System and method for a reduced water consumption vacuum toilet

A system and method for reducing water consumption in a vacuum toilet on an aircraft. The system and method includes a weight sensing device that is mounted to an aircraft toilet and is responsive to weight applied to the bowl of the toilet. The weight sensing device detects that a toilet user is seated upon the toilet seat when it measures a weight applied to the toilet bowl. On the other hand, the weight sensing device detects that a toilet user is standing when the weight sensing device does not measure a weight applied to the toilet bowl. A flush control unit is connected to the weight sensing device and controls the amount of water utilized in the flushing of the toilet. When the toilet is flushed and the weight sensing device has detected that a toilet user has sat upon the toilet seat, the flush control unit commands that a standard amount of water be used to remove the solid waste of the toilet user. On the other hand, when the toilet is flushed and the weight sensing device has detected that the toilet user has stood, the flush control unit commands that a small amount of water be used thereby saving water.

FIELD OF THE INVENTION
 This invention relates generally to vacuum toilet systems for aircraft, or
 other modes of transportation, and, in particular, to a system for
 reducing the amount of water consumed in the flushing of a vacuum toilet
 system.
 BACKGROUND OF THE INVENTION
 In the operation of commercial aircraft, it is necessary to provide
 on-board toilet facilities for use by the passengers and crew. These
 toilet facilities include vacuum toilets which presently require a
 specific amount of water to operate. However, an aircraft has only one
 limited water supply to meet all the water needs of the passengers and
 crew during the flight of the aircraft for drinking, food preparation, and
 other uses, as well as for use by the aircraft toilets. The amount of
 water stored in the aircraft is proportional to the trip duration and the
 number of passengers and crew that will be on-board the aircraft. Further,
 an ample amount of water is necessary to ensure the comfort of the
 passengers and crew. Unfortunately, a large amount of fuel is required
 just to transport the water supply which is a significant operational cost
 and decreases the efficiency of the aircraft. Therefore, a need exists to
 reduce the amount of water required to be flown by the aircraft to
 increase the overall efficiency of the aircraft and reduce operational
 costs.
 As previously discussed, a significant amount of water is used during any
 given aircraft flight to operate the aircraft toilets. A reason for this
 is that when the toilet is flushed, no differentiation is made between
 flushing with solid matter due to defecation, and without solid matter,
 such as when a man stands and urinates. When the toilet is flushed, a
 standard amount of water is always used, approximately 8 fluid ounces,
 which assumes that solid matter, such as tissues and feces, is present.
 This large amount of water is necessary, when solid matter is present,
 because if a lesser amount of water is used, deposits will rapidly build
 upon the walls of the connecting conduits and critically impair the
 operation of the toilet system. However, when a male uses the toilet to
 urinate, substantially less water is required for flushing, and using a
 standard amount of water is excessive. The applicants estimate that
 approximately 45% of toilet use is by men standing and urinating resulting
 in a large amount of wasted water. Thus, aircraft currently carry a large
 load of unnecessary water, and correspondingly use a significant amount of
 unnecessary fuel to transport it. Therefore, it would be beneficial to
 have an aircraft toilet system that can vary the amount of flush water
 used depending upon whether the user has urinated or defecated.
 An apparatus to control the amount of water used in the flushing of a
 toilet is described in U.S. Pat. No. 4,707,867 to Kawabe et al. The Kawabe
 apparatus utilizes a light beam sensor to detect whether a person has sat
 upon a toilet seat and uses this information to determine the volume of
 flush water to be used. The light beam sensor emits a light beam to a
 reflector which reflects the light beam back to the sensor creating an
 unbroken horizontal light beam path. The light beam sensor and reflector
 are located slightly above the toilet such that the light beam path is
 correspondingly located at the same position. Hence, when a user occupies
 the toilet, the light beam can neither reach the reflector nor be
 reflected back to the sensor, and the sensor thereby detects the presence
 of a user seated on the toilet. If the light beam is interrupted for
 greater than 90 seconds, the apparatus assumes the toilet user has
 defecated and utilizes a large amount of water, otherwise, the apparatus
 assumes the toilet user has urinated and utilizes a small amount of water.
