Evaluation of an industrial welding robot's pose performances.

Vacarescu, Valeria ; Draghici, Anca ; Argesanu, Veronica 等

1. INTRODUCTION

The industrial robot applications require the execution of work tasks with an established precision. The evaluation of the robot's precision it made through performance characteristics. In the robotics field are presented different methods and techniques to determine the performance characteristics of the industrial robots. So, (Braglia et al., 2000), proposes a global evaluation of the robot's performances through a method based on the Dimensional Analysis Theory. (Ahmad et al., 2008) used a laser interferometer to determine the pose performances for a robot type Fanuc. (Hung-Hsing, 2008) presents the methods and techniques for posture estimation of an autonomous mobile robot, using a laser scanner. This paper proposes the evaluation of the pose performance for a welding robot, in different points within the working space, using an optical method with two digital tachimeters. It is evaluated the influence of target's location in the working space over the robot's pose accuracy and repeatability. So, it can be determined the optimal location of the robot's end-effector for the given industrial application. The algorithm and the method were presented in (Vacarescu, 2009). Comparable with the manufacturer specifications, the experimental results validate the used testing method.

2. METHODOLOGY

The analysed robot is designed for welding and sharing operations. It's a robot which has only rotation joints, generating a spherical work space. For testing the robot's pose characteristics, is selected the base coordinates system. The robot's movements are reported to this coordinates system. For measurements are used two electronic tachymeters. The device is a complex instrument, produced by Carl Zeiss, which allows the measurement of the horizontal angles (Hz) and vertical angles (V), but also allows measuring distances through telemetry. So, the device is a theodolyt and also a telemeter. To perform the measurements regarding the robot's pose performances, the robot's end-effector was equipped with a calibrated target cube, with an edge of 50 mm (fig. 1). For measurements were used two electronic tachimeters REC-Elta, placed in the robot's work space as in fig. 1. The base measuring principle is simple: one point of the calibrated target cube (in this case a cube's corner), fixed on the robot's end-effector, is situated on the intersection of two aiming lines with two tachymeters, T1 and T2, (fig. 2).

[FIGURE 1 OMITTED]

Measuring the angles of the two aiming lines in horizontal plan ([[alpha].sub.H]= Hz) and in vertical plan ([[alpha].sub.V]= V), using the two electronic tachymeters, as it can be seen in fig.2, and then using the triangulation methods, are determined the coordinates of the aimed point. This steps are repeated for the target cube's corners, N1, N2, N3 and N4 and in this way are determined their coordinates. These coordinates are expressed reported to a reference system with it's origin in the tachimeter's T1 optical centre (fig. 2). To calculate the aimed point's coordinates is necessary to know the position of the optical centre of the second tachymeter. This position is determined through telemetry between the two devices. In this way were determined the optical centre's coordinates for the second tachymeter, T2(0; 5,98592; 0,02721) reported to tachymeter T1 (fig.2). It is mentioned the fact that these coordinates are expressed in [m], the measuring unit for distance selected in the initial menu of the device's software. In this way was created the measuring base, which remains unchanged during the experimental protocol.

For all the measurements realised on the analysed robot, the target is represented by the four corners of the calibrated cube, marked with numbers and also represented by the robot's welding wire electrode head.

[FIGURE 2 OMITTED]

3. THE EXPERIMENTAL DETERMINATIONS

All the experimental tests were made under the recommendation of (***, 1998). All the tests were made in normal environmental conditions in which the robot works (in industrial environment), the devices are equipped with temperature and pressure sensors, and the correction of the measured parameters is realised automatically depending of the environmental conditions.

[FIGURE 3 OMITTED]

The tests were made for 100% of nominal velocity and for a load exceeding the nominal load (proxy. 113%), because was also considered the weight of the calibrated cube with its device. The measurements of the pose characteristics (positioning and orientation) were made for five testing positions (points P1, P2, P3, P4, P5), placed in the most used area of the robot's working space, recommended by ISO 9283:98. The placement of the testing points reported to the reference pose (initial pose) can be observed in fig. 3 a) and b).

