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Capacitors
A usage guide
As a rule, BEAM
robots tend to use capacitors
in two, very different roles -- small ones in Nv
neurons and solar
engines (filter capacitors), and bigger ones as energy
storage devices (storage capacitors).
Filter capacitors
The following 4 sizes of filter caps seem to be most often
used in BEAM
bots (you'll want to stock up on these):
- 0.1 uF -- normal Nv
timing usage
- 0.22 uF -- Nv
nets with longer delays
- 0.47 uF -- across motor power leads (power
stabilization)
- 4.7 uF -- storage cap internal resistance mitigation
(i.e., attached in parallel to main storage cap)
Usage guide
Photopoppers
(and BEAMhttp://encyclobeamia.solarbotics.net/articles/beam.htmlbots
with motors, in general) require the use of storage
capacitors with internal resistance of less than half an
ohm. As a general rule, a decent cap for driving a motor is
longer than it is wide by a factor of 2 or more. (eg. 10 mm
diameter, 20 mm long). This is due to the way that they're
constructed. Long and skinny generally means low internal
resistance.
You can, of course, also put a small-but-efficient
capacitor (namely, a filter cap -- maybe 4.7 uF) in parallel
to your storage cap in order to reduce its effective
internal resistance.
Choosing the right size storage capacitor
First you need to think about what your application is.
Do you want a motor to spin for a long time or just a a few
turns? If you want it to spin for a long time then you're
going to need a capacitor with higher capacitance. If you
want a motor to spin for only a quick burst then a smaller
capacitor is what you want.
Nothing comes for free, of course, so the bigger the
capacitor you have, the longer it will take to charge up and
move. So a 2200uF (0.0022 F) capacitor might take only a
second to charge but a 1F capacitor could take 8
minutes.
Here's a few sizes commonly used
- Dual SE photopopper -- 3300 uF (0.0033 F)
- Solaroller -- 0.033 - 0.047 F (depends on solar cell
size)
- Symet -- 3000 uF (3 x 1000 uF)
For more information...
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Ben Hitchcock also has a good writeup of capacitor
use in BEAM
robotics here;
meanwhile, Audio Magazine published an interesting
article on "Picking Capacitors," which was put on
the web here.
For an exhaustive (almost compulsive) set of data
on capacitors, make sure you check out the
CapSite.
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