Abstract:
The present invention relates to a detector device and, more particularly, to a detector device comprising a self-diagnosis means for monitoring the correct performance or otherwise of the device. During testing, the device, which normally operates using the Doppler shift principle, is arranged to influence the phase and/or amplitude of the received signal in response to an applied test signal. The received signal may be a Doppler shifted version of a transmitted signal. Combining the phase and/or amplitude shifted version of the received signal and the signal from the local oscillator produces an IF signal that is indicative of the phase and/or amplitude shift. If the device is operating correctly, that signal should have a predetermined range of characteristics. If the self-diagnosis means determines that the produced signal falls outside of the predetermined range of characteristics, an alarm is raised to alert, for example, maintenance personnel to the problem.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a detector device and, more particularly, to such a device comprising a self-diagnosis capability.  
         BACKGROUND TO THE INVENTION  
         [0002]    Present day motion detector devices rely upon the Doppler principle, as is well known within the art, to detect the presence of a moving object within a protected volume or volume to be surveyed. Conventionally, such motion detector devices use at least microwave technology, that is, typically X-band or K-Band signals transmitted and received via a transceiver. Post-reception, the received signals are suitably processed to identify the presence of a Doppler signal that can be attributed to movement within the protected volume.  
           [0003]    Relatively sensitive devices such as, for example, the mixer, which typically takes the form of a mixer diode, can be easily damaged by inappropriate handling or excessive signal strengths. Therefore, care needs to be taken in the fabrication and the installation of the motion detector devices. Notwithstanding any such care, the mixer diodes may, post-installation, be inappropriately exposed to signals or handling that may cause damage. It will be appreciated that any such damage may, in the worst case, prevent the motion detector device from operating correctly in that, from the perspective of the output of the mixer diode, there will be no change in the signal due to any Doppler shifted received signal. Alternatively, the performance of the mixer diode may be degraded in some way so that it produces excessive noise which reduces the sensitivity of the motion detector device.  
           [0004]    It is an object of the present invention at least to mitigate some of the problems of the prior art.  
         SUMMARY OF INVENTION  
         [0005]    Accordingly, a first aspect of the present invention provides a detector device comprising at least a field-distortor, responsive to an input signal, for influencing at least one characteristic of a first electro-magnetic signal carried by a first conductor; and a mixer for combining at least the influenced first electro-magnetic signal (the RF signal) with a second signal (the local oscillator signal), to produce a combined signal (IF signal) having a characteristic determined by the input signal.  
           [0006]    In a preferred embodiment, the field-distortor for influencing at least one characteristic of the first electro-magnetic signal is operable to vary at least the phase and preferably the phase and magnitude of the first electro-magnetic signal when that first signal is carried by the conductor.  
           [0007]    Preferably, an embodiment is provided in which the field distortor comprises a semi-conductor device disposed adjacent to the conductor carrying the first signal. It will be appreciated that the semi-conductor device should be positioned sufficiently close to the conductor so as to be able to influence the electro-magnetic field of the first signal being carried by the conductor.  
           [0008]    In a preferred embodiment, the degree of influence exerted by the field distorter varies with variations in the applied signal. Preferably, the applied signal is modulated at a predetermined frequency. The predetermined frequency is preferably between 10 Hz and 80 Hz, these being typical Doppler frequencies that would be generated by a human moving within a protected volume. Additionally, the applied signal is arranged to switch the field distorter between substantially open and substantially closed circuit conditions.  
           [0009]    In preferred embodiments, the field distortor does not radiate or generate an electro-magnetic field in response to the input signal.  
           [0010]    In a preferred embodiment the semi-conductor device is a diode.  
           [0011]    It will be appreciated that the output of the mixer, having had one of the input signals influenced by the field distorter, can be used by a signal analysis system comprising, for example, a micro-processor and associated software, to determined whether or not the mixer and/or the device as a whole is functioning correctly. Suitably, an embodiment additionally comprises a signal analyser for analysing the combined signal to determine whether that combined signal has predetermined characteristics. In preferred embodiments the predetermined characteristics include at least an oscillation having a frequency that is associated with the frequency of the input signal.  
           [0012]    To test the operation of the mixer an appropriate input signal should be applied to the field distorter. Suitably, an embodiment provides a signal generator for generating the input signal to be applied to the field distorter for influencing the electro-magnetic field.  
