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tuzzer
02-16-2010, 11:06 AM
Hi,


I am building a robot off Lynxmotion BRAT. In order to keep the centre of gravity low, I am thinking of distributing the cells on its giant feet.

I am just wondering if you would have any suggestion for which batteries I should buy and what's your approach in regulating them.

We are planning to get a few Li-ion batteries in pairs (7.4V) and connect them to a central power management board. On it, we will have three regulators: one to microcontroller (3.3V), one to servos and transponder (6V), and one to the camera (5V). Please give me any advise on anything that I should be aware of. Other than having three power regulators there on the power management board, are there any other additional circuitry that I should be adding? In addition, do you recommend splitting the batteries around the feet of the robot? And where would you suggest me to buy those battteries?


I know that's a lot to ask, but this is my first time building such system and is trying to get as much done the first try as possible due to limited budget.


Thanks!


Matthew

lnxfergy
02-16-2010, 01:06 PM
I'd be concerned that if you put the batteries in the feet, there wouldn't be enough weight transfer and the bot couldn't walk at all.... Jim @ Lynx might know better, somebody has probably tried this and can confirm/rebut this theory.

-Fergs

4mem8
02-16-2010, 02:35 PM
also watch what type of regulator u use for the servos as these could draw a few amps if under load. I have a similar problem with my scout that uses 14 hsr 5990tg servos and was thinking of a similar approach for the batteries. but with some on the legs as well with covers over them. will have to see. but make sure you have the right reg for the servos.find out what amps the servos draw when all working and loaded up. or calculate 1 servo under load with a meter then x how ever many servos you are using.

darrellt
02-16-2010, 07:31 PM
My brat has batteries in the feet. The walking all depends on how much weight you have up top. If you have a heavy head the foot weight can be beneficial. Also with heavy feet you have to use slower, longer strides due to the inertia. I recommend experimenting with different mounting locations at full battle weight.

As for the power system, I started out regulated but switched Lithium-Iron-Phosphate cells that have close to an ideal voltage unregulated. They also have a nice big flat spot in their discharge curve so I stay near 6.5V for most of the discharge.

Details are in my build thread: http://forums.trossenrobotics.com/showthread.php?t=3559

tuzzer
02-17-2010, 08:59 PM
Thank you very much for your advise.

I actually have already gotten a high power switching regulator for the servos that can handle even the current draw in case all servos are stalled (can handle up to 7A) <http://www.robotshop.ca/duralite-6v-7a-regulator-high-current-switch.html>


I would prefer having the battery down at the feet because we have the airsoft guns, bb, camera, xbee, as weel the microcontroller and power managment boards on top of the legs. It seems more reasonable to put the battery lower to ground.

One concern that I have is the power of the servos. For now, I am still using the original HS-422 servos that came with the BRAT. Seeing that most people changed to more powerful servos, i suspected that I might also need more powerful servos if I have my battery on the feet. Which servos do you think would serve the purpose? Would a standard servo work or should I go to digital servos? I prefer standard servo because I have not used digital servos before and they are a bit more expensive. I was reading about why digital servos are more powerful is because that it updates the servos in short intervals. So does that mean that I have to change my program if I use digital servos?

Thanks.

Matthew

tuzzer
02-17-2010, 09:05 PM
Also, is there any suggestion for good Li-ion batteries?

Robot Dude
02-22-2010, 04:10 PM
Thank you very much for your advise.

I actually have already gotten a high power switching regulator for the servos that can handle even the current draw in case all servos are stalled (can handle up to 7A) <http://www.robotshop.ca/duralite-6v-7a-regulator-high-current-switch.html>


I would prefer having the battery down at the feet because we have the airsoft guns, bb, camera, xbee, as weel the microcontroller and power managment boards on top of the legs. It seems more reasonable to put the battery lower to ground.

One concern that I have is the power of the servos. For now, I am still using the original HS-422 servos that came with the BRAT. Seeing that most people changed to more powerful servos, i suspected that I might also need more powerful servos if I have my battery on the feet. Which servos do you think would serve the purpose? Would a standard servo work or should I go to digital servos? I prefer standard servo because I have not used digital servos before and they are a bit more expensive. I was reading about why digital servos are more powerful is because that it updates the servos in short intervals. So does that mean that I have to change my program if I use digital servos?

Thanks.

Matthew

Digital servos respond to different servo pulse ranges than analog servos. I'm not going into specifics because even the digital servos have different ranges. It depends on which servo. If you change servos in your bot you will likely need to fine tune any walking sequences already written.

The digital servo updates the servo MOTOR more frequently than an analog one. This allows the motor to hold it's output shafts position with greater strength. ;)