Electricity: motive power systems – Positional servo systems – Program- or pattern-controlled systems
Reexamination Certificate
2002-04-19
2004-11-02
Gibson, Randy W. (Department: 2837)
Electricity: motive power systems
Positional servo systems
Program- or pattern-controlled systems
C318S568170, C318S568210, C700S245000
Reexamination Certificate
active
06812665
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a system and method for calibrating a virtual robot tool center point (TCP) or virtual work-object frame and more particularly to the use of relative measurement to perform in-process workcell calibration.
BACKGROUND OF THE INVENTION
Industry is now seeing a dramatic increase in robot simulation and off-line programming. In order to use off-line programming effectively, the simulated workcell has to be identical to the real workcell. This requires a more efficient and accurate method for robot calibration. By making use of calibration, the simulated robot workcell will clone the real workcell in a simulation model, so that the off-line generated robot program from a simulated workcell will be accurate enough and can be directly downloaded to a real robot controller to drive the real robot with maximum accuracy and without further modification.
A variety of attempts to develop a better robot calibration system and method to improve robot accuracy exist in the prior art. Currently used techniques, however, are typically tedious, time consuming and expensive. This is because most of the prior art calibration methodology so far is based on absolute calibration.
“Absolute calibration” refers to the method by which an external coordinate measurement system is utilized to measure the absolute position, often referred to as a global coordinate system. Since the external system measures the coordinates of a point in the workspace, the absolute method can validate any path accuracy. However, absolute position measurement has many drawbacks including the fact that it is time consuming, expensive and sometimes fails to meet accuracy requirements. One example in the prior art is to use an optical coordinate measurement system (OCMS) to calibrate the robotic workcell, which is a very expensive and time-consuming way of calibrating the robot.
In contrast to absolute calibration, some development has been made in the area of “relative calibration”. Relative calibration is a method in which a standard reference target is used as the precision reference for the correction of robot kinematic error. This “standard reference” provides high-precision relative geometric quantities such as length, circularity and linearity. A standard reference could simply be a bar, a cube, a cylinder, or a ball. During the calibration, the robot is driven to make the tool center point (TCP) follow the geometry of the selected standard reference. This standard reference therefore provides a constraint on the TCP process. Due to the kinematic error, this constraint would be violated if the nominal kinematic model were used to calculate the Cartesian coordinates from the same joint angles. Minimization of the constraint violation (constraint error) will give the values of error parameters. In the present invention, this standard is called “relative reference.”
However, all known relative calibration techniques are only for one component calibration. There is no relative calibration technique to deal with the overall workcell calibration. Accordingly there is a need for an economical calibration method and apparatus to deal with overall workcell calibration.
Moreover, the tool center point (TCP) may change due to tool wear or tool changes. The workpiece itself can introduce a significant amount of error or uncertainty due to workpiece variation or deflection during the manufacturing process. Real time calibration for each workpiece can eliminate this effect. Accordingly, there is a need to develop a method and apparatus, which must be cost effective and capable of in-process operation and real-time implementation.
SUMMARY OF THE INVENTION
In order to overcome the shortcomings and drawbacks of conventional calibration systems and methods to make calibration cost-effective, efficient and easy to use, the objective of the present invention is to create a novel method and device for robotic workcell calibration. The present invention will provide an economical, robust calibration system that will have the ability to calibrate the major considerations involved in any robot system including calibration on a real-time basis during manufacturing processes.
Generally speaking, there are two types of setups in robotic workcells. One consists of the robot holding the tool and workpiece being fixed on the worktable. This is called “Moving TCP”. The other type consists of the robot holding the workpiece and the tool is fixed on the floor. This is called “Fixed TCP”.
In a fixed TCP-based robotic workcell, the forward kinematic chain includes the robot (robot based coordinate), the gripper (work-object coordinate) and the workpiece (object coordinate); the backward kinematic chain includes the tooling system (tool coordinate). In an ideal case, the errors of real or virtual contact points between the tooling and the object are zeros along the working path.
All of the errors from the two kinematic chains can be divided in two parts: “forward chain error” and “backward chain error”. Forward chain error includes the robot error, the gripper-setup error, and the object-installation error. Backward chain error includes tool-table error and tooling fixture error. The role of calibration is to eliminate or correct all of these errors in order to create highly accurate paths for robot operation. The same principle applies to a moving TCP-based robotic workcell.
In a conventional absolute calibration environment, the goal is to calibrate all the components related to a global absolute reference, in order to eliminate all of these errors separately. Absolute workcell calibration includes robot TCP calibration, tooling calibration and work-object coordinate calibration, wherein each is performed individually. Each calibration process will measure all the Cartesian coordinates to determine the error between the nominal and true value.
Unlike conventional absolute calibration methods, the relative calibration method of the present invention treats all of the errors as relative error between the tooling and the working object compared to a relative reference. Measuring this relative error and finding a way to correct this error is a major advantage of this invention. As long as the relative error is eliminated compared to the relative reference, the workcell is calibrated related to the relative reference and the perfect path will be generated.
Prior to beginning the relative calibration, a computer aided design (CAD) model of the workpiece is downloaded into a data collection and computing device such as a programmable controller or computer.
There are five steps for completing the relative calibration.
The first step is tool center point (TCP) calibration. This consists of performing a TCP calibration using the robot as a measurement tool. The calibration is accomplished by mounting a calibration target within the workcell and in a position that the robot can reach from various orientations. The calibration target can be a sphere, cylinder, cubic or any other definable geometric shape. The robot is programmed to touch the calibration target surface from various angles with a CMM touch probe. All contact positions are recorded. The TCP is calculated from the measurements using a non-linear least squares optimization algorithm.
The second step is to set up a relative reference between the robot and a sample-working object. The relative reference is established by having the robot hold a finished sample of the working object (workpiece) while a series of measurements is performed to compensate for the error between the perfect CAD model and the finished sample to obtain a relative reference. When the actual implementation of the relative method is considered, the enforcement of TCP to follow the standard geometry becomes the biggest concern since the achievable accuracy of a “standard reference” can be very high with moderate manufacturing cost. This compensation process will make the standard reference in a cost-effective way. The actual TCP path becomes the equivalent reference when the
Gan Zhongxue
Sun Yunquan
Tang Qing
ABB AB
Colon Santana Eduardo
Franklin Eric J.
Gibson Randy W.
Venable LLP
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