ReP

Repetative Pipettor

Background

The ReP was created to automate repetative pipetting tasks in a simple and cost-effective way. It utilizes a rack and pinion design to move the pipette's plunger up and down. It can be calibrated to aspirate and dispense at any given rate, frequency, and volume. It is intended to be held upright with a ring stand clamp or any other clamped arm to allow for easy maneuverability and adaptability. The ReP is built around an Eppendorf pipette. The DC motor is controlled with a Raspberry Pi Nano and an L298N motor controller. The structure itself is 3D printed from PLA.

Included

In this repository, I have included:

1. STL files for the 3D prints,
2. wiring and construction guides, and
3. the code for the raspberry pi

1. 3D Printing

There are seven total pieces that need to be printed: axle_extension, motor_holder_clip_side1_assembly, clip_side2_c, housing, pinion, and rack_assembly. All of the STL files are in the STL folder. None of the pieces require any special printing instructions. PLA is the recommended material given its structural stability and cost. Support material will be required. 20% infill and 0.2 layer height is adequate. Once the piecees have finished printing, thuroughly remove all of the support material especially around the dove tails.

2. Wiring and Construction

The final hardware diagram and orientation of 3D printed parts are shown in Figures 1 and 2, respectively. The following guide will walk through the process of wiring and constructing step by step.

1. Solder pins 16, 18, 20, and 22 onto the Nano.
2. Either hot glue, super glue, or screw the motor controller and Raspberry Pi Nano onto the clip_side1_c such that the corners line up with the protrusions on either side of the part. When attaching, be sure the the Nano is facing such that the SD card is facing up and the USB and HDMI ports are facing out. For the motor controller, the heat sink should be facing down. If gluing, make sure to allow the piece to sit for a minute or so to fully dry.
3. Attach all wires as shown in the hardware diagram in Figure 1. The wires leading to the motor can remain unattached for now.

Figure 1: Hardware Diagram

4. Remove all support material from the 3D printed parts. May need to dremel parts down as necessary depending on printing accuracy.
5. Insert the motor into the motor_holder with the axle lined up with the top dove groove.
6. Put the rack_assembly into the housing and line up the pinion inside it as well.
7. Carefully slide the housing with the rack_assembling and pinion into the top dove groove of the motor_holder. It is important to line up the flat edge of the pinion with the flat edge of the motor's axle and may take some finessing.
8. Slide clip_slide1_c into the bottom groove of the motor_holder.

Figure 2: Assembly of 3D printed parts.

10. Attach the two leads from the motor controller to the two nodes on the motor. Figuring out which wire goes to which node will need to be determined by trial and error. If the wires are connected to the wrong nodes, the motor will simply rotate in the opposite direction. The correct orientation will need to be tested by running the up or down program to ensure it goes the correct direction. If the plunger goes down when the up program is ran, simply switch the wires.

3. Code

The code must be uploaded to the raspberry pi. I save all of my codes in a folder on the Desktop for easy access. This can be done manually through a USB drive or by remotely accessing the pi and transfering the files through an app such as Transit. I have included a sample code that has an up, down, stop, and loop function that asperates and dispenses 100 ul at a rate of 100 reps per 5 minutes (or 1 rep per 3 seconds) for 15 minutes. It will need to be calibrated to each pipette specifically and will most likely differ slightly from pipette to pipette. Once the code is loaded onto the pi, the following code can be used to run the ReP code provided.

cd Desktop/code/
sudo python ReP_code.py

If you need to edit the code, the following line can be used to edit the code within the terminal.

nano ReP_code.py

A futher explanation of the code provided is in the code/README and there are comments annotated in the code itself.