Abstract:
An apparatus and method is provided for detecting motion or displacement of an object in a monitored zone. The apparatus is disposed between a load and a power source and comprises a transmitter for providing a pulsed signal within a monitored zone. The pulsed signal interacts with objects in the monitored zone and provides a return signal. A receiver receives echoes from a return signal of the pulsed record signal, and a microcontroller circuit processes the echoes. The processing involves retrieving and comparing phase and amplitude information associated with the echoes.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to a method and system for controlling lighting fixtures in a room via a motion sensor. More particularly, the invention relates to the detection of displacement in a room using ultrasonic pulses and envelope detection techniques to accurately detect displacement in favorable and unfavorable environments.  
         BACKGROUND OF THE INVENTION  
         [0002]    Many commercial, industrial, and government facilities require a significant number of lighting fixtures for adequate illumination, and therefore use a significant amount of power to operate the fixtures. In an effort to reduce costs in powering the light fixtures, as well as address environmental conservation concerns, a number of lighting control systems are used which employ sensors to automatically and selectively power the light fixtures on and off. Such lighting control systems are especially useful to automatically power down lights used infrequently, and thereby minimize lights remaining on unnecessarily after users have vacated the area. Thus, lighting control systems can provide significant energy and cost savings.  
           [0003]    Currently, different types of occupancy sensors such as passive infrared (“PIR”) ultrasonic, microwave and acoustic sensors, for example, are used for lighting control systems. The PIR sensor activates lighting fixtures whenever a moving or additional heat source is detected. The ultrasonic sensor emits ultrasonic vibrations at frequencies of 25 kHz or higher and listens to the return of echoes. If a significant Doppler shift is detected, it indicates a high probability that there is movement in the room. The lighting fixtures are then activated in response to the detected movement. Based on a preset time interval, the light fixtures are activated to illuminate the room for a period of time that is typically between three and sixty minutes in duration. The motion sensitivity of the sensors is usually set by users upon the initial installation of the sensors.  
           [0004]    PIR sensors, however, are characterized by a number of disadvantages. First, PIR sensors cannot detect motion behind barriers in a room. For instance, if a secretary is standing behind a file cabinet, the PIR sensor cannot detect motion occurring behind the file cabinet. Therefore, it may appear to the sensor that the secretary is no longer in the room, and the lights will be powered off once the preset time period for illumination has expired.  
           [0005]    Secondly, PIR sensors are susceptible to “dead spots” which are areas in the room where the PIR sensors are less sensitive to heat sources. The dead spots usually occur in areas that have obstructions or at the fringes of the range of the PIR sensor.  
           [0006]    Ultrasonic sensors suffer from the following disadvantages. Firstly, ultrasonic sensors are subject to false tripping where the lights can be powered based on false readings. The cause of false tripping is usually heating and air conditioning units moving air flow. The change in air temperature effects the return echoes by introducing phase and amplitude changes which, in turn, changes the arrival time of the echoes. Since the echoes do not arrive when expected, the ultrasonic sensors assume that movement has been detected in the room.  
           [0007]    Secondly, ultrasonic sensors typically use continuous wave ultrasonic signals. Ultrasonic sensors using continuous wave signals respond to any detected motion in a room. There is no discrimination between a small object close to the ultrasonic sensor and a larger object that is further away. In other words, there is no range discrimination using continuous wave ultrasonic signals.  
           [0008]    Thirdly, ultrasonic sensors do not perform as well in noisy environments. The noise can give false readings, causing the lights to power off at an inappropriate time.  
           [0009]    Fourthly, conventional ultrasonic sensors draw a lot of current. It would be preferable to operate an ultrasonic sensor with as little current as necessary.  
           [0010]    Therefore, a need exists for an occupancy sensor that can detect objects behind obstacles in a room. The occupancy sensor should also be able to address dead spots in a room. In addition, the occupancy sensor should also be able to address the problems associated with the effects of heating and air conditioning on air flow. Further, the occupancy sensor should be able to operate in noisy environments, as well as draw minimal current.  
         SUMMARY OF THE INVENTION  
         [0011]    The above and other objectives are substantially achieved by an apparatus and method employing a circuit for detecting motion within a monitored zone.  
