Abstract:
An ultrasonic transceiver device and remote output devices controlled by the transceiver for use by domestic pets. The transceiver utilizes a fixed interval edge detect modulation system to lock out stray environmental noise thereby, avoiding false triggering and greatly increasing the working range. Output devices include: a wireless pet containment system in which ultrasonic transmitters are mounted on a series of posts defining the perimeter of the containment area and a receiver device adapted to be worn by a domestic animal. Upon receiving the transmitted ultrasound the receiver device sounds a warning tone followed by an electric shock to train the animal to stay within the perimeter; a sonic alarm system to deter a domestic animal from entering a restricted area whereby, an ultrasonic transmitter is worn by the animal and a receiver with an alarm output is placed in a stationary location to sound an alarm chasing the animal from the area in which the receiver is placed; a remote pet training device whereby a hand held transmitter is carried by a human trainer and a receiver unit is worn by the pet, the receiver produces a set of training tones in response to the modulated ultrasound produced by the transmitter; and a remotely operated pet door which automatically opens under its own power when it receives a signal from an ultrasonic transmitter worn by a pet.

Description:
FIELD OF THE INVENTION 
     The field of the invention pertains to an ultrasonic transceiver to control output devices for use by and for pets and more particularly to a transceiver with modulation characteristics to improve range and avoid false triggering in the control of a wireless pet containment system; a sonic alarm for area restriction; a remote pet trainer and a remotely operated, fully automatic pet door. 
     BACKGROUND OF THE INVENTION 
     This invention has the purpose of facilitating a means by which pets can self activate automatic devices and allow humans to operate the devices remotely. The scope of this invention includes a modulated ultrasonic transceiver utilizing a fixed duration edge detect means to extend the range of the device by distinguishing between a low level transmitted signal and environmental noise. This is employed to control a wireless pet containment system; a sonic alarm for area restriction; a remote pet trainer and a remotely operated, fully automatic pet door but, its uses can easily be extended to other output devices. 
     PRIOR ART 
     Conventional devices utilizing ultrasound for remote sensing, such as ultrasonic burglar alarms, have been severely limited by the directionality, acoustical shadowing and limited range inherently characteristic of ultrasonic devices. For that reason they have been largely replaced by radio frequency and infrared transceivers. 
     In the pet field, electronic containment systems and remote trainers use radio frequency based transmitters and receivers. These devices are expensive to produce and severely restricted by the fact that they create interference to other radio frequency transmissions. To avoid this problem, manufacturers must use extremely low power devices, practical for only very short range operation, or subject the design to restrictions placed upon it by regulating agencies such as the Federal Communications Commission. The use of ultrasound has been largely ignored as an alternative for the reasons listed above. 
     Electronic pet containment systems typically employ a radio frequency transmitter and an antenna in the form of a buried cable to enclose the perimeter of the containment area. As the pet wearing an R.F. receiver approaches the buried cable, a warning tone is sounded. Then, if the animal has not retreated from the perimeter after a predetermined time, an electric shock is delivered. Such systems present certain difficulties such as having to burry the cable. 
     Remote trainers typically use a hand held radio frequency transmitter to activate a single output (usually an electric shock or an irritating sound pulse) at the receiver worn by the pet. The single output of conventional systems is always used as a negative reinforcing stimulus rather than a positive enforcement of a specific behavior. 
     Conventional systems using sound to chase intruding animals from a restricted area use either infrared detection to sense the presence of the intruder (in much the same manner as a burglar alarm) or radio frequency. In addition to the fact that both such systems are relatively expensive to manufacture, the infrared system has the disadvantage that it will trigger on any warm body including humans and is therefore not selective. A system of this type may work well in keeping the family dog off the couch but, it would also drive the people of the household away form the couch as well. The radio frequency systems are selective but, to date are limited to a range of less than four feet to avoid expense and unwanted R.F. interference. 
     It would therefore be desirable to create a remote transceiver system for pets to facilitate containment, training and area restriction which has relatively long range capability, which does not interfere with radio frequency devices nor is subject to the governmental restrictions of radio frequency devices, which is selective to an individual transmitted signal and which is relatively inexpensive to mass produce. The invention described herein provides such a system. 
