GB2452951A - Control station receives condensed data from wirelessly linked sensing units which store raw data for transmission on receipt of raw data request signal - Google Patents

Abstract

Sensing units 40 placed at spaced locations each comprise: a sensor 41, 42; means 47, 49B for receiving wireless synchronisation signals from a source common to all the sensing units; buffer 45 for storing raw data from the sensor at sampling times determined by the synchronising signals; processor 44 for processing and condensing the raw data; wireless means for transmitting the condensed data; and means 46, 49A for causing the wireless means to transmit raw data from the buffer in response to receipt of a raw data request signal. Control station 10 comprises means for receiving the condensed data transmitted by the sensing units and means for transmitting a raw data request signal. The control station may identify from the condensed data, which may be statistical data, a period for which raw data is required and, in response, transmit a raw data request signal, possibly sequentially to different sensing units.

Description

A Remote Sensing System This invention relates to sensing system comprising a number of remote sensing units which are wirelessly linked to a control station.

The invention was developed in response to the need for a system capable of sensing and processing physical quantities such as temperature, pressure and acceleration at large plants or structures located in hazardous environments. Examples include onshore and offshore platforms and bridges and the sensing systems are typically needed to monitor conditions and to generate models for the identification of faults or potential problems.

Systems having wireless communication between a control station and the sensors are strongly favoured as they are easier to install; provide less of an obstruction to those working on site and make it more practicable to produce a sensing system that is safe for use in hazardous areas such as an explosive atmosphere.

A problem with wireless systems is that the data transfer capacity between the sensors and the control is more limited than in wired systems, mainly because each sensor and associated transceiver must be battery powered.

The present invention is particularly suited to systems which record dynamic physical properties, i.e. properties prone to change rapidly over a short time period. One example is the sensing of acceleration associated with vibration. Because changes in acceleration can occur relatively quickly in a vibrating environment, it is necessary to sample the sensor typically hundreds of times a second in order to accurately model the vibration. This contrasts with less dynamic properties such as temperature which, in many situations, is unlikely to change significantly over the course of a second.

Because of the high sampling rate, sensors measuring dynamic physical properties will produce large amounts of raw data. Transmitting all raw data from all of the sensor units to the control station requires a high bandwidth connection which in turn requires greater power and results in shorter battery life.

This power demand conflicts with the need to place these sensors in areas which are, for much of the time, inaccessible and thus require a prolonged battery life.

In systems measuring dynamic properties, it is important to ensure that the data recorded from the sensors is highly synchronised so that accurate modelling can be performed. Expressed another way, the control needs to have data from the sensors that is synchronised so that correct deductions can be made regarding the timing of events distributed across the object or environment that is being monitored.

The present invention was developed with a view to creating a sensing system which is capable of transferring synchronised raw data from the sensors to the control whilst prolonging the battery life as far as possible.

According to the invention there is provided a sensing system comprising: (a) sensing units at spaced locations each sensing unit comprising (i) a sensor; (ii) means for receiving wireless synchronisation signals from a source common to all the sensing units; (iii) a buffer arranged to store raw data from the sensor at sampling times determined by the synchronising signals; (iv) a processor for processing the raw data and producing condensed data relating thereto; (v) wireless means for transmitting the condensed data and, (vii) means for causing the wireless means to transmit raw data from the buffer in response to receipt of a raw data request signal; and (b) a control station comprising (i) means for receiving the condensed data transmitted by the sensing units, and (ii) means for transmitting a raw data request signal.

The "condensed" data referred to above will normally be statistical information for example the mean square acceleration, or mean square velocity or mean square displacement in each second. However, many other possibilities exist. For example the condensed data could simply be the peak value of acceleration or of velocity or of displacement observed in each second. The only essential requirement is that the condensed data be of a significantly smaller size than the raw data so that the bandwidth supported between the sensor units and the control can be smaller and so the battery life prolonged.

Misalignment of the raw data from the different sensor units can occur as a result of drifting of a clock that is internal to a processor within each sensing unit. Therefore, it is preferable that the synchronising signal be used by the processor to synchronise the internal clocks and thus the sampling times.

So as to ensure that the data transmitted from each sensor in response to a raw data signal request relate to the same time period, the synchronising signal may also be used to associate a time marker to a position of the ring buffer. Preferably, this position relates to a current writing position on the buffer at the time the signal is received.

To minimise the quantity of raw data that needs to be transmitted, a preferred system comprises a means associated with the control station to analysis the condensed or statistical data and to transmit a raw data request signal if some threshold value is exceeded or if some other characteristic in the condensed data is observed, indicating an event or condition of special interest deserving a more detailed investigation.

