Motor power regulation

[pittuck]

Hi again only me,

I have got my motors working now will put a pic in a bit later to try it out with PWM but another 'problem' has poped up.

I just realised that a old problem has come back to haunt me, at the moment i run the motor supply directly from 6AA rechargeable batteries (NiCad - cheap ones £1 each but only on 3rd chage cycle...). After charging i get about 7.8V and this levels to about 7.2V after 15minutes running with no load on the motors.

Problem is this, i will probably use timing to excute turns. But with a voltage different ov 0.6V this could make for over / under turning. I am willing to sacrefice some speed to regulate the motor supply, but still have the problem of regulating it!

ok then, the maximum the motors can produce is 2A each (talking max stall current, dont know how it occours, probably using a vice and a wrench to hold it....) Anyhow. So i need a max of 4A regulation.

I have heard of using a zener in parrallel? what voltage would i get / would it handle the current!

So to recap, LDO regulation to about 7V, max 4A load. Over to you.

Thanks,

- Martyn

 

[Nigel Goodwin]

Hi again only me,

I have got my motors working now will put a pic in a bit later to try it out with PWM but another 'problem' has poped up.

I just realised that a old problem has come back to haunt me, at the moment i run the motor supply directly from 6AA rechargeable batteries (NiCad - cheap ones £1 each but only on 3rd chage cycle...). After charging i get about 7.8V and this levels to about 7.2V after 15minutes running with no load on the motors.

Problem is this, i will probably use timing to excute turns. But with a voltage different ov 0.6V this could make for over / under turning. I am willing to sacrefice some speed to regulate the motor supply, but still have the problem of regulating it!

ok then, the maximum the motors can produce is 2A each (talking max stall current, dont know how it occours, probably using a vice and a wrench to hold it....) Anyhow. So i need a max of 4A regulation.

I'm rather dubious about a 2A stall current?, are these motors really tiny?.

But in any case, regulating the supply isn't going to help you a deal. Whatever you do the motors won't run consistently at the same speed, in order to do accurate turns (or even straight lines!) you need somekind of feedback from the wheels - and even this assumes the wheels never slip!.

 

[pittuck]

i know i got encoders, but i want to make sure nothing can go wrong again. it gets a bit expensive....

 

[pittuck]

Ok well here is a bit more info

I am using the swallow drive train, nice unit and nice size also pretty easy to use.

www.vapourscripts.com/temp/mouse.wmv here is the robot, this was to show odd the lights, but i dont use em when dev'ing, but u should get an idea of scale, u can also see the wheel sticking out the bottom.

**broken link removed** here is a 'naked' picture. Oh and on of the batteries is there.

Unfortunatelly i am charging the batteries ATM otherwise i would be playing with the sensors... lol (And the charger only takes 4 batteries, so i gotta charge in 2 sessions, i do have a charger to do 6 and might use it later, but its not a very good one...)

Thanks,

- Martyn

 

[pike]

I'm rather dubious about a 2A stall current?, are these motors really tiny?

I ctually did a test on something on the size of those motors and I recorded a stall current of 1.3 amps!!

Then I did a test on motors taken from a hand-held power drill. stalled the engine and pulled 66AMPS, with a computer PSU!!! The current was so much that the tip of my dmm had melted!!!

 

[Oznog]

Hi again only me,

I have got my motors working now will put a pic in a bit later to try it out with PWM but another 'problem' has poped up.

I just realised that a old problem has come back to haunt me, at the moment i run the motor supply directly from 6AA rechargeable batteries (NiCad - cheap ones £1 each but only on 3rd chage cycle...). After charging i get about 7.8V and this levels to about 7.2V after 15minutes running with no load on the motors.

Problem is this, i will probably use timing to excute turns. But with a voltage different ov 0.6V this could make for over / under turning. I am willing to sacrefice some speed to regulate the motor supply, but still have the problem of regulating it!

ok then, the maximum the motors can produce is 2A each (talking max stall current, dont know how it occours, probably using a vice and a wrench to hold it....) Anyhow. So i need a max of 4A regulation.

I have heard of using a zener in parrallel? what voltage would i get / would it handle the current!

So to recap, LDO regulation to about 7V, max 4A load. Over to you.

Thanks,

- Martyn

Nope, you can't use a zener, it shorts out the power supply and will basically melt.

What are you using for control? An ideal solution would be to read the power supply voltage and cut down a bit on the PWM duty when the voltage is high, so the motors will see the same average voltage regardless of the supply voltage.

 

[bmcculla]

If you add a current sense resistor and an opamp and RC filter you can measure the current and integrate it to get rid of the PWM so you just get a voltage proportional to the current through the motor. You can use your micorcontroller to adjust the PWM to apply the appropreate current. This setup is nice because motor torque is proportional to current, not voltage. With this setup you can safely overvoltage your motor to get better torque at high motor speeds.