Abstract:
A method and device for object detection are disclosed. In one aspect, a method comprises transmitting a plurality of signals into a region; measuring a noise level during a noise measurement time interval corresponding to each respective one of the plurality of transmitted signals; generating a threshold signal dependent on the noise level; and comparing a first plurality of signals received by a sensor, each of the received signals corresponding to a respective one of the plurality of the transmitted signals, with the respective threshold signal. In another aspect, a device comprising components adapted to carry out the steps of object detection is disclosed. In one example, a thresholding circuit is adapted to generate a threshold signal have a level corresponding to substantially a peak-to-peak level of the measured noise level.

Description:
[0001]    This disclosure generally relates to detecting objects in monitored regions, and more particularly relates to methods and devices for detecting objects in environments where interfering signals, or noise, may be present. 
         [0002]    Sensors, such as optical sensors, have been used to automatically monitor the presence of objects in certain defined regions for applications such as industrial safety and automation. In one example type of application, an optical device has a transmitter that emits pulsed light signals into a monitored region and a sensor that detects signals resulting from the interaction, such as reflection or scattering, between the transmitted light signals and any object located in the region. When an object is located in the monitored region, light signals above a threshold level may be detected as a result of transmitted light being reflected or scattered into the sensor. However, when light signals above a threshold is detected, the detected signals may be from noise sources, such as neighboring transmitters or lighting sources, rather than an object located in the monitored region. Various methods and devices have been used to reduce the chances of false assessment, e.g., determining that an object is in a monitored region when it is not, due to noise. 
       SUMMARY 
       [0003]    This disclosure relates to methods and devices for detecting objects in a region (or detecting a retro target). In one aspect, an object detection method comprises transmitting a signal, such as an optical signal, from a transmitter into a region; measuring a signal, corresponding to the transmitted signal, received by a sensor; determining whether the received signal satisfies a condition; determining whether noise of at least a threshold amount is present; and determining whether an object is located within the region depending on whether the received signals meet the condition, the condition being different when noise present than not present. 
         [0004]    A plurality of transmitted signals can be transmitted sequentially during a plurality of repetition time intervals, and the signals received by the sensor can be measured during each repetition time intervals and during a signal measuring time interval beginning at or after the onset of each transmitted signal. The condition may be, for example, that the received signal is above a threshold level (the “dark detect threshold”) for indicating that an object is located in the region, or below a threshold level for indicating that an object is not located in the region. Noise can be measured during a noise measurement time interval in one or more of the repetition time intervals by measuring the signals received by a sensor, which can be the same sensor described above for measuring signals during the signal measurement time interval. A determination that noise of at least a threshold amount is present can be made, for example, when signals above a noise threshold level is detected during one or more noise measurement time intervals. 
         [0005]    A determination that an object is located in the monitored region (or that a retro target is present) can be made, for example, if the signals measured during the signal measurement periods are above the threshold, optionally also for a predetermined number of consecutive repetition time intervals. When noise is present, a higher threshold is used to determine that an object is located in the monitored region. For example, the threshold can be set to have a specified relationship to the measured noise level. In one example, the threshold can be set approximately to the peak-to-peak noise level. 
         [0006]    Once a determination that an object is located in the monitored region (or that a retro target is detected) is made, the signals measured during the signal measurement time intervals must be lower than a threshold level (the “light detect threshold”), optionally in a predetermined number of repetition time intervals (consecutive or a minimum fraction of a total number of intervals), before a determination is made that an object is not located in the monitored region. 
         [0007]    Thus, the difference between “dark detect” and “light detect” thresholds, or threshold hysteresis, can be dynamically set, or varied depending on the noise level. 
         [0008]    In another aspect of this disclosure, a device for detecting an object in a region (or detecting a retro target) comprises a transmitter, a sensor for detecting signals from the monitored region during the signal measurement time intervals, a noise detector and a controller. The controller is adapted to operate the transmitter to transmit signal, operate the sensor to measure the received signals and operate the noise sensor to measure noise. The controller is configured and adapted to determine whether an object is located in a region as described above. In one aspect of the disclosure, 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1   a  schematically shows an object detecting device and its operating environment according to one aspect of this disclosure. 
