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[Albion]

I'm building a device to vary the C/G of my single piston tank sub. I'm using a low speed 380 sized motor, driving through a 4:1 pulley system to a 4mm threaded shaft. Due to being a single piston design the C/G vaires as the tank fills and empties, so  iwant to be able to trim out the boat at some of the in between points. During a quick bench test the motor speed seems high and probably too fast to get accurate control over the mass. The voltage is nominal 13.2V and the motor is probably rated 6-7V. I'm guessing if i put a resistor in the circuit it will slow the motor down, but want some idea of what vlaue i should use without experimenting too much. there isnt much load on the motor only shifitng about 300g of lead backwards and forwards.

[essex2visuvesi]

I'm building a device to vary the C/G of my single piston tank sub. I'm using a low speed 380 sized motor, driving through a 4:1 pulley system to a 4mm threaded shaft. Due to being a single piston design the C/G vaires as the tank fills and empties, so  iwant to be able to trim out the boat at some of the in between points. During a quick bench test the motor speed seems high and probably too fast to get accurate control over the mass. The voltage is nominal 13.2V and the motor is probably rated 6-7V. I'm guessing if i put a resistor in the circuit it will slow the motor down, but want some idea of what vlaue i should use without experimenting too much. there isnt much load on the motor only shifitng about 300g of lead backwards and forwards.

Maybe a smaller motor would be better?

[Subculture]

Servos work fine for this. Proportional and cheap.

[Albion]

Smaller motor - probably faster

Servo's yeah, but Andy, you've seen my other builds, fairly solid structures, and besides i have already built most of the hardware side of things. I just need to wire it up now. I'm using an old Engel BTS for control as it gives me end stops. 


I guess check the amps, and work out nominal system resistance, treble the figure I get, and put in a resistor of something near that value

[Subculture]

I think somewhere between 20-30 ohms should do it- you should only need a couple of watts to move that backwards and forwards. 10 watt wirewound resistor, mounted on a metal plate. Reminds me of the old controllers on my Tamiya cars!

[Albion]

I think somewhere between 20-30 ohms should do it- you should only need a couple of watts to move that backwards and forwards. 10 watt wirewound resistor, mounted on a metal plate. Reminds me of the old controllers on my Tamiya cars!

interesting, i got it all put together last weekend and tired it without a resistor. it shuttles the ballast weight across in about 1.5 seconds, approx 100mm. this is sort of ok and should work. The i found an old circuit board with some resistors on and ripped them out measured and gave them a go, the 22ohm worked very nicely. however it fell off. i assumed the solder joint wasnt good and the wire had jerked it off. Wrong, it had melted the solder joint, burnt my finger very nicely trying to pick it up. So i guess i should put a power resistor in there!

[Subculture]

You bet-  pulling about 6-8 watts through that resistor (depending on the motor resistance), which is why I recommended a 10 watt resistor, and mounting it on something that'll carry the heat away! (IIRC you have an aluminium piston tank....)

For long life I calculate the wattage through the resistor and double it for the rating of resistor. However your application is only getting light duty so you can get away with something a little nearer the hairy edge.

Andy

[Mankster]

Have you tried a weight on the end of the threaded piston rod? Much easier as its automatic.
BTW where do you sail in Sing? I am there or passing through around 3 times a year, may take a small sub with me next time.

[Albion]

Subculture - the hull is quite narrow, so no way to mount on the aluminium P tank. See photo of set up, i might extend the motor mounting plate and use a thicker piece. its only gonna be used to juggle the weight occasionally

Mankster - had considered that idea but its done now and works a treat. There's a few places, Pandan reservoir, lower seletar reservoir and bedok reservoir. Why all reservoirs? Well until a few years ago all our water came from our neighbour Malaysia. There are three big pipes cross the causeway, two are river water coming in, and one is freshwater going out. They give us water, we clean it up and give them some back, simples. However whenever there is a dispute, Malaysia will threaten to turn the taps off! So any freshwater area is turned into a reservoir to maximise our storage. You may have see the Grand prix runs close to some water, this was formerly a deep water inlet and in the 60's 70's the liners would dock here. Now there is a barricade and the water is fresh.

[Subculture]

Yes that will work okay. You could also use lower wattage, higher ohmage resistors in parallel which would be less bulky. 3 watt wire wound resistors are much less cumbersome than ceramic encased 10 watt types.