CAM TIMERS
To control the motors and other mechanisms,
when I was young I bought
cam timers from industrial surplus stores. A small motor on the end drives round the drum
with a series of cams in contact with microswitches. By adjusting the cams, any circuit
connected to the microswitches can be made to switch on and off as the drum rotates.
Cam timers are now obsolete (though they can still sometimes be found on
ebay). Its still satisfying to be able to see how it's working,
particularly if you are a bit intimidated by electronics. |
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PLCs
More flexible and reliable than a cam timer is a Programmable
Logic Controller. These industrial devices are primarily made for factory automation,
controlling pneumatics, but are very adaptable. You can read a piece I wrote
about them for Make magazine here. I use a Mitsubishi fx range that
new cost
from £100 (4 outputs) to £350 (14 outputs) but increasingly buy mine from
Ebay, which is much cheaper. They are programmed in ladder logic either
with a hand-held programmer or on a PC. It took me a while to get the hang of the
programming language, but the struggle was definitely worth it. They are
amazingly robust and reliable. I've fried a few outputs connecting them to
switched mode power supplies (with the inrush current - most notably a
15watt DVD player connected to a 5 amp rated output). However despite
having over 50 PLCs running every day in different locations (most running
for over ten years) I've only ever
had one fail completely (which was replaced free under Mitsubishi's
lifetime guarantee). PIC chips can
also be used for control. They are very cheap, and more versatile
than PLCs - they cycle much faster (so they can do things like multiplexed
displays). They can also easily accept analogue inputs and outputs (making
them suitable for motion control). I've done a few things with them but
its easy to get bogged down with their added speed and analogue
capacity. I've tried several different ones. PIC (by arizona
microchip) were the first microcontrollers but I don't find their software
intuitive. Arduinos are easier to use and
because of their success, there's a great selection of 'shield' boards to
plug in. These provide relay outputs, servo drivers, mp3 players,
accelerometers, the list is endless. There are also large online libraries
of programs so its often possible just to modify something that already
exists. If you get stuck there's also a big online community to ask for
help. Raspberry Pies are more
powerful and can run HD video as well as everything else. I haven't
used them myself. This is partly because its useful for all my machines to
run from the same system. Its also because years ago when I first thought
of learning C++ a friend advised me to stick with what I understand if it
does what I need. Its better to understand one language thoroughly than
several languages partially. I don't enjoy learning new programming
languages so I followed his advice. For
people like me who aren't really into learning programming languages, it
makes sense to do the most with the least possible programming time, so I
stick to my PLCs where possible. PLCs are also
constantly getting faster and more adaptable so I've never really had the incentive to
change completely. |