     The current state of the art with respect to electronic pet doors is limited to devices which are operated by a magnet or a very low power R.F. transmitter worn on a pet&#39;s collar. The limited range of these transmitters restricts the design to smaller units suitable only for cats or small dogs. The transmitter used in this type of device merely acts to unlock a hinged panel to be physically pushed open by the pet and returned by gravity or a spring mechanism. The pet door device presented here uses the transmitted ultrasound to trigger the activation of an electric motor and transport mechanism to move a panel in and out of the door opening in a somewhat similar fashion to that of a power car window thereby, facilitating added security from intruders and weather and automatic operation requiring no human effort. 
     SUMMARY OF THE INVENTION 
     This invention makes practical use of the directionality inherent in ultrasonic transmission and offers a novel means of modulating the transmitted signal to increase the working range from a conventional twenty-five feet to over three hundred feet. This is achieved by delivering the ultrasonic signal output in periodic bursts. The receiver is designed to detect these bursts by sensing a repeated low to high transition at the amplifier input at predetermined intervals corresponding to the periodic rate of generation of ultrasonic bursts at the transmitter. Thus, sporadic environmental noise in the frequency range of the transceiver is essentially filtered out allowing the amplifier in the receiver to trigger on extremely low level signals. 
     Applied to the development of a wireless pet containment system, the present invention requires no antenna nor buried cable and specifically optimizes and makes use of the inherent directionality of ultrasound to approximate a linear beam of ultrasound forming a wireless boundary. The signal modulation and edge detect receiver virtually eliminate the possibility of false triggering which would allow the device to needlessly shock the animal. Additionally, it is possible to simultaneously send two individually modulated ultrasonic signals, one of higher amplification for a wider transmitted beam and one of lower amplification for a narrower transmitted beam, to separately control the warning tone and shock outputs at the receiver, making it impossible for a clever animal to run through the perimeter and out of range during the time delay from warning tone to shock used by conventional devices. 
     The invention in its present form also provides a useful alternative to the use of R.F. transmission in the creation of a remote training device by greatly extending the range of conventional ultrasonic transceivers. The result is a remote training device with a useful range comparable to that of a conventional R.F. transceiver but, without the possibility of interfering with the reception of radio, television and other R.F. signals. The training device then uses one or more preset ultrasonic pulse rates to trigger one or more tones for use in obedience training of pets. 
     The directional characteristics of ultrasound are of great value applied to the operation of an automatic pet door by allowing an acceptable frontal activation range when the pet is facing and approaching receiver but, greatly reducing the probability of false triggering of the receiver when the pet is just passing by at even closer range than the frontal activation distance. The modulated pulsed signal and edge detect receiver technique are employed to create an ultrasonic key code so that the device may be operated only by the pet wearing a transmitter with those specific modulation characteristics to be sensed by the receiver. This type of pet door achieves the advantages of preventing stray animals and intruders from entering the home through the pet door (a common problem with conventional mechanical pet doors) and provides protection against strong wind which can easily blow through the hinged flap of a conventional pet door. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1 schematically illustrates the ultrasonic pet containment system. 
       FIG. 2 schematically illustrates the sonic alarm used for area restriction. 
       FIG. 3 schematically illustrates the remote pet trainer. 
       FIG. 4 schematically illustrates the automatic pet door. 
       FIG. 5 is a cross-section of the automatic pet door taken along lines  1 — 1    5 - 5  of FIG. 4. 
       FIG. 6 is a vertical cross section of the automatic pet door taken along lines  2 — 2    6 - 6  of FIG. 5. 
       FIG. 7(a) is a block diagram of the ultrasonic transmitter. 
       FIG. 7(b) is a block diagram of the ultrasonic receiver. 
       FIG. 8 schematically illustrates an alternative embodiment of the pet containment system. 
       FIG. 9(a) illustrates the burst modulated ultrasonic wave form typically emitted by the ultrasonic transmitter showing the high to low transitions sensed by the receiver in the absence of environmental background noise. 
       FIG. 9(b) illustrates the burst modulated ultrasonic wave form typically emitted by the ultrasonic transmitter showing the high to low transitions sensed by the receiver in the presence of background noise of the same carrier frequency as the transmitter with typical sporadic peaks which are of greater amplitude than the transmitted ultrasonic signal. 
       FIG. 10 is a circuit diagram of the operational amplifier employed as part of the ultrasonic receiver. 
       FIG. 11 is an alternate embodiment of the pet containment system using two separate signals to individually control warning tone and shock outputs. 
         FIG. 12  is a cross-section of the receiver assembly. 
       FIG. 13 is an alternate embodiment of the receiver assembly. 
       FIG. 14 is a cross-section of the transmitter casings and positioning post used in the pet containment system. 
       FIG. 15 is a circuit diagram of the ultrasonic transmitter with modulator. 
