extra resistors where needed.  How large the extra resistors should be depends on the range, but there usually is a limit of some 0.1% of change for the usual DMS.  With 1 M resistors switch resistance should not be a major problem, it more like leakage limited. If needed the next step would be using something like 300 Ohm + 50 Ohm in parallel with a few 100 K resistors.

DMS bridges may use quite some power, so that self heating can be important.
At 350 Ohms I would consider wire wound resistors and possibly bulk metal foil if needed.
The larger resistors for signal steps are less critical and could likely be thin film types.

For some tests one could consider a real dummy type, that has the bridge part only for a much smaller voltage and just series resistors to get the overall resistance.  so more like 170 Ohms resistors at each end and than a bridge with 10 Ohms resistors. This version could get away with much less critical resistors and still get a very stable reading.

jbb:
I think it's fair to say that if you want a continuously adjustable, high accuracy bridge simulator, you're going to have a difficult time.

Maybe you could split this into two sorts of tests: a lower precision adjustable type (maybe simple digital potentiometers would be sufficient) for debugging / exploring, and a number of discrete high-accuracy 'strain references' for precision testing?

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