           [0010]      FIG. 1   b  schematically shows an object detecting device and its operating environment according to another aspect of this disclosure. 
           [0011]      FIG. 2   a  schematically shows the various signals leading to a determination that an object is located in a monitored region according to an aspect of the disclosure. 
           [0012]      FIG. 2   b  schematically shows the various signals leading to a determination that an object is not located in a monitored region according to an aspect of the disclosure. 
           [0013]      FIG. 3  schematically shows threshold hysteresis according to an aspect of the disclosure. 
           [0014]      FIG. 4  shows a diagram of a circuit for dynamically setting the dark detect threshold according to an aspect of the disclosure. 
           [0015]      FIG. 5  schematically shows the time intervals for the various steps in a method for detecting an object according to an aspect of the disclosure. 
           [0016]      FIG. 6  schematically shows an example of dynamic threshold hysteresis setting, where the dynamic hysteresis is set at approximately the peak-to-peak noise level, according to an aspect of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Machine sensing finds a wide range of applications. Devices for automatically detecting the presence of objects in a monitored region, for example, are used in applications such as industrial safety and automation. For such applications, it is desirable to quickly and accurately determine whether an object is present in a monitored region. 
         [0018]    In one example application, an object detection device includes an optical emitter (or transmitter) and receiver (or sensor), which may be contained in the same housing (i.e., the device may be self-contained). The emitter emits a train of pulsed light signals and the device measures the signals received by the sensor to detect light reflected or scattered back from an object in the monitored region in order to determine if there is an object present. 
         [0019]    To attain a desired degree of reliability of object detection, a process sometimes referred to as “demodulation” is used. In such a process, the signals received by the sensor are measured repeatedly and the measured signals must meet a certain requirement (such as being above or below a threshold level) a minimum number of times before the device determines whether an object is present in the monitored region. For example, a requirement can be that the sensor must receive signals above a particular threshold level (“dark detect” threshold) for a number (sometimes referred to as “demodulation count”) (e.g., four) consecutive repetition time intervals (sometimes referred to as “rep-rates”) for the device to determine that an object is present in the monitored region and to change the state of an object-presence indicator from the “dark” state to the “light” state. Here, “light” state means the device has determined that an object is present in the monitored region (and the measure signals resulted from the transmitted signals being reflected or scattered by the object); and “dark” state means no object is present in the monitored region. 
         [0020]    Conversely, for an object detecting device to change from a “light” state to a “dark” state, there must be no light (or no light above a threshold level (“light detect” threshold)) returned for, e.g., four (4) rep-rates. 
         [0021]    However, even with demodulation, chances for false detection exist when noise is present. Measured signals may reach levels above threshold due to noise, including interfering light pulses from other nearby sensor devices and environmental noises such as light from building lighting fixtures. Objects may thus be determined to be present in a monitored region when they are not. 
         [0022]    Example methods and devices disclosed in the present disclosure provide dynamic thresholding (or dynamic threshold hysteresis), i.e., using a threshold level that is dependent on the detected noise level, to achieve a high level of reliability. In certain examples, a dark detect threshold that bears a specified relationship to the noise level is used. In one example, the output signal of an analog noise peak detection circuit is used as to increase the dark detect threshold, such that a signal approximating 2× the maximum noise level (during a noise detection period) is used as the threshold. 
         [0023]    As an example, referring to  FIG. 1   a , in a diffuse mode, an object detecting device  100 , which can be an optical detector, includes an optical transmitter (not explicitly shown) disposed in the housing  102 , which is mounted on a support  104 , for transmitting pulsed light  106  to a monitored region  10 . If an object  20  is present in the monitored region  10 , the object  20  interacts with the transmitted light  106 , and at least a portion of the product of that interaction, such as scattered or reflected light  108 , can be received by a sensor (not explicitly shown), which can also be disposed in the housing  102 , in the device  100 . In another example, depicted in  FIG. 1   b , in a retro- or polarized retro mode, the object detecting device  120  transmits a signal  106  to a retro target  130  and receives a reflected signal  140  when no object is present in the monitored region. When an object enters the monitored region, the transmitted signal  106  and/or reflected signal  140  are blocked by the object  20 , and the sensor in device  120  does not receive any signal reflected by the retro target  130 . 
