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Recording on a Wire
Posted Date:
Total Responses: 0
Posted By: sharathsathisan Member Level: Silver Points/Cash: 7
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Introduction
Magnetic recording has proven to be a quick, safe, and robust method for storing and retrieving information. From the first voice recordings on Poulsen's wire recorder (Figure 1), to the tape recording machines used by radio stations in the 1940's and 1950's that freed the stations from having to produce all of their programs live, to the modern hard disk drive that can store billions of bits of digital information in an area smaller than a quarter, we can see the application of the fundamental principles of magnetism.
Poulsen's wire recorder.
Figure 1. A diagram of Poulsen's design for a wire recorder.
Magnetic materials may be roughly categorized as "hard" or "soft." Magnetically hard materials are able to retain a magnetic moment even after an applied magnetic field is removed. Magnetically soft materials have negligible magnetic moment in the absence of an applied field. Both magnetically hard and magnetically soft materials have a role to play in any magnetic recording system.
At the heart of the recording system is the storage medium, where the signal is stored for later retrieval. This material is necessarily magnetically hard. The choice of material is very important. If the coercivity (Hc) is too high, then it will be difficult to record our signal. If the product of the saturation magnetization (Ms) and the volume (V) is too small, then the replay signal will be small. Additionally, one must worry about the mechanical properties of the storage medium. Corrosive resistance, magnetic grain size, and surface roughness are some examples of important material parameters to measure. Materials must be able to retain their magnetic moments for a reasonable length of time over a suitable range of temperatures and in the presence of stray magnetic fields. The materials chosen for modern tape drives and for disk drives are very different because they have differing requirements for performance and environment.
The magnetic transducer (the recording head) is responsible for writing to the magnetic medium. Reading and writing are two very different processes. However, they are sometimes handled by a single transducer, serving both functions (as will be the case for your wire recorder). Fifteen years ago, a single head both read and wrote in a disk drive. Today there are separate heads for each role. Handling each task with a separate transducer allows engineers to optimize each head for its specific purpose.
The write head needs to convert an electric current into a magnetic field. Ideally, the field generated should be as strong as possible and its strength should fall off as quickly as possible as one moves away from the head. A good basic design for a write head is a coil of wire wrapped around a soft magnetic material. This type of head is known as an inductive head because the coil of wire works as an inductor and generates magnetic flux, which is conducted and concentrated by the soft magnetic material to the pole tip(s). Modern write heads still use this principle. The most classic of head designs is the ring head, wherein the soft magnetic material is in the shape of a toroid (i.e., ring) with a small gap cut into it. The soft material has high permeability and conducts magnetic flux easily, whereas the gap has a low permeability, which causes the magnetic flux to fringe away from the gap area. This fringing field at the gap is used to write to the recording medium.
The read head converts magnetic flux into a voltage potential. It must be sensitive enough to provide a useable signal, and it must respond quickly to changes in magnetic flux. It is possible to use the same head used for writing to read the magnetic fields from the medium. In the reverse of the writing process, magnetic flux is coupled into the soft magnetic core and induces a voltage potential across the coil. In the read configuration, the voltage (v) across the coil is proportional to the change in flux (df/dt) experienced by the coil. This is expressed as Faraday's Law:
v = -N df
dt
N is the number of turns in the coil. The negative sign indicates that the voltage is opposite in polarity to the change in magnetic flux. Here, it is important to note that anything that increases the rate of change of the flux will generate a larger output voltage. Often the easiest method for increasing the output voltage is to move the recording medium past the head faster. Increasing the number of turns is also a convenient method for increasing output amplitude.
Currently, hard disk drives use a type of read head that has proven to have much greater sensitivity than the inductive read head. This head operates on the principle known as magnetoresistance. Here, the sensor is a piece of material whose resistance changes in proportion to the magnetic field it experiences. Magnetoresistive heads are not sensitive to the rate of change of the magnetic flux because they sense flux directly.
The Wire Recorder
There are three components to a wire recorder:
1. the wire (recording medium),
2. the read/write head (and electronics), and
3. the mechanics for moving the wire past the head (or the head past the wire).
As you will see when you do this project, each poses its unique challenges!
Pictured below is a Telegraphone, the first commercially-produced wire recorder. Your wire recorder will be a greatly simplified version, but will function essentially like the Telegraphone.
Annotated photograph of a Telegraphone.
Figure 2. A Telegraphone, the first commercially-produced wire recorder.
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