Piezoelectricity

Can you be more specific? Are you talking about mass deforming a piezoelectric sensor or something sitting on a piezo transducer (sound making device).

its a mass deforming piezo sensor for generating voltage.!!

I'm still not sure what you want to know. I believe the device you want is called a strain gauge. There are also piezoelectric pressure transducers available. The theory of operation can be searched if you use the key words mentioned above in italics. Several suppliers have application information.

I like to use www.google.com for my search engine of choice.

This the piezoelectric plate I was talking about. When you apply force on it voltage generated. I would like to know if there are any formula relating both.!!

Actually a piezoelectric device generates some voltage when it is bent. The amount of response will depend on the characteristics of the particular crystal used including its physical dimensions.

Load cells are optimized for weight measurement and are made with high enough precision that the response between samples of the device is very similar.

The device shown in your picture appears to be a Piezo designed for making sound but it would also be usable for measuring force or weight. Unfortunately, you would have to calibrate the circuit for each device of this type as it will not likely match the next one for that application. By calibrate, I mean that you will have to "determine the formula" experimentally.