GB2358534A

GB2358534A - Testing of doppler alarm sensors

Info

Publication number: GB2358534A
Application number: GB0001460A
Authority: GB
Inventors: Stephen W Redfern; Paul A Tyson; Peter P Blunden
Original assignee: Dynex Semiconductor Ltd
Current assignee: Dynex Semiconductor Ltd
Priority date: 2000-01-21
Filing date: 2000-01-21
Publication date: 2001-07-25
Also published as: EP1118873A1; GB0001460D0; US20010019305A1

Abstract

Apparatus for testing a Doppler alarm sensor 13 includes a receiver 14, means 17, 18 for modifying a signal received by the receiver to simulate a "moving target" return signal, means 19 for varying the power of the modified signal, a transmitter 15 for transmitting the modified signal to the sensor, and means 20 for detecting a signal output by the alarm sensor. The sensor 13 is mounted on an opposite side of an anechoic chamber 11 to the transmitter 15 and the receiver 14. The apparatus includes a controller 20 to reduce the attenuation of an attenuator 19, forming part of the power varying means, from a high level until the modified signal received by the alarm sensor 13 is large enough to trigger an output signal.

Description

2358534 TESTING METHOD AND APPARATUS This invention relates to a method

of, and apparatus for, testing an alarm sensor; and in particular to a method of, and apparatus for, testing an alarm sensor suitable for use in a microwave frequency vehicle intruder alarm.

Vehicle intruder alarms which rely on detection of the Doppler shift of a reflected microwave signal to indicate movement within a space have been available for several years. One such alarm is described in the specification of our UK Patent Application

GB-A-2 322 029, the contents of which are incorporated herein by reference.

A continuous wave vehicle intruder alarm sensor of this type responds to the movement of an object within the aperture of an antenna of the sensor, and is typically used to detect movement of human beings or parts thereof within a protected area such as the interior of a vehicle. This type of sensor will only respond to moving targets.

This type of vehicle intruder alarm sensor can be tested using a moving mechanical object such as a rotating vane, which generates a Doppler return signal which is dependent upon the speed of movement and angular position of the rotating vane relative to the sensor. One known method of doing this is shown in Figure 1, which shows an anechoic chamber 1 having first and second windows 2a and 2b. A vehicle intruder alarm sensor 3 is positioned across the window 2a, and a movable shutter 4 is positioned in the vicinity of the window 2b. The shutter 4 is rotatable, by means of a motor 5 and a lever system (not shown), so as to be movable into, and out of, a position in which it lies directly between the sensor 3 and a rotatable comer cube reflector 6 positioned outside the chamber 1. The reflector 6 is rotatable, about a vertical axis, by means of a motor 7, and a controller 8 is provided for controlling the two motors. A feedback line 9 provides signals output by the sensor 3 to the controller 8.

With this arrangement, electromagnetic energy returned to the sensor 3 following reflection by the reflector 6 has a range of different Doppler offset frequencies and 2 amplitudes, due to the differing speeds of the reflecting surfaces of the reflector and size of the aperture which opens and closes in front of the reflector. The alarm sen of UK Patent Application GB-A-2 322 029, in common with other Doppler sensor3 set to register a trigger signal when a predetermined threshold signal level has b( reached. This trigger signal is the signal fed back to the controller 8, and its leve known as the sensitivity of the sensor. Consequently, a sensitivity value car ascribed to the device based on the position of the shutter 4 when the trigger poini reached.

This known type of arrangement has the following disadvantages: - I. The system needs to be quite large in order to house the motors 5 and 7 and rotatable reflector 6.

2. It is difficult to achieve good measurement repeatability without very prv, control of the shutter 4.

3. Measurements are only made slowly, due to the inertia of the shutter 4 and' drive mechanism (the motor 5 and the associated lever system, and the motor 7).

4. The system generates electrical and acoustic noise, vibration and heat.

5. The system requires regular servicing.

The aim of the invention is to provide a method of, and apparatus for, testing an sensor which reduces or eliminates these disadvantages.

