Learn to grip objects using the UR5 robot and a ROBOTIQ adaptive robot gripper and perform some simple gripping tasks.

Transcription

1 ME 5286 Robotics Labs Lab 3: Gripper Control Duration: 1 Week (2/12 2/16) 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 or H drive) and 2) Remove all of your programs from the robot controller 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. Use a gripper offset in the Z-direction of 148.9mm and a mass of 0.9kg. Objective: Learn to grip objects using the UR5 robot and a ROBOTIQ adaptive robot gripper and perform some simple gripping tasks. Figure 1:ROBOTIQ Adaptive Robot Gripper 2-Finger 85 1

2 Prelab: - Download and look over the ROBOTIQ gripper manual. Specifically look at the commands to communicate with the gripper Lab Procedure: Task 1: Open and close the gripper using PolyScope and RoboDK Python API. Task Steps 1) Turn on the robot and follow the example provided in the gripper manual, Gripper Node p ) Review the additional function options on pages and try a few of them using the Script Code block on PolyScope. You will be using several of these functions in the later tasks and labs. (i.e. test rq_close(), rq_close_and_wait(), rq_open(), rq_open_and_wait() ). 3) Use RoboDK Python API to open and close the gripper. Do not include any move commands in your script for this task. Simply make sure you can run rq_close(), rq_close_and_wait(), rq_open(), rq_open_and_wait() within your Python script. Se a. Note: You will NOT be able to visualize the opening and closing within RoboDK itself, as RoboDK doesn t support this. You will have to test it on the actual robot but you can write the code beforehand. Task 2: Grip the load cell assembly (Figure 2) and create a relationship between grasping force with a force and speed input. For this task you will be calibrating the gripper s force in Newtons as a function of the speed and force setting in RoboDK Pyhton API. Note: Python uses an 8bit (0-255) number to vary the force and speed setting while PolyScope varies from 0%-100%. Task Steps 1) Determine the location of the load cell on the table by free driving the robot to the location of the load cell. Position the robot so it grips the load cell assembly as shown in Figure 2. 2) You will now be comparing the gripper s grasping force with that stated in the gripper s manual Figure (Figure 3, in this manual). In order to do this you will be grasping the load cell assembly and measuring the force reading while you vary the gripper s commanded force and speed. In order to compare and recreate this plot you will grasp the load cells with the following force settings: [0, 1, 50, 100, 127, 128, 150, 200, 255]. For 2

3 the speed setting you will pick three speed setting before the corner or change in gripper s change in slope (~ ), one at the corner, and three above the corner. 3) Write a Python program which grasps the load cell assembly using the nine force settings and seven speed settings. You should have 63 total grasps in this program and each grasp should start with the gripper fully opened and finish with the load cell gripped for two seconds. 4) To record a dataset of the gripper s force, open a session of PuTTY on the computer and press Open to record the forces of the load cell into a file. 5) Once you have recorded the data for the 63 grasps you will need to parse the recorded dataset for the steady state gripping force of each grasp. The steady state gripping force occurs after the impulse and transients of each grasp has died out. (Writing a MATLAB script to do this will save you a lot of time.) Figure 2: ROBOTIQ gripper grasping the load cell assembly. The gripper is oriented at RX = 0, RY = 3.146, and RZ = 0. 3

4 Figure 3: Measured grip force according to speed & force setting for 95 HV hardness material (6061-T6 aluminum). Figure from the Gripper's manual. Task 3: Grab objects of varying stiffness and move them from one fixture to another. This is your first pick and place task and you will be grabbing ping pong and golf balls and moving them from one tray to another. You must not break the ping pong balls. Task Steps 1) Start a new Python script in RoboDK named ME5286_Lab3_Task3_[last name]_[first name] 2) Determine the coordinate transformation from the robot s base frame to pallet 1 and pallet 2. A sample figure illustrating the pallet s frames and the robot s frame are shown in Figure 4. 3) Determine the location of the six balls sitting in the pallet 1 s frames which can be seen in Figure 5 and the pallet drawing (Appendix). 4) Write a program which transfers all the six balls (three ping pong balls and three golf balls) from pallet 1 to pallet 2 and keeps the balls in the same configuration. The program should be written in such a way that the position and orientation of the pallets could be moved for other applications without re-writing the code. In other words, you need to 4

