Storing information through magnetic patterns was first demonstrated to record audio. Consequently, this idea has become applied for different items like floppy disks, audio/video tapes, hard disks, and magnetic stripe cards. This post concentrates on Magnetic stripe cards used extensively for financial transactions and access control across the globe.
Reading magnetic stripe cards requires significant analog circuitry besides digital logic to decode data. Recording of information in the magnetic cards is digital and is also completed by magnetizing particles along the length of the stripe. Reading the magnetic card successfully is really a challenge due to the fact the amplitude of sensor signal varies together with the speed at which card is swiped, the caliber of the card, and the sensitivity of magnetic read head. Moreover, frequency also varies using the swipe speed. This requires passport reader to adapt to these changes and process the sensor signal without distortion. This post explains mechanisms for handling variations in the sensor signal.
So that you can know the results of card swipe speed, the grade of the credit card, and sensitivity of the sensor, it is essential to know how details are stored on a card along with the way it is sensed from the read head. In magnetic-based storage systems, details are represented by pole patterns with a magnetizing material like iron oxide. Figure 1 shows a magnetic stripe coated with magnetizing material. The particles inside a magnetizing material may have some specific alignment or might be in random directions if this is not previously subjected to a magnetic field by using a particular orientation. However, when subjected to an outside magnetic field, particles about the stripe are aligned using the external applied field.
In practical systems, a magnetic write head is commonly used which is simply a coil wound around a core. The magnetic field orientation can easily be programmed by controlling the current direction in the coil. This can help to generate north-south pole patterns on the card. The narrower the air gaps in between the poles, the greater the density of information, which can be programmed in the card.
In F2F encoding, if a pole transition occurs somewhere between the bit period, it is actually logic 1 else it is actually logic . For instance, as shown in Figure 3, in case the bit period is ? of course, if a transition transpires at ?/2, then its logic 1, else it is logic . See that the length occupied by logic 1 and logic about the card is same. However, the bit period ? varies together with the swipe speed and that should be made up when reading the credit card.
The reading process is exactly reverse. It needs a read head which is similar to the passport reader arrangement shown in Figure 2. Be aware that you will have one sensor for each track. As soon as the card is swiped, the magnetic field from the stripe induces voltage inside the read head coil. Figure 5 shows the waveform obtained from the read head.
The signal peaks at every flux transition. This is because of the high density of magnetic flux with the pole edges. As you can see, facts are represented from the location of signal peaks. A peak detector circuit can decode this signal or perhaps a hysteresis comparator with all the thresholds kept very close to the signal peak. However, additional processing is necessary before we are able to give this signal towards the detector circuit for your following reasons:
Swipe speed: Swipe speed is specified in inches/sec (IPS). Generally, a magnetic card reader is needed to function properly within the swipe speed range of 5 IPS to 50 IPS. The amplitude in the sensor signal varies with the swipe speed: a rise in swipe speed leads to a heightened rate of change of flux cut with the coil inside the 89dexlpky head, creating increased amplitude from the signal. On the other hand, if the swipe speed is slow, the signal amplitude is lower which could result in difficulty in reading your data.
Expertise of the card: As time passes and based on the usage, card quality degrades with decreased magnetic field strength and distortion as a result of dust and scratches about the card. Together, these decrease the amplitude of your sensor signal.
Because of every one of these parameters, magnetic card reader can be between several 100s of uV to 10s of mV. This range can be compensated employing an amplifier. However, it can not be a fixed gain amplifier. As soon as the swipe speed is high along with the card quality is good, the amplifier output can saturate to the rails. And once the signal saturates, information, which is the time difference between two successive peaks, is lost. Thus, it is essential to faithfully amplify the sensor signal without saturating or altering the wave shape. This calls for a configurable gain amplifier to ensure that we can easily tune the gain in the fly. To do this, the system must be able to sense as soon as the signal is weak.