 This apparatus uses a complicated electronic circuit having a plurality of
 different timers, which are used to time the interruption of the light
 beam and to control the timing of the opening and closing of a flush
 valve, thereby controlling the amount of water used. Although, this type
 of apparatus works well for its intended purpose, it has many drawbacks
 that would make it unsuitable for use in an aircraft toilet system.
 A disadvantage of the Kawabe apparatus is that it is not suitable for use
 with aircraft because the aircraft lavatory system, as well as, the
 aircraft toilet system, would have to be substantially altered to
 accommodate it. For example, the light beam sensor would have to be
 mounted to one wall of the aircraft lavatory and the reflector would have
 to be mounted to another wall of the aircraft lavatory. Also, this type of
 light beam sensor is likely to become contaminated over time and require
 servicing. Furthermore, the use of a light beam sensor requires a
 complicated and expensive electronic circuit which has a plurality of
 different timers to time the interruption of the light beam. Lastly,
 another disadvantage of this apparatus, is that the sensor, the electronic
 circuit, and the associated wiring, could potentially cause
 electromagnetic interference with the existing on-board electronics of the
 aircraft. Therefore, the addition and integration of the Kawabe apparatus
 to an aircraft lavatory and toilet system could be costly and could
 potentially require additional testing and retrofitting to verify that it
 does not interfere with the already existing on-board electronics of the
 aircraft. Thus, the Kawabe apparatus is just not readily adaptable for use
 with an aircraft.
 Accordingly, it should be appreciated that there is a need for a system for
 reducing water consumption in a vacuum toilet, suitable for use in an
 aircraft, that detects whether a toilet user has sat on the toilet seat,
 or has stood, and uses that information to control either a longer or
 shorter period of flow of rinse water allowing for a significant reduction
 in average water consumption.
 SUMMARY OF THE INVENTION
 The present invention provides a system for reducing the water consumption
 in a vacuum toilet on an aircraft by detecting whether a toilet user has
 sat on the toilet seat, or has stood, and uses that information to control
 either a longer or shorter period of flow of rinse water allowing for a
 significant reduction in average water consumption while maintaining
 current standards of toilet system cleanliness.
 The system for the reduced water consumption vacuum toilet of the present
 invention includes a weight sensing device that is mounted to an aircraft
 toilet and is responsive to a weight applied to the bowl of the toilet.
 The weight sensing device detects that a toilet user is seated upon the
 toilet seat when it measures a weight applied to the toilet bowl. On the
 other hand, the weight sensing device detects that a toilet user is
 standing when the weight sensing device does not measure a weight applied
 to the toilet bowl. A flush control unit is connected to the weight
 sensing device and controls the amount of water utilized in the flushing
 of the toilet. When the toilet is flushed and the weight sensing device
 has detected that a toilet user has sat upon the toilet seat, the flush
 control unit commands that a standard amount of water be used. On the
 other hand, when the toilet is flushed and the weight sensing device has
 detected that a toilet user has stood, the flush control unit commands
 that a small amount of water be used thereby saving water.
