GB2589849A

GB2589849A - Sewer blockage detection

Info

Publication number: GB2589849A
Application number: GB1917602.3A
Authority: GB
Inventors: Gooberman-Hill Stephen; Gaio Michele; Yotov Vergil
Original assignee: Amey Ventures Ltd
Current assignee: Amey Ventures Ltd
Priority date: 2019-12-02
Filing date: 2019-12-02
Publication date: 2021-06-16
Anticipated expiration: 2039-12-02
Also published as: GB201917602D0; GB2589849B

Abstract

A capacitive sensor assembly which comprises a finger-like first sensor unit 320 and 322 comprising one or more capacitive sensors (404 and 402 figure 4) and a second sensor unit substantially identical to the first sensor unit. These are fixed in a spaced apart position on a substrate, parallel to one another and perpendicular to the flow in a sewer 108. The sensors and substrate are made from waterproof material. The sensors may measure the depth of sewage and comprise multiple capacitive sensor pads arranged in a column. The sensors may communicate with a control unit 100 in a blockage detection system and the control unit may send an alarm when the flow of waste rises. A control unit 100 for the detection system is also disclosed which may compute a wavefront speed and a backwave. The control and signal processor may have a low power mode or a timer and the detection unit may be configured to infer an upstream-downstream relationship of the sensor assemblies.

Description

SEWER BLOCKAGE DETECTION

BACKGROUND

[0001] The present technology is concerned with sewers and in particular, narrow-bore sewers. Narrow-bore sewers are designed to carry away fluids from a small number of properties, and in dry weather are not expected to carry a continuous fluid flow, as the intermittent nature of sewage sources (toilet flushes, sinks emptying etc.) is not sufficiently moderated by the number of originating properties to provide a continuous aggregate flow.

[0002] A sewer may become blocked, due to a build-up of sediment on the sewer floor, or the build-up of material on the sewer walls. Alternately sewer blockages may be caused by cracking or other damage to the sewer pipe, or by material such as rag becoming caught and forming an obstruction. A full or partial blockage of a sewer may lead to flooding; raw sewage may escape from the sewer (for example at an inspection chamber), leading to at best a nuisance and at worst a public health hazard.

[0003] The embodiments described below are not limited to implementations which solve any or all of the disadvantages of known sewer blockage detection apparatus.

SUMMARY

[0004] The following presents a simplified summary of the disclosure in order to provide a basic understanding to the reader. This summary is not intended to identify key features or essential features of the claimed subject matter nor is it intended to be used to limit the scope of the claimed subject matter. Its sole purpose is to present a selection of concepts disclosed herein in a simplified form as a prelude to the more detailed description that is presented later.

[0005] A capacitive sensor assembly for use in a narrow bore sewer is described.

The capacitive sensor assembly comprises: a first sensor unit having a finger-like form and comprising one or more capacitive sensors; and a second sensor unit substantially identical to the first sensor unit; and a substrate holding the first and second sensor units such that longitudinal axes of the first and second sensor units are substantially parallel and such that the first and second sensor units are spaced apart on the substrate by a fixed distance; and wherein the first and second sensors units and the substrate are formed from waterproof material suitable for immersion in domestic sewage, such that in use, the substrate holds the first and second sensor units such that the longitudinal axes of the first and second sensor units are substantially perpendicular to a direction of flow of sewage in a narrow bore sewer. Many of the attendant features will be more readily appreciated as the same becomes better understood by reference to the following detailed description considered in connection with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

[0006] The present description will be better understood from the following detailed description read in light of the accompanying drawings, wherein: FIG. I is a schematic diagram of a narrow-bore sewer serving three homes; FIG. 2 is a schematic diagram of a narrow-bore sewer and showing an inspection chamber; FIG. 3 is a schematic diagram of a control unit and also showing a pair of sensor units deployed in an inspection chamber of a narrow-bore sewer; FIG 4 is a schematic diagram of an upper and lower surface of a sensor unit; FIG 4A is a schematic diagram of a pair of sensors; FIG. 5 is a flow diagram of a method of operation at a control unit such as the control unit of FIG. 3 and showing a low power state and a high power state; FIG. 6 is a graph of fluid depth against time as detected by a pair of sensor units deployed in a narrow-bore sewer; FIG. 7 is a flow diagram of more detail of the signal processing operation 504 of FIG. 5; FIG. 8A is a flow diagram of more detail of the wavefront speed computation of operation 706 of FIG. 7; FIG. 8B is a flow diagram of more detail of the detect backwaye operation 710 of FIG. 7; FIG. 9A is a schematic diagram of an adhesive film haying a pair of sensor units integral in the adhesive film; FIG. 9B is a schematic diagram of a narrow-bore sewer having a pair of sensor units integral in the sides of the narrow-bore sewer.

Like reference numerals are used to designate like parts in the accompanying drawings.

DETAILED DESCRIPTION

[0007] The detailed description provided below in connection with the appended drawings is intended as a description of the present examples and is not intended to represent the only forms in which the present example are constructed or utilized. The description sets forth the functions of the example and the sequence of operations for constructing and operating the example. However, the same or equivalent functions and sequences may be accomplished by different examples.

[0008] The inventors have recognized that the detection of blockages within a narrow-bore sewer is made more complex by the dynamics of the fluid in the sewer pipe. Close to the downpipe, the flow can be described as being in a buoyant zone: here solids are carried floating within a deeper and more turbulent fluid flow. As the fluid progresses down the pipe, the fluid depth reduces as the wavefront spreads out, until the fluid is not deep enough to carry the solids. At this point the flow transitions to a sliding zone: solids act as dams, each with a lake of fluid behind it. The lake provides some lubrication which can help the movement of the solid, which slides down the base of the pipe under the pressure of the lake behind it. The sliding zone typically starts 7-10m downstream of the downpipe. The nature of the sliding zone (a series of slowly and intermittently moving solids, each with a small lake behind it) makes it difficult to distinguish expected behaviour in the sliding zone from the presence of a partial blockage (which will also cause a lake to form).

