Registers – Coded record sensors – Particular sensor structure
Patent
1988-08-22
1991-05-07
Trafton, David
Registers
Coded record sensors
Particular sensor structure
235492, 235493, G06K 708
Patent
active
050138987
DESCRIPTION:
BRIEF SUMMARY
This invention relates to data-storing devices, e.g. in the form of tokens, and apparatus for communicating with such devices.
EP-A-147099 describes examples of data-storing tokens and apparatus for communicating with such tokens. In one embodiment, a token interrogator is inductively coupled to a token and transmits a carrier signal which is received by the token and used to generate power for operating the token. Data is transmitted to the token by selectively interrupting the carrier. The token transmits data to the interrogator by altering its impedance, in particular by shorting-out its antenna, which is sensed by a receiving coil in the interrogator.
According to one aspect of the present invention there is provided an interrogator for, transmitting power to a data-storing device and for receiving data from the device, the interrogator comprising a resonant circuit arranged for inductive coupling to the device in order to transmit said power.
The invention will be described below primarily in the context of its application to portable data-storing tokens, but is applicable also in other areas.
It has been found that the use of a resonant circuit can have significant advantages in the particular field of powering and interrogating tokens in a contactless manner. One problem with this area arises from the fact that it is necessary for the token to receive an adequate but not excessive amount of power from the interrogator and this can be very difficult to achieve, especially when limited power is available, because manufacturing tolerances may cause significant variations in the token characteristics and the positioning of the tokens in relation to the interrogator may tend to vary. It has now been found that such problems can be avoided or substantially mitigated by the use of a resonant circuit in the interrogator. In particular, it has been found that the power transferred to a token in such a system can be arranged to vary very little over a relatively large range of distances between the token and the interrogator. In a preferred embodiment to be described below, the token interrogator comprises means for supporting the token in proximity to an interrogator inductance forming part of a resonant circuit, the support means being such that the distance between the token and the inductance can vary between pre-defined limits, and the power transferred to the token varies by not more than 20% irrespective of the position of the token within those limits.
The token interrogator has a drive circuit which drives the resonant circuit in order to transmit power to the token. The total impedance presented to the drive circuit is, in the presence of a token, formed by a combination of the impedances of the resonant circuit and the token circuit, which are inductively coupled. The mutual inductance will vary in dependence on the distance between the token and interrogator inductances. The total impedance presented to the drive circuit can be considered as a combination of X.sub.i and R.sub.i, which are respectively the reactive and resistive impedances of the resonant circuit itself, and X.sub.t and R.sub.t which are the reactive and resistive impedances of the token as they appear to the drive circuit. The values of these quantities will be dependent upon circuit components (i.e. values of inductances and capacitances, sizes of coils, numbers of turns, etc.), and the frequency at which the resonant circuit is driven, as well as the distance between the token and interrogator inductances.
It has been found that the power transferred to the token can be made relatively independent of distance between the token and interrogator inductances by making R.sub.t substantially equal to, or preferably slightly less than, R.sub.i when the distance between the token and interrogator inductances is a maximum, and by making R.sub.t greater than R.sub.i when the distance is a minimum. Also, the drive circuit is arranged so that the current supplied to the resonant circuit increases as the total impedance presented to the dr
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Mars Incorporated
Trafton David
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