R/C Naval Combat

Since the other thread is getting hard to follow with all the different topics, I would just like to set this one up for discussing the electric propulsion via battery or capacitor using a small electric motor and prop. To seed the connversation, here is something that worked reasonably well on the bench.

This was made by one of my more electrically inclined buddies. We initiially tried one cap for the whole thing but it just didn't work very well at all.

It basically pulse width modulates the motor so you get a constant average voltage across the motor. the reason for the two caps is that in order to run the pic needs a little over 2V and in order to drive the fet, it needs another .75-1V on top of that. With the two caps as shown, the first one provides the power to the pic and voltage to drive the fet and the second one drives the motor. This allows the other one to run from 5V down to 2V (it is a 2V motor) before the motor starts to slow down.

with some of the tiny 10-20F aerogel caps available, this might actually work. ideally we would use a 1V motor and some of the 2.2-2.7V 10-20F caps for propulsion, and a smaller 5.5V cap to run the pic and the fet




the pic used the following code

quote:

---

DEFINE OSCCAL_1K 1
ANSEL = 0 'all inputs digital
CMCON = 7 'comparators off
@ device pic12F675, INTRC_OSC_NOCLKOUT, wdt_on, pwrt_on, MCLR_OFF, protect_off
x var word
y var word
pm var gpio.2 ' define LED pin
Include "modedefs.bas" ' Include serial modes
y = 0
x = 0
loop:
ADCIN 0,x
y = 255-x ' x is a number between 0 and 255, y will be the inverse
y = y + 50
y = y min 255
pwm pm, y, 10 ' PWM
GoTo loop 'Do it forever

---

I'm using a cheap GSW esc from tower hobbies. It's only about $10. I'm using a bare bones "arduino" pcb with ATMEGA168. Right now I'm trying to work out the mechanics for dive planes for depth control. I'm between either an r/c plane style wire linkage or using bevel gears. Lego makes some tiny bevel gears that may work.

I dont know much about electricity and that kind of stuff, but it seems to me that besides recharging it(which you would have to do with a gas-propelled torp anyway), that it would be a great idea.

Ideally, this would be hooked up to a regulated voltage source (or sources if the two caps are used at different voltages) on the ship to keep the caps topped off. then on launch a switch (not shown in schematic) would close, starting torp.

-Greg

Here's the board I intend to use. http://moderndevice.com/RBBB_revB.shtml The whole kit is only $10 or we can get the pcb's for $2. My prototype torpedo is a bit bigger than expected at 1.2" OD but the upside is that my tiny servos fit inside without being cut down. I'm going with a wire linkage for the control surfaces since the parts were handy.

Greg,
Meant to get back to you over email. Your circuit looks good, quite a nice way to constrain how much power flows from the cap. Pat your friend on his back.

The code however, I don't really like much.

First off; X&Y, really?, are there no more descriptive terms out there?
Second, Can't you just find a chip with a PWM on board; set it & forget it?

we tried hardware pwm, but the chips he had on hand did not gracefully change. and since to compensate for the decreasing voltage of the drive capacitor, the pwm has to constantly change.

he even noted that it was really crappy code, but he was just trying to get it hacked out and go home... but hey, it worked.

Do you have any pictures of the whole setup?

I'm glad a good way to run a motor with caps has been found, but my concerns are still size and weight. Will this fit in anything remotely resembling a scale torpedo?

I'm really not too much of stickler when it comes to scale size, just look at the barrels in 1/144, they are obviously oversize but really don't look out of place, but really we should try to limit ourselves to reasonably scale size (i.e. a 3/4" wide, 10" long torp)

How are you guys going to keep the torps form spinning if you drive them with a prop?
Even if you can solve the electrical issue, this might be a problem.

Bending the torpedoes' fins in the opposite direction of the props torque is likely the easiest solution to that.

we only breadboarded it. so far I have found suitable caps in the 10mm OD X 30mm long range and motors in the 4-7mm diameter range. best setup sofar I think will be 1X drive cap, 10F 2.5V (10mmOD X 30mm Long) 2X squat .22F 3V caps 11.5mmOD, ~3mm long (run in series) to drive the pic and fet. pic and fet together take up another 10mm. so that ends up with a drive section ~.4-.5 inces in diameter and 1..8-2in long. we had some great tiny fets about 1.5mm X 3mm long...

Well,I just ordered some stuff to play with. the following part numbers from digikey (and sizes)

589-1000-ND 2.7V Radial Cap, 3F 8mmOD X 20mm Long
P10791-ND 5.5V Cap, 0.1F 10.5mmODX 5mm Long

between those two we may have a good propulsion system there... assuming that the pager motors can deal with the load we want to put on them.

just got some caps in the mail.... here is a picture with a dime for size comparison.

the large cap is a 2.7V 3F cap and the small one is a 5.5V 0.1F cap.

The small one will drive the pic & fet (not shown) and the large one will drive the motor.

Assuming I can get sufficient power out of a pager motor at 1VDC, that gives me about 8 joules of energy to play with. so even if I loose 99% of that, it should still be enough to drive a torp for a handful of seconds.

So we ordered up an additional component to further reduce complexity...

To get rid of the 5.5V .1uF cap, I found a boost converter that will be able to supply the pic and fet with 5V so long as the large cap is above ~.7VDC That will get rid of the largest diameter cap and reduce our minimum required ID to 8mm.

I am waiting on samples to come in the mail.

-Greg

This is sounding very interesting. What torpedo length is the goal for the design?

Not sure yet... this is just the propulsion phase, who's goal is as small as possible... then once we can drive the things, I was going to work on the warhead phase (the much more difficult part)

Yeah, that makes sense. I didn't realize that the little pager motors required a special driver to make them run :(

they don't. but if you are driving them with a capacitor, you need to recoagnize that the capacitor drops in voltage for a given energy discharge much faster than a battery (at least one out of the initial drop region), so if you want a constant speed on the prop, the voltage to the motor has to be maintained constant as the cap discharges.

to do this, we have a pic chip that pulse width modulates an average voltage set in firmware. the pic and fet that do this require more voltage to run than the supercap can provide, so we add an additional chip to up convert the voltage from the cap to a sufficient level to power the pic and the fet. This removes the requirement for a second power source (such as another cap).

I should also say that this allows us to turn off the torpedo after a certain run time.... which will help keep folks from loosing them.

I know about the RC time constant :) But I'm not planning on a motor run time of more than 5-10 seconds initially. Too much runtime isn't really a concern at this point :)