EGR 315 Design Project - 1 - Executive Summary Vibrations can have an adverse effect on the accuracy of the end effector of a multiple-link robot. The ability of the machine to move to precise points scattered around its work envelope repeatedly over time is essential to its purpose. Any vibration occurring at the manipulator will jeopardize this precision. The purpose of this design study is to examine the vibrations at the end effector, determine the sources of these vibrations, and suggest design improvements to reduce the effect of the vibrations. The study was done by obtaining some preliminary knowledge of similar studies done in this area, researching the machine being studied, taking experimental data using accelerometers and a data acquisition system, analyzing the data taken, and determining the source and solution to vibrations seen at the manipulator. Several design solutions were proposed, including the addition of a vibration damping material between the links of the robot, and some suggestions for future design studies involving mechanical aspects of the robot design.
EGR 315 Design Project - 2 - Table of Contents Introduction... 3 Literature Review... 3 Research Approach... 4 Experimental Details... 6 Data Analysis and Observations... 9 Design Studies... 11 Conclusions and Future Work... 12 References... 13
EGR 315 Design Project - 3 - Introduction The focus of this design project is to analyze the effect of vibrations on the end effector of a five axis Mitsubishi RV-M1 robot. Specifically, the focus was on the movement of the gripper at the end of the movement to a certain point. When the robot stops moving there is a considerable amount of vibration in the end effector, so much that it is visible to the naked eye. This movement may have adverse effects on the accuracy of the robot, which is one of the key advantages of using robotics. The purpose of this design project is to measure the vibration seen at the end effector and determine the source of the vibration. Once this information was gathered, design solutions were proposed to isolate the sources of vibration from the end effector. The following report contains information on studies that have been performed in this field, the research done on the specific topic, the data collected during experimentation, analysis of this data, and design solutions based on the research and data. Literature Review Much of the research that has been done in the field of robot manipulators has dealt with the effect of vibrations on the end effector of flexible arm robots. Internet searches were the main source for information regarding this topic. Two articles regarding this subject were found, both focusing on similar testing. Tests were done measuring the accuracy of the end effector after being given a specific point. The error seen in the position of the effector has been attributed to the accuracy of the motors controlling the rotation of the links of the robot arm (5). The studies did measurements on end effector position before and after adding vibration damping in the links. Specific
EGR 315 Design Project - 4 - information on how these vibrations were isolated was very brief, and details regarding the exact method of doing so were not available. Research Approach The research done on this project involved examination of robotics manuals, texts on robotics, several websites devoted to the use of robotics, and experimentation with the robot being considered. The main focus of the research involved determining the equations of motion for the robot arm. There was no discrete information regarding the equations for the specific robot considered in this project, which is why robot manuals were examined. The manuals had information regarding the development of equations of motion for robots, but the mathematical methods involved were not familiar to the analysts. Equations for a two-arm manipulator were provided, and these were used as the theoretical basis of the design project. The equations included terms accounting for the angular acceleration of the joints, the effect of gravity on the system, and the coriolis effect. For our system, it was sufficient to ignore the effect of gravity since the robot is fixed to the table, and to neglect the coriolis effect because we were moving the arm at slow speeds which did not produce significant centrifugal forces. Due to the fact that the theoretical system matrix was that of a two-link manipulator and the experimental system had five degrees of freedom, it was not realistic to compare the results of the two and determine whether or not the model was an accurate representation of the actual system. Another constraint to the analysis was the wide range of motion that the robot can possibly move through, and the large operating envelope, which is displayed in Figure 1.
EGR 315 Design Project - 5 - Figure 1 Operating Envelope of Mitsubishi RV-M1 Robot (7) To simplify our analysis and keep within reasonable time constraints, one particular motion of the robot arm was chosen to do analysis of. The goal was to do analysis on the worst-case scenario; the movement that produced the maximum magnitude of vibration in the end effector. This determination was made based on visual analysis of the vibrations seen with varying angles between the links. The resulting case was that of the arm extended to the limit of the operating envelope, where the end effector was farthest from the base. In this configuration, the robot has the freedom to move in two axes: up and down and left and right. Because of this freedom of motion, analysis was done on the vibrations seen while moving in each direction, with measurements made on both axes during each trial.
