Abstract:
An ultrasonic transmitter of an ultrasonic occupancy sensing device has adjustable ultrasonic signal output amplitude to prevent overload of an ultrasonic sensor associated with the ultrasonic occupancy sensing device. A circuit for controlling the operating voltage to a power driver of the ultrasonic transmitter allows field adjustment of the output thereof so that an optimal level (amplitude) for the transmitted ultrasonic signal may be found in an area of actual use (e.g., field adjustable).

Description:
TECHNICAL FIELD 
     The present invention relates generally to ultrasonic occupancy sensing, and more particularly, to controlling the output of an ultrasonic transmitter used in an ultrasonic occupancy sensing device. 
     BACKGROUND 
     Ultrasonic occupancy sensing devices are used to detect the presence of moving objects such as a person entering an area of interest, e.g., a room, and when such movement is detected perform a function such as turning on lights in the room. The ultrasonic occupancy sensing device radiates (transmits) high frequency sound waves that are undetectable to the human ear. These sound waves bounce off surfaces, including people. Motion is detected via shifts in frequency that are detected as “Doppler shift” when receiving the reflected sound waves and comparing the frequency thereof to the transmitted sound wave frequency in a frequency mixer and then through a low pass filter. 
     The relative acoustic strength of the high frequency sound waves is affected by many factors including square footage of desired coverage, partitions, drapes, carpeting, furniture, potential reflection patterns, and the efficiency of the transducer converting electrical energy into acoustic energy. If adjustments in detection sensitivity are required, present technology ultrasonic occupancy sensing devices use either a potentiometer (manual) or algorithms in a microcontroller (automatic) to adjust the amplitude of the received signal. Adjusting the amplitude of the received signal is critical to avoid saturation, e.g., overload, of the receiver circuitry and to accommodate various noise sources such as heavy airflow from a supply register in the ceiling and/or wall. 
     As multiple ultrasonic occupancy sensing devices are added to adjacent spaces, the total amount of ultrasonic energy increases. The ultrasonic total energy can saturate the controlled areas to the point where manual adjustments become very difficult. The ultrasonic sensors have a synergistic effect that increases with the increase in acoustic energy, making adjustment of the received signal strength more and more difficult. The increase in ultrasonic signal density also makes isolating control of discreet areas very difficult and inconsistent over time as conditions change. Variations in performance are observed as the configuration of an area changes, such as differing number of perimeter doors being closed at different times. 
     Some ways to mitigate the aforementioned problems have been to use different types of ultrasonic occupancy sensing devices designed for specific size areas with transmitter output amplitudes appropriate for the square footage of the specific area of use. Use of ultrasonic transducers, each driven at a different ultrasonic frequency have been used so that the ultrasonic energies do not accumulate and saturate the total areas being monitored and controlled. Since frequencies and ultrasonic power outputs are not selectable, a plurality of different ultrasonic occupancy sensing devices must be used. 
     SUMMARY 
     Therefore to mitigate the aforementioned problems, it is desirable to be able to reduce the output of an ultrasonic transmitter so as not to overload the ultrasonic sensors. According to the teachings of this disclosure, a circuit for controlling the operating voltage to a power driver of the ultrasonic transmitter allows field adjustment of the output thereof so that an optimal level (amplitude) for the transmitted ultrasonic signal may be found in the area of actual use (e.g., field adjustable). 
     According to a specific example embodiment of this disclosure, an ultrasonic transmitter having adjustable output amplitude comprises: a carrier oscillator; a power driver having an input coupled to the carrier oscillator; an acoustic transducer coupled to the output of the power driver; and a controlled voltage supply having an adjustable voltage output coupled to the power driver, wherein an amplitude of an acoustic signal from the acoustic transducer is controlled by adjusting the voltage output from the controlled voltage supply. 
     According to another specific example embodiment of this disclosure, a method for adjusting output amplitude of an ultrasonic transmitter comprises the steps of: driving an acoustic transducer with a power driver; driving the power driver with a carrier oscillator; and adjusting a voltage supply having a variable voltage output coupled to the power driver, wherein an amplitude of an acoustic signal from the acoustic transducer is determined by a value of the variable voltage output from the controlled voltage supply. 
