Dimension Engineering motor driver

[Kohanbash]

Hi all
Since Dimension Engineering advertisers on this site:

I just purchased some sabertooth 2X25 and sabertooth 2X10 motor drivers.
They were easy to use to get my motors spinning and integrated into a generic RC controller very easily.
The only issue I had was that they say that the battery ground(B-) and the signal GND(0v) are tied internally. However if you only wire the battery GND and not the signal GND it is left floating and the signal wire can act as an antenna sending parasitic signals to your controller causing the motors to spin.

lesson: always tie both GND's to GND

[SomeSaba]

OMG!!@#@!@ I HAD so much trouble with that... i had 2x10 ty so much

[ryan]

I just bought a sabertooth 2x25 motor driver too. Do you know the electronics principle behind its function?

[ArcMan]

It's a dual H-bridge using power MOSFETs.

I designed and built my own power MOSFET drive, but was having trouble blowing up MOSFETs and frying gate resistors.  I believe the problem lies in some bad MOSFETs I got off of ebay.  My drive did work for a little bit.

Anyway, I also bought a Dimension Engineering Sabertooth 2X25 drive.  I have it driving wheelchair motors for my yard robot.  So far I've only done a bench test, but the performance of the Sabertooth is quite a bit better than my home-made drive.  Very good low speed torque, fast stopping, quick throttle response.  And a great small package.  The only complaint I have is that I emailed their tech support a question and it was never answered.

[ryan]

Are power mosfet that small? The ones we usually use to build our h bridge is big??

Yard Bot? A Lawn mower?

[ArcMan]

Are power mosfet that small? The ones we usually use to build our h bridge is big??

Yard Bot? A Lawn mower?

Big and small is relative.  The body of a 60A power MOSFET is no bigger than your fingernail.

I will use my yard bot for mowing, hauling mulch, etc.  It will start off as radio-controlled, but I want to move on to tele-operation.  I hope to get the basic rolling chassis running this weekend.

After I get the tele-operation working, I'll think about connecting to the web and outsourcing my lawn mowing 

[ryan]

Cool. What motors are you using as mowing motor?

[ArcMan]

I'm using a 6.5 HP Briggs & Stratton gasoline engine.  A powerful engine (or electric motor) with a properly designed steel cutting blade is the only way to get a nice-looking cut.

[ryan]

Do you have an in-depth description how the sabertooth 2x25 driver works or a link about how a dual H-bridge works? I have googled for it and all that I get is chips specs.  I need to include it in my report. I'll try to send an email to sabertooth too but I don't really think they'll let their "secret" out.

Thanks

[ArcMan]

It's not indepth, but Wikipedia has an article on the basics of H-bridges.  I wrote some of the article.

In a nutshell, an H-bridge uses MOSFETs or BJTs to turn the voltage on and off to a motor very rapidly - typically >= 20 kHz.  By varying the duty cycle of the on-off waveform, the average voltage to the motor is increased or decreased resulting in variable speed.  H-bridges allow variable speed operation in both forward and reverse directions, which requires 4 transistors.  Drawing the circuit with the motor and 4 transistors makes an "H" - which is where the circuit got its name.

The beauty of an H-bridge is that it is a very efficient way to control the speed of a motor and it provides full motor torque at any speed.

[Admin]

However if you only wire the battery GND and not the signal GND it is left floating and the signal wire can act as an antenna sending parasitic signals to your controller causing the motors to spin.

Hmmmm odd, because John (the owner of DE) specifically told me they were both connected. He talked about instances where people connected the battery to GND(0v) and fried the board - although that ground works, the wire tracings are too small to handle the power.

The only complaint I have is that I emailed their tech support a question and it was never answered.

John tells me he always makes a point to help every customer, even those annoying customers that don't read manuals
Your email was probably lost accidentally.

I'll forward this post to him and see what he has to say about the ground issue.



edit:

or a link about how a dual H-bridge works?

http://www.societyofrobots.com/schematics_h-bridgedes.shtml

[Kohanbash]

However if you only wire the battery GND and not the signal GND it is left floating and the signal wire can act as an antenna sending parasitic signals to your controller causing the motors to spin.

Hmmmm odd, because John (the owner of DE) specifically told me they were both connected. He talked about instances where people connected the battery to GND(0v) and fried the board - although that ground works, the wire tracings are too small to handle the power.

Hi
I made that post a while ago...
The problem was that with the battery ground going to ground and the signal ground hanging the motor would randomly start moving slightly. When I ran the signal ground to ground everything worked properly (motors did not start running randomly).

Before tying the signal ground to ground I put a meter beetween the signal and the signal ground and saw small voltage fluctuations which I believe was causing the motors to spin.
I am not sure but I think I also ohmed the two grounds out and saw an open circuit.