EGR 315 Design Project - 6 - Experimental Details To analyze the vibrations in the system, accelerometers were placed on the robot at the end gripper, the elbow and on the upper arm area. In the vertical direction, an accelerometer with the serial number 23351 and a conversion rate of 98.6mV/g was used and connected to channel 0. In the horizontal direction, an accelerometer with the serial number 23354 and a conversion rate of 110.3mV/g was used and connected to channel 1. When testing, the grippers on the robot were always closed and markings on the robots axes of rotation were used to make the data collection as repeatable as possible. Each accelerometer used was connected via a microdot cable to a signal conditioner. This signal conditioner is used the cut down on outside electrical noise and to clean the signal to a form that can be used by the data acquisition board. Each signal conditioner is connected to the data acquisition board (DAQ), which is then connected to the PCI card slot on the laptop. Once all connections are complete, and the laptop is running, Microsoft Excel is opened and the DAS Wizard->DAS Tasks tool is chosen. Using that tool, the rate of 2000 samples per second for a total of 5 seconds is set up for the correct DAQ board. After running the task that was set up, at least 3 times for the gripper, elbow and the upper arm and for movement vertically and horizontally, some problems were noted in the data. First, at times, perfectly horizontal voltages were noted. Obviously, these were not actual accelerations, but readings caused by interference or shorts in the circuitry. Additionally, we noted that when the robot was powered off, much of that interference disappeared. As seen in figure 2, the noise level is very low.
EGR 315 Design Project - 7 - No Power To Robot 0.003 0.002 0.001 Volts 0 0 2000 4000 6000 8000 10000 12000-0.001-0.002 side top -0.003-0.004-0.005 Figure 2. However, the best that we were able to obtain when the robot was powered on can be seen in Figure 3. Robot Powered On 0.006 0.004 0.002 Volts -0.002 0 0 2000 4000 6000 8000 10000 12000 side top -0.004-0.006 Figure 3. As can be seen from these figures, the input from the accelerometers during no movement is not zero, even when the robot is not on. However, when it is turned on,
EGR 315 Design Project - 8 - there is an increase in this ambient noise. The voltages peak at over.005 Volts or 5 mv. At times, this noise was significantly larger than what is seen in Figure 3 and was quite often a horizontal line, indicating a constant voltage from a source other than the accelerometers input. Another problem encountered can also be noted from the above two figures. Other than the fact that they are never constant at 0, indicating no movement and acceleration, the two inputs are also always offset from one another. Sometimes the input form the top is above that of the side, and at other times vice versa. This makes it nearly impossible to easily subtract out known noise from the input to get to the real acceleration inputs. Another problem encountered was separating out the vibrations due to the systems modes, and which ones are caused by any internal servo motors and vibrations caused by their spinning.
EGR 315 Design Project - 9 - Data Analysis and Observations Direction of Movement and sensor placement Up and Down, gripper sensor on side Frequencies peaks seen at 70.4, 140, 210.2 Up and Down, gripper sensor on top 70.5 Left and Right, gripper on side 70, 140 Left and right, gripper on top Minor at 140, equivalent to 60 Up and Down, measured at elbow at side Up and Down, measured at elbow on Top Left and Right, measured at elbow on side Left and riht, measured at elbow on top Up and Down, measured at Upper arm on side Up and Down, measured at Upper arm on top 70, 140, 210, 280, 350 70, minor at 140, 210, 280, 350 70 None 70, 140, major at 210 70, minor at 140, 210 Left and right, measured at Upper arm on side Left and right, measured at upper arm on top 140, 195 70, 140 both minor Table 1.
