Introduction
[0001] The invention relates to static alarm devices such as smoke, heat and/or carbon monoxide
(CO) alarm devices.
[0002] The sensing technology of such devices is typically of good quality and is reliable.
However, problems can arise due to use of the devices whereby an adequate flow of
air to be sensed does not reach the active sensor, for example smoky air not reaching
an optical chamber sensor of a smoke alarm device, hot air not reaching a temperature
sensor, or air with CO not reaching a CO sensor. This can arise if someone inadvertently
puts an obstacle in place, such as for example taping over a vent of a device while
painting and decorating, covering over a device with a false ceiling, or placing a
large item of furniture close to the device.
[0005] The invention is directed towards providing enhanced obstacle detection for alarm
devices which are suitable to be mounted in static positions to detect a condition
of ambient air.
Summary
[0006] We describe an alarm device as set out in the accompanying claims 1 to 15.
[0007] We describe an alarm device to detect a condition of ambient air, the alarm device
comprising:
a housing having a longitudinal axis and containing a sensor and having vents for
access by ambient air to the sensor,
a signal processing circuit with a processor linked with the sensor,
a power supply for the circuit and the sensor,
an obstacle detector for detecting presence of an unwanted obstacle to flow of ambient
air to said sensor.
[0008] Preferably, the obstacle detector comprises:
an ultrasonic transducer mounted in or on the device to have a field of emission outside
of the device,
a processor in the circuit and linked with the ultrasonic transducer to monitor ultrasonic
return values and process them to determine if any have been reflected by an unwanted
obstacle, wherein the processor is configured to generate an alert to indicate presence
of such an obstacle which may affect access of ambient air to the sensor through the
vents, and
a guide mounted to the housing to reflect emitted ultrasonic waves in radial directions
relative to the longitudinal axis.
[0009] The alert may be internal, to prevent proceeding with an obstacle detection test,
and/or it may be a user alert such as an audio or visual alert.
[0010] Preferably, the guide comprises a guide element mounted to the housing so that it
is spaced-apart from the housing. Preferably, the guide element is mounted by a plurality
of pillars. Preferably, the guide element has a first curved surface facing the housing,
and said guide element first curved surface may be generally concave.
[0011] Preferably, the housing has a curved surface facing the guide element, and the housing
curved surface may be generally convex. Preferably, the guide element is mounted substantially
symmetrically to the device longitudinal axis.
[0012] Preferably, the guide element is dish-shaped with a narrower end facing the housing.
Preferably, the guide element has a second curved surface facing away from the device
housing. Preferably, said second curved surface is generally convex.
[0013] Preferably, the guide element has a thickness and a density to act as a secondary
ultrasonic source upon ultrasonic waves being incident on the first surface.
[0014] Preferably, the vents are arranged around at least some of the circumference of the
housing with a field of emission of ultrasonic waves guided by the guide. Preferably,
at least some of the vents are arranged so that at least some ultrasonic waves pass
through the vents.
[0015] Preferably, the vents include vents which are primarily facing radially and vents
which at least have a directional component facing axially.
[0016] Preferably, the housing includes a barrier to render application of tape to the vents
difficult. The barrier may comprise a barrier element mounted by pillars so that it
is spaced apart from at least some vents. The barrier element may be annular.
[0017] Preferably, the barrier comprises a plurality of elements which extend from the housing
between vents. Preferably, the ultrasonic transducer is mounted in a resilient cover
and said cover engages in an aperture of a substrate which is in turn mounted to the
housing.
[0018] Preferably, the resilient cover has a groove which engages a side edge of the substrate
aperture. Preferably, the ultrasonic transducer is connected to a conductor on a substrate
by a flexible wire link.
[0019] Preferably, the processor is programmed to detect ambient air instability either
before or as part of an obstacle detection operation. The processor may be configured
to record a return signal amplitude value for each of a plurality of sample points,
and to quantify variance across said values, and if said variance exceeds a threshold
determine that there is excessive ambient air instability.
