Sabertooth 2x32 Controller Voltage Question

Question

Hi All, I am working on a wheelchair based robot that utilizes 2 24 VDC motors and will be controlled by EZ B V4 controller and a Sabertooth 2X32 motor controller. The problem Im having is that when I hook up the positive 24VDC line to the Battery + terminal I get positive potential through all remaining terminals. So, basically if I put a volt meter probe on the detached negative and the other on and sabertooth controller terminal I get 24VDC, which I think would cause a short. I tried calling and emailing Sabertooth Tech support with no response for a couple of days, so any help at all is highly appreciated ?

Thank you.

ptp · Answer

@Rob-bot, If i got the right picture:  IF you you read 24 volts on your voltmeter is not equivalent to have a short circuit on your board. Your voltmeter is closing a loop, asfaik Sabertooth does not have a power off button, so the electronic (h-bridge) is reacting to the energy flow. If you want to check for a short circuit switch to Resistance (ohms) and measure the resistance between Sabertooth's bat(-) and bat(+) , if you get zero, or a low resistance something is wrong. I would add a fuse between the battery (+) and the sabertooth (+), if something goes south, the fuse blow up and you protect your setup.

Dave Schulpius · Answer

I agree with ptp that fusing is critical in protecting your project. However the Sabertooth is a whole different animal. Dimension Engineering suggests not placing a fuse between your power source and the Sabertooth. Here's an explanation direct from the horse's mouth:  If this is more of a hobby project instead of an industrial product, most people do not use fuses. The Sabertooth has built-in current limiting. We prefer that you fuse the motors rather than the battery leads, or if for safety reasons you need to fuse the battery that you do it at a much higher current than the motors. The reason is this: Because Sabertooth is a regenerative driver, it relies on having a battery to put regenerated energy back into when you command a stop (and at other times). If you draw enough power to blow a battery fuse, all of a sudden the Sabertooth has nowhere to put the energy. It will see the fault, though, and try to stop the motor. It doesn't, however, have anywhere to put the energy (the mechanical energy of the vehicle and the electrical energy carried in the windings and the caps). So blowing a battery fuse will sometimes kill the driver -- which we would replace under warranty, but it's still annoying for all parties. The better way to do it is to fuse the motor leads. If a motor fuse blows, the motor will freewheel, the Sabertooth will sit there contentedly, and the only thing you will have to replace is the fuse. A very large fuse (100 amps) on the battery lead will protect the wiring in the event of a battery lead short or motor driver failure.

Rob-bot · Answer

PTP and Dave S., Thank you so much for you fast replies. I will take both of your advice and check the ohms between B+ and B- and add fuses to the motor lead. What size fuze is appropriate for these motors/controller? I'm at work now, but will check 1st thing in morning when I get home. Thanks again.

ptp · Answer

@Rob-Bot, I would put 100 amps between the battery (+) and the Sabertooth. The high value will allow the regenerative circuit to work and protect your setup. Quote: A very large fuse (100 amps) on the battery lead will protect the wiring in the event of a battery lead short or motor driver failure.

Rob-bot · Answer

PTP, I left the OEM battery harness, which has a 60A blade style fuse, change to 100A? What about the motor lead? What size fuze do you recommend for each motor? Thanks.

ptp · Answer

@Dave Thanks for sharing the info, i got curious regarding the "regenerative driver" i need to add to my list. I can't find a Sabertooth schematics, is it public ? Regarding setups with high amps, i pay a lot of attention to the wiring and i add fuses to extra protection. One of my bots has 4 x 12v 9 A SLA batteries in parallel, i added a 10 A fuse between each battery and the main circuit. Both motors (left & right) have a fuse of 8 Amps. If the bot gets stuck blows the motor fuse, it's annoying but broken fuse is better than a fire or short circuit. To prevent blowing up the motor fuses, i have a micro-controller monitoring the motor controller amps when the value is too high cuts the motor power. It seems the Sabertooth does that, although they recommend a motor fuse. Don't ignore the safety recommendations.

Dave Schulpius · Answer

If you place fusing between the motor and the sabertooth then you would be matching the amp draw of the motor. I also add about 20% or so for overhead so you don't blow the fuse if you reach that full amp draw. You're running a Sabertooth that is rated for 32 amps plus your overhead = 40amp fuse.  Edit: Woops. Here's the correct answer: Peak motor amp draw + 20% overhead = correct fuse size. @ptp, I don't ever remember seeing a drawing of the sabertooth's inners. They probably want to protect that information.

ptp · Answer

60 Amps is OK, if blow up you need to check what happened, if is the regenerative driver you need to go up. Sabertooth has a built-in current limiting, i presume the model 2x32 will cut the power if try to pull more than 32 Amps (continuous). The motor fuse should be aligned with your motor's max recommend amps.

ptp · Answer

Regarding the motor fuse if you put a fuse of 10 amps, and your motor is stalled, the amps will go up and the 10 fuse blow up. If you don't have the fuse the Sabertooth will cut the power when the safe limit is reached for your model. This can be an "operational problem" replacing fuses every time your bot gets stuck... To avoid that check the sabertooth manual. Quote: The current and temperature limit of the Sabertooth 2x32 can be set for each motor channel.  Set the motor limit below the motor fuse, that way the fuse will help you with catastrophic or multiple failures e.g. (motor controller, motor stalled etc etc)

Rob-bot · Answer

Ok, I just got home and checked the resistance between B+ & B- and the initial reading is approx. 5 mega ohms and slowly drops. And this seems to be the same reading on all terminals to the B+. Does this mean it should be safe to connect the negative? Or will it short circuit? Thanks.

ptp · Answer

It's OK.

