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Omni mixer


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Daniel
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Joined: 30 Aug 2005
Posts: 2729
Location: Gold Coast


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Omni mixer

The mixer in Sidewinder got fried at the last competition. It was an OMX-3 that I got from Robot Market place 5 or 6 years ago. I went to buy a replacement and remembered it was $45. I probably thought that was good when I brought the first one but after buying a normal mixer for $3 last month it seems a bit much now.
So I'm wondering if anyone can teach me how to make a mixer. I think that it would be a simplish thing to play with since I think it is just one of those chip things that gets programed on a compter. You'll have to explain the basics because I'm only a dumb mechanical draftsman who has previously refused to learn anything electrical.

Post Wed Apr 27, 2011 11:54 am 
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seanet1310



Joined: 08 Nov 2006
Posts: 1265
Location: Adelaide


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May not be much help right away but i am currently working on a autonomous omnidirectional robot 3 wheeled right now.
When I have it driving and some free time I may be able to adapt the code to run on an AVR atmega.

Else if you do it yourself i would suggest using an AVR microprocessor as Steve, Aaron, Brett and myself have experience on this type of chip.
If not AVR then maybe a PIC, I have used two external interrupts with timing in a previous project on a PIC and I think it was the micro of choice for Valen. Although PICAXE from memory has a pulsein command and will return a value inequivalent to the pin ontime(may be wrong).
Aduino's have lots of libraries available so probably have an input libraries and PWM servo output libraries documented and easy to use online but mostly come as a development board and chips are far bigger then you require.

I would try and use external interrupt pins to read in the time of each inputs pulse. I am guessing that is how steve is currently doing it in the 1 BESC for an ant bot just posted.

The next step is applying a lookup table to these values to compute an approximate output for each of the 3 wheels then use PWM (hardware or software. Hardware if you run a high clock speed may have low accuracy. Depends on the chip used)

This is 4 wheels but may be of some use. http://www.societyofrobots.com/robot_omni_wheel.shtml
lookup table for 4 wheel omnidirectional robot.
http://www.societyofrobots.com/programming_trigtable.shtml

Within a week or so I hope to have my autonomous bot taking inputs and driving around itself in an omnidirectional way so I will keep this thread updated with any code I develop if you wish.


C or BASIC i think would be your best bets for programing language unless you are know with another one or you could do this by assembly.

Do not expect to save much $$$ if you plan on programing it yourself, Programmers often cost a few $$. You will need some form of pcb or development board. the microprocessor itself and a couple of capacitors.

Sorry if this is disjointed, It was written in 3 sittings.
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Post Wed Apr 27, 2011 12:34 pm 
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dyrodium
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Joined: 24 Aug 2004
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Location: Sydney


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Just a wild stab in the dark, there's some homebrew programmers for those hobbyking 6ch radios, perhaps they can be mixed 3 ways? Worth looking at, if it works it removes needing a mixer all together.
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Post Wed Apr 27, 2011 12:50 pm 
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Daniel
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Location: Gold Coast


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i use a futaba 9CAP and I picked up someones hobby king radio at the last meeting and it felt so light and cheap that I don't fell comfortable using one.

I had to go to wikipedia to learn how a servo works. I feel ashamed telling people I build robots but have no idea about the electronics inside them. I want to make an electronic thing to feel better about it.
This is what I learned. So a reciever puts out a series of on/off pulses at 50Hz. Every 20ms the current turn on and off. If the servo measures the current on for 15ms it says the servo is in the middle. If its 17.5ms its fully up and 12.5ms is fully down. The chip in the mixer just adds or subtracts the pulse lengths and sends out the result, right?

Post Wed Apr 27, 2011 1:31 pm 
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seanet1310



Joined: 08 Nov 2006
Posts: 1265
Location: Adelaide


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quote:
Originally posted by Daniel:
i use a futaba 9CAP and I picked up someones hobby king radio at the last meeting and it felt so light and cheap that I don't fell comfortable using one.

I had to go to wikipedia to learn how a servo works. I feel ashamed telling people I build robots but have no idea about the electronics inside them. I want to make an electronic thing to feel better about it.
This is what I learned. So a reciever puts out a series of on/off pulses at 50Hz. Every 20ms the current turn on and off. If the servo measures the current on for 15ms it says the servo is in the middle. If its 17.5ms its fully up and 12.5ms is fully down. The chip in the mixer just adds or subtracts the pulse lengths and sends out the result, right?


Check your numbers. Middle is often on for 1.5ms. and the others are also 10 times bigger then should be although some servos have different thresholds I often go 1ms total 1 way. 2ms total other 1.5 center. The 20 ms is the total period of the signal so it should look like http://www.er-online.co.uk/minisumo/images/servo%20pulse.jpg

The mixer will take this information and combine it appropriately depending on the many types of mixing of signals you wish to preform. If you build a 3 wheel omnidirectional bot you will need to take into account angle so could not just add and subtract.

To see an example maybe check the old Pic Motor controller wikki page. http://www.robowars.org/wikka/PicRelayController
Although It may not be clear how without programing experience.
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Post Wed Apr 27, 2011 1:52 pm 
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marto
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Joined: 08 Jul 2004
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Just tell me how you want it mixed will modify existing code I have then post it.

Steve
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Post Sat Apr 30, 2011 11:34 am 
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kkeerroo
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Joined: 17 Jun 2004
Posts: 1459
Location: Brisbane


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I don't think you need the mixer. Just mix the front two wheels on your radio as normal and have the rear wheel run off the yaw stick via a heading hold gyro. When you move the right stick left or right the gyro will turn the rear wheel to keep the nose on the same heading which will cause the robot to strafe sideways. You may need to turn the limits down on the left and right to prevent the front motors out turning the rear. You don't need all three motors to turn the robot. So just running the rear by itself to turn will be enough.
You have all the parts so give it a go before going to the trouble of making a mixer.
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Post Sun May 01, 2011 4:24 pm 
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