RoboSavvy Forum

Dynamixel Potentiometer Lifetime Problem Solved

2012-09-29T03:46:16+01:00

Hi Befi;
I want to use AS5045. Please tell me how to contact you for programming and schematic of the modification.

Statistics: Posted by MOHIT JINDAL — Sat Sep 29, 2012 3:46 am

Dynamixel Potentiometer Lifetime Problem Solved

2010-06-22T19:57:53+01:00

Any further development since March ?
Seems that the magnetic rotation encoder is gradually gaining popularity (see openservo and reprap)

Statistics: Posted by limor — Tue Jun 22, 2010 7:57 pm

Dynamixel Potentiometer Lifetime Problem Solved

2010-04-11T20:23:51+01:00

Hi, thanks for posting comprehensive discussion regarding the potentiometer problem. I am looking forward for more info especially on linear potentiometer

EMAIL TO SPAMMERS:
From: limor
To: sales@spectrasymbol.com
dateTue, Apr 13, 2010 at 11:35 AM
subjectyour softpot product

we are a UK distributor of robotics products and your SoftPot potentiometer product line looks quite interesting but I'd like some clarifications first regarding your resorting to annoying spamming techniques to drive traffic to your site. see this link

Statistics: Posted by mickey05 — Sun Apr 11, 2010 8:23 pm

Dynamixel Potentiometer Lifetime Problem Solved

2010-03-16T14:54:12+01:00

Hi everyone!

We just received two repaired/modded servos (1 RX28, 1 RX64) from Bennet and I wanted to share our first impression.
We haven't done any intensive testing, yet, but we've tried out moving to different goal positions and recalibrating using Bennet's tool.
The servos are working as promised and are compatible to the normal Robotis ones.
The calibration tool works fine (even though I hope we'll have to use it only once per servo .

Once we have done some more testing I'll post more data. (may be in a few weeks)

Many thanks to Bennet!

Cheers,
Dorian

Statistics: Posted by Doms — Tue Mar 16, 2010 2:54 pm

Dynamixel Potentiometer Lifetime Problem Solved

2010-03-12T14:55:27+01:00

I won't publish it yet because there are some things to do. Mainly cleaning up the code and GUI. Maybe monday a version for publishing will be ready.
But for feeding your curiosity I post the interesting part:

Code:: def write_calib_click(self, widget, data=None): DEG_PER_STEP = 360.0 / SENSOR_STEPS ANG_MES0 = float(self.p2ang0.get_text()) ANG_MES1023 = float(self.p2ang1023.get_text()) ANG_OFFSET_MES = float(self.p2midang.get_text()) DEG_RANGE_MEASURED = ANG_MES1023 - ANG_MES0 if DEG_RANGE_MEASURED < 0: DEG_RANGE_MEASURED += 360 if DEG_RANGE_MEASURED < 200: dia = gtk.Dialog("Did you accidentally swap the two angles?") dia.add_button("Did you accidentally swap the two angles? (0 and 1023)", 0) dia.show() return DEG_RANGE_ERROR = DEG_RANGE_MEASURED - DEG_RANGE_WANTED STEP_RANGE_ERROR = DEG_RANGE_ERROR / 2 / DEG_PER_STEP CALIB_DOWN = int(CALIB_DOWN_DEFAULT + STEP_RANGE_ERROR) CALIB_UP = int(CALIB_UP_DEFAULT - STEP_RANGE_ERROR) CALIB_OFFSET = int(1024 - ANG_OFFSET_MES / DEG_PER_STEP) self.servo1.reset_servo() print "DEG_RANGE_MEASURED ", DEG_RANGE_MEASURED, " DEG_RANGE_ERROR ", DEG_RANGE_ERROR, " STEP_RANGE_ERROR", STEP_RANGE_ERROR, " CALIB_OFFSET ", CALIB_OFFSET

Statistics: Posted by befi — Fri Mar 12, 2010 2:55 pm

Dynamixel Potentiometer Lifetime Problem Solved

2010-03-12T14:02:29+01:00

I would be interested in looking at your open source to better understand what it is you are doing.

Statistics: Posted by Bullit — Fri Mar 12, 2010 2:02 pm

Dynamixel Potentiometer Lifetime Problem Solved

2010-03-12T13:54:40+01:00

Ok I think I wasn't clear enough. The calibration program only works with servos I have modified with the Austria Microsystems sensor. The program is completly useless for standard servos. Therefore I will only deliver it with the modified servos. For giving an impression how the calibration works:

You set the servo position to 0, 511 and 1023 and note the respective angles. After that it's just a single click for calibrating the servo.

Statistics: Posted by befi — Fri Mar 12, 2010 1:54 pm

Dynamixel Potentiometer Lifetime Problem Solved

2010-03-12T13:28:16+01:00

Very interesting.
Please point us to your open source.

Statistics: Posted by Bullit — Fri Mar 12, 2010 1:28 pm

Dynamixel Potentiometer Lifetime Problem Solved

2010-03-12T12:52:37+01:00

For the question about mounting the magnet on the shaft:

The big one is for the RX64, the small one for the RX28.

I'm making the calibration software open source. This helps you to freely chose the middle-position of the servo and also the angular range.

