Abstract:
There is provided a sensor array, comprising a pair of transducers, a first transducer, configured to transmit pressure waves; and a second transducer, acoustically coupled to the first transducer, configured to detect pressure waves incident upon the sensor array from the environment. The array further comprises control circuitry, coupled to the first and second transducers, configured to: control the first transducer to transmit pressure waves; simultaneously control the second transducer to monitor the transmission of said pressure waves; and verify correct operation of the array in the event that said pressure waves are detected. The invention therefore provides a sensor array with a failsafe mechanism.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to sensor arrays. 
       BACKGROUND ART 
       [0002]    Existing ultrasonic proximity sensors typically consist of a few piezoelectric ceramic transducers, each of which performs both the transmit and receive functions. A short burst of ultrasonic energy is transmitted from the transducer. If the burst hits an object, it will be reflected back towards the transmitting element. If the object is within the sensor&#39;s range, then this is received by the transducer as an echo, and the distance to the object can be calculated from the time taken between the transmitted pulse and the received echo. 
         [0003]    Traditional ultrasonic proximity sensors rely on a single transducer which performs both the transmit and receive functions (or an array of such transducers). However, this scheme has a number of drawbacks. First, in order to detect echoes the transducer must switch to receive mode as soon as a pulse is transmitted. In practice there is a time delay in switching between the transmit and receive modes of the transducer, however, which translates to a minimum sensing distance. For example, at a transmit frequency of 40 kHz, the minimum sensing distance may be as high as 100 mm, although the minimum sensing distance is also dependent on the type of sensors used and how they are driven. Objects that are closer than the minimum distance create echoes which are incident on the transducer at a time when the receive function is yet to be enabled. These objects are therefore not sensed, potentially leading to collisions. 
         [0004]    In addition, it is difficult if not impossible to detect if the sensor array has a fault. For example, consider the situation where one transducer is faulty in its receive function, and therefore fails to report incoming echo signals. This means that when a pulse is sent out and no return signal is registered, it is not possible to tell if this is because there is no object to echo from, or because there is a failure in the transmitter/receiver etc. In the absence of any other indications, a user will assume the sensor is functioning correctly and that there are no nearby objects. He or she will only become aware of a sensor fault at the point of collision with an object. 
         [0005]    Ultrasonic proximity sensors are used in a number of different fields. Examples include car-parking sensors in the automotive industry, fill level indicators in agricultural grain silos, and proximity detection in medical devices such as radiotherapy machines (see U.S. Pat. No. 7,046,765, for example). In all of these technical fields, and particularly the latter, a failure in the proximity sensor can have grave consequences for the user or those nearby, as well as the equipment to which the sensor is attached. 
         [0006]    Clearly it is important that any sensor malfunction in these applications is detected as soon as it occurs. 
         [0007]    Some devices have separate transmit and receive transducers (see PCT application WO 2009/136196, for example). However, these systems are generally designed to minimize coupling between the transmitter and the receiver, and suffer from the same fault detection problems as single transducer designs. 
         [0008]    There is a further problem in that most ultrasonic ranging solutions rely on ceramic piezoelectric transducers. These transducers tend to be relatively high cost, limiting the number of sensors available, and pushing the design towards a single transducer for both transmit and receive. They also have a tendency to resonate or “ring” more than other transducers. This limits the minimum sensing distance as the ringing makes it difficult to distinguish echoes within the detected signals (i.e. the signal to noise ratio is low). Only once the ringing has fallen to an acceptable level is it possible to pick out echo signals from the noise. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention provides a sensor array, comprising a pair of transducers, a first transducer, configured to transmit pressure waves; and a second transducer, acoustically coupled to the first transducer, configured to detect pressure waves incident upon the sensor array from the environment. The array further comprises control circuitry, coupled to the first and second transducers, configured to: control the first transducer to transmit pressure waves; simultaneously control the second transducer to monitor the transmission of said pressure waves; and verify correct operation of the array in the event that said pressure waves are detected. 
         [0010]    The present invention therefore provides a sensor array with a failsafe mechanism. The entire signal chain can be verified without providing many additional components over conventional apparatus. Indeed, the separation of transmit and receive functions to different transducers allows objects that are extremely close to be detected. 
         [0011]    In an embodiment, the array further comprises memory, for storing a sample of the second transducer response to the transmission of pressure waves by the first transducer in an environment with no nearby objects. The control circuitry is further configured to: receive detected signals from the second transducer resulting from said pressure waves; subtract said stored sample from said detected signals; and process the signal remaining from said subtraction. 
         [0012]    This embodiment therefore provides a sensor array which can compensate for the resonance caused by transmitted pulses, allowing objects that are extremely close to the array to be detected. 
