DC motors upgrade

Lutamos

I am using the continues rotating servo that came in the boe-bot kit, I have them connected to a track set but they seem to be too weak. That is with a 7.2v battery connected to them. Isnt the torque on those servos like 43 in*oz I was wondering if I should go with a dc motor or a larger servo?. I was thinking about replacing the servos with these dc motors.

www.solarbotics.com/products/gm9/specs/
www.solarbotics.com/products/gm17/

attached is what the track design looks like so if you guys think a more powerful torque motor would be better.


Thank, you

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Thank, you
Lutamos

Timmoore

Depends on the problem. I have used continous rotation servos though not with those tracks and those tracks with the gm9 motors.
I tend to find servos very slow, the gm9 with those tracks were not bad once I have cleaned up the idler wheels and bolts so they would spin freely. Note I only had 1 idler so free idlers are even more important in your case. I found that the idler will tighten on the bolt sometimes and the motor would slowdown a lot.
With the gm9 I originally used 6V battery, I uped it to 7.2V which made it faster. It still isn't the fastest thing I have seen though.

Lutamos

Well the main problem is it seems like the servo is having issues turning the tracks like it is too weak. I was looking for a stronger motor, and both the idler cogs are loose. The tracks are not tight ether. The whole chase fills like maybe 2bl at the most. Would the design of the tracks loose the torque? So the GM9 motors work good then or better than the servo motor?

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Thank, you
Lutamos

John Abshier

The GM9 motors will require a motor driver that continuous rotation servos do not need. You could get a more powerful servo and modify it for continuous rotation. I just modified a GWS servo and it was not hard. Other servos may be harder.

John Abshier

Prettybird

I had the same problem too. Parallax servos soo small for larger stuff. I just retrofitted some hi torque servos but it took some work. Metal gears and bearings are a must and my first mini monster boe grew to around 3 lbs. They have to be disassembled, the restriction pin filed off and had to pot the shaft pot hung outside. The pots then had to be ajusted for the null position. Found out the servo pulses were different values but I finally got it working last night. It was worth it. My bot now moves easily and even slipped on the mattracks on the grass. They same size as the servos used in the boebot and cost around $40. Still working on the software differences. If interestted you cam IM me and I can give more details.
The more you learn, the more you find you don't know. It is neverending. Confucous 550 BC

Timmoore

Does the servo turn fast enough without the tracks? Its probably around 1 rev per sec. the gm9 is about the same speed but about 50% more torque.
If its fast enough without the tracks then you need more torque and either a bigger servo or the gm9 would do. If its not fast enough even without load then you need a faster motor. Most servos I have seen are about 1rps though I have seen some that are about 40% faster.If you want faster than that then a dc motor is the best bet. I have used pololu metal gearbox motors a lot and like them, see http://www.pololu.com/catalog/category/51, there is a chart of each motors rpm and torque.
Which one to use depends on the speed and torque you want.
The tracks I use with the gm9 has 19 links, yours looked like 27links so should move approx 50% faster than mine for the same rpm, with the gm9 its slow, though you can switch the motors in the gm9 to RM2 for extra speed if you need it.
As John mentioned if you use a dc motor then you need a driver, I have used the L293D with the gm9 and pololus mini-metal motors. If you use the RM2 with the gm9 you probably need to use the l298 or mc33887 drivers both work well for a slightly higher powered motor.

Prettybird

·Here is a pic of my boe conversion. Remember more amps (power) needed to move bigger things. My next bot will be DC motor for sure. You can get a hobby motor 7.2 V 2 amp geared to 160 rpm with encoder for around $40. The hi·torque servo motors are advertised·for R/C monster trucks. The next step is learning H bridge chips.

