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An Accuracy and Repeatability of a Robot made with V-Slot Extrusion with built-in Linear Rails

Recently, aluminum extrusion with a built-in V-rail linear bearing draws increasing attention, especially among the hobbyist and makers, and is applied into many of their projects. The aluminum extrusion is an open source, light weight, and coming with extensive accessory. This significantly reduces cost and gives flexibility to their projects. Its performance is reasonable and adequate for operations among hobbyist and university’s projects. However, the precise accuracy of this kind of robot is usually unavailable. Accordingly, the purpose of this paper is to investigate and analyse performance of the V-Slot extrusion with built-in linear rails robot in terms of accuracy and repeatability. The tools, in this investigation, can be obtained within the ecosystem and the process can be done by the user. This performance result will help general users to define the scope of the operation that can be achieved with reasonable result when using this robot. The methodology, in this experiment, is Trajectory analysis approach that commands the robot to move in both linear and circular, with various travel distances, in a single run, and determine the defects. Effects of travel speed are also investigated. From experimental result, the accuracy of the experimental robot is 0.45 mm in the entire range of motion. This methodology can be applied to other machines with their accuracy not better than 0.1 millimetre due to the precision of tools that are used.


Introduction
Since 1980, industrial robots were designed with high accuracy by using high-grade materials [1], intricate assembly process, and advanced controller. They are comparatively expensive among automation devices. By the technological advancement, robot's performance is enhanced, while its cost continues to decrease. As the robot's cost to perform ratio reduces, the robots have been widely introduced into new applications. Recently, aluminum extrusion with a built-in V-rail linear bearing catches more attention, especially among the hobbyist and makers, as it can be seen in many activities; 3D printer (1), Farmbot (2), Braille Printer in Figure 1. The aluminum extrusion is an open source, light weight, low cost, and coming with extensive accessory. Its ecosystem is very interesting. This gives flexibility to both the design and how we source parts, which can be obtained by various vendors. The performance of this robots also appropriates for daily activities which only sub-millimetre precision is adequate. Though, for this robot, its resolution is usually given by provider, its accuracy and repeatability are usually missing and cannot be exactly identified since the robot was fabricated under ecological communication. The international standard ISO 9238 [2], that is used to determine  [3], requires precision, expensive tools and timeconsuming process, is not applicable for the user in aluminum extrusion's ecosystem. Furthermore, the accuracy and repeatability are for the robots, not the task. This paper focuses on the investigation of accuracy and repeatability of aluminum extrusion robots while it is performing on specific tasksgripping and moving--by using trajectory analysis approach. Trajectory analysis approach is designed to be a simplified method that requires only ecological equipment and lessens performing time, which is accessible by user. Trajectory analysis approach is the accuracy and repeatability investigating method focusing on the shifting of value between desired trajectory and actual performing trajectory of the robot.

The Cartesian robotic system
The robot used in this experiment is fabricated by Thai StartUp company "Easy Builds". The components of robot shown in Figure 2 are built with 3030 V-Slot aluminium Extrusion profile, a feed screw and rollers. The thread of the screw is 1 mm. Each axis is driven by NEMA stepper motor with micro stepping drive and Arduino board that can give the resulting velocity up to 50 mm/s for X and Y axes and 10 mm/s for the Z axis with 0.0025 mm resolution. The workable space is 150×150×150 mm 3 . The controller board is RUMBO 3D printer control board with Repetier firmware.

Design of experiment
To investigate the accuracy and repeatability of the experimental Cartesian robot in moving and gripping task, trajectory analysis approach has been assigned. The Z-axis of this experimental robot is in vertical direction against gravity field while X-and Y-axis are in horizontal plane. Thus, the characteristic of motion robot's arm in XY-plane will be differ from the motion in Z-axis. Furthermore, the G-code [4], which is the ISO standard language used to control the machine, commands the motion with linear or circular trajectory. It is noted that G01 is for linear movement, G02 and G03 for clockwise and counter clockwise circular curve respectively, G04 for time delay which could stop the motion of all axes for a given amount of time. For the accuracy investigation, we command the robot to move in circular motion with various radius, e.g. 10,20,30,40,50,60 mm and radial motion with various directions. E.g. and each set up of the robot is commanded with two velocity, high and low velocity. In this designed moving pattern, both linear and circular trajectories are investigated in a single run. For the repeatability test, we command the robot to move repeatedly in circular motion for 30 mm in an entire workspace in horizontal plane. In the task, the robot grips a pen in its hand and draws lines on the paper. A 0.5 mm felt-tip pen is used. For the image processing process, scanning ink trace graphical is result in a resolution of 300 dots per inch (about 0.085 mm resolution) to measure all necessary dimensions in sub-millimetre tasks; the circle's radius, length of the line. Moreover, for an accuracy investigation, constructing the circumferential band that cover the ink trace to represent the minimum and maximum radius of the ink trajectory; the width of the band will

Experimental result
The experimental accuracy is demonstrated in Figure 3. From the experiment, in the work space of 150x150x150 m 3 , the accuracy, as analysed in Table 1, of this robot is 0.45 millimetre under the entire range of travel velocity. The accuracy linearly degrades as the robot's travel velocity increases. If the travel velocity is too high (2400 mm/min in the repeatability test, as in Figure 4 and summarized in Table 2), the repeatability degrades quickly. The tools, that are used in the proposed technique, give the variation of result around 0.1 millimetre which could be used to examine the accuracy at about 0.5millimetre resolution. This is sufficient for this type of robot that its accuracy is about 0.5 millimetre and for our case that the resulting accuracy is only 0.45 millimetre. The effect of travel's distance also linearly degrades the accuracy. This could be corrected at the step/millimetre in the controller's firmware. The accuracy is better when the robot travels along its hardware's axis, in X-direction in this case, and slightly poorer in the incline direction. The linear trajectory gives better accuracy. These preliminary results may be used to design the task's trajectory for a better accuracy.

Conclusion
In this investigation, the robot with V-slot extrusion with 150x150x150 m 3 workspace, give the resulting task accuracy of 0.45 millimetre under the entire range of motion. This task accuracy is a lot lower than 0.0025 mm resolution of the robot actuation system. However, this performance is enough to accomplish most of operation in hobbyist's works and university's projects which are only submillimetre tasks. The travel's distance and speed also affect the accuracy of the robot. If the circular trajectory is too big, the robot may degrade its accuracy quickly. The effect of travel speed is observed in the repeatability test as we test the robot with lower and higher velocity, compared to in the accuracy test. The repeatability degrades as the robot moves faster.