         [0013]    The sensory array may have a plurality of such transducer pairs, with each pair being acoustically decoupled from the others. 
         [0014]    For ease of manufacture, in one embodiment the transducers comprise a piezoelectric material, such as polyvinylidene fluoride (PVDF) film. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    An embodiment of the present invention will now be described by way of example, with reference to the accompanying figures in which; 
           [0016]      FIG. 1  is a schematic drawing of a sensor array according to embodiments of the present invention; 
           [0017]      FIG. 2  is a schematic drawing showing part of the control circuitry according to embodiments of the present invention; 
           [0018]      FIG. 3  is a flowchart of a method according to an embodiment of the present invention; and 
           [0019]      FIG. 4  is a flowchart of a method according to another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0020]      FIG. 1  shows a sensor array  10  according to embodiments of the present invention. The array can be used as part of a proximity sensor, but may have uses in other fields such as gesture recognition, for example. 
         [0021]    The array  10  comprises a plurality of transducer pairs  12 - 16 , and control circuitry  20  in communication with each of the pairs. Each pair comprises one transducer  12   a  that is connected to transmit circuitry within the circuitry  20 , and another transducer  12   b  that is connected to receive circuitry within the circuitry  20 . In the illustrated embodiment, the transducers  12   a,    12   b  are otherwise identical. However, in other embodiments the array may employ pairs of non-identical transducers specifically adapted to perform transmit and receive functions. 
         [0022]    The transducers within each pair are acoustically coupled together, such that a transmission of pressure waves (e.g. ultrasonic waves) from the transmit transducer  12   a  is detected directly (i.e. without reflecting from a nearby object) by the corresponding receive transducer  12   b  of that pair. The pairs themselves are acoustically decoupled, however, to the extent that is possible. Thus, the transmission of pressure waves by a transmit transducer  12   a  in one pair does not result in the direct detection of those pressure waves in receive transducers of other pairs  13 - 16 . Of course, the receive transducers may nonetheless detect echo signals resulting from the transmissions of other transmit transducers. 
         [0023]    The transducers can be manufactured from any suitable material known to those skilled in the art. Piezoelectric materials are particularly suitable, due to their ease of manufacture. Of these, polyvinylidene fluoride (PVDF) film exhibits a suitably low ringing effect. 
         [0024]    The connections between the transducer pairs  12 - 16  and control circuitry  20  can be made on a flexible printed circuit board, for example. 
         [0025]    In the illustrated embodiment, the pairs  12 - 16  are arranged side by side in a long, linear array, as this provides the simplest way of acoustically decoupling one transducer pair from the others. Alternative arrangements are contemplated, however. In addition, although five transducer pairs are illustrated in  FIG. 1 , the invention contemplates any number of pairs, including as few as one pair. 
         [0026]    In operation, the control circuitry  20  controls the transmit transducers  12   a  of each pair to transmit bursts of pressure waves (e.g. ultrasonic waves) radiating away from the array  10 . The receive transducers  12   b  listen for echo signals that have reflected off nearby objects, and output these signals to the control circuitry. By determining the time delay between the transmitted pulse and the received echo, the distance to the nearby object can be calculated. In order to transmit energy most efficiently, the transmit transducers  12   a  can be driven sinusoidally at a frequency equal to their resonant frequency. 
         [0027]    The transducer pairs can be operated according to a particular timing scheme, to ensure that the echo signals can be unambiguously related to a particular burst of ultrasonic waves. For example, in one embodiment, the transmit transducers may be pulsed sequentially from one end of the array to the other. 
         [0028]      FIG. 2  shows further details of the receive circuitry within the control circuitry  20  according to embodiments of the present invention. 
         [0029]    The control circuitry  20  comprises a plurality of inputs  22  for receiving signals from the respective receive transducers  12   b.  Each input is provided to a multiplexer  24 , which generates a corresponding multiplexed signal, for example, using time division multiplexing. The multiplexed signal is provided to a low noise amplifier  26 , and the amplified output converted to digital with a high-speed analogue-to-digital converter  28 . The digital signal is then provided to a digital signal processor (DSP)  30  for processing. 
         [0030]    The DSP  30  may perform many functions. For example, the signal processor  26  may determine the distance of the array from a nearby object by measuring the time between a pulse being transmitted and an echo signal being detected. In order to improve its detection capability, the DSP  30  may access a memory bank  32  of stored nominal signal responses, and subtract the detected signal from the stored signal. The memory  32  may be on board (as illustrated) or external. The stored signals are previously recorded samples of the receive transducer  12   b  responses to the transmission of pressure waves by the transmit transducer  12   a  in an environment without nearby objects. That is, the transmission of pressure waves by the transmit transducer  12   a  of a pair causes an initial detection of those pressure waves and/or of vibrations transmitted via the acoustic coupling between the transmitter and receiver by the corresponding receive transducer  12   b.  It is also common for the transmit transducer to “ring” for a period of time after it is initially driven. This ringing is detected by the receive transducer, and can interfere with detection of incoming echo signals. The difference between the recorded response and the real-time detected signal should therefore show the echo signals more clearly. 