To prevent conflict of the synchronised signals with the raw data transmission, the synchronisation signal may be broadcast via a separate transmitter at a frequency differing from that used to transfer raw/statistical data.

One example of the invention will now be described by way of example with reference to the following drawings in which: Figure 1 is a schematic illustrating a sensing system constructed in accordance with the invention for sensing vibration in an offshore oil platform; Figure 2 shows, in more detail, one of the routers of Fig 1; Figure 3 shows, in more detail, one of the sensor units of Fig 1; and Figure 4 illustrates, schematically, various functions performed by the computer of Fig 1.

Referring firstly to Fig 1 there is shown a sensing system network having a control station 10 located at a relatively safe part of the platform. This is connected, via an Ethernet network connection 20 (which may include one or more network switches, not shown) to a plurality of wireless routers 30. The wireless routers 30 are also located at a relatively safe part of the platform and provide a wireless link between the control station 10 and sensing units 40 which are located at strategic, relatively hazardous points on the platform. Connection between a router 30 and sensing units may also occur via a wireless relay 50 used to extend the operable distance between the control station 10 and the sensors 40 and/or used to create a mesh to increase the resilience of the system against obstructions to a signal. The wireless routers 30, sensing units 40 and relay 50 communicate using the Zigbee wireless protocol.

The control station 10 is formed by digital controller programmed to control the network 20 and to provide the functions illustrated on Fig 4. Programming may be achieved using procedures which are well known in the art. Referring to Fig 4, the computer provides the function of a synchronisation signal generator 11 which applies synchronising signals, to be described later, onto the network 20. Statistical data from the network is received at 12 where it is analysed to produce values that may relate to the condensed data received from each of the sensor units individually, or which relate to some interrelationship or combination of data from all sensor units.

These values are applied to a threshold detector 13. When a threshold, set at 13, is exceeded, a raw data request signal is generated at 14 and applied to the network 20.

Raw data received in response to such a request is analysed at 15 to produce information which is presented to a user via a user interface 16.

Referring now to Fig 2, the wireless router unit 30 comprises a processor 31 such as a Microchip Ethernet microcontroller (PlC 1 8F67J60) connected to the Ethernet link 20 and a programmed processor/transceiver module 32 connected to antenna 33 and adapted for communication of data using the Zigbee standard. An example of a suitable module is the EM250 microchip manufactured by Ember. The programming of the processor 31, which may be achieved using C language, uses procedures well known in the art. Also housed in the router 30 is a separate transmitter 34 and antenna 35 for transmitting a synchronisation signal from the computer control station 10.

The primary function of the router 30 is to receive signals from the control station 10 via the Ethernet connection and to re-transmit them to the sensor units 40 via a wireless connection and vice-versa.

Referring to Fig 3, the sensor unit 40 comprises two sensors 41, 42 for measuring acceleration and temperature respectively, a battery 43, processor 44, a ring buffer 45 capable of storing 10 seconds worth of data from acceleration sensor 41, a Zigbee processing unit/transceiver module 46, a synchronisation signal receiver 47 connected to the processor 44, an analogue to digital Converter (ADC) 48 and two antenna 49A, 49B associated respectively with the transceiver 46 and receiver 47.

Other sensor units 40 may have any number of sensors for measuring physical properties considered appropriate for their position. Examples include those for measuring pressure (including dynamic pressure), load, strain and chemical identification etc. During use, the accelerometer 41 produces, in response to vibration of the platform structure, an analogue output proportional to acceleration. This is sampled by the ADC 48 at sampling times synchronised with the clock of the processor 44. This sampled raw data is sent by the processor 44 to the ring buffer 45 and to processor 44.

Processor 44 uses the raw data to calculate condensed data such as the peak acceleration in each second and the mean square acceleration per second which is transmitted via Zigbee transceiver module 46 and antenna 49A to the router and so to the control station 10 on a regular basis.

Each sample of raw data sent to the ring buffer 45 is stored in an assigned slot. Once the buffer has been filled, incoming raw data is written over existing slots in the order that the slots were previously filled. This cycle repeats until the information from the buffer 45 is recalled.

The temperature sensor 42 is connected directly to the processor 44, which is programmed to sample the sensor 42 at a fixed period, for example once every 0.5 seconds, or upon request from the control station 10.

