Team:NUS Singapore-A/Hardware/Pump

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Peristaltic Pump

The peristaltic pump displaces fluid, such that bacterial culture is kept flowing through the cuvette. This allows the sensor to take continuous measurements.

Design - Improvement!

This peristaltic pump is an improvement on the design by the 2015 Aachen iGEM team. We chose to build on their design since their pump was also designed for continuous pumping in a bioreactor. Three modifications were made.


The first modification replaces their pentagonal spring rotor base with a circular rotor base (Figure 1).


Figure 1. Pentagonal spring rotor figure (left) against our rotor design (right)

This was so that we could countersink flat-head bolts into the rotor structure, compared to having raised bolts such as pan-head bolts (Figure 2).


Figure 2. Aachen 2015’s rotor with a pan-head bolt installed (left) and our rotor with a flat-head bolt installed (right). The choice of bolt for the Aachen rotor was arbitrary and is for illustration purposes. Their wiki simply instructs one to use an M3 bolt there. Our flat-head bolt is flush with the bottom face of the rotor. The height of our rotor’s base was thickened slightly to accommodate the countersinking feature.

When we recreated Aachen 2015’s pump, we discovered that no matter how carefully we mounted the rotor onto the shaft of the stepper motor, one screw head would always drag along the pump plates, implying that the rotor was tilted at an angle.This caused the pump to have insufficient torque to move fluid through the silicone tubes.

The screw attaching the rotor to the stepper motor shaft is a flat end screw, and the rotor should not be tilted at an angle. Troubleshooting further would yield diminishing marginal returns, as other possible sources of error such as inaccuracies in 3D-printing would not be within our power to resolve. Moreover, the angle was very slight and otherwise undetectable by visual inspection. An obvious solution would be to decrease the height of the central rotor column, and then shift the entire rotor assembly upwards, but this would not be ideal. As the load moves further along the shaft, the angular deflection of the shaft increases. While the increase in deflection would definitely be negligible in this case, such a solution would be bad design practice. Hence, we elected to countersink the bolts because this approach allows us to easily visually inspect the rotor to ensure clearance between it and the faceplate of the stepper motor.


However, a flat-head bolt of equivalent size could not be countersunk directly into the Aachen design as there was too little material remaining to hold the bolt in place. We then simplified the rotor base, and thus produced what you see in Figure 2.

Our second modification was to add material for the fastening screw. A drawback of the 2015 team’s design was the weakness of the bolt and nut combination fastening the rotor to the stepper motor shaft (Figure 3).


Figure 3. The wall separating the fastening nut and the shaft was less than 0.5mm thick. When we attempted to 3D-print the Aachen 2015 rotor, our 3D printer was not precise enough to produce such a thickness, or rather, thinness. While the rotor was still functional in this state, we felt that this design had room for improvement. The type of fastening nut here used was arbitrary.

We thickened the wall separating the fastening nut and the shaft by 2.75 mm, placing the nut 6mm away from the centre of the shaft (Figure 4). We also thickened the wall between the bolt head and the nut. This design can thus better withstand mechanical stresses, and is more durable.


Figure 4. The circled parts show where we have added material around the fastening bolt for a sturdier structure.

The third modification we made was to reposition the inlet and outlet for the silicone tubing.

Aachen 2015’s design stacked several Plexiglass layers on the stepper motor’s faceplate, forming a housing to hold a silicone tube inside a circular path. We noticed that Aachen 2015’s silicone tubes had a smaller outer and inner diameter than the tubes we planned to use, as the working volume of their bioreactor is less than ours. After analyzing their design, we conjectured that we only needed to modify the thickness of the pumping layer (Figure 5) to accommodate our own silicone tubing.


Figure 5. Original shape of pumping layer.

However, our silicone tube exerted forces on the walls of the pumping layer and caused the walls to deflect (Figure 6).


Figure 6. Diagram showing the direction of normal forces exerted by our silicone tubing on the pumping layer. The direction of deflection is the same as the force here.

Because of this, the silicone tube could not be pinched closed (occluded), and the pump was unable to force the fluid to move through the tube.

We thus repositioned the inlet and outlet, combining them into one opening (Figure 7).


Figure 7. New shape of pumping layer.

Now, when the silicone tube attempts to regain its original, non-deformed shape, the lines of action of the forces it exerts at the opening will be much closer to the metal fasteners (Figure 8), causing a much smaller bending moment. Additionally, the force exerted by the silicone tube is now shared by 4 fasteners instead of 2 as the shape is now continuous.


Figure 8. Diagram showing the direction of normal forces exerted by our silicone tubing on the new pumping layer.

After this modification, the pumping layer no longer deflected visibly, and operation was smooth.

Testing

We validated the functionality of this component and characterized it by plotting the mass flow rate as a function of RPM.


  1. Place a length of silicone tubing in the peristaltic pump. Put one end into a water reservoir. Place another end into a clean, empty beaker. Put the empty beaker on an electronic balance.
  2. Use the code to set the RPM.
  3. Turn the peristaltic pump on.
  4. Wait approximately 5 seconds for the flow to stabilize. When the flow has stabilized, start the stopwatch while simultaneously taring the electronic balance.
  5. Read the mass off the display on the electronic balance every 10 seconds, for 5 minutes total.
  6. After completing Step 4, stop the stopwatch and empty the beaker. Refill the water reservoir if necessary.
  7. Repeat Steps 3-5 twice more.
  8. Represent the measurements with a scatter plot. Find the trendline. The gradient is the mass flow rate [g/s]. While kg is the correct SI unit, we find g to be more helpful here.
  9. Record mass flow rate and RPM.
  10. Repeat steps 4-9 for different RPMs.
  11. Plot a graph of flow rate against RPM. You may now use this graph to find out what RPM you should enter in the code for your desired mass flow rate.

Our pump was able to displace fluid consistently for different mass flow rates. Please visit Results:Cell-Machine Interface for the results of our characterization of this pump.

Construction

Ever wanted a peristaltic pump of your own? Now you can have one, get started here!

All fasteners required are M3 size.


  • 12V AC adapter x 1
  • Stepper motor x 1
  • Motor drive x 1
  • ID=3 mm OD=7 mm radial ball bearing x 8
  • ID=3 mm OD=5 mm food grade heat-resistant silicone tube x 1
  • 200mm x 200mm x 3mm clear acrylic sheet x 1


Figure 9. Pump assembly


Figure 10. Rotor assembly for one countersunk bolt. Note that washer size as shown is inaccurate.


Files for lasercutting are available in DXF format, while files for 3D printing are available in STL format. You may download them all as a ZIP file here.





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