 In a preferred embodiment, the weight sensing device used is a pressure
 sensor which responds to the force applied to the toilet bowl by the
 toilet user sitting upon the toilet seat and generates a pressure signal
 that is proportional to the amount of force. A pressure derivation circuit
 is connected to the pressure sensor to receive the pressure signal. Upon
 receipt of the pressure signal, the pressure derivation circuit compares
 the pressure signal to a threshold value. The threshold value corresponds
 to a minimum weight required to be applied to the toilet bowl to indicate
 that a toilet user has sat upon the toilet seat. The pressure derivation
 circuit generates a standard flush signal if the pressure signal is
 greater than the threshold value indicating that a toilet user has sat on
 the toilet seat and has most probably defecated. Otherwise, the pressure
 derivation circuit does not generate a signal indicating that the toilet
 user has not sat upon the toilet seat and has most likely urinated. A
 flush control unit is connected to the pressure derivation circuit and
 controls the amount of water utilized in the flushing of the toilet. When
 the toilet is flushed and the flush control unit has received the standard
 flush signal, a standard amount of water is used to remove the solid waste
 of the toilet user. On the other hand, if the toilet is flushed and the
 flush control unit has not received the standard flush signal, a reduced
 amount of water is used because the toilet user has most likely urinated.
 The use of this system provides an important advantage, in that, by
 differentiating when a toilet user has either defecated or urinated, and
 adjusting the amount of water used accordingly, a significant amount of
 water is saved. The standard amount of water used for defecation in an
 aircraft toilet is approximately 8 fluid ounces, which is enough to
 maintain the cleanliness of the toilet bowl and maintain waste system
 reliability by sufficiently removing deposits from the walls of the
 connecting conduits of the toilet system. On the other hand, when the
 toilet is flushed with the reduced amount of water for urination, which is
 approximately only 3 fluid ounces, this lesser amount of water is still
 sufficient to clean the toilet bowl and maintain waste system reliability
 while saving 5 fluid ounces of water. In fact, the applicants predict that
 about 45% of toilet use on an aircraft is due to men standing and
 urinating, and that the use of this system can therefore result in an
 approximate 28% savings in water use by the toilet. Further water savings
 are accrued because the reduced amount of water will be used when the
 toilet is flushed prior to use or for simply flushing used tissue, both of
 which are common occurrences. Thus, by utilizing this system less water is
 required to be carried by the aircraft and consequently less fuel is
 needed. This can translate into significant cost savings for the aircraft
 operator. Furthermore, because less water is needed for a given aircraft
 flight, more space is available in the aircraft to be utilized for
 passengers and cargo. Also, because the same amount of water is used when
 a toilet user defecates, the system still maintains the current standard
 of toilet system cleanliness and waste system reliability.
 In a preferred embodiment, the pressure sensor utilized is a load cell
 containing a strain gauge device which can be easily mounted to the
 existing supporting frame of the toilet. To accomplish this, a support
 structure is mounted to the supporting frame of the toilet, beneath the
 underside of the toilet bowl, and the load cell is secured to the support
 structure. The support structure secures the load cell to the supporting
 frame so that the load cell can measure the deflection of the toilet bowl
 in response to the force applied to the toilet bowl by a toilet user
 sitting upon the toilet seat. Advantageously, the support structure is
 easily installable onto an aircraft toilet without any alteration being
 required of the surrounding aircraft lavatory. Thus, the installation of
 the load cell only affects the toilet and no additional retrofitting of
 the aircraft lavatory or electromagnetic interference testing is required.
 Furthermore, upon installation of the load cell to the toilet, the load
 cell can be manually preset to provide a zero or null response. The
 structure which accomplishes this presetting feature includes a bracket,
 an adjustment screw, and a lock nut. The bracket is welded to the
 underside of the toilet bowl and the adjustment screw is threaded to the
 bracket so that the position of the adjustment screw can be altered to
 properly engage the load cell. The adjustable positioning of the
 adjustment screw permits the application of a desired amount of initial
 pressure against the load cell so that the load cell can be properly
 preset upon installation. Further, the lock nut secures the adjustment
 screw in the desired position, once the load cell has been properly
 preset, to retain the load cell in the proper preset condition. This
 procedure compensates for standard component manufacturing and assembly
 tolerances and eliminates these errors. Once this presetting procedure is
 finished, it does not need to be repeated unless the toilet is later
 disassembled.
 In a preferred embodiment, when the toilet system is powered-up, the
 pressure derivation circuit automatically adjusts for errors that