           [0012]    The apparatus is disposed between a load and a power source and comprises a transmitter for providing a pulsed record signal within a monitored zone. The pulsed signal interacts with objects in the monitored zone and provides a return signal which is a record. A receiver receives echoes from a return signal of the pulsed signal, and a microcontroller circuit processes the echoes. The processing involves retrieving phase and amplitude information associated with the echoes.  
           [0013]    In accordance with an embodiment of the present invention, the microcontroller compares amplitude and phase information associated with echoes of a first record to amplitude and phase information associated with echoes of a second record.  
           [0014]    In accordance with another embodiment of the present invention, a difference in at least the phase or amplitude indicate that displacement of an object occurred in the monitored zone.  
           [0015]    In accordance with another embodiment of the present invention, the load is activated upon detection of displacement by the apparatus. The load can be associated with at least one of a lighting system, heating ventilation and air conditioning system, security system and the like.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The details of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:  
         [0017]    [0017]FIG. 1 illustrates a lighting control system mounted on a wall for controlling suspended lighting fixtures, and constructed in accordance with an embodiment of the present invention;  
         [0018]    [0018]FIG. 2 is a schematic diagram of an envelope detection circuit used to determine the displacement of an object for the lighting control system of FIG. 1 in accordance with an embodiment of the present invention;  
         [0019]    [0019]FIG. 3 is an output signal for the envelope detection circuit in accordance with an embodiment of the present invention;  
         [0020]    [0020]FIGS. 4A and 4B depict, respectively, a modified exclusive OR circuit for the envelope detection circuit and associated signals in accordance with an embodiment of the present invention;  
         [0021]    [0021]FIG. 5 is graph from an oscilloscope showing various output signals for the envelope detection circuit in accordance with an embodiment of the present invention;  
         [0022]    [0022]FIG. 6 is a microcontroller for using envelope detection to determine displacement of an object in accordance with an embodiment of the present invention; and  
         [0023]    [0023]FIG. 7 is a flow chart of a method for using envelope detection to determine displacement of an object in accordance with an embodiment of the present invention. 
     
    
       [0024]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    A switching control system  10  constructed in accordance with the present invention is shown in FIG. 1. The switching control system  10  is implemented with lighting fixtures for illustrative purposes and is therefore hereinafter referred to as a lighting control system  10 . The control system, however, can be used with a number of different types of loads such as heating ventilation and air conditioning (“HVAC”), security and temperature control systems. The lighting control system  10  is secured to a wall  12  preferably 41 to 53 inches vertically from the floor. The height is selected to enable the motion sensor (not shown) in the lighting control system to detect when an occupant  16  is walking in proximity of the sensor. However, it will be appreciated by those skilled in the art that the lighting control system  10  can be ceiling mounted without departing from the scope of the present invention. As will be described below, the lighting control system  10  controls the powering up and down of lighting fixtures  14  which are typically mounted overhead to a ceiling  18 .  
         [0026]    While the lighting control system  10  is shown in FIG. 1 secured to a wall in a room with ceiling mounted lighting fixtures, the system  10  can be installed in indoor areas, for use with or without overhead lighting fixtures (e.g., floor lamps can be used). In an embodiment of the invention, lighting control system  10  can be used in outdoor areas. Furthermore, lighting control system  10  can be mounted on various surfaces such as the ceiling or on a vertical support or an angled wedge and at various heights to detect, for example, persons sitting in or walking about the “lighted area”. The term “lighted area” defines the area served by the lighting fixtures  14  controlled by a lighting control system  10 , and does not necessarily imply that the fixtures  14  are powered up.  
         [0027]    The lighting control system  10  will now be discussed with reference to FIG. 2 which is a schematic diagram of an envelope detection circuit  20  used to determine displacement of an object by the lighting control system  10  of FIG. 1 in accordance with an embodiment of the present invention. Specifically, the envelope detection circuit  20  comprises a power supply circuit  22 , a timing circuit  30 , a transmitter driver circuit  36 , a phase lock loop chip  34 , an amplifier circuit  42 , a carrier half-wave rectification circuit amplifier  82 , a hard limited circuit amplifier  84 , and a modified exclusive OR circuit  46 , and a microcontroller  50 .  
