Abstract:
A method for measuring sensor signals, has the steps: a) receiving a signal packet with a plurality of sampled sensor signals; b) determining a signal level from the sampled sensor signals; c) determining signal variations within the packet; d) comparing the determined signal variations with a predetermined noise threshold and if the variations are below the noise threshold then using the signal packet for further processing and if the variations are above the noise threshold then summing the signal level and repeating steps a) to c) and determining a predetermined number of repetitions has been reached and if so averaging the summed signal level.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/635,508 filed on Apr. 19, 2012, which is incorporated herein in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to energy efficient measurement of sensor signals, in particular sensor signals that are subject to noise and in particular capacitive sensor signals. 
       BACKGROUND 
       [0003]    Human device interfaces include touch sensing and gesture detection systems that are based on capacitive touch sensing and/or capacitive proximity detection. In many systems these capacitive sensors and associated detection circuitry need to be powered on all the time and respective power consumption is kept at a minimum often within the range of ordinary discharge currents of chargeable batteries. Other sensor technologies face similar problems. 
         [0004]    In any sensor based technology, the sensor signals are often subject to environmental influences, in particular noise signals are superimposed on the sensor signals proper. Noise sources include, for example, continuous noise sources, pulsed noise sources, broadband and narrowband noise sources, frequency-, amplitude-, and phase-modulated noise sources, variable and/or fixed frequency noise sources. Extracting the actual sensor signal, for example, through filtering, may become problematic. This can become even more difficult if the sensors are implemented in a mobile device and may require additional power which could render power consumption inefficient. Hence, there exists a need for efficient data acquisition in sensor based systems, in particular mobile systems. 
       SUMMARY 
       [0005]    According to an embodiment, a method for measuring sensor signals, may comprise: a) receiving a signal packet comprising a plurality of sampled sensor signals; b) determining a signal level from said sampled sensor signals; c) determining signal variations within the packet; d) comparing the determined signal variations with a predetermined noise threshold and if the variations are below the noise threshold then using the signal packet for further processing and if the variations are above the noise threshold then summing the signal level and repeating steps a) to c) and determining a predetermine number of repetitions has been reached and if so averaging the summed signal level. 
         [0006]    According to a further embodiment, if during the repetition of steps a) to c) it is determined in step c) that the signal variations of a current signal packet are below the predetermined noise threshold, then discarding the summed signal level and use the signal level of the current signal packet. According to a further embodiment, the number of repetitions can be a predetermined number between 1-256. According to a further embodiment, a signal packet may comprise sampling of a received signal synchronous with a transmitted signal. According to a further embodiment, a signal packet may comprise sampling values of the maximum and the minimum peak value of a carrier. According to a further embodiment, the method may further comprise delaying the repetition measurements by inserting a delay between subsequent measurements. According to a further embodiment, the delay can be varied with each repetition. According to a further embodiment, the delay can be increased with each repetition. According to a further embodiment, the delay can be decreased with each repetition. According to a further embodiment, the total delay of all delays during the repetition of the signal packets can be equal the time between two subsequent signal packets. According to a further embodiment, the method may further comprise validating the measurement. According to a further embodiment, validating the measurement may require a minimum amount of repetitively receiving sets of valid signal packets. According to a further embodiment, the minimum amount of sets of already averaged signal packages can be set between 1-256. According to a further embodiment, the signal level is compared with a threshold level to determine a state of the sensor. According to a further embodiment, the method may further comprise validating the determination whether a sensor state changes. According to a further embodiment, validating the determination of sensor state change may require either a minimum set of measurements with a processed signal level below the threshold level while the current state of the sensor is “ON”, or may require a minimum set of measurements with a processed signal level above the threshold level while the current state of the sensor is “OFF”. According to a further embodiment, the method may further comprise delaying the repetition of validation measurements. According to a further embodiment, the method may further comprise varying the delaying between subsequent validation measurements. According to a further embodiment, the method may further comprise determining whether the signal level is overloaded or distorted and repeating the measurement up to a predetermined number of times to obtain an undistorted signal level and limit the current consumption. 
