Electric Kontiki motor power supply and PWM questions

[default]

Sounds fun but a dangerous way to fish! A year ago when I first
broached the Kontiki thing I think someone suggested the butane
powered spud/melon gun to launch the line out. Compressed air would
work too but if butane or another flamable gas was used then I would
expect that a stainless steel (flame-proof) leader connected to the
projectile followed by a length of heavy line to take the shock of the
launch followed by the backbone would be required. That, a long
ignition switch wire and a well placed dune to hide behind wouldn't go
astray either.

Regards,

Rubicon.

That does sound safer than what we are doing.

A couple of times the bait hook and leader slipped out of the pouch
and once caught the pouch as the sling reached the end of the travel -
fortunately the leader snapped and the weight went into the ocean and
the hook came back on the pouch.

I haven't fooled with spud cannons but that sounds like a good idea
too.

Or rockets?

 

[default]

On Fri, 22 Dec 2006 13:37:18 GMT, (Rubicon) wrote:

With the spud gun you'd still need some wadding to seal the gases in
the bore of the barrel, so it'd make sense to just use a spud and put
the bait and leader in after the spud was rammed down the barrel.

When I was a kid we'd make black powder and fire a cannon with wooden
bullets (reinforced copper tube cannon with a spark plug igniter). To
seal the gases we'd wrap the wood dowel with electrical tape and ram
it down the barrel. As soon as we had it perfected the parents put a
stop to it.

We hollowed out some dowels and packed them with slower burning powder
and shot "tracers" into the night sky - in the city - that seemed to
get their attention. It was loads of fun while it lasted. We moved
on to making rockets.

 

[Rubicon]

With PWM, you turn the motor on/off very rapidly,
and use duty cycle to control the speed. It is really
the only way to go for this project. Since it is
the only way to go, we really can't talk about saving
power or reducing average current, as that implies
a comparison to something else.

The concern is not exhausting the batteries to the
point that the microprocessor (mp) won't run. The
motor needs hundreds of times the current that the
mp needs. When the batteries drop low enough that
they can't run the motor, they still would be able
to run the mp.

Therefore, exhausting the batteries is of no concern
when looking at keeping the mp running. What is of
concern is the temporary voltage drop created when
you first turn on a motor that is not moving. You
get a very large surge current, far in excess of the
motor's steady state draw. That causes the voltage
to drop briefly, until the motor gets going.

Your LDO voltage regulator needs at least 5.6 volts in
to provide 5 volts out. If you use a 7805, you need at
least 7.5 volts in. So the idea is to keep at least
7.5 volts available to the mp when the voltage from
the batteries drops due to the surge when the motor
is first turned on. The job of the capacitors at the
input of the regulator is to do that.

Your LDO regulator is capable of 100 mA, so we must
assume you are keeping the mp from drawing more than
that. Using 100 ma as worst case, and figuring your
capacitors at 80% of their value, they will keep the
regulator satisfied for ~ 300 mS with the 7805 regulator,
and longer with the LDO LM2931.

Whether 300 mS is sufficient I can't say. I don't know
how long it will take the motor to get up to speed, nor
how low it will drag the voltage at startup. Among other
things, the duration of the start up time (getting the
motor up to speed) will depend on the load on the propeller
and the condition of the batteries. Weak batteries will
have a higher internal resistance, and therefore a
greater voltage drop.

A separate battery for the mp avoids the problem, but may
not be the best solution. It could be that a separate
battery is not needed - the existing circuit may do the job
nicely. Do you have any specs on the motor operation at
startup?

Ed

Ed,

Thankyou for thaking the time especially at this time of year to reply
to my post.

Unfortunately I have no specs apart from 12VDC/30A 30lb thrust and I
can't seem to find any more on the net either. The motor itself isn't
stamped with a make or model number and the supplier has no other
specs for it. I got the motor from a friend who wanted in on the
project and he contacted the supplier by phone to try and get more
info to no avail.

The LDO will have to be changed to a LM2940 5V/1A as I've found that
when all is fine the current draw is under 100mA but with side
pressure, as in a weighted line being connected to the servo release
mechanism the current draw can go up to 0.5A.

The PIC will also have no external circuit powered/activated apart
from the PWM circuit during motor startup.

I see that there are "Soft-start" features in many DC motor
controllers and I'll also look into that feature.

Regards,

Rubicon.