This invention relates to a pet door system and to a method of operation thereof.
Many owners of pets such as cats and dogs fit a pet door (usual referred to as a cat flap or a dog door) to their houses so that their pet(s) can leave and re-enter the house at will. As explained in patent document GB 1426698 (Pedrick), a problem with a simple cat flap is that other unwanted animals can also enter the house, and that earlier patent document proposed a chromatically-selective locking system for a cat flap. Of course, that earlier system is ineffective against unwanted animals having the same color fur as the “authorized” pet. Another known type of system has a magnetically-actuated locking mechanism which responds to a magnet attached to the collar of the authorized pet. Of course, such a system is ineffective against unwanted pets whose owners have fitted similar pet door systems to their houses and attached similar magnets to their pets' collars. More recently, locking systems have been used which employ a radio frequency identification RFID device carried by the pet. The RFID device can respond to an RFID reader fitted adjacent the pet door and transmit a unique ID to the reader. The system can then compare the received ID with one or more authorized IDs and unlock the pet door only if there is a match.
In a typical inductively-coupled passive RFID system, the RFID reader includes an RLC resonant circuit including a coil antenna as its inductor. The reader's resonant circuit is driven at a driving frequency, and the antenna radiates energy. A resonant circuit in an RFID device adjacent the reader receives some of the radiated energy and uses the received energy to power the RFID device. When powered up, the RFID device modulates the loading that it places on the reader's resonant circuit typically with a repeating message each consisting of a stream of binary bits which include the unique ID of the RFID device. The reader demodulates the antenna signal and decodes the message. Therefore when used in a pet door system, if the ID in the decoded message matches the authorized ID, or one of the authorized IDs, registered in the system, the pet door is unlocked.
In order to maximize the energy radiated by the RFID reader, the driving frequency at which the reader's resonant circuit is driven should be equal to the resonant frequency of the reader's resonant circuit. Also, in order to maximize coupling between the RFID reader and device thus maximize message quality, the driving frequency of the reader and the resonant frequency of the RFID device's resonant circuit should be equal.
In the case where a pet door system with an RFID reader is supplied with a single RFID device, for example for attachment to the pet's collar, the driving frequency of the reader and the resonant frequency of the reader can be set at the factory to match the resonant frequency of the RFID device. However, even so, once the pet door system is installed, the driving frequency, and in particular the resonant frequency, may then drift over time. For example, the following can each have a significant effect on the driving frequency and/or resonant frequency of the reader: (a) the type of house door or wall to which the pet door is fitted, (b) siting a large household appliance such as a washing machine near the pet door, (c) temperature, (d) aging of components and (e) power supply voltage.
In the case where a pet has been ‘microchipped’ with an implanted RFID device, it would be desirable for a pet door system to be able to communicate with the implanted RFID device, rather than requiring a separate RFID device attached to the pet's collar. However, different types of microchips with different nominal resonant frequencies are used for implanting into pets. Although an RFID device reader driven at one frequency can in some circumstances communicate with an RFID device having a different resonant frequency, the data quality is often poor, and repeated reading may be required in order to obtain a complete, error-free message.
The signal strength of the demodulated signal in an RFID reader is dependent on the distance between the reader and the device, and the amount of loading modulation provided by the RFID device. Different types of pet microchips provide different amounts of loading modulation. However, the amplitude of the signal to be decoded is preferably approximately constant. It is possible to provide an amplifier/attenuator between the demodulator and the decoder which provides some form of automatic gain control. However, this adds to the circuit complexity and manufacturing cost and desirably would be avoided.
Aims of the present invention, or at least of specific embodiments of it, are to provide an RFID pet door system and to a method of operation thereof which does not require the RFID reader to be tuned at the factory, which can cope with tuning drift, which can be used with different types of RFID device having different resonant frequencies, and which does not require any automatic gain control amplifier or attenuator between the demodulator and downstream circuitry.