[0020] Preferably, the processor is configured to determine a series of difference values
for each pair of successive values for a sample point, and to derive a sample point
variance value representative of variance for each sample point, and to compare the
derived sample point variance value with a threshold. Preferably, the derived sample
point variance value is a sum of the difference values for a sample point.
[0021] Preferably, the processor is configured to perform a plurality of scans each with
a plurality of sample points, and to determine a multi-scan derived variance value
derived from said sample point variance values. Preferably, a multi-scan derived variance
value is an average of all sample point variance values.
Detailed Description of the Invention
[0022] The invention will be more clearly understood from the following description of some
embodiments thereof, given by way of example only with reference to the accompanying
drawings in which:
Figs. 1, 2, and 3 are top perspective, top plan and side views respectively of an
optical alarm device;
Fig. 4 is a block diagram of the main functional parts of the device;
Fig. 5 is a cut-away perspective view showing the device housing, and Ultrasonic ("US")
wave guide element pathways for US waves for obstacle detection, especially in relation
to vents for ambient air entry;
Fig. 6 is a cut-away perspective view of an ultrasonic transducer, and Fig. 7 is a
perspective view of its mounting PCB in which the transducer is mounted in a manner
to minimise transfer of vibration to the PCB and to other components;
Figs. 8 to 11 are diagrams illustrating operation of the obstacle detector in terms
of emission of ultrasonic waves;
Figs. 12, 13 and 14 are perspective, side and cut-away views of an alternative alarm
device with an obstacle detector; and
Fig. 15 is a table of data from eight consecutive scans, each with 38 sample points
captured in still air, for air turbulence detection, Fig. 16 is a plot of the eight
scans overlaid using the data from Fig. 15, Fig. 17 is a table of values including
processed values, Fig. 18 is a plot showing by interrupted lines the differences in
plots of amplitude vs. time where conditions are unstable in terms of air turbulence,
and Fig. 19 is a plot of variance across multiple sample points for stable air variance,
stable air average variance, unstable air, and unstable air average variance.
[0023] Referring to Figs. 1 to 3 an alarm device 1 comprises a housing having an overall
cylindrical shape with a longitudinal axis, and with a base 2 and vents 3 arranged
circumferentially around a top part of the device. In this specification the term
"top" means the end opposed to the mounting base, even though the device may in practice
be mounted on a ceiling with the "top" facing downwardly.
[0024] The device 1 includes an obstacle detector having an ultrasonic ("US") transceiver
or "transducer" mounted on a circuit board to emit US waves, which are routed by a
guide 4 having pillars 5 supporting a dish-shaped guide element 6. The guide element
6 is mounted centrally around the longitudinal axis of the device. It has curved lower
(first) and upper (second) surfaces 10 and 11 respectively. A generally concave lower
surface 10 faces the device body 2, and a generally convex surface 11 faces away from
the device body 2. The concave surface 10 faces a top generally convex surface 12
of the housing 2 facing the guide element 6.
[0025] The invention is not concerned with the sensing details as they can be of any known
type for detecting heat, smoke, or gas which enters the vents 3. In this case the
active sensor is a smoke detector with an optical chamber.
[0026] As best shown in Figs. 1, 3, and 5 the vents 3 comprise a ring of circumferentially
arranged radial vents 3(a) and, immediately above, a ring of vents 3(b) which are
sloped to have a directional component facing radially and a directional component
facing axially (parallel to the longitudinal axis). There is a rim 9 over the vents
3, in the form of a ring mounted on pillars 13, spaced apart from the vents 3. In
this specification the term axial is intended to mean parallel to the longitudinal
axis, which in Fig. 3 is a central vertical line through the centre of the device
1.
[0027] Referring specifically to Figs. 4 and 5 the device 1 has an ultrasonic transducer
16 driven by a driver circuit 17, and is connected to a receiver amplifier circuit
18, in turn connected to a signal processing circuit 15 for delivery of signals to
a microprocessor (not shown). In this case the microprocessor is a low power, 8-bit
processor.