         [0028]    The power supply circuit  22  comprises a first power regulator  24  connected to a capacitor C 12  and an adjustable power regulator  26  connected to capacitor C 13  and resistors R 24  and R 25 . The first power regulator  24  and adjustable power regulator  26  are connected to second power regulator  28  and to an external power source (not shown). A filter capacitor C 14  is connected across the input power source. A capacitor C 11  is connected across the output of the second power regulator  28 . The adjustable power regulator  26  is preferably an adjustable power regulator model no. LM317LZ, and first and second power regulators  26  and  28  are preferably a power regulator such as model no. LM78L05ACZ. Both types of regulators are manufactured by National Semiconductor Corporation.  
         [0029]    The power supply circuit  22  receives preferably about 12 to 24 volts DC. The first power regulator  24  preferably provides about five volts DC to circuits within the lighting control system requiring a DC input signal, including the microcontroller  50 . The adjustable power regulator  26  preferably provides about ten volts to the transmitter drive circuit  36  which drives the transmitter  38 .  
         [0030]    In an embodiment of the present invention, an alert indication can be provided by the microcontroller  50  to a user which indicates that the lighting control system  10  needs to be serviced. The alert indication can be a visual indication, audible indication or a combination of the two.  
         [0031]    The timing circuit  30  comprises a timing chip  32  which is preferably a timing integrated chip model no. 555 manufactured by Motorola Inc. of Schaumburg, Ill. The timing chip  32  is connected to capacitors C 1  and C 2 , resistor R 1 , adjustable resistor R 2 , resistor R 3  and adjustable resistor R 4 . An inverter  86 , which inverts an output signal from the timing chip  32 , provides an output signal I/O  1 . Output signal I/O  1  which is shown in FIG. 3 is preferably about 1.5 ms in duration and occurs in about 60 ms intervals.  
         [0032]    In accordance with an embodiment of the present invention, the timing function of timing circuit  30  can be implemented by microcontroller  50 . Using micro-controller  50  to perform the timing function provides for having an adjustable I/O  1  signal that can be adjusted to conform with a changing room size. For example, a threshold value is compared to the recovered echo to determine the size of the room based on the return time of the echo. For instance, some conference rooms can be sectioned off based upon the number of users in a room. When a large number of users are expected, the collapsible walls can be folded away to accommodate the large number of users. Thus, the microcontroller  50  can adapt to a changing environment based on a changing room size.  
         [0033]    The phase lock loop chip  34  operates in a conventional manner and is preferably a phase lock loop chip model no. MC14046B manufactured by National Semiconductor. The phase lock loop chip  34  is connected to resistors R 21  and R 22 , capacitor C 15  and adjustable capacitor C 9 . The adjustable capacitor C 9  is capable of adjusting the frequency of the signal provided by the phase lock loop chip  34 . The frequency provided by the phase lock loop chip  34  is preferably about 32.8 kHz. This frequency can be higher or lower than 32.8 kHz. However, using a lower frequency can affect users with hearing aids.  
         [0034]    The output signal from the phase lock loop chip  34  and the output signal from inverter  86  is provided to NAND gate  68 . NAND gate  68  is a conventional NAND gate and gates the 32.8 kHz signal from oscillator chip  34  to the transmitter drive circuit  36 . In accordance with an embodiment of the present invention, the signal outputted from NAND gate  68  is a gated 1.5 ms burst every 60 ms. Specifically, inverter  86  disables NAND gate  68  except for when inverter  86  outputs signal I/O  1  and allows the 32.8 kHz signal from the phase lock loop chip  34  to pass through NAND gate  68 . It will be appreciated by those skilled in the art that, although the transmit signal is described as a burst, it can also be a chirp that changes in frequency over time.  
         [0035]    A resistor R 6  connects NAND gate  68  with a transistor Q 1 . NAND gate  68  provides the 32.8 kHz burst signal to transistor Q 1 . It is the 32.8 kHz burst signal which drives transistor Q 1 . Transistor Q 1  is connected to NAND gate  70 , which is also connected to NAND gate  74 . Transistor Q 1  is also connected to NAND gate  72  and capacitor C 3 . Capacitor C 3  is connected to the transmitter  38  on one side, and NAND gate  74  is connected to the transmitter  38  on a second side.  
         [0036]    NAND gate  70 , NAND gate  72 , NAND gate  74  and capacitor C 3  comprise driver circuit  36 . NAND gates are used in driver circuit  36  to drive one side of the transmitter  38  high when the other side of transmitter  38  is low and vice versa. Transmitter  38  can be driven from either direction so that when pin  10  of NAND gate  74  is high and pin  11  of NAND gate  72  is low, there is preferably about 10 volts from pin  10  of NAND gate  74  to pin  11  of NAND gate  72 . Similarly, when pin  11  of NAND gate  72  is high and pin  10  of NAND gate  74  is low, there is preferably about 10 volts from pin  11  of NAND gate  72  to pin  10  of NAND gate  74 . Specifically, transmitter driver circuit  36  provides 20 volts peak to peak using a 10 volt power supply.  