[0009] US 4,546,346 describes a sewer blockage detection device which can be screwed onto a threaded clean out access opening of a sewer pipe. The detection device comprises a pneumatic switch activated by a flexible diaphragm when a waste-water level inside the sewer pipe rises above a threshold level. In the event of a blockage of the sewer pipe, the blocked material exerts pressure on the flexible diaphragm which closes the pneumatic switch and sets off an alarm.

[0010] US 1,164,882 describes a rod of electrically conductive material which extends into a chamber part of a sewer pipe. If sewage rises sufficiently within the chamber, the sewage causes the lower contact ends of the rod to submerge. This closes an electric circuit, since the sewage is conductive, and an alarm is triggered.

[0011] GB 2 423 365 describes a detection unit with a magnetic float and a switch.

The detection unit is in a sewer and the magnetic float is arranged to ride up and down a support member, whereas the switch is located inside the upper end of the support member. The magnetic float rises with the level of the sewage and, when it reaches a threshold, triggers the switch and activates an alarm.

[0012] A disadvantage with each of those known sewer blockage detection systems is that they comprise measurement and control engineering and/or mechanical or electrical parts inside the sewer which may get damaged by the sewage. In narrow bore sewers, the introduction of an apparatus into the sewer pipe comprises a potential obstruction on which rag and other materials can get caught. Additionally, the measurement and control engineering and/or mechanical or electrical parts may get damaged during an unblocking operation when a plumber's snake or other cleaning object gets forced through the sewer pipe.

[0013] Other implementations of sewer blockage detection systems depend on the use of ultrasonic sensors to detect a change in the level of fluid in the pipe. Ultrasonic systems sense the fluid level at a distance, without the need for any part of the device to sit within the sewer pipe. A key disadvantage of such implementations is the active nature of the ultrasonic sensor; it requires significant power to drive an ultrasonic sensor: in typical implementations battery capacity is limited, resulting in either a short battery lifetime or less frequent sampling of the fluid level.

[0014] One typical response to the problem of powering an ultrasonic sensor is to combine an ultrasonic level sensor with a mechanical or electrical level switch. The level switch is preset to engage at an appropriate level, triggering the ultrasonic level sensor to measure the fluid level in the sewer at a preset temporal resolution.

[0015] The inventors have recognized various problems with ultrasonic sensor systems which are combined with mechanical or electrical level switches. Such a mechanism is still reliant on a mechanical or electrical switch in the body of the sewer pipe and so is prone to cause blockages and is prone to damage. Furthermore, the setting of the fluid depth required to engage the level switch is difficult. If it is set too low, it will both create a potential significant blockage in the pipe, at a level which fluid regularly reaches, and will also trigger the ultrasonic level sensor often, compromising battery life. Alternately, if it is set too high, then the ultrasonic level sensor will only trigger once the fluid level has reached a depth at which an alarm should be raised; in this case the ultrasonic level sensor provides little new information, as the level sensor has already triggered an alert or alarm requiring remedial action.

[0016] FIG. 1 is a schematic diagram of a narrow-bore sewer serving three homes (only two homes are visible in FIG. 1 since one home is behind another). Each home 110 has a downpipe connected to a narrow bore sewer 106 that drains into a shared narrow bore sewer pipe 108 under a road connecting the homes. One of the narrow bore sewers 106 is shown with dotted lines to indicate its presence underground and it connects to a home 110 which is not visible since it is behind another home 110. Waste water from washing machines, baths, toilets and other appliances 112 in each home passes into the narrow bore sewer 106 and flows by gravity into the shared sewer 108 under the road.

[0017] Manhole covers 104 are located at intervals along the shared narrow bore sewer 108 (such as every 40 to 50 metres). Each manhole cover 104 seals the top of an inspection chamber which allows inspection of the shared narrow bore sewer 108 as described in more detail with reference to FIG. 2 below. Located near to each manhole cover 104 is a control unit 100 which is in communication with an operations centre 102 via any suitable communications network. Each control unit 100 is a small robust device which is powered by batteries or another power source at the control unit 100. In an example, which does not limit the scope of the technology, the control unit is 8cm by 4cm by 3cm. In the example of FIG. 1 a manhole cover 104 is indicated by a dotted line over narrow bore sewer 108 downstream of a join between a narrow bore sewer 106 from a home 110. On the underside of the manhole cover is a control unit 100 indicated as a rectangle in FIG. 1. More detail about the control units 100 is given later in this document. The control unit 100 receives data from a pair of sensors in the shared narrow bore sewer 108 and sends the data, and/or analysis of the data, to the operations centre 102. Although only one control unit 100 and manhole cover 104 is shown in FIG. 1 there are many more in practice. Each control unit 100 sends data to the operations centre 102. The operations centre knows the topology of the narrow bore sewer, the pairs of sensors, and the control units.

[0018] The operations centre 102 is remote of the control unit 100. The control unit 100 is able to trigger an alarm at the operations centre 102 when a blockage is detected or predicted in the shared narrow bore sewer 108. When an alarm is triggered an engineer is able to investigate the problem or predicted problem and carry out appropriate maintenance of the narrow bore sewer.

[0019] FIG. 2 is a schematic diagram of a narrow-bore sewer 106 draining from a home 110 into a shared narrow bore sewer 108 and showing an inspection chamber 204 in the shared narrow bore sewer 108. The shared narrow bore sewer is under a road as illustrated and the inspection chamber travels from a manhole cover 200 in the road, down through the ground, to the shared narrow bore sewer.

[0020] A pair of sensors 206 are in the part of the narrow bore sewer which is part of the inspection chamber. The sensors are powered by a power source in the inspection chamber which is a small battery or other power source (not shown in FIG. 2). The sensors 206 are in communication with a control unit 100 which is mounted inside the inspection chamber, either on the underside of the manhole cover 200 or on the inspection chamber wall directly underneath the manhole cover 200. The control unit 100 is in communication with an operations centre 102 as described with reference to FIG. I. [0021] FIG. 3 is a schematic diagram of a control unit 100 and also showing a pair of sensors 206 deployed in an inspection chamber 204 of a shared narrow-bore sewer 108. Only part of the inspection chamber 204 is visible in FIG. 3 as the upper part of the inspection chamber and the manhole cover are outside the view point. In FIG. 3 there is no sewage in the narrow bore sewer so that the pair of sensors 206 is fully visible.

