ME 5286 Robotics Lab Lab 4: Flashlight Assembly Duration: 3 Weeks (Mar 4 Mar 29; the 3 weeks does not include spring break)

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1 ME 5286 Robotics Lab Lab 4: Flashlight Assembly Duration: 3 Weeks (Mar 4 Mar 29; the 3 weeks does not include spring break) Note: Two people must be present in the lab when operating the UR5 robot. Read all warnings and cautions in the manual. Once you are done with all your tasks for the day, 1) Save all your programs to permanent storage (i.e. flash drive) and 2) Remove all of your programs from the robot tablet (if you uploaded any). Failure to remove your programs from the robot controller will result in a loss of points. There is a TCP offset and mass created by the gripper and the load cell. For both robots, use a gripper offset in the Z-direction of mm. Use a mass of 1.5kg for Robot 1 and 1.85kg for Robot 2. Objective: Use the UR5 robot and a pneumatic chuck to assemble a flashlight. Lab Procedure: For this lab you will be programming the robot using the RoboDK Python API to assemble a flashlight. The flashlight has undergone subassemblies before assembly with the robot, reducing the assembly to four parts. The four parts that you will be assembling are the head, barrel, tail cap, and battery as shown in Figure 1. Figure 1: Flashlight parts that will be assembled during the lab. 1

2 In order to assemble the flashlight, you will need to clamp the head of the flashlight into the pneumatic chuck mounted to the table, which is controlled by digital I/O in the robot controller. A sample script for using the pneumatic chuck is provided in the Appendix and is posted to the course website. The layout of the components is shown in Figure 2. The four flashlight components will be placed onto a tray. The drawing file for this tray can be seen in the Appendix. Figure 2: The frame of the robot, {XR, YR}, relative to the frame of the chuck, {XC, YC}, and tray, {XT, YT}. The origin of the tray frame is located in the center of the hole in which the flashlight head is mounted, as outlined in the Appendix. The flashlight tray, as seen in Figure 3, has mounting holes around each flashlight component insert. These are for you to mount fixtures on to the flashlight tray to customize or make your assembly process easier. Creativity points will be awarded to students who do any creative modifications that improve the assembly process. Examples of modifications could include improving the fixturing, sensing, or control of the robot. The steps of the assembly should go as follows: 1) Place the head into the chuck. 2) Clamp the chuck using the Digital I/O. 3) Place the barrel onto the head. 4) Screw the barrel onto the head until 3 Nm is reached. Use the tighten_torque() function to do this. Ignore the torque readings from the first few rotations as high forces will occur when you try to insert onto the threads. See Appendix for more information on monitoring torques. 5) Insert the battery pack into barrel. Make sure you place the battery pack into the flashlight with the correct polarity. 6) Place the tail cap onto the barrel 7) Align and screw the tail cap until a torque of 2 Nm is reached. 8) Unclamp the assembly with the Digital I/O. 9) Remove the assembled flashlight from the chuck and place it in the head location of the flashlight tray. 2

3 Notes: - Verify that all the flashlight components are oriented in the same manner for each test. - Tightening the threads with the robot takes a lot of time. After you have the threading started, you can pause your threading task, manually thread the next few rotations and then resume the robot doing the threading. This will save you considerable time during the robot assembly task development. When you take a video of your assembly or demo your assembly the robot needs to perform the whole assembly without human assistance. - Ignore the torque measurement during the first few thread rotations. The torque will be high during the initial threading but will come down. - Start the assembly at slow speeds. Once you have the flashlight assembly working, then try to assemble the flashlight as quickly as possible. The speed setting that we started out with are: speed_ms = 0.25 [m/1], speed_rads = 0.25 [rad/s], acc_mss = 0.2 [m/s^2], blend_radius = [m]. - When you release a part, it may stick to the gripper and become misaligned. Think about gripper release methods which will minimize this issue. - The positions in Table 1 provide you with a starting point for the location of parts. You will need to fine tune these locations in order to assemble the flashlight. - Adding delays into your assembly program can be helpful. - Save multiple versions of your code. If you have parts of the assembly working, save it. Keep track of version numbers of your code. - Sometimes RoboDK will delete Python code. Make sure to save a copy of your Python code external to RoboDK. - Slowing down the robot s movements before inserting a part will result in a more accurate assembly and will not trigger the protective stop as easily. - The robot has the load cell attached but the cable is disconnected. If you are a motivated student and would like to incorporate impedance control to your flashlight assembly process, please contact a TA. It will be your duty to disconnect the load cell when you are done. Using impedance control would be one way to earn creativity points. - Note that get_joint_torques() is a command to get the joint torques from the robot itself (called within tighten_torque function). You will not need the external load cell to do this. The external load cell could be used if you would like to implement impedance control. - The robot often tries to rotate past its joint range when using its inverse kinematic solvers. Adding poses defined by joint angles will reduce this issue. - Especially during screwing operations, be careful not to pull any cables that are attached to the robot. Coming up with a practical solution for handling the cords or dealing with any other problematic part of the assembly process can be counted toward creativity points. 3

