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Hi Eric
I was going to put the second s/g on the opposite side to the original one so as to be in compression. The original one is on top so is in extension.I have reduced the gain so as to get only 1v to 1.5v change between rest and full load seems a lot more stable. But will have to change micro to read this change. Had difference between breadboard and circuit board but added cap between + - on the opa.Has been on test for one hour and the output changes by 10mv up then down slowly this is acceptable as i am displaying 0-9 1 volt divided by 10 this is done in the pic which is already working but thats another story .
Regards
bee
Hi Eric
I was going to put the second s/g on the opposite side to the original one so as to be in compression. The original one is on top so is in extension.I have reduced the gain so as to get only 1v to 1.5v change between rest and full load seems a lot more stable. But will have to change micro to read this change. Had difference between breadboard and circuit board but added cap between + - on the opa.Has been on test for one hour and the output changes by 10mv up then down slowly this is acceptable as i am displaying 0-9 1 volt divided by 10 this is done in the pic which is already working but thats another story .
Regards
bee
hi,
I would use the 2nd half of the OPA to give extra gain at the same time reduce the gain of the 1st stage.
Use the ZEROing method discussed earlier to remove any zero offset from the bridge when not loaded.
By adjusting the gain of both OPA's you should be able to get a 0V thru +5V change for the PIC.
Remember you will get some drift in readings due to temperature changes in the bridge circuit.
If you have spare SG's use them in the other bridge arms BUT not fixed 'firmly' to the crank,, no tension/comp but subject to the same termpr change as the active SG's.
Hi Eric
The circuit is working reasonably well as i have built it on a vero board and it all works allthough the opa is working hard with the gain. I will use the second opa as soon as i work out the best configuration.I am not to concerned with temp variations as it is more a comparison between cranks as both at same temp.Very pleased as it gets better everytime i mess around with it.Any ideas on the second opa appreciated.
Regards
bee
Hi Eric
The circuit is working reasonably well as i have built it on a vero board and it all works allthough the opa is working hard with the gain. I will use the second opa as soon as i work out the best configuration.I am not to concerned with temp variations as it is more a comparison between cranks as both at same temp.Very pleased as it gets better everytime i mess around with it.Any ideas on the second opa appreciated.
Regards
bee
Hi Eric
The OPA i am using is the mcp 6292 dual sorry for the confusion as the pin out in the drawing is a 741 this is all i could find in eagle.See page one from data sheet attached.
Regards
bee
Hi
Have found correct pinout in eagle not correct device but same pinout. If IC1A is configured as a differential amp? what would IC2B be configured as a non inverting amp?.Have attached updated working schematic and also how i think the second amp should be configured not sure about values for R1,R7,R8 and by how much to reduce gain of IC1A etc.Thanks
bee
Hi
Have found correct pinout in eagle not correct device but same pinout. If IC1A is configured as a differential amp? what would IC2B be configured as a non inverting amp?.Have attached updated working schematic and also how i think the second amp should be configured not sure about values for R1,R7,R8 and by how much to reduce gain of IC1A etc.Thanks
bee
hi bee,
Looked at the new dwgs, the 2nd dwg with the NI OPA would be OK.
As you say, the gain of the 1st stage should be reduced, I would set the gain no more than about 100 [or 200 tops] and the 2nd stage to about 100.
Make the 2nd stage gain adjustable to give SPAN control.
This would give an overall gain of about 10,000/20,000.
Do you want me to help with the resistor calculations.?
I see in your dwg.4 that only one SG is used in the bridge.??
EDIT:
Added dwg.
Hi Eric
Still using only one s/g as i got it working reasonably well with one.I still have the 1.5k resistors on the bridge arm with s/g to reduce current is that ok as it seems to still work ok.The caps 100nF/100uF are to stablise supply correct.If you could help with res values would be good, are the values on the drg approximate.Forgive my ignorance but what is AOT on the feedback loop IC1A
Regards
bee
Hi Eric
Still using only one s/g as i got it working reasonably well with one.I still have the 1.5k resistors on the bridge arm with s/g to reduce current is that ok as it seems to still work ok.The caps 100nF/100uF are to stablise supply correct.If you could help with res values would be good, are the values on the drg approximate.Forgive my ignorance but what is AOT on the feedback loop IC1A
Regards
bee
hi,
AOT or SOT are 'accept on test' and 'select on test'.
All it means if you want to reduce the cost of using the VR1 pot, once you have the required gain from the 1st stage, you could replace the 6K8 and VR1 with one fixed value resistor. [measure VR1 ohms value when done].
The caps are essential, if possible add another 100nF across pins 8 and 4 of the OPA.
You are working with very low signal levels and high gains, so you must try to minimise noise.
Thanks for that i will build this circuit and post back my findings as soon as possible.Original circuit set up on crank arm testing pressure and sending data via wireless to display (seven seg) working nicely.Have to build one for other crank arm and send different data to turn on left/right display
Regards
bee
Hi Eric
I have altered the circuit to use both amps and it looks good.Still testing the other circuit
with single amp and it is working good also. So will have to make a decision if one amp is good two must be better.Thanks for your help again
Regards
bee