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| Research | Indoor GPS Metrology System and Modular Robot Metrology |
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| Objective |
Robots are becomming an integral part of many modern manufacturing processes.
With their ability to perform repetitive tasks, while maintaining the flexibility to be reprogrammed to perform new tasks.
Simulation tools can be used to quickly and easily investigate robot applications and even perform programming off-line using accurate 3-D computer models.
Real-life robotic devices, however, can have inaccuracies up to 10 mm, obviously unwanted if product specifications are lower than 1 mm.
This gap between the simulations and real life can only be closed with measurement and calibration systems.
The objective of this project is to develop a new 3-D, non-contact metrology system to close this gap enabling massive cost reduction with simulatin data.
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| Approach |
I. Indoor GPS Metrology System
The RRG has developed the 'SR (Space and Robot) Probe' solution.
This is specifically designed for intelligent scanning using recently available indoor GPS technology.
The indoor GPS metrology system is a laser based reverse engineering system.
A battery-operated transmitter uses laser and infrared light to create one-way position information,
the relative azimuth and elevation from the transmitter to the receiver.
The receiver has photodiodes inside its module and senses the transmitted laser and infrared light signals.
With the addition of a second transmitter of known location and orientation,
users can calculate the position of the receiver in the base coordinate system. By adding two more transmitters,
the system will have four laser transmitters having its accuracy maximized.
The signal is transferred through a wireless network connection providing mobility to the operator.
This is shown in Figure 1.
Figure 1. Indoor GPS Metrology System with SR Probe
The SR Probe shown in Figure 1. is a portable measurement tool equipped with three receivers in optimum locations and a ball probe tip.
The operator holds the SR Probe in his hand and moves the probe tip over the object.
The X, Y and Z coordinates of the probe tip are measured in real time with high accuracy.
Different probe tips of all sizes and shapes can be used on the probe to have access to all areas.
The buttons on the probe allow the operator to start and stop the scanning process.
The indicators on the probe give information concerning visibility and scanning mode.
The basic robot measurement principle involves attaching detectors to the end-effector on the robot either using the robot probe or attaching detectors directly to a robot tool.
The robot is then moved to a number of predefined poses and the position actually reached compared to the ideal position reached by a nominal robot software model is compensated.
From this comparison, various compensation of performance data can be calculated which can be used to improved the accuracy or optimize the robot task.
The robot measurements, which the Indoor GPS Metrology System can perform, are applicable to one or two areas, robot testing and robot metrology.
II. Robot Testing
The Indoor GPS Metrology System with RRG robot testing software, the osciloscope of robots, will allow the measurements of path and position accuracies, robot speed, compliance, and mechanical play.
These tests will allow the feasibility of a robot process to be checked saving tramendous tests and iteration costs.
The tests will also be combined with ISO software to guide you through the ISO 9283, robot certification process.
The RRG robot testing software will have an ability to perform five types of robot tests. Dynamic positioning, if a robot is programmed to visit the same pose several times, what is the speard on the actual achieved poses?
Hysteresis, if the robot approaches a pose from two different directions, is there a difference in obtained pose due to mechanical play?
Overshoot, when arriving the pose, do important inertia effects occur and how long does the robot need to stabilize?
Path, any path can be accquired and eventually can be compared to the desired path. Grid, the absolute accuracy of the robot can be tested by visiting a set of poses and comparing them to the command poses.
The user is free to choose the poses and trajectories for these tests as well as the applied robot velocity, payload, and controller settings.
II. Robot Metrology and Workcell Calibration
The Indoor GPS Metrology System can be used in conjuction with the RRG Metrology software. This allows the calibration of not only the robot but entire robot cell to provide the link between simulation and the real world.
The RRG Metrology software provides the ability to perform three types of calibration routines, robot calibration, tool calibration, and the environment calibration.
The System Level Metrology (SLM) involves the measurement of 25 to 60 robot positions, which are then compared to a nominal robot software model.
From these measurements, an indication of the accuracy of the robot is produced. Compensation data can also be calculated which when applied to the robot controller compensates for the inherenet robot error
and allows the robot to achieve accuracies of better than 1 mm.
So, how is the robot compensated for to improve accuracy? From the measured robot positions, the system has an accurate model of the real robot. The robot controller still only has the nominal robot model.
If the robot controller is told to go to a certain position, it uses inverse kinematics to calculate the joint values required to get to this position. Since it is using a nominal robot model, the position actually obtained will have the error.
To get the robot to move to the correct position, the accurate model measured by the Indoor GPS Metrology System will be used to perform inverse kinematic calculation giving the required joint values for the real robot.
By inputing these joint values back into the nominal robot model on the RRG Metrology software and performing forward kinematics, a new compensated positions have obtained.
It is this new position that when used by the robot controller with a nominal model gives the required joint values and thus moves to the required position.
In effect, you are trying to drive the robot to a different offset position so that it has actually reached a position you want.
Tool calibration can be quickly performed using the detectors attached to the tool used in the robot calibration and the Space Probe.
The tool center point frame is measured using the detector as a dynamic frame from the measured robot. This corrects for any errors in the nominal tool model due to mechanical variances.
Environment calibration utilizes the CMM capabilities of the system to take measurements of the positions of fixtures and other environmental features with respect to the robot.
Again using the detectors on the robot as a reference creating a frame from the measured robot base to the fixture, the system now knows the positions of the robot base frame, tool frame, and fixture frame.
It is these three frames that a user references when programming the robot.
By using these three calibration routines, nominal off-line programs can be downloaded directly into the robot and, through the use of the compensation data, can be used almost immediately with no manual touch-up requirement.
This is achived by comparing the nominal positions of the robot, tool, and fixture in a simulation to the actual measured positions.
The use of this technique provides the real cost and time saving for implementing robot programs in the shop floor and accessing an implementing change in a highly utilized plant.
Figure 2. shows that the potential applications of the metrology system range from actuator metrology to robot, manufacturing workcell, and mobile platform calibrations.
Figure 2. Applications of Indoor GPS Metrology System
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| Research and Results |
I. Development of Indoor GPS Metrology System
The laboratory implementation of the developed Indoor GPS Metrology System is shown in Figure 3.
Figure 3. Laboratory Environment
The general concept and kinematics of the metrology system as well as the derivations of an error budget for the general device are available.
II. Metrology and Calibration of Modular Robots
Experimental results of geometry and compliance error identification on a robot will be updated in the near future.
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| References |
| [1] |
Kang, S. and Tesar, D., "A Noble 6-DOF Measurement Tool With Indoor GPS For Metrology and Calibration of Modular Reconfigurable Robots," IEEE ICM International Conference on Mechatronics, Istanbul, Turkey, 2004. |
| [2] |
Kang, S., Pryor, M., and Tesar, D., "Indoor GPS Sensors Applied To Actuator Metrology and Manufacturing Cell Calibration," $250,000 Proposal submitted to National Science Foundation, 2003 - Awarded. |
| [3] |
Kang, S. and Tesar, D., "Metrology Test Environment for Modular Actuators," Thesis, 2000. |
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| Publications |
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| Contact |
For more information, please contact Seong-Ho Kang
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Page Last Updated: 04/9/04
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