OpenServo.com

[Toortanga]

I'm much more used to designing larger motor drivers that don't have to fit inside a tiny plastic box so I normally never leave out things like flyback diodes, push-pull gate drivers, and floating gate drives.

So I'm wondering what exactly I can get away with for the DC motor in an RC servo? Like without fully developed gate drivers I don't think you can have switching speeds much higher than 20kHz without having the FET of a reasonable size overheat or flyback becoming a big issue because of the lack of Schottkys.

I'd attach an image of my current schematic for review but I don't see how. It's just a P/N H-bridge with compliementary BJT emitter-follower gate drive for the low-side NMOSFETs that is to be PWM'd and a pull-up resistor/NMOS level shifter for the high-side PMOSFETs that aren't PWM'd. To be sign magnitude drive. No schottky diodes, just the parasitic diodes of the FETs for flyback, and the high-side PMOSs can tolerate a gate voltage that exceeds the motor voltage so there shouldn't be any problem with the level shifters pulling down the gate voltage too much.

[ginge]

Hi Toortanga, and welcome.

I am no expert on electronics, but here is some of my experience on this subject.

You are right that switching frequencies over 20khz tend to create problems with heat and current draw through switching losses. However, for small motors this isn't normally a problem as the gearing of the horn tends to offset this deficiency in speed.

Given your observations I am not too sure what your end goal is. Are you looking to drive a regular RC servo with a different bridge design than the stock OpenServo, or are you looking at doing something slightly different?

"I'd attach an image of my current schematic for review but I don't see how."
You could use imageshack or flickr to host the image.

Cheers,

Barry
_________________
http://www.headfuzz.co.uk/
http://www.robotfuzz.co.uk/

[Toortanga]

I work with dsPICs, not Atmels, and I need 9-bit RS-485 interface which neither OpenServos or the RX-10 have. The RX-10 is also slightly too non-standard (and strong/heavy) since this is going on a plane. THe problem with a regular sero is that can't run off 10-14V and I don't think I can afford the space of a BEC on the plane.

SO I kind of need a programmable RX-10 in RC servo form but with a 9-bit interface rather than the one it comes with (0xFF, 0xFF to indicate an RX-10 address will follow is a bit inefficient).

WHat does OpenServo PWM the motor at? As for a regular RC servo, I'm not sure if the motor is PWM'd at all or if it just has fine "on/off" control since I'm unsure if companies would stick gate drivers into their servos to save on cost. I'm not entirely sure how Futaba managed to squeeze a BLDC drivers into their new BLDC servos...but they're too expensive to buy and rip a part.

I have to wait a day before I can post links/images.

[Toortanga]

I'm not sure why I am still unable to post links to my schematic (I have more than 2 posts and it's been more than 2 days hasn't it?)

http://img221.imageshack.us/my.php?image=image1gk7.gif

[wurpfel]

maybe you look for a "external" PCB.
I made I2Cbusnodes (ATMEL ATt15 or ATt26) with integrated power-unit.

you can use the L6205-dual-driver (4A and 40V) for one stepper or two brushed motors.

also for the 3ph-motors: you can check commercial dropin-modules.
or look on old CDR-boards for 3ph drivers, works often up to 1A and 20V!


the poties can be read-out by your pic using the ADs..
or check the austriamicrosystems hall-encoder up to 12bit resolution


a 3ph-esc is a snap when you know the rotor position..
for taller engines I use smartFETs and a logic-to-FET-driver. they offer current-sence and temperatur-protection up to 49V and 50+A.
_________________
not to old to play with scrap