Wednesday, June 15, 2005


Polymorph Carriage Tests

The first tests have been done on the Polymorph carriage, oiled lightly, with the feet trimmed to 75% of the circumference and de-stressed with a hair dryer. Smoothly overcoming the friction between the four saddles requires a force of 2.6N and 1.2N keeps it moving:

The carriage itself weighs 140g and I started to wonder just how much power we're going to want to expend on moving the carriage around at this point?

As an example, the tiny little 3V-6V Meccano motor I have here consumes 200mA startup current at 3V and will move the carriage. Assuming that's around 50% efficient, thanks to the gearbox, and a load on the carriage that is expected to be about 300g, the dilligent student should be able to work out the maximum speed and acceleration. It's been a good few years since I did physics at school...

Would anyone like to tell me:

a) The likely top speed.
b) What's the accceleration, and
c) Is this good enough?

Vik :v)
PS Spot the cameo appearance of the FDM'd pinch wheel assembly.

Isn't it more of a power=force*distance/time
therefore power/force=distance/time so
.3 W/1.2 N=.25 m/s? I may be wrong - haven't been doing teriffically in my physics classes - but I think that's it.
I do remember that acceleration is a somewhat harder thing to get from power, if only a bit. More tanlged.
Hmmm... one meter every four seconds? certainly not tremendously fast, but I can see that being fast enough for basic operation.
I think some practical experiments are going to be called for here.

Remember that this is the smallest motor I've got - I'm trying to establish what the requirements are at this point, and I can add more horsepower simply by running at 6V.

Vik :v)
Thanks for putting me straight on that. Like I said, long time since I did physics and I was never a star pupil at it :)

The friction is higher than I'd like, but the thing needs to grip the rails well enough so that it doesn't wobble about. It's not just resting on the top of the rails, but wrapped around them and hanging on so it's not just a downward-acting force generating the friction.

Vik :v)
A belt drive is just as stable - most printers use this technique. I'll be experimenting with one this weekend, and Ed successfully used a belt drive on his axis prototype.

What positional accuracy did you get out of the Lego?

Vik :v)
For comparison, the width of a tooth on a printer drive belt is usually in the order of 1.8mm

What is the Lego like?

Vik :v)
All this, of course, fails to account for any friction in the drive system, which is going to be an old HP DC motor and rotation encoder out of an ancient printer. Mostly on account of that's what I've got.

This is an ungeared 12V motor, and I'll have a go at wiring it up to thrash the carriage back and forth. We'll soon see what wears out first, and I'll be able to figure out some real-world speeds.

Vik :v)
Right, Ash and I have tried it for real. We used a nice, big 12V motor off an old HP printer of Adrian's, and put it on a makeshift carriage and bar assembly.

It moves things around at one heck of a speed. Far more than I need, and in desperate need of some PWM controlling!

We're going to need robust limit switches on this thing, probably backed up with an anchor and short chain.

Vik :v)
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