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Robotic Sushi - FAO Davy

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Postby Davy » Tue Jul 04, 2006 12:04 pm

Hi Andy,

The "swimmer" PIC and board are on thier way to you.

It has ahead,left, right and stop controlled by switch input. Any switch would work - even controlled by servo. It could even be a single channel fish! PWM inputs will take a little longer.

Let me know how you get on with it and I'd appreciate some info on the mechanical linkage.

See you at Norwich Sub Day??

David
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Postby Sub culture » Tue Jul 04, 2006 3:01 pm

Dave,

That's great news!

Wow, really have to get my finger out now, and get this thing built.

I should hopefully be at Norwich.

So to control this fish, I'll need a switcher unit with four outputs, correct? These feed into the PIC input channels for stop, go, turn right and turn left.

Or have I got the wrong end of the stick here?

Andy




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Postby Davy » Wed Jul 05, 2006 5:10 am

Hi Andy,
A switcher may be a bit over the top. You'll see what I mean when you have a go with the board. Earthing a pin puts the function on.

It would be possible to activate 3 micro switches or push buttons with a servo arm to get the functions. If this seems a little in-elegant, the PWM version should not be too many weeks away!

My first thoughts were to see if the fish swims forward, right left etc. (I still find it hard to believe!) and if a servo is powerul enough So maybe we should aim for an initial "lash-up" trial in the home test tank!

Let's get swimming!

David
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Postby Sub culture » Wed Jul 05, 2006 8:58 am

Dave,

Servos are pretty torquey these days.

Check this one out- 417 oz in. torque!! -

http://servocity.com/html/hsr-5995tg_ultra_torque.html

Now I don't think we'll need anything like that, but it's comforting to know that those kind of things are now available. Not cheap though.

Now regarding a switcher. This is a unit i had in mind-

Image

It's our way, and the outputs act as grounds, so perfect. It's also tiny and quite cheap (about a tenner).

Works off a single channel. More info here (in German I'm afraid)

http://www.cti-aichtal.de/shop/anleitung-ps4a.pdf

Andy
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Postby Davy » Wed Jul 05, 2006 9:39 am

Hi Andy,

At least it's not in Chinese!

That could well do the job and for a tenner I could hardly get the parts. :(

Slight health warning though since my German will not get me more than a beer! Don't put more than 5v on the Swimmer PIC.

Get that fish tail flapping!

:D

David
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Postby Sub culture » Wed Jul 05, 2006 10:54 am

This is what I had in mind-

Image
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Postby Himszy » Wed Jul 05, 2006 11:05 am

If the 4 ch switch is, left, right, forwards, backwards, then how to you go forwards and left?

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Postby Sub culture » Wed Jul 05, 2006 12:51 pm

No reverse.

Swim forward
Swim left
Swim right
Stop.

Four functions.

It's basic, but it does work well, because I've used that system in the little toy RC shark.

Unfortunately the RC shark is very small, has terrible batteries, and the range of the radio on mine was abysmal, although others have reported more success with theirs- perhaps I got a duff one.

With a scratch built fish, I can use a decent radio, better batteries, plus I can ballast the thing easily, and make it a decent size.
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Postby Himszy » Wed Jul 05, 2006 1:46 pm

////////Where's the delete post button....//////////



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Postby Wheelerdealer » Thu Jul 06, 2006 5:52 pm

Sounds neat, I'll have to see this fish when its done. 7.2 will shorten the life of the tail servo considerably, best stick with 6v.
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Postby Sub culture » Fri Jul 07, 2006 5:39 am

Why do you say 7.2 volts will shorten the life of the servo considerably?

I'm referring to special Hitec robotic servos, which are designed for use with this voltage.

Andy
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Postby Wheelerdealer » Fri Jul 07, 2006 5:58 pm

Sub culture wrote:Why do you say 7.2 volts will shorten the life of the servo considerably?

I'm referring to special Hitec robotic servos, which are designed for use with this voltage.

Andy

Missed that bit, thought you were usung a normal servo :D
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Postby Sub culture » Sat Jul 08, 2006 6:06 am

As it so happens I purchased a servo yesterday.

I plumbed for a Hitec HS-635HB. This is a high torque servo with 'karbonite' gears. These are exceptionaly rugged gears which wear much better than either nylon or metal.

On 5.0 volts the thing is virtually unstoppable.

It cost £20 from Als Hobbies (good price) and is fully ballraced with a lovely smooth action.

I considered a digital servo was complete overkill for this application.

Image

Here is a little picture of what I have together so far.

On the top left is the afore mentioned servo, next to that is the little PIC board sent through from Dave Forrest.

Next to that is a very nice little BEC board I purchased with the servo. This little thing is a switch-mode device that can supply either 5 or 6 volts at 3 amps from a 5.5 to 23 volt power source.

You can select the output voltage using a small jumper which you can just see on the bottom of the board.

It's made by Dualsky and cost about £10.

The three small boards to the bottom right are some little switchers I'm making up. These trigger the various functions on the PIC board.

I did find an off-the-shelf switcher, which is tiny and cheap. However I considered the means of controlling the switching didn't quite work for the fish. So I looked around for an alternative.

These baords are designed by Ken Hewitt. The design can be downloaded here, and it is very simple and cheap (about £1 per switch) to put together. I'm waiting for some more components to arrive to finish the boards off, which should hopefully arrive in the next couple of days.

I'm using a much smaller logic level MOSFET for the switches, than specified in the design. The original was overkill for this application.

They are bulkier than a commercial design, but that isn't an issue as I have plenty of room inside the fish.

Andy
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Postby Himszy » Sat Jul 08, 2006 7:46 pm

Andy,

What's changed on a servo to increase its torque? Can't just be a case of changing the gear ratio surely?

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Postby Sub culture » Sun Jul 09, 2006 6:03 am

Hotter motor, slightly lower gear ratio (high torque servos are usually marginally slower than high speed) and thicker gears.

Andy
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