Octopod servo controller

[cyberdead]

Hy,
I'm opening this topic 'cause, I'm working on an Octopod robot, but I'm not to good in electronics.
So question is: I designed a controller for 24 servos with a PIC microcontroller. The problem is that the servos will work with 6V and I have a 7.4V Li-Pol Battery. I thinking about, using in the IC an LM7806 regulator to transform 7.4V to 6V. It is okay, but the regulator can give only 500mA but the 24 servo need lots more. They consume 280mA at no load operation. The total consumption is 24*280mA (they are connected parallel to Vcc), so regulator is a bad solution.
Any ideas, how to solve the problem?

[Bullit]

Something like this may be suitable. You can even order free samples.

[StuartL]

The biggest problem I can see with most power regulation solutions is that the overhead for the voltage regulation will be about the difference between your typical voltage and your desired supply voltage. Once your supply voltage drops below about 7.2v (i.e. not far from the average voltage of two LiPo cells) the overhead of the regulation is going to mean that you'll get less than 6v at the outputs.

Can you check your servo specs with the manufacturer and see if they'll run off LiPo voltages (6.4v - 8.4v for a two cell pack)? Lots of radio controlled model servos are designed to run off higher voltages as traditionally they were used in RC cars which had a 7.2v or 8.4v NiCd/NiMh pack.

It's worth looking...

[cyberdead]

Thanx Bullit, that stuff would be really suitable, for my application, it is small, so i can build it in my robot.
But after Stuarts reply, i started to thing about the voltage drop problem, and i got an idea, what actually my friends idea. In mobile phones, the DC-DC ( http://en.wikipedia.org/wiki/DC_to_DC_converter ) converter is a common used circuit. With it, we can store energy, and we can increase, and/or if i'm right, we can stabilize voltage, so myproblem is maybe solved.
Thank you for the clever replies.

Pete

[cyberdead]

I surfed a little on the net, and i see that i have a common problem. DC-DC converter regulators have the same problem, they cant give that much power what we need. Actually, i watched after my servos (11 HS-55, 16 HS-81 ->27 servos) max current drain, they can be more than 500mA (HS-55) and more than 800mA (HS-81). Summa: more then 18A needed. with my 25C Li-Po isnt that a problem, but i cant find any regulator, solutions we have talked about arent enough. So problem isn't solved.

[StuartL]

How about using a 3S LiPo for 9.6v - 12.6v and use a good high current switching regulator? If the regulator is good it won't waste too much power in the step down...?

[cyberdead]

Finally, after hours of surfing, i got something useful.
http://www.edn.com/article/CA434875.html
So, if we connect the regulators parallel like in the link, we can get more current. They use 7812 regulators in the PDF, but i looked for that type and it has max output 100mA, so we can use instead LM7806 (it regulates to 6V).
I dont know its backdraws (voltage drop) yet, but i will build it and will post my experiences.
Thanks for help. See you later.

[StuartL]

Don't use the 78xx range of voltage regulators, they aren't switching regulators and instead 'waste' the extra power as heat in the voltage drop. You'll end up with a very inefficient heater if you do it that way

There are pin-compatible switching regulators, I don't recall any part numbers but google should be able to help.

[cyberdead]

Thanks to you, Stuart, i had to surf other couples of hours BUT i found again something. http://datasheets.maxim-ic.com/en/ds/MAX8664.pdf or http://datasheets.maxim-ic.com/en/ds/MA ... AX8599.pdf
It can provide 20-25A maximal output current and it's a step down regulator.
What do you think?:)

[StuartL]

Thanks to you, Stuart, i had to surf other couples of hours BUT i found again something. http://datasheets.maxim-ic.com/en/ds/MAX8664.pdf or http://datasheets.maxim-ic.com/en/ds/MA ... AX8599.pdf
It can provide 20-25A maximal output current and it's a step down regulator.
What do you think?:)

They look like superb little chips, although the analogue support circuitry looks interesting

If you're up to the analogue side of that I'd jump on it, otherwise the likes of the 8665 with its built in MOSFET looks like a really good bet; although it seems to be limited to 25A @ 5.5vDC.

Any of the above solutions look useful

[i-Bot]

I find that working with these switching regulators is quite difficult and time consuming, especially sourcing the components. Suitable components especially capacitors get quite expensive in small quantities.

Since all the servos don't need to have the same supply, I would use 4 off 5A BEC each powering 6 servos. Common the ground for all BECs and the servo controller.

[cyberdead]

This small, thing is a great idea. So, if i'm right, will need 4 LiPo-s with 4 BEC-s. Now only thing is to recalculate the invers dynamics, cause new batteries arent too light, and the robots weight is near to limit, i'm affraid servos cant drive the legs. Other way is, to use smaller cells, but then the robot is going to live only 10-18 minutes,before rechargeing ... But this is my problem.
So i will use this BEC thing Thanks I-Bot.

[i-Bot]

You can run multiple BEC off the same batteries provided they can provide the current. Some BECs specify they can run in parallel sharing both battery and load, but I suggest to split the servo load over the multiple BEC.

[cyberdead]

I searched after this BEC a bit. I red that it has a great servo limit. With 7.4V battery we can attach 3 servos...because the dissipation, which comes with the regulation. I found this while i have searched: http://www.amainhobbies.com/product_inf ... s_id/13163
It supplies 15amps max 20, and it can be used with 2s LiPO-s.
Its a really suitable solution, i think.

[Joseph Conrad]

Hello dude,

I too am a robo Savvy like u. I too had many struggles in building robo initially. Later i came across Cypress Products, and i could build few robots. I feel good about it.
TO know more abt it check http://www.cypress.com/