- Work this week:
- rethinked the approach for the gripper. Originally, I was going for a spring mechanism: the motor winds up the spring after making contact with the object, and force is controled by how much the spring is wound. The issue is that it is mechanically complex. I was not able to come up with a good implementation of this concept.
- If I can control the amount of torque at the motor directly, I don’t need a spring mechanism.
- Currently, I am looking into smart servos that can do force control, such as the Dynamixel series: https://www.robotis.us/dynamixel/?srsltid=AfmBOoqgSxak-qbqm3m2aevmbsqF1EVobBmwjjBQxEVpmByu927cBMaB
- I am also looking into using stepper motors and BLDC motors: https://docs.simplefoc.com/torque_control
- I also added a design requirement for gripper force:
- 300g payload * 10m/s^2 = 3N gravity force. Typical static friction coefficient value for glass on metal contact is 0.5. We will have a higher coefficient of friction because we plan on using rubber for the gripper. However, I will use 0.5 to be safe. This means we need 3N/.5 = 6N normal force at the gripper.
- The gripper design we chose (https://www.thingiverse.com/thing:2661755) uses a rack-and-pinion setup. The pitch diameter is approximately 1.2cm. This means the torque required at the motor will be 6N * (1.2cm/2) = 3.6 Ncm
- for example, typical servo motors such as https://tinyurl.com/yc4nuvbe has 12kgcm of torque, or about 120Ncm. Therefore, there will be more than enough torque.
- Conclusion: I can use a regular servo motor, or a DC motor. Originally I thought torque requirements meant I could only use a DC motor.
- designed and printed motor brackets for the gantry:
- Modified an existing design for the gripper, built and assembled. This gripper only supports position control, not forrce control. For the MVP this might be sufficient. I tested this on some common household objects and even without a rubber sleeve, the gripper will be able to hold on to most objects with a flat side. (round objects are more difficult, but adding a rubber sleeve should alleviate that)
- Finished design of X and Y axis 3D print parts. I have attached a particular difficult part, as well as the overall assembly below. Pulleys and belts are not shown because I don’t know how to model belts in SolidWorks.
- Currently, I am working on designing the wrist and Z-axis mechanism for the gantry.
- My goal for the next week:
- finish building the XY parts of the gantry and attaching the gripper to it.
- design and print wrist roll joint mechanism.
- write firmware to control gripper motor
- have a working gantry system consisting of X, Y, wrist and an open/close gripper. This will be our MVP configuration. For the final product, I will need to add Z axis and force controllable grippers
- the MVP gantry will be able to communicate to the central PC via serial. It will be able to execute move and grip commands.
Leland Mersky, Jack He, Zhenghao Jin