Solving Cantilever Load Cell Drift Issues

[scott_alexsk]

Hello,

I recently put together a standard (or mostly so ;)) cantilever load cell for some tests I want to do (basically I am making a screw driven tensile machine). The cantilever has two strain gages mounted on it. These strain gages are wired in a half bridge configuration. In addition to this circuit I have a nulling potentiometer. The two standard outputs from the bridge circuit are taken and put into an instrumentation amp with a gain of about 750. I understand that typically these types of setups are unforgiving in terms of noise, but this is not my problem. The output of the circuit is not so noisy (at least with the level of precision I am concerned with), but it has an annoying drift(which varies in rate). Additionally there seems to be a significant delay in the settling of the voltage after I apply the load. When I jostle around the lead wires connected to the circuit typically the voltage will settle within a reasonable amount of what it was before, though occasionally it shifts by a much higher amount. From the tests I have done and loads I have applied (just static weights), it seems that the output is linear minus the drift. It might also be useful to know that this is a very thin Al-cantilever with the strain gages not covered and mounted with epoxy (I've been thinking that maybe I should cover the entire surface of the strain gage w/ epoxy, but this would mean that I would potentially lose a gage and have to remount one(again) if a lead broke or crossed or whatever). Any advice, speculation, and questions are very welcome.

Thanks,
-Scott

 

[scott_alexsk]

Well, I think I found the main source of the problem as being loss lead connections from the strain gage. The signals seem considerably more stable now. What would you guys recommend for having good connections to a breadboard? Should I dip some solder onto each lead and try to make a permanent connection? Is there a standard way of handling this? Should I just use tape? At this point it seems that this might be a better thread for the electronics section.

Thanks,
-Scott

 

[oswaler]

Hello Scott,

Thank you for sharing your cantilever load cell setup and the issues you are facing. Based on your description, it seems that the drift in your output voltage is likely caused by a combination of factors, including temperature changes, mechanical stress, and possibly even electromagnetic interference.

To address the temperature issue, you may want to consider using a temperature compensation circuit or a temperature sensor to account for any changes in temperature that may affect the strain gages. This can help stabilize the output voltage and reduce drift.

In addition, the mechanical stress on the cantilever and the strain gages can also contribute to drift. It is important to ensure that the load is applied evenly and consistently to the cantilever, and that there are no external forces causing stress on the gages. You may also want to consider using a strain gauge adhesive that is specifically designed for high-stress environments.

Electromagnetic interference can also be a factor in drift. Make sure that your setup is shielded from any nearby sources of interference, such as power cables or electronic devices. You can also try using twisted pair cables for your connections to help reduce interference.

Covering the strain gages with epoxy may also help reduce drift, as it can provide protection from external factors and help stabilize the gages. However, as you mentioned, there is a risk of damaging or losing a gage during the process. If possible, it may be worth trying to cover the gages with a thin layer of epoxy to see if it has any effect on the drift.

I hope these suggestions are helpful in solving your cantilever load cell drift issues. Let me know if you have any further questions or if you would like to discuss any other potential solutions.

Best,