[0022] Each sensor 320, 322 comprises a laminate, flexible structure with a similar shape to a ruler. The sensors 320, 322 are substantially identical such that relative differences between data sensed by the two sensors may be used to compute wavefront speeds and to detect back waves as explained in more detail below. In the example of FIG. 3 a lower face of the sensor is in contact with a wall of the sewer and each sensor is positioned so that its longitudinal axis runs generally from the base of the sewer pipe towards the top of the sewer pipe in order to measure a depth of sewage in the pipe. Each sensor is connected to a power source 300, such as a battery, local to the sensor. In the example of FIG. 3 each sensor is adhered to the wall of the sewer using a suitable adhesive which is resistant to sewage. Each sensor is a capacitive sensor which senses depth of sewage in the narrow bore sewer 108 by virtue of a change in capacitance at the sensor caused by the presence of sewage.

[0023] The sensors comprise a wireless transceiver which is able to send sensor readings from the sensors using wireless communications to a control unit 100. The sensors are also able to receive signals from the control unit.

[0024] The control unit 100 comprises a communications module 302, a processor 312, a signal processor 304, a memory 306, an interrupt driver 308 and a power unit 310. The control unit provides one or more power outputs for powering sensors. Some or all of these elements 302 -312 may be combined in some examples. The communications module 302 is a wireless transceiver which enables communication between the control unit 100 and the operations centre 102. The communications module 302 also enables communication between the control unit 100 and the pair of sensors 206.

[0025] The signal processor 304 processes signals received from the sensors 206 as described in detail with reference to FIGs. 7, 8A, 8B, 8C below. The signal processor 304 is implemented using any suitable combination of one or more of: software, hardware, firmware. Where the signal processor is implemented in hardware an FPGA, ASIC or other suitable hardware is used. The memory 306 is any suitable type of memory such as RAM, or other computer memory.

[0026] The interrupt driver 308 controls which one of two possible modes the control unit and sensors 206 are in at any one time. The two possible modes are a low power state and a high power state.

[0027] FIG. 4 is a schematic diagram of one of the sensors of FIG. 3 and shows an upper face 320 and a lower face 400 of the sensor. The upper face of the sensor has an optional scale marked in a similar manner to a ruler. The lower face 400 of the sensors has a plurality of parallelogram shaped sensor areas 404 arranged in a column and where the column is flanked on each of its longitudinal sides by a continuous sensor 402 strip running substantially along the length of the sensor. As a depth of sewage travels up the sensor from an end which is lowest in the sewage pipe (see arrow 400) it is always in contact with at least two of the parallelogram shaped sensor areas 404. A relative capacitance between adjacent parallelogram shaped sensor areas 404 is used to calculate a depth of the sewage in the pipe. A capacitance at the continuous sensor strips 402 is used as a ground reference.

[0028] Note that the sensor of FIG. 4 is one example only and other configurations of capacitive sensor are used in some cases. In particular, the parallelogram shaped sensors areas 404 are not essential although are found to give good working results in practice.

[0029] FIG. 4A is a schematic diagram of a pair of sensors such as the pair of sensors in FIG. 3. The pair of sensors comprises a first sensor, to be deployed upstream and depicted in FIG. 4A as the column under the rectangle labelled A Unit. The pair of sensors comprises a second sensor, to be deployed downstream of the first sensor and depicted in FIG. 4A as the column under the rectangle labelled B Unit. The direction of flow of sewage in the narrow bore sewer under gravity is indicated by an arrow in FIG. 4A. Each sensor comprises an array of capacitive sensor pads and in FIG. 4A these are labelled Al to A4 in the first sensor and labelled BI to B4 in the second sensor. Each sensor has a ground reference pad depicted in FIG. 4A as a rectangle running the length of the vertical array of capacitive sensor pads.

[0030] FIG. 4A shows a series of identical parallelogram shaped capacitive sensors arranged in a vertical pattern with a graded overlap. The combination of multiple capacitive sensors with graded overlaps acts as an interpolation and error reduction mechanism enabling a high-resolution measurement of the fluid depth within the pipe. A preferred implementation mounts these sensors in an adhesive-backed Film, which can easily be stuck onto the inside wall of a sewer pipe within an inspection chamber.

[0031] Moreover, the implementation described in FIG 4A has an additional advantage as each individual sensor is connected to a corresponding input pin on the measuring data logger, an appropriate software implementation triggers the control unit to wake from a very low-power deep-sleep state when the fluid level in the pipe rises or falls sufficiently to energise or de-energise a single pin on the control unit. This reduces the power consumption of the system by enabling it only to wake when the fluid level in the pipe changes.

[0032] FIG. 5 is a flow diagram of a method of operation at a control unit 100 such as the control unit 100 of FIG. 3 and showing a low power state 500 and a high power state 514. The control unit 100 is in either the low power state 500 or the high power state 514 at any one time and is unable to be in both states at the same time. The interrupt driver 308 controls transitioning between the states and determines which state the control unit 100 is in at any one time.

[0033] During the low power state 500 the control unit 100 draws minimal power from power unit 310. During the lower power state 500 there is enough power to enable the interrupt driver 308 to monitor for signals from the sensors 320, 322 but otherwise the power drawn is minimal since in the low power state 500 the communication component 302 and the signal processor 304 do not operate. In the low power state 500 there is enough power to keep the contents of the memory 306 of the control unit. In contrast, in the high power state 514, the control unit is fully powered and the communications component 302 and signal processor 304 operate.

