Strain gages, pics and lcds

[kitedude]

Hi,

I'm wondering if any body can give ma some tips on how best use strain gages.

The reason i ask is i've to build a pwer meter for a bicycle.
My basic design will be;

full wheatstone bridge (located on the crank arm of the bike)with a dc supply (4 strain gages). i was going to use an ac supply but i couldn't generate a clean enough signal, as i'm using a pp3 battery.
the output from the bridge is going into a 16f877 pic. i'm also using a reed switch to calculate speed og the rotating pedals,this is also beign fed into the PIC. I'm doing the calculation in the pic and then transmitting it using an tx433 to a unit on the handlebar. this unit will be attahced to the chainwheel and hence will be rotating.

on the handle bar unit there is a rx433 with the output going in to another 16f877. also going into the pic is another reed switch to measure velocity. i'm then doin some more calculations and displying the results on a serial LCD Module.

if anybody can see any faults or places were i might have a problem please speak up.

 

[Russlk]

The output from the strain guage bridge is going to be extremely small (microvolts). You will need a low noise instrumentation amp to get the signal up to where a D/A can read it.

 

[Nigel Goodwin]

You mention 4 strain gauges, presumably you are using 4 wheatstone bridges?, you can't really use strain guages for all 4 arms of a single bridge.

Also, I notice you are using 16F877's, these are rather large (ABSOLUTELY MASSIVE), a 16F876 is a great deal smaller for the loss of a few I/O pins, or there are even smaller FLASH chips now with 10 bit A2D, 18 pin and even 8 pin ones.

 

[kitedude]

thansk for your reply,

i'm using 4 strain gages in a 1 wheatstone bridge.i'm uner the impression from the omega hane book that this is possible once they are locatred right.

Nigel, i realise that the 16f877 is large, i had intended to use an 16f88

but hadn't got a devbelpoment board, so i just choose the 877 as i had a development board for this.

see here for an explanation of a full wheatstone bridge using 4 strain gages

**broken link removed**

 

[Nigel Goodwin]

thansk for your reply,

i'm using 4 strain gages in a 1 wheatstone bridge.i'm uner the impression from the omega hane book that this is possible once they are locatred right.

Yes, if you only want one output, I presumed you were wanting to monitor both crank arms independently - which you couldn't do with one bridge.

Funnily enough I used to know a guy who worked on the railways (I don't know if he still does or not) fitting strain gauges to railway bridges and monitoring them.

Nigel, i realise that the 16f877 is large, i had intended to use an 16f88

but hadn't got a devbelpoment board, so i just choose the 877 as i had a development board for this.

Right, it's a good idea, and 14 bit PIC code will easily move to another of the 14 bit core range (proving you don't use any peripherals present on the 877, and not on your target processor - but I'm sure you knew that!).