         [0037]    Transmitter  38  is a conventional ultrasonic transducer that outputs preferably a 32.8 kHz, 1.5 ms burst that occurs preferably about every 60 ms as shown in FIG. 5 waveform  62 . Transmitting a signal burst requires less current than providing a continuous signal. The prior art uses a continuous signal, and, thus, requires more current.  
         [0038]    Initially, the first few transmit records can be used to estimate the room size and determine the position of objects that are presently in the room.  
         [0039]    The room echo is received at receiver  40 . Receiver  40  is a conventional receiver and provides the echo to amplification circuit  42 . Amplification circuit  42  comprises amplifier  78  and amplifier  80 . amplifier  78  is a first stage amplifier connected to resistors R 7 , R 8 , R 9 , R 10 , and capacitor C 4 . Specifically, amplifier  78  is preferably a 32.8 kHz carrier amplifier.  
         [0040]    In accordance with an embodiment of the present invention, the received echo can be amplified using amplifier  78  and a band pass filter. For example, a feedback capacitor can be connected across resistor R 10  to provide a band pass filter.  
         [0041]    The amplified output of amplifier  78  is preferably provided to a second stage amplifier. The second stage amplifier, which is amplifier  80 , further amplifies the output from amplifier  78 . Amplifier  80  is connected to resistors R 11 , R 12 , R 13 , R 14  and C 5 . In accordance with an embodiment of the present invention, a band pass filter can also be used with amplifier  80  via a capacitor across R 14 . It will be appreciated by those skilled in the art that amplification of the received echo can be performed using a single amplifier without departing from the scope of the present invention.  
         [0042]    The amplified signal from amplifier  80  is provided to amplifier  82  and to amplifier  84 . Amplifier  84  is connected to resistors R 15 , R 16  and C 6  and is a hard limited amplifier. The gain is high which is an open loop. Information from amplifier  84  is contained in the zero crossings from the output signal.  
         [0043]    Amplifier  82  is connected to variable resistor R 17 , resistors R 18 , R 19  and capacitor C 7  and performs half-wave carrier rectification on the amplified signal from amplifier  80  and also removes the DC offset voltage from the signal.  
         [0044]    The rectified signal from amplifier  82  is provided to low pass filter circuit  44  comprising resistor R 20  and capacitor C 8 . Low pass filtering the rectified signal removes the high frequencies from the signal producing output I/O  2  which is the amplitude envelope of the rectified signal. The current envelope from the current echo is compared to the previous echo from the previous record by the microcontroller  50 . Specifically, the micro-controller  50  looks for changes in amplitude between the current and previous amplitude envelopes. For example, in FIG. 5, waveform  64  shows an increase in amplitude about 20 ms after the effects of the transmitted signal dissipates for the current amplitude envelope. Microcontroller  50  then determines, based on a threshold value for changes in amplitude, whether motion has occurred in the room.  
         [0045]    The phase lock loop chip  34  also provides a 32.8 kHz output signal to inverter  88 . Inverter  88  provides an inverted 32.8 kHz signal to NAND gate  76  which is combined with an output signal from amplifier  84 . The output from NAND gate  76  is provided to inverter  90  which inverts the signal. The inverted signal from inverter  90  is provided to low pass filter circuit  46  which comprises resistor R 20  and capacitor C 10 . The modified exclusive OR circuit is shown in FIG. 2 and in greater detail in FIG. 4A where the low pass filtered signal is represented as output I/O  3  which is the phase envelope of the hard limited signal from amplifier  84 .  
         [0046]    In an embodiment of the present invention, the function of the modified exclusive OR circuit  48  can be performed using a quad exclusive OR integrated circuit, for example chip model no. MC14070B manufactured by National Semiconductor. The quad exclusive OR integrated circuit can be used to replace inverters  88  and  90  and NAND  76 . In addition, the quad exclusive OR integrated circuit can be used to replace NAND  68  and inverter  86 .  