In our experiments we saw that especially the original RX28 servos are very bad calibrated. The typical angular range error was about 25°.
By using our solution each servo behaves exactly like every other servo. They reach exactly the 300° angular range and keep it.

Statistics: Posted by befi — Fri Mar 12, 2010 12:52 pm

Dynamixel Potentiometer Lifetime Problem Solved

2010-01-26T16:07:27+01:00

Hi Dorian

It seems as you have also gone very far in analyzing the servos!

In fact there are two calibration methods inside the servo. One is the linear method you mentioned (using the UP und DOWN calibration and another hidden parameter), the other is a bit more complicated and is indeed not linear. AFAIK all our servos (RX-28 and RX-64) are using the non-linear method.

And you are right, the sensor from Austria Microsystems is linear. It does not offer a method for calibrating its charactaristic.
Because of that, we used a method to 'trick' the robotis calibration to work together with these sensors.

We haven't yet put a price tag on our modification as the price will depend a lot on the demand. But it will definetly be just a fraction of the price for a new servo.

We hope for you that there will be some interesting opponents for you at the German Open (even if the probability is fairly low... ). But I'm looking forward for our next encounter!

Bye,
Bennet.

Statistics: Posted by befi — Tue Jan 26, 2010 4:07 pm

Dynamixel Potentiometer Lifetime Problem Solved

2010-01-26T15:26:26+01:00

Hi Bennet, hi Hamid,

this is Dorian form the Darmstadt Dribblers.
First of all congratulations on this very nice solution!
As you know, we are experiencing the same servo problems.
But we have not gone as far in modding as you have!
Our main focus is on improving the servo firmware.

But I have a question about your solution.

Since you are not modifying the firmware, I assume you are doing the 'calibration' on the sensor to make it's analog output signal 'emulate' the one of the original potentiometer.

My understanding of the original firmware suggests, that Robotis is calibrating an upper and lower limit of the ADC input for the 0 resp. 300 degree positions of the poti.
These limits typically are around 30 and 980 (if I recall correctly) reducing the available values to about 980 - 30 = 950.
Since the ADC of the Atmega8 is only 10 bits and the servo position is also reported in 10 bits (0..1023) the remaining 950 values of the ADC have to be mapped onto the 1024 values of the servo position.
From talking to Robotis engineers and some experiments I found that this mapping does not seem to be linear.

Have you made similar experiments? And what are the results?
How do you cope with the non-linear mapping in the Robotis firmware, since your new sensor is probably linear?

Thanks by the way for offering your solution to others!
Have you already though about a price for the modding?

We are very sorry to hear that we wont see you at the German Open!
So best of luck with your preparations for Singapore!

Cheers,
Dorian

Statistics: Posted by Doms — Tue Jan 26, 2010 3:26 pm

Dynamixel Potentiometer Lifetime Problem Solved

2010-01-26T14:07:29+01:00

i-Bot wrote:
Do you modify the code in the servo ? I know Limor was interested to talk with people who had done that for one of his projects.

First versions required a changed firmware but we found a solution for doing all the adaptions without changing the firmware. This implies that we now have an in-depth knowledge of the robotis firmware

Bullit wrote:
It looks like you are using Austriamicrosystems AS5046.

http://austriamicrosystems.com/eng/Prod ... ers/AS5046

I'm curious how are you mounting the magnet on the main shaft?

It's true that I'm using a sensor from AM, currently the AS5043. The mounting is done by a custom part for the RX-64 which replaces the original plastic part. Since all parts are now integrated in our Robots I am not able to show you a picture. For the RX-28 the construction allows to mount the magnets directly onto the axis.

Statistics: Posted by befi — Tue Jan 26, 2010 2:07 pm

Dynamixel Potentiometer Lifetime Problem Solved

2010-01-26T13:42:36+01:00

It looks like you are using Austriamicrosystems AS5046.

http://austriamicrosystems.com/eng/Products/Magnetic-Encoders/Rotary-Encoders/AS5046

I'm curious how are you mounting the magnet on the main shaft?

Statistics: Posted by Bullit — Tue Jan 26, 2010 1:42 pm

Dynamixel Potentiometer Lifetime Problem Solved

2010-01-26T11:45:20+01:00

That's very high credibility on this forum. Thx to you both for posting.

The pictures of the modification look really clever and very well executed.

Most of the RX servos are in college and Universities, so I guess most interest will come from there. I wish I could afford the RXs and the modification myself!

Do you modify the code in the servo ? I know Limor was interested to talk with people who had done that for one of his projects.

Statistics: Posted by i-Bot — Tue Jan 26, 2010 11:45 am

Dynamixel Potentiometer Lifetime Problem Solved

2010-01-26T11:27:56+01:00

Hi everyone,

I confirm Befi's identity. He is a member of my team and has done a great job to solve the problem we had for a while with the servos.

It is really sad. Just after you are finished with the software development, one or more servos start to vibrate. This is then slowly transfered to all other servos and after a while you have a totally useless robot.

I hope his solution can also help others.

Best regards,
Hamid.

Statistics: Posted by hamid_m — Tue Jan 26, 2010 11:27 am