Multi-jointed robot deviation under load determination

Patent metadata

Field	Value
Publication number	US-10449676-B2
Application number	US-201515555406-A
Country	US
Kind code	B2
Filing date	Mar 23, 2015
Priority date	Mar 23, 2015
Publication date	Oct 22, 2019
Grant date	Oct 22, 2019

Abstract

Determining deviation of a multi jointed robot under load using a tribological contact between an end of the robot and any available hard constraint near the robot, involves pressing the end against the constraint, and then soliciting a movement of the end relative to the constraint in a tribologically resisted direction to apply a force that does not overbear the resistance. By measuring the force and a position encoded by the robot, a deviation of the robot under the corresponding load is determined. Correction terms may be required for deformation of the tribological surface and/or constraint. The constraint may be tooling or parts subjected to an intended process. The deviation at many measurement poses of the robot, each in multiple resisted directions, within the ordinary operating space of the robot, was used to derive compliances of the robot, and a kinetostatic model.

First claim

The invention claimed is:

1. A method for determining a deviation of a multi-jointed robot subjected to a load, the method comprising:
moving an end of the robot to a measurement pose adjacent a constraint;
applying a first force to press the robot end into tribological contact with the constraint;
soliciting the robot to move the end in one direction that is resisted by the tribological contact while the tribological contact is maintained, where the solicitation exerts a second force that is insufficient to overbear the tribological contact's resistance; and
quantifying the second force in the resisted direction, and a variation in position of the robot from the robot's encoders resulting from the solicitation.

2. The method of claim 1 further comprising a preliminary step of mounting a friction bearing element (FBE) onto the robot directly, or via: an end effector, a production end effector, a part added to a production end effector, an end effector with one or more parts added or removed, a tool, a production tool, or a production tool with one or more parts added or removed, and wherein moving the end of the robot involves moving the FBE into position adjacent the constraint.

3. The method of claim 2 or wherein the FBE has a contact surface of a material to prevent marking or material transfer during the tribological contact at a force having a magnitude of 1 KN or greater.

4. The method of claim 3 further comprising:
determining a displacement of the robot end during the tribological contact by:
using a well characterized deformation of the selected material, and fitting at least the first and second forces to a model of the deformation;
measuring a deformation of the selected material relative to a surface of the constraint at the measurement pose; or
measuring a change in position of a metrological target on the robot end with respect to a reference frame that is fixed with respect to a grounding of the robot, and

determining the deviation by calculating a difference between the determined displacement from the variation in position, and associating the second force with the difference.

5. The method of claim 1 wherein
moving the robot end comprises moving the end over a free surface of the constraint to the measurement pose, and
a shape, size, and conformability of an area of the end that makes the tribological contact with the constraint, relative to a curvature of the free surface, permits tribological contact to be made at any location on the free surface.

6. The method of claim 1 wherein soliciting the robot to move while the tribological contact is maintained comprises commanding the robot to maintain the first force in a direction normal to the constraint throughout the tribological contact, where the first force is greater than the second force times a pre-established constant of proportionality.

7. The method of claim 1 wherein the second force has a lower magnitude than the first force and is applied quasistatically.

8. The method of claim 1 wherein soliciting the movement comprises:
directing the robot in a force control mode to exert the second force, the force control mode being enabled by a force sensor: provisioned with the robot; mounted to the end; in an end effector; or in a friction bearing element, and
forwarding at least select force control feedback from the force sensor to a processor for executing a computer program for determining the deviation, and associating the second force with the determined deviation.

9. The method of claim 8 wherein directing the robot in force control mode comprises:
operating the robot in a force control mode to apply the first force, and exert the second force, while keeping the robot free of moments in pitch and yaw;
operating the robot in a force control mode to apply the first force, and exert the second force, where an effective joint in the robot end precludes the robot from applying moments in pitch and yaw; or
operating the robot in a force control mode to apply the first force, and exert the second force, and may apply a torque in pitch and yaw, where the torque in pitch and yaw are also quantified, and forwarded to the processor.

10. The method of claim 8 further comprising the processor determining the deviation by:
receiving the variation in position of the robot determined from a difference between encoder readouts at first and second stable positions, the second stable position being produced when the robot stabilizes after the second force is applied;
receiving from the force sensor the select force feedback, which includes at least the second force;
using a third force, which is a force applied at the first stable position, and the second force to compute a difference in force; and
calculating a deviation of the robot and associating the deviation with the difference in force.

11. The method of claim 10 wherein the first stable position is the measurement pose adjacent the constraint, and at most negligible force is applied in the first stable position, whereby the third force is null, the difference between the second and third force is the second force.

12. The method of claim 1 wherein the soliciting and quantifying are repeated for a plurality of the resisted directions, including diametrically opposed directions for tribological contact at each of one or more measurement poses.

13. The method of claim 1 further comprising improving the tribological contact by one or more of the following:
cleaning a surface of the constraint and a part of the robot that meet to form the tribological contact prior to moving;
changing a pressure in a sealed fluid chamber near the tribological contact to produce enhanced surface contact by the pressure difference; or
engaging a magnetic tool positioned behind the constraint.

14. An apparatus for measuring deviation of a multi-jointed robot under load, the apparatus comprising:
a robot with an end flange;
a friction bearing element (FBE) mounted to the end flange directly, or via an end effector or end tooling;
a force sensor within the robot, or mounted to the end flange directly, or via an end effector, end tooling, or the FBE; and
a computer processor with a memory, in communication with the force sensor and the robot,
wherein the computer processor has program instructions for:
directing the robot to move the FBE into a measurement pose adjacent to a constraint located within an envelope of the robot;
applying a first force to press the FBE into tribological contact with the constraint;
soliciting the robot to move in one direction that is resisted by the tribological contact while the tribological contact is maintained, where the solicitation exerts a second force that is insufficient to overbear the tribological contact's resistance; and
quantifying the second force and a change in position of the robot from the robot's encoders due to the resistance to the solicitation, and using same to determine a deviation of the robot.

15. The apparatus of claim 14 further comprising at least one of:
a joint mounted between a flange of the robot and an interface where the tribological contact is made, that has at least one rotational degree of freedom, about an axis perpendicular to a local normal of the interface, to neutralize moments about the axis; and
a metrological target mounted to the robot between the end flange and an interface where the tribological contact is made, the metrological target adapted to be used for measuring a deformation of the FBE during the tribological contact, and wherein the computer processor is adapted to subtract the measur

Assignees

• Nat Res Council Canada

Inventors

• Monsarrat Bruno
• Fortin Yves

Classifications

• G05B2219/39041

  Calibrate only for end position · CPC title

• B25J13/085 Primary

  Force or torque sensors (B25J13/082, B25J13/084 take precedence) · CPC title

• B25J9/1653

  parameters identification, estimation, stiffness, accuracy, error analysis · CPC title

• G05B2219/39183

  Compliance compensation · CPC title