         [0047]    [0047]FIG. 4B is an output signal for the modified exclusive OR circuit  48  for the envelope detection circuit  20  in accordance with an embodiment of the present invention. Input signal A is the output of the phase lock loop chip  34  which is preferably a 32.8 kHz signal, and input signal B is the output from the hard limited circuit amplifier  84 , inverter  88  inverts the input signal A and provides output signal C, which is an inverted input A signal.  
         [0048]    As the phase difference between inputs A and B increases, the output signal E becomes larger. That is, as input signal B shifts to the right in the direction of the arrows relative to signal A, the output signal E becomes wider. Therefore, the wider the output signal E, the larger the phase difference between input signals A and B.  
         [0049]    Table 1 is a truth table showing the relationship between input signals A, B, and output signals D and E and a conventional exclusive OR circuit.  
                                           TABLE 1                                   IN   OUT   IN   OUT   OUT   XOR           A   C   B   D   E   Exc OR                           0   1   0   1   0   1           0   1   1   0   1   0           1   0   0   1   0   0           1   0   1   1   0   1                      
 
         [0050]    Since NAND gate  76  is a NAND gate, the output will always be high except when both inputs are high. When both inputs are high, the output of NAND gate  76  will go low, as reflected in Table 1, where input signals C and B are high and the output signal D is low. Inverter  90  is an inverter and inverts the output values for D and provides output signal E. Output signal E is only high when both input signals are high. The output of inverter  90  is different from a traditional exclusive OR gate, the output of which is only high when one of the input signals are high. If both input signals have the same value, then the exclusive OR output is low.  
         [0051]    Referring now to FIG. 5 which shows a graph from an oscilloscope showing various output signals for the envelope detection circuit  20 , waveform  62  is the output I/O  1 , waveform  64  is output I/O  2  and waveform  66  is output I/O  3 . Waveform  62  is the 1.5 ms enabling gate signal for the 32.8 kHz burst signal sent from transmitter  38 . Waveform  64  is the amplitude envelope of the return echo for the 1.5 ms burst signal. The dotted lines show the amplitude envelope for a subsequent return echo. As can be seen, the amplitude for the subsequent return echo is much larger than the amplitude of the previous return echo, in the area of the echo record corresponding to the distance from the sensor to where the motion occurred. This implies that there is movement in the room. However, a change of amplitude for the return echo envelope can result from moving air or turbulence and homogeneities in temperature and relative humidity of the air, which results in interference, scattering and refraction of the transmitted signal in the room. For example, the air conditioning system could have been turned on. The changes affect the echoes returning to the receiver  40 .  
         [0052]    Output I/O  3  can be used by the microcontroller  50  to detect motion in a room also. The phase envelope of the previous record is compared to the phase envelope of the present record. The solid line for waveform  66  is the previous phase envelope for the previous record, and the dotted line is the phase envelope for the current record. If there was no change in phase, the dotted line and solid line should be superimposed on each other. Since there is a noticeable shift, it indicates that there is motion in the room.  
         [0053]    The microcontroller  50  can compare the results from I/O  2  and I/O  3  to determine whether there was any displacement in the room. For example, in accordance with an embodiment of the invention, at a specific location in the echo record, a significant difference in phase, but no significant difference in amplitude, can be an indication of a false reading. In accordance with another embodiment of the invention, a significant difference in amplitude, but no significant difference in phase, can be an indication that there is a probability of displacement. In accordance with still another embodiment of the invention, no significant difference in amplitude or phase indicates a high probability that no displacement occurred. In accordance with another embodiment of the invention, a significant difference in amplitude and phase, can be an indication that there is a high probability that displacement occurred.  
         [0054]    Turning to FIG. 6, an alternative embodiment for performing the amplitude and phase envelope detection functions of the envelope detection circuit  20  is depicted. Specifically, FIG. 6 depicts the microcontroller  50  suitable for use in the lighting control system  10 . The microcontroller  50  comprises a microprocessor/Digital Signal Processor(DSP)  52 , as well as memory  54  for storing programs for performing various envelope detection functions. The microprocessor/DSP  52  cooperates with conventional support circuitry  56  such as power supplies, clock circuits, analog to digital (A/D) and digital to analog (D/A) conversion circuitry, filtering circuits such as high pass, low pass and the like, as well as circuits that assist in executing the envelope detection functions of the present invention. A user interface device  58  such as a sensitivity adjuster is provided to adjust the sensitivity of the lighting control system  10 . In accordance with an embodiment of the invention, the sensitivity adjuster can comprise, but is not limited to, a potentiometer, a dip switch and a key pad.  