[0028] As shown in Figs. 5 to 7 the US transducer 16 has a cap 21 housing the active piezo
element and being surrounded by a vibration isolator in the form of a rubber sleeve
23 which protrudes downwardly to provide an internal space 25 for flexible leads 26.
The isolator 23 has a circumferential groove 27 near its lower end, the groove 27
having a width matching the thickness of a mounting PCB 22. The transducer 16 is mounted
in a hole 24 of the PCB 22, with the edge of the hole 24 engaging in the groove 27
of the rubber isolator casing 23.
[0029] As best shown in Fig. 5, the surface of the US transducer 16 is substantially co-planar
with the housing surface 12, and this provides for the US waves to be generated at
or near the external surface of the housing device body 12. The US waves therefore
do not interfere with internal components within the device body, and advantageously
the transmission and reception of US waves have clear paths from the on-axis transducer
location and reflected from the guide element 6.
[0030] A major advantage of placing the transducer 16 surface substantially flush with the
housing surface 12 instead of inside the housing, is that it minimises echo signals
caused by the ultrasound waves reflecting off the internal details of the housing.
These unwanted signals could cause false detections of objects due to the housing
itself and could be randomly increased if turbulent warm air passing through the housing
refracts the ultrasound in such a way that more of the sound energy is reflected back
to the transducer than during static airflow conditions.
[0031] Also, the mounting arrangement minimises vibration from the US transducer to the
PCB 22 by virtue of damping by the sleeve 23. Also, the flexible electrical connector
leads 26 provide electrical connection to a terminal 28 mounted on the lower surface
of the PCB 22. This further ensures that mechanical vibration is not transmitted to
the PCB. This has the effect of reducing signal "ringing" while also reducing stress
on electrical joint components, helping to ensure reliability. "Ringing" is a known
condition of US transducers, where the generated oscillations continue even after
the excitation source has been removed. For detection of near-field objects, ringing
should be reduced such that it does not interact with the returning echo signal of
interest. For a near object to be correctly detected, the ringing is preferably therefore
stopped as soon as possible after excitation.
[0032] The guide element 6, being placed so close to the US source 16 (less than 5 mm) acts
as a secondary ultrasonic source upon ultrasonic waves being incident on the first
surface 10. This aspect is assisted by the guide element 6 having a thickness of only
about 2.8mm, being located just less than 5mm at its closest from the ultrasonic source
16, and being of moulded plastics material (polycarbonate).
[0033] The top surface 11 generally has an overall taper extending proximally towards the
housing 12 and radially inwardly to the longitudinal axis (centrally and downwardly
as viewed in Fig. 3 for example), and as noted above has locally between its rim and
the centre (longitudinal axis) a slight convex shape as viewed in section. The slight
convex shape is not essential, but it is preferred that the element 6 has an overall
dish shape tapered distally from the housing 12 and radially from the longitudinal
axis (upwardly and outwardly as viewed in for example Fig. 3). The lower surface 10
is also tapered generally proximally and radially towards the housing at the longitudinal
axis (downwardly and inwardly as viewed for example in Fig. 3). Locally between the
element rim and the longitudinal axis the surface 10 has a slight concave shape as
viewed in section, but again this is not essential. Again, the overall guide element
configuration is dish-shaped.
[0034] Use of the obstacle detector is now described with reference to Figs. 5 and 8 to
11, which are diagrammatic views illustrating paths of US waves emitted by the transducer
16. Advantageously, the US waves propagate radially from the longitudinal axis of
the device, out from a space between the guide element 6 lower surface 10 and the
housing 2 top surface 12. This wave propagation path includes above and below the
rim 9 and through the vents 3(a) and 3(b). Blockage or taping of either the axial
or radial vents creates a new surface off which the outgoing US wave can strike, resulting
in an echo that is measurable, thereby detecting blockages. This detects for example
taping over the vents (which may be inadvertently left in place by a decorator). Also,
as shown in Figs. 11 (a) and 11(b) such waves will encounter an obstacle Y parallel
with the longitudinal axis, in this case vertical. Upon encountering an obstacle,
the waves are reflected back much more quickly than if there were no obstacle. The
transducer 16 operates with the receiver amplifier circuit 18 to detect such early
reflections in a manner which is well known
per se in the art.
