GB2615892A

GB2615892A - Sensor for an intruder alarm

Info

Publication number: GB2615892A
Application number: GB2300665.3A
Authority: GB
Inventors: Galbraith Colin; O'connell Michael
Original assignee: Orisec Ltd
Current assignee: Orisec Ltd
Priority date: 2023-01-17
Filing date: 2023-01-17
Publication date: 2023-08-23
Anticipated expiration: 2043-01-17
Also published as: GB2615892B; GB202300665D0

Abstract

A door or window detector comprises a 3-axis accelerometer whose output is processed to monitor at least two different trigger conditions and to cause an alarm condition if any one of the trigger conditions is met. A first trigger condition is met if the accelerometer output continuously differs from a first reference value by more than a difference threshold value for a first period of time, such as when the leaf is prised open slowly. A second trigger condition is met if the difference between the accelerometer output and a second reference value exceeds a trigger threshold value and the output from the accelerometer crosses a third reference value more than a predetermined number of times within a second period, such as when the leaf is being drilled. A third trigger condition is met based on a sum exceeding a threshold value, the sum being a sum of differences between samples from the accelerometer output and a fourth reference value, the samples taken from the accelerometer output over a third period of time, such as when the leaf is being hit or kicked. The effects of gravity based on the detector installation orientation may automatically be accounted for.

Description

SENSOR FOR AN INTRUDER ALARM

The present invention relates to a door / window sensor for an intruder alarm, particularly to a sensor which can be mounted to a door, window, or similar, and reliably detect attempts to open the door or window by different methods.

BACKGROUND TO THE INVENTION

It is well known to provide sensors on doors and windows in an intruder alarm system. A typical door sensor comprises a reed switch, or another magnetic sensor, for example a Hall effect sensor, on one part, and a magnet on a second part. One part (typically the magnet) is fixed to the door and the other part is fixed to the door frame, so that the magnet closes the reed switch or is detected by the Hall effect sensor when the door is closed. When the door is opened, the magnet is moved away from the reed switch or Hall sensor and an alarm condition is generated.

However, this type of sensor will not trigger an alarm condition for example if a panel is kicked out of a door to gain entry by entering through a hole made in the door.

Particularly in the case of modem high-security doorsets with multi-point locking mechanisms, a forceful attack may break a hole in the door before it breaks the door away from the frame. Even if the attack eventually breaks the door away from the frame and therefore triggers the alarm, this may be a significant period of time after the attack started. This will delay the alarm, delay a response to the alarm, and is likely to result in increased loss and/or a lower likelihood of apprehending the intruder.

It is also known to provide shock sensors. These sensors detect shocks to the door or window which may indicate an attempt to break the door, for example by kicking it or hitting it. Hence these shock sensors will cause an alarm condition on detection of a violent shock, even if the door does not come away from the frame. This should ensure an earlier alarm, i.e. when the attack has started but perhaps up to several minutes before an intruder has actually succeeded in gaining entry. However, a shock sensor will not activate the alarm if the door is, for example, slowly prised open with a crowbar, or if entry is gained for example by picking the lock or even stealing a key.

GB2512577 describes a detector using a MEMs accelerometer, which is said to be able to detect both shock and motion. However, the detector has to be calibrated to the surface on which it is installed and to that end is provided with a "learn" mode.

It is of course possible to provide both types of sensor on a single door or window, but this increases the cost of the system, and also increases the number of devices which may be unsightly in some environments. In practice it is not often done.

It is an object of the invention to solve these problems, in particular to provide a single sensor which will reliably cause an alarm condition when a door is kicked or hit, or when it is prised open.

STATEMENT OF INVENTION

According to a first aspect of the present invention, there is provided a detector for use in an intruder alarm system, the detector being suitable for mounting to a door or window, the detector comprising: an accelerometer adapted to provide output of instantaneous proper acceleration experienced by the detector; and processing means arranged to receive the output from the accelerometer, the processing means being adapted to monitor at least two of the following trigger conditions, and to cause an alarm condition in the case that any one of the monitored trigger conditions is met: a first trigger condition being met if the output from the accelerometer continuously differs from a first reference value by more than a difference threshold value for a first period of time; a second trigger condition being met if the difference between the output from the accelerometer and a second reference value exceeds a trigger threshold value and then the output from the accelerometer crosses a third reference value more than a predetermined number of times within a second period of time; and a third trigger condition being met based on a sum exceeding a sum threshold value, the sum being a sum of differences between samples from the output of the accelerometer and a fourth reference value, the samples taken from the output of the accelerometer over a third period of time.

