Project Magnetic Particle Manipulation & Observation System

This project required the design and development of a 3 axis electromagnetic coil system to manipulate and observe magnetic particles. The customer required a completely integrated solution designed from the ground up. The complete system was designed in SolidWorks providing the customer with 3 alternative designs prior to commencing the prototypin & build. The system design included a epi-illuminated upright microscope with high-speed camera, 3 axis electromagnetic Helmholtz coil assembly, synchronised waveform generator and power amplifiers, along with sample manipulation stages. Custom designed user interface software allowed easy control of the whole system.

Key Specification Points:

  • 3 Axis Uniform AC Electromagnetic Field over an area 50x50mm.
  • Fields should be synchronised with USB control of the Frequency, Phase & Amplitude.
  • Magnetic Fields ranging up-to 25mT for frequencies 1 - 300Hz.
  • A simple PC User Interface should be provided to control the Frequency, Phase & Amplitude.
  • A built-in oscilloscope should provide confirmation of the coil excitation signals.
  • The coil assembly should be designed to allow a sample (microscope slide) to be inserted from either side & its position controlled.
  • The microscope should be an upright configuration with epi-illumination, supporting Olympus objectives.
  • The optical components should be mounted on a damped optical bench.
  • The coil assembly should be easily lowered to allow objectives to be changed.
  • The sample should be able to be electronically manipulated in the X and Y axis.
  • The microscope Z axis focus should be electrically controlled.
  • A high speed camera capable at least 1000 FPS should be implemented and remotely triggered.

The Completed System

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Key Development Stages

Stage 1 - Coil Assembly Design & Testing

The coil geometries and configuration were derived via a process of emprical measurement and simulation. The X & Y axes were based on a rectangular HelmHoltz geometry and the Z axis on a circular geometry. Many revision of coil were constructed and tested, each wound in-house, this process allowed each coil pair to be gradually optimised providing cross-validation of our simulation model.

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For each test cycle the AC magnetic fields were measured using a Gauss meter and the current through the coils measured with a floating differential scope probe. Along with the magnetic field being the primary optimisation parameter, the other two were winding resistance and microscope sample/objective accessibility. The resulting assembly was mounted on a jack to allow the objective to be changed.

The coil assembly housing was manufactured from PTFE which served to withstand the generated heat and protect the user during operation.

Stage 2 - Amplifier & Signal Design

Custom PCB amplifier boards were designed and constructed based on the Apex Microtechnology amplifier range. These are then mounted into a 3U rack housing along with their control circuitary and power supplies. Each rack housing featured local temperature control. Multiple 150W AC/DC power supplies were used to achieve +/- 200VDC rails for the amplifiers. In the system 3 rack mounted amplifiers were implemented, one of each axis. A custom USB 3 channel waveform generator was designed and built. This provided 3 synchronised sinewaves which were generated using DDS IC's and controlled via a C# User Interface on the integrated PC.

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Stage 3 - Optical Design & Camera

The microscope was built around the coil assembly with an extended optical path to reach into the centre of the coils. The design was an upright configuration with an epi-illumination using a single wavelength light source (445nm) to reduce the number of optical paths and improve focusing. The microscope (objective) is fixed with respect to the coil, but rather the sample can be moved within the X and Y axes. The microscope was fitted with Nikon trinoculars so that the user could observe the experiment either through the eye pieces or via the high-speed camera.

The camera selected was the Mikrotron Motionblitz, when operated in Quad mode it can achieve a frame rate of 1700 FPS across the full image sensor, even higher depending on the Area of Interest or Resolution selected. This was ideal for observing fast particle kinetics. Image/Video capture could be initiated using a software or external trigger button.

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Stage 4 - Sample Manipulation

The sample manipulation arm can house a standard microscope slide 75 x 25mm. The sample arm is manually inserted into the coil assembly using a simple incremental guide. Greater precision is then achieved using an electronic XY axis. The objective is mounted on an electronic Z axis to provide precision focusing.

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Stage 5 - Final System Design & Construction

Once the key sub-assemblies of the system had been prototyped and characterised the final design was completed within SolidWorks. This allowed the remaining parts to be designed and to ensure that all the components and parts fitted together. In the early stages using Solidworks allowed detailed visualisations to be discussed with the project team. This system follows a modular concept. Therefore if new requirements emerge through use then parts and sub-systems can be easily re-designed and changed/modified.

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