The present invention provides apparatus for testing a Doppler alarm sensor, apparatus comprising a receiver for receiving signals output by the sensor, means modifying the signal received by the receiver to simulate a "moving targef' r r signal, means for varying the power of the modified signal, a transmitteill 3 transmitting the modified signal to the sensor, and means for detecting a signal output by the sensor.

In a preferred embodiment, the apparatus further comprises an anechoic chamber, the sensor being mounted in one wall of the chamber, and the transmitter and the receiver being mounted in an opposite wall of the chamber.

Advantageously, the moditdng means comprises a phase shifter and a signal source, the signal source driving the phase shifter with a repetitive wave form. The apparatus may finther comprise an amplifier, preferably positioned between the receiver and the phase shifter.

Conveniently, the power varying means is an attentuator positioned between the phase shifter and the transmitter, and the apparatus ftirther comprises a controller for controlling the attentuator in dependence upon signals fed back to the controller from the sensor.

Preferably, the controller is such as to reduce the attenuation of the attentuator progressively from a first high level until the modified signal received by the sensor is large enough to trigger an output signal for transmission to the controller.

The invention also provides apparatus for simulating a "moving target" return signal for transmission to a Doppler alarm sensor, the apparatus comprising a receiver for receiving signals output by the sensor, a phase shifter for modifying the signal received by the receiver, and a transmitter for transmitting the modified signal to the sensor.

The invention further provides apparatus for measuring the sensitivity of a Doppler alarm sensor, the apparatus comprising a transmitter for transmitting a signal to the sensor, an attenuator for progressively reducing the attenuation of a signal input to the transmitter.) and means for sensing the output of the sensor, the sensitivity of the alarm sensor being a function of the attenuation of the attenuator when the sensor is triggered.

4 The invention still fin-ther provides a method of testing a Doppler alarm sensor, tj method comprising the steps of outputting a signal by the sensor, receiving said oulp signal at a receiver, modifying the output of the signal from the receiver to simulal e 1 44moving target" return signal, controlling the power level of the modified si 1, transmitting the modified signal to the sensor, and detectin a signal output by 'it] k, 9 sensor.

The invention also provides a method of simulating a "moving target' return signal f)r transmission to a Doppler alarm sensor, the method comprising the steps of outputling a signal by the sensor, receiving said output signal at a receiver, modifing the si, received by the receiver using a phase shifter, and transmitting the modified signaT o the sensor.

The invention still fin-ther provides a method for measuring the sensitivity of a Dop I alarm sensor, the method comprising the steps of transmitting a signal to the sert (r, progressively attenuating the attenuation of a signal input to the transmitter, and sensflg the output of the sensor, the sensitivity of the alarm sensor being a function of tl te attenuation of the attenuator when the sensor is triggered.

The invention will now be described in greater detail, by way of example, vyi 11 i reference to the drawings, in which:

Figure I is a schematic representation of a known apparatus for testing an alarm senso Figure 2 is a schematic representation of an alarm sensor testing apparatus constru. ted in accordance with the invention; and Figure 3 is a circuit diagram of a pin diode attentuator forming part of the apparatu Figure 2.

Referring to the drawings, Figure 2 shows an anechoic chamber I I having a continu s wave vehicle intruder (microwave) alarm sensor 13 to be tested positioned withjn a window 12 provided in one wall of the chamber. The anechoic chamber 11 is Ii with radar-absorbing material effective at the operating frequency of the sensor 13 Ld up to the third harmonic thereof The size of the chamber I I and the material of the absorber need to be commensurate with the operating frequency of the sensor 13 which, in the case of the sensor described in the specification of UK Patent Application GB-A-2 322 029, is 2.45 GHz. The chamber 11 is about 4 cubic metres in volume, and the window 12 is placed in the centre of one face thereof.

Two patch antennas 14 and 15 are positioned on the internal surface of the opposite face of the chamber 11, the antennas being directly in the boresight of the sensor 13. The two antennas 14 and 15 are arranged sideby-side, and electrical connections 14a and 15a pass from the antennas to the outside of the chamber 11. The antenna 14 is a receiver antenna, and the antenna 15 is a transmitter antenna.