The detector monitors at least two of the first, second and third trigger conditions, but preferably monitors all three of the first, second and third trigger conditions and causes an alarm condition in the case that any one of the first, second and third trigger conditions is met.

The detector according to the invention is effective for detecting a number of different types of attack which may indicate an intrusion. The first trigger condition may be met for example if a door or window is opened by prising it open slowly, or even after opening the lock by picking it or using a stolen key. The second trigger condition may be indicative of use of a vibrating tool, for example a drill to drill through a lock or to make a hole in the door to open it from a handle or turn on the inside. The third trigger condition may be indicative of the door or window being hit or kicked.

The accelerometer may be a multiple axis accelerometer. In other words, the accelerometer may provide outputs of acceleration experienced by the detector along multiple different axes. Commonly, a three-axis accelerometer provides accelerometer measurements along three mutually-orthogonal axes.

Where multiple axis outputs are provided, each trigger condition may be monitored on the multiple outputs simultaneously, with the trigger condition being considered met if the respective condition is met on any of the axes. So, for example, the first trigger condition is met if the output from the x-axis accelerometer continuously differs from a first reference value by more than a difference threshold value for a first period of time, or if the output from the y-axis accelerometer continuously differs from a first reference value by more than a difference threshold value for a first period of time.

For the first trigger condition, the first reference value is essentially a steady-state or quiescent value from the accelerometer output. Different first reference values may be applied to different axis outputs on a multiple axis accelerometer. In particular, a three-axis accelerometer will often be installed with one axis in the up-down direction, i.e. in the direction in which gravity acts. One axis will experience a constant proper acceleration due to gravity of about 1g. The steady-state value on that axis will therefore be about 1g, with the steady-state value on the other axis being close to zero.

If the accelerometer is installed in a position other than with one axis exactly in the up-down direction, then components of gravitational acceleration will act on at least two of the axes and the components will be more than zero but less than 1g.

In other embodiments, the output from three (or another number) of accelerometers on multiple axes may be treated together as a single combined output, with the first trigger condition accordingly being met if the single combined output continuously differs from a first reference value by more than a difference threshold value for a first period of time. For example, in such embodiments the output from three accelerometers may be treated as a vector having a magnitude and a direction. The magnitude of that vector could then be treated as a single output which can be compared to a reference, the trigger condition being met when that magnitude differs from that reference continuously for the first period of time. In yet other embodiments, continuously differing from a reference could mean that, for the continuous period (of at least the first period of time), at any one time at least one axis has to differ from the reference by at least the difference threshold value.

The first reference value is preferably automatically set. This can be done for example by monitoring the output(s) of the accelerometer(s) and taking an average of the or each output over a period of time when the output does not vary beyond a quiescent range threshold. In other words, by looking at the output at a time when it is not changing very much, and taking that as a steady-state reference. The output of an accelerometer mounted to a non-moving door or window in a building may change for example by up to about 20-30mg, and this output can essentially be considered as noise in the system. The first reference value is preferably updated automatically from time to time, since it is possible that the detector may move slightly on its mountings (or even, the door or door frame may move slightly), changing the orientation of the detector with respect to gravity. The device may therefore be in effect self-calibrating, requires no manual set-up, and has a high degree of tolerance to different installation positions.

The first trigger condition is effective for detecting movement, for example when a door to which the detector is fitted is opened. In embodiments, the first period of time may be a few tens of milliseconds, for example, 20 -60ms. The difference threshold value may be for example between 0.2 and 0.8g. Embodiments may have these values as adjustable parameters to provide different sensitivity settings for the detector.

