Message thread on Webots Programming

big lag (100ms) between measured joint position and target, using Atlas model (webots 7.4.3)by gb sun (77) 2016-11-11 17:10:44
Hello Everyone,

As the title shows, there is aroung 100ms delay from command to measuremnt of joint positions (attached). The time_step for controller is 10 ms. This seems to be causing problems for walking stability. What might be wrong??? Thank you very much.

Best Regards,
Guangbin

Attachment: atlas_angles.png.
Re: big lag (100ms) between measured joint position and target, using Atlas model (webots 7.4.3)by Fabien Rohrer (Webots PRO - 1173) 2016-11-14 08:39:05
Re: big lag (100ms) between measured joint position and target, using Atlas model (webots 7.4.3)by gb sun (77) 2016-11-14 14:47:11
Thank you for the answer. But the problem seems different from touch sensor delay. The 100ms delay of joint command seems too big. Is there a way to adjust the delay?

Best Regards,
Guangbin
Re: big lag (100ms) between measured joint position and target, using Atlas model (webots 7.4.3)by Stefania Pedrazzi (Webots PRO - 460) 2016-11-15 14:26:05
Hi,

I'm sorry but I don't see the measurements you are talking about in your plot because there are too many lines and no legend.

How are you setting the joint position? using the Motor API or the Supervisor API?
Note that the Motor API sets the target position, but the joint is not immediately moved to the target position.
Instead the movement from the current to the target position is computed based on the physics of the object and the Motor specifications, like the maximum force/torque, maximum velocity etc.

If you suspect a bug, I would suggest you to post your project so that we can inspect it.
But note that Webots 7 is no longer supported, you should check on Webots 8.

Best regards,
Stefania
Re: big lag (100ms) between measured joint position and target, using Atlas model (webots 7.4.3)by gb sun (77) 2016-11-20 16:44:15
Hi Stefania,

Thank you very much for the kind reply and really sorry for the late response.

Yes, I used wb_motor_set_position to set positions. I always measured the real angle positions just before a set position command. I understand it is normal to take some time to reach a target. But It seems to be too big delay (100ms).

I wrapped my project and shared it in this url:
https://www.dropbox.com/s/kov31g19eqndbd4/atlas0.rar?dl=0
Please try the 'atlas_simple_bending.wbt'. Then a file named 'save.mat' is generated. You can run plt.m to show the curves of measured and desired angles.
Webots 8 might be a good option, but unfortunately my graphics card could be too slow to run it.

Best Regards,
Guangbin
Re: big lag (100ms) between measured joint position and target, using Atlas model (webots 7.4.3)by Stefania Pedrazzi (Webots PRO - 460) 2016-11-21 09:37:40
Hi,

I checked your world using both Webots 7.4.3 and Webots 8.5 and the plot I get is different from the one you posted (see file attached). The behavior seems reasonable andI don't see any particular issue applying the desired position.
The grid lines are missing but it also seems that in your plot the lag is smaller than 100ms.

Note that you should wait until the desired position is reached before setting the next one in order to get more accurate and stable results.

Best regards,
Stefania

Attachment: plot.jpg.
Re: big lag (100ms) between measured joint position and target, using Atlas model (webots 7.4.3)by gb sun (77) 2016-11-21 10:56:49
Thank you so much for the quick reply. I think you possibly need change the 3rd line in plt.m to the following:
figure;h1=plot(Angle_meas); hold on; h2=plot(circshift(Angle, 0),'--'); hold off
Where the offset 10 points was previously used to find the delay.

Then you should find the big delay exists.

Your plot has been offseted by 10 points (as can be seen around the beginning period of the plot).

I totally agree that the next command should be issued after the previous one is done. However, the big delay might affect the stability. If anyway to handle this, please kindly teach me. Thank you very much.

Best Regards,
Guangbin
Re: big lag (100ms) between measured joint position and target, using Atlas model (webots 7.4.3)by gb sun (77) 2016-12-08 05:51:19
The problem was already solved by changing pid gains to much higher values. I report here in case it be useful to other users. Thank you very much.

Best Regards,
Guangbin
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