The technology center of a large company was subcontracted by a
manufacturer of avionics components to build an electronic device to
monitor the shock, vibration, and temperature in a compartment of an
avionics bay in an Air Force plane. The prime contractor was tasked
with collecting this data in a series of test flights in order to
troubleshoot a failure in one of their components.
The subcontractor's engineering staff was busy with other
projects, so they hired me to perform a significant part of the
work. I worked directly with the prime contractor to ensure I
understood their requirements. My client had already decided upon an
commercial-off-the-shelf (COTS) processor board, and I designed a
custom ultra-low power board to collect the data and transfer it to
the memory on the processor board. The whole electronics package was
about 3" x 5" x 2".
I identified a 3-axis accelerometer, and selected the temperature
sensors and major components, such as the amplifiers and power
regulators. I researched and obtained large capacity 9-volt
batteries, and then designed a battery-pack for the unit. I cascaded
a switching regulator followed by a linear regulator to provide
clean power, suitable for low-noise measurement, while maintaining
good conversion efficiency to conserve battery energy. I needed to
monitor peak shock, but I didn't have enough printed circuit board
real estate to design a full-blown, continuous peak-following
circuit. So, I did the next best thing: I used a PIC microcontroller
to continuously sample the accelerometer and find the peaks in the
readings. I was able to sample the accelerometer fast enough to meet
the shock measurement specifications. I also used the PIC to
communicate with the main processor board using the 3-wire Serial
Peripheral Interface (SPI).
Packaging the electronics with the connectors and the
battery-pack was a major challenge. The customer provided very
detailed volume outline specifications for the package because it
had to fit into an irregular slot. Fortunately, I found a designer
and machinist up to the task.
Finally, I took C code fragments from various sources in the
project and put them together with my own C code to operate the
unit, collect and store data, and interface with the user through an
RS232 port.
This project is a good showcase of my experience with project
management, product definition, interaction with multiple clients,
microcontroller hardware and firmware development, linear design,
digital design, mechanical packaging, and software development.
