Why won’t it work?

Gramps

Ok, first (hopefully not dumb) question..........


My steering motor is a 12v- 2 amp windshield wiper motor. (About 100 degrees turning end to end)
To control it with the Stamp, use a $50 parallax HB-25 motor controller.
OR:
How 'bout this..?

Disassemble a servo. Remove the gearing.
Remove the pot and extend the wires and glue it to the wiper motor shaft.
Disconnect the servo motor wires and extend them to two 5 volt relays with diodes in line to sense polarity reversing.
Relays control the wiper motor.
Heavy lifting for under $15!

erco

MY KINDA GUY! Yep, I love this relay/servo hybrid approach, and I have been meaning to try it. Sure seems like it should work, my only question is will the relays buzz endlessly when the pot is slightly off-position. If the motor is pulling high current, that's not good for the relay contacts. Please let us know how it works if you try it.

Clearly the apple didn't fall far from the tree Gramps/OBC Dad!

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·"If you build it, they will come."

PJAllen

Hysteresis (dampening) (damping)·would keep the relays from hunting and buzzing off-position, but also have an effect·upon the response, too.

Post Edit -- malaprop

Post Edited (PJ Allen) : 4/16/2010 12:55:44 PM GMT

erco

Such as a small RC across the relay coil...?

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·"If you build it, they will come."

HollyMinkowski

I love ideas like this gramps!
You are my kind of tinkerer

I'd probably make a homemade H-bridge from
large discreet transistors to control a 2amp motor.
And rig up an optical shaft encoder to keep precise
track of motor movement.
But that's just me.

I once entertained the idea of controlling
a starter motor with a prop...but it was all
just so messy and greasy

Gramps

Thanks for your feedback!
Couple of problems with trying the idea.
My bot’ is in Mongolia and i’m in Korea.
The only servo here is the good one that came with the Stamp.
An old servo with smoked gears might be a good test case.
Ya, Was wondering about buzzing, too. Thought, maybe, just leaving the little servo motor in the circuit might help.
Holly, Please send a sample H bridge that would work in this app. The ones i looked at on the web were pretty complicated.

HollyMinkowski

Gramps,

This page has good drawings of the type of 6 transistor
bridges I usually make. Of course you would need to substitute
larger transistors for the 3906, 3904 as they won't even handle 1 amp.

It takes two output pins to drive them...just make certain your code
NEVER sets both pins high at the same time or you will have SMOKE

These designs also allow you to deliver an ac output using
a controller chip and it's single polarity power supply....very useful
sometimes.

www.beam-wiki.org/wiki/Mark_Tilden's_6-transistor_H-bridge

erco

@Holly: As my Heathkit HERO Jr. says, "Smoking transistors is bad for my health..." Your statement about "thangs blowin' up when both pins are high" reminds me of one of my favorite posts from PhiPi: hardware should be designed so that a software mistake can never cause a hardware failure. Good policy, it will inevitably happen and release the magic smoke.

@Gramps: I'm a relay user myself, and I'm convinced that real men·build H-bridges from relays! Attached is my favorite design that uses small transistors to switch two SPDT relays. This method WON'T smoke from a software error. You can size the relays to power any size motor you like. The relays give full forward & reverse, as well as dynamic braking and motor coasting (two very different ways to stop a motor). Try it, you'll like it! Just get the parts from your local Korean Radio Shack, unless the Mongolian Radio Shacks are better!

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·"If you build it, they will come."

Gramps

erco, i like your H bridge much!

your local Korean Radio Shack, unless the Mongolian Radio Shacks are better!

LOL!

Gramps

Still thinking about overshoot and hunting with my servo hack. Can't we just program in a little slop?

PJAllen

Worry about that then.· You'll have to experience it real-time.

erco

Gramps: As I said, this has been on my to-do list for a while, per my thread at http://forums.parallax.com/showthread.php?p=880514 . I have several projects going on now, and hope to get to this soon, but if you beat me to it I won't be too upset! Let us know how it goes!

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·"If you build it, they will come."

Peter Jakacki

H-bridges can be built with basic components and they can be made foolproof too. I have a smoke-proof circuit which just uses three 10c transistors for the drivers plus the MOSFETs of choice so it is possible to easily have a 10A bridge or more for only a few dollars. What's more each half-bridge is controlled from a single I/O line and can float when the I/O line floats too.

I have built quite a few different ones before but this one is designed to be run at high switching frequencies without heat-sinks. My simulations show that I can easily run to 100kHz especially if I allow for a little inductance in the supply to limit the switching peaks. OK, I haven't actually built one yet but because of my experience and the sims I am very confident of this one. I will probably cobble one together on some breadboard to check it out although I already have the pcb designed.

I use these as tiny 1.8" x 0.8" quad half-bridge modules for driving high-current 24V motors at 10 amps from the Propeller. My external control circuits also include a dedicated micro on the outputs and I/O lines that monitors fault conditions and overrides the control lines if needed.

If anyone is interested in a real tough and cheap MOSFET bridge then have a look at this web page I did up docs.google.com/View?id=ddgq8k4b_1d7j4d2fh

Remember, no heat-sinks and no smoke, and cheap.

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*Peter*

Gramps

erco, Yes, i see you got the jump on me!
Well, we think alike, that is, keep it simple, cheap and innovative!
I have another month in Seoul, and hope to get through the WAM instruction book. Than on to the States perhaps for the summer. Might get up to see Jeff and even make an EXPO.

later, phil