Vibration Detection and Characterization
Design Requirements
The purpose of this project was to create a device that would improve the build and testing workflow of a specific active vibration isolation(AVI) cell being produced by our sponsor, Aerotech. It was intended to replace a previous system that had become obsolete due to lost source code. The desired specifications were as follows:
Detect vibrations with a magnitude <.1 g and a frequency <10 Hz.
Distinguish between an acceptable and unacceptable level of vibrations
Run from wall power
Be maintainable/repairable from paper schematics (i.e. no source code to lose)
This picture shows the completed system on its testbed for simulating the AVI system.
Sensor
The vibration sensor consisted of a 100 g steel mass attached to a vertical spring steel beam. This spring-mass system was tuned to resonate at the expected vibration frequency in order to amplify the motion. A magnet on top of the mass allowed its displacement to be measured by a Hall Effect sensor.
Signal Processing
I designed the circuit that processed the signal from the Hall Effect sensor.
The signal was first compared to a tuneable zero point using a differential amplifier. The result was then passed through a low-pass filter/amplifier and a full-wave rectifier to eliminate noise and determine absolute displacement. The signal was then compared to a set threshold value for acceptable vibration, illuminating a warning light if that value was exceeded. A separate window comparator provided an indication to allow tuning without additional tools.
The Design Process
Most of the signal processing techniques I used I had to teach to myself over the course of the project. I was chosen to lead the circuit design because I was willing to learn those techniques and because I had experience with digital circuits.
That experience helped me when it came time to convert the early prototypes into the final design. I used Autodesk Eagle to layout the circuit board, which was then milled. Off the shelf components were used to reduce cost and ensure future repairability.
Manufacturing Process
My other main responsibility on the team was to act as the liaison with university resources such as the machine shop and electronics lab. They produced the circuit board, mass, beam, beam mount, and other custom components. Some of the components, like the mass shown here, were produced from verbal design specifications.
Design for Manufacture
Others, like the beam mount shown in this drawing, were produced from mechanical drawings. The beam mount was the project’s most complex custom mechanical component, and its design illustrates one of my most important roles as liaison: I needed to hear and convey feedback on producibility. The beam mount was originally designed to be easily replaceable, but its design was too expensive to produce. The machinist suggested a design revision to allow a more robust mount, which was implemented in the final design.
