June
2006, Issue 191
Measurement
System for Weight and Dimensions
Renesas
M16C Platform Design Contest 2005
WEIGHT
MEASUREMENTS
Shipping
companies don’t require precise measurement data. Most
items are shipped in the ubiquitous cardboard box. By
measuring boxes, you can measure almost anything that’s
shipped. Weights are rounded up the nearest pound. Sizes
are rounded up to the nearest inch.
I
first focused on weight measurements. I started with
an existing postal scale. I bought a new DIGIWEIGH DW-36XP
digital scale on the Internet for about $10. It’s a
basic scale that generates a simple LCD digital readout.
When I received it, I took it apart and began probing
for a way to adapt it to this project (see Photo 2).
|
Photo
2—The DW-36XP scale is perfect for this project.
The metal bar with the hole in it is the strain
gage. The white material over and under the hole
measures the metal bar’s deformation. The additional
wires are routed out the back to communicate with
the Weasure board. |
The
measuring component in the scale is a strain gage, which
is a metal bar with a flex point (a hole drilled in
it). A resistive material coats the bar above and below
the hole. As a load is applied, the bar flexes, causing
small changes in the resistive characteristics of the
bar’s coatings.
Reading
the strain gauge directly requires some carefully calibrated
analog electronics, so I looked for a higher-level output
that I could take from the scale. The scale, which doesn’t
provide any documented digital output, uses a proprietary
MPU hidden under an epoxy blob. I expected to find an
analog value that changed with the force applied, but
some probing with an oscilloscope turned up something
just as useful: a PWM signal. The signal’s low period
is directly proportional to the applied weight. (This
is conveniently labeled T3 on the DW-36XP scale’s circuit
board.) The low pulse is approximately 20 ms wide with
no load. It increases by about 2 ms per pound of force.
This signal, which is already at the same 5-V level
used by the Renesas M16C/62P microcontroller, works
perfectly with the microcontroller’s built-in PWM timer.
