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j.m.krell
08-19-2008, 01:44 AM
Hi,

My name is Jordan Krell and I am from Plankinton, South Dakota. I am starting a project and to reach my goal I need to be able to sense and read the vibrations that an object outputs. I purchased a LDT0 vibration sensor from Parallax (the Piezo Film Vibra Tab Mass). In the information sheet, it is specified how to use it as a triggering device but I would like to use it to measure the vibrations and not as a triggering device.

In the application note from the manufacture, it describes how to use it as a vibration sensor and states that a charge amplifier is needed for this. My first question is what is a charge amplifier or how is one constructed and secondly, is one neccessarily needed? Thank you very much and have a great day!

Jordan M. Krell

Tracy Allen
08-19-2008, 02:53 AM
What aspects of the vibration do you need to read? If it is only frequency or whether the vibration is above or below a certain threshold, that is relatively easy. To quantify the amplitude of vibration you will need an op-amp and an analog-to digital converter, at least RCtime.

The sensor is like a small capacitor and when it is flexed, a small amount of charge is generated in the piezo plastic . The charge collects on the electrodes plated on the opposite sides of the plastic and makes a voltage. The movement of charge is also small current, so if the electrodes are shorted, that current flows from one plate to the other. The current (or voltage) alternates + and - depending on the direction of flex. The sensor can be hooked up in an op-amp circuit that amplifies the current or the voltage. A charge amplifier integrates the current with another capacitor in the feedback path, and that can be made very sensitive and proportional to the time course of charge generation in the sensor.

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Tracy Allen
www.emesystems.com (http://www.emesystems.com)

j.m.krell
08-19-2008, 12:07 PM
Thank you very much as I greatly appreciate it. I would like to determine if the frequency is within a certain range. What exactly is an op-amp circuit or how can one be constructed and also the charge amplifier. Once again, thank you very much as I greatly appreciate it.

Jordan

Tracy Allen
08-20-2008, 02:04 AM
If the vibration is large enough, all you have to do is connect the sensor to a Stamp pin and use the PBASIC COUNT command. That will work only for vibrations that generate over 1.4 volts of signal. For medium vibrations, you can bias the Stamp input pin and the sensor near threshold, so that it can respond to signals on the order of 0.1 volt. For weaker vibrations you would need the op amp. What sort of vibrations are you looking at in terms of amplitude and frequency?




10kΩ potentiometer set ~1.4 volts for high sensitivity.
+5 ---/\/\/\/\/\/\------ Vss
| 10 MΩ
`---/\/\---o----- p0
|
=====
|
Vss

DO
COUNT 0,1000,result
DEBUG DEC result,CR
LOOP


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Tracy Allen
www.emesystems.com (http://www.emesystems.com)