[0035] The "field of view" for the single transducer 16 therefore includes all radial directions
from the device longitudinal axis, allowing detection of obstacles both on the device
itself (such as tape) or nearby.
[0036] Moreover, the guide element 6 also acts as a secondary US wave source due to its
vibration in response to the waves which are incident on its lower surface 10 facing
the transducer 16. This is primarily due to the short distance between the US transducer
16 and the guide element 6 (less than 5mm at its closest, and preferably in the range
of 1.5mm to 4mm) and it is also helped by the fact that the guide element is thin
enough to vibrate. In this case the thickness of the element is about 2.8mm and it
is of plastics (polycarbonate) composition, thereby allowing it to vibrate. This causes
US waves to propagate axially from the device, as shown in Figs. 9 to 11. This allows
further extension of the field of view, allowing the obstacle detector to detect horizontal
obstacles X such as shown in Figs. 10(a) and 10(b). The returning echo signal (Fig.
10(b)) encounters the guide element 6 upon which the vibration is re-transmitted back
to the US transducer 16 for detection. The return signal is further enhanced from
horizontal obstacles X by way of the angled edge of the outer rim 9 and/or angled
vent fins.
[0037] Another advantageous aspect of the device 1 is that by providing vents which face
radially and also vents which at least partially face axially there is less chance
of all of the vents being accidently taped. This is further improved by virtue of
the rim 9, which acts as a spaced-apart barrier to help reduce chances of the vents
being taped over such that air entry is completely blocked or blocked to an extent
that severely hinders the operation of the alarm.
[0038] With the rim 9 effectively preventing convenient access to the radial and axial vents,
tape applied around the circumference of the unit is no longer sufficient to block
some or all of the vents from air entry, since there still remains a viable air entry
path in the axial direction.
[0039] Referring to Figs. 12 to 14 an alternative device 100 has a housing 102 with a guide
104 akin to the guide 4 of Figs. 1 to 13. In this case there are vents 103 which face
both radially and axially. Spacers 130 are arranged to extend generally axially between
the vents 103, thereby helping to prevent taping of the vents 103 while also allowing
propagation of US waves through and over the vents.
[0040] Another advantageous aspect of the device is that the processor is programmed to
detect ambient air changes and turbulence, thereby providing an indication of reliability
of the US obstacle detection measurement. An obstacle detection test may be inaccurate
if the ambient air is unstable, for example by flowing at an excessive flow rate,
and/or rapid changes in air temperature (relative to the steady state temperature
of the unit or object under test), by for example a nearby air conditioning unit.
Such air flow may render the obstacle detection inaccurate because the relevant changes
in air characteristics (temperature, velocity, refractive index) would affect US beam
uniformity, signal intensity and propagation time to and from the obstacle. This makes
accurate US detection of objects, and also calculation of object distance from echo
return time, unreliable.
[0041] In this embodiment the air stability test is integrated with the obstacle detection
test, the processor firstly analysing the US return signal values to initially determine
if the ambient air is sufficiently stable. If sufficiently stable, the processor proceeds
to use the values to determine if there is an obstacle.
[0042] It is also envisaged that the processor may carry out an air stability test as a
discrete test to decide whether to carry out an obstacle detection test. For example,
the processor may be programmed to carry out an obstacle detection routine once per
day, thereby consuming valuable electrical power only for a number of seconds once
per day.
[0043] For each obstacle detection test the processor performs a number of ultrasonic scans
separated by a small (about 1msec) delay. To reduce the effect of signal noise, the
results of the scans are summed and averaged to a single data set upon which obstacle
detection logic is performed. Since averaging requires multiple scans, the system
uses these multiple scans more advantageously to determine the ambient air stability,
thus acting as an indicator of reliability for obstacle detection measurements.
[0044] In each scan, the US transducer is driven with a number of pulses, after which the
processor samples and stores the amplitude of the returning signal as a function of
time. The processor then performs calculations to quantify the extent of amplitude
variation between the same sample points on successive scans. Variance or Standard
Deviation across all scans is then calculated for each data point.