Referring back to Fig 2, the processor 31 is programmed to generate two signals. The first is a wireless synchronisation signal which is synchronised with the signal from synch generator 11 (Fig 4) and is transmitted via antenna 35 to sensor units 30. The second is a "raw data request" signal to be described later.

Referring to Fig 3, the synchronisation signal from a router 30 is received by antenna 49B and receiver 47 and fed to the programmed processor 44. This causes the clocks of the processors 44 in all the sensor units to be reset simultaneously, ensuring that the sampling of the sensors by the ABC 48 are synchronised. If the clock of a processor 44 has deviated, any information stored in the previous shortened sample is discarded. Abnormalities in the data caused by this can be smoothed over' using processing techniques during subsequent analysis at the control station.

A second marker' synchronisation signal is also periodically sent from the computer via the same communication route as the first synchronisation signal. Receipt of the marker signal causes each processor 44 to mark the current writing slot of the buffer 45 with a time marker X. This marker is moved each time a marker signal is received.

Referring to Fig 4, the statistical data sent by each sensor unit's processor 44 is continuously processed by a processing mechanism 12 to look for an event' signified by a particular sized peak or the exceeding of limit values. Such an event might be a rocking motion of the platform (or part thereof) caused by the impact of a large wave and is detected by threshold detector 13. Upon the identification of an event, a mechanism 14 causes a raw data request to be sent to each sensor unit 30 in turn for the raw data in their respective ring buffer 45 corresponding to a time period over which the event was noticed. To ensure that the sensors send data corresponding to the same period, the request specifies specific sectors of the buffer relative to the time marker X. For example, if the start of an event was noticed at time X + 1000, the request might be for all raw data stored from X-1000 to the current write segment.

Alternatively, the request may be for all of the data in the buffer to be sent including information as to the time marker. The threshold exceedance could equally well be performed at the sensor unit 40. This would involve the transmission of a threshold event detection signal via the transceiver 46 to the control station 10 and the control station 10 would then send a raw data request out to all sensor units 40.

Upon receiving a raw data request, the processor 44 in each sensor unit stops writing to the buffer. The processor 44 then waits for the receipt of specific request for it to upload the relevant information in the buffer 45. Because the bandwidth of communication channel is narrow, this may take several minutes. Once the transmission is complete the buffer is instructed to accept new data from the sensor 41.

Once the data has been uploaded to the control station 10 it is stored (not shown) for future analysis and can be displayed or otherwise presented to a human operator via the interface 16.

The request for raw data may also be accompanied by a request for ancillary data from the temperature sensor 42. This information is then stored in store 13 associated with the acceleration data.

Claims (11)

1. Claims 1. A sensing system comprising: (b) sensing units at spaced locations each sensing unit comprising (i) a sensor; (ii) means for receiving wireless synchronisation signals from a source common to all the sensing units; (iii) a buffer arranged to store raw data from the sensor at sampling times determined by the synchronising signals; (iv) a processor for processing the raw data and producing condensed data relating thereto; (v) wireless means for transmitting the condensed data and, (vii) means for causing the wireless means to transmit raw data from the buffer in response to receipt of a raw data request signal; and (b) a control station comprising (i) means for receiving the condensed data transmitted by the sensing units, and (ii) means for transmitting a raw data request signal.
2. 2. A sensing system according to Claim 1 characterised in that the condensed data is statistical data.
3. 3. A sensing system according to Claim 1 or 2 characterised in that the control station comprises means for identifying, from the condensed data, a period for which raw data is required and for transmitting a raw data request signal in response to such identification.
4. 4. A sensing system according to Claims 1, 2 or 3 characterised in that the raw data request signal is sent sequentially to different sensing units.
5. 5. A sensing system according to any preceding Claim characterised in that the buffer is a ring buffer.
6. 6. A sensing system according to any preceding Claim characterised in that the raw data request signal identifies a time period for which sensed raw data is required and characterised in that the sensing unit is adapted to transmit raw data from the buffer appropriate to that time period.
7. 7. A sensing system according to any preceding Claim characterised in that the synchronisation signal is used to synchronise a clock of the processor.
8. 8. A sensing system according to any preceding Claim characterised by means for generating and using a second synchronisation signal to record a time marker at a point on the ring buffer.
9. 9. A sensing system according to Claim 8 characterised in that the time marker is recorded at the writing point at the time that the signal is received
10. 10. A sensing system according to any preceding Claim characterised in that the means for receiving wireless synchronisation signals is a receiver separate from that used to transmit data.
11. 11. A system substantially as described herein, or as illustrated in the figures.