Rob-bot · Answer

Ok, will try hooking up the negative and pray for the best. I'll keep ya posted either way. Thanks.

Rob-bot · Answer

So far, so good. I hooked up the negative and it seems like it's fine I have the LEDs blink 4 times, but no response from the controller. Right now I have the dip switches set for radio control mode for testing purposes.and I'm not getting any response I just get the 4 blinks on the two LEDs. Any ideas?

thetechguru · Answer

You need to set it to simplified serial to use the ARC Sabertooth control.

Rob-bot · Answer

Even though I want to test with RC control 1st?

Dunning-Kruger · Answer

@Rob-bot Are you using an RC radio in RC mode? That will work. However with the EZB4 as Alan mentioned you need to use simplified serial...

Rob-bot · Answer

Yes, testing with RC, but will soon change to E-Z B V4 once I learn a little more about the controller. I had one switch wrong, so now I am getting RC communication, but noticed the brake is stuck on. In DEScribe I added brakes to P1 and P2 and uploaded, but they are still stuck in the default locked position. Any ideas on this? Thanks

Rob-bot · Answer

PTP, Thank you for all of your help. I found your answers most helpful, but also appreciate all who contributed too. I will start a new thread later on how to figure out why I cant get the brakes to release, but need to get some sleep first. Thank God for this forum and its contributors. I hope one day soon I can also help others here too.

Dunning-Kruger · Answer

I thought a constant voltage to the brake lead was needed to keep them released? I assume it is a safety feature (or dead man switch) to prevent the wheel chain from rolling in case of power failure, user error or malfunction... However, I do believe it is possible to remove the brake mechanism altogether. I am pretty sure some people here have done this...

Dave Schulpius · Answer

I've never worked with the breaking feature of the Sabertooth. However I have read a few things about a braking system on power wheelchairs and using a Sabertooth to convert them into a bot. We know little about your bot and how you have it set up and wired. It sounds like you may be using all the original motors, brakes and wiring from the power wheelchair? You say you added the Breaks in the Power Output tab in the DE software? Did you set the Turn-On, Turn-Off Delay and the E-Stop delay below the Mode window? You may have to adjust these to get the brakes to work as needed. Also are you supplying power to your breaks? I've read that the default of a power wheelchair braking system is locked on when there is no power in case the power system shuts down (for obvious reasons). EDIT: (I now see RR mentioned this as I was writing my post. I'd pay attention to what he suggests). Here is a cut from the the ST 32 manual: In this mode, the power outputs are used to operate electromagnetic brakes. P1 corresponds to motor 1 and P2 corresponds to motor 2. When the motor stops, the brakes activate after the Turn-On Delay. When the motor is commanded to move again, the brakes deactivate immediately, but the motor does not turn until the Turn-Off Delay has elapsed. If an emergency stop is commanded, the brakes will activate after the E-Stop delay has elapsed.  Lastly, if you have rewired to the braking system did you install blocking diodes? If not you need them. If so you may have them in the wrong direction. Hope this is helpful.

Rob-bot · Answer

Guys, Thanx for the replies. Yes, the braking system is the original that came with the motors. I noticed people who have the earlier sabertooth models just removed the braking system being those models didn't have that option. I did consider removing the brakes but are afraid the robot may roll freely on a hill etc... I think you guys are right about I just need to adjust the settings, but not sure why a diode is necessary? Can you explain please. Thanks.

Dave Schulpius · Answer

Two things happen with the diodes, they allow DC to get to the brakes in forward/reverse and they don't allow current back to the motors from the brakes. The diodes are required and the breaks will not work correctly without them.

Rob-bot · Answer

Dave, I see what your saying. I wonder if there are diodes already on the controller for that being P1 & P2 are designated outputs only. As far as the motor leads go, I will have to remove the brake cover to see if their is already a diode installed by the manufacturer. If needed, would 60A diodes be sufficient?

Dave Schulpius · Answer

Again, I've never wired or controlled a setup like this and I don't know how you have yours wired. The diodes may not be needed If you have you break wiring completely separate from the motor power feed and going directly to the sabertooth 32's p1 and p2. The drawing I posted show how to add the needed diodes if the breaks and motor share the same power feed from the sabertooth. The size of the diodes need to be sized to the amount of amps that this part of the circuit is passing. That usually depends on the amp draw of all devices added together. You also should use fast switching diodes. Good luck.

Rob-bot · Answer

Ok Dave. I appreciate yours and everyone else contribution in helping answer my questions. I plan on becoming an active member of the forum as from previous threads I read were very Informative. You're a great bunch of fellow robot enthusiasts and I look forward to sharing my progress pics from the robot I am building.

Thanks, Rob

Dave Schulpius · Answer

Outstanding! Welcome!

Rob-bot

Sabertooth 2X32 Controller Voltage Question

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