Designs and code for an open source simple parallel gripper, designed as part of PhD work at the Australian Center for Field Robotics
This repo contains the solidworks designs, STL files, assembly instructions and code required to build a simple yet high performance parallel gripper. The intention of this design is to be accessible and low cost for educational and research applications. You should be able to construct this design in around 4 days for less than $500.
The current design has a mounting pattern for the Universal Robotics arms which consists of 4 m6 bolts evenly spaced on a 50mm diameter circle.
It is designed to be parametric in the rail length. In our experience a 300mm rail works well for a wide range of objects while keeping the fingers out of the camera field of view when open. If you are space constrained a 200mm rail is a good compromise. Much larger should also be possible for custom applications, provided the actual rail is strong enough to support it.
Our gripper design mounts a Realsense D435 depth camera above it, a very common setup in robotics. This operates independently of the gripper software and can be included or left off as you see fit. Those mounting points are m3 and 45mm apart with a 10 degree downwards angle, these could be used for mounting other sensing or actuation modules instead.
This is not intended to be an off the shelf solution. Using very fine tendons without bearings will not support cycle limits in the thousands but we have not found any issues running this hundrends of times. There are many improvements that could be made to the design, but very few of these come without additional complexity or cost. It does take a bit of practice and tuning to get the tendon (fishing line) tension right, and there is still a small amount of backlash. It is possible to add tensioning springs but these tend to get broken if something goes wrong. If you require very precise (<3mm) positioning or torque control this is not a good solution.
If you intend to actually construct this design start by reading the whole of BuildInstructions.pdf. The major components are:
- mx28T dynamixel servomotor
- 12mm linear rail and 2 carts
- OpenCM9 controller board and EXP485 to power the servomotor
- High strength fishing line
You will also need access to:
- A reasonable quality 3D printer
- A soldering iron for adding heat set inserts
Because we use the mx28T in multi turn mode there is an issue that it must be within the same 360degree rotation each time it is powered off. To ensure this, schematics are included for a capacitor power board so that the servo can be driven to the correct position when it loses power.
The addition of this board is optional but recommended. If you are powering the system from a UR controller with 24V we found 8 x 4700uF capacitors to be sufficient for a single gripper operation.
Very basic firmware is provided in the 'Software' folder. This gets loaded onto the OpenCM9 controller (which is basically an arduino). This then listens for serial commands in the form of a letter and number. You will need to calibrate the servo position once the gripper is constructed in order to use this functionality. The most basic commands are:
-
'o' for open
-
'c' for close
-
'm' for mid position
-
'd'xxxx for a width in mm
-
'p'xxxx for a position
-
's'xxxx for a speed
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't'xxxx for a torque
See the GripperSoftware.ino code and mx28T eManual for more details
In order to use this gripper with the Robot Operating System (ROS) a basic action service is provided. You will need to upload the gripper firmware then add the parallel_gripper folder under Software>parallel_gripper to your catkin workspace. Ensure you rerun catkin_make and source devel/setup.bash. You should then be able to run
rosrun SimpleSliderHand gripperServer.py
then
rosrun SimpleSliderHand gripperClient.py
and press enter to change the gripper state.
In addition to the ROS action server. A URDF of the UR5 Robot (currently just the gripper, this is a ToDo) with this gripper attached is also provided as an example for planning with the gripper on.