In many embodiments, the accelerometer output may be sampled. For example, a suitable sample rate may be around 200Hz (i.e. a sample every 5ms). In such embodiments, an output continuously differing from a reference by more than a difference threshold value for a first period of time means that a minimum number of consecutive samples (the minimum number of consecutive samples corresponding to the first period of time) must all differ from the reference by more than the difference threshold value.

The second trigger condition is met if the difference between the output from the accelerometer and a second reference value exceeds a trigger threshold value and then the output from the accelerometer crosses a third reference value more than a predetermined number of times within a second period of time.

The second trigger condition is in effect a two-stage process. Firstly, the difference between the output from the accelerometer and a second reference value must exceed a trigger threshold value. In this context, the second reference value has a similar purpose to the first reference value and in many embodiments will be exactly the same value, set and in many cases updated in exactly the same way. The second reference value is effectively a steady-state or quiescent value of the relevant accelerometer output, so by referencing a difference rather than an absolute value the effect of the constant force of gravity on the accelerometer can be discounted. Equally, if the accelerometer could be fitted to the door in a way which guaranteed that it detected on an axis or axes which were exactly orthogonal to gravity, then the second reference value could be zero which is the same as comparing the absolute value of the accelerometer output to the trigger threshold value.

In the context of an accelerometer output which is sampled, only a single sample needs to have a difference exceeding the trigger threshold value.

Once the trigger threshold value has been exceeded, the output continues to be monitored. The second trigger condition is then met if the output from the accelerometer crosses a third reference value more than a predetermined number of times within a second period of time. The accelerometer output crossing a third reference value means that the accelerometer output changes from a value which is greater than the third reference value to a value which is less than the third reference value, or vice versa.

The third reference value has a similar purpose to the second reference value in that it is treated as the "zero" or steady-state of the accelerometer output, and therefore discounts for example constant gravity. However, in some embodiments the third reference value may be different from the second reference value. In particular, the third reference value may be updated more often, so that it is a more temporally local reference. For example, in one embodiment, the third reference value is calculated as the average of the 20 samples (100ms at a 200Hz sample rate) immediately preceding the sample which first exceeded the trigger threshold value.

The trigger threshold value may be for example between 0.075g (for very high sensitivity) and 1.75g (for low sensitivity). It is envisaged that embodiments will have the trigger threshold value as an adjustable parameter providing different sensitivity settings for the detector. The predetermined number of times (which the output must cross the reference to trigger an alarm) may be from around 30 times (for high sensitivity) to around 100 times (for low sensitivity). Again, embodiments may allow this parameter to be varied for adjusting the sensitivity of the detector. The second period of time may be around 1-2s but again this could be varied.

Where the accelerometer is a multiple output accelerometer, again the outputs could be monitored independently to see if any one output meets the second trigger condition, or alternatively combined to be treated as a single output to be monitored against the second trigger condition.

The third trigger condition is met based on a sum exceeding a sum threshold value, the sum being a sum of differences between samples from the output of the accelerometer and a fourth reference value.

In some embodiments, meeting the third trigger condition requires firstly that a difference between the output from the accelerometer and a fifth reference value exceeds a second trigger threshold value, and then that the sum exceeds a sum threshold value, the sum being a sum of differences between samples from the output of the accelerometer and a fourth reference value, the samples from the output being samples within a third period of time after the second trigger threshold value is exceeded.

Preferably, samples are only included in the sum where the difference exceeds a reference limit. For example, the reference limit may be around 10-60mg. If a relevant sample differs from the fourth reference value by more than the reference limit, then the difference is included in the sum, if not, then it is not included in the sum. To put it another way, differences of less than the reference limit are treated as zero differences.

The fourth reference value has a similar purpose to the third reference value. Accordingly it may be set in the same way in embodiments, and in particular it may be updated more often than the first and second reference values, so that it is a more temporally local reference.

Where the third trigger condition requires the output from the accelerometer to exceed a trigger threshold, the third and fourth reference values may be calculated as the average of the 20 samples (100ms at a 200Hz sample rate) immediately preceding the sample which first exceeded the trigger threshold value.

The fifth reference value has a similar purpose to the first and second reference values, and in some embodiment some of the first, second and fifth reference values may be the same value, or set in the same way.

