I am developing the signal conditioning of the conductive polymer which gives the resistance change when the force/strain is applied to the sensor. To develop a signal conditioning, first the transfer function of the sensor should be defined. so how can i calculate my transfer function in terms of strain.
For something like this, you don't calculate it, you measure it.
You need to set up some mechanical system to allow you to vary the strain accurately, perhaps clamps and a ruler. Then you need some system to measure the resistance, perhaps a DMM. Plot resistance against strain. Over a small range, it can be approximated by a linear relation. Over a large range, I don't expect that will be possible.
While you are measuring, you will need to see the effect of various confounding factors that might make your transfer function less accurate. The effect probably has hysteresis (sensitivity to strain history), temperature sensitivity, and time (creep). Once you know the sensitivity to these, you'll have an idea of what accuracy you can expect from the system.
To develop the conditioning circuit you just initially need to measure the signals at the extremes of force you might apply. You don't need to undertake research into the full transfer function until towards the end of the design unless, of course, the TF is highly non-linear.
how can i calculate my transfer function in terms of strain
You may possess data that allows you to calculate it but, in my experience, it is better to measure it and modify firmware constants to account for non-linearities and overall gain.
One of the first things you do need to decide upon is the value and method of excitation. Things to consider: -
- Voltage excitation (fairly simple and stable)
- Current excitation (better linearity in quarter bridge configurations)
- Value of excitation - more voltage or current usually means lower gains in amplifiers but, it means fewer operating hours if powered by a battery
- AC excitation can offer some advantages but is probably not needed.
Amplifier considerations: -
- After excitation is determined, the overall gain needs to be established so that at extremes of force, the circuit output doesn't saturate.
- Frequency response
- Accuracy, reliability, repeatability, drift.
If you are digitizing the signal you need to choose an appropriate ADC.