       FIG. 16A is a diagram of the automatic pet door control circuit power supply. 
       FIG. 16B is a diagram of the automatic pet door control circuit motor controller. 
       FIG. 16C B is a diagram of the automatic pet door control circuit operational amplifier. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is an ultrasonic transceiver designed specifically to control various output devices for use by domestic animals. The output devices, which are also the subject of this invention, include a wireless pet containment system; a sonic alarm for area restriction; a remote pet trainer and a remotely operated, fully automatic pet door. The basic ultrasonic transceiver, shown in block form in FIGS. 7(a) and (b), utilizes a conventional quartz crystal controlled oscillator  1  to generate the carrier frequency. Ultrasonic frequencies of 32.7 KHZ and 40 KHZ are convenient because microphones, piezo speakers and quartz crystals of these values are readily available. The oscillated signal is fed to a modulator  2 . FIG. 15 is an electrical schematic of the oscillator and modulator which together employ a “hex schmitt trigger” and “quad dual input schmitt NAND” integrated circuit to create a pulsed output consisting of periodic bursts of high frequency ultrasound of the type shown in FIG. 9(a). Various other modulation techniques may be employed to develop individual coded signals each to activate a separate functional output. These may include frequency modulation, amplitude modulation, pulse rate modulation, the generation of a pulse code or other techniques. The means employed in the embodiment of FIG. 15 simply varies the time interval between output bursts. The output amplitude is equal to the maximum plus and minus voltage supplied to the device which is typically limited to the maximum voltage rating of the integrated circuits of three to six volts direct current. The pulsed signal may be fed to an operational amplifier  3  to increase the working voltage supplied to the output speaker  4 . The preferred embodiment employs a narrow band piezo transducer for the speaker due to its high efficiency. The electric current demand of such a device may be easily optimized to less than fifty microamperes facilitating the use of a small light-weight power source  5 , such as a lithium coin cell battery, which can easily be worn by a pet with an acceptable useful life at the 50 microampere current consumption of six months to one year. 
     The receiver utilizes a narrow band piezo electric microphone  6  fed to a multi-stage operational amplifier  7  of the variety shown in FIG. 10. The use of discrete components, instead of commercial grade integrated circuits in this embodiment, allows quiescent electric current consumption of the receiver to be also limited to under 50 microamperes providing acceptable battery life from a light-weight lithium source  11 . 
     The demodulation technique employs an edge detector which senses the low to high transition of the initial burst received at the amplifier. FIG. 9(a) shows that in the absence of background noise, the transition is measured from the zero state to the peak amplitude of the initial burst. As shown in FIG. 9(b) background noise shifts the zero baseline at each edge to the background level. The low to high transition is then sensed as the difference between the background level and the peak amplitude of the pulse. Once the first edge is detected, a timer is started with its duration equal to the interval between bursts of the transmitted signal. While the timer is running, the sensor is disabled so changes in background are not mistaken for another edge. At the end of the timing period the sensor resumes within a narrow time window. If a second edge is sensed within this time window it is taken as receipt of the modulated signal the output device  9  is activated. The technique may be repeated for multiple edge detect cycles before the output is activated, for added insurance against the probability of detecting a transition in the background noise which happens to occur within the edge detect time window. This technique greatly increases the working range of the device since it requires only a very small transition in ultrasound amplitude to trigger the device. The magnitude of that transition may be much less than the background ultrasound level. The fact that environmental background noise is generally sporadic with large momentary peaks, represents a problem to conventional devices which trigger on fixed signal levels and are apt to confuse a large background peak with the actual transmitted signal. In this device, background noise may be much larger than the minimum edge amplitude and indeed higher, at its peak, than that of the transmitted signal itself without affecting the sensitivity or range of the device. Even in worst case conditions where some unusual source of background noise level is high and of long, steady duration, it may only serve to mask some of the transmitted signal ultimately reducing range yet, it is still impossible for the device to trigger falsely on the background noise. 