The second trigger threshold value may be for example between 0.075g (for high sensitivity) up to around 1.175g (for low sensitivity). Suitable values for the second trigger threshold value are in around the same range as suitable values for the first trigger threshold value, and in some embodiments the first and second trigger thresholds may be the same parameter. However, in other embodiments, the first and second trigger threshold values may be different. In particular, both may be adjustable parameters for setting the sensitivity level of the detector, and it is useful to make them individually adjustable so that the sensitivity of the second and third trigger conditions can be set individually. This allows the device to be "tuned" to a particular installation to maximise sensitivity while avoiding false alarms caused by environmental factors.

The third time period may be similar to the second time period, for example 1-2s.

According to claim 19 there is provided a method of detecting an intrusion event at a door or window, and according to claim 20 there is provided a computer program product, for example instructions on a non-transient computer readable storage medium. All of the features and advantages described above may be applied equally to these aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made by way of example only to the accompanying drawings, in which: Figure 1 shows a typical output from a three-axis accelerometer which would meet the first trigger condition as described above; and Figure 2 shows a typical output from a three-axis accelerometer which would meet the second and/or third trigger conditions as described above.

DESCRIPTION OF PREFERRED EMBODIMENTS

An embodiment of a detector according to the invention makes use of a 3-axis digital output MEMS accelerometer, model number ADXL362 available from Analog Devices, Inc of Norwood, MA, USA. The accelerometer output is processed by a suitable microprocessor which is programmed to monitor the conditions according to the invention. In turn the microprocessor outputs signals to an interface with an alarm system control panel.

It will be appreciated that alternative accelerometer devices may be used in other embodiments. However, the ADXL362 provides certain features which are useful for an efficient implementation which in particular may conserve battery life in a battery-powered wireless detector. Although a high-level description of various features of the ADXL362 is provided herein, the skilled person is referred to the manufacturer's datasheet for implementation detail.

The first trigger condition, as described above, is met if the output from the accelerometer continuously differs from a first reference value by more than a difference threshold value for a first period of time. In practice, with a digital output accelerometer such as the ADXL362, this means that a predetermined number of consecutive samples must differ from the first refence value by more than the difference threshold value.

The ADXL362 may be set in a referenced configuration for activity detection. In this configuration, the first reference value is set so that activity is detected when acceleration has deviated from the initial orientation. In other words, the effect of gravity on the device and the steady-state acceleration due to gravity, which will be experienced by the device depending on the orientation in which it is installed, can be automatically taken account of.

The accelerometer is configured to send an "activity detection" signal to the processor when the output on at least one of the axes exceeds the first reference value by more than the difference threshold value for a first period of time. The processor of the detector may then enter an alarm condition, i.e. start signalling an alarm event to the alarm system controller.

Figure 1 shows an example of the output from the three axes of the accelerometer, when the device is fixed to a door and the door is opened normally. It can be seen firstly that one of the axes, which is orientated in an up-down direction, experiences a steady-state gravitational acceleration of around 1g. The other two axes experience a steady-state acceleration which is relatively close to zero, although both are clearly not exactly zero, with one being about 0.18g above zero and the other being around 0.02g below zero (i.e. experiencing acceleration in the other direction). This can be explained simply by the fact that the accelerometer is unlikely to be mounted exactly vertically in any given installation, and hence each of the axes experiences a component of gravitational acceleration.

Note also that the two accelerometer axes which experience only a small component of gravitational acceleration, because they are mounted roughly along horizontal axes, show a relatively large signal when the door is opened (starting around sample 150). On the other hand the vertically-mounted axis shows only a small signal. This is expected since opening a normal door would not involve any acceleration at all along a vertical axis. Again, the small signal which is present can be explained by the device not being mounted precisely vertically. Alternatively, an imperfectly fitted door (or, for example a door fitted on rising butt hinges) may indeed move slightly in the vertical direction when opened.

In any case, the effect of the referenced configuration for activity detection is that the steady-state offset on each axis is taken account of automatically.

The sensitivity of the device may be configured by setting values for the difference threshold value and for the first period of time. In the embodiment using the ADXL362, these values may be set by writing the activity threshold and activity time registers. Preferably, selectable presets may be programmed into the device to provide "high", "medium" and "low" sensitivity options to an installer, although it will be appreciated that more options could be provided if required, even by allowing an installer to directly set the difference threshold value and the first period of time. Examples of suitable presets are shown in the table below.

