Introduction
As robots have become more sophisticated, they have been able to be applied to more and more industrial processes. The 6Dof (6Dof) robot can perform complex motions and thus can demonstrate good performance in many complex industrial tasks such as palletizing, handling, gluing, and welding. The 6-DOF robot can lift and flexibly manipulate heavy loads and precisely control the load through complex geometric movements.
Based on its good performance and flexibility, 6-DOF robots are used in combination with various tools for many different tasks. However, every time a new tool is introduced, accurate recalibration is required before the robot can be used. Recalibration is often time-consuming and inaccurate, hindering the smooth operation of industrial processes and delaying production.
Servotronix has created an effective position teaching method that can quickly calibrate new tools used by 6-DOF robots without having to rely on manufacturer measurements or external sensors. This method is simple, accurate, and very effective for practical applications.
calibrationThe 6-DOF robot needs to hold and move the tool when performing tasks. In order to achieve satisfactory performance, the robot must know the exact position of the tool when working. Every time a different tool is assembled, the robot must be calibrated again precisely.
There are different ways to calibrate a 6-DOF robot. For example, contact with reference parts, use distance sensors, and use laser interferometer measurements. In addition, an external sensor, such as a photographic system, can be mounted at different locations on the robot to obtain the precise location of the robot's calibration reference object.
These methods are time consuming and complicated. Servotronix has developed a simpler method and has achieved excellent results. We will explain this method below.
Determine tool center pointWe use the kinematic calibration method to determine the tool center point (TCP), and all robot positions are defined based on this point. TCP is defined in the world coordinate system - Cartesian coordinate system that can locate any point in the world. This coordinate system will always remain stationary with respect to the robot.
The tool coordinate system defines the position and attitude of the tool, and the zero point of the coordinates is set at the tool's center point (TCP). The robot's TCP will move to the programmed position as it performs a Cartesian motion. Changing the tool will change the coordinate system of the tool, so it needs to be recalibrated so that the new TCP can reach the target position accurately.
In many robotic applications, TCP's motion trajectory means a complex path within the robot's workspace, usually a straight line path to a tool that changes attitude. The tool itself needs to be replaced occasionally, even if it needs to be replaced frequently. Each time the tool is changed, a new set of geometric parameters must be determined and configured before the robot can resume operation.
In most industrial applications, location teaching is the most practical method for robot task programming. When using this method, you must have high-precision tool parameters (usually from the manufacturer), including the tool's angular offset (yaw, pitch, and roll) and Cartesian offset, in order to generate a straight line with a controllable tool attitude. path.
Unfortunately, operators often find that the geometrical identification of tools is subject to certain constraints, such as: (1) There is no information from the manufacturer on tool dimensions; (2) There is no hardware assistance available; (3) Unable to Learn how to install the tool to the robot flange. In the face of these limitations, every time a tool is changed, the operator must waste a lot of time to calibrate the tool.
Accurate Evaluation Simplified CalibrationServotronix has developed a method for quickly and accurately estimating tool geometry without the need for external sensors, vision or other assistance, and without the need for disassembly tools. In this positional teaching method, the operator simply puts the TCP of the 6-DOF robot in several different poses, and then automatically inputs the Servotronix tool size evaluation algorithm. The algorithm can quickly determine the precise calibration parameters of the new tool so that the tool can be quickly put into use.
The accuracy of this calibration method will increase as the tool pose sample increases. Our experiments show that using an inverse-conversion matrix may not necessarily produce the desired result, but using a least-squares method will produce an accurate calibration value.
Servotronix's methodWe used a 6-DOF robot with a tool, six Servotronix high performance CDHD servo drives, and a Servotronix softMC controller for testing. Our method only involves analyzing the calculation without disassembling the tool. We only evaluate the XYZ dimensions and assume that the tool's endpoints are in a constant Cartesian position.
It is self-evident that all robot poses that point to the same location must be on a sphere and the tool endpoint is in the center of the sphere:
TCP can be calculated by measuring the point on the sphere.
Where t represents the center
(1) R2 = (X - Xt)2 + (Y - Yt)2 + (Z - Zt)2
There are four unknown parameters (R, Xt, Yt, Zt) in equation (1). The X, Y, and Z values ​​are calculated by kinematics positive solutions. To achieve acceptable accuracy, our method requires the use of at least four points to define a sphere. Therefore, four such settings would be:
By using the equation subtraction, we can not only eliminate the unknown variable R, but also eliminate all non-linear components in the equation. This will generate a set of polynomial equations of degree 1, which can be solved by least-squares fitting. If more than 4 points are used, more equations and higher accuracy will result.
This step-by-step method takes only a few minutes to complete and requires a total of at least four measurements, as follows:
Servotronix's method is fast, accurate, and economical and can be calibrated without disassembly tools. This method eliminates the need for dedicated hardware and saves the time and effort required to install new tools. Machine manufacturers can easily use this method to quickly, accurately, and almost costlessly recalibrate the tools of a 6-DOF robot, thereby enhancing the robot's smooth operation in a wide range of applications and speeding up production.
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