Exploring Data Storage: a Readable Lego Hard Disk Drive Model
By Sarah Braden
Introduction
In a world increasingly saturated with computers and electronic media, it is important for today's
students to understand the scientific principles behind basic data storage structures, like the hard
disk drive (HDD). The HDD is one of the most common tools for digital memory storage with
hundred of millions of HDDs being manufactured worldwide each year [1]. To start with a bit of
history, the first magnetic hard drive was invented at IBM in 1956. The drive consisted of 50 21inch disks and could store 5 megabytes [2]. After exponential growth of drive capacity and
density, the average disk drive exists as we know it today, capable of storing hundreds of
gigabytes with 3.25-inch disks. Over the past three years, the rate of increase of information
density has declined, prompting researchers to develop new methods of storing data [1].
Educators can use hard disk drives as a teaching tool, not only to link concepts in physics and
nanoscale science with the general principles behind hard disk drives, but to foster an interest in
the development of technology and continued research. Our group, inspired by the success of
the Lego AFM [3] and the appeal of Lego bricks, decided to construct a read-only Lego hard
drive model to help teach students about physics and magnetic storage.
Undergraduate students in the Chandraskehar Mesoscopic Physics group at Northwestern
University, with the support of the National Center for Learning and Teaching in Nanoscale
Science and Engineering (NCLT), created this lab project to bring a new hands-on perspective to
learning about HDDs. If a curious student wishes to explore the innards of a HDD, it results in
the demise of the drive. The disks are very sensitive and are hermetically sealed to protect from
dust, condensation, and other contamination. The hard disk's read-write heads fly on a cushion
of air only nanometers above the disk surface. If the tip crashes into the surface of the one of the
disks, the resulting scratches will destroy the storage capabilities of the drive. Dissecting a
broken HDD can help students learn more about the actual structure, but it will not teach them
how the drive works. As with other nanoscale devices, inaccessibility makes hands-on studying
impractical. Therefore using a model is an excellent option. Figure 1 shows an early version of
the model. The rest of this article will describe the concepts taught in the lab, the construction of
the model, results from the lab, and areas of improvement.
What is a hard disk drive?
Initially, students need to learn a little background information on HDDs before they construct
their own model. HDDs record and read data through changing or detecting the magnetization of
the material on the platters (disks). Allowing the students to inspect a dismantled hard drive or
showing pictures of the various components, such as the read/write head, the actuator arm, the
platters, the spindle, and the actuator, will help them translate what they learn through the model
to the actual drive. It will also help to go over the function of each part and the accompanying
vocabulary used when describing hard disk drives. Figure 2 shows where the various parts are
located in an actual HDD. The read/write head uses electromagnetic induction to read the
magnetization of the material directly under it, and it writes by generating a strong localized
magnetic field to change the magnetization of the platter material. The highly reflective disks, or
platters, are made of aluminum or glass coated with a thin magnetic layer which actually stores
the data. The spindle rotates the platters at speeds from 3,600 rotations per minute (rpm) to
15,000 rpm [1]. The actuator, which uses very strong magnets, moves the actuator arm radially
across the platter at high speeds. The actuator arm suspends the read/write head above the platter
at a distance of anywhere from 3-25 nanometers [4]. When taking apart a HDD, warn the
students about the strength of the magnets, which can pinch skin, erase credit cards, and harm
other magnetic storage media. The magnets from inside the HDD and the smaller magnets the
students use later must be handled with care.
Electromagnetic induction, a basic physical concept behind the operation of the HDD, is the
production of a voltage across a conductor that is in a changing magnetic field, or in the case of
our model HDD, the production of a voltage across a conductor when the conductor moves
through a stationary magnetic field. The voltage across the conductor will cause a current to
flow through the conductor, which in this case is a coil of wire. As the read head moves over the
platter’s small magnetic dipoles created by areas of local magnetization, it is exposed to a
changing magnetic field.
Construction
After an overview of the components of an actual HDD, the students will be ready to build their
own models out of Lego bricks. While fostering creativity and ingenuity is a goal, it does help to
supply a finished Lego model for reference. This model will help speed the building process so
the students can progress to the encoding and reading parts of the lab.
The model is fairly easy to construct. The actuator arm attaches to a support using rotating Lego
pieces. The spindle also uses rotating Lego pieces, and we found that providing support for the
top of the spindle helped stabilize the disk. The read head, shown in Figure 3, is a coil of wire
taped to the underside of the actuator arm with wires that extend down the actuator arm and
support. The ends of these wires are soldered to the ends of a microphone jack, providing input
to the laptop and Audacity program [5]. Students will position the read head over the part of
their disk that contains magnets. In our model, the spindle rotates by manual means. Students
can choose just to spin the disk like a top, or winding a string around the axle of the spindle,
which when pulled gently produces a fairly stable and consistent rotation. The disk itself can be
made out of any sturdy, thin material, such as corrugated cardboard. Here we used thin slices of
Plexiglas. To speed up the lab process, wind the coils and solder the wires ahead of time.