         [0007]    According to another embodiment, a method for measuring sensor signals, may comprise: a) receiving a signal packet comprising a plurality of sampled sensor signals, wherein the signal packet comprises sampling of a received signal synchronous with a transmitted signal; b) determining a signal level from said sampled sensor signals; c) determining signal variations within the packet; d) comparing the determined signal variations with a predetermined noise threshold and if the variations are below the noise threshold then using the signal packet for further processing and if the variations are above the noise threshold then: summing the signal level and repeating steps a) to c) and determining a predetermine number of repetitions has been reached and if so averaging the summed signal level, wherein if during the repetition of steps a) to c) it is determined in step c) that the signal variations of a current signal packet are below the predetermined noise threshold, then discarding the summed signal level and use the signal level of the current signal packet. 
         [0008]    According to a further embodiment of the above method, the method may further comprise delaying the repetition measurements by inserting a delay between subsequent measurements. According to a further embodiment of the above method, the method may further comprise validating the measurement, wherein validating the measurement requires a minimum amount of receiving valid signal packets. According to a further embodiment, validating the determination of sensor state change may require either a minimum set of measurements with a processed signal level below the threshold level while the current state of the sensor is “ON”, or may require a minimum set of measurements with a processed signal level above the threshold level while the current state of the sensor is “OFF”. According to a further embodiment of the above method, the method may further comprise determining whether the signal level is overloaded or distorted and repeating the measurement up to a predetermined number of times to obtain an undistorted signal level. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  shows sample values of a measurement packet of an undisturbed signal and a signal with superimposed noise. 
           [0010]      FIG. 2  shows repetitive measurements of packets when a strong interference is present. 
           [0011]      FIG. 3  shows a flow chart diagram for a conventional sensor processing method. 
           [0012]      FIG. 4  shows a flow chart diagram of an improved method according to an embodiment. 
           [0013]      FIG. 5  shows the principle for interference frequency suppression in frequency and time domain. 
           [0014]      FIG. 6  shows another flow chart diagram of an improved method according to an embodiment. 
           [0015]      FIG. 7  shows a flow chart with integrated validation measurement. 
           [0016]      FIG. 8  shows a flow chart for handling overload. 
           [0017]      FIG. 9  shows an exemplary sensor system for detecting three-dimensional gestures. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    According to various embodiments, the average forming process for synchronous sampled measurement values, in particular in a system receiving sensor signals, can be made adaptive. Such a system can be for example a touch sensing system or in particular a gesture detection system which uses proximity detection and does not necessarily require a physical touch. For example a low frequency electric field may be generated using an electrode plate that is fed a low frequency signal, for example a 100 kHz AC or square-wave signal. Further electrode plates may be used to detect disturbances caused by an object entering the electric field. The signals received can be pre-processed and signals can be assembled in packets for further processing. 
         [0019]    According to various embodiments, generally, a packet including a low number of wanted signal periods is measured and averaged wherein for each wanted signal period preferably the sampling instant is set synchronous to the transmitter signal to acquire the maximum value of the positive half-wave and the minimum value of the negative half-wave is measured. According to various embodiments, the more the measured packet is interfered with the more packets are measured and averaged. Furthermore, the evaluation of the interference of a packet can be performed through the evaluation of the scattering of the sample values of the positive and negative half-wave, respectively. The respective greater deviation or variance or standard deviation may be used in the evaluation to get an indicator for the noise. 
         [0020]    If a sample value in a packet is distorted or overloaded, the packet will be discarded and measured again. If multiple packets are discarded, then this will be an indicator for a strong noise or interference and the number of measured packets will be limited to limit the power consumption according to various embodiments. 
         [0021]    If there exists only a small disturbance or little noise, a single packet is already enough for meaningful measurement result and the current consumption will be minimal. With increasing noise/interference, the current consumption will increase wherein multiple packets are required to be measured and averaged. This will result in an energy efficient measuring method according to various embodiments. 
