Synchronising DC motors

[Novata]

Hello all,

I am embarking on a project whereby I need to synchronise 4 DC motors together in phase. 2 will rotate clockwise and the other 2 will rotate anticlockwise. It is absolutely imperative that these motors never even rest out of phase when powered down.
I also need to control the speed, but I could probably do this with a varibiable resistor.
I am a qualified electronics service engineer, but it has been a long while since I have dabbled with electronics.
Ideally I would like a circuit diagram or a small explanation on the methods I need to accomplish this.
Your help is greatly appreciated.

Many Thanks.

[KMoffett]

You didn't indicate speeds or loads, but I doubt that you will be able to achieve your goal electronically, especially maintaining sync of standard dc motor after power-down, even with absolute position encoders. Are gear or timing belt systems out of the question.

Ken

[Novata]

Gears are an option, I wanted to avoid using gears altogether.
Thanks for your help

[Biffo]

you could use a photocell and light on the motor to make sure it went to the right position?.

A notch on the motor would block the light to the photocell, this would tell your circuit to keep moving the motor, until came it came to the point on the motor where you had a drilling hole in the notch that allowed light to reach the photocell.

[Novata]

Ah, so it is possible. I wanted to have a control servo system, so if I could get the motors to generate a pulse on each rotation then feed all four pulses into my control circuit and then there should be no problem right?
I know of servo systems as I used to service proffessional broadcast vtr's, and there has to be very accurate timing with regards to the control pulse on the tape, the head switching points and the overall tape speed.
So, say I have this photocell to generate this pulse, would you happen to know of any useful circuit diagrams that would aid me to do this?
As I previously mentioned, I'm not very well brushed up on the electronics side of things. Sorry.

Thanks for the help anyway. It is greatly appreciated.

[Biffo]

yes you mentioned tape machines-just thought of something else you could use is 'Hall effect' to get a pulse from your motor, basically you just put a little magnet on wherever you want the switching position to be on your motor and then something to pick it up as it passes- an old cassette tape head usually will do, obviously connected to a little pre amp.
If you think the motor will disturb the magnetism then you could build the head on a platform with the magnet opposite . Then have a blade on the motor in required position-which of course as the blade goes through the head and magnet gap causes a disturbance or pulse.

but never mind that, here is an example of what could be a ready made solution?

http://www.active-robots.com/products/m ... s/1001.pdf



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[Novata]

Wow, thanks Biffo, that is just what I'm after. I really appreciate this.
I knew I'd come to the right place

Now one last favour. Do you know of any good electronic educational sites with regards to me relearning electrical theory?
Ideally, I'd like to know of any cheapish software I could run where I could design a circuit and the software could give me the results. This is for only home projects.

It would be an electrical CAD software I guess. I will have a look myself but if you know of any good cheap or even freeware software then your recommendation would be a great help.

Thanks once again Biffo.

Novata.

[Biffo]

How about 'The Open University' ? study at home and have qualifications at the end of it.

With a computer, a ' circuit simulation program ' is usefull. IE you connect components up on screen and it will tell you the result ..but dont expect miracles.
The most famous free one is ' Spice' and you should be able to find it here

http://www.smps.us/tools.html

but as with everything you get what you pay for, and a free program can/may be difficult to set up , while something for about 200 should be easy to install and get right down to business.



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[Novata]

Thanks Biffo,

You've been a great help. Appreciated.

Novata