Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
1999-03-23
2001-11-20
Hofsass, Jeffery A (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S572200, C340S572400, C455S277100
Reexamination Certificate
active
06320508
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an arrangement for a system for the contactless transmission of data and/or energy between a base station, including an antenna resonant circuit, and a transponder.
The base station transmits data and energy to a transponder via an electromagnetic field. The highest transmission efficiency is achieved when the resonance frequency of the antenna resonant circuit is tuned to a transmission frequency which is defined by a generator in the base station and whereto the resonance frequency of the transponder resonant circuit is also tuned.
EP 0 625 832 A1 describes a system for the wireless transmission of data and energy in which the antenna resonant circuit is automatically tuned to a fixed transmission frequency by way of connectable capacitances.
Detuning of the antenna resonant circuit occurs due to temperature-induced drift phenomena in the elements of the resonant circuit or due to manufacture-related deviations. Consequently, the resonance frequency of the resonant circuit is no longer tuned exactly to the transmission frequency, so that the antenna current is reduced and hence the transmitted field strength also decreases. Such detuning could possibly preclude correct transmission of data and energy to the transponder, or could substantially reduce the distance that can be allowed between the base station and the transponder. In order to avoid these functional restrictions, either components having small tolerances must be used or the systems must be manually retuned. Complex circuits for automatic tuning of the transmission frequency could also be employed. Another possibility of ensuring reliable transmission is to increase the operating power of the system. However, stronger antenna drivers are then required; like the other possibilities, this increases the cost of the system.
When the resonance frequency of an antenna resonant circuit of the base station is accurately tuned to the transmission frequency, the impedance of the antenna resonant circuit consists only of the real ohmic resistance, so that the maximum current is reached and hence also the maximum field strength. The maximum current possible is limited by the antenna drivers and the power supply. Usually public rules are imposed as regards the maximum field strength. Antenna resonant circuits reach the field strength required for a data and energy transmission in a tolerance range. This tolerance range characterizes the range in which the resonance frequency may deviate from the transmission frequency without endangering reliable transmission. There is a measure for the maximum permissible deviation of the resonance frequency of the antenna resonant circuit from the transmission frequency. If the resonance frequency of the resonant circuit lies outside this tolerance range, transmission of data and energy cannot take place.
When the antenna resonant circuit is operated with a higher power, the current flowing and the maximum field strength increase if the antenna resonant circuit is tuned. The tolerance range for the resonance frequency in respect of the deviation from the transmission frequency is greater in antenna resonant circuits operating with a higher power than in antenna resonant circuits operating with less power. However, stronger drivers are required for the higher power. In addition to this additional expenditure, antenna resonant circuits operating with a higher power also generate a stronger electromagnetic field when the antenna resonant circuit is tuned, so that public rules might be violated.
SUMMARY OF THE INVENTION
Therefore, it is an object of the invention to provide an arrangement which generates a substantially constant and adequate field strength within a tolerance range, independently of the detuning of the resonance frequency or the transmission frequency.
This object is achieved as disclosed in the characterizing part of claim
1
.
Reliable transmission is ensured when the antenna resonant circuit is tuned to the transmission frequency. Transmission of data and energy is possible even when the antenna resonant circuit is not tuned exactly to the transmission frequency of the system. The deviation from such exact tuning of the resonance frequency to the transmission frequency, for which transmission is still possible, is referred to as said tolerance range.
A resonant circuit operated with more power reaches a high field strength upon resonance. Such an increase of the field strength, however, is no longer necessary when the required field strength is reached. In order to avoid a further increase of the field strength, the voltage is limited after the necessary field strength has been reached. Because these systems usually utilize amplitude modulation which is suppressed by voltage limitation, the voltage is limited in one half-wave only. Such limiting is achieved by the arrangement which is connected parallel to an element of the antenna resonant circuit and limits the voltage in one half-wave. As a result of this limitation, the antenna current no longer increases strongly after a limit value has been reached so that, also in the case of a deviation of the resonance frequency of the antenna resonant circuit, the field strength is so high that reliable data and energy transmission is possible but public rules are not violated. The antenna drivers need not be proportioned so as to be larger, because the antenna current does not increase further when a limit value is reached. The resultant field strength remains constant across the range of the voltage limitation which lies within the tolerance range. A constant field strength is thus achieved, independently of the deviation of the resonance frequency from the transmission frequency in the range in which transmission of data and energy to the transponder is possible.
The arrangement according to the invention may also be considered as a circuit Q limiter, since the circuit Q of the resonant circuit inherently becomes smaller when the voltage across a resonant circuit element is limited. The circuit Q of a series resonant circuit is dependent on the ratio of the voltage across a resonant circuit element to the generator voltage at resonance. In the case of a parallel resonant circuit, the circuit Q is dependent on the ratio of the current flowing through a resonant circuit element to the applied current at resonance. When a resonant circuit has a high Q, the resonance increase becomes smaller in the case of slight detuning and the resultant field strength quickly decreases, so that reliable transmission of data and energy is no longer ensured. This ratio changes when the resonant circuit has a lower Q.
The limitation arrangement according to the invention is used in antenna series resonant circuits in the described manner. In the case of antenna parallel resonant circuits, a limitation arrangement is preferably connected in series with a resonant circuit element so as to limit the current flowing through said resonant circuit element.
A further advantage of resonant circuits having a low Q consists in the smaller relative phase shift occurring upon detuning, so that load modulation can be performed in a wider range. The frequently used envelope demodulation can also be applied in an extended range since, because of the lower Q of the resonant circuit, a phase shift occurs only in response to a degree of detuning which is higher than in the case of resonant circuits having a higher Q.
REFERENCES:
patent: 3820103 (1974-06-01), Fearon
patent: 4302846 (1981-11-01), Stephen et al.
patent: 5729236 (1998-03-01), Flaxl
patent: 5923251 (1999-07-01), Raimbault et al.
patent: 0625832A1 (1994-11-01), None
Böh Frank
Giesler Thomas
Hofsass Jeffery A
U.S. Philips Corporation
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