         [0024]    An electronic controller (not explicitly shown in  FIG. 1   a  or  1   b ), which can reside inside, partially inside or outside the housing  102 , controls the transmission of the transmitted light  106  and measures the signals received by the sensor and makes determinations characterizing the signals and noise received by the sensor, and on whether an object is present in the monitored region  10  based on the signals and noise received. 
         [0025]    For example, as illustrated in  FIG. 2   a , in an example method for determining, a transmitter (or emitter) is driven to transmit a signal  210  comprising a sequential series of light pulses  210   a - f , each having a pulse width, T pulse    212 , and transmitted during a repetition time interval, T per    214 . For each transmitted pulse  210   a - f , the signal received by the sensor is measured during a signal measurement time interval  232 , which can begin, for example, at or after the onset of each transmitted pulse  210   a - f . See, also,  FIG. 5 . The measured signals  230   a - f  are compared with an object detection threshold level  220 . If a measured signal exceeds the threshold level  220 , a condition consistent with detecting reflection of a transmitted signal by an object in the monitored region is determined to be met, as indicated by a change of state in a light-pulse-detect signal  240 , e.g., one of light-pulse-detect pulses  240   a - f . Upon detecting a certain number, n ( 242 ), of light-pulse-detect pulses in a certain number of repetition time intervals, an object is determined to be present in the monitored region, as indicated by a change of state in a “light-event” signal  250 . In the example shown in  FIG. 2   a , if light-pulse-detect signals are detected in four (n=4) consecutive repetition time intervals  214 , the light-event signal  250  changes from a first state  252  to a second state  254 . 
         [0026]    The example in  FIG. 2   a  illustrates a time sequence that begins with the detection device in a “dark” state (corresponding to the first signal level  252  of the light-event signal  250 ), i.e., where no object was determined to be present in the monitored region, and the device is configured to detect when the monitored region becomes occupied by an object. In the “dark” state, the object detection threshold (“dark detect” threshold) level  220  is set at a higher value  222 . Upon determining that an object is present in the monitored region, the device switches to a “light” state (corresponding to the second signal level  254  of the light-event signal  250 ). Upon transition from a “dark” state to a “light” state (or transition point  256  in the light-event signal  250 ), the threshold level transitions ( 226 ) to a lower level  224  (“light detect” threshold). The device becomes configured to determine when the monitored region becomes empty. 
         [0027]    An example of light-to-dark transition, i.e., a process for determining that the monitored region is no longer occupied, is illustrated in  FIG. 2   b . Here, the signal  230  received by the sensor is compared with the lower, light detect, threshold level  224 , and if the signal  230  is below the threshold level  224 , the light-pulse-detect signal does not change state (e.g., remains low). Upon determining that a certain number, m ( 342 ), of light-pulse-detect pulses in a certain number of repetition time intervals are low, or have undergone no change of state, an object is determined to be not present in the monitored region, as indicated by a change of state in the “light-event” signal  250 , from the second state  254  to the first state  252 . In the example shown in  FIG. 2   b , if light-pulse-detect signals are “low” in four (m=4) consecutive repetition time intervals  214 , the light-event signal  250  changes from second state  254  to the first state  252 , and the detection device  100  switches to the “dark” state. The object detection threshold level  220  is switched ( 326 ) to the higher, dark detect, level  222 , and the device becomes configured to determine when the monitored region becomes occupied. 
         [0028]      FIG. 3  schematically shows an example of a threshold hysteresis loop, in which the dark detect threshold  320  (or  222  in  FIGS. 2   a  and  2   b ) and light detect threshold  330  (or  224  in  FIGS. 2   a  and  2   b ) (both referenced to a reference point  310 , which in this example is 0V) are alternately used as described above. At least the dark detect threshold  320  is noise dependent. 
         [0029]    According to another aspect of the disclosure, noise received by the sensor is also measured during at least one repetition time interval  214 . The noise can be measured from the same sensor that is used to detect the reflected signal during the signal measurement time interval  232 , or it can be measured indirectly from a different sensor the output which bears a known relationship to the sensor for reflected signal. 
