MFM Challenge

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This page provides a sample module for the MFM Challenge. Extensions, modifications, and other implementation options can be found at: MFM Extensions.

Contents

Background

One of the most fundamental challenges in nanoscience is visualization of a sample. Features of interest in nanoscience investigations are often too small to be resolved by even the most powerful light microscopes. To circumvent these fundamental limitations scientists turn to tools like electron microscopes and atomic force microscopes. Electron microscopes function somewhat analogously to light microscopes, except that the light waves are replaced with an electron beam, and the detector (your eye in the case of a light microscope) is replaced with a device sensitive to the reflected electrons, such as a charge-coupled device (CCD).

Physical structure is not, however, the only property of interest to nanoscience. It is also important to understand other properties, such as the magnetic forces at work within a sample. To understand such forces, researchers can make use of probe microscopes such as the Magnetic Force Microscope (MFM). A MFM functions by scanning a very tiny magnetic tip over the surface of a sample. On the back of the tip is a reflective element. The entire assembly is attached to a flexible cantilever beam. A laser is bounced off of the reflective element and into a series of photodetectors arranged opposite the beam source. When the tip is dragged over a magnetic area, it deflects. The deflection of the tip, and subsequently the reflective element, deflects the laser beam to different areas of the detector array. By understanding the material properties of the cantilever, it is possible to use these deflections to measure the strength of the magnetic forces between the tip and the sample. This helps researchers develop a 'magnetic picture' of a given sample.

Scanning electrom microscope (SEM) image of a MFM probe.
Scanning electrom microscope (SEM) image of a MFM probe.

The Challenge

Your goal is to create a working model of a MFM from a LEGO NXT kit. You will be given three sealed samples containing an arbitrary configuration of magnets and asked to determine the placement of the magnets within the sample. These samples will be of varying complexity, consisting of an 'easy', 'intermediate', and 'difficult' sample. As you would expect, the complexity of the magnetic patterns will increase with the difficulty rating. Successfully determining the configuration will require you to create a way of scanning the provided sample with your MFM tool and interpreting the results. This can be done in one of two ways. You can either create a fixed cantilever, laser source, and detector along with a movable sample table, or create a fixed sample table with movable detector assembly. In either case you should be able to place samples given to you into your device, start your processing program, and not need to intervene in the operation again until all results are stored on the NXT unit and it is time to interpret them.

Materials

In order to successfully complete this project you will probably need (though you are not limited to):

  • LEGO NXT kit
  • Small mirror
  • Small magnets for testing
  • Laser pointer
  • Thick string
  • Electrical tape

Construction

We have provided a sample device to give you some direction in beginning your designs. The sample is a fixed sensor, mobile table design which is functional, but only when properly calibrated. Step by step instructions for its construction can be found here. It can serve as a starting point for your project, but you should be aware that it is optional and if you should choose to use it we highly recommend that you modify it in as many ways as you can. If you do not, you will likely have great difficulty determining the 'intermediate' sample, and will probably not be able to determine the 'difficult' sample at all. Keep in mind that your primary goal is to determine the magnetic composition of the sample. In order to do this you will need a device that enables you to keep track of where the probe's tip is in relation to the sample at all times and relate this position to feedback in a light sensor. You will also need to store this information in such a way that the results are human readable.

Possible Modifications

  • You may want to experiment with different sizes of working envelope, to tailor it to the sample size.
  • You may want to attempt to modify the 'belt drive' to prevent the string from tangling
  • You should think about ways to detect which direction the cantilever is deflected in (think polarity)
  • You may want to develop a quick way to calibrate the laser/receiver assembly

Documentation

Your design should be fully documented. This includes step-by-step instructions similar to the ones for the sample MFM design provided. You should also include a document containing a written rationale for your design decisions. Any code you write should be fully commented and submitted along with your build instructions and design rationale.

We highly recommend the use of the LDraw suite of utilities for LEGO CAD work. Instructions for installing the suite can be found on the LDraw community's getting started page. You may also want to look into the 'unofficial files' section on their web site, as many of the technic parts used in the NXT kit are still held only in this archive. You should be aware that the package has a somewhat steep learning curve, but has many similarities to professional CAD packages. Give yourself time to learn to use it properly and you will be able to create useful instruction sets like the sample MFM package.

Goals and Grading

The underlying goal of this project is to complete the design and documentation of your MFM. The end goal is to be able to use the MFM you construct to determine the composition of the provided magnetic samples.

Grading is as follows:

  • Demonstrating a functional MFM (in the sense that it moves): 40 points
  • Documentation of your MFM design: 20 points
  • Correctly describing the easy sample: 10 points
  • Correctly describing the intermediate sample: 20 points
  • Correctly describing the difficult sample: 10 points

Partial credit will be awarded. A functioning run of your device will be watched by your instructor. You will be asked to submit the output from your device as proof that the device detected the characteristics of the sample, and not the human operators.

Example

The following are images of a fully built MFM created by following the instructions provided in the 'Construction' section above. There is also a video of its operation available.

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