         [0031]    The algorithm is set out below: 
         [0000]      voltage_recorded[n]−voltage_incoming[n]=ring_cancelled response [n],
 
         [0032]    where n is the time (or clock cycles) between the sample time and the transmit pulse. 
         [0033]    The detected response can be expected to change gradually over time, as the transmitters and receivers age. The control circuitry therefore periodically re-records the response at times when no object is nearby. 
         [0034]    This operation may be performed by the processor  30  itself, using its memory  32 . The processor  30  may use the number of pulses in the echo signal, as well as the frequency, to discriminate what is a valid echo and what is noise after the ringing has been cancelled out. If no valid echo signals are detected, the memory can be updated with the new stored signal. Alternatively, the response may be re-recorded during self-test steps when the device is initialised. 
         [0035]    The memory  32  may store recorded samples for each receive transducer. 
         [0036]      FIG. 3  is a flowchart of a method according to embodiments of the present invention. 
         [0037]    The method starts in step  100 , in which the transmit transducer  12   a  of a pair is controlled to transmit a burst of pressure waves. This can be implemented as part of an error detection mode or during normal operation of the sensor array. 
         [0038]    In step  102 , the receive transducer  12   b  is activated to monitor the transmission of step  100 . As previously described, the receive transducer is acoustically coupled to the transmit transducer, and thus receives the transmitted pressure waves directly. The goal of the receive transducer in this step is not to detect echo signals from a nearby object (which would be received after a time delay proportional to the distance of the object), but to detect whether the pressure waves were transmitted at all. Therefore this step may take place simultaneously with step  100 . 
         [0039]    In step  104 , the control circuitry  20  determines whether a transmission was detected. If a transmission was detected, the transmit transducer  12   a  and the receive transducer  12   b  are both deemed to be functioning correctly (step  106 ). If no transmission is detected, this could be because the receive transducer and/or the transmit transducer are faulty, and the fault can be reported to a controller of the system, for example (step  108 ). 
         [0040]    In order to detect faults, the processor  30  may look at both the magnitude and the time of the ringing, as well as its frequency to determine that the signal chain is working correctly. The direct response from the transmission is typically orders of magnitude larger than any echo response or other incoming signal. Under ordinary operational conditions, the response varies only by a small amount. If there were any faults in the signal chain, however, the response would vary substantially, much more than any change due to an echo of a nearby object. A fault would most likely result in substantial reduction or complete loss of the echo response. 
         [0041]    The present invention therefore provides a sensor array with a failsafe mechanism. The entire signal chain can be verified without providing many additional components over conventional apparatus. Indeed, the separation of transmit and receive functions to different transducers allows objects that are extremely close to be detected. This has particular merit in technical fields where collision avoidance is of paramount importance, such as radiotherapy for example. Thus, the present invention also provides for a radiotherapy apparatus comprising a proximity sensor as described above. 
         [0042]      FIG. 4  is a flowchart of a method of operating the sensor array  1  according to an embodiment of the invention. 
         [0043]    The method starts in step  200 , where the first transducer  12   a  is driven to transmit a pulse of pressure waves. 
         [0044]    In step  204 , signals are detected by the receive transducer. These detected signals will typically include the response of the receive transducer to the transmissions themselves, i.e. the ringing, and may also include echo signals reflected from nearby objects. 
         [0045]    In step  206 , the recorded calibration signal is subtracted from the detected signal. The difference is then processed to detect the presence of echo signals (step  208 ). If there are no echoes, it can be assumed that there are no objects in the vicinity, i.e. that the detected signal represented the receive transducer&#39;s response to the ringing only. In that case, the recorded signal is replaced with the detected signal in step  210 . 
         [0046]    If the difference signal contains echo signals, the recorded signal is not updated, and the difference signal is processed (step  214 ) as usual, for example to determine the distance to the object. 
         [0047]    This embodiment therefore compensates for the resonance caused by transmitted pulses, allowing objects that are extremely close to the array to be detected. In addition, this compensation can be continuously updated by intelligently recording the response when no objects are present in the vicinity, thus allowing for drift in the transmit and receive transducer responses. 
         [0048]    It will of course be understood that many variations may be made to the above-described embodiment without departing from the scope of the present invention.