Storing information through magnetic patterns was first demonstrated to record audio. Since then, this idea has become requested different goods like floppy disks, audio/video tapes, hard disks, and magnetic stripe cards. This short article targets Magnetic stripe cards used extensively for financial transactions and access control throughout the world.
Reading magnetic stripe cards requires significant analog circuitry besides digital logic to decode data. Recording of data about the magnetic cards is digital which is performed by magnetizing particles along the size of the stripe. Reading the magnetic card successfully can be a challenge due to the fact how the amplitude of sensor signal varies together with the speed from which card is swiped, the standard of the card, and the sensitivity of magnetic read head. Moreover, frequency also varies together with the swipe speed. This requires passport reader to evolve to those changes and process the sensor signal without distortion. This informative article explains mechanisms for handling variations from the sensor signal.
In order to comprehend the outcomes of card swipe speed, the quality of the credit card, and sensitivity of the sensor, it is very important recognize how information and facts are stored on a card as well as the way it is sensed from the read head. In magnetic-based storage systems, facts are represented by pole patterns on a magnetizing material like iron oxide. Figure 1 shows a magnetic stripe coated with magnetizing material. The particles in the magnetizing material probably have some specific alignment or could possibly be in random directions if it is not previously exposed to a magnetic field with a particular orientation. However, when put through an outside magnetic field, particles around the stripe are aligned with the external applied field.
In practical systems, a magnetic write head is used which happens to be only a coil wound around a core. The magnetic field orientation can be easily programmed by governing the current direction inside the coil. It will help to produce north-south pole patterns about the card. The narrower the air gaps involving the poles, the higher the density of web data, which can be programmed in the card.
In F2F encoding, if your pole transition happens in between the bit period, it is actually logic 1 else it really is logic . By way of example, as shown in Figure 3, in the event the bit period is ? and when a transition takes place at ?/2, then it is logic 1, else it is logic . Observe that the length occupied by logic 1 and logic about the card is same. However, the bit period ? varies with the swipe speed and also this needs to be accounted for when reading the credit card.
Now the reading process is exactly reverse. It takes a read head which is just like the card dispenser arrangement shown in Figure 2. Be aware that you will have one sensor for each and every track. As soon as the card is swiped, the magnetic field in the stripe induces voltage within the read head coil. Figure 5 shows the waveform taken from the read head.
The signal peaks at every flux transition. This is because of the top density of magnetic flux in the pole edges. As you can see, details are represented with the location of signal peaks. A peak detector circuit can decode this signal or possibly a hysteresis comparator together with the thresholds kept very close to the signal peak. However, additional processing is required before we can 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 required to function properly within the swipe speed array of 5 IPS to 50 IPS. The amplitude of your sensor signal varies using the swipe speed: an increase in swipe speed leads to an elevated rate of change of flux cut with the coil within the 89dexlpky head, resulting in increased amplitude from the signal. In contrast, once the swipe speed is slow, the signal amplitude is lower which could result in difficulty in reading the data.
Excellence of the card: With time and in accordance with the usage, card quality degrades with decreased magnetic field strength and distortion on account of dust and scratches in the card. Together, these lower the amplitude of your sensor signal.
As a result of all of these parameters, magnetic stripe card reader can be between several 100s of uV to 10s of mV. This range might be compensated utilizing an amplifier. However, it cannot be a set gain amplifier. If the swipe speed is high along with the card quality is useful, the amplifier output can saturate to the rails. So when the signal saturates, information, which is the time distinction between two successive peaks, is lost. Thus, it is important to faithfully amplify the sensor signal without saturating or altering the wave shape. This involves a configurable gain amplifier to ensure that we could tune the gain in the fly. To achieve this, the machine must be capable of sense if the signal is weak.