[0034] There are 2 different measurement mechanisms, one in the high power state (514) and one in the low power state (500) [0035] In the high power state 514 the signal processor measures the depth of sewage in the pipe as described above by comparing the relative capacitance of adjacent sensor areas 404. In the high power state 514 an interrupt is triggered at the interrupt driver 308 if a voltage on a sensor pad 404 changes sufficiently (effectively crossing a trigger threshold from "off' to "on' or vice versa).

[0036] The latter mechanism, using the interrupt driver 308 is also usable in the low-power state 500, to wake the control-unit 100 up and move it into the high power state 514 when a sufficiently large change (either increasing or decreasing depth) occurs in the fluid depth.

[0037] When the control unit 100 enters the low power state, interrupts are set based on the current depth of sewage. In the example of FIG. 4A the sensor areas 404 are labelled Al, A2, A3,... on sensor 320 (upstream and B1,B2, B3, ... on sensor 322 (downstream). If the fluid level in the pipe 102 is sufficient to put sensors Al and A2 into the "on' state, but not sensor A3, then the interrupt driver sets interrupts for A2 entering the "off' state and A3 entering the "on" state. When the interrupts are triggered a significant change in the fluid depth in the pipe is detected during the low power state.

[0038] Suppose the control unit 100 is in a low power state 500. The interrupt driver 308 checks (at decision diamond 502) whether to wake the control unit from the low power mode. The check comprises monitoring for one or more of the following criteria: a voltage change on an input pin of the control unit 100, a hardware timer expiry of a timer in the interrupt driver 308, a change in capacitance which is greater than a threshold amount detected at one or both of the sensors 320, 322. If the check is successful, that is, one of the criteria are detected, then the interrupt driver transitions the control unit 100 from the low power state 500 to the high power state 514. If the check is unsuccessful, that is, none of the criteria are detected, then the control unit 100 remains in the low power state 500 and the interrupt driver continues to monitor for the criteria.

[0039] In the high power state 514 the signal processing unit 504 operates to process signals received from the sensors 320, 322. The signal processing unit 504 carries out one or more types of signal processing such as noise removal, compression, aggregation, pattern recognition, statistic computation. More detail about particular types of signal processing done by the signal processing unit 504 is described with reference to FIG. 7 below.

[0040] The signal processor monitors for an alarm situation as indicated by decision diamond 506. If no alarm situation is detected then the processor 312 checks 512 if there has been a sufficient interval of time since a last data upload from the control unit 100 to the operations centre 302. The check if there has been a sufficient interval of time is done by inspecting a timer and comparing the timer contents with a stored threshold time interval. If there has not been a sufficient time interval then the processor 312 transitions the control unit back to the low power state 500. If there has been a sufficient time interval then the interrupt driver triggers the signal processor to send 510 data to the operations center 510 using communications unit 302. In some cases the data is sent in encrypted form using conventional encryption technology. The data which is sent comprises sensor data which has been processed by the signal processor and comprises one or more of: denoised sensor data, smoothed sensor data, aggregated sensor data, statistics computed from the sensor data, sensor data patterns or other information derived from the sensor data. Once the data has been sent at operation 510 the interrupt driver triggers transition of the control unit 100 from the high power state 514 to the low power state 500.

[0041] As mentioned above, the signal processor monitors for an alarm situation as indicated by decision diamond 506. If an alarm situation is detected the signal processor triggers the communications unit 302 to send 508 an alarm to the operations centre. An alarm situation is detected by detecting one or more of: a depth of sewage above a threshold value, a wavefront speed above a threshold value, a backwave. A backwave is a body of the sewage fluid in the narrow bore sewer which flows in a direction opposite to the gravity defined downstream direction in the narrow bore sewer.

[0042] After the alarm has been sent 508 to the operations centre the interrupt driver triggers the communications unit 302 to send data 510 to the operations centre as described above. Then the interrupt driver triggers transition of the control unit 100 from the high power state 514 to the low power state 500.

[0043] FIG. 6 is a graph of empirical data obtained from a pair of sensors 320, 322 deployed in a narrow bore sewer in the arrangement of FIG. 3. The y axis of the graph represents depth of sewage in the narrow bore sewer in millimetres. The x axis of the graph represents time in the 24 hour clock notation. The line 600 plots data sensed by a sensor which is upstream as compared with the other sensor of the sensor pair. The line 602 plots data sensed by a sensor which is downstream as compared with the other sensor of the sensor pair.

[0044] The regular spikes in fluid depth in the upstream sensor plot 600 are the flows in the narrow bore sewer from a toilet flush. A blockage is introduced into the system at 12:39:00. As a result of the blockage there is a lack of flow detected by the downstream sensor (see the plot 602) while flow spikes in the upstream plot are still observed. Additionally, a lake is detected forming in an upstream section of the sewer, evidenced by the underlying base fluid level rise of approximately 20mm between 12:40:00 and 12:58:00. After 13:00:00 the toilet is not flushed again, and the lake behind the blockage slowly drains. Between 12:55:00 and 13:05:00 some flow in the downstream sewer is observed corresponding to falls in the upstream lake level -this is due to water pressure partially clearing the blockage, resulting in flows being observed downstream.

[0045] The empirical data in FIG. 6 demonstrates the accuracy of the sensor pair and the ability to detect the blockage at time 12:39:00 by detecting: a lack of flow detected by the downstream sensor while flow spikes are detected by the upstream sensor. The data in FIG. 6 also shows how the control unit and sensor pair are used to detect the presence of a blockage by detecting the an underlying base fluid level rise; and are used to detect partial clearing of a blockage by detecting the combination of a drop in upstream base fluid level and a flow in the downstream sewer.

[0046] FIG. 7 is a flow diagram of more detail of the signal processing operation 504 of FIG. 5. In FIG. 7 an upward pointing triangle shape (Fork) is used to denote splitting the control flow of the signal processor into two or more parallel streams. In FIG. 7 a downward pointing triangle shape (Join) denotes pulling together parallel execution flows of the signal processor.

