Patent ID: 6323769
Filing Date: 2001-11-27
Classification: G01V,G06Q,G08B,Y10T

Abstract:
An apparatus for interrogating a magnetically coded tag, the apparatus comprising:(a) means for generating a magnetic field, said magnetic field comprising a first region at which a component of the magnetic field resolved in a first direction is zero, wherein within regions contiguous with said first region the component of the magnetic field resolved in said first direction is sufficient to saturate at least a part of the magnetically-coded tag; and(b) an electrical circuit having a signal source at frequency 2fand an output means for providing an electrical output, said signal source being coupled to said output means by (i) means for halving the frequency of said source signal, (ii) a first filter means arranged substantially to reject a signal at frequency 2f,(iii) a tuned circuit means tuned to energy at frequency f and including a transmitter coil adapted to radiate energy towards the magnetically coded tag positioned so as to be influenced by said radiated energy and a receiver coil for receiving energy radiated by the tag in response to the received energy, and (iv) a second filter means arranged substantially to reject energy at frequency f and to pass energy at frequency 2f, wherein said output means is arranged to provide an output which is a function of the amplitude of the output of said second filter means and the phase difference between the signal source and the output of said second filter means, wherein means for generating the magnetic field is associated with said transmitter and receiver coils such that, in use, a magnetically coded tag moving through the first region will also interact with said transmitter and receiver coils.