[0045] It is useful to, for example, sum or average the variance, or standard deviation
values to yield a single value for the entire scan. This value is compared to a threshold
to determine if the ambient air properties were stable or unstable during the time
of the scans.
[0046] In stable air conditions the variation from measurement to measurement should be
small across the full scan, yielding a low average variance value. As instability
in the air increases the variance increases. The threshold level is determined empirically,
based on acceptable limits of detection and repeatability of the system.
[0047] If the result of the calculation is less than the threshold, the scan is "Stable"
and so the average scan is accepted as a valid obstacle detection measurement. If
the result is "Unstable", the obstacle detection measurement is not accepted and the
measurement should be repeated. This may be repeated at intervals until a valid ("Stable")
measurement is obtained.
[0048] Fig. 15 is a table of return signal amplitude values for 6 of the 36 sample points
in 8 scans. Each column is a scan and each row contains the return values for a certain
sample point across all of the scans. For example, Row 3 of the Fig. 15 table shows
the return signal amplitude values for the first sample point for all 8 scans.
[0049] Referring to Fig. 16 variations in amplitude of the return signals is shown.
[0050] To determine the level of variability, the processor measures the absolute difference
(or 'delta' in reference to Fig. 15) between a particular sample point return signal
value of one scan and the next, comparing each scan to the previous, for all 8 scans
(in this case). There may be one column for each delta value across the 8 scans, so
therefore 7 columns of delta values, as shown in Fig. 17. The right-hand column of
Fig. 17 has the value for the sum of the deltas for each row (i.e. the sum of the
seven-delta series of values for each sample point).
[0051] Fig. 18 shows examples of amplitude signals for stable and unstable conditions. The
higher and lower amplitudes shown for the unstable condition indicate how the signal
can vary relative to the true signal (stable condition) as a result of air instability,
caused by, for example, an air-conditioning blower. In this case the amplitude variability
occurs rapidly (for example, less than 1 sec), so fast successive scans can therefore
measure these variances. This is in contrast to other environmental instability such
as room temperature changes which generally occur over minutes or hours and would
generally have little effect on the amplitude of the return signal (but may change
the echo return time if compensation for temperature is not included).
[0052] Unwanted amplitude variation of a signal is effectively noise, and an alternative
approach may be to minimize such noise by averaging the signals. However, if the variation
in amplitude is a significant fraction of the true amplitude, then the average signal
may be a poor representation of the actual signal. This is especially true if the
number of scans or samples is limited by hardware, power consumption or memory, such
as with inexpensive 8-bit microcontrollers. Experimentally it was found that averaging
of 16 times was insufficient to remove the noise caused by air instability from air
conditioning unit and could lead to false detections or misinterpretation of the signals.
Hence, the approach of determining and analysing variance is preferred.
[0053] Fig. 19 shows the values of variance represented by arbitrary units. The average
variance for stable air is approximately 2 units and that for unstable air is about
10 units. These units allow the processor to have a threshold setting for average
variance above which it decides that the air is excessively unstable.
[0054] The following is an example of simplified pseudo code for the turbulence testing
performed by the processor.
- Drive transducer with x number of pulses.
- Start sampling via ADC.
- Take 40 samples with time interval of 50µsec (total time of sampling is therefore
2 msec per scan).
- Store samples in array.
- Wait 5 msecs.
- Repeat the above procedure 16 times to yield an array of 16 x 40 samples.
- For each sample point, calculate the absolute difference between each point on successive
scans.
- Sum the 16 absolute difference values to yield a single value for each of the 40 sample
points. The resulting array is 1 x 40.
- This array can be averaged by dividing each value by 16 or used as is.
- The resulting array can be further simplified to a single value by averaging all samples
into one, i.e. sum the 40 sample points and divide by 40.
- This "turbulence" value gives an indication of the average absolute difference across
all sample points.
- This single value is compared to an "instability threshold".
