low-cost programmable servo controller?

I'm looking for a small, low-cost programmable robot controller with 4 servo outputs. Ideally, something based on an Atmel chip, but that's
not a firm requirement.
Pololu's Baby Orangutan controller is great, tiny and only $25, but it has onboard motor controllers rather than servo controllers. So, unless I'm missing something, I'd have to add a serial servo controller for another $20. Still not too bad -- but can anyone suggest something better, maybe some all-in-one deal?
Thanks, - Joe
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wrote:

http://www.pololu.com/products/pololu/#servcon
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shb*NO*SPAM*@comporium.net (Si Ballenger) wrote:

Yes, that's the $20 serial servo controller that I mentioned having to add to the Baby Orangutan to make a complete solution (i.e. one that's programmable, and doesn't need a computer telling it what to do).
But can you suggest anything cheaper than this plus the Baby Orangutan? Maybe something that does it all in one board?
Thanks, - Joe
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Joe, what are your requirements ? as in max $$ to spend, and a brief description of what you want the board to do? We might be able to help more if you can scribble that out for us.
Having said that, I just bought a brand new complete serial version BoeBot kit off of eBay for around $80 just to have a cheap bargain basement controller with servo capability for a much larger robot I am developing. The BOE has 4 connections for servos designed into the board, and offers some amount of programmability for sensor inputs, etc. I specifically went for the BoeBot because I am familiar with it from prior use and knew it had the servo connections broken out on the board, plus comes with 4 AA battery power pack, etc. Includes 2 continuous motion servos, also. The thing I like about the Parallax products is that due to thier marketing exposure I can resell them back on eBay when my need for them is over. It could get you going until you find th100% ideal solution.
Hope that helps a little bit! JCDeen
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Sorry, I thought I was clear about that: I want a programmable controller that with four (or more, obviously) servo outputs. As for max $$, well my current best plan is a Pololu Baby Orangutan ($25) plus a Pololu Micro Serial Servo Controller ($20) = $45. So I'm asking whether there's something less than $45 that will do the same job.

Hmm... I'll look into that. Thanks for the tip!
Best, - Joe
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You were clear. My response to you was not so clear! :-)
What I should have said is "what do you want the microcontroller to do besides control 4 servos?". But, having said that, I can see where I also might have asked the exact same question the way you did --- and probably have on this same forum. My apologies. And good luck!
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Oh, I see! Nothing much. Maybe run through a demo sequence of moves, or listen to wireless control signals of some sort, and fire off the corresponding sequence of servo motions. Nothing that would tax any microcontroller, I imagine, but more than a nonprogrammable servo controller can do by itself.
Thanks, - Joe
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You will notice that the Basic Stamps are *not* servo controllers. They can send a pulse out to a servo, but then they're dead for doing anything else until that pulse finishes. If you look inside the cpu during that time, you'll see it's doing nothing except waiting for the pulse to end. By time you get to running 4 servos, with 20-msec refresh rate, you've used almost 50% of the cpu duty cycle.
That being said, I love the Stamps, but they're not too good for any kind of multitasking. They will start to bog down trying to do everything else alongside the servo pulses.
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If I use an Atmel controller (like the Baby Orangutan), then maybe I could control the servos myself. Starting and stopping pulses shouldn't be a big deal -- I can just set up an event queue, and probably manage lots of servo outputs that way.
But, unless I really complexify my code, or use something like the RTOS described in the latest SERVO mag, it's going to be a pain in the neck to do anything else while managing this event queue. Or rather, if I start doing something else, and that something takes too long, then the pulse trains could get screwed up, with unhappy results. So it'd really be nice to just hand off the desired servo position and speed to a separate chip, in a "set and forget" sort of way.
Best, - Joe
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Joe Strout wrote:

Joe:
You are slowly reinventing the modular approach to doing things. A dedicated microcontroller to keep the pulses going out to 4 servos does not cost very much. I'll point you to my solution:
<http://gramlich.net/projects/rb2/servo4/index.html
Enjoy,
-Wayne
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Well, not if I can help it -- I'd much rather buy something someone else has invented!

