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Motors and speed controllers
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Knight



Joined: 30 Mar 2006
Posts: 111
Location: Sydney, Australia


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Motors and speed controllers

I knew i was going to have more questions so i'll make it quick:

The 300W motor for Oatley electronics, is it any good as a drive motor? if not why not?

Also the Oatley (or Jaycar) speed controlers? Would they work in a simple cheap bot? (note i'm asking if they would work not perform perfectly, i'm looking to get in here, not build a top of the line bot (K.I.S.S.))

Post Fri Mar 31, 2006 10:52 pm 
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assassin



Joined: 27 Jun 2004
Posts: 1105
Location: SunshineCoast


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The more experenced bot builders may disagree but the 300watter as a pair for drive motors might be to heavy for a featherweight. The other guys will confirm.

One thing thou, if you design a bot that has only drive motors and armor(what I call a slam bot) you could get away with it. But you still will need some gearing!

Post Fri Mar 31, 2006 11:09 pm 
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Philip
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Joined: 18 Jun 2004
Posts: 3842
Location: Queensland near Brisbane


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Assassian is correct. Drill motors give a better power to weight ratio. You could use the Oatley motors and have a low current requirement. This would simply mean that you sacrifice some weight that could have been used for armour and the like.

You might want to make sure that the Oatley speedos can go instantly from full speed forwards to full speed reverse. This is nessessary in our sport.

I doubt the Jaycar speedo would work in a robot. You cannot always believe the current rating of speed controllers.

Some people use the triggers of cordless drills as speedos. There are how to guides somewhere on this forum.
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Post Sat Apr 01, 2006 4:34 am 
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Knight



Joined: 30 Mar 2006
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Location: Sydney, Australia


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Well with the speed controlers, would it be possible to buy a standard radio car speed controller, and use it to drive a mosfet?

Post Sat Apr 01, 2006 12:11 pm 
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Philip
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I don't know enough about mosfets to give advice on them. I know that Andrew Aidulus has built a speedo that uses the PWM from a servo to power a drill mosfet.

I personally just bought speedos that other roboteers had been using with great success. They cost me about AU$150 for the two of them and I have been using them for a few years now.
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Post Sat Apr 01, 2006 7:18 pm 
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dyrodium
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Joined: 24 Aug 2004
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Location: Sydney


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There's afew options, the most common are either a specific made robot controler (IBC for $325 shipped in aus) or victors, sidewinder etc, or more "home brew" approaches such as driving switches of servos from the radio (ie, microswitching) or andrew's funky work with servo mosfet driving. Really depends on the budget. Smile
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Post Sat Apr 01, 2006 9:16 pm 
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Knight



Joined: 30 Mar 2006
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Location: Sydney, Australia


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Much as i wish it wasn't, i'm still a typical student, so budget is small to non existant.

I do have some skills in electronics (father has a degree in it and has taught me from when i could hold a soldering iron, and i'm doing it for my HSC) and i have found some speed control circuits that i could build but most if not all of them don't have a reverse function.

I'm thinking that much as i want the ability to reverse, for the time being that may be the way to go.

What was mentioned about ripping the insides out of a drill for a speed controler?

Post Sat Apr 01, 2006 9:52 pm 
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dyrodium
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Joined: 24 Aug 2004
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Same deal as me then. I say start with making up a servo switching unit, this is one made recently. http://img.photobucket.com/albums/v424/dyrodium/P3150256.jpg
I noticed your in sydney. You should make your way out to our next competition in blacktown in a few weeks. Smile
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Post Sat Apr 01, 2006 10:10 pm 
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Knight



Joined: 30 Mar 2006
Posts: 111
Location: Sydney, Australia


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The only problem i see with servo switching is the whole fail safe issue.

Personally i was planning to do something like this:
http://www.math.niu.edu/~behr/RC/speed-ctl.html

goto that website and look at the second one down:
"miniature high-rate speed control, designed by Keith Walker"

Thats not a hard one to build, except i don't think it goes backwards

Post Sat Apr 01, 2006 10:14 pm 
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dyrodium
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Failsafing is an issue, and if you can make a speed controler like that then sweet, go ahead with it. The servo switching simply allows you to have something up and running as soon as possible. Smile
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Post Sat Apr 01, 2006 10:17 pm 
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Philip
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Joined: 18 Jun 2004
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I have read a lot of build reports where a roboteer tries to build their own speed controller to save money. Typically they end up spending the same or more money than buying a commercially available product. There are also issues withreliability.