     FIG. 14 shows the working elements of the receiver enclosed within tubular upper casing  10 . Speaker  4  is mounted inside tubular ultrasound shaping element  12  which is projected through a hole in the casing. The purpose of ultrasound shaping element  12  is to narrow the projected ultrasound envelope to more closely resemble a beam of ultrasound. Mounted within the casing is printed circuit board  13  with protruding contact spring  19  and leaf contact  20  contacting, respectively, the negative and positive terminals of lithium coin cell battery  14 . Battery retainer  15  is threaded into the casing to hold the battery in place and seal off one end of the casing. Cover  18  is pressed into the opposite end of the casing to seal that end. Lower casing  23 , identical to casing  10  and housing identical elements, is fitted into counter-bore  40  in the upper casing and allowed to rotate relative to that casing for proper positioning until secured in a fixed position by set screw  17  passing through a threaded hole in the counter-bored wall and into groove  16  thus, locking upper and lower casings rigidly together. Post  20  is fitted into counter-bore  41  of the lower casing. Set screw  42  of the lower casing passes through a threaded hole in the casing&#39;s counter-bored wall and into groove  22  in the post locking lower casing and post rigidly together. 
     FIG. 12 shows the receiver assembly in which housing  28  encases microphones  54  and  55  spaced 180 degrees apart to provide a direct line of sight to posts to the left and right of the animal as it approaches the boundary. Holes  69  and  70  provide sound paths to microphones  54  and  55 . Acoustical speaker  52 , pressed into integrally molded retaining cylinder  66 , generates the warning tone emitted through hole  52 . Battery holder  51  secures and contacts lithium coin cells  56  and  57  with access to the batteries provided by battery cover  53  secured to housing  28  with screws  62  and  63 . Microphones  54  and  55 , battery holder  51  and speaker  52  are wired to printed circuit board  50  providing the sensing, amplifying and output circuitry. Threaded electrodes  58  and  59  are soldered directly to printed circuit board  50  and protrude through holes in housing  28  and holes in strap  29  and secured to the housing with nuts  58  and  59 . This also secures strap  29  relative to housing  28  as it passes through integrally molded bezel slots  67  and  68 . 
     FIG. 1 schematically illustrates the interlaced ultrasound envelopes  26  created to form a perimeter when the receiver post  20  and like posts are placed in the ground with upper and lower receiver casings,  10  and  23  respectively, positioned to form the corners of the perimeter. The actual boundary line  27  is defined be the intersection of the inside leading edges of the ultrasound envelopes. Receiver casing  28  is attached to a domestic animal by strap  29  with buckle  30  looping through the receiver casing and around the animal&#39;s neck. 
     FIG. 8 schematically illustrates an alternate embodiment of the pet containment system whereby, a single transmitter inside casing  23  is mounted on post  20 . The transmitter casing is positioned so that the transmitted ultrasound envelopes a specific area off limits to the pet. Receiver casing  28  and mounting arrangement to the animal is identical to that shown in FIG. 1. 
     FIG. 11 schematically illustrates the use of dual signal levels and modulations to define two separate boundary perimeters. Outside boundary area  31  is the area covered by the stronger signal. The modulation of this signal causes the receiver to generate the warning tone. That tone is delivered to the animal wearing the receiver when it crosses within outer perimeter  32 . Inside boundary area  33  is the area covered by the weaker signal. The modulation of this signal causes the receiver to generate an electric shock. The shock is delivered to the animal wearing the receiver when it crosses within inter perimeter  34 . This dual boundary technique is more effective in training the animal than the time delay method because the shock boundary is more clearly defined for the animal. Efforts on the part of the animal to run through the boundary before the shock is delivered are fruitless and the resultant containment system is more positive. 
     In the field of pet training, it has been shown by noted animal behaviorists that cats dogs and other animals respond positively to tones as training stimuli. The results of testing the training device described herein indicate that both dogs and cats can distinguish individual tones if separated by at least 1000 hertz. Each tone may then be used to positively enforce a specific behavior or command e.g. sit, stay, heal, come, fetch, etc. Negative stimuli, such as a loud piercing, tone or an electric shock may be used conjunctively to deter bad behavior and enforce the “NO!” command. 
     The training device of FIG. 3 utilizes the transceiver of FIGS. 7(a) and (b) with outputs at the receiver capable of producing multiple tones with pitch separation of at least 1000 hertz, and an electric shock corresponding to preset modulated signals produced at the transmitter. The receiver is housed within casing  28  and strapped to the animal as in FIGS. 1 and  8 . The receiver assembly of FIG. 13 is similar that of FIG. 12 in all respects except that it utilizes a single microphone  65  oriented to face the ground when housing  28  is attached to the animal so that it may more easily sense the part of the transmitter signal which is reflected off the ground. This allows the device to be used effectively even when the animal is not facing the trainer so long as the line of sight between transmitter and animal is not obstructed. 
