POST No. 2406757
MX28T not moving to its goal position
2018-04-10 14:03:38 MedexRobot


I am using the MX28T with 12 V supply voltage.

It is lifting / rotating a weight of 300 grams against the gravity.

I have rotated with the rpm of 10 and 20..

In both the cases i tried to rotate from 0 degrees to 50 degrees. So i set the goal position( Value : 569 ) for 50 degrees in the dxl memory.

When i executed the command to rotate, it moved upto 48.25 degrees and stopped. It is the same case with both the rpm..

Why it is not moving upto 50 degrees? Is there any other settings to be made additionally?

If i give the angle as 10 degrees, i can get the rotation up to 9.8 degrees.. 

How do i get the precise position? 

2018-04-10 14:03:38
2018-04-11 09:39:07 Will Son


As the load gets heavier, it becomes difficult to follow the exact goal position against gravity.

If the 300gram object is attached closer to the Dynamixel, it'll more precisely follow the goal position.

It is better approach to calculate the actual torque required by Dynamixel. 

Usually it is recommended to use about 20% of stall torque.

As Dynamixel uses the PID controller, you can increase P gain to generate more torque which will generate bigger torque near the goal position.

However, you might still not be able to get the exact goal position and this might cause some vibration depending on your system configuration.

In that case, you can adjust D and I gains.

Thank you.

2018-04-11 09:39:07
2018-04-11 19:38:49 Medex Teknologies Private Limi

Yes thank you...

When i set the ( P = 0, I = 0. D = 0 ), i have got the very poor backlash, somewhat free rotation. The shaft rotates when i rotate it manually by hand. 

Then i set the ( P = 32, I = 0. D = 0 ), i have got some decent backlash, somewhat near 2 degrees of free rotation. 

Then i tried with ( P = 100, I = 0. D = 0 ), i got same decent backlash, somewhat near 2 degrees of free rotation. 

But motor starts to vibrate on its own. How do i set the PID gain value hence i will get very good backlash and also correct goal position? 

Is there any relation to be followed while setting the P,I,D values?

2018-04-11 19:38:49
2018-04-12 09:44:36 Will Son

Each P, I, D gain values contributes various effects.

You can find more details in Wikipedia, especially look for the Manual tuning section which describes the effect of each gain with an animated graphic.

There is no magical value that works for all system, so you have to undergo some trial and error procedure by adjusting each gains.

Again, if the required torque to lift the object gets larger, PID won't be able to compensate the position discrepancy.

2018-04-12 09:44:36
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