The testing points were stored in the robot's computer by learning. After that the robot was commanded to go back in the learned points in the following way: P1-P5-P4-P3-P2-P1. There were realised 30 measurement cycles. In each position were aimed all the corners of the target cube, N1, N2, N3, N4, using the two tachymeters (fig.2). The angles [[alpha].sub.H], respectively [[alpha].sub.V] were measured. The coordinates x, y, z, of the aimed points N1, N2, N3 and N4 were calculated. This coordinates represent the initial data for the algorithm which calculates the pose accuracy and repeatability. All these tests were made moving the robot in the testing points, commanding simultaneously all robot's actuators. So, the error of the end-effector cumulates the effect of the errors produced by all robot's movements. Based on the obtained measurements were determined: the positioning accuracy, the positioning repeatability, the orientation accuracy, and the orientation repeatability, for each of the five points from the robot's working space. The values of these parameters are presented in table 1 respectively in figure 4.

4. CONCLUSIONS

Based to the experimental results can be formulated the following conclusions:

--analysing the pose parameter's values experimentally obtained in the all of the five testing points of the robot's working space, it can be observed that the parameters have minimal values in P3 and P5. In conclusion these positions are optimal for industrial applications using this robot in the analysed working space area;

--so, it is useful to identify the optimal area for conceiving a flexible manufacturing cell, from the robot's working space;

--the method used in this paper for determining the industrial robots pose characteristics is validated through the test's results. The obtained experimental values are comparable with the manufacturer specifications;

--the used method and the used algorithm allow the matricial expression of the pose characteristics, mathematically language used in the robotics field;

--the method is easy applicable in the industrial environment (the devices being portable) and the measurement base can be realized very easy;

--it can be seen the generality character of the method and the algorithm used in this paper, for other types of industrial measurements, but also in medical applications using targets (markers) with proper forms.

[FIGURE 4 OMITTED]

In the future work, the method and the algorithm used in this paper will be adapted for postural measurements, using two CCD cameras for investigating the operators in the dental medicine in the project: PC 91-0222 /18.09.2007 "The ergo engineering of the work place--Applications in the dental medicine".

5. REFERENCES

Ahmad, R. I. et. al (2008). The performance analysis of industrial robot under loaded conditions and various distance, International Journal of Mathematical Models and Methods in Applied Sciences, Issue 2, Vol. 2, pp. 277-283, 2008, ISSN 1998-0140

Braglia, M. & Gabbrielli, R. (2000). Technical note: Dimensional analysis for investment selection in industrial robots, 2000, VOL. 38, NO. 18, Issue 2, Volume 2, 2008, pp.4843-4848 International Journal of Production Research ISSN 0020 [+ or -] 7543, Taylor & Francis Ltd

Hung-H. L. & Ching-C. T. (2008). Laser Pose Estimation and Tracking Using Fuzzy Extended Information Filtering for an Autonomous Mobile Robot, Journal of Intelligent and Robotic Systems, vol. 53, issue 2, oct. 2008, pp. 119-143, ISSN 0921-0296

Vacarescu, V. et al. (2009). An Optical Method for the robot's performances testing, The 20th International DAAAM Symposium--Proceedings, ID1069, 25-28th November 2009, Vienna, Austria

*** (1998) International Standard ISO 9283:1998, Manipulating industrial robots--Performances Criteria and test methods, Geneve, Switzerland Tab. 1. The robot's pose performances Parameters Pose 1 Pose 2 Pose 3 Pose 4 Pose 5 AP [mm] 0,42983 0,43814 0,18814 0,32470 0,27815 [AP.sub.a] [[.sup.0]] 0,00113 0,00051 0,00120 0,00060 0,00261 [AP.sub.b] [[.sup.0]] 0,00660 0,00076 0,00030 0,00032 0,00050 [AP.sub.c] [[.sup.0]] 0,01033 0,00029 0,00170 0,00013 0,00060 RP [mm] 0,70155 0,62989 0,55469 0,79285 0,51407 [RP.sub.a] [[.sup.0]] 0,01395 0,00958 0,07722 0,01369 0,15929 [RP.sub.b] [[.sup.0]] 0,01123 0,01104 0,01172 0,01052 0,01051 [RP.sub.c] [.sup.0]] 0,90680 0,51664 0,38388 0,37919 0,37632