           [0013]    It will be appreciated that the second signal fed into the mixer is conventionally a divided portion of the output of a local oscillator; the other portion having been, in operation, radiated by a transmit antenna. In the absence of motion, the RF signal on the input to the mixer is at the same frequency as the LO signal. The mixer then operates as a phase detector and produces a DC voltage at its output that is related to the phase difference between the LO and RF signal and the magnitude of the RF signal. Hence, by disturbing at least the phase and, preferably, the phase and magnitude of the RF input signal to the mixer, an output signal indicative of that disturbance will be produced by the mixer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:  
         [0015]    [0015]FIG. 1 illustrates schematically a motion detection system having a self-diagnosis capability in accordance with a first embodiment of the present invention; and  
         [0016]    [0016]FIG. 2 illustrates a detector device for use in such a motion detection system in accordance with an embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    Referring to FIG. 1 there is shown schematically a motion detection system  100  comprising a detector device  102  which co-operates with a detection system  104  to detect relative motion between the device and a body  106  within a monitored volume  108  and to produce a corresponding alarm signal via an output device  110  in response to the detection of any such relative motion. The output device maybe an audio-visual output device.  
         [0018]    The detector device  102  comprises a local oscillator  112  for generating K or X-band microwave signals that are radiated into the monitored or protected volume  108  via a transmit antenna  114 . A portion of the output of the local oscillator signal  112  is directed towards a mixer  116  which combines that portion with an RF input signal  122 , carried on a conductor  118 , that has been received by a receive antenna  120 . Typically, in the presence of relative motion between the device and the body, the RF input signal  122  is a reflected and Doppler shifted version of the transmitted signal  124 . The output of the mixer, that is, the IF signal  126 , is input into the detection system  104  which, if appropriate, can instruct an alarm-fault generator  130  to generate an appropriate alarm signal to be output via the audio-visual output device  110 .  
         [0019]    The detector device also comprises a field distortor  132  for influencing the electro-magnetic field of the RF input signal being carried by the conductor  118 . Preferably, the field distortor  132  takes the form of a diode that, in response to an applied signal  136 , causes a change in the electro-magnetic field carried by the conductor. In particular, the field distortor influences the phase of the RF input signal (at the RF input port) of the mixer  116 .  
         [0020]    In the absence of motion, the mixer  116  acts as a phase (detector) and produces a DC output signal in response to receiving an input signal on the port that would ordinarily receive the received signal  122 , that is, on the RF input port of the mixer. This input signal is a combination of the received signal  122  reflected from static objects within the protected volume  108  and signals generated within the detector device  102  due to internal reflections.  
         [0021]    Referring to FIG. 2 there is shown in greater detail a circuit  200  for realising an embodiment of the detector device  102  according to the present invention. The circuit  200  comprises a local oscillator circuit  202 , that is coupled, via a decoupling capacitor  204  to a is transmit antenna feed portion  206  and a receive antenna portion  208  of the circuit. The device also comprises a test portion  210 . The various portions  202  to  210  are screened via appropriate planar metalisations  212  having holes drilled therein to suppress unwanted spurious electro-magnetic emissions from the various circuit elements.  
         [0022]    A series of connection tabs  214  are provided to allow power to be supplied to the circuit elements  202  to  210  and to allow the IF signal to be received as well as to allow a test signal to be applied to the test portion  210 . The connection tabs  214  comprise a ground connection tab  216  and an IF output connection tab  218 . The IF output connection tab  218  also serves as the output for the signal generated as a result of testing the operation of the receive portion  208 . It will be appreciated that the test diode  242  tests the whole of the operation of the detector, that is, the local oscillator, the mixer and the transmit and receive antennas. A test signal connection tab  220  is arranged to provide the test signal to the test portion  210  of the device. Power is supplied to the local oscillator circuit  202  via a power connection tab  222 .  
         [0023]    The local oscillator circuit  202  comprises, in a preferred embodiment, a dielectric puck  224  disposed adjacent to a pair of strip lines  226  and  228  that are connected to one another via the gate and drain of a FET transistor. Power is supplied to the transistor via the power connection tab  222  and a resistor  230 . The output of the transistor is coupled via the decoupling capacitor  204  to the transmit and receive portions  206  and  208  where that power is divided in a predetermined ratio. The power carried by a microstrip line  232  of the transmit portion  206  is coupled via a slot in a ground plane (not shown), preferably, to a pair of patch transmit antennas (not shown) as is well known within the art. An example of a detector device within which the present invention can be incorporated is shown in UK patent application no. GB2253108, the content of which is incorporated herein for all purposes.  
         [0024]    Similarly, power is coupled to a microstrip line  234  of the receive portion via a corresponding slot in a ground plane (not shown) and an associated pair of patch receive antennas (not shown).  
         [0025]    It can be appreciated that the diode mixer  236  of the receive portion  208  receives power from the local oscillator via a corresponding microstrip line  238  and also from the microstrip line  234 . The output of the mixer diode  236  is fed to the IF output tab  218  for further processing by the detection system  104  and, more particularly, by a signal analyser  128 .  
