Robotic arm humming in admittance control

[xuxiang817]

Hi,

I have successfully run the script admittance_control.script. It's amazing!

Question 1

But when i drag the TCP, the robot emits a humming sound. The humming intensifies when dragging rapidly.
Is this normal behavior ?

My robot :
robot mode: UR5e
tcp mass: 4KG
tcpoffset=[135.76，135.76，126.51，0.0146，0.0437，-2.3713]

My input params:
mass_scaling = 0.5
damping_scaling = 0.5
mass_list = [40, 40, 40, 4, 4, 4]
damping_list = [120, 120, 120, 12, 12, 12]

dead_band = [3, 0.1, 3, 0.1]
compliance_vector = [1, 1, 1, 1, 1, 1]
stiffness_params = [0, 0, 0, 0, 0, 0]
target_wrench = [0, 0, 0, 0, 0, 0]

base_to_compliance_frame = p[0, 0, 0, 0, 0, 0]
tool_flange_to_compliance_center = get_tcp_offset()

Question 2

Based on the parameters entered in Question 1
If you want to achieve rotation around the TCP center, you must do it this way:
tcpoffset = get_tcp_offset()
tool_flange_to_compliance_center = [tcpoffset[0], tcpoffset[0], tcpoffset[0], 0, 0, 0]
Not
tool_flange_to_compliance_center = tcpoffset

why ?

Thanks

[urrsk]

Hi @xuxiang817
Thanks for the question.

Question 1)
It is normal that you have to tune that mass, damping and stiffness according to your objective. So please tune the values so it does not make the humming. The humming is not good for the robot.
I expect that part of your humming is due to your definition of the compliance center. Which you have defined 229 meters way from the tool flange. This will significantly scale up the impact of the mass inertia.

Please verify that you actually want a TCP of 229 meters?!
or did you mean tcpoffset = p[0.13576，0.13576，0.12651，0.0146，0.0437，-2.3713]?

At UR we use SI unites in general, so in this case you define your pose with meters and radians.

Question 2)
Interesting that you have problems with the orientation. I will look into that.
Though, please confirm that
tool_flange_to_compliance_center = [tcpoffset[0], tcpoffset[0], tcpoffset[0], 0, 0, 0]
should be:
tool_flange_to_compliance_center = p[tcpoffset[0], tcpoffset[1], tcpoffset[2], 0, 0, 0]

[xuxiang817]

Thanks for you reply.

You are right. I made a basic mistake.

my tcpoffset = p[0.13576，0.13576，0.12651，0.0146，0.0437，-2.3713]
tool_flange_to_compliance_center = p[tcpoffset[0], tcpoffset[1], tcpoffset[2], 0, 0, 0]

My robot :
robot mode: UR5e
tcp mass: 4KG
tcpoffset = p[0.13576，0.13576，0.12651，0.0146，0.0437，-2.3713]
URSoftware 5.12.3.xxxxx

About Question 1 , This issue maybe be resolved through a version update. The image below shows: 5.14.X release notes.

About Question 2, If you use it this way
tool_flange_to_compliance_center = p[tcpoffset[0], tcpoffset[1], tcpoffset[2], 0, 0, 0] It can rotate normally around the TCP center.
Otherwise,
tool_flange_to_compliance_center = p[tcpoffset[0], tcpoffset[1], tcpoffset[2], tcpoffset[3], tcpoffset[4], tcpoffset[5]] it won’t rotate around the TCP, and dragging feels out of control.

One more question:
if my compliance center is defined below the TCP (e.g. 0.3 m along the TCP Z direction), and it is not rigidly connected to the TCP, would your solution still work?

If I define a compliance center and its own compliance frame, does tool_flange_to_compliance_center need to be computed dynamically?

for example:
my new compliance center

tcp_to_sphere_center = p[0, 0, 0.3, 0, 0, 0]
initial_tcp_pose = get_actual_tcp_pose()
compliance_center= pose_trans(initial_tcp_pose,tcp_to_sphere_center)

while True:
     local wrench_at_tool = adm_apply_dead_band_smooth(adm_get_flange_wrench(), dead_band[0], dead_band[1], dead_band[2], dead_band[3])
     local current_flang_pose = get_actual_tool_flange_pose()
     local tool_flange_to_compliance_center= pose_trans(pose_inv(current_flang_pose), compliance_center)
     local wrench_at_compliance_center = wrench_trans(tool_flange_to_compliance_center, wrench_at_tool)
         .......
end

thanks

[urrsk]

@xuxiang817
Q1)
Yes please update to the latest. There have been a lot of good stuff been added since 5.12. Including the fix of speedl!

Maybe you might also be interested in direct_torque control we added lately in 5.24 as well. See https://github.com/UniversalRobots/URScript_Examples/tree/main/impedance-control

Q2)
Okay, what you are say is that when you define a rotation part to the compliance center, the controller does not work as intended?

Q3)
The compliance center is defined from the tool flange to the compliance center (Where you are holding your hand for guiding). It should work no matter what tcp offset is given.

[xuxiang817]

@urrsk Thank you for your help！

Q2）
I missed a crucial detail: compliance_vector = [0, 0, 0, 1, 1, 1]
My goal is to make the TCP rotate around the TCP center.
If you use it this way
tool_flange_to_compliance_center = p[tcpoffset[0], tcpoffset[1], tcpoffset[2], 0, 0, 0] It can rotate normally around the TCP center.
Otherwise,
tool_flange_to_compliance_center = p[tcpoffset[0], tcpoffset[1], tcpoffset[2], tcpoffset[3], tcpoffset[4], tcpoffset[5]] it won’t rotate around the TCP, and dragging feels out of control. More precisely, when I drag the TCP , I wanted it to Rx+, but it actually Rx-, Its rotation direction is incorrect.

Q3）
My goal is the same as in Q2. However, I need to redefine a virtual compliance center as the sphere center.

If possible, could I communicate with you via email?

[urrsk]

If possible, could I communicate with you via email?

Yes, write me on rsk at universal-robots.com