- If "turbulence" value > "instability threshold" raise flag to alert system or user,
abort obstacle detection.
- If "turbulence" value < "instability threshold", proceed to process the previously
stored 16-scan data for purposes of obstacle detection.
[0055] The invention is not limited to the embodiments described but may be varied in construction
and detail. For example, even though the US guide provides a wide field of view for
the single transducer there may be one or more additional transducers. It is not essential
that the top surface or the bottom surface of the guide element be curved as viewed
in a plane from the longitudinal axis towards the edge, and if either is curved the
shape may be convex or concave between the longitudinal axis and the edge.
Summary Statements
[0056]
- 1. An alarm device to detect a condition of ambient air, the alarm device comprising:
a housing (2) having a longitudinal axis and containing a sensor and having vents
(3) for access by ambient air to the sensor,
a signal processing circuit (15) with a processor linked with the sensor,
a power supply for the circuit and the sensor,
an obstacle detector for detecting presence of an unwanted obstacle to flow of ambient
air to said sensor, the obstacle detector comprising:
an ultrasonic transducer (16) mounted in or on the device to have a field of emission
outside of the device,
a processor (18, 19) in the circuit (22) and linked with the ultrasonic transducer
to monitor ultrasonic return values and process them to determine if any have been
reflected by an unwanted obstacle, wherein the processor is configured to generate
an alert to indicate presence of such an obstacle which may affect access of ambient
air to the sensor through the vents, and
a guide (4) mounted to the housing to reflect emitted ultrasonic waves in radial directions
relative to the longitudinal axis.
- 2. An alarm device as in 1, wherein the guide comprises a guide element (6) mounted
to the housing (2) so that it is spaced-apart from the housing, and the guide element
is dish-shaped with a narrower end facing the housing, and the housing has a curved
generally convex surface (12) facing the guide element.
- 3. An alarm device as in 2, wherein the guide element is mounted by a plurality of
pillars (5), is mounted substantially symmetrically to the device longitudinal axis,
and is spaced from the ultrasonic transducer by less than 5mm.
- 4. An alarm device as in 3, wherein the guide element has a surface (10) facing the
housing which has a concave curve from the longitudinal axis towards the edge of the
guide element, and the transducer (16) has a surface which is substantially flush
with the housing surface (12).
- 5. An alarm device as in any preceding, wherein at least some of the vents (3(b))
are arranged so that at least some ultrasonic waves pass through the vents.
- 6. An alarm device as in 5, wherein the vents include vents (3(a)) which are primarily
facing radially and vents (3(b)) which at least have a directional component facing
axially.
- 7. An alarm device as in any preceding, wherein the housing includes a barrier (9)
to render application of tape to the vents difficult.
- 8. An alarm device as in 7, wherein the barrier comprises a barrier element (9) mounted
by pillars (13) so that it is spaced apart from at least some vents, and optionally
the barrier element is annular.
- 9. An alarm device as in 7, wherein the barrier comprises a plurality of elements
(130) which extend from the housing between vents (103).
- 10. An alarm device as in any preceding, wherein the ultrasonic transducer is mounted
in a resilient cover (23) and said cover engages in an aperture (24) of a substrate
(22) which is in turn mounted to the housing.
- 11. An alarm device as in 10, wherein the resilient cover (23) has a groove (27) which
engages a side edge of the substrate aperture, and wherein the ultrasonic transducer
is connected to a conductor (28) on a substrate by a flexible wire link (26).
- 12. An alarm device as in any preceding, wherein the processor (18, 19) is programmed
to detect ambient air instability either before or as part of an obstacle detection
operation, and wherein the processor is configured to record a return signal amplitude
value for each of a plurality of sample points, and to quantify variance across said
values, and if said variance exceeds a threshold determine that there is excessive
ambient air instability.
- 13. An alarm device as in 12, wherein the processor (18, 19) is configured to determine
a series of difference values for each pair of successive values for a sample point,
and to derive a sample point variance value representative of variance for each sample
point, and to compare the derived sample point variance value with a threshold.
- 14. An alarm device as in 13, wherein the derived sample point variance value is a
sum of the difference values for a sample point.
