RoboSavvy Forum

Octopod servo controller

2008-09-22T13:07:55+01:00

Thanks for reply, i havent read the for a while so there happened many things. Now about the power supply. I use a simple 5cell NimH hobby battery, which has 6v of voltage. It's not to good but enough for testing. But, i heard many things about the so called fuel cells. Is there anyone who knows more about it? I would like to know, is there any fuel cell with the size of 5cmx5cmx5cm approximately. and with the weight under 120g, with voltage of 6v?

Statistics: Posted by cyberdead — Mon Sep 22, 2008 1:07 pm

Octopod servo controller

2008-08-12T18:20:06+01:00

Hi
First of all, originally a BEC was a Battery Eliminator Circuit and eliminated the need for separate radio and servo batteries, I think the terminology is now a little muddled, but BECs supplied regulated voltage for the electronics while motors ran from raw battery voltage.

The amainhobbies product is a high power regulator not a BEC.

According to one Hitec rep, Hitec servos don't smoke until run on over 12v and many servos now are spec'ed for more than 6v.
Switching regulators are a pain, and sometimes don't like supplying motors.

The problem with running servos on higher voltages is not that the control electronics will blow up but that the motor will take more current which may be too much for the H-bridge transistors even on light loads. Basically if your servos are not able to carry your robot you need bigger servos or less weight or a different leg design.

The easiest way of dropping the voltage is a high current Si diode, or two, each diode will drop ~0.7v on no load. Two diodes in series will drop a 2 cell LiPo battery down to 6v.

David

Statistics: Posted by robots42 — Tue Aug 12, 2008 6:20 pm

Home Built

2008-08-12T13:21:31+01:00

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/

Statistics: Posted by Joseph Conrad — Tue Aug 12, 2008 1:21 pm

Octopod servo controller

2008-06-25T12:11:46+01:00

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.

Statistics: Posted by cyberdead — Wed Jun 25, 2008 12:11 pm

Octopod servo controller

2008-06-24T23:00:53+01:00

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.

Statistics: Posted by i-Bot — Tue Jun 24, 2008 11:00 pm

Octopod servo controller

2008-06-24T21:47:31+01:00

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.

Statistics: Posted by cyberdead — Tue Jun 24, 2008 9:47 pm

Octopod servo controller

2008-06-24T15:46:02+01:00

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.

Statistics: Posted by i-Bot — Tue Jun 24, 2008 3:46 pm

Octopod servo controller

2008-06-24T14:12:14+01:00

cyberdead wrote:
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

Statistics: Posted by StuartL — Tue Jun 24, 2008 2:12 pm

Octopod servo controller

2008-06-24T09:41:38+01:00

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?:)

Statistics: Posted by cyberdead — Tue Jun 24, 2008 9:41 am

Octopod servo controller

2008-06-24T08:42:32+01:00

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.

Statistics: Posted by StuartL — Tue Jun 24, 2008 8:42 am

Octopod servo controller

2008-06-23T23:50:55+01:00

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.

Statistics: Posted by cyberdead — Mon Jun 23, 2008 11:50 pm

Octopod servo controller

2008-06-23T19:41:23+01:00

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...?

Statistics: Posted by StuartL — Mon Jun 23, 2008 7:41 pm

Octopod servo controller

2008-06-23T10:28:11+01:00

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.

Statistics: Posted by cyberdead — Mon Jun 23, 2008 10:28 am

Octopod servo controller

2008-06-23T09:18:31+01:00

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

Statistics: Posted by cyberdead — Mon Jun 23, 2008 9:18 am

Octopod servo controller

2008-06-23T06:52:23+01:00

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...

Statistics: Posted by StuartL — Mon Jun 23, 2008 6:52 am