Vibration Sensor Development
Why a Vibration Sensor Sensor?
The vibration sensor is a piezoelectric sensor that generates a voltage when the film flexes. The greater the flex, the greater the voltage generated by the film. We realized that the PIR sensor is not sensitive enough to let us know if someone is actually in the room. Since the table in the lounge tends to wobble a little bit when bumped (or even if someone is writing on it) we decided to detect these vibrations with this piezoelectric film. The film is going to hang under the table and, when bumped, a voltage can be detected.
Testing
We tested the sensor's sensitivity and how well it works.
Our Findings
We found that each sensor can only detect vibration effectively on one axis because of the way it is oriented. In order to detect all vibrations, one sensor will hang to detect X axis motion and another will hang perpendicular to this to detect Y axis motion. Watch the video of us testing the vibration sensor!
To Do
As the video above shows, there is a good deal of noise when the sensor is not picking up any vibrations. We have noted from testing that most of the noise is high frequency noise and most of the voltages generated by vibration are much lower frequency. Soon, we are going to implement a low pass filter in order to eliminate most if not all of this high frequency noise. If need be, we can then implement some amplifier to better detect even small voltages.
Amplification
Due to the vibration sensor having negative voltage that needs to be amplified a simple op-amp would not suffice. We talked to tecknick for suggestions, and he helped us find the correct amp chip based on input offset current, imput common-mode voltage range, input bias current, and output voltage swing(TLV2782). Also the schematics for the correct way to amplify the system, including a buffer, was presented.
This design is used so it can amplify the positive and negative singles that are received. Also, a 10 mega ohm resistor was added to filter the single. This sensor is so sensitive, as well as having a high impedance, that this resistor is used as a form of filter in itself. This circuit not only solved the sensitivity and the effectiveness of the vibration sensor, however also fixed the problem of having a noisy single.
This is the finished result of one vibration sensor completely hooked up.