[0047] When the control unit 100 is woken from a low power mode and is receiving data from sensor A the signal processor carries out the method of FIG. 7. Sensor A is the one of the pair of sensors which is deployed upstream of the other sensor. Fork 702 is reached whereby the signal processor carries out operations 704 and 706 in parallel. Operation 704 comprises calculating a depth (referred to as a flow level) of the sewage in the sewer at the location of the upstream sensor. To calculate the depth the signal processor determines how many of the sensor pads of the upstream sensor are in contact with sewage by examining the capacitance differences between individual ones of the sensors pads and the ground reference sensor strips in the upstream sensor. In addition the signal processor uses information about the relative capacitance of each sensor pad compared to the pads above and below, which gives a measurement of the percentage fill on that pad. Operation 706 comprises calculating a wavefront speed of the sewage in the sewer at the location of the upstream sensor. More detail about how the wavefront speed is computed is given with reference to FIG. 8A below. Note that it is not essential to compute operations 704 and 706 in parallel. Operations 704 and 706 are in series in some cases which are workable but have lower accuracy that the parallel arrangement of FIG. 7.

[0048] Once the wavefront speed has been computed at operation 706 fork 708 is reached whereby the signal processor carries out operations "detect backwave" 710 and join 712 in parallel.

[0049] Operation join 712 comprises waiting until operations 704, 706, 708 and 710 have completed before forming a unified flow of control which proceeds to operation 506 of FIG. 5.

[0050] FIG. 8A has more detail about how the signal processor computes wavefront speed at operation 706 of FIG. 7. The wavefront speed calculation is triggered if the wake-up from the low-power state was triggered by an increasing fluid depth (see decision diamond 798 in FIG. 8A). It is not triggered if the fluid depth is decreasing, or if the system has been woken by a timer time-out.

[0051] The signal processor starts 800 a timer when the control unit 100 is woken from a low power state. Consider a corresponding pair of individual sensor pads. A corresponding pair of individual sensor pads is an identically numbered pair as depicted in FIG. 4A such as the pair Al B1 or the pair A3 B3. The following description refers to pair Al B1 for example only and any corresponding pair of sensor pads is used in other examples. The signal processor starts 800 a timer when the control unit 100 is woken from a low power state and is receiving data from sensor pad Al (the upstream sensor pad) above a threshold value. The timer 800 thus starts when a wavefront begins to move from the region of sensor pad M towards sensor pad Bl. The signal processor also sets 802 an interrupt on sensor pad Bl. Sensor pad B1 is the downstream sensor of the sensor pad pair. The signal processor checks at decision diamond 804 whether the timer has timed out. If the timer has timed out then the signal processor proceeds to operation 712 of FIG. 7.

[0052] If the timer has not expired then the signal processor checks at decision diamond 806 whether the interrupt on sensor pad B1 has been triggered. Interrupt on sensor pad B 1 is triggered if sensor pad B1 (the downstream sensor) detects the presence of sewage above a threshold depth. If not then the signal processor returns to operation 804. If the interrupt on sensor pad B1 has been triggered then the signal processor stops the timer at operation 808 and computes a speed of a wavefront at operation 810. The speed of the wavefront is given by the distance between sensor pad Al and sensor pad Bt (which is known from the manufacturing phase) divided by the time taken for the wavefront to reach sensor pad B t from sensor pad Al (which is the time indicated on the timer).

[0053] FIG. 8B has more detail about how the signal processor detects the occurrence of a backwave at operation 710 of FIG. 7. The signal processor has already computed the wavefront speed and it now starts a timer at operation 820. The timer begins when a wavefront has passed the upstream and downstream sensors. The signal processor sets an interrupt on sensor pad Al (the upstream sensor) at operation 822. The signal processor checks at decision diamond 824 whether the timer has timed out. If so, the signal processor moves to operation 712 of FIG. 7.

[0054] If the timer has not timed out the signal processor checks 826 if the interrupt on sensor pad A] has been triggered by the presence of sewage above a threshold depth at sensor pad Al. If not the process moves back to decision diamond 824. If the interrupt has been triggered at sensor pad Al, by the presence of sewage above a threshold depth at sensor pad At, then the timer is stopped. The timer thus records the time elapsed between a wavefront travelling from the downstream sensor pad B I to the upstream sensor pad Al. The signal processor computes the speed of the backwave by dividing the known distance between sensor pad Al and sensor pad BI by the time recorded by the timer. The signal processor computes the height of the backwave by sensing the depth of the sewage at sensor pad B1 at the time the timer is stopped at operation 828. Once the height and speed of the backwave have been computed at operation 830 the process moves to operation 712 of FIG. 7.

[0055] FIG. 9A is a schematic diagram of a flexible film 900 in which a pair of capacitive sensors are embedded or mounted. The capacitive sensors 320, 322 are as described with reference to FIG. 4 or are of another configuration. The capacitive sensors 302, 322 are separated by a fixed distance 902 which is known to the control unit 100 and/or operations centre 102. The flexible film 900 incorporating the capacitive sensors is suitable for adhering to the inner surface of a narrow bore sewer. As a result the capacitive sensors are deployed in a narrow bore sewer in a low cost and effective manner. Since the capacitive sensors and film are laminate and adhere to an inner surface of the sewer, there is a low risk of debris and rag collecting on the capacitive sensors. There is also a low risk of damage to the sensors from sewage flow.

[0056] FIG. 9B is a schematic diagram of a section of narrow bore sewer with integral capacitive sensors 320, 322. The narrow bore sewer is made of plastic and the capacitive sensors 320, 322 are integrated into the wall of the narrow bore sewer during manufacturing. An injection molding or plastics extrusion process is used to manufacture the sewer pipe with the integrated capacitive sensors 320, 322. When the sewer pipe is deployed there is no risk of the capacitive sensors causing a blockage since the capacitive sensors are part of the wall of the sewer pipe itself [0057] In various examples a capacitive sensor system is installed within an inspection chamber (a preferred implementation system consists of multiple sensors mounted in a single film); timing the detection of a fluid level rise energising the same sensor on both pads enables the speed and direction of the wavefront to be calculated.