     The preferred embodiment of the sonic alarm system is shown in FIG. 2. In this embodiment, transmitter and receiver positions used in the pet containment system are switched so that the receiver of FIG. 7(b), which now activates a sonic alarm output device and visual flashing light each mounted within casing  73  with microphone  74 , alarm speaker  75  and flashing light element  78 , is stationary. The transmitter of FIG. 7(a) is mounted within housing  72  and attached to a domestic animal by strap  76  with buckle  77  looping through the receiver casing and around the animal&#39;s neck. In this arrangement, transmitter housing  72  may be made quite small since there is no need for a tone generating speaker or shocking device providing a more comfortable and light weight device to be worn by the animal. Receiver casing  73  may now be easily positioned at any desired location such as couch, chair, kitchen table, etc. The variable gain control of amplifier  7 , FIG. 7(b) now acts to determine the working range of the system. 
     FIG. 4 is a schematic representation of the automatic pet door controlled by the transceiver of FIGS. 7(a) and (b) utilizing a transmitter arrangement identical to that of FIG. 2 wherein the transmitter of FIG. 7(a) is mounted within housing  72  and attached to a domestic animal by strap  76  with buckle  77  looping through the receiver casing and around the animal&#39;s neck. FIG. 16 shows the a typical working circuit of one embodiment of the automatic pet door. Here there is no need to use discrete transistors in the receiver to conserve power since the device is ultimately powered by household alternating current. 
     FIG. 4  illustrates an automatic pet door that is controlled by the transceiver of FIGS.  7 (a) and (b). The door utilizes a transmitter arrangement identical to that of FIG.  2  wherein the transmitter of FIG.  7 (a) is mounted within housing  72 . For attachment, housing  72  is attached to strap  76  and strap  76  is looped around the animal&#39;s neck and secured with buckle  77  as discussed above. Receiver  82  is mechanically attached to pet door as shown in FIG.  4  and discussed below. With respect to the electrical configuration for receiver  82 , FIG.  16 B shows a typical working circuit of one embodiment of receiver  82  for the automatic pet door shown in FIG.  4 . Note that for the pet door there is no need to use discrete transistors in the working circuit for the receiver to conserve power. This is because receiver  82  for the pet door is ultimately powered by household alternating current.   
     FIG. 5 shows that door casing  80  houses all of the elements of the receiver and door assembly. In this embodiment, an output device of FIG. 7(a) is a normally open relay  124 , FIG. 16B, which routes power to motor  96 , FIG. 5, when the transmitter signal is received by microphone  82 . Movable panel  81 , which is normally within opening  83  and completely obstructing it, is then guided upwards to allow the animal to pass through opening  83 . FIG. 5 shows that door casing  80  of FIG. 4 is constructed by joining left frame member  90 , right frame member  92 , top frame member  91  and bottom frame member  93 . Said frame members, in this embodiment, are custom wood mouldings but, could easily be made from extruded aluminium or plastic. Grooves  94  and  95  shown in FIG. 6, in left and right frame members  90  and  92  respectively, act to guide movable panel  81  along its path. Electric motor  96  is held in position by mounting bracket  97  and drives spool  98 , which is rigidly fixed to motor shaft  99  with set screw  100 . Cable  101  is thereby wound around spool  98  upon the counter-clockwise motion of electric motor  96 . Cable  101  is looped around pulley  102  and prevented from slipping off pulley  102  by cable guide  103 . Pulley  102  and cable guide  103  are rigidly attached to movable panel  81  with shoulder screw  104 . Cable  101  is then fed to one end of locking pin  105 , looped through a hole in that end and held in place by cable crimp  106 . Locking pin  105  is held in place by bracket  108  and allowed to translate linearly in holes  106  and  107 . Bracket  108  is rigidly fixed to movable panel  81  by screws  109  and  110 . Compression spring  111  fitted around stepped shaft of locking pin  107  act to push locking pin  107  toward left frame member  90  and into hole  101  thus, preventing the movable panel from being moved out of opening  83  while there is slack in cable  101  as when motor  96  is idle. Thus, it is not possible for an intruder to manually lift movable panel  81 . As cable  101  is wound around spool  98  when motor shaft  99  is rotated, the weight of movable panel  81  acts to create tension in cable  101 . Said tension acts against compression spring  111  pulling locking pin  107  away from left frame member  90  and out of hole  101  freeing movable panel  81  and allowing it to move upward with cable  101 . Front and rear plates  112  and  113 ,  FIG. 6 , respectively are locked into grooves  127  and  114  respectively and act to enclose the assembly. Holes  115 ,  116 ,  117 ,  118 ,  119  and  120 , FIG. 5, are provided to mount the assembly to a household door or wall. 
