RoboSavvy Forum

The OpenServo

2007-02-13T12:56:19+01:00

Just a quick update on the OpenServo project:-

We now have bezier curve motion profile functionality! The servo will carefully follow the defined curve along its path.

Details here.
http://openservo.com/moin.cgi/Utilities

In addition I have created a large amount of documentation for construction through to use of an OpenServo. Code Samples will be ready soon.
http://openservo.com/moin.cgi/StepByStep

Barry

Statistics: Posted by ginge — Tue Feb 13, 2007 12:56 pm

The OpenServo

2007-01-24T00:29:40+01:00

Hi all,

There are a few people in this thread that have noted no documentation for OpenServo, so I have attached a few links to the docs.

http://openservo.com/moin.cgi/TWIProtocol
http://openservo.com/moin.cgi/ServoDocumentation
http://openservo.com/moin.cgi/ServoFAQ

It would seem Basic Stamp code is what people want to see, I guess I had better order one.

Barry

Statistics: Posted by ginge — Wed Jan 24, 2007 12:29 am

The OpenServo

2007-01-24T00:15:55+01:00

You can stack the FETs on the board to give you more current handling. You should be able to stack 2 or 3 of those chips jay found... This should be fine up to a certain limit (i.e when they catch fire) but nowhere near 91 Amps!

Barry

Statistics: Posted by ginge — Wed Jan 24, 2007 12:15 am

The OpenServo

2007-01-19T15:19:39+01:00

Billy,

You're right, it wouldn't be cheap. Rule No. 1 in robotics: Nothing is cheap. Or easy. Good luck though. Electronics is really fun once you start to get the hang of it.

Statistics: Posted by RobotJay — Fri Jan 19, 2007 3:19 pm

The OpenServo

2007-01-19T06:00:46+01:00

That sounds expensive! haha. One of my new years resolutions is to learn enough basic electronics to make such a device on my own. We'll see how that goes though and maybe I'll get back to you later. haha.

Statistics: Posted by billyzelsnack — Fri Jan 19, 2007 6:00 am

The OpenServo

2007-01-17T15:37:57+01:00

Without creating a new, custom design, the largest drop-in replacement I can find is here. This FET is only rated at 9A, which is MUCH lower than the 91A necessary for your gearmotors.

Sorry to say, but you will need a custom servo driver. Fortunately for you, the OpenServo can be fairly easily redesigned to give you what you need. E-mail me if you're interested in me redesigning a custom servo driver for you.

Statistics: Posted by RobotJay — Wed Jan 17, 2007 3:37 pm

The OpenServo

2007-01-17T08:27:21+01:00

What's the biggest FET that could be put on it without changing the design?

How much would you want for one? I bought a bunch of motor+gearboxes that were for paper shredders. The motor is a pretty large guy. I think it is this motor..

http://www.johnsonelectric.com/product/ ... odID=75--A

I did a rough calculation and it would have 8248 oz/in of torque! Should be about the same speed as a normal servo too at 50rpm.

Here's the blog entry I did on it awhile ago.. http://www.z425.com/robot/diy-servo-project/

Anyway.. I guess what would be the largest you could go and how much? btw. I don't really care about the footprint too much.

EDIT: haha.. I just looked at the stall current on that motor. 91 amps!

Statistics: Posted by billyzelsnack — Wed Jan 17, 2007 8:27 am

The OpenServo

2007-01-16T22:18:07+01:00

First of all, to Derek: Holy crap. I never realized that the HSR-5995's drew so much current. I bet batteries get drained SUPER fast at that rate. Either way, it would be a simple matter to drop in a higher rated MOSFET.

To Bren,
I'll start a new thread in the "homebrew" section with pics of my very first Robo-1 attempt. Hope you and the community enjoy.

DO NOT try to assemble an OpenServo by hand, unless you have God-like hand stability and the patience of 5 saints combined. All the components are surface mount and they are TINY. I personally cannot even solder the largest components by hand. I had to buy a lot of fairly expensive equipment just to be able to produce these. I am selling completed boards for $20/piece. E-mail me at j.ragsdale@pendragonrobotics.com if you want to order some. I offer discounts if you buy 16 at a time (enough for a 16 DOF biped,) but I suggest buying just one or two to play with and learn with before buying enough to fill out a whole biped.

Here's a quick peek at what the 4 wires are for:

1---> SDA (see explaination below)
2---> Battery (+)
3---> GND (-)
4---> SCL

These are the necessary wires for low-level I2C programming. Basically, SDA is how the information is sent in and out of the servo, and SCL is just a series of +5v clock pulses that all the servos use to synchronize with each other.

Unfortunately, there is no clear, easy, documented way to communicate with the OpenServo. Currently, the best method is to use the U2C-12 bridge from Diolan.

It is possible to use a BASIC Stamp 2 to communicate with the OpenServos, although it is not as easy as using the bridge from Diolan. Also, there is no documentation at the OpenServo website to specifically do it. The BASIC Stamp 2p series of stamp have built in I2C capabilities, but again, you would have to play with it for a while, and read a whole lot of datasheets to get it to work. You should start a thread in the OpenServo forums about using stamps to control the OpenServo, and we can possibly write the necessary documentation together.

I am currently working on an I2C bridge that that uses a wireless BlueTooth conection from Sparkfun. (i'm sure you can find other people around the community using the same set-up... its so easy to work with.) This is still a work in progress, and probably won't be finished until early March. When I finish that, I plan to fully support it with documentation and software.