         [0055]    The microcontroller  50  also comprises input/output circuitry  60  that forms an interface between the microprocessor  52 , transmitter driver circuit  36 , transmitter  38  and receiver  40 . The input/output circuitry  60  can interface with the lighting fixtures  14  such that the lighting fixtures can be powered on when displacement is detected. The lights will remain on as long as the displaced object or person remains in the room or movement of the displaced object or person is detected within a predetermined time interval.  
         [0056]    The microcontroller  50  is depicted as a general purpose computer that is programmed to perform, in general, the envelope detection functions of the envelope detection circuit  20 . Specifically, the microcontroller  50  performs the timing functions of timing circuit  30  and NAND gate  68 , the oscillator function of the phase lock loop chip  34 , the carrier rectification functions of amplifier  82 , the hard limiter functions of amplifier  84 , the low pass filtering of low pass filter  44 ,s filter  44 , and the modified exclusive OR functions of modified exclusive OR circuit  48 , in accordance with the present invention. The invention, however, can be implemented in hardware, in software, or a combination of hardware and software. As such, the envelope detection functions described above with respect to the various figures are intended to be broadly interpreted as being equivalently performed by software, hardware, or a combination thereof.  
         [0057]    The present invention will now be discussed with reference to FIG. 7. FIG. 7 is a flow chart of a method  92  for using envelope detection to determine displacement of an object in accordance with an embodiment of the present invention. The method  92  is initiated with a burst being transmitted by the transmitter  38  at step  94 . If this is the first time the lighting control system is being used in the room, a series of burst signals will be sent to form an image of objects presently in the room. It should be appreciated that bursts are being transmitted and not a continuous 32.8 kHz signal as in the prior art.  
         [0058]    At step  96 , the echo of the burst is received by the receiver  40 . Depending on how the envelope detection circuit  20  is designed and optioned, a portion of the return echo can be discarded. Although a single pulse is transmitted, the echo continues to return over a 60 second record from various parts of the room. For example, the transmit pulse can encounter a chair in the front of the room and later encounter the back wall. The echo from the chair will return first and the echo from the back of the wall will return later in time.  
         [0059]    If transmit pulses occur too frequently, it is possible that the next transmit pulse can encounter the echo from the chair. If this occurs, there can be interference. Thus, the echo return record length must be long enough for room echoes to dissipate. Dissipation of the room echoes takes about 60 ms for most rooms. During the first 10 ms of the record, the transmitted signal overloads the receiver front end. Hence part of the record is ignored.  
         [0060]    At step  98 , the received echo is amplified. The amplification can be performed in two stages or in a single stage. In accordance with an embodiment of the present invention, a band pass filter is used to reduce noise and impairments in the return echo.  
         [0061]    At step  100  the amplified echo is carrier rectified and low pass filtered in order to remove the carrier and look at the amplitude envelope.  
         [0062]    At step  102 , the amplitude of the current envelope for the current record is compared to the amplitude of an echo for the previous record. Changes in amplitude between the two envelopes can indicate that a displacement occurred in the room.  
         [0063]    At step  104 , the amplified echo from step  102  is hard limited to examine the zero-crossings of the amplified signal. The hard limited signal is provided to the modified exclusive OR circuit at step  106  where the phase envelope for the return echo is retrieved.  
         [0064]    At step  108 , the phase of the echo for the current record is compared to the phase of the echo for the previous record. If there is a difference in phase between the two envelopes it indicates that displacement occurred in the room.  
         [0065]    It should be appreciated by those skilled in the art that steps  100  and  102  can be done in parallel in real time with steps  104 ,  106  and  108  without departing from the scope of the present invention.  
         [0066]    At step  110 , the microcontroller  50  compares the results from the amplitude envelope and the phase envelope to determine whether a displacement actually occurred. Having two means of determining whether displacement occurred eliminates many of the problems that occur when a lighting control system is used in a noisy or changing environment. As each pulse is transmitted, the returning echo is compared to the echo of a previous record.  
         [0067]    In accordance with an embodiment of the present invention, the envelope detection circuit  20  can store the results of a number of comparisons to get an improved estimate of whether displacement occurred.  
         [0068]    Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention can be described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification and the following claims.