[PRL] Morning blog post: don't make me pull the plug!

[Mitchell Wand]

Another good story for the freshmen....

What was that? <http://codemines.blogspot.com/2007/02/what-was-that.html>

By Mark Bessey
When I was a young lad I had, shall we say, a more "carefree" attitude
towards software development. This is the story of how I learned to be a bit
more methodical in my approach to certain types of issues.

The project I was working on was the motion control system for a robot. Now,
I should probably clarify that, so you don't get the wrong idea - we're not
talking about C3PO or R2D2, here. This was an industrial robot, used for 3-d
imaging. The robot itself was made out of slabs of cast iron bolted
together, was probably 8 feet tall, 4 feet wide, and ten feet long, and
weighed in the neighborhood of 4 tons.

The problem I was having was that the motion control was somewhat
unresponsive - you'd move the joystick, and the translation table or the
optical head would slowly start to move, and when you got to where you
wanted to go, it'd keep on moving for a little while before coming to a
stop.

As I was looking at the code, I found what I thought was the problem - I had
simply put the wrong coefficient in for one of the control equations, so we
weren't getting the proper exponential factor applied to the requested
motion. A quick edit and recompile, and I was ready to test the new code.

I started the system up, and very slightly moved the joystick. The
translation table started to creep forward. I then pushed the stick over a
little farther, and the table accellerated. So far so good.

And then, something very bad happened. When I put the stick back to the rest
position, the table didn't slow down. In fact, it kept speeding up. I tried
pulling back on the stick, but that didn't seem to have any effect. I
managed to turn off the power to the motors just before the table hit the
hard stops at the end of its travel.

So this 600 pound cast-iron table slams into the rubber bumpers at the back
of the machine, going something like 30 feet per second. The whole machine
rings like a gong, and all work in the entire shop grinds to a halt as
everybody looks over to see this multi-ton machine gently rocking back and
forth. I was really worried that I'd wrecked at least part of a very
expensive machine, but a later calibration run showed that the mechanical
parts of the robot were just fine.

It turns out that there were two problems in the system - the incorrect
exponential on the input side that I'd corrected, and an additional
incorrect damping factor on the output side. The upshot of all this is that
once the system was up to speed, it took a very long time to slow down, but
the bug on the input side ensured that it never got up to more than a tiny
fraction of the maximum speed.

The next time I needed to make a change in those calculations, I did a "dry
run" with the motors disconnected first...
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