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Author Topic: SERVO HELP  (Read 7068 times)

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Offline ltmhallTopic starter

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SERVO HELP
« on: October 07, 2006, 08:56:15 PM »
Iím Building an autonomous mobile robot using differential drive and have a few questions. Iím using two continous rotation modified Hitec 422 servo motors, a Basic Stamp II (education board), two Dubro 2.75 in. wheels, and a NARP 4cm omni direction wheel.

1) If Iím using two HS 422 servo motors and a Basic Stamp II do I need an H-bridge. I was planning on connecting my servo motors to an L293 chip then connecting the L293 to a Basic Stamp, but a classmate told me I didnít need one, I could connect the servos straight to the Basic Stamp. Please Help!

2) If I need to connect my servos to an H-bridge do I actually open my modified servos and connect the two pins of the motor to the H-bridge.

3) I read a regular caster is a very great ideal for the third wheel, does anyone know a good way to mount an omni directional wheel to an aluminum chassis.

ANY HELP WOULD BE APPRECIATED

Offline Gopher

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Re: SERVO HELP
« Reply #1 on: October 08, 2006, 12:07:14 AM »
Your friend is right; an H-Bridge is only needed to power a motor directly, one of the advantages of servos is that they contain their own internal circuitry eliminating the need for an h-bridge.

As for how to attach the omnidirectional wheel, all I'll say is don't be afraid to improvise. Robotics really isn't brain surgery; if it doesn't work, try something else. With circuitry and motors you might blow one if you do something wrong, but when it comes to the mechanics, don't sweat it too much. If the wheel falls off, the front of the bot falls 2.75", otherwise nothing happens except you have to try again.

Offline ltmhallTopic starter

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Re: SERVO HELP
« Reply #2 on: October 08, 2006, 01:19:03 PM »
So I would connect the servos to the spot on the board of education for connecting servo's(I saw an exampe in the What's a Microcontroller? book ). Also do I use an external 5 volts for the servos. I read somewhere its not recommended to use the regulated 5 volts from the board.

Offline Gopher

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Re: SERVO HELP
« Reply #3 on: October 08, 2006, 01:45:14 PM »
Not sure about the power, so I won't complicate the issue by guessing, just say that it wouldn't be much trouble to provide separate power so I'd say better safe than sorry.

I assume by "board of education" you mean the microcontroller board, in which case you are correct about connecting the servos.

Offline dunk

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Re: SERVO HELP
« Reply #4 on: October 08, 2006, 11:18:42 PM »
yup, you want to use a separate voltage regulator for your servos.
everything would probably work ok with the one on your microcontroller board but servos (as with any motor) can create power ripples so it's better to use a separate regulator.
you can try it without but if you find the microcontroller doesn't seem to operate as it should then power problems are the likely cause.

there's some info here on voltage regulators:
http://www.societyofrobots.com/schematics_powerregulation.shtml
http://www.societyofrobots.com/electronics_advanced_components_tutorial.shtml#voltreg
also do a search for voltage regulators.

dunk

Offline ltmhallTopic starter

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Re: SERVO HELP
« Reply #5 on: October 09, 2006, 05:35:48 AM »
Could I supply four 1.5 volt batteries to power the board, then connect my servos to the servo headers located on the board.
« Last Edit: October 09, 2006, 12:44:00 PM by ltmhall »

Offline Gopher

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Re: SERVO HELP
« Reply #6 on: October 09, 2006, 03:36:31 PM »
Yeah, 4x1.4v batteries in serial will give you 6v, though I recomend not using standard alkalines, as they drain so quickly; a more expensive set of rechargable batteries will pay for itself pretty quickly. You'll want to use a voltage regulator to throttle the 6v down to 5v, and you still shouldn't power the servos from the board directly, just connect their control line to the board. Servos and motors can have spikes in their power consumption and also can create noise in the power signal. This can cause the microcontroller to misbehave or, in the worst-case scenario, possibly even damage it.

Offline ltmhallTopic starter

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Re: SERVO HELP
« Reply #7 on: October 10, 2006, 08:18:40 AM »
So I could use a 3 pin male/male header and connect my servo to it. Then connect my power pin to a 7805 5 volt supply, connect my ground pin to the vss terminal and connect my last pin to a pin on the basic stamp. Also connect my 7805 supply ground to vss on board of education.

And repeat this for the other servo with a different 7805 5 volt supply.

Offline Admin

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Re: SERVO HELP
« Reply #8 on: October 10, 2006, 09:18:36 AM »
You dont need to regulate voltage to a servo. Just connect it directly to power.

Your microcontroller however needs regulated power.

Offline dunk

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Re: SERVO HELP
« Reply #9 on: October 10, 2006, 10:01:37 AM »
but you can use a regulator like the 7805 to get a nice even 5 volts from a 6 volt supply (as ltmhall is suggesting).

ltmhall, your have it right in your previous post.
basically you can use lots of different DC supplies as long as you connect all the ground terminals.

in practice, i usually use one for all the on board microcontrollers (and other electronics) and a separate source for all the servos.
when i say "separate source", usually using a separate voltage regulator but still sharing the same battery is sufficient.

dunk.

Offline ltmhallTopic starter

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Re: SERVO HELP
« Reply #10 on: October 11, 2006, 05:15:28 AM »
Will I have to add a resistor to one of the basic stamp out puts or reverse one of the servos leads to make both rotate in the same direction when I add wheels to them? I heard from a Friend I will have to do something like this.

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Re: SERVO HELP
« Reply #11 on: October 11, 2006, 06:39:52 AM »
Nope, the wiring will be exactly the same. To get the wheels to go in different directions, all you do is give each servo a different signal pulse.

For example, a neutral no rotate signal would be maybe 1.4ms, to go forward would be 1.5ms, and for reverse 1.3ms. Those numbers of course are dependent on the servo, but its the generally idea.

Offline ltmhallTopic starter

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Re: SERVO HELP
« Reply #12 on: October 12, 2006, 08:27:47 AM »
Are there anyways I can run two modified servos without a microcontroller. Can I use a circuit built around a 555 timer.

Offline Admin

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Re: SERVO HELP
« Reply #13 on: October 12, 2006, 08:35:00 AM »
I believe gopher has something to say about not using microcontrollers  ;)

Yes, you can use a 555 timer . . .
But the question is, do you want to control the system using software, or hardware?

Offline ltmhallTopic starter

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Re: SERVO HELP
« Reply #14 on: October 12, 2006, 09:37:29 AM »
I haven't bought my microcontroller yet. I just wanted to too see how my chassis with the wheels and motors attached would perform. WHAT IS GOPHER.

Offline Admin

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Re: SERVO HELP
« Reply #15 on: October 12, 2006, 09:42:59 AM »
If you want to see how a servo will perform before using a microcontroller, you can either use a function generator, or any hobby remote control kit.

I perfer the latter cause I have one, and its fun drivin the bot around  :)

Offline Gopher

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Re: SERVO HELP
« Reply #16 on: October 13, 2006, 10:10:48 AM »
Gopher is an old pre-web, text-based internet protocol; it served the same purpose as www, and online companies and universities would provide gophers as a way to access their different departments and features. I think Admin meant me, though, and I'm a who, not a what. :) My current 'bot doesn't have a microcontroller, but I'm certainly not an expert on not using microcontrollers; I'm actually a programmer professionally. I'm not using a MC because I want to challenge myself and get more experience on the hardware side of things.

 


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