Path-modifying control system managing robot singularities
A controller for robot arms and the like having mechanical singularities identities paths near the singularities and modifies those paths to avoid excessive joint movement according to a minimization of tool orientation deviation to produce alternative paths that minimize changes in the tool orientation such as can affect application such as welding, sealant application, coating and the like.
1. A robot control system for a multi-axis robot having multiple links including an end link defining a tool position, the links movable with respect to each other at joints defining axes and having sensors and actuators for movement of the joints about the axes by the actuators according to the sensors subject to joint movement rate limits, the joints and links configured so as to present at least one singularity, the controller comprising:
an electronic circuit communicating with the actuators and sensors at each joint and executing a stored program to:
(a) receive path instructions defining a path of desired changes in tool orientation along with desired positional velocity occurring along this path;
(b) identifying at least one unmodified path portion proximate to the singularity exceeding at least one joint movement rate limit at the desired positional velocity;
(c) identifying a modified path portion providing the desired velocity and the desired tool position without exceeding the at least one joint movement rate limit of (b) according to a minimization of a maximum accumulated deviation of a tool orientation of the modified path portion from a tool orientation of the unmodified path portion taken over the entirety of the modified path portion.
2. The robot control system of claim 1 wherein the modified path does not intersect the singularity.
3. The robot control system of claim 1 wherein the modified path intersects the singularity.
4. The robot control system of claim 1 wherein step (c) identifies alternative modified path portions providing the desired velocity and the desired tool position without exceeding the at least one joint movement rate limit of (b) according to a minimization of a maximum deviation of a tool orientation of the modified path portion from a tool orientation of the unmodified path portion; and
further including the step of receiving path instructions describing at least one region where path rerouting is prohibited and wherein step (c) identifies the modified path portion to minimize the deviation among the alternative modified paths not including the at least one region.
5. The robot control system of claim 1 wherein step (c) further identifies a modified path portion according to a minimization of a length of the modified path portion.
6. The robot control system of claim 1 wherein step (c) further identifies a modified path portion that limits at least one of joint maximum acceleration and joint maximum velocity among joints of the multi-axis robot to at least one predetermined value.
7. The robot control system of claim 6 wherein step (c) further identifies a modified path portion according to a predetermined lower limit of a length of the modified path portion.
8. The robot control system of claim 1 wherein the three axis robot wrist provides a spherical roll-pitch-roll configuration and the singularity occurs when the two roll axes are collinear.
9. The robot control system of claim 1 wherein the robot provides an offset wrist.
10. The robot control system of claim 1 further including a robot providing a total of six joints.
11. A robot control system for a multi-axis robot having multiple links including an end link defining a tool position, the links movable with respect to each other at joints defining axes and having sensors and actuators for movement of the joints about the axes by the actuators according to the sensors subject to joint movement rate limits, the joints and links configured so as to present at least one singularity, the controller comprising:
an electronic circuit communicating with the actuators and sensors at each joint and executing a stored program to:
(a) receive path instructions defining a path of desired changes in tool orientation along with desired positional velocity occurring along this path;
(b) identifying at least one unmodified path portion proximate to the singularity exceeding at least one joint movement rate limit at the desired positional velocity;
(c) identifying a modified path portion providing the desired velocity and the desired tool position without exceeding the at least one joint movement rate limit of (b) according to a minimization of a maximum deviation of a tool orientation of the modified path portion from a tool orientation of the unmodified path portion;
wherein step (c) models the robot as a closed kinematic system consisting of a base comprising a first link in that system supporting a first point supporting a first joint of the robot, through the joints of the robot then through a three axis virtual wrist sharing one axis with a final axis of the robot and then through a virtual joint connecting to a link on the path that is rigidly connected to the first link.
12. The robot control system of claim 11 wherein the virtual three axis wrist is constrained so that a first and third joint of the three axis wrist are constrained to move by equal and opposite angles.
13. The robot control system of claim 12 wherein virtual three axis wrist is constrained so that a second joint of the virtual wrist is perpendicular to a second joint in the robot wrist comprising the final three joints of the robot.
14. The robot control system of claim 11 wherein the three axis virtual wrist provides a spherical configuration.