     According to yet another specific example embodiment of this disclosure, an ultrasonic occupancy sensing system comprises: a carrier oscillator; a power driver having an input coupled to the carrier oscillator; an acoustic transducer coupled to the output of the power driver; a controlled voltage supply having an adjustable voltage output coupled to the power driver, wherein an amplitude of an acoustic signal from the acoustic transducer is controlled by adjusting the voltage output from the controlled voltage supply; an acoustic sensor; a bandpass frequency filter coupled to the acoustic sensor; a programmable gain amplifier (PGA) having an input coupled to the bandpass frequency filter; a frequency mixer having a first input coupled to an output of the PGA and a second input coupled to the carrier oscillator; a frequency filter coupled to an output of the frequency mixer; a Doppler shift detector having an input coupled to the frequency filter; motion determining logic coupled to an output of the Doppler shift detector; a receiver sensitivity adjustment circuit coupled to the PGA for controlling gain of the PGA; and an acoustic power adjustment circuit coupled to the controlled voltage supply for controlling a voltage therefrom, whereby acoustic power output from the acoustic transducer is adjusted; wherein when a Doppler shift is detected from a difference between a signal frequency from the carrier oscillator and a received signal frequency from the acoustic sensor an external load control is actuated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description, in conjunction with the accompanying drawings briefly described as follows. 
         FIG. 1  illustrates a schematic plan view of an area having ultrasonic occupancy sensing devices installed for detecting motion therein; 
         FIG. 2  illustrates a schematic block diagram of an ultrasonic occupancy sensing device as utilized in  FIG. 1 , according to the teachings of this disclosure; 
         FIG. 3  illustrates a more detailed schematic diagram of an ultrasonic transducer, power driver and a voltage control circuit shown in  FIG. 1 , according to a specific example embodiment of this disclosure; and 
         FIG. 4  illustrates a more detailed schematic diagram of an ultrasonic transducer, power driver and a voltage control circuit shown in  FIG. 1 , according to another specific example embodiment of this disclosure. 
     
    
    
     While the present disclosure is susceptible to various modifications and alternative forms, specific example embodiments thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific example embodiments is not intended to limit the disclosure to the particular forms disclosed herein, but on the contrary, this disclosure is to cover all modifications and equivalents as defined by the appended claims. 
     DETAILED DESCRIPTION 
     Referring now to the drawings, details of example embodiments of the present invention are schematically illustrated. Like elements in the drawings will be represented by like numbers, and similar elements will be represented by like numbers with a different lower case letter suffix. 
     Referring to  FIG. 1 , depicted is a schematic plan view of an area having ultrasonic occupancy sensing devices installed for detecting motion therein. An area  108 , e.g., conference room, office, closet, bathroom, etc., is shown having two access doors  106 , a plurality of light fixtures  104 , windows  112 , and an ultrasonic occupancy sensing device  102  that transmits and receives ultrasonic acoustic waves  110 . The location of the ultrasonic occupancy sensing device  102  is selected for optimum coverage and sensing of movement in the area  108 , e.g., people entering and occupying the area  108 . 
     Referring to  FIG. 2 , depicted is a schematic block diagram of an ultrasonic occupancy sensing device as utilized in  FIG. 1 , according to the teachings of this disclosure. The ultrasonic occupancy sensing device  102  comprises an acoustic sensor  234 , a bandpass filter  212 , a programmable gain amplifier (PGA)  214 , a frequency mixer  216 , a lowpass or bandpass filter  218  (hereinafter “frequency filter  218 ”), a Doppler shift detector  220 , motion determining logic  222 , an ultrasonic frequency carrier oscillator  228 , a power driver  226 , an acoustic transducer  224 , a controlled voltage supply  230 , and a digital processor  232 . The acoustic transducer  224  generates the ultrasonic acoustic waves  110  at a frequency determined by the carrier oscillator  228 . The power driver  226  amplifies the signal frequency from the carrier oscillator  228  sufficiently to cause the acoustic transducer  224  to generate the ultrasonic acoustic waves  110  at a desired amplitude. The controlled voltage supply  230  provides for settable control of the output from the power driver  226  to the acoustic transducer  224  so as to obtain the desired amplitude of the ultrasonic acoustic waves  110 . 
     The acoustic sensor  234  receives the ultrasonic acoustic waves  110  (both direct and reflected) and converts them into electric signals that are applied to the bandpass filter  212 . The bandpass filter  212  restricts alternating current energy therethrough to frequencies within the bandpass of the filter  212 , e.g., about 1 kilohertz (kHz) bandwidth, centered at the frequency of the carrier oscillator  228 , e.g., 25 to 27 kHz, 32.768 kHz, 40 kHz, etc. The gain of the PGA  214  is controlled by the digital processor  232  so as to set the receive sensitivity of the ultrasonic occupancy sensing device  102 . The greater the receive sensitivity, the greater the range of motion detection, but also the greater the chance of nuisance tripping from noise sources, e.g., supply air ducts, adjacent ultrasonic occupancy sensing devices (not shown), etc. 