         [0022]    A further advantage lies in the fact that there is no need to distinguish between continuous source of interference and pulsed sources of interference which results in a simpler code. In case of a pulsed interference source, if a packet is measured within the time period without interference, the resulting measurement can be used immediately without the need to measure and average further packets. Repeating the measurement would be in fact detrimental because a time period with interference could possibly negatively influence the measurement. 
         [0023]      FIG. 1  shows sample values of a measurement packet of an undisturbed signal on top and a signal with superimposed noise on the bottom. In the shown example, four signal periods are measured within a packet. As shown in  FIG. 1 , each sample point is taken at the respective maximum or minimum. The respective four consecutive maxima and minima are identical in this example as shown in the top curve. When noise is added as shown in the bottom curve, the maxima and minima sample points vary within the respectively marked variation ranges. 
         [0024]      FIG. 2  shows repetitive measurements of packets when a strong interference is present. The top curve shows the interference or noise, whereas the bottom curve in  FIG. 2  depict the signal packets. Through the fast repetition of packet measurements, a time period is found in which no or very little interference/noise is present. In this example five packets have been measured until a valid result has been obtained. The first four packets include the strong interference signal but as shown in  FIG. 2 , the fifth signal is free from interference. 
         [0025]      FIG. 3  shows a flow chart diagram for sensor processing, wherein the measured signal is compared with a threshold to determine the actual sensor state according to a conventional method. Step  310  represents the signal acquisition in which a signal packet including synchronous sample points is obtained. In following step  320 , the signal level are processed. In step  330  it is checked whether the signal level exceeds a predetermined threshold. If so, in step  340  the current state of the sensor is determined. If the state is “OFF” then the routine switches the state to “ON” in step  360  and other skips this step. Similarly, if step  330  is negative, a current sensor state is checked in step  350  and if it is “ON” the state is changed to “OFF” in step  370  and other wise step  370  is skipped. The routine continues with step  380  in which a sleep mode is activated. The sleep mode may be deactivated in step  390  and the routine returns to step  310  to receive the next signal packet. 
         [0026]    According to  FIG. 3 , the object of an improved method is to obtain a fast response time and use very little power. An improved method should furthermore be able to handle a variety of different interference sources as discussed above. Generally, the filter could be designed to have a long time constant to produce good filter results. However, long time constants will result in long reaction times and require a lot of power. Hence, an adaptive solution according to various embodiments is proposed which adapts to the respective interference source to reduce the power requirements in respective situations. 
         [0027]    Conventional systems use synchronous demodulation wherein the received signal is sampled synchronous with the transmitted signal. In particular, in a sensor system that uses an E-field as mentioned above and shown in  FIG. 9 , a transmitter electrode Tx  930  is used to which for example a 100 kHz carrier signal is fed by a signal generator  950 . The signal generator  950  can be a microcontroller port, for example generating a square wave signal. Also, the signal generator can be an oscillator generating a sinusoidal signal. Sinusoidal signals can also be obtained through respective filtering of other signals such as the mentioned square-wave signal. Thus, the sampling through an evaluation device  960  can be synchronized with the transmitted signal, for example, to measure the maxima and minima of the respective electric field signal. To this end, the evaluation device  960  also receives the generated transmission signal from signal generator  950 . In a purely digital circuit, the synchronization can be performed by software or through respective control registers. Multiple receiving electrodes may be implemented and signals received from these sensors are then further evaluated by the evaluation device  960 . For example as shown in  FIG. 9 , a first receiving electrode Rx 1   900  may be provided which is coupled with an amplifier  905  All electrodes can be simple electrodes plates, for example, formed on a printed circuit board by etching the copper plane respectively. Further electrodes Rx 2  and Rx 3  ( 910  and  920 ) and associated amplifiers  915  and  925  may be provided. The electrodes are not limited to a specific shape as shown in  FIG. 9 . Other shapes and number of electrodes may apply according to the respective system. Finally, a shield electrode  940  surrounding transmitter electrode  930  can be provided which may be preferably fed by an inverted transmission signal through inverter  955 . The present application is not limited to the system shown in  FIG. 9 , but could also be used with any other sensor systems. 
