Force monitoring

Jason_g

I am working on a project that will require a tool change with Universal Robot. We are hoping to accomplish this without adding additional guarding due to the pinch point as we're moving into swap tools. What would be best practice for monitoring the force as were moving from waypoint 1(in front of tool) to waypoint 2(at tool)? If an obstacle is detected (finger or hand) we either kill program with popup, or momentarily move back to waypoint 1, and then try again.  

matthewd92

That is simple to do.  You don't do it inside of a thread. 

When you get to that point in your program or subroutine you will have your destination under an if statement.  That if statement will be monitoring the force at all times.  I usually use a flag below the waypoint so that I know that I made it or not and then can take action based on that result.

Also, make sure that the check box for "Check Expression Continuously" is checked, this basically turns the if statement into a while loop so effectively while force is below some number keep doing this, as soon as its not exit this situation.  So if on the way to the second point my force exceeds the threshold I immediately exit the if statement and so the flag never gets set to True.

Here is a quick example of how I might approach something like this

 



Does this help?

JeanPhilippe_Jobin

@Sebastien
There is no short-term plan to modify our FT sensors in such a way that it could be considered as safety device.

@Jason_g
About your need, I would simply use the reduced force/speed mode as suggested previously and make sure that the force/pressure respects the safety standards (i.e. ISO/TS-15066). This should not be too complicated since I don't think that a lot of force is needed to "couple" the tool changer parts' together. So you reduce force/speed zone could be quite broad to avoid pinching with body parts.

But for those who would have an application where a significant force would need to be apply, here is what you could do:

• Move the robot to a safe distance (no pinching possible with any body part at no time) from the surface it needs to apply a high force/pressure
• Change the robot to reduce force/speed mode
• Move the robot towards the surface until the distance between this surface and the robot is smaller then the smallest body part that can fit in (usually the fingers, so ~ 5 mm)
• Change to normal force mode
• Move the robot towards the surface, make contact and apply requested force/pressure

Sebastien

@Jason_g
I would try to use the Robotiq FT300 sensor. You could simply have a thread in the program constantly monitoring Fx, Fy or Fz depending on what you need to look at. This would be quite straight forward and it would be more accurate then using UR robot. However, the only limitation is that the force feedback from the sensor is not 'safety rated'. So I don't think that solves it. @JeanPhilippe_Jobin what do you think? Any plans on having the FT sensor be considered as a safety device?

The other method I am thinking of doing it would be to have an input going to the safe input of the robot when it is going to perform tool change. You could then configure this input as the reduced mode input. Then in the reduced mode settings you could have force limitations according to what you need during the tool change operation. Be careful to when selecting the device that will toggle the safety input for reduced mode. Its safety performance needs to match the risk level associated with the tool change operation.

Hope that helps!

matthewd92

@Jason_g there are a coupe of things that I would try first that are built into the robot.  For starters the robot has built-in force monitoring that we use a ton of to limit actions on the robot based on the force that it is seeing.  Works very similar to the FT300 or FT150 from Robotiq but without the fine resolution, its only good for +/- 10N.  But I would try it as its free and easy to sprinkle around the program where you need it.

Another thing that you could do is place the tool changer on the other side of a safety plane.  Then when the robot crosses that plane you can automatically have it go into reduced force/speed mode.  We do this when we need to interact with humans or want to enter the human workspace without having to add a light curtain or other means of entering reduced mode.

I agree with @Sebastien you need to make sure that whatever you wind up doing meets safety requirements in your area and that a risk assessment has been performed.  His idea for using an output on the robot to trigger a safety input could work as well, especially if the tool changer cannot be placed outside of the normal working area of the robot, i.e, not behind its own safety plane.

Jason_g

Thank you. @matthewd92 my next thought was using a safety plane when @Sebastienmentioned using an input for reduced mode.

I did attempt utilizing "force()" command with an if statement. If force is greater then said value we move back to waypoint 1, or halt. The two problems I kept running in to was it seems that were monitoring the force at all times. Would it be best to do the move towards tool within its own subroutine?  I only want to perform this monitoring and action during the move from waypoint 1 to waypoint 2. When I tried doing this within a thread, I get a popup saying "program is already being controlled by another thread".

tkelch

You could also consider designing the location and enclosure around a tool changer - forcing the robot to collide/pinch a forearm or torso (higher pain points) before it can get to your finger. This could be done by having the tool changers on a different base than the robot, so in order to reach the tool changer, someone would need to walk in-between the robot and the tool changer - having the robot hit the person prior to being able to get to the tool changer.

Funky way to look at safety, but it may work.