[0058] Narrow-bore sewers are expected to carry an intermittent fluid flow. A change in the frequency or depth profile of the flow indicates the presence of a blockage upstream. If the sewer pipe upstream is completely blocked, then there will be no intermittent flows downstream. A partial upstream blockage will act as a choke point in the sewage flow, spreading the intermittent flow, reducing the depth of the flow and spreading it through time. This is detected by a capacitive sensor downstream as a change in the depth profile. Similarly, changes in the composition of the fluid (for example an increase in viscosity) are detected in the depth profile of the intermittent flow. In the remainder of this section, a method of detecting a blockage located upstream of the sensor system is referred to as an upstream blockage detector, and a method for detecting a blockage located downstream of the sensor system as a downstream blockage detector.

[0059] The advantage of the upstream blockage detection method is that it that it has no dependence on the lake that forms upstream of a blockage.

[0060] A downstream blockage detector not only detects the presence of a lake, but is also used to infer the nature and position of the blockage. Tree roots penetrating the sewer wall typically cause a partial or complete blockage when rag and solids gets caught on the roots. This type of blockage is soft in nature and adsorbs and dampens the wavefront. By contrast a hard blockage, such as that caused by a sewer crown collapse, will reflect the wavefront. A bacicwave is detected from upstream when the backwave passes the sensor, travelling back up the pipe. The approximate position of the blockage is detected from the wave velocity and the timing between the initial wavefront and the reflected wave.

[0061] A single sensor system is unsuitable as a downstream blockage detector; there is no means of distinguishing between a lack of flow caused by an upstream blockage, and a lack of flow because an upstream property is not in use (for example if the residents are away on holiday).

[0062] Therefore a sensor network comprising of multiple sensor systems in a sewer pipe or network of sewer pipes gives benefits; the sensor systems can be located at significant distances from each other in adjacent inspection chambers (typically placed at up to 40m intervals). If a blockage occurs too far down the pipe for an upstream lake to be detected, its presence may still be inferred by comparing the flow at adjacent sensors.

[0063] In an example the control unit sets an interrupt timer to bring itself out of a deep-sleep state. This enables the control unit to wake periodically if no change in fluid level in the sewer pipe has been detected for a sufficiently long time.

[0064] In an example the control unit sends out a signal to the operations centre to report the lack of activity in the sewer pipe on waking from the interrupt triggered by the timer.

[0065] In an example, the control unit can be interrupted, and brought out of a deep-sleep state into a normal state if a capacitance change occurs on a sensor pad in the A sensor unit, corresponding to an un-energised pad becoming energised, or a unenergised pad becoming un-energised. This corresponds to the fluid level in the sewer pipe rising or falling sufficiently to change the energisation of a pad.

[0066] In an example the control unit sets an interrupt timer on waking. This enables the control unit to return to a deep sleep state if no change in fluid level in the sewer pipe has been detected for a sufficiently long time.

[0067] In an example the control unit returns to deep-sleep mode when interrupted by timer expiring. Before entering deep-sleep mode, all interrupts described hereafter are cancelled, and the interrupt timer is set.

[0068] In an example the control unit calculates the fluid depth in the sewer pipe on waking [0069] In an example the depth of fluid in the sewer pipe is continuously sampled at an appropriate resolution, and the trace of the fluid depth is sent to a server at an operations centre.

[0070] In an example, the control unit starts a timer on being woken from deep sleep in the case where the fluid level in the sewer pipe has risen, and sets an interrupt to be triggered when a sensor pad on a downstream sensor unit corresponding to the sensor pad on an upstream sensor unit that caused the wake-up is triggered.

[0071] In an example the timer is stopped on the interrupt being triggered.

[0072] In an example the speed of the wavefront of the fluid in the sewer pipe is calculated based on the time elapsed between corresponding A and B sensors units being energised, as recorded by timer, and the distance between sensor units A and B in the normal direction of the flow in the sewer pipe [0073] In an example the speed of the wavefront is sent to the operations centre by the control unit.

[0074] In an example statistics of the fluid depth and wavefront speed are held by the control unit and reported up to the server at suitable intervals [0075] In an example an interrupt is set to be triggered by the de-energisation of a pad in the B sensor unit is detected, corresponding to a fall in the level of fluid in the sewer pipe.

[0076] In an example, on the triggering of interrupt, an interrupt is set to be triggered by the re-energisation of the same pad whose de-energisation triggered interrupt.

[0077] In an example the control unit starts a timer on interrupt being triggered, and sets an interrupt to be triggered when the pad on the A unit corresponding to the one that caused the interrupt to be triggered. This corresponds to the detection of a back-wave travelling up the pipe in the direction opposite to that of normal flow. A back-wave only occurs when there is a hard blockage in the pipe.

[0078] In an example the speed of the back-wave in the fluid in the sewer pipe is calculated based on the time elapsed between corresponding B and A sensors units being energised, as recorded by timer, and the distance between sensor units B and A against the normal direction of the flow in the sewer pipe [0079] In an example the control unit signals the server to alert it to the presence and speed of a back-wave in the pipe when intermpt is triggered.

[0080] Alternatively or in addition to the other examples described herein, examples include any combination of the following: [0081] A capacitive sensor assembly for use in a sewer, the capacitive sensor assembly comprising: [0082] a first sensor unit having a finger-like form and comprising one or more capacitive sensors; and [0083] a second sensor unit substantially identical to the first sensor unit; and [0084] a substrate holding the first and second sensor units such that longitudinal axes of the first and second sensor units are substantially parallel and such that the first and second sensor units are spaced apart on the substrate by a fixed distance; and wherein the first and second sensors units and the substrate are formed from waterproof material suitable for immersion in domestic sewage, such that in use, the substrate holds the first and second sensor units such that the longitudinal axes of the first and second sensor units are substantially perpendicular to a direction of flow of sewage in a sewer.