     The amplified received signal from the PGA  214  is applied to the frequency mixer  216  where it is mixed with a signal from the carrier oscillator  228 . The frequency mixer  216  produces signals at the sum and difference frequencies of these two input signals and feeds them to the input of the frequency filter  218 . The frequency filter  218  removes the sum frequency, generally twice the frequency of the signal from the carrier oscillator  228  and passes the difference frequency to the Doppler shift detector  220 . When there is no movement in the area  108  there will be no frequency change (Doppler shift) in the reflected receive signal (generated by the ultrasonic acoustic waves  110 ), therefore, the difference frequency will be substantially zero (0) Hertz and the Doppler shift detector  220  will have substantially no output signal therefrom. However, when there is movement in the area  108  the reflected receive signal (generated by the ultrasonic acoustic waves  110 ) will be shifted in frequency and the difference frequency from the mixer  216  will be greater than zero (0) Hertz, thereby causing the Doppler shift detector  220  to generate an output signal therefrom. This output signal from the Doppler shift detector  220  is processed in the motion determining logic  222  that will generate a control signal for controlling an external load, e.g., turning on the light fixtures  104  in the area  108 . 
     The digital processor  232  may be used to control both the gain of the PGA  214  and the voltage level to the power driver  226 . A single adjustment control  236  may be used to concurrently control both the gain of the PGA  214  and the voltage level to the power driver  226  (for amplitude control of the ultrasonic acoustic waves  110 ) for ease and simplicity in making range adjustments in the field. Alternatively, a separate control  238  may be used for control of the voltage level to the power driver  226  thereby allowing independent control of the gain of the PGA  214  (receive sensitivity) and transmitted amplitude of the ultrasonic acoustic waves  110 . The digital processor  232  may also perform the functions of the motion determining logic  222 , thereby incorporating those functions into a single integrated circuit device, e.g., a mixed signal (both analog and digital) microcontroller. 
     Referring to  FIG. 3 , depicted is a more detailed schematic diagram of an ultrasonic transducer, power driver and a voltage control circuit shown in  FIG. 1 , according to a specific example embodiment of this disclosure. The power driver  226  may comprise inverters  226   a  and  226   b  for driving the acoustic transducer  224  input terminals alternately high and low at the frequency of the carrier oscillator  228 . A plurality of inverters  226   a  and  226   b  may be paralleled together for adequate drive power to the acoustic transducer  224 . A transistor  344 , e.g., a metal-oxide semiconductor field effect transistor (MOSFET), may be used as a buffer between the output of the carrier oscillator  228  and the inputs of the plurality of inverters  226   a  so as to provide a high impedance to the carrier oscillator  228  for reduced loading effect thereon. The resistors  340 ,  346  and  348  are used to provide proper biasing and drain pull-up for the transistor  344 . A filter capacitor  342  provides direct current filtering of the voltage to the plurality of inverters  226   a  and  226   b.    
     The output voltage from controlled voltage supply  230  is determined by a control signal from the digital processor  232 . This control signal may be analog or digital. The value of this output voltage determines the acoustic wave amplitude from the acoustic transducer  224 . The controlled voltage supply  230  reduces the voltage value of the voltage source to the lower voltage value desired for the acoustic wave amplitude, and may be any one of a number of open or even closed loop voltage regulator designs, e.g., linear and switch mode, as known to those skilled in the art of voltage regulator design. A more detailed embodiment for a controlled voltage supply  230  is described hereinafter. 
     Referring to  FIG. 4 , depicted is a more detailed schematic diagram of an ultrasonic transducer, power driver and a voltage control circuit shown in  FIG. 1 , according to another specific example embodiment of this disclosure. The carrier oscillator  228 , power driver  226  and acoustic transducer  224  function as described hereinabove. The controlled voltage supply  230  comprises switching transistors  454  and  456 , driver transistor  464 , diode  460 , and resistors  450 ,  452 ,  458 ,  462 ,  470  and  472 . The control signal from the digital processor  232  is a pulse width modulation (PWM) signal wherein the on and off duty cycle times of the PWM signal determine the average voltage at node  474  and the filter capacitor  342 . The digital processor  232  may easily and inexpensively provide this digital PWM control signal. When the PWM control signal is at a logic “1” transistor  456  is on and applies voltage from the voltage source to the node  474  and the capacitor  342 . Transistor  454  is off. When the PWM control signal is at a logic “0” transistor  456  is off and does not allow voltage from the voltage source to be applied to the node  474  and the capacitor  342 . The voltage at node  474  depends upon the “on” times of the transistor  456 . 
     Although specific example embodiments of the invention have been described above in detail, the description is merely for purposes of illustration. It should be appreciated, therefore, that many aspects of the invention were described above by way of example only and are not intended as required or essential elements of the invention unless explicitly stated otherwise. Various modifications of, and equivalent steps corresponding to, the disclosed aspects of the exemplary embodiments, in addition to those described above, can be made by a person of ordinary skill in the art, having the benefit of this disclosure, without departing from the spirit and scope of the invention defined in the following claims, the scope of which is to be accorded the broadest interpretation so as to encompass such modifications and equivalent structures.