The value in the "Activity Time" register sets the number of consecutive samples that must have at least one axis greater than the activity threshold for an activity event to be detected. In this example the sample rate is 200 samples per second, i.e. a sample every 5ms.

Sensitivity Level Difference threshold value (activity threshold register) First period of time (activity time register) 3 750 10 samples (50ms) (low sensitivity) 2 500 7 samples (35ms) (medium sensitivity) 1 250 5 samples (25ms) (high sensitivity) As explained above, the second trigger condition is met if the difference between the output from the accelerometer and a second reference value exceeds a trigger threshold value and then the output from the accelerometer crosses a third reference value more than a predetermined number of times within a second period of time.

This second trigger condition can be conveniently implemented using "triggered mode" of the ADXL362. In this mode, a number of samples surrounding a trigger event are saved in the ADXL362's FIFO buffer. In an embodiment, the ADXL362 may be configured to save a total of 300 samples -20 samples from before the trigger event and 280 samples following the trigger event. These parameters could of course be changed in different embodiments.

The trigger event is when the output (of at least one axis in this embodiment) exceeds the second reference value by more than the trigger threshold value. The second reference value is preferably set automatically to take account of the steady state gravitational acceleration experienced by the accelerometer, similar to the first reference value (and may well be the same value set at the same time in the same way).

The third reference value, in this embodiment, is calculated after the trigger event has occurred. It is calculated by averaging (for each axis) the first 20 samples in the FIFO buffer, i.e. the 20 samples (100ms at a 200sample/s rate) immediately preceding the trigger event. Hence the third reference value represents a temporally local reference value, and will take into account not only gravity but, for example, the force of wind acting on an exterior door.

Once the third reference value has been calculated, the remaining time series of samples (280 samples in this embodiment) is analysed to count the number of times the signal crosses the third reference value (i.e. the number of times in the time series when sample n is above the third reference value and sample n+1 is below the third reference value, or vice versa). If the number of crossings counted exceeds the predetermined number of times, then the trigger condition is met and an alarm signal is generated and set to the alarm system controller.

The trigger threshold value and the predetermined number of crossings may be tuned to set the sensitivity of the device. A table of suitable presets is provided below, although again it will be appreciated that different, more, or fewer options may be provided in different embodiments. In particular, in this embodiment the second period of time is the same for all sensitivity settings, and is 1.4 seconds (corresponding to the 280 samples saved after the trigger event). However, in other embodiments the second period of time may be settable. In principle a higher sensitivity is achieved by reducing the second period of time while keeping predetermined number of crossings the same.

Sensitivity Trigger threshold (mg) Predetermined number of times third reference value crossed 8(10w) 1750 100 7 1500 95 6 1250 90 1000 80 (medium) 4 500 55 3 250 30 2 150 30 1 (high) 75 30 The third trigger condition is met based on a sum exceeding a sum threshold value, the sum being a sum of differences between samples from the output of the accelerometer and a fourth reference value. In this embodiment, once the trigger event has occurred and 300 samples (20 from before the trigger event and 280 following the trigger event) have been loaded into the FIFO buffer, it is the 280 samples following the trigger event that are added up. The fourth reference value is taken as an average of the 20 samples preceding the trigger event (i.e. the fourth reference value is the same as the third reference value). The differences between the fourth reference value and each one of the following 280 samples are then added together, and compared to a sum threshold value.

Preferably every difference in the sum is taken as a positive value -i.e. it does not matter whether the value of the sample is above or below the fourth reference value, it is the modulus of the difference which is added to the sum.

In this embodiment, differences which are less than a reference limit are ignored when calculating the sum. The reference limit is a further tuneable parameter which can be adjusted according to the desired sensitivity of the detector.

Figure 2 shows an example signal from a three-axis accelerometer which may be expected to cause the second and/or third trigger conditions. Figure 2 shows what is loaded into the accelerometer's FIFO buffer, and made available to the connected microprocessor, when the trigger threshold is exceeded. Accordingly it includes 20 samples from before the trigger event and the 280 samples immediately following the trigger event.

