Storing information through magnetic patterns was basically demonstrated to record audio. Since that time, this idea is applied for different items like floppy disks, audio/video tapes, hard disks, and magnetic stripe cards. This post is focused on Magnetic stripe cards used extensively for financial transactions and access control around the world.
Reading magnetic stripe cards requires significant analog circuitry besides digital logic to decode data. Recording of web data about the magnetic cards is digital and it is carried out by magnetizing particles along the length of the stripe. Reading the magnetic card successfully is really a challenge mainly because that this amplitude of sensor signal varies together with the speed from which card is swiped, the grade of the credit card, and the sensitivity of magnetic read head. Moreover, frequency also varies together with the swipe speed. This involves magnetic card reader to adapt to such changes and process the sensor signal without distortion. This post explains mechanisms for handling variations in the sensor signal.
As a way to comprehend the outcomes of card swipe speed, the standard of the credit card, and sensitivity from the sensor, you should know how information and facts are stored on a card as well as how it is sensed with the read head. In magnetic-based storage systems, information is represented by pole patterns with a magnetizing material like iron oxide. Figure 1 shows a magnetic stripe coated with magnetizing material. The particles within a magnetizing material might have some specific alignment or might be in random directions when it is not previously subjected to a magnetic field having a particular orientation. However, when put through an external magnetic field, particles in the stripe are aligned using the external applied field.
In practical systems, a magnetic write head is commonly used which can be nothing but a coil wound around a core. The magnetic field orientation can easily be programmed by governing the current direction from the coil. This assists to produce north-south pole patterns in the card. The narrower air gaps in between the poles, the greater the density of web data, which may be programmed around the card.
In F2F encoding, if your pole transition happens between the bit period, it can be logic 1 else it is logic . By way of example, as shown in Figure 3, if the bit period is ? and in case a transition happens at ?/2, then its logic 1, else it is actually logic . Realize that the length occupied by logic 1 and logic around the card is same. However, the bit period ? varies together with the swipe speed and this needs to be included when reading the credit card.
The reading process is exactly reverse. It will require a read head which is just like the passport reader arrangement shown in Figure 2. Keep in mind that there will be one sensor for every track. Once the card is swiped, the magnetic field through the stripe induces voltage within the read head coil. Figure 5 shows the waveform obtained from the read head.
The signal peaks at each and every flux transition. This is due to our prime density of magnetic flux in the pole edges. As you can see, facts are represented through 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 necessary before we can easily give this signal on the detector circuit to the following reasons:
Swipe speed: Swipe speed is specified in inches/sec (IPS). Generally, a magnetic card reader is required to function properly from the swipe speed range of 5 IPS to 50 IPS. The amplitude of the sensor signal varies using the swipe speed: an increase in swipe speed results in an elevated rate of change of flux cut through the coil from the 89dexlpky head, resulting in increased amplitude of your signal. In comparison, as soon as the swipe speed is slow, the signal amplitude is lower which could cause difficulty in reading the data.
Excellence of the card: As time passes and in line with the usage, card quality degrades with decreased magnetic field strength and distortion on account of dust and scratches in the card. Together, these lessen the amplitude from the sensor signal.
As a result of all of these parameters, magnetic stripe card reader may be between several 100s of uV to 10s of mV. This range could be compensated employing an amplifier. However, it can not be a set gain amplifier. Once the swipe speed is high and the card quality is nice, the amplifier output can saturate on the rails. And when the signal saturates, information, the time difference between two successive peaks, is lost. Thus, it is very important faithfully amplify the sensor signal without saturating or altering the wave shape. This involves a configurable gain amplifier to ensure we could tune the gain about the fly. To achieve this, the system must have the capacity to sense if the signal is weak.