Robot feature tracking devices and methods

Details Robot feature tracking devices and methods Robot feature tracking devices and methods

1999-12-20

2002-08-06

Cuchlinski, Jr., William A. (Department: 2666)

Data processing: generic control systems or specific application

Specific application, apparatus or process

Robot control

C700S095000, C700S259000, C700S262000, C318S568100, C318S568200, C318S568110, C318S568120, C318S568180, C318S577000, C219S121610, C219S121620, C219S121630, C219S121640, C219S121820, C219S121830, C901S041000, C901S042000

Reexamination Certificate

active

06430472

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to robot feature tracking devices and methods, and more particularly to an assembly, a system and a method for providing additional positioning ability to a tool at an end of a robot arm, and improving the positioning accuracy of a robot tool over a feature to be processed. The invention applies for example to laser processing, such as laser welding, and to arc welding. It also applies to other types of processing that involve the guidance of a tool over a joint or feature to be processed.
BACKGROUND
It is well known that process robot tasks are often programmed using the method of play back of a taught path. If the work piece to be processed by the robot is not accurately positioned and oriented to correspond with this taught path, the robot will not position its tool accurately over the work piece and flaws will result.
The current solution to this problem is to install a sensor in front of the robot tool and to link this sensor with the robot through a special interface. In a welding operation, for example, the sensor measures the position and orientation of the joint, and communicates this information to the robot to correct its trajectory and tool orientation at the right time and place.
One problem is that many robots are not equipped with this type of interface. They cannot be linked with a sensor for joint or feature tracking.
Another problem is the calibration that is required in order to define the physical relation between the tool center point and the sensor. This relation must be well defined to allow the control unit of the sensor to accurately control the position and orientation of the tool while getting position information about the joint some distance in front of the tool. This calibration is usually performed by accurately positioning the tool center point over a reference object. If the operator does not accurately position the tool center point during this operation, the calibration will not be accurate. The robots usually have a very good positioning repeatability but poor absolute positioning accuracy. This means that the tool center point can be repeatedly sent back to the same position with a good accuracy, but the coordinates of this position in space will not be known accurately. The robot also makes an error when it informs the sensor about its current position during joint or feature tracking because of the response time of the robot arm and because of its mechanical elasticity. In the case of arc welding with a filler wire, the calibration problem is further complicated by the fact that the filler wire is not always straight when it gets out of the tool tip. It often gets out with a variable curve so that the tip of the wire does not correspond to the position of the tool center point. In the case of laser welding, the focal point of the laser beam moves relative to the theoretical position of the tool center point because of imperfections in the optical path.
U.S. Pat. No. 4,952,772 (Zana), U.S. Pat. No. 4,954,762 (Miyake et al.), U.S. Pat. No. 4,969,108 (Webb et al.), U.S. Pat. No. 5,006,999 (Kuno et al.), U.S. Pat. No. 5,014,183 (Carpenter et al.), U.S. Pat. No. 5,015,821 (Sartorio et al.), U.S. Pat. No. 5,066,847 (Kishi et al.), U.S. Pat. No. 5,463,201 (Hedengren et al.), U.S. Pat. No. 5,465,037 (Huissoon et al.), U.S. Pat. No. 5,582,750 (Hamura et al.) and U.S. Pat. No. 5,624,588 (Terawaki et al.) provide examples of welding control systems and methods of the prior art, some of which including error correction algorithms. Yet, none of them provides easy robot path correction for joint and feature tracking by an industrial process robot, which would be applied even at very high speed and without directly intruding into the robot control itself. Likewise, none of them satisfactorily solves the problem of accurate computing of the sensor to robot tool center point geometric relation, in static and dynamic operating modes, which is so critical to high speed joint tracking due to the use of the delayed shift method usually applied when a laser vision system is used in front of the robot tool.
SUMMARY
An object of the invention is to provide easy robot path correction for joint and feature tracking by an industrial process robot, which can be applied even at very high speed and without directly intruding into the robot control itself.
Another object of the invention is to provide additional positioning ability to a tool at an end of a robot arm.
A subsidiary object of the invention is to allow a robot to perform joint and feature tracking even if the robot is not equipped with the proper interface.
Another object of the invention is to provide a solution to the problem of accurate computing of the sensor to robot tool center point geometric relation, in static and dynamic operating modes, which is so critical to high speed joint tracking due to the use of the delayed shift method usually applied when a laser vision system is used in front of the robot tool.
According to the present invention, there is provided a motorized slide assembly for providing additional positioning ability to a tool at an end of a robot arm. The assembly comprises a slide arrangement having a base and a sliding element movable along a predetermined course relative to the base. A motor is mounted onto the slide arrangement. A drive device is connected to the motor for moving the sliding element along the course upon operation of the motor. Fasteners are provided for fastening the base of the slide arrangement to the end of the robot arm, and for fastening the tool onto the sliding element.
According to the present invention, there is also provided a motorized slide system for providing additional positioning ability to a tool at an end of a robot arm. The system comprises a motorized slide assembly including a slide arrangement having a base and at least one sliding element movable along a predetermined course relative to the base. A motor is mounted onto the slide arrangement. A drive device is connected to the motor for moving the sliding element along the course upon operation of the motor. Fasteners are provided for fastening the base of the slide arrangement to the end of the robot arm and for fastening the tool onto the sliding element. An encoder is operatively coupled to the motor to provide motor positional information. A control unit is provided for the motorized slide assembly. The control unit includes a communication interface for receiving sensor related data, a I/O interface for receiving and transmitting synchronization signals, a CPU for controlling positions of the sliding element, a memory, a servo-amplifier circuit for powering the motor, a slides control for controlling the servo-amplifier circuit in response to the CPU and the motor positional information provided by the encoder, and a bus circuit interconnecting the communication interface, the I/O interface, the CPU, the memory and the slides control together.
According to the present invention, there is provided a compensation method for compensating errors made by a control unit of a robot sensor when evaluating a relation between a position of a robot guided tool behind the sensor and a position of a feature to be followed by the guided tool. The method comprises the steps of recording position data generated by the sensor during a dry pass of the guided tool over the feature, the position data representing consecutive positions of the feature detected by the sensor, and subtracting the recorded position data from joint position errors computed by the control unit during a feature tracking operation where the guided tool is operated to process the feature.
According to the present invention, there is provided a control unit for a robot sensor tracking a feature to be processed with a robot tool positioned behind the robot sensor. The control unit comprises a sensor interface having a sensor control output and a video input. A memory is connected to the sensor interface. A CPU is connected to the sensor interface and the memory. A communication

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