     Referring now to FIGS.  4-6   , the pet door of the present invention is shown in greater detail. Specifically, door casing  80  is constructed by joining the opposing ends top frame member  91  to one end of left frame member  90  and one end of right frame member  92 . The other respective ends of left frame member  90  and right frame member  92  are attached to opposing ends of bottom frame member  93  (See FIG.    5)   . With this configuration, the door casing defines a door opening  83 , and movable panel  81  is slidably mounted to door casing  82  within door opening  83 . To do this, vertical grooves (not shown) are formed in the respective inner surfaces of left frame member  90  and right frame member  92 . These vertical grooves interfit with movable panel  81  and act to guide movable panel  81  vertically along its upward/downward path. Left frame member  90 , right frame member  92 , top frame member  91  and bottom frame member  98  are preferably custom wood moldings, but could easily be made from aluminum or plastic materials.    
       The pet door of the present invention further includes an electric motor  96  that is fixed to top frame member  91  with mounting bracket  97 . Motor shaft  99  extends outwardly from motor  96 , and spool  98  is rigidly fixed to motor shaft  99  with set screw  100  (See FIG.  5 ) so that spool  98  rotates with motor shaft  99  during operation. To raise and lower movable panel  81 , cable  101  is attached to motor  96  and to movable panel  81 . More specifically, one end of cable  101  is attached to spool  98 . Cable  101  is then looped around a pulley  102  and a cable guide  103  that are rigidly attached to movable panel  81  with shoulder screw  104 . The other end of cable  101  is fed to one end of locking pin  105  and attached with cable crimp  106 . Cable guide  103  prevents cable  101  from slipping off pulley  102  during operation of the pet door.    
       Pin bracket  108  is rigidly fixed to movable panel  81  by screws  109  and  110 , and locking pin  105  is slidably mounted to bracket  108 , as best seen in FIG.  5 . This allows locking pin  105  to move in translation into locking pin hole  107  which is formed in left frame member  90 . A compression spring  111  is fitted around locking pin  105  and biases locking pin  105  into locking pin hole  107 , thus, preventing the movable panel from being moved out of door opening  83  while there is slack in cable  101 . With this configuration, it is not possible for an intruder to manually lift movable panel  81 .    
       Front and rear plates  112  and  114 , FIG.  6 , respectively interfit with grooves  127  and  113  and act to enclose the upper portion of the assembly. Door mounting holes  115 - 120 , FIG.  5 , are provided to mount the assembly to a household door or wall.    
       During operation, receiver  82  receives an input signal that is received from the transmitter within transmitter housing  72  only when the pet is facing the pet door, as discussed above. Receiver  82  receives this signal, and, provided the received meets the predetermined parameters also discussed above, closes normally open relay  124 , FIG.  16 B, which further closes to route power to motor  96 . As motor  96  is activated and rotates, attached spool  98  turns, and cable  101  is wound around spool  98  when motor shaft  99  is rotated. As cable  101  is wound around spool  98 , a tension force is created in cable  101 . The tension force in cable  101  counteracts and overcomes the force from compression spring  111  that biases locking pin  105  into locking pin hole  107 . Accordingly, locking pin  105  is pulled out of locking pin hole  107  and away from left frame member  90 , which frees movable panel  81  for upward movement as cable  101  is gathered around spool  98 .   
     In this embodiment activation of electric motor  96  ultimately acts lift movable panel  81 . When power is removed from electric motor  96 , as in the absence of a signal sensed by the ultrasonic receiver of FIG. 7(b), movable panel  81  will return to its initial position under the force of gravity. A shown in FIG. 16B, the speed of return may be controlled by the addition of resistor  121  and diode  122  across the legs of electric motor  96  to create an electro magnetic field in the internal coil of electric motor  96  which is resistive to the motion of motor shaft  99  in the direction of unwind thus, slowing the downward motion of the panel. 
     FIG. 16B further shows that the coil to relay  124  is energized upon sensing the ultrasonic signal. Timer  125  holds relay  124  on, providing power to motor  96 , for a selected amount of time after the signal is lost. Electrical energy is stored in capacitor  123  on the counter-clockwise motion of motor  96  as movable panel  81  is hoisted upward. This same energy is released upon the deactivation of relay  124  providing a momentary surge of current to move motor  96  in the clockwise reverse direction in order to overcome any static friction in the system and start movable panel  81  on its downward path under the force of gravity.