Yes, but like the Pololu Micro SSC, this isn't programmable by itself either; I'd need to combine it with something else that does the "thinking." Which, I agree, is modular, but less than ideal for this project (a very small robot with tight space and cost constraints).
Best, - Joe
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Joe Strout wrote:

Joe:
I can not begin to guess what all of your constraints are. If you tell us the entire project, we might be able to give you better advice. If you list all of your constraints, we can give better suggestions. By the way, the word "animatronics" is used to talk about systems where a bunch of servos are run through a set of preprogrammed moves (i.e. not much feedback.) If you search for "animatronics servo" using your favorite search engine, something might turn up.
-Wayne
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Well, I think I've listed them all: cost under $45, programmable, controls at least 3 servos, and takes up as little space as possible. That's about it. Seems like one could make a board that had one chip (plus support stuff) to act as the programmable microcontroller, and another chip to drive a handful of servos; if these were all surface-mount parts, it should be pretty small and cheap. Kinda like what Baby Orangutan would be if the motor driver were replaced with a servo driver. That's what I'm looking for -- but it seems as though it doesn't exist.

Thanks, that's a good idea! A quick search seems to indicate that the animatronics community uses serial servo controllers, generally with a PC as the master controller. Makes sense for them -- they don't need portability.
However, that led to this very interesting comparison table:
<http://www.oricomtech.com/svc-comp.htm
It's not quite complete (omits the Pololu MSSC for example), but still pretty handy.
Best, - Joe
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Joe Strout wrote:

You have not been very specific about how small you need to get. I can easily imagine using two PIC16F688 chips in DIP14 sockets to get the job done. This could be made with point-to-point wiring on a small piece of perferated board. SMT would be smaller, but you would need to create a PCB for the chips.
[snip animatronic stuff]
-Wayne
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Joe Strout wrote:

There are any of a number of free trutorials on writing AVR and PIC based serial servo controllers. I think WG has provided one, but a quick Google should show lots more. For example, this was the first hit for the query 'serial servo controller':
http://www.rentron.com/SerialServo.htm
Since you're building the controller out a PIC, if the design has extra room (memory, CPU time), you might be able to do everything in one very inexpensive chip. Even if you add the $$ for a low-cost PIC programmer, you're still ahead compared to $45 for the Pololu pair. I doubt you will get any cheaper than that, as they have some of the lowest prices around.
Most people do not want an "all-in-one" pre-programmable servo controller, which is why you aren't finding many commercially-available options. There are a few; look at Pontech, for example.
-- Gordon
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Gordon McComb wrote:

Actually, that's why I built my Bot-Cop controller board. It has a 20-channel servo controller, with velocity-control, on the same pcb as a 24-pin Stamp/etc module. Gives you general programmability in HLL without incurring the overhead of generating pulses for 20 servos. The idea worked out really well for my hexapod walker, BTW. The pcb is a little large for Joe's app, from the sounds of it.
http://www.oricomtech.com/prod2/bcop-svo.htm http://www.oricomtech.com/projects/nico6prj.htm
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Thanks Dan -- that's pretty neat. The size doesn't look unreasonable (which is to say, I can probably shoehorn it in if I'm sufficiently motivated).
I really like the way you made it so it can run as a normal slave module if no 24-pin controller is installed. That offers a nice smooth upgrade path -- start without it, then when you want autonomous operation, buy one of those modules and plug it in.
Of course I suppose I get the same effect by starting with a Pololu MSSC (which I already have) and then adding a controller like the Baby Orangutan. But then I have to rig up my own connection between the boards, and my own DB-9 connector adapter, etc. You've got a leg up in terms of plug-and-playness.
It's certainly a neat option -- thanks for pointing it out!
Best, - Joe
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Parallax has a new chip out called the "Propeller". It has eight 32 bit processors (COG's) on one chip. With eight processors you can get some true multitasking done. Also, the price is nice and like the Stamp chips the development software is free. I'm looking to use this as the controller chip on a robot project. See link => http://www.parallax.com/propeller/index.asp
Steven

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And very recently (last weekend) geeks.com had the full BoeBot serial kit on sale for $59 each. I picked up 4 of them for much the same reason you describe, the parts alone are certainly worth more than that, but I wanted the BoeBot as an intro into robotics for myself anyway. It is my first robot.
As I write this my BoeBot is wandering autonomously around the house, looking for three specific IR sources (each source has a unique PRF), the sources must be found in a certain order and a specific "button" pressed to kill each source source. The next source is not a valid target until the source before it is detected, sought out, and shut down.
Right now the search is random, and I am making no attempt to cover ground only one time, but I have a couple of ideas to try for that in the next couple weeks or so.
C!
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