Strange motor output behaviour

[daveha]

Love the crumble kit. great product.
Setting up a robot workshop and come across a strange, and intermittent problem.

Setup 4.5v battery pack, two standard motors.

1. Run a simple program to turn 2 motors on and off together running at 75%. 2 seconds on, 2 seconds off, forward, then reverse

Runs well on the USB, take it off the motor lights just flash really fast, not drive from the motors, just a little whining

2. put a 5.5v battery pack on, no difference

3. spin one of the motors by hand when it is meant to be on. both start to work (so handcrank to start)

so it just looks like there is a problem supplying enough current….except

4. i then started with a 25% power and it worked, then up to 40% it also worked normally.

In previous tests had it working fine at 100%.

Put me out of my misery…what schoolboy error have i made?

[Joseph]

Hi,

Sorry I’m only just replying to this!

The motor driver has a current limit of 1.5A. If this current is exceeded, the power is cut for a short time. Depending on the size of the motor, the startup current might exceed the limit, so the motor doesn’t start.

Having said that, the fact the USB connection makes a difference suggests it’s an under-voltage problem (i.e. the voltage drops too low when the motors start).

Do you have a link to the motors you are using?

Joseph

PS There was an older version of the software that didn’t output full power to the motors (bug). So might explain why it was previously working at 100%

[daveha]

Hi there,
I don’t have any details of the motors, although we did get them to work if we paralled up two battery packs. Each battery pack had 3 AA alkaline batteries.

[Joseph]

Thanks for the info – it’s very helpful.

It definitely sounds like a voltage drop. I do have a couple of software tweaks that may help in future though (basically, very quickly ramping to a new power value, instead of instantaneously changing it).

Joseph

[Chrisk]

Hi,

I’ve had exactly the same problems with a Crumble / robot kit. Looking over it with a multimeter, there is no voltage drop either on supply or motor output, when driving the motors at full power.

Tellingly though, with just one motor hooked up, it can run at 100%, or 60% with both. Guessed it must be a regulator or current limiter. Any more info or work-arounds would be great.
Thanks

Chris

[Chrisk]

Sorry, just to be clear. Setting 2 motors between 75% and 100%, no measurable voltage on either output. Voltage holds up well at 60% though. Input voltage stays steady regardless of motors/power.
Thanks

[Joseph]

Hi,

Thanks for the info. The voltage drop is usually much worse just as the motor starts up – so it’s difficult to see on a multimeter.

Do you have a link to the motors or robot kit you are using?

Thanks,

Joseph

[warren]

hi

What motor/gearboxes do you recommend?

warren.

[Author]

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