I will admit that the OpenServo website is fairly hard to navigate at first, because it's so hard to know where to start. The forums have much greater amounts of info than the main site, so make sure you are reading through those as well. I suggest posting in the forums with ANY questions you happen to have. The admins are nice, and there is no such thing as a dumb question. Chances are, if you're asking it on the boards, then there are about 50 lurkers who are asking the same thing to themselves. Hope this answers all your questions. Talk to you soon.

-Jay

Statistics: Posted by RobotJay — Tue Jan 16, 2007 10:18 pm

The OpenServo

2007-01-16T19:20:29+01:00

Robo1 wrote:
DerekZahn - what servo do you use that could pull more then 3A!

The HSR-5995TG has a stall current of 5.2A at 7.4v. If I were to try to build my own someday for Bing 4 (an idea that sounds kind of fun) I'd try for even higher power... although using a higher voltage motor would be better than increasing the current.

Statistics: Posted by DerekZahn — Tue Jan 16, 2007 7:20 pm

The OpenServo

2007-01-16T18:49:53+01:00

Another thing when I mean a dummies guide I use a stamp processor for trying out low level stuff so example code and a diagram would be good.

if you want me to move these question over to open servo I would understand.

thanks bren

Statistics: Posted by Robo1 — Tue Jan 16, 2007 6:49 pm

The OpenServo

2007-01-16T18:46:40+01:00

DerekZahn - what servo do you use that could pull more then 3A!

RobotJay two question:

One could you post a proper picture of you robot (the on in you author image). with some detail looks good.

second, are the two wire send e.g. position, recieve e.g. current, position, force.

How much for one open servo board with the parts already attached, shakey hand soldering

If I did buy this what sort of docs is there I've looked at the site and there is some but a dummy guide would be good for the communication protocol. I'm a good high level programmer but low level communication isn't my strong point but at I'm trying to learn.

you guy keep the good work going over at open servo hopefully I can start to contribute once I have a demo going and get a little bit more stuck in.

bren

Statistics: Posted by Robo1 — Tue Jan 16, 2007 6:46 pm

The OpenServo

2007-01-16T17:15:34+01:00

Yeah, in quantity 10 they are 2.65 instead of 0.85 (US$), but 3A stall isn't enough for high-performance servos. Nice to know there's an alternative that could be dropped in for high-end servos though, even if it adds $3.60 per board.

Statistics: Posted by DerekZahn — Tue Jan 16, 2007 5:15 pm

The OpenServo

2007-01-16T17:08:10+01:00

The IRF7389 would be a drop-in replacement for the 7309. The price though, is 4 times as much (through DigiKey, at least).

And yes. There are a TON of uses for the current sense ability. I am certain that eventually it will become standard on most digital servos.

Statistics: Posted by RobotJay — Tue Jan 16, 2007 5:08 pm

The OpenServo

2007-01-16T16:57:10+01:00

A couple of other uses for the current sensing:

* To help determine "natural" poses by trying to minimize current use.
* Controller software can look for extended periods of high current use and issue a warning to help avoid servo overheating.

I decided to add some current monitoring to Bing in order to get some of these advantages (pretty soon I'll start wondering why I didn't just replace the Hitec boards with OpenServo boards!)

For higher current, would it be possible to substitue the IRF7389 for the 7309?

Statistics: Posted by DerekZahn — Tue Jan 16, 2007 4:57 pm

The OpenServo

2007-01-16T16:36:15+01:00

Bren,

I just recently received an ICS servo from Limor, and I will attempt an install of the OpenServo PCB. As far as I can tell, +12v will be no problem. As long as the motor does not draw more than 3A at stall, it won't burn out the MOSFETs. All the other components are run from a 5v regulator, which can handle voltages up to 20v.

Not only can the OpenServo PCB be used to replace standard servo PCBs, but it can also be used to create your own servos. All it takes is a gearhead motor, a 4.7k Ohm linear-type potentiometer, and an OpenServo PCB. If, for some reason, you needed to be allowed a higher current draw, you can just drop in a different (probably slightly more expensive) MOSFET, and you could drive a motor with a lot more power.

To discuss Limor's concern over the necessary 4-wires: It is entirely possible to use only 3 wires. In fact, I am almost certain that we can even use the same PCB, we just would need to write new firmware for it. I think we might even be able to do something like this: if the servo doesn't receive an I2C command after 3 seconds, then it'll go into standard PWM mode, and only accept PWM position commands. Ultimately, I want the OpenServo to be an all-around replacement for other more expensive digital servos. If someone just needs a cheap digi, they can use these. If someone needs more precise control of their servos, they can use these. It's being worked on.

The biggest problem with the OpenServo is that none of us are getting paid to work on it. All of the people involved have day jobs, and only work on the OpenServo in their leisure time, so progress is slow. But the more people we have involved, the faster new features will be added.

The OpenServo has 2 main advantages over other digital servos:
1) Current sensing resistor on-board. When the servo begins to move a load, it draws a certain amount of current. The larger the load, the more current the servo will draw. If you biped is walking, and the foot encounters an obstacle, then there will be a current "spike" from the involved servos. You could simply write code for "If a servo's current spikes, stop motion and return to the pose where there was no current spike." ALSO, knowing how much current your servo is using will allow you to know precisely how much battery power you are using.
2) Open architecture. Students and hobbyists can learn about closed-loop motion control without having to worry about copyright infringement, resin covering important parts, etc.

Anyways, the OpenServo project is, and will always be, a work in progress. Anyone can take what they need from it. That's the beauty of Open Information Sharing.

Statistics: Posted by RobotJay — Tue Jan 16, 2007 4:36 pm