[0085] In examples the substrate is a wall of a sewer and the first and second sensor units are integral in the wall.

[0086] In examples the substrate is a plastic film.

[0087] In examples the substrate has adhesive on one face suitable for adhering the substrate onto the inner surface of a sewer.

[0088] In examples the first sensor unit and the second sensor unit are configured to measure a depth of sewage in the sewer.

[0089] In examples the first sensor unit and the second sensor unit each comprise a plurality of capacitive sensor pads arranged in a column and a capacitive ground reference sensor running the length of the column.

[0090] Tn examples there is communication means for sending information from each of the first and second sensor units to a control unit.

[0091] A sewer blockage detection system comprising: a capacitive sensor assembly as described above and a control unit, the control unit configured to operate in either a low power mode or a high power mode.

[0092] The sewer blockage detection system mentioned above wherein the control unit comprises an interrupt driver to control transition between the lower power mode and the high power mode on the basis of one or more of a voltage change on an input pin of the control unit, a timer expiry of a timer in the interrupt driver, a change in capacitance which is greater than a threshold amount detected at one or both of the sensor units [0093] In examples the control unit comprises a signal processor arranged to compute a depth of water in the sewer based on capacitance measured in individual ones of a plurality of capacitive sensor pads in at least one of the sensor units.

[0094] In examples the control unit is configured to send an alarm to an operations centre when flow of waste water in the sewer detected by the sensor units significantly varies from a historical pattern detected by the sensor units.

[0095] In examples the control unit is configured to send an alarm to an operations centre when flow of waste water in the sewer is detected above a threshold depth.

[0096] In examples the control unit is configured to send a trace of waste water depth in the sewer over time to an operations centre.

[0097] A control unit for use in a sewer blockage detection system, the control unit comprising: [0098] a communications unit for receiving data from a first sensor unit and a second sensor unit of a capacitive sensor assembly as described above; [0099] a signal processor configured to compute each of: a depth of waste water in the sewer from signals received from the capacitive sensor assembly, and a wavefront speed of waste water in the sewer, where the wavefront is a leading edge of a body of waste water travelling between the sensor units.

[00100] In examples the signal processor is configured to compute the depth of waste water and the wavefront speed in parallel.

[00101] In examples the signal processor is configured to detect a backwave, where a backwave is a leading edge of a body of waste water travelling in a direction against a gravity determined direction of flow of waste water in the sewer.

[00102] In examples the signal processor is configured to detect the backwave after it has computed the wavefront speed.

[00103] In examples the signal processor is configured to compute the wavefront speed by starting a timer on being woken from a low power mode as a result of the first one of the sensor units detecting waste water over a threshold depth, setting an interrupt to be triggered when the second one of the sensor units detects waste water over the threshold depth, stopping the timer when the interrupt is triggered, and computing the speed using a value of the timer and information about a distance between the first and second sensor units.

[00104] In examples the signal processor is configured to compute a speed of a backwave, where a backwave is a leading edge of a body of waste water travelling in a direction against a gravity determined direction of flow of waste water in the sewer, by starting a timer on being woken from a low power mode as a result of the first one of the sensor units detecting waste water over a threshold depth, setting an interrupt to be triggered when the first one of the sensor units detects waste water over a second threshold depth, when the interrupt is triggered, stopping the timer and computing a speed using a value of the timer and information about a distance between the first and second sensor units.

[00105] In examples the signal processor is configured to compute a depth of the backwave from data received from the first capacitive sensor when the timer is stopped.

[00106] In examples the signal processor is configured to implement a timeout on the timer and, if the interrupt is not triggered before the timer timeout occurs, to quit computing speed of a backwave.

[00107] The sewer blockage detection system, in some examples, has an operations center server in communication with a plurality of control units, each control unit associated with a capacitive sensor assembly, the server configured to infer a upstream-downstream relationship of the capacitive sensor assemblies from data received from the control units [00108] The sewer blockage detection system mentioned above whereby the server compares statistics of flows received from pairs of control units where one of the control units is downstream with respect to the other control unit of the pair, and raises an alert to an operator if the statistics of flows reported by the downstream control unit has changed significantly when the pattern of flows reported by the upstream unit has not changed significantly.

[00109] The sewer blockage detection system mentioned above whereby the control units are in a sewer network having a tree structure where sewer pipes join and their flows are aggregated together as they run downstream, and wherein the server aggregates signals from a plurality of control units upstream of a single downstream control unit and compares the aggregation result and the data from the single downstream control unit.

[00110] The sewer blockage detection system of mentioned above where the server is configured such that if the fluid level in the sewer does not return to the level before a wavefront, or returns to the original level much slower than previously, then an alert is raised to indicate the presence of a blockage downstream of the sensor.

[00111] The methods described herein are performed, in some examples, by software in machine readable form on a tangible storage medium e.g. in the form of a computer program comprising computer program code means adapted to perform all the operations of one or more of the methods described herein when the program is run on a computer and where the computer program may be embodied on a computer readable medium. The software is suitable for execution on a parallel processor or a serial processor such that the method operations may be carried out in any suitable order, or simultaneously.

[00112] Any range or device value given herein may be extended or altered without losing the effect sought, as will be apparent to the skilled person.

[0011 3] Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.

[00114] It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. It will further be understood that reference to 'an' item refers to one or more of those items.

[00115] The operations of the methods described herein may be carried out in any suitable order, or simultaneously where appropriate. Additionally, individual blocks may be deleted from any of the methods without departing from the scope of the subject matter described herein. Aspects of any of the examples described above may be combined with aspects of any of the other examples described to form further examples without losing the effect sought.

[00116] The term 'comprising' is used herein to mean including the method blocks or elements identified, but that such blocks or elements do not comprise an exclusive list and a method or apparatus may contain additional blocks or elements.

[00117] It will be understood that the above description is given by way of example only and that various modifications may be made by those skilled in the art. The above specification, examples and data provide a complete description of the structure and use of exemplary embodiments. Although various embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the scope of this specification.