The receiver antenna 14 is connected, via the connector 14a, to a linear amplifier 16, whose output is fed to a phase shifter 17. The phase shifter 17 is constructed, in a known manner, using balanced varactor diodes, although other means of electronic phase shift generation are possible. The phase shifter 17 is driven from a signal source 18 which creates a continuous low frequency phase change. The signal source 18 is a low frequency function generator, which may be implemented in software using a DAC board and a computer. The low frequency waveform generated by the source 18 can be a sine wave, a triangular wave, or any other wave form which is repetitive and has varying states such as would be generated by a Doppler return signal.

The transmitter antenna 15 is connected, via the connector 15a, to the output of an attenuator 19, the output of the phase shifter 18 being fed to the input of the attenuator.

A controller 20 is provided for controlling the attenuator 19 in dependence upon signals received by the controller from the sensor 13 via a line 13a.

In use, the phase shifter 17 is set up to give around 3600 of peak-topeak phase excursion in response to the cyclic modulating signal supplied by the signal source 18.

The amplified and modulated output signal from the phase shifter 18 is fed into the attenuator 19, and then to the transmitter antenna 15 positioned in the chamber 11. In this way, an artificial Doppler sideband is added to the constant wave signal supplied by 6 the sensor 13 via the receiver antenna 14 and the amplifier 16. This signal is fed bac to the sensor 1 3 via the transmitter antenna 15, and is detected as a "moving targ e' return signal.

The purpose of the attenuator 19 is to adjust the level of the signal which is transmilte to the sensor 13 so that its trigger point can be measured. This trigger poini determined by the sensitivity of the sensor 13. Thus, a more sensitive sensor 13 ivil trigger on detecting an event at a lower input power level. The attenuator 1 S controlled by the controller 20 in response to signals fed back from the sensor 13]v a 10 the line 13a. Measurement of the sensitivity is achieved by stepping the attenuator 19 througi a range of attenuation values until the sensor triggers and the output is detected. T1k, attenuator 19 is initially set to a high value, and then the attenuation is progressiNe. 15 reduced until the trigger point is reached. The attenuator 19 is adjustable in very sniell steps (0. 1 dB) so that an accurate measurement can be made. The attenuation valu, 'S software controlled by a computer and a DAC card. As shown in Figure 3, the attenuator 19 is realised using a pair of pin diodes 21 l n a 20 balanced configuration between 900 hybrid couplers 22. In this case, the attenuator P will deliver around 20 dB of maximum attenuation when current is driven through tl diodes 21. If necessary, more than one attenuator stage could be used to providT it least 40 dB of range. 25 The amplifier 16 should be operated well within its linear operating region, so as o avoid the generation of any harmonic products which may corrupt the sensitivi y measurement. The amplifier 16 should have a gain of approximately 40 dB, in order p provide an adequate signal level to transmit back to the sensor 13. This gain leAl depends on the sensitivity of the sensor 13, the size of the chamber 11, the gain of fl 30 antenna 14 (?), and the losses in the other components of the circuit. The amplifiet 6 can be built from discrete or integrated components in silicon or gallium arsenide. n 7 this way, an amplifier 16 having a noise figure less than 5 dB and a 1 dB compression point of 17 dBm can be realised.

The frequency of the low frequency modulating signal supplied by the source 18 should be comparable with that generated by the Doppler return which the sensor 13 would see in a normal application. This Doppler frequency will depend on the operating frequency or wavelength of the sensor 13 and the speed of a typical moving target in accordance with the Doppler equation:

af = 2V/.X where Af is the Doppler frequency in Hz, V is the target speed in m/s, and X is the radar wavelength in m.

For the sensor described in the specification of UK Patent Application GBA-2 322 029 when used in a vehicle security system, the Doppler frequencies typically lie in the range of I to 40 Hz. When using the apparatus described above with reference to Figure 2 to test such a sensor, a frequency of 5 Hz is used, although other frequencies are possible.