Good luck to you if you decide to build your own.
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Post Sun Apr 02, 2006 3:04 am 
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Knight



Joined: 30 Mar 2006
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Location: Sydney, Australia


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Yeah i can understand that, but i'm using a tested circuit (found a better one than the one above) ( http://www.bobblick.com/techref/projects/sv2pwm/sv2pwm.html ) and a common circuit, an H bridge, for the reversing. Both should work.

I enjoy playing with electronics, and its something i'm reasonably good at, so i think i will try.

But thanks for the warning

Post Sun Apr 02, 2006 12:27 pm 
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Philip
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Then good luck and enjoy yourself.
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Post Sun Apr 02, 2006 1:01 pm 
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Spockie-Tech
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Joined: 31 May 2004
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Location: Melbourne, Australia


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The RC-Servo Pulse to PWM conversion is relatively easy to do with a simple microprocessor setup,

The PicAxe's or the Ateml AVR Micro used in the IBC and OSMC will do it for about $15, the not so easy part is keeping the Micro running reliably in the typical combat robot environment, surrounded by crawshes, bangs, Radio Noise, Huge Electrical Spikes, and a wildly varying power supply as the batteries struggle with the load of the drive being stalled several times a second in a ramming match.

Once you have the brains sorted out, the *Really* hard part to go is to get the Mosfet H-Bridge Power-side of the circuit to survive.

The difference between pushing a baby-motor powered hobbyist robot along the floor at a leisurely pace chasing the cat and the sort of pounding a combat robots speed controller has to endure is like asking a Diahatsu to win a demolition derby.

To deal with the sort of current surges a combat bot does, the Mosfets have to be top-shelf (very low resistance) units, and the gates have to be driven with perfection, timing and power wise. Anything else will guarantee frustration as the magic smoke escapes again and again.

All the details for the IBC / OSMC style design are available for free on the net (look at the IBC manual on the RoboWars page for the schematic or the OSMC documentation here - http://www.robotpower.com/downloads/ ,

but as someone who has done it, and pointed out by Philip, I can assure you you will *not* save money trying to do it yourself.. Unless you want to spend a *lot* of time and burnt parts learning the art of power-electronic motor control, your best course is to buy a proven speed control. It only takes your home-brew to fail once in combat and you will likely suffer more damage than you would have spent buying a good one.

The "Electronize" controllers are about the cheapest proven combo, and the IBC is about the simplest to use.

The Bob Blick designs arent even in the ball park for combat bot use. No Failsafe, the H-Bridge uses Transistors rather than Mosfets which makes it far too wimpy to handle combat power motors, and the analog servo-pwm conversion is finicky and drifts around.

If you really want to build your own, read up on the OSMC documention link above.
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Post Mon Apr 03, 2006 12:55 pm 
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Totaly_Recycled
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Joined: 15 Jun 2004
Posts: 1346


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Makeing your own speed controlers can be interesting .Aarons(Knightrous oxide) speed 50 relay desighn is cheap simple and easy to make. So far only one has been tested in Sproing and was working fine , but some times glitches. We havent investigated it properly yet as i still think it is more bodgy soldering on my part or the fact that i havent put the diodes on the relays and they could be spikeing the chip .This circute could easily be used in conjunction with a pwm signal through a fet for speed control .

I have built several diferent types of controlers from servo controled micro switches useing drill trigers for pwm through to servo board controled rotary swiched desighn's and servo boards switching relays and pwm-ing fetts .

The servo controled types have been the most reliable but geting a zero point is dificult and they dont have true fail safe but usualy will stop when they lose radio signal as they need at least.5 of a second of proper signal to hold the relay H bridge on before pwm from the fets goes across them.

I haven't attemped any true fet type H bridges yet as the control circurty is just to complicated .

Relays will handle a lot of current provided you can switch them on before puting power across them and turn them off after pwm is shut down that way the dont cop a lot of arcing .
So far i havent smoked any fetts in in my atempts at controlers but i usualy go over board though and use 3 or 4 in parallel where only one is realy needed .
But like every one else my advice for a proper battle hardy controler a comercialy built unit is probaly the best way to go ---but dont let that dishearten you if you want to experiment and have some one handy like you said, who is good with electronics to help you. Go ahead and have a go ..you might even come up with something beter than what is already available Smile


Last edited by Totaly_Recycled on Mon Apr 03, 2006 5:33 pm; edited 1 time in total

Post Mon Apr 03, 2006 4:06 pm 
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