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Miniaturized Magnetic Stirrer II

MAE 156B Winter 2018 Sponsored Project

University of California, San Diego

Sponsored by IKA Works

The executive summary can be found here: Executive Summary

Background

IKA Works headquartered in Stefaun, Germany is a leading company for laboratory technology. The professional stirring machines IKA produces are widely used in research labs all over the world. One of the products that IKA is manufacturing and selling, the magnetic stirrer is a laboratory device commonly used in chemistry and biology research to efficiently stir and mix solutions evenly. The magnetic stirrer works by sequentially changing the polarity of a set of coils in the device. Sitting on the device is a beaker with a stir bar, which is simply a magnetized bar that varies in size depending on the solution spinning. By manipulating the coil polarity, the stir bar in the solution can rotate in a circle, which in turn “stirs” the liquid.

Last year, a team of 5 students from the Mechanical and Aerospace Engineering Department at UC San Diego were sponsored by IKA Works and successfully created a wirelessly controlled, inductively charged, sealed device for stirring 4 vials with small volumes of liquid. Building on this achievement, the device was further developed. Now the size of stirrer was expanded to contain more coils, so that it could be used for synchronous stirring of more vials. Rather than being able to stir only small volumes, the device could be controlled through physical interaction to alternate between stirring both small and large volumes. The housing of the device was completely sealed to protect electrical components within it from damage caused by unintended chemical spills.

Figure 1. Final design including all components

Objective 

Primary objective for MAE and ECE：

• • Develop a magnetic stirrer that utilizes electromagnetic coils to alternate between spinning both small (0.5 mL to 5 mL) and large (5 mL to 250 mL) volumes of liquid.

  • Expand the current 9 coil array to spin more vials simultaneously. Explore using 81 coils.

  • Keep the overall thickness of the device near the same from last year’s group (~10mm).

  • The device should be controlled through physical interaction.

  • Have the device completely sealed so that it is able to survive IP67 testing.

  • Ensure the housing of the device is compatible with inductive charging.

  • Incorporate a Printed Circuit Board in the device.

  • Optimize the electrical components to meet an 8 hour battery life.

Secondary objectives：

• • Control the device through a phone application.

Components of Final Design

Individual Coil

• • Generate strong enough magnetic field, at least 0.002 T per coil, to attract the stir bar effectively enough to reach rotating speed as high as 1500 RPM

  • Small size with height no larger than 4 mm

  • Consistent dimensions and property

The coils used for the project were all manufactured in Germany and provided by the sponsor.

Table 1. Data table of the coil

Coil Array

• • 25 stirring positions for stirring small volumes.

  • 4 stirring positions for stirring large volumes.

  • Reach 1500 rpm stirring speed.

  • Allow steady and continuous rotation of the stir bar.

The coil array describes how the electromagnetic coils are laid out and connected  together either in series or in parallel within the stirrer. The coils connected in series would change their polarities at the same time, while the ones connected in parallel could be controlled separately. The coil array is composed of two sets of 10 coils in series and eight sets of 2 coils in series.

As seen in the figures above, the blue circle represents the vial with liquid solution inside of it, and the black line represents the stir bar which is placed in the vial. As the polarity of the coils change, so does the stir bar’s rotation. By continually changing the polarity in the order of the steps shown above, the device will allow the stir bar to rotate continuously, thus allowing the liquid to stir. The two stirring modes for small and large volumes have different polarity configurations.

Magnetic Cores and Plates

Cores

• • Increase the magnetic strength of the coils.

  • Easy to machine and assemble.

  • Machined out of 99.9% pure iron.

Plates

• • Capture the divergent magnetic field lines coming out of the electromagnetic coils.

  • Concentrate the magnetic field to the desired stirring positions.

  • Very thin, thickness no larger than 0.8 mm.

  • Easy to machine and drill holes in it so that cores can be press fitted in to the plates.

  • Manufactured out of 1075 spring steel made out of 97% iron.

The bottom plate is a single piece with holes separated by 25 mm from center to center. The top plates were designed to have a rectangular shape and were aligned symmetrically such that each stirring position had 4 corners of different plates.

Printed Circuit Board

• • Minimize wiring inside the device.

  • Area footprint small enough to fit inside housing of the device.

  • Thickness to be within 2mm.

  • Contains all of the electrical components needed for the device, except for the batteries and the inductive charging set.

On the PCB shown in Figure 6 below, 8 H-bridges were attached. An H-bridge is an electronic circuit that enables a voltage to be applied across a load in opposite direction. Those components are essential for the magnetic stirrer to allow the polarity of the coil change from one to another.

Components soldered on the PCB include:

• • 36 coil array

  • ESP 32 Microcontroller

  • LCD Display

  • H-Bridges

  • I/O extenders

  • Capacitive Touch Pads

  • Capacitive Touch Sensor

  • Resistors

  • Capacitors

  • Buck Converter

Figure 8. 5000 Lithium Ion Polymer Battery

Table 2. Dimensions of the battery

Housing

• • Compatible with inductive charging.

  • Completely sealed and pass IP67 testing.

  • Corrosive resistant material.

  • Device thickness should stay nearly the same as last year’s (10mm).

Glass

• • Approximately 1 mm in thickness to reduce thickness of the overall device

  • Safe so if the glass shatters it does not cause harm to those around

Performance results

The device was unable to function properly due to a printed circuit board issue. Therefore the final delivered product was not able power on the 36 coils. However 9 of the 36 coils were tested throughout the quarter and the performance of this found below: 

Table 3. Modele Performances

In testing, the stirrer was able to spin 4 vials and one large vial. With a fixed printed circuit board the final device should theoretically be able to spin 25 small vials and 4 large vial simultaneously. The overall thickness was restricted to 12mm, which was close to that from last year’s group, 10 mm. The device was completely sealed and survived IP67 testing.

Performance result using 9 Coils from the 36 Coil Array