5 create a program which has a variable that takes in the position and orientation of each pallet. Figure 4: Overview of the table and pallet 1 and pallet 2 with respect to the robot. Note that this image is not to scale and may differ from what you see in the lab. Figure 5: Two pallets shown. The pallet on the right contains the arrangement of the three golf balls and ping pong balls. Pallet 1 and pallet 2 are identical. The drawing file for the pallet can be found in the Appendix. Deliverables: - A single PDF in memo format which includes: Task 1 Task 2 Task 3 1) Describe the difference between rq_close() and rq_close_and_wait(). 1) Using the results from Task 2, Recreate Figure 3 from the Gripper manual. Compare your gripping results with Figure 3. Are there any differences and when do they occur? 2) Does speed have an effect on the force calculation? 3) Theoretically is the gripper able to crush the ping pong ball? If so, what are the maximum force and speed settings that you can use before crushing the ping pong ball? A ping pong ball is made out of celluloid, detailed properties on ping pong balls can be searched for online. For this calculation use your plot from question 1. 5

6 1) What is the transformation from the robot s base frame to pallet 1 and pallet 2? How did you determine the transformation? 2) What gripping force and speed did you use for the different ball types and why did you choose these settings? 3) Describe your algorithm for Task 3 along with details of its portability. Show and explain all equations that you used. General UR5 Questions 1) Write the rigid body transformation matrix to the world frame, {X w, Y w, Z w }, positioned on the work table, from the base frame {X 0, Y 0, Z 0 } of the UR5 robot. In other words, find T 0 W using the information labeled in Figure 7. 2) Label the rotational axis of the UR5 robot, J i, for j = 1 to 6 on Figure 7. Make sure to include the annotated Figure 7 in your memo. 3) The zero configuration of the UR5 is shown in Figure 8. On Figure 8 clearly indicate the location of the origin of each joint coordinate frame on the robot and number them. Draw the coordinate frames that move with each joint for the six rotational joint axes in the corresponding boxes. Use the convention we discussed in class (same as textbook). Be sure to include this in your memo. 4) The elements of the A matrices from the (i-1) th to the i th joint can be calculated by using the Denavit- Hartenburg variables; fill in θ, d, a, and α for each A matrix in Table 1. Do this for the zero configuration from part C and Figure 8. Use the appropriate geometric dimensions, from Figure 7. Use the convention discussed in class (same as textbook). Be sure to include this table in your memo. 5) Suppose a stereo ranging camera is set up above the UR5 robot work table as shown in Figure 6. The stereo camera is able to provide the x, y, z position of objects residing on the table, such as the ping pong balls. Find the homogenous transformation between the world frame and the camera frame, T W C. 6) Determine the location of the part in the camera frame when the part is defined in the world frame as shown in Figure 6, that is determine {X C P, Y C P, Z C P }. 6

7 Figure 6: The robot setup on the table. The world frame and the camera frame are shown in both the top and front view. Note that the world and the robot base coordinate frame are not the same. - A single zip file named [last_name]_[first name]_lab3.zip containing a PDF your report and all robot files (RoboDK, Universal Robot Projects) uploaded to Moodle. 7

8 Figure 7: The UR5 robot shown in its "home" configuration [0, -90, 0, -90, 0, 0] deg. The world {X_W,Y_W,Z_W} and the robot base coordinate frame {X_0,Y_0,Z_0} are both provided in this image. All measurements are taken with respect to the base frame. 8