A table of suitable presets for the third trigger condition is shown below. Again, different embodiments may have different presets or even allow the installer to set the parameters directly.

Sensitivity Reference limit (mg) Trigger Sum threshold value threshold (mg) 8 (low) 60 1750 30000 7 50 1500 25000 6 40 1250 19000 (medium) 30 1000 15000 4 20 500 10000 3 10 250 7000 2 10 150 6000 1 (high) 10 75 5000 In a preferred embodiment, the second and third trigger conditions are configured together as one sensitivity setting. This is convenient since, firstly, the second and third trigger conditions attempt to detect a similar event -a shock. By combining the second and third trigger conditions a more sensitive detector can be made without increasing susceptibility to false alarms. Secondly, the trigger level is a parameter which can be set in a register of the ADXL362 accelerometer, and similar functions may be available in alternative devices as well. The ADXL362 can be set so that when the trigger level is reached, samples both preceding and following the trigger event are loaded into a buffer and made available to a connected microprocessor.

Accordingly the table below shows example presets which set the sensitivity of both the second and third trigger conditions.

Sensitivity Reference limit (mg) Trigger threshold (mg) Sum threshold Predetermined value number of times third reference value crossed 8 (low) 60 1750 30000 100 7 50 1500 25000 95 6 40 1250 19000 90 (medium) 30 1000 15000 80 4 20 500 10000 55 3 10 250 7000 30 2 10 150 6000 30 1 (high) 10 75 5000 30 In some embodiments the sensitivity of the first trigger condition may also be configured alongside the second and third trigger conditions. However, the first trigger condition is designed to detect movement, as opposed to shock, and therefore it may be appropriate to tune its sensitivity separately. Nevertheless, the combination of the first trigger condition with the second and/or third trigger conditions provides a single detector which can detect a wide range of possible attacks on a door or window, protecting a property.

Embodiments have been described using the ADXL362 accelerometer. This device has certain convenient features, as have been described, which can be taken advantage of in implementations of the invention in particular to reduce the overall power consumption. This may be important, for example in a battery-powered wireless sensor. However, other accelerometers capable of measuring instantaneous proper acceleration may be used. For example, the LIS3DHTR from STMicroelectronics is a digital output motion sensor which is fundamentally similar, and comes at a lower cost. The power consumption is greater than the ADXL362 and so it is more suitable for use in wired sensors which are powered by an external power supply (ultimately powered by mains electricity and usually with a central battery backup at the alarm control panel).

In some embodiments, a door contact sensor may be incorporated into the device, for example in the form of a reed switch and a magnet, or a hall effect sensor to detect a proximal magnet. This method of detecting an open door is well known, and may be useful in combination with the trigger conditions using the accelerometer as described.

To put it another way, devices of the invention may have a fourth trigger condition which is met when a door contact sensor detects that a magnet has been moved out of proximity of the contact sensor.

The embodiments described above are provided by way of example only, and various changes and modifications will be apparent to persons skilled in the art without departing from the scope of the present invention as defined by the appended claims.