Post Edited (Prettybird) : 7/27/2009 1:30:20 AM GMT

Lutamos

The speed is fine looks like there isn't enough torque. It stalls on little things like pencils. I understand I need a motor contorlor I was thinking of getting this one:
www.parallax.com/Store/Accessories/MotorServoControllers/tabid/160/CategoryID/35/List/0/Level/a/ProductID/491/Default.aspx?SortField=ProductName%2cProductName
I was looking at servos pricing they seem bit to expensive for what I am wanting here since there like 35puls. With two gm9 motors and the motor controller be about $40ish. I am thinking of buying the dc motors since servos are too costly.
Thank you for your advise!

Nice robot prettybird, what is that Cylinder shaped thing above the ping sensor?

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Thank, you
Lutamos

Timmoore

I have used that controller and its fine. If you want something cheaper and have spare pins on the prop then look at the L293D (http://www.sparkfun.com/commerce/product_info.php?products_id=315). I use it with 6 pins for 2 motors whereas the mdsmc only needs 1 pin.

W9GFO

Timmoore said...

The tracks I use with the gm9 has 19 links, yours looked like 27links so should move approx 50% faster than mine for the same rpm

Why would that be?

Rich H

Timmoore

W9GFO, your right, no difference, should have had coffee first.

Prettybird

As I mentoned, The hi torque servo works great but some work to set up. The speed is a little faster but can easly be slowed down not losing any power. I am still tweeking. Before my boe couldn't even climb a 1" peice of wood. Now it is like night and day. The treads will slip before the motor stalls and has metal gears. The pulse train is not the same but easily tuned if you know the software. It is all in the pot ajustment and has a wider range then the Parallex servo. It needs a bigger ontime. Very sensitive to small number changes in the pulsout number too. Even has torque at low speed. I think these servos outgrew my bot. Now I can add mabe a few more things. Only 3 I/O pins left using the Basic Stamp homework board. With the power problem solved, I can now concentrate on learning the software.

Lutamos

That chip looks nice except I would have to buy two of them since the motors are rate stall current at 600ma. which still with two of those chips and a extra prototype bored still be a fraction of that Pololu Motor controller I was looking at. One big problem though how would I go about hooking those chips up and programing them in Pbasic. I looked at the pdf on that page but it was just a data sheet and didn't make much sense to me. Any documentation on how to program these would be nice.
I would love to use servos though but there too expensive. I would like to keep this motor upgrade around $20 since I am just a college student. I take it with the servos with metal gears would I have to take a Dremel tool to the nob stop on the gear so it would make full 360?

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Thank, you
Lutamos

Timmoore

Its 1A per output, so it handles 600ma per motor without problem. I use 1 to drive 2 gm9 without problem. Controlling it from pbasic may be a problem, the 1-4A inputs are just high or low depending on the direction you want to motor to go. 1,2EN and 3,4EN I use to control the speed of the motor. I output a PWM signal at about 1000Hz and vary the pulse form 0 to 100% - 0 is stopped, 100% is full speed. So it depends on whether you can output a pwm signal on 2 pins at the same time as doing whatever else you need to do. Try looking at this thread looks like they are trying the same thing with a l293d http://forums.parallax.com/forums/default.aspx?f=5&m=262446
also take a look at http://www.phanderson.com/stamp/tutorial_5_2.html

Post Edited (Timmoore) : 7/28/2009 12:24:37 AM GMT

Lutamos

O did not see 1A per output that is nice. Since it just highs and lows for controlling it I doubt I need this thing to go anywhere between 0% and a 100%. It's not like the motor is that fast anyway, looks like I will be buying that chip then!

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Thank, you
Lutamos

Prettybird

For the high power servo, you could use a dremel to flatten the pin or a small hobby file. It is not hard steel. Also have to file a slot under the servo wires to hang out the pot. The rest is setup. For the price I don't think you can get any cheaper and looking at you design, it would have more then enough power. I got mine up and sort of running in 4 hrs. I also got some small DC motors geared down that work from 3v to 12v with·ALOT of torque but haven't figured a way to attach an output shaft yet.·Another one of my future projects.·I can't stall out with my fingers at 4v. I just got in the mail today. Been running one now for 6 hrs. at 12v and no heat. Testing for·24 hrs. as I type this.



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