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Larry Pieniazek wrote:
> This is the classic "how do you calibrate the measuring tool if you
> don't have a standard to start from". We have no idea how much
> variability there is in things like springs and piston friction and
> pivot friction, which in turn can mean that two testers get different
> measurements of the same pump, or two identical pumps give different
> measurements on the same tester...
Here are thoughts on the variability in the tester issues posed.
The most important sources of variability in the RCX Controlled Air Compressor
Tester and precautions to be taken are as follows:
1. The compressor under test itself may be temperamental in its performance. Repeat
the test several times.
2. Recording arm
The pneumatic cylinder stiffens with age perhaps quicker in the hot and humid
climate where I live. (Anyone found a safe and effective lubricant for them?)
Differences in load imposed. Being a purist, I used 2 Lego weighted bricks to
maintain a constant load.
3. Pneumatic switch
Initially the pneumatic valve was operated by hand which generated lots of
variability. Use a motorised switch a la official design.
4. The simple strip chart recorder (from 8888 Idea Book) is perhaps the most
important single cause of variability in the whole tester set-up. The paper speed
is likely to vary with the type of paper and tension in the friction wheels. Need a
constant paper speed mechanism here.
5. Power supplies. Using batteries is a no, no. The RCX should be powered from a
mains adapter.
6. Operator variability
Although it is "RCX Controlled", there are quite a lot of things that has to be
done by the human operator. Build the tester, set it up, connect up the hoses and
wires, fix the paper roll, attach the writing pen, change the compressors between
the tests, etc. And finally, the operator (using a built-in "RCX"?) has to
interpret the charts.
To overcome the variability in a single tester problem, I used the "bracketing"
principle. That is, the test sequence ABA is followed, where:
A. Test on a reference compressor (Mike's pump is now the benchmark, no less).
B. Test on the given compressor (a challenger, presumably).
For the test results on the given compressor to be deemed valid, the two tracings
with the reference compressor must be the same or very close. Of course, you could
increase the test sequence (ABABA, ABABABA, etc).
But, please, don't get too carried away. After all we're only testing a Lego
compressor, not a multi-million dollar space rocket ;-)
As to variability between different testers, well, will have to go build another
tester...
--
C S Soh
http://web.singnet.com.sg/~cssoh
... where air is power
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Message has 1 Reply:
 | | Re: RCX Controlled Air Compressor Tester
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| (...) Set up a pump, a valve, and a cylinder like this. pump------Valve---...---cylnder \___...___/ Then put a few drops of oil into each tube AFTER the valve. Turn the valve so the cylinder goes up, and push the pump. Then set the valve to go down. (...) (25 years ago, 4-Oct-99, to lugnet.robotics)
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