Deltabot Sorting Machine
- Background :
Late this year I bought a delta 3d printer. I was fascinated by the design and mechanism, how it moves so dynamically by actuating its three arms.it accelerates and decelerates quickly maintaining the accuracy of the printing head position.
It made me think to study it and why not make my own delta robot. The occasion came up in this year’s project, I proposed the idea to my team and they liked it.
So we intend to make a deltabot with a different purpose than 3d printing, a pick and place machine sorting items according to different characteristics (color, weight…..)
2. The Delta Mechanism :
A delta mechanism is composed of three parts:
- The BASE:
A triangular shaped platform which houses the actuators in our case we chose stepper motors for their accuracy and repeatability, because all the weight resides in the base the arms of the robot can be very light reducing there inertia so they can accelerate very fast in an accurate way.
- The End-Effector:
A smaller triangular shaped platform which translates in the XYZ axis but stays parallel to the base platform, it also houses a suction device or a smell actuated arm to pick and drop the sorted items.
- The Arms:
Three symmetrical arms that move independently, Composed of an arm that is actuated from the base, a parallelogram acting as the forearm which is the connected to the end effector all its joints are ball joints. The parallelogram locks the end effector platform parallel to the base which is the difference between a delta bot and a Stewart platform and the ball joints give the end-effector the 3 Degrees of freedom DOF to move in the XYZ planes.
- Actuation Type Choice:
There is two types of actuation for these three arms either linear or revolute, common 3D printers use linear actuation, and industrial deltabot used for pick & place use revolute actuation. I chose the revolute actuation because it’s easier to make with minimal mechanical items and also cheaper as linear rods or wheels on aluminum profiles tend to be more expensive to buy than to directly drive the arms from the actuator.
3. Motion Study:
Only by rotating first part of the arms we are able to change the position of the end effector platform as shown in the figure below
Input 3 rotations è output 3 translations on the X Y and Z axis.
Inverse kinematics was applied to determine the output angle of each servo for a desired position of the effector (xg,yg,zg). Each leg was solved individually. Cylindrical polar coordinates are used to take advantage of the circular symmetry. Each leg is projected into a 2D plane then basic geometry was used to determine the final servo output angle.
4. Design and Parts List
1. Mechanical Parts:
We will be needing a strong frame to support the weight of the motors and the base it needs to rigid and heavy to eliminate all the vibration when the arms are moving quickly so we can either use steel or aluminum profiles to make it.
For the Base and the end effector an acrylic (plastic) or a wooden board can be used
For the upper arm 3d printing the part can be easier as it’ll have many mounting points one side for the motor and the other side to connect the ball joint, if we don’t have access to a 3d printer we will be using acrylic.
For the parallelogram we need small rods need to be very light and 4 ball joints each.
The ball joint problem:
It may be difficult to find small ball joints (embout a rotule) on the market so we are not lucky we can make our own in a very clever way by combining two perpendicular pivot axes on the X and Y using 4 ball bearings.
The end effector will be equipped with an actuator to hold the items were picking preferably it’s a suction module but we can use a servo actuated gripper.
2. Electronics Components:
• As actuators we can use either servo motors or steppers I prefer steppers as they are more accurate in both position and acceleration, we need three in total
• If we are using steppers drivers are needed (DRV8825 or A4988)
• A microcontroller (an arduino Uno or Mega)
• A color sensor
• Weight sensor
Essential parts List Proposal: