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Author Topic: Servos  (Read 6236 times)

MEgg

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Servos
« on: September 30, 2015, 06:00:28 PM »
Did anybody ever try these servos:
http://www.crustcrawler.com/motors/AX12/index.php?prod=63
?

Thx in advance
1st project: Dagu 5 Rover + Dagu - 4 Channel DC Motor + Red Back Spider robot controller + Raspberry B+
Chassis + wheels: https://picload.org/image/dggroior/20150831_028.jpg
current: https://www.keepandshare.com/userpics/m/a/r/k/usegg/2016-04/sb/img_3480-79682018.jpg

Nemesis

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Re: Servos
« Reply #1 on: October 02, 2015, 01:00:55 PM »
Nop, I didn't try them... but for the price, use the drs-0101 from Dongbu instead : http://www.dfrobot.com/index.php?route=product/product&product_id=963
I have a bunch of them and they are just great for the price !

Duane Degn

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Re: Servos
« Reply #2 on: October 02, 2015, 03:51:36 PM »
I haven't used the AX-12A but I have a good number of AX-12+ (the older model) servos.

They're expensive but they're pretty cool. You get all sorts of feedback and control with these things.

If you purchase some, I'd suggest getting them from a plumber free shop like Trossen Robotics. Edit: Crustcrawler appears to be free of the plumber also.

Trossen has lots of good information about Dynamixel servos.

HobbyKing has started selling some smart servos.

I have the non-"D" version of this one.

It uses a RS-485 signal which is more immune to interference. I have the servo wired up but I haven't written the test code for it yet.
« Last Edit: October 02, 2015, 06:09:09 PM by Duane Degn »

erco

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Re: Servos
« Reply #3 on: October 02, 2015, 05:42:08 PM »
If you purchase some, I'd suggest getting them from a plumber free shop like Trossen Robotics.

I'm afraid to ask what "plumber-free" is, but I can almost guess. IOW, we should buy from American distributors?

Smashley

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Re: Servos
« Reply #4 on: October 02, 2015, 05:45:42 PM »
I believe that is a reference to Luigi's brother, who happens to be CEO of some other website that will remain nameless  :)

Duane Degn

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Re: Servos
« Reply #5 on: October 02, 2015, 06:08:05 PM »
I think I'm wrong about CrustCrawler being infected.

I can only find information about Lynxmotion being corrupted. I must be misremembering about Crustcrawler.

I Crustcrawler is plumber free then it's likely a good place to purchase stuff. I know I've purchased from them in the past.

I think Mike G. (not Green) (from Parallax forum) was a big shot at Crustcrawler at one time. My apologies to Crustcrawler.

Duane Degn

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Re: Servos
« Reply #6 on: October 13, 2015, 11:50:45 PM »
I've been experimenting with my AX-12+ servos. I was hoping to use the one servo as a force feedback input device but I found if the torque was on continuously to push back against my hand, the servo quickly overheated and turned off.

My first attempt to get the HobbyKing servo which uses a RS485 connection didn't work. The problem could have been the cheap PCB I had purchased from China. I'll give it another try with a different com chip.

fad0

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Re: Servos
« Reply #7 on: November 07, 2015, 12:57:46 PM »
I'm looking at the AX-12W. These servos look pretty complicated to use.  I'm assuming each servo needs a dedicated UART and frame packets are sent to control and received for feedback.  Do I have that correct?

Duane Degn

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Re: Servos
« Reply #8 on: November 07, 2015, 10:31:38 PM »
I'm looking at the AX-12W. These servos look pretty complicated to use.  I'm assuming each servo needs a dedicated UART and frame packets are sent to control and received for feedback.  Do I have that correct?

IIRC the AX-12W is very similar to the AX-12A. It's the version intended to be used to drive wheels (hence the "W").

The communication protocol is the same as the AX-12A. A single UART is used to communicate with up to 253 servos. Each "servo" gets set to a different ID number so when commands are sent only the servo with the ID number in the message will act on the command.

I'm not sure if I'd recommend the wheel version of these servos. It seems like one could get a pretty decent motor with encoder for the price of an AX-12W.

I suppose the main advantage to these motors/servos is the ability to send speed commands and the motor will automatically control the speed.

I used AX-12+ (the older version of AX-12A) to power my omnibot. I wasn't very impressed with the speed control. I particularly thought the low speed control was not very accurate.

I had to make adjustments to the speed algorithm to compensate for the speed control. Still I was able to get my omnibot to "spin straight" as seen below.


Is there a particular reason you want to use the AX-12W?

BTW, the communication protocol of the Dynamixel servos is a very good protocol to learn how to use. It sends raw data values instead of ASCII characters. This is a big savings when it comes to the number of bytes required to send the data.

erco

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Re: Servos
« Reply #9 on: November 08, 2015, 03:04:28 AM »
That's an awesome straight-line demo, Sir Duane! Sounds easy until you try it. Was that using a compass for heading, or brute force calibration? You're definitely smart enough to have used the encoders to keep everything in sync. Fine job, me so jealous!

fad0

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Re: Servos
« Reply #10 on: November 08, 2015, 11:16:02 AM »
I agree.  Really cool the way it moved.  Reminded me of the Cyclonebot https://www.youtube.com/watch?v=wkUT8UcUFaw.  It was designed using the Altera Cyclone FPGA (I think the Cyclone III), which I was one of the designer of.

I want to move up to much bigger platform than the BOE bots.  Something Arlo size or maybe slightly bigger...around knee height.  But I'm not sure if I'm ready...

I guess not matter what, I should move over to motor/encoder for wheel drive.  The Rover 5 looks like a great deal, but again, I want something bigger.

Duane Degn

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Re: Servos
« Reply #11 on: November 23, 2015, 05:01:06 PM »
or brute force calibration?

It was our old friend trial and error.

This robot doesn't have any encoders I can access. The AX-12+ "servos" have a speed command but it's not very precise at low speeds (or at high speeds).

 

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