         [0028]    The more signal periods are recorded the better the filter effect. Such a method must use many periods of synchronous sampling to filter out frequencies that are close to the carrier frequency to be able to operate with a wide variety of interference sources. This means that if a typical carrier frequency of for example 100 kHz is used, as common in E-field measurement systems that use proximity detection for example in gesture detection systems, the reaction times can easily become greater than 1 sec. This is however not acceptable for a user, in particular if basic operating elements are implemented by such sensors, such as on/off switches. 
         [0029]    According to various embodiments, the level of the actual present noise/interference is determined and an adaptation of the filter length for average forming is performed to minimize the influence of interference/noise sources. To this end, signal packets with typically a few signal periods, for example, 2 to 256 periods, are used for averaging. These packets are demodulated synchronously. If the variation in the sampling values within the few periods is not greater than a predetermined noise source, then it is assumed that the sensor system is not under the influence of a noise/interference source and the signal level is formed from the average of the few signal periods as shown in  FIG. 1 . If the variation in the sampling values is greater than the system noise, then it is assumed that the system is under the influence of a noise/interference source and N−1 further signal packets are determined. 
         [0030]      FIG. 4  shows the associated flow chart diagram of an improved method according to various embodiments. Steps  400  to  445  show the processing of the incoming signal packets. All signal levels are summed (Sum) and after the acquisition of N−1 further signal packets, the sum is divided by the number of total signal packets N. Typically, N can be in the range of 2 to 256 repetitions. By these additional method steps, the filter length is enlarged and the variation in the averaged signal levels is reduced. If the measurement is influenced by a continuous interferer, then a total of N signal packets will be received. In step  420  it is determined if a noise source is present from the calculated signal variation obtained through steps  405 - 415 . If there is a noise source present, then the routine branches to step  425  where the signals are summed up. In step  435  the number of repetitions is checked and incremented in step  400 . If enough repetitions have been performed, the routine branches to step  445  where the signal level is determined by the dividing the calculated sum by the number of repetitions. The routine then continues in step  430  and  440  where the sum and repetition variables are reset. In step  450  the calculated signal level is checked. The following steps  455 - 480  correspond with steps  340 - 390  of  FIG. 3 . These steps  455 - 480  can also be substituted by equivalent determination steps. The changing of the sensor state as shown in these steps is merely one possible embodiment. There are many equivalent routines to set respective flags or register values that determine the function of the evaluated signals. 
         [0031]    In case of a pulsed interferer, a maximum of N signal packets may be received if the pulsed interferer is active during all data acquisitions. If a noise pulse is shorter, then n (n≦N) signal packets are received until a signal packet is free of interfering influence as for example shown in  FIG. 2  with the fifth packet. The summation of the previously received signal levels is discarded and only the single signal packet will be used for further threshold level comparison. By means of the cancellation condition current consumption and the system reaction time can be reduced. 
         [0032]    Furthermore, an interference frequency suppression for interference frequencies can be implemented wherein the frequency distance f a  to the carrier frequency can be the same or multiples of the interval frequency f p  between the acquisition of two signal packets as shown in  FIG. 5 . The top graph in  FIG. 5  shows the frequencies of the carrier and two interfering sources. As shown in the bottom part of  FIG. 5 , an envelope modulation is created on the sampled summation signal of F carrier  and f interferer  with a frequency of f a =|F carrier −f interferer |. Because the signal values have to be processed after acquisition, always a time distance Tp develops which is equally long in each repetition. Typical values for T p  are between 100 μs and 1 ms which corresponds to an interval frequency of 1 kHz-10 kHz. If the interference signal has a frequency distance to the wanted frequency such that f a  is close to frequency f p  or multiples thereof, then through taking in multiple signal packets will lead to no filter effect because sampling takes place always at the same phase position of the envelope. If however the time between two signal packets is varied then the phase position of the signal acquisition time will change with respect to the phase position of the envelope. A respective flow chart is shown in  FIG. 6 . Similar steps as designated with the same reference symbols. Typically a variation of time between two signal packets can be realized by inserting of a delay T v  in step  610  which increases with each repetition to further change the phase position in each single repetition. Alternatively the delay can be randomly varied. Preferably, the total delay T g  of all delays during the repetition of N−1 signal packets is equal to the time T p , such that the signal packets are delayed to be equally distributed over the time period. 