4 Table 1: Fixture locations with respect to the robot base frame. The Z direction is omitted in this table. Fixture/ Component X (mm) Y (mm) (1) Tray (2) Chuck Deliverables: - A single report which addresses the following issues and includes: 1) An abstract, introduction, methods, results, discussion, and conclusion section. 2) Description of the steps and method you used to assemble the flashlight. 3) How would you describe the effectiveness of assembling the flashlight? Is your code good enough for an industrial flashlight assembly? Why or why not? 4) How quickly were you able to assemble the flashlight? Provide the time in seconds that it took for your assembly. 5) Is there anything creative you added to the flashlight tray to make the assembly easier? Explain what you added and how it helped the assembly process. 6) Describe how you would speed up the assembly without increasing the robot s speed. Assume that you are in an industrial setting and have ten thousand dollars left in your budget. 7) Based upon your experience with the UR5 robot, what are other assemblies that you feel confident doing? Provide details on how you would accomplish these other assemblies. - A single zip file named [last_name]_[first name]_lab4.zip uploaded to Canvas which contains: 1) A PDF of your report. 2) All robot files (RoboDK, Universal Robot Projects). 3) A video of your robot assembling the flashlight. The format for the video should be.mp4,.avi, or.m4v. - Demos of your assembly will be evaluated by Professor Donath from April A hard copy of your report and the digital submission are due 24 hours after your demo. A grading rubric will be provided at a later date. Issues may come up and questions may be posed by students. When appropriate, we will share these with others in the class. It is your responsibility to check your and Canvas announcements. 4

5 Appendix: Barrel Battery Tail Cap Head Figure 3: Drawing file of the flashlight tray that holds parts for 1 flashlight. The tray coordinate frame is centered where one head piece of the flashlight is located. All units are reported in inches. Figure 4: Drawing file of the chuck assembly. All units are reported in inches. 5

6 Figure 5: Sample script for clamping and unclamping the chuck. 6

7 Using Pneumatic Chuck Commands in your Code: The post processor Universal_Robots.py, which is what your Python code goes through to generate URScript code in the lab, has been updated on the computers in ME 50B. They have been modified to include the clamp() and unclamp() functions shown above in Figure 5. Additionally, it declares global variables for pin_clamp = 0 and pin_unclamp = 2. Essentially, this means that as long as you are generating your code from the computers in ME 50B, the only lines of code you need to clamp or unclamp the pneumatic chuck are: robot.runcodecustom('clamp()', INSTRUCTION_INSERT_CODE) robot.runcodecustom('unclamp()', INSTRUCTION_INSERT_CODE) Note: You may notice the INSTRUCTION_INSERT_CODE argument is different from the INSTRUCTION_CALL_PROGRAM we used in lab 3. It is suggested that you use INSTRUCTION_INSERT_CODE since it will type exactly what you specify in the post processed code. INSTRUCTION_CALL_PROGRAM doesn t allow spaces and other characters. Additionally, you will want to use robot.runcodecustom('declare Variable', INSTRUCTION_INSERT_CODE) to declare variables within your code that are not supported by the post processor, such as a variable to get the joint torque values of the UR5 robot. Steps to Change Post Processor: 1. Right click on the UR5 robot, select Options in the drop-down menu (see below) 7

8 2. In the top right of the panel that pops up, click parameters (see below) 3. Another pop up called Robot Parameters should appear, and you should see a post processor section. Next to Post Processor you should see a drop-down menu. Click the drop-down menu and select Universal_Robots_me5286 (see below). 4. This is the post processor that will automatically add the clamp() and unclamp() functions. 5. Some of you may get an error when you try to Generate robot program using this post processor saying that there is no file in C:\Users\X500\temp\filename.py. a. To fix this, save the Python program you are working on in the path C:\Users\X500\temp\ (where X500 is the X500 of the user logged in) b. Reopen the Python program saved at this path in RoboDK. 8

9 Screwing on Flashlight Components while Monitoring Torque: Monitoring the torque while screwing on components isn t straight forward in RoboDK Python API. It would require rewriting a large portion of the post processor used in RoboDK or using many robot.runcodecustom() commands to construct while loops. Instead, a function was created in the post processor that takes care of twisting on the component while monitoring the torque of the 6 th joint of the robot. This function is automatically added to the end of your post processed code when using the computers in ME 50B. The function is called tighten_torque() and should ONLY be called once you have almost fully twisted on the flashlight component without checking torque (only a turn or so left), then this function will finish tightening the piece on. Once the measured torque exceeds the torquelimit you set, the robot will quit twisting the piece on. You will have to insert this function into your code by using RunCodeCustom(). The definition of the function is below; def tighten_torque(torquelimit, wrist3_startangle, wrist3_endangle, jointaccel, jointspeed, num_checktorque, gripperforce, gripperspeed, gripperopen): Where the arguments are; torquelimit = Max allowable torque for 6th joint on the UR5 wrist3_startangle, wrist3_endangle = Start and end angles of the 6th joint for twisting (rad) jointaccel = Acceleration of the joints jointspeed = Speed of the joints num_checktorque = The number of times you want to check the torque during the twist gripperforce = Force of the gripper (1-100) gripperspeed = Speed of the gripper (1-100) gripperopen = The amount you want the gripper to open (1-100) Using this function in your Python code may look like this: robot.runcodecustom('tighten_torque(5, 0, 1.57, 2, 2, 1, 100, 100, 50)', INSTRUCTION_INSERT_CODE) 9