tare circuit for digital scale

[terrence thompson]

I have made a scale to accept a wheelchair from the components of a digital bathroom scale mounted on a stiffenned piece of 16m mbf board 900mm * 900mm. The scale works acceptably well since the wheelchair isloaded to exactly the same position each time.
I would like to add a tare device to tear off the weight of the wheelchair but my electronics knowledge doesn't go to that.
The 4 strain gauges have three wires and and are set out on the circuit board in a manner that suggests two complete wheatstone bridges. ( I can only see the back of the board) I had thought that I could create another adjustable bridge and feed it into the circuit but I am unsure if it should be in parrallel with the other bridges or in series. Can anyone help please?

 

[ericgibbs]

I have made a scale to accept a wheelchair from the components of a digital bathroom scale mounted on a stiffenned piece of 16m mbf board 900mm * 900mm. The scale works acceptably well since the wheelchair isloaded to exactly the same position each time.
I would like to add a tare device to tear off the weight of the wheelchair but my electronics knowledge doesn't go to that.
The 4 strain gauges have three wires and and are set out on the circuit board in a manner that suggests two complete wheatstone bridges. ( I can only see the back of the board) I had thought that I could create another adjustable bridge and feed it into the circuit but I am unsure if it should be in parrallel with the other bridges or in series. Can anyone help please?

hi,
You could add an analog tare by adding a 'high' value variable resistor across one arm of the strain gauge bridge.

What are the approx resistive values of the strain gauge elements.?

 

[Mikebits]

This would be a software thing I think. To tare the wheelchair, one would have to place only wheelchair on the scale and set zero reference based on this weight, then any additional weight added to scale is added based on reference zero point.

I guess I don't understand your question fully as this seems like a software issue, your not trying to do this with only hardware are you?

 

[Mikebits]

Sorry Eric, guess we were posting at the same time...

To OP, listen to Eric, he is one sharp guy.

 

[MrAl]

Hi,

The tare weight is usually assumed to change with time, but i dont really know
what accuracy you are after either. Because of this, it is usually done with a
digital method where one reading is taken with no object placed on the scale
and another one after the object is placed on the scale. Of course the first
reading is then subtracted from the second to get the actual weight of the
object.
What else you should tell us is how you are actually converting the signals to
a digital readout, assuming that is how you are doing it. Depending on how
you are doing it (microcontroller, discreet logic, etc.) a different technique
will apply. It's simple to understand that for example if you are using a micro
then software would be the best solution, but if discreet logic then you may
have to add a few ic packages.
So, perhaps you can explain how you are actually converting the analog signals
to a digital readout for the existing design that you have now.