The apparatus described above operates on the principle of generating a simulated "moving target" Doppler return signal electronically, and using this return signal to measure the sensitivity of the sensor 13. Thus, the sensor 13 generates its normal microwave frequency signal, and this is detected by the receiving antenna 14. The output of the receiver antenna 14 is amplified by the amplifier 16 whose output is modified by the phase shifter 17 and modulated by the source 18. This signal is then fed back to the transmitter antenna 15 via the attentuator 19, and the return signal is picked up by the sensor 13. The sensor 13 triggers when the received Doppler signal reaches a level that exceeds the sensitivity level of the sensor. The apparatus can readily be set up to provide an accurate sensitivity measurement by suitably calibrating the variable parameters of the system against sensors whose sensitivities have previously been accurately determined.

8 It will be apparent that the apparatus described above with reference to Figures 2 ar d 3 could be modified in a number of ways. For example, the patch antennas 14 and. 5 could be replaced by any other suitable antennas such as slot antennas or bip)] antennas. Similarly, the particular form of the attenuator 19 shown in Figure 3 is nit essential, and other ways of achieving the required attenuation function ur d - r programme control, including mechanical methods, are also possible. Indeed, control range of the attentuator depends on the range of sensitivities required to measured. When the sensor 13 triggers, the value of attenuation provided by attentuator is detected, and this attenuation value is translated to a measurement of sensitivity of the sensor in dB. In practice, the higher the attenuation value when sensor 13 triggers, the more sensitive the sensor is under test. The attenuation valu s called the sensitivity, and is measured in dB.

The apparatus described above could also be adapted for testing types of Doppler ah]i sensor other than microwave alarm sensors.

Although the apparatus described above is configured to measure the sensitivity of 9 k, sensor 13, it will be apparent that it could also be used to measure the output power the operating frequency of the sensor without removing the sensor from the te apparatus. These parameters are required in order to check compliance with the rad 6 type approval requirements (ETSI 300, 440 etc). Measuring these parameters can Be used to confirm that the sensor is operating correctly.

Claims

9 Claims

1. Apparatus for testing a Doppler alarm sensor, the apparatus comprising a receiver for receiving signals output by the sensor, means for modifying the signal received by the receiver to simulate a "moving target" return signal, means for varying the power of the modified signal, a transmitter for transmitting the modified signal to the sensor, and means for detecting a signal output by the sensor.

2. Apparatus according to claim 1, in which the apparatus further comprises an anechoic chamber, the sensor being mounted in one wall of the chamber, and the transmitter and the receiver being mounted in an opposite wall of the chamber.

3. Apparatus according to either preceding claim, in which the modifying means comprises a phase shifter and a signal source, the signal source driving the phase shifter with a repetitive wave form.

4. Apparatus according to any preceding claim, in which the apparatus further comprises an amplifier, which is preferably positioned between the receiver and the phase shifter.

5. Apparatus according to any preceding claim, in which the power varying means is an attentuator positioned between the phase shifter and the transmitter, and the apparatus further comprises a controller for controlling the attentuator in dependence upon signals fed back to the controller from the sensor.

6. Apparatus according to claim 5, in which the controller is such as to reduce the attenuation of the attentuator progressively from a first high level until the modified signal received by the sensor is large enough to trigger an output signal for transmission to the controller.

7. A method of testing a Doppler alarm sensor, the method comprising the steps of outputting a signal by the sensor, receiving said output signal at a receiver, modifying the output of the signal from the receiver to simulate a "moving target" return Sig controlling the power level of the modified signal, transmitting the modified sign. the sensor, and detecting a signal output by the sensor.

8. Apparatus for simulating a "moving target" return signal for transmission I I io Doppler alarm sensor, the apparatus comprising a receiver for receiving signals o p it 11 I by the sensor, a phase shifter for modifying the signal received by the receiver, ar d a transmitter for transmitting the modified signal to the sensor.

9. Apparatus for measuring the sensitivity of a Doppler alarm sensor, the appara 5 comprising a transmitter for transmitting a signal to the sensor, an attenuator f)r progressively reducing the attenuation of a signal input to the transmitter, and means I f) sensing the output of the sensor, the sensitivity of the alarm sensor being a functioi) f the attenuation of the attenuator when the sensor is triggered.