         [0030]    In one example, as illustrated in  FIG. 4 , a signal and noise evaluation circuit  400  is used to measure both reflected signals and noise from the same sensor (not shown). The output  402  of the sensor is received into an amplifier  410 , the output  412  of which is fed to an amplifier  410 , which is coupled by a capacitor  411  to a second amplifier  414 . The output of the second amplifier  414  is fed to one input  416   b  of a comparator  416  for comparing the received signal (such as  108  in  FIG. 1   a  or  140  in  FIG. 1   b ) with a threshold voltage at another input  416   a . The output  418  of the comparator  416  is at one level that is indicative of the presence of an object or a retro target when the magnitude of the signal at signal input  416   b  is greater than the magnitude of the threshold level at the reference input  416   a ; the output  418  of the comparator  416  is at another level that is indicative of the absence of an object or a retro target when the magnitude of the signal at signal input  416   b  is smaller than the magnitude of the threshold level at the reference input  416   a.    
         [0031]    The remainder of the circuit shown in  FIG. 4  generates the threshold voltage threshold voltage from the output of the second amplifier  414  during, for example, the noise detection period. An amplifier  420 , together with resisters R f    422  and R in    424 , form a third amplifier for amplifying noise signal from the second amplifier  414  and outputting the amplified signal to the input  430   a  of a peak detector circuit  428 , which includes a comparator  430 , diode  432 , switch  440  (which can be an electronic switch controlled by a microprocessor or microcontroller), output resistor  450  and capacitor  460 . The resistor R in    424  is biased at terminal  426  at a voltage corresponding to the noise-free dart detect threshold ( 640  in  FIG. 6 ) so that the peak detector circuit  428  detects the peak noise only when the noise level is sufficiently above the noise-free dark detect threshold. The this example, the output resistor  450  is connected between the output  428   a  of the peak detector circuit  428  and a fixed voltage supply  450   a , which in this case is biased at the same voltage as the terminal  426  of the resister R in  ( 424 ), i.e., a voltage corresponding to the noise-free dart detect threshold  640 . The peak detector circuit  428  is put in a peak-detect mode when the switch  440  is put in a closed position  440   a  and connects the cathode of the diode  433  to the input  430   b  of the comparator  430 . 
         [0032]    In operation, in one example, the switch  440  sets the peak detector circuit  428  in the peak-detect mode during a noise detection period when the sensor is in the dark state. In the peak-detect mode, the output  428   a  of the peak detector circuit  428  is at a level  620  (see  FIG. 6 ) that approximately the peak noise level. When it is time to fire the emitter and detect the received signal, the switch  440  is switched to an open position  440   b . The voltage at the output  428   a  is initially at approximately the peak noise level but decays toward a voltage corresponding to the noise-free dark detect threshold  640  with a time constant determined by the capacitance value of the capacitor  460  and resistance value of the output resister  450 . For a sufficiently short period of time for received signal detection, the voltage at the output  428   a , and therefore the input  416   a  of the comparator  416 , is approximately the peak noise level. The dark detect threshold level  620 , or the threshold hysteresis  650 , is therefore dynamically set according to the level of noise present. 
         [0033]    As illustrated in  FIG. 5 , noise can be measured in a noise measurement time interval (or “noise evaluation window”)  520  within one or more repetition time intervals  214 . In one example, noise measurement can be done by peak detection, which is performed by a noise detection circuit, such as the one shown in  FIG. 4 . The noise measurement time interval  520  can begin after the signal measurement time interval  232 , preferably with an intervening period  510  to allow any remaining signal left over from the signal measurement time interval  232  to settle. 
       CONCLUSION 
       [0034]    A method and device have been disclosed, in which a threshold level for determining that a signal has been received is dynamically set based on the noise level detected. For determining that an object is present (or, alternatively, that no object is blocking a transmitted light or light reflected from a retro target) a higher threshold level is used; a lower threshold level is used when no noise is present. The increased threshold hysteresis improves the reliability of object detection. 
         [0035]    Because many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.