9 Figure 8: A top down view of the UR5 robot is shown in the zero configuration of each joint configuration [0, 0, 0, 0, 0, 0] deg. The robot base frame is {X 0, Y 0, Z 0 } and is provided on this image. Identify and label the location of the origin of each joint coordinate frame on the robot and draw the joint coordinate frames in the corresponding boxes, each of which is a top down view of the robot at the joint. Table 1: D-H Table for the UR5 robot Joint θ [rad] d [m] a [m] α [rad] EE

10 Appendix: Golf and Ping Pong Ball Pallet Figure 9: Dimensions for the pallet in Task 3. All units are in inches. How to Program Robotiq Gripper Movements in Python It s important to know what happens when you create your Python code within RoboDK for the purpose of controlling the UR5 robot. Once you hit Run on robot within RoboDk, your code goes through a Post Processor (see: This essentially converts your Python program into a language the robot can understand. For the UR5, this language is called URScript. For Lab 1 and Lab 2, you were able to simply use Run on robot in RoboDk to get your Python code to run on the UR5. However, since we are using the gripper this is no longer the case. Instead, you will be using FileZilla to transfer files to the UR5. Instructions on how to do this is outlined in the Appendix of Lab 1 as well as on the course webpage under Miscellaneous Instructions. Be sure to delete your file when done! In order to get our gripper commands to be entered into the post processed code from RoboDK, we need to get familiar with another Python command. This command is 10

11 robot.runcodecustom(). What robot.runcodecustom() does is add any text you put as an argument to the function into your post processed program. For example, typing robot.runcodecustom('rq_close()',instruction_call_program) in your Python code will add the line rq_close() in your post processed script. Take the below Python code: Once it gets converted to URScript (UR5 language) it looks like this: As you can see, the gripper commands are entered into the code and the actual UR5 will execute these commands, although RoboDK will not show the gripper opening / closing. To see what your Python code will look like (not required but sometimes useful) in URScript language, all you have to do within RoboDk is right-click on your program and click Generate robot program (see Figure 10 below), a screen should pop up with your post processed code. Figure 10. Generating Robot Program in RoboDK 11

12 Here are the general steps you ll follow for Lab 3 and Lab Write your Python script using RoboDK Python API. This includes using the command robot.runcodecustom( Desired_Command',INSTRUCTION_CALL_PROGRAM) to control the gripper. (must have INSTRUCTION_CALL_PROGRAM as argument) 2. Visualize the UR5 movements within RoboDk (the gripper won t move) to ensure that the robot is moving about the workspace as you expect. 3. Once you are satisfied with the robot s movements, save your code using Generate robot program. The robot should both generate a post processed script for the UR5. 4. Transfer the file over to the UR5 by using FileZilla (Appendix Lab 1). 5. Press Program Robot on PolyScope (touchscreen pendant) 6. Press Empty Program 7. On the tabs near the top of the screen, click Structure Advanced Script. 8. Press the Command tab button, change the drop down menu from Line to File and click the box. Navigate to your script file that you uploaded. 9. Press the Play button to run the script on the robot. 10. You can also change the force and speed of the gripper using rq_set_force(force_value) and rq_set_speed(speed_value). For a full list of Robotiq gripper commands see the Robotiq manual. Issues with using pause() in RoboDk with Post Processor There has been issues with the command pause(time_s) not being transferred over to post processed URScript code. To avoid this, try using robot.runcodecustom('sleep(time_s)', INSTRUCTION_CALL_PROGRAM) where time_s is the amount of time you want the robot to pause in seconds. Tips for setting for force and speed values: In your Python code, you cannot simply define your force and speed value vectors and try to update your gripper force / speed values in a for loop as below; This doesn t enter the gripper_force or gripper_speed values into the rq_set_force() or rq_set_speed() functions. Instead it just keeps the text gripper_force[0] as seen below. 12

13 Unless these variables are defined elsewhere in your post processed URScript code, they mean nothing to the UR5 and will throw out an error. You have to tell Python that you intend to put a value inside your text string, and then tell it what that value is. Instead the proper syntax would be; This generates the post processed URScript with the actual force and speed values; You should follow a similar format in your code. 13