View Full Version : [Question(s)] Motor control

06-29-2009, 07:27 PM
Hi everyone,

i am trying to build a motorized robot kind of thing - a few motorized wheels would do the trick i figured.
the thing is, i have only worked with servos so far. i know for a servo you need a servo controller, but for a motor you need a servo controller and also a motor controller.

but i am looking for something simpler, less accurate, and hopefully cheaper!
my question is: is it possible to connect a moderate motor in power (such as the lynxmotion ghm 02 http://www.trossenrobotics.com/store/p/4259-Gear-Head-Motor-12vdc-50-1-120rpm-6mm-shaft-.aspx) to some kind of a simple switch on / switch off, serial-ly controlled board? something simple and cheap that will get the job done on a motor like this, or an even more powerful one!? or is it just a dream......

06-29-2009, 09:06 PM
Can you give us a little more info on what you are trying to do? Are you building a small desktop robot or something to take over the world? How much weight do the motors need to move?

06-30-2009, 01:09 AM
Options are wide-open. You can use a simple transistor-relay to switch on the motor through serial port - if you have a true hardware serial port (or one of the newer FTDI usb-serial ones) you can write a tiny program that toggles the Tx pin to turn the motor on/off.

But that's just on / off. With a bit more work you can go full forward/full reverse (another transistor/relay and another serial port pin).

But as Jes said, a little more info could help steer us in the right direction to help you accomplish exactly what you want.

06-30-2009, 02:22 AM
Most anything can be done. The question is: should it? ;) Seriously though - you need to pick up your motors, build-up the base, and just throw a simple power switch on it and see the results of what you're asking for. I suspect you may not be happy with that.

07-01-2009, 02:57 PM
Would a phidget interface kit 0/0/4 work?


07-01-2009, 11:32 PM
We still don't have enough information to say one way or the other.

07-02-2009, 11:32 AM
Hey everyone, sorry to take so long to reply and clarify my question - it was Canada day yesterday and i was out all day!

i want to build a motorized robot - the whole thing will weight about 10 - 15 pounds. i think the motors i specified earlier up in the post should do the job of spinning the wheels. slowly, but they will get the job done.

i want to do this so that i learn more about switching, and about motors (as opposed to servos)...

so yea, i guess i should be looking at a switching device as sugested by scud or scowby. connecting a motor to a switching device such as the 8/8/8 phidgets i/o (http://www.trossenrobotics.com/store/p/5341-InterfaceKit-8-8-8.aspx) should do the trick (turning on and off) for motors such as the one i mentioned above? how will i know if a motor is too 'powerful' for this board?

07-02-2009, 11:44 AM

In theory you might be able to get it to move 2mm/sec with 72mm wheels. But that's at stall and the motors are going to burn up. [Assuming 2 motors, 5deg incline max] A little hard to calculate at the extremes of the formulas.

For those motors, you should be considering a 4-5lb robot as max.

But you're going to need to suck it up and buy a motor controller.

And no, that relay board isn't going to give you forward/reverse.

07-02-2009, 11:27 PM
To give a little perspective..

A standard servo is around 50 oz/in of torque.
That gearhead motor is 123 oz/in of torque.

I'd not think that a couple of standard servos could move around 15lbs very quickly ( or at all. )

07-03-2009, 04:37 AM
What's the definition of a "standard servo"? Even a little 475HB will do 76+ oz-in. A 645MB is around 133oz-in. And some of the more powerful standard digitals are 333oz-in or more.

07-04-2009, 12:36 AM
In the RC circles I run in "standard" specifies both the size as well as a torque rating of around 50oz/in. Something like a Futaba S3003. If we want to talk about something better we'd mention it by name. I should of been more precise for this forum though.

07-04-2009, 01:55 AM
Ok - yeah, that's helpful. I can't imagine modern servos that wimpy. Micro-servos crush that these days. Even the wimpiest $15-20 standard-form-factor servos are doing half again that.

These S148 Futaba I use for testing are probably in that 50oz-in range - but they don't qualify as "built in this century" ;)

07-04-2009, 02:36 AM
Hey Bodhi,

Check out this page... (http://www.robotmarketplace.com/products/motors_geared.html) There are a lot of different motors to choose from...and they aren't all that far above your current price range.

As for controlling them...

Ok...I am the king of cheap...I'm cheaper than cheap...

For a while I used these weird speed controllers...they had 5 speeds...2 forward, 2 back...and stop...though stop wasn't a speed...and didn't stop anything. In anycase they used these ceramic resistors for the slow forward and back, and an H-bridge to turn the motors on and off. You had to buy a servo, and hook the servo up to the controller. The servo horn would switch the controller from stop... to slow...to fast...The problems were as follows.

- The servo could never seam to find the right spot...and the linkage would get wonky making it really un-reliable...
- It was damn near impossible to switch the motors on at the same time...
- The resisters would get really really hot...I mean...really hot...and the wood that I had them mounted on would smolder a lot.
- Finally they would somehow get stuck in the on possition and we would end up chasing the robot trying to un plug it.

I also made switch controllers using double throw switches...also not worth it...they worked the same as the above deal, only they didn't have slow...so the robot was full on...or off...and servos that were cheap weren't strong enough to switch the switches...

Finally I tried the cheap Electronic speed controllers from Ebay. totally not worth it. They go up in smoke so fast your head will spin. They almost never seam to be rated for what they say they are...

Ok...if you plan to do this either in the short or long term...get an ESP (electronic speed controller).

I pretty exclusively use the Vantec RDFR 21 (http://www.robotmarketplace.com/products/VT-RDFR21.html). I know its expensive...its really expensive...but it is so worth it. It controls both sides and does all of the mixing for you. The sweet part is that once you have the wiring all set up, you can use them over and over and over again.

I have two of them...no three...but I use them in a lot of different bots...controlling relatively big motors and I've never had a problem.

I know...


07-04-2009, 02:44 AM
That Vantec is a nice speed controller - but overkill for a lot of applications. Don't get me wrong - I love overkill. LandShark is running an RS80D (http://www.robotmarketplace.com/products/RS-RS80D.html)

For most small-bot applications, the Sabertooth (http://www.trossenrobotics.com/store/p/5466-Sabertooth-dual-25A-motor-driver.aspx)or the even smaller Sabertooth 10A (http://www.trossenrobotics.com/store/p/5105-Sabertooth-dual-10A-motor-driver.aspx) will do pretty nicely.