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IKA Lab Disc

http://www.ika.com/owa/ika/catalog.product_detail?iProduct=3907500

Objective

Design and fabricate a magnetic stirrer capable of stirring multiple small vials (0.5-5ml) of solutions with viscosities similar to that of water. The specific requirements are as follows:

• • Four stirring positions minimum

  • 50mm x 50mm x 5mm maximum size for 4 coil array 

  • 300 rpm to 1500 rpm stirring speed

  • 5mm thickness maximum for entire device housing 

Further, explore the possibilities for mobile app-control, battery power, inductance charging and modular communication. 

Components 

Magnetic Plates and Cores

• Guide the magnetic field lines

• Diagonally adjacent Top Plates behave as opposite poles

• Bottom Plate confines magnetic field lines to the top surface

• Higher Relative Magnetic Permeability = Stronger Magnetic Field

Table 2. Magnetic permeability and relative cost of materials

Final Cores:

• 1008 Low Carbon Steel 

• Low Cost 

• Machined on CNC Lathe 

• (3.72mmH x 3mmD) 

Final Plates:

• 1018 Low Carbon Steel 

• Low Cost 

• Cut on Waterjet

Magnetic Coils 

Table 1. Coil Specifications 

• • The Battery Powered module Coils draw only 400mA to ensure a sustainable battery life. 

  • The Bluetooth Enabled module Coils draw approximately 600mA. 

The final design utilizes 32 AWG Magnet Wire: 

• High enough resistance for current draw to ensure sustainable battery life 

• Low resistance creates strong magnetic field with low voltage 

ABS Dissolving Method:

• Eliminates the need for a bobbin 

• Utilizes extra height for more wire turns

In our efforts to reduce the height of our device, our sponsor suggested dissolving the ABS bobbins to further shave off 1mm of height from the coils. We decided to implement this suggestion by gluing the coils in after spooling to allow it to retain its shape and then dissolve the ABS bobbins using Acetone. We successfully tested this method on a single coil and verified that the magnetic wire retains its enamel.

Dissolving ABS Plastic with Acetone

Remaining Coil Sealed with Polyurethane Adhesive

Electronics

Battery Powered Module:

• Adafruit Pro Trinket: Microcontroller 

• Adafruit Powerboost 500: Used to step up 3.7V from battery to 5V used by H-bridge 

• Adafruit L293D H-Bridge: Allows polarity of coils to be reversed 

• Adafruit 3.7 V Lithium Ion Polymer Battery: Powers module 

Bluetooth Enabled Module: 

• Raspberry Pi Zero: Microcontroller with Bluetooth functionality 

• Adafruit L293D H-Bridge: Allows polarity of coils to be reversed 

Casing 

Design Requirements: 

• 9 mm thick for Battery Powered module 

• 5 mm thick for Bluetooth Enabled module 

Features:

• • Component mounts 

  • Channels for wiring 

  • Ports for power supply 

Considerations:

• Future designs to be aluminum cut or injection molded 

• Egonomic design for consumers 

CAD of Front, Back & Section Views: Battery Powered Module & Bluetooth Module

Final Design & Performance

Median Decoupling Speed of Stirrer in Varying Configurations

Recommendations 

• Implementation of custom PCB and microcontroller 

  • Reduce overall size 

• PCB Printed Coils 

  • Reduce volumetric footprint 

  • Increase portability and ease of transport 

• Higher magnetic permeability materials for cores and plates 

  • Reduce size of coils needed

  • Decrease the current draw from the power source 

  • Possibility of a Card-thickness module with a appropriate material (Pure Iron 99.99% Hydrogen annealed) 

• Double Wrapped Coils 

  • Twice the Magnetic Field and half the resistance 

  • Lower the amount of voltage needed for a higher field 

• Draft Angles on Housing 

  • To easily separate an injection molded housing

Prototypes 

The initial risk reduction verified that we could quickly and inexpensively produce prototype magnetic coils. 

Risk Reduction Test Bed 

The second prototype successfully implemented stirring of a single vial using four coils. 

Circuit Schematic Diagram

The next few prototypes focused on testing the difference between using wires of varying gauge size.

Before implementing the 9-coil array, a 6-coil array with 2 stirring positions was built to quickly find out possible issues with sharing magnetic coils between stirring positions.

The next few prototypes successfully implemented 4 position stirring using a 9-coil array. It was observed that although three vials could be reliably stirred simultaneously, the addition of the fourth vial caused the negihbouring stir bars to decouple from the coils. The system was also highly sensitive to the positioning of the vials. 

CAD of proposed Magnetic Plate & Core design

The distance at which the magnetic stir bars interfere was determined to be approximately 25mm apart by physically moving one vial closer to the other on our prototype as it was stirring. Based on this measurement, the coil and bobbin dimensions were redesigned using a CAD template as reference and the coils were packed as close as possible without causing interference. 

CAD Schematic of Coils, Vials & Grid Layout

The next important prototype worked using a PWM signal and 8-directional control of the coils to smoothly spin the stir bars, like butter.

The Pattern of the plates were also experimented with and tested relative to one another.

Star Pattern Upper Plates & Circle Pattern Lower Plate

Rectangluar Pattern Upper Plates

The last acrylic prototype was designed using 32 AWG Magnet Wire, coiled on ABS bobbins, which were then dissolved off in acetone. The plates were made of 1018 Steel and the cores out of 1008 Steel. This prototype worked at 5V, and became the basis of the final deliverable modules.

9 Coil ABS Dissolved Bobbin Prototype