Do you have to set the robot's payload every time you pick up an object?

Tim_Smith

Hi DoF community,

I started the Universal Robots Academy training modules online. In the 5th module, they explain how to use the SET command in a program, they were using a gripper that requires a Digital Output to open/close the gripper. In the SET command, they were also changing the robot's payload when the gripper picked-up a part and when the gripper placed the part.

Does the robot's payload need to be changed every time the gripper picks up an object? How will this change the robot's behaviour?

matthewd92

We generally set the payload if it changes by more than 250 grams. What you notice is that the robot will be smoother in operation, especially at higher speeds and towards the end of the reach of the robot. Also, depending on what the difference is you may find that the robot is havingnprotective stops because the momentum seen during accel or decel does not match what is expected based on the payload that the TCP is set to. 

Another thing to also consider is making sure that the CoG for the TCP is set correctly. I believe by default it puts the CoG at the end of the TCP coordinates.  

To make it easier if I am using a gripper that requires a digital output to operate I will write custom script functions for the open and close command and then embed the tcp weight setting there so all I need to do is say open_gripper() or close_gripper() in the program and the weight is set correctly

Etienne_Samson

Agreed with @matthewd92, you can ignore small payload variation but for both performance and to avoid any unexpected safety stop you need a dynamic adjustment of your payload.

When using the Robotiq Gripper you should always have a logic like this:

1. Move robot
2. Gripper close
3. Check for object
4. Adjust payload if you got the object
5. Move robot
6. Gripper open
7. Adjust payload

lakshmip001

For the maximum payload, it says it is calculate with a coefficient of 0.3. I would like to check that is it possible for us to change to something that have a higher coefficient of friction such that I can increase the weight of the item the gripper can hold?

lakshmip001

Also the grip opening is 85mm and 140mm. is it ok for us to change the grip plates at the front to something so that I can grip a wider item? As I see that the grip plate on each finger is secured by 2 bolts each. Will there be any problem with this? I would need to grip something that is 268mm wide.

Etienne_Samson

@lakshmip001 yes you can play with the friction coefficient to increase your payload. The payload calculation is detailed in the instruction manual section 6.2. Same thing with the custom fingertips, some dimensions are specified. The graph is lacking a dimension in X (what you are trying to do), you should respect the same dimensions then the Y specifications.

When looking at those charts you will probably notice that grip force decrease with an increase in finger opening. Thus if you use the 2-F-140 and increase it's opening to maximum you will be able to pick you part of 268 mm but only with a grip force of 40 N.