Claims

CLAIMS1. A detector for use in an intruder alarm system, the detector being suitable for mounting to a door or window, and the detector comprising: an accelerometer adapted to provide output of instantaneous proper acceleration experienced by the detector; and processing means arranged to receive the output from the accelerometer, the processing means being adapted to monitor at least two of the following trigger conditions, and to cause an alarm condition in the case that any one of the monitored trigger conditions is met: a first trigger condition being met if the output from the accelerometer continuously differs from a first reference value by more than a difference threshold value for a first period of time; a second trigger condition being met if the difference between the output of the accelerometer and a second reference value exceeds a trigger threshold value and then the output from the accelerometer crosses a third reference value more than a predetermined number of times within a second period of time; and a third trigger condition being met based on a sum exceeding a sum threshold value, the sum being a sum of differences between samples from the output of the accelerometer and a fourth reference value, the samples taken from the output of the accelerometer over a third period of time.
2. A detector as claimed in claim 1, in which the processing means is adapted to monitor all three of the first, second and third trigger conditions and to cause an alarm condition in the case that any one of the first, second and third trigger conditions is met.
3. A detector as claimed in claim 1 or claim 2, in which the accelerometer is a multiple-axis accelerometer, having an output corresponding to the acceleration on each of multiple axes.
4. A detector as claimed in claim 3, in which each bigger condition is monitored in relation each output, with the respective trigger condition being met if the respective condition is met in relation to any one of the outputs.
5. A detector as claimed in claim 3 or claim 4, in which the first, second, third, and/or fourth reference values are set individually in relation to each of the multiple outputs of the multiple-axis accelerometer.
6. A detector as claimed in any of the preceding claims, in which the output of the accelerometer is sampled at a rate of at least 100Hz, preferably at least 200Hz.
7. A detector as claimed in any of the preceding claims, in which the second reference value is the same as the first reference value.
8. A detector as claimed in any of the preceding claims, in which the third reference value is calculated after the difference between the output of the accelerometer and the second reference value exceeds the trigger threshold value, by averaging the output of the accelerometer over a period of time immediately preceding the time at which the difference first exceeded the trigger threshold value.
9. A detector as claimed in claim 8, in which the period of time immediately preceding the time at which the difference first exceeded the trigger threshold value is between 50ms and 200ms
10. A detector as claimed in any of the preceding claims, in which the trigger threshold value is between 0.075g and 1.75g.
11. A detector as claimed in any of the preceding claims, in which the predetermined number of times which the output must cross the third reference value to trigger an alarm is between 30 and 100.
12. A detector as claimed in any of the preceding claims, in which the second period of time is between 0.5s and 2s.
13. A detector as claimed in any of the preceding claims, in which meeting the third trigger conditions requires firstly that a difference between the output from the accelerometer and a fifth reference value exceeds a second trigger threshold value, and then that the sum exceeds the sum threshold value, the sum being a sum of differences between samples from the output of the accelerometer and the fourth reference value, and the samples from the output being samples within a third period of time after the second trigger threshold value is exceeded.
14. A detector as claimed in claim 13, in which the second trigger threshold value is the same as the first trigger threshold value.
15. A detector as claimed in claim 13 or claim 14, in which the fifth reference value is the same as the first reference value.
16. A detector as claimed in any of claims 13 to 15, in which the third period of time is between 0.5s and 2s.
17. A detector as claimed in any of the preceding claims, in which samples are only included in the sum of differences where the difference exceeds a reference limit.
18. A detector as claimed in claim 17, in which the reference limit is between 10mg and 60mg.
19. A method of detecting an intrusion or intrusion attempt at a door or window, the method comprising the steps of: mounting an accelerometer on the door or window, the accelerometer being adapted to provide output of instantaneous proper acceleration experienced by the accelerometer; and processing the output from the accelerometer to monitor at least two of the following trigger conditions, and to cause an alarm condition in the case that any one of the monitored trigger conditions is met: a first trigger condition being met if the output from the accelerometer continuously differs from a first reference value by more than a difference threshold value for a first period of time; a second trigger condition being met if the difference between the output of the accelerometer and a second reference value exceeds a trigger threshold value and then the output from the accelerometer crosses a third reference value more than a predetermined number of times within a second period of time; and a third trigger condition being met based on a sum exceeding a sum threshold value, the sum being a sum of differences between samples from the output of the accelerometer and a fourth reference value, the samples taken from the output of the accelerometer over a third period of time.
20. A computer program product comprising instructions which, when executed on a processor provided with a signal from an accelerometer, will cause the processor to process the output from the accelerometer to monitor at least two of the following trigger conditions, and to cause an alarm condition in the case that any one of the monitored trigger conditions is met: a first trigger condition being met if the output from the accelerometer continuously differs from a first reference value by more than a difference threshold value for a first period of time; a second trigger condition being met if the difference between the output of the accelerometer and a second reference value exceeds a trigger threshold value and then the output from the accelerometer crosses a third reference value more than a predetermined number of times within a second period of time; and a third trigger condition being met based on a sum exceeding a sum threshold value, the sum being a sum of differences between samples from the output of the accelerometer and a fourth reference value, the samples taken from the output of the accelerometer over a third period of time.