         [0033]    A further feature according to various embodiments is the suppression of interferers whose frequency is close to the carrier frequency as shown in the flow chart of  FIG. 7 . Again, the same reference symbols refer to the same steps of  FIGS. 4 and 6 . The above described methods for interferer recognition and filter lengthening will not be effective with interferers whose frequency is close to the carrier frequency. The reason for this is that such interferers only generate a small variation in the measured signal within few signal periods of a signal packet. Therefore, no repetition measurements are introduced and the interferer will not be filtered from the carrier signal. For this reason, the measurement sequence is repeated m times (validation measurement) at each expected sensor state change and also a variable delay element for phase position change will be integrated. Typical values m for the validation measurements are 1-16. For the validation measurements the maximum filter length of N−1 repetitions is used. In addition it will be determined whether it is a validation measurement. To this end, the flow chart according to  FIG. 7  shows an additional step  710  inserted between steps  420  and  430 . In this step  710  it is checked whether the validation is greater than 0. If no validation is required, the routine continues with step  430 , otherwise branches to step  425 . Between steps  455  and  465  as well as between steps  460  and  470  the same check is inserted with respect to the validation variable. If the validation variable is smaller than a predefined number of m validations then the routine branches in both cases to step  750  in which the validation variable is incremented. In following step  760 , the validation variable is added to the delay variable and the routine continues with at the start with step  405 . Finally, between step  480  and step  405 , the validation variable is reset to 0 in step  740 . 
         [0034]    Signal packets which include a measurement value with overload are discarded and will be measured again so long until a maximum number of K overload repetitions are reached as shown in the flow chart of  FIG. 8 . Signal acquisition and level processing is performed in steps  810  and  820 , respectively. A signal overload is determined in step  830 . If no overload occurs then the routine continues with step  420  as previously described. If an overload occurred then the routine branches to step  840 . If within K repetitions as determined by step  840  no signal packet is acquired which is not overloaded then the acquisition of further signal packets is terminated and in the respective measurement cycle no further evaluation will take place and the routine branches to step  475 . Otherwise the overload repetition variable is incremented in step  850  and the routine returns to its start in step  810 . 
         [0035]    Signal packets that are overloaded will falsify the average forming and will lead to a wrong result. The repetitions in case of overload are limited to a number of K (typically 1-4) to minimize the total current consumption in a heavily interfered mode. 
         [0036]    Furthermore, the current consumption can be limited in a heavily interfered environment by determining whether at the last x measurements no result could be obtained because of interference. If this is true then a tedious interferer could be present and the number of repetition measurements N−1 can be heavily reduced until a valid measurement is available. 
         [0037]    According to various further embodiments, instead of a continuously increasing delay Tv between two signal packets, alternating constant delays can be used or the delay could be turned off alternating to invert the phase position of the wanted signal between two signal packets. 
         [0038]    Optionally, the filter length could be increased such that instead of averaging over multiple signal packets, averaging takes place over multiple signal periods in a packet. The chosen filter length depends on signal variations. 
         [0039]    Instead of the use of a fixed number for the further acquisition of N−1 signal packets, a function (f(signal variation) could be used which allows for a variable number of N signal packets. 
         [0040]    For the validation measurements for an expected state change, a subset u (u&lt;m) of successful validation can be predetermined to declare a validation as valid. 
         [0041]    The sensor system according to various embodiments adapts with the proposed digital signal filter dynamically to the respective interfering environment and allows for a minimum filter length and therefore small current/power consumption in an undisturbed mode. In a disturbed mode the system is robust with respect to may different type of interferences through the above mentioned filter designs.