Storing information through magnetic patterns was demonstrated to record audio. Since that time, this idea has been applied for different goods like floppy disks, audio/video tapes, hard disks, and magnetic stripe cards. This article focuses on 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 on the magnetic cards is digital and is completed by magnetizing particles along the size of the stripe. Reading the magnetic card successfully can be a challenge because of the fact how the amplitude of sensor signal varies together with the speed where card is swiped, the grade of the credit card, and also the sensitivity of magnetic read head. Moreover, frequency also varies using the swipe speed. This requires card dispenser to adapt to those changes and process the sensor signal without distortion. This short article explains mechanisms for handling variations inside the sensor signal.
So that you can be aware of the negative effects of card swipe speed, the quality of the card, and sensitivity in the sensor, it is important to know how details are stored on the card in addition to the way is sensed from the read head. In magnetic-based storage systems, information and 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 a magnetizing material may have some specific alignment or could possibly be in random directions if it has not been previously exposed to a magnetic field having a particular orientation. However, when exposed to an external magnetic field, particles about the stripe are aligned using the external applied field.
In practical systems, a magnetic write head is utilized that is simply a coil wound around a core. The magnetic field orientation can be programmed by controlling the current direction within the coil. It will help to make north-south pole patterns in the card. The narrower air gaps in between the poles, the higher the density of web data, that may be programmed about the card.
In F2F encoding, if a pole transition occurs between the bit period, it is actually logic 1 else it is actually logic . For instance, as shown in Figure 3, if the bit period is ? and in case a transition occurs at ?/2, then it is logic 1, else it really is logic . Notice that the length occupied by logic 1 and logic on the card is same. However, the bit period ? varies with all the swipe speed and this has to be accounted for when reading the card.
Now the reading process is just reverse. It takes a read head which is just like the passport scanner arrangement shown in Figure 2. Remember that you will see one sensor for every track. As soon as the card is swiped, the magnetic field in the stripe induces voltage in the read head coil. Figure 5 shows the waveform from the read head.
The signal peaks at each and every flux transition. This is because of our prime density of magnetic flux on the pole edges. As you can tell, information and facts are represented from the location of signal peaks. A peak detector circuit can decode this signal or possibly a hysteresis comparator with all the thresholds kept not far from the signal peak. However, additional processing is necessary before we can easily give this signal towards the detector circuit for that 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 array of 5 IPS to 50 IPS. The amplitude in the sensor signal varies together with the swipe speed: an increase in swipe speed results in a higher rate of change of flux cut through the coil from the 89dexlpky head, resulting in increased amplitude of the signal. In contrast, once the swipe speed is slow, the signal amplitude is lower which could result in difficulty in reading the data.
Expertise of the card: Over time and in line with the usage, card quality degrades with decreased magnetic field strength and distortion as a result of dust and scratches in the card. Together, these lessen the amplitude of your sensor signal.
Due to each one of these parameters, TTL magnetic card reader might be between several 100s of uV to 10s of mV. This range can be compensated using an amplifier. However, it should not be a fixed gain amplifier. As soon as the swipe speed is high as well as the card quality is useful, the amplifier output can saturate on the rails. And once the signal saturates, information, 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 calls for a configurable gain amplifier so that we are able to tune the gain about the fly. To get this done, the device must be capable of sense once the signal is weak.