Younger children should not solder unless under direct adult supervision. In our lab each coil
had approximately 60 winds and was a little more than 1 centimeter in diameter. Some questions
the students should consider at this point:
- How will the design of the spindle and actuator arm impact the performance of the model?
- How far away should the read head be from the platter?
- What is the best way to spin the platter?
- How stable is the design?
Encoding and Reading
When the students have completed building the models they should begin placing magnets on the
disks and reading the output on the screen of the Audacity program. Figure 4 shows examples of
signals. The first task is placing a single magnet (using double-sided tape) on the platter, spin
the platter while recording, and establish which orientation of the magnet (a magnetic field
pointing up or down) is a 1 or 0. Again, it is important to warn the students to work carefully
with strong magnets. At this point students may have two problems: the signal is too weak, or it
is too difficult to read. As the students solve these problems they will learn. If the signal is too
weak, students can try amplifying the output using the computer or they can adapt their model so
that the read head is closer to the magnets. In real HDDs a shorter distance between the
read/write head and the platter corresponds to being able to increase the density of information
on the disk, to a certain extent. If the signal is still too weak, the students should be prompted to
consider how electromagnetic induction works. When a single magnet moves underneath the
coil, the magnetic field in the center of the coil of wire changes from zero, to some value, and
then back to zero. Faraday’s law of electromagnetic induction states that a larger change, or a
faster change, corresponds to a larger voltage difference in the coiled wire. The microphone
pickups measure the voltage difference on either end of the wire and the voltage difference is
directly displayed on the Audacity screen. Hopefully the students will realize that if they spin
the platter faster, the amplitude of their signal will increase. After the students establish that they
can clearly read a “1” signal and a “0” signal, then they can progress to encoding actual numbers
onto the platter, using binary code. Figure 5 shows a student preparing to spin the platter and
record data. Before the lab, the students should be given an introduction to counting in binary
from 1 to 10 and examples of the uses for binary code.
Results
Our first “test” group consisted of twelve college freshmen, working in groups of two. The
fastest group was able to construct a working model hard drive from our kit, arrange magnets on
the disk in a pattern, and read the output from the Audacity program in 40 minutes. The students
made many observations that correlate to actual problems and concerns in the construction of
actual hard drives. They noted how the proximity of the magnets to the read head coil affected
the signal, and also how the proximity of the magnets to one another affected the signal. In
Figure 6 shows one group testing how close the magnets could be placed before the signal was
indecipherable. This corresponds to the current issue of increasing the density of information
stored on HDDs. Another student made the connection between the coils on the model hard
drive and the coils in guitar pickups, which both rely on electromagnetic induction. The models
also had limitations, which the students identified. For example, at higher spin speeds, the
platters would wobble, making the signal harder to read. Our second test group was ten middle
school students, ages _ to _, also working in groups of two. The middle school students did not
have as much success in terms of being able to encode numbers in binary, although most of the
students were able to get stable “1” and “0” signals within an hour.
Problems
We found that additional rings of magnets (or additional tracks) were hard to read due to
interference from other magnets. Magnets placed closer to the center of the platter move past the
read head at slower speeds, which decreases the amplitude of the signal. The placement of the
magnets also affects the balance of the platter, which can increase the wobble, making the data
harder to read. Occasionally computer settings were an issue.
Future Goals
In the future it would be nice to construct a read and write Lego hard drive model would not only
demonstrate the basic principles behind reading data on a hard disk, but also analog to digital
translation, programming, and microprocessors.
Citations
[1] “Technological impact of magnetic hard disk drives on storage systems”
by E. Grochowski and R. D. Halem
[2] “computer memory.” Encyclopaedia Britannica. 2007. Encyclopaedia Britannica Online.
March 23, 2007. <http://www.britannica.com/eb/article-252743>
[3] Turner et al. “Seeing the Unseen: The scanning probe microscope and nanoscale
measurement.” The Science Teacher. pp. 58-61. December 2006
[4] Magnetic Storage Systems Beyond 2000, George C. Hadjipanayis, p. 487
[5] The free Audacity software can be downloaded at http://audacity.sourceforge.net/
"LEGO® is a trademark of the LEGO Group of companies which does not sponsor, authorize or
endorse this work"
Table of Materials
1/8” microphone jacks
8 1/2” disks
Neodymium (NdFeB) disc magnets 7/16” x 1/8”
double sided tape
thin copper wire (what gauge?)
Lego bricks
Lego baseplate
laptop computer with free Audacity software
super glue
soldering iron
string
Figures
Figure 1. An early proof-of concept model
Figure 3. Showing the proper attachment of the coil of wire to the underside of the lego actuator
arm. Notice the audio input cable attached to the ends of the wire.
Figure 4. Sample output collected using the Audacity program. The first box is a zero, the
second box is a one. The third box is a zero followed by a 1.
Figure 6. Students experiment with the placement of magnets. How close can the magnets be
before the signal is undecipherable?
Figure 5. Students prepare to run their model. This group was successful and encoded the
number three onto their disk using the 1’s and 0’s represented by magnets placed with the north
side of the magnets up or down.
Figure 2. Basic anatomy of a hard disk drive. Image from
http://commons.wikimedia.org/wiki/Image:Hdd_od_srodka.jpg