EGR 315 Design Project - 10 - When looking at the above table, it is easy to see a pattern. Nearly every time we looked at the fourier transform of a set of data, there was either 70 Hz or 140 Hz as a peak. At first, there was concern that these frequencies were not actually nodes. Since there are internal motors that run the arms, it was a concern that these frequencies were actually the frequency at which the motors were running. Once the manual for the robots was found, it was discovered that the robots servo motors run at 30W. Mitsubishi Electric s website lists a wide variety of servo motors, some of which run at 30W. However, those motors which run at 30W run at a max of 3000 rpm (6). To get to a frequency of 70 Hz, one would need to run at a speed of 4200 rpm. The motors were thus eliminated, at least to some extent, since they would have to be running well beyond their design to be running at 4200 rpm. Another concern was that electrical noise was adversely affecting our results. Thus, 60 Hz and multiples there of were watched closely. However, no peaks at 60 Hz or any multiples there of were discovered. Although some peaks were seen at 6-10Hz, these did not appear consistently and were most likely rigid body modes and appeared because we could not completely secure the robot. The peaks at 70 Hz and multiples there of appeared in virtually every area of measurement and during side to side and up and down movement of the robot. When looking at multiple data sets and fourier transforms for the same times of movements and the same sensor placement, it was amazing how much each looked alike. Even when comparing what seemed like completely separate sets of data and types of motion, the same peaks would occur at multiples of 70 Hz.
EGR 315 Design Project - 11 - Design Studies Based on research and experimental data recorded, there is limited design solutions to the system being analyzed. Several sources can be attributed to causing the problems this project has set out to solve. The first is the mechanics of the robot. It uses DC motors to control the link arms, and these have inherent limitations that cannot be overcome within the scope of the project. More accurate and smoother running actuators could be implemented, but the scope of this study is to analyze the conditions that exist within the current configuration. The controller of the robot is another source of error. There is a certain time delay between the event of the user pressing a button to move the arms and the actual movement of the robot itself. In a similar fashion, there is a time delay when the user wants the robot to stop and when it actually stops moving. Once again, a more accurate and faster transmitting system could be implemented to alleviate some of the problems, but that is not in the scope of the project. The design solution that is best suited to this application is the isolation of the vibration causing elements from the end effector. This can be done with isolation rings, which are simply ring-shaped elements, composed of rubber or other vibration absorbing material. These could be placed at the joint of each link along the arm of the robot, each one essentially isolating one link from the other. The net result would be a significant reduction in the vibrations transmitted to the end effector, which is the end goal of the design study.
EGR 315 Design Project - 12 - Conclusions and Future Work Based on the results of the design study, it can be concluded that vibrations do have a significant effect on the end effector of a multiple-link robot. The source of these vibrations is mainly the actuators controlling the movement of the joints; in this case DC servo motors. There are other sources of concern that may attribute a smaller amount to the excitation of vibrations in the end effector, but solutions to these problems were not part of the problem definition. An effective solution to reduce the effects of vibration in the manipulator is the use of a damping material between each link of the robot, to isolate the manipulator from the sources of vibration. Future design studies could focus on aspects of the design that were within the scope of this project. Design of a more accurate and faster transmitting control device for the robot is one area that research can be done, along with designing a more accurate actuating system to move the links of the robot.
EGR 315 Design Project - 13 - References 1.) Fu, K.S., R.C. Gonzalez and C.S.G. Lee. Robotics: Control, Sensing, Vision, and Intelligence. McGraw Hill. 1987. pp. 82-103. 2.) Craig, John J. Introduction to Robotics: Mechanics and Control. Addison Wesley. 1989. pp.104-119. 3.) Thomson, William T. and Marie Dillon Dahleh.Theory of Vibration with Applications. Prentice Hall. 4.) Learning Ballistic Movements of a Robot Arm. Internet source. http://www.prt.fernuni-hagen.de/lehre/kurseprt001/course_main/node29.html 5.) Kobayashi, Nobuyuki and Kenji Tamechika. Vibration Suppression Control of Flexible Robot Arm. Aoyama Gaukin University, Setagaya-Ku, Tokoyo, Japan. 6.) Mitsubishi Servo Motor Line-up. Mitsubishi Electric. Internet source. http://www.nagoya.melco.co.jp/english/servo/lineup/motor/svmtr01l.htm 7.) MovemasterRV-M1 Micro Robot. Rixan Associates, Inc. Internet source. http://www.rixan.com/rv_m1robot.htm.