Question
Asked November 19 2014

Community Question

Hi all.

My K-9 2.0 build is just about complete but I've come up against an unexpected issue. After running a couple of drive and script tests to reduce the overall top speed on my controller last week, I had to stop as the drive motors were losing power due to a dud battery. So I ordered a new one and it came today so back to testing. Well, not quite. What I thought to be a dud battery causing loss of power was not the cause as the new, fully charged battery does exactly the same thing. So here are a few details.

Two 12v motors powered by a 12v lead acid battery, now a new 11.1v LiPo.

Motors are the same used in kiddies ride in cars, and got plenty of horsepower left over after robots total weight.

Running through an L298 H-bridge, connected to an EZ-B4 (EZ-B running off its own 7.4 supply).

Using H-bridge with PWM control on EZ-Builder.

H-bridge Speed control connected to EZ-B4 via a 2 to 1 (Y) cable

Wheels go back and forward while HD servo does the steering.


During tests, going forwards and reverse at full speed everything is fine, even doing a few turns. But when I start to use variable speed using a joystick, after a few seconds the bot quickly, but gradually loses power until it comes to a complete stop. I wait 5 seconds and power is back for a couple of seconds then loses power again. But If wait 15 or 20 seconds, and at full speed, everything is fine until I drop the variable speed again and it happens all over again.

I jacked it off the ground and tried it and it never loses power. So, things I'm pretty certain about. I'm sure it's not the overall weight of the bot causing to much traction, as I mentioned before, the motors are made to power a child's ride on car. The bot doesn't weigh that much more than a kids car itself, without a child, so there's more than enough power. Besides, going in straight lines first off the bot goes very quickly (too quick in fact) for a long time.

I'm now positive it's not the battery, as it does the same thing with a brand new fully charged one.

So from what I can make out, it has to be the H-bridge doing something. Is this the case or am I missing something else? It is wired up correctly and connections are good. Has anybody else come across this in the past or can any of you lovely people advise me to what could be causing the temporary drop in power?

As always any help any of you guys can offer would really be appreciated, as this above all else is really bugging me. *mad*

Many thanks,
Steve. :)
November 19 2014
Thanks Rich. I'm going to order one now. Just seen a 15 amp one for only a few pennies more so opting for that one. So it's just a case of connecting PWM 1, dir 1, PWM 2, and dir 2 single pins on the EZ-B (D0,D1,D2,D3 for example)?
November 19 2014
Steve,

You might want to check if you have a 'high resistance source'. Look at all of the wiring and connectors between the batteries, controller, and the motors. Make sure all the power wires are the correct size for the maximum load plus a good safety margin.

After running the system a while, check for hot spots. A poor or corroded connection could cause poor performance.
November 19 2014
@OldBotBuilder.

Thank you for your reply. That's a good point you make about the wiring. It was one of the first things I did check, along with all terminal connections and all seemed well and cool to the touch. I did just check again after your post to be sure and all is well. The wires I'm using are rated for about 50 volts max, so I'm good there. :)
November 19 2014
It's not the voltage rating, it's the current capacity. Look at: http://www.tedpella.com/company_html/wire-gauge-vs-dia.htm
You will see that for a maximum drain of 11 Amps you will need at least a wire size equivalent to AWG 20 (.81280 mm dis.)
November 19 2014
Oops, Thanks for putting me straight about that. The wire I'm using for power and the motors are sufficient. I just had a look and the wire harness (came with the motors) are rated at AWG 12 so should be good up to 18 amps.

Could someone confirm what I wrote at the end of post #11 is correct?

Cheers. :)

EDIT:

As an aid to help others, what I wrote in post #11 is indeed correct.