Universal Robots Forum

Position droop during long periods of wait (30min+)

Dear,

We have a UR10 CB3. We have a load of approx. 3 to 4kg on the robot, which is constant (a fixed tool). We run a program, then pause the robot by letting it move to the same location over and over for 40min (during our lunch time). When we return and give the robot a new coordinate to move to, we need to restart the program because the UR controllor complains the robot has deviated (slightly) from its path… How can this be?

The load is stable and does not alter, it’s efectively not moving at all, the UR has it’s servo loop for positioning constantlty running…

It’s possible that we may have the wrong estimate weight of the tool. The problem is that there are some cables attached which are hard to judge in terms of weight. The cables are stationary, so only move when the tool does.

Has anyone experienced this before? Could it be wrong tool weight estimation? Or is there another problem? Can we somehow read-out the robots’ force sensors to get a better estimate of the load?

Love to hear from your experiences.

Regards,

Max

Hello,

Yes, probably that is because of the wrong payload. As you are using a CB3, it doesn’t have torque sensors, the torque in this unit is calculated from the Voltage and Current values.

I strongly suggest you to get the right value of your payload, besides probably fixing this error, it will avoid damaging your servos. The payload and the CoG (Center of Gravity) are used to calculate the power of each server, getting one of these wrongly adjusted, well…

Ok, thanks. We’ve adjusted and the problem has been reduced significantly. But can we read-out the weight at the tool flange to get the right setting? We’re just guessing now…

As you have a CB Series, the torque reading in the flange is not nearly as precise as the E Series. I do recommend you to get your tools weighted and also the correct values of the CoG. If you don’t get that right, you probably will never fix the problem.

(If you are in an emergency, and have no ideia how it weights but needs to get the robot running, I would suggest you to put the robot in the test force mode towards the flange and all your tools in the flange, you declare the payload as 0kg, and try to adjust the force values the nearest to get the balance right. )

Again, this is not recommended as the values aren’t as precise as it needs to be, which will probably damage the robot in a long term.