Claims

CLAIMSI. A capacitive sensor assembly for use in a sewer, the capacitive sensor assembly comprising: a first sensor unit having a finger-like form and comprising one or more capacitive sensors; and a second sensor unit substantially identical to the first sensor unit; and a substrate holding the first and second sensor units such that longitudinal axes of the first and second sensor units are substantially parallel and such that the first and second sensor units are spaced apart on the substrate by a fixed distance; and wherein the first and second sensors units and the substrate are formed from waterproof material suitable for immersion in domestic sewage, such that in use, the substrate holds the first and second sensor units such that the longitudinal axes of the first and second sensor units are substantially perpendicular to a direction of flow of sewage in a sewer.
2 The capacitive sensor assembly of claim 1 wherein the substrate is a wall of a sewer and the first and second sensor units are integral in the wall.
3. The capacitive sensor assembly of claim 1 wherein the substrate is a plastic film.
4. The capacitive sensor assembly of claim 3 wherein the substrate has adhesive on one face suitable for adhering the substrate onto the inner surface of a sewer.
5. The capacitive sensor assembly of any preceding claim wherein the first sensor unit and the second sensor unit are configured to measure a depth of sewage in the sewer.
6. The capacitive sensor assembly of any preceding claim wherein the first sensor unit and the second sensor unit each comprise a plurality of capacitive sensor pads arranged in a column and a capacitive ground reference sensor running the length of the column.
7. The capacitive sensor assembly of claim 1 comprising communication means for sending information from each of the first and second sensor units to a control unit.
8. A sewer blockage detection system comprising: a capacitive sensor assembly as claimed in any of claims I to 7 and a control unit, the control unit configured to operate in either a low power mode or a high power mode.
9. The sewer blockage detection system of claim 8 wherein the control unit comprises an interrupt driver to control transition between the lower power mode and the high power mode on the basis of one or more of a voltage change on an input pin of the control unit 100, a timer expiry of a timer in the interrupt driver, a change in capacitance which is greater than a threshold amount detected at one or both of the sensor units.
10. The sewer blockage detection system of claim 8 or claim 9 wherein the control unit comprises a signal processor arranged to compute a depth of water in the sewer based on capacitance measured in individual ones of a plurality of capacitive sensor pads in at least one of the sensor units.
11. The sewer blockage detection system of claim 8 or claim 9 wherein the control unit is configured to send an alarm to an operations centre when flow of waste water in the sewer detected by the sensor units significantly varies from a historical pattern detected by the sensor units.
12. The sewer blockage detection system of claim 8 or claim 9 wherein the control unit is configured to send an alarm to an operations centre when flow of waste water in the sewer is detected above a threshold depth.
13 The sewer blockage detection system of claim 8 or claim 9 wherein the control unit is configured to send a trace of waste water depth in the sewer over time to an operations centre.
14. A control unit for use in a sewer blockage detection system, the control unit comprising: a communications unit for receiving data from a first sensor unit and a second sensor unit of a capacitive sensor assembly as claimed in any of claims 1 to 7; a signal processor configured to compute each of a depth of waste water in the sewer from signals received from the capacitive sensor assembly, and a wavefront speed of waste water in the sewer, where the wavefront is a leading edge of a body of waste water travelling between the sensor units.
15. The control unit of claim 14 wherein the signal processor is configured to compute the depth of waste water and the wavefront speed in parallel.
16. The control unit of claim 14 wherein the signal processor is configured to detect a backwave, where a backwave is a leading edge of a body of waste water travelling in a direction against a gravity determined direction of flow of waste water in the sewer.
17. The control unit of claim 16 wherein the signal processor is configured to detect the backwave after it has computed the wavefront speed
18. The control unit of claim 14 wherein the signal processor is configured to compute the wavefront speed by starting a timer on being woken from a low power mode as a result of the first one of the sensor units detecting waste water over a threshold depth, setting an interrupt to be triggered when the second one of the sensor units detects waste water over the threshold depth, stopping the timer when the interrupt is triggered, and computing the speed using a value of the timer and information about a distance between the first and second sensor units.
19. The control unit of claim 14 wherein the signal processor is configured to compute a speed of a backwave, where a backwave is a leading edge of a body of waste water travelling in a direction against a gravity determined direction of flow of waste water in the sewer, by starting a timer on being woken from a low power mode as a result of the first one of the sensor units detecting waste water over a threshold depth, setting an interrupt to be triggered when the first one of the sensor units detects waste water over a second threshold depth, when the interrupt is triggered, stopping the timer and computing a speed using a value of the timer arid information about a distance between the first arid second sensor units.
20. The control unit of claim 19 wherein the signal processor is configured to compute a depth of the backwave from data received from the first capacitive sensor when the timer is stopped.
21. The control unit of claim 19 wherein the signal processor is configured to implement a timeout on the timer arid, if the interrupt is not triggered before the timer timeout occurs, to quit computing speed of a backwave.
22. The sewer blockage detection system of any of claims 8 to 13 comprising an operations center server in communication with a plurality of control units, each control unit associated with a capacitive sensor assembly, the server configured to infer a upstream-downstream relationship of the capacitive sensor assemblies from data received from the control units.
23. The sewer blockage detection system of claim 22 whereby the server compares statistics of flows received from pairs of control units where one of the control units is downstream with respect to the other control unit of the pair, and raises an alert to an operator if the statistics of flows reported by the downstream control unit has changed significantly when the pattern of flows reported by the upstream unit has not changed significantly.
24. The sewer blockage detection system of claim 23 whereby the control units are in a sewer network having a tree structure where sewer pipes join and their flows are aggregated together as they run downstream, and wherein the server aggregates signals from a plurality of control units upstream of a single downstream control unit and compares the aggregation result and the data from the single downstream control unit.The sewer blockage detection system of claim 22 where the server is configured such that if the fluid level in the sewer does not return to the level before a wavefront, or returns to the original level much slower than previously, then an alert is raised to indicate the presence of a blockage downstream of the sensor.