         [0026]    The test portion  210 , in a preferred embodiment, comprises the series arrangement of a resistor  240  and the test diode  242 . The diode  242  is disposed within close proximity to a conductor or microstrip line  244  which couples power from the receive microstrip line  234  to the receive RF input of the diode mixer  236 . The diode  242  is positioned sufficiently close to the microstrip line conductor  244  such that, in the presence of the signal applied to the test signal connection tab  220 , the variation in diode  242  characteristics is sufficient to vary the degree of influence experienced by the electro-magnetic field of the signal being coupled from the receive microstrip line  234  to the receive RF input of the diode mixer  236  in the vicinity of microstrip line  244 . In a preferred embodiment, the electro-magnetic field perturbation caused by the change in diode characteristics in response to the applied signal imparts at least a phase change to the signal carried by the microstrip line  244 . As the microstrip line  244  couples power to the RF port of the mixer, that is, the receive RF input of the diode mixer  236 , a corresponding phase change occurs at the RF port of the mixer.  
         [0027]    It will be appreciated in the absence of motion, that is, in the absence of the received signal being a Doppler shifted version of the transmitted signal, that the mixer diode  236  operates as a phase detector which generates a DC voltage that is dependent upon the relative phases and magnitudes of the signals present at the local oscillator input port and the RF input port of the mixer. The two signals are in fact each the resultant of a vector summation of all signals appearing at each of the two ports. Therefore by influencing one of those signals, as in the preferred embodiment, the phase of the resultant signal at the RF input port can be changed. In turn this will change the DC voltage present on the IF output tab  218 .  
         [0028]    Preferably, the test signal applied to the test signal connection tab  220  is arranged to be a TTL signal (0V to +5V) at a frequency that is within the bandwidth of the amplifiers contained within the signal analyser  128  that is connected to the IF output tab  218 . For example, embodiments may use a test signal having a frequency of 30 Hz.  
         [0029]    The detection system  104  uses the signal analyser  128  to monitor the IF output  218  to determine whether or not an intrusion into the protected volume  108  has been detected during conventional operation of the system  100 . In the event that the signal analyser  128  determines that an intrusion into the protected volume  108  has been detected, the signal analyser  128  instructs the alarm-fault signal generator  130  to produce at least one of an audible or visual alarm signal via the output device  110  or to output an alarm condition signal for further processing by, for example, a central control centre.  
         [0030]    However, during a test mode of operation of the detection system, the signal analyser  128  supplies an appropriate test signal to the test signal connection tab  220  during the normal operation of the detector device  102 , that is, during the conventional transmit and receive operations of the detector device  102 . The signal analyser  128 , again, monitors the IF output tab  218  and in the absence of the IF output signal meeting predetermined criteria, the signal analyser  128  instructs the alarm-fault generator  130  to produce a fault indication signal that is output via a fault indicator  138 . It will be appreciated that the testing preferably takes place in the absence of motion of a body within the protected volume. The predetermined criteria include the magnitude of the IF signal being disposed above or below a particular threshold. The fault indication signal can equally well or additionally be output to a central control centre.  
         [0031]    Advantageously, the correct operation or otherwise of the detector device  102  can be monitored in a predetermined and controlled manner and also in the absence of external test equipment that is conventionally used to monitor such operation.  
         [0032]    It will be appreciated that a conventional diode, such as a BAS  16 , can be used in the preferred embodiments. However, the embodiments are not restricted to such an arrangement. Embodiments can equally well be realised in which other devices could be used such as, for example, a microwave diode, a FET, a further mixer or the like, providing the characteristics of the device vary in response to an applied test signal in such a manner as to influence the electro-magnetic field carried by the microstrip lines to the RF input of the mixer.  
         [0033]    Although the above embodiments have been described with reference to detecting movement within a protected volume, the present invention is not limited to such an application. It will be appreciated by those skilled in the art that the invention is equally applicable to relative motion between the detector device and a body whether the body is stationary and the detector is moving or both the body and detector are moving relative to each other.  
         [0034]    The above embodiments have been described with reference to a field distortor that influences or modifies the electro-magnetic field passing through it. It does not radiate electro-magnetic energy as a microwave oscillator would for example, at least not intentionally or not to a sufficient degree to influence the field carried by the conductor. However, it will be appreciated that embodiments of the present invention can be realised in which the field distortor does radiate an electro-magnetic field, in response to an input signal or otherwise, which influences the signal or electro-magnetic field present at the RF input port of the mixer. The field present at that port will be generated by superposition of the generated field and the received RF field. This superposition may be in addition to the influencing of the field described above in the preferred embodiments.  
         [0035]    The reader&#39;s attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.  
         [0036]    All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.  
         [0037]    Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.  
         [0038]    The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.