- 15. An alarm device as in any of 12 to 14, wherein the processor (18, 19) is configured
to perform a plurality of scans each with a plurality of sample points, and to determine
a multi-scan derived variance value derived from said sample point variance values,
and wherein a multi-scan derived variance value is an average of all sample point
variance values.
1. An alarm device to detect a condition of ambient air, the alarm device comprising:
a housing (2) having a longitudinal axis and containing a sensor and having vents
(3) for access by ambient air to the sensor,
a signal processing circuit (15) with a processor linked with the sensor,
a power supply for the circuit and the sensor,
an obstacle detector for detecting presence of an unwanted obstacle to flow of ambient
air to said sensor, the obstacle detector comprising:
an ultrasonic transducer (16) mounted in or on the device to have a field of emission
outside of the device,
a processor (18, 19) in the circuit (22) and linked with the ultrasonic transducer
to monitor ultrasonic return values and process them to determine if any have been
reflected by an unwanted obstacle, wherein the processor is configured to generate
an alert to indicate presence of such an obstacle which may affect access of ambient
air to the sensor through the vents, and
a guide (4) mounted to the housing to reflect emitted ultrasonic waves in radial directions
relative to the longitudinal axis, and
wherein the ultrasonic transducer is mounted in a resilient cover (23) and said cover
engages in an aperture (24) of a substrate (22) which is in turn mounted to the housing.
2. An alarm device as claimed in claim 1, wherein the guide comprises a guide element
(6) mounted to the housing (2) so that it is spaced-apart from the housing.
3. An alarm device as claimed in claim 2, wherein the guide element is dish-shaped with
a narrower end facing the housing.
4. An alarm device as claimed in claim 3, wherein the guide element is mounted by a plurality
of pillars (5), and is spaced from the ultrasonic transducer by less than 5mm.
5. An alarm device as claimed in claim 4, wherein the guide element has a surface (10)
facing the housing which has a concave curve from the longitudinal axis towards the
edge of the guide element, and the transducer (16) has a surface which is substantially
flush with the housing surface (12).
6. An alarm device as claimed in any preceding claim, wherein at least some of the vents
(3(b)) are arranged so that at least some ultrasonic waves pass through the vents.
7. An alarm device as claimed in claim 6, wherein the vents include vents (3(a)) which
are primarily facing radially and vents (3(b)) which at least have a directional component
facing axially.
8. An alarm device as claimed in any preceding claim, wherein the housing includes a
barrier (9) to render application of tape to the vents difficult.
9. An alarm device as claimed in claim 8, wherein the barrier comprises a barrier element
(9) mounted by pillars (13) so that it is spaced apart from at least some vents, and
optionally the barrier element is annular.
10. An alarm device as claimed in any preceding claim, wherein the resilient cover (23)
has a groove (27) which engages a side edge of the substrate aperture.
11. An alarm device as claimed in claim 10, wherein the ultrasonic transducer is connected
to a conductor (28) on a substrate by a flexible wire link (26).
12. An alarm device as claimed in any preceding claim, wherein the processor (18, 19)
is programmed to detect ambient air instability either before or as part of an obstacle
detection operation, and wherein the processor is configured to record a return signal
amplitude value for each of a plurality of sample points, and to quantify variance
across said values, and if said variance exceeds a threshold determine that there
is excessive ambient air instability.
13. An alarm device as claimed in any claim 12, wherein the processor (18, 19) is configured
to determine a series of difference values for each pair of successive values for
a sample point, and to derive a sample point variance value representative of variance
for each sample point, and to compare the derived sample point variance value with
a threshold.
14. An alarm device as claimed in claim 13, wherein the derived sample point variance
value is a sum of the difference values for a sample point.
15. An alarm device as claimed in any of claims 12 or 14, wherein the processor (18, 19)
is configured to perform a plurality of scans each with a plurality of sample points,
and to determine a multi-scan derived variance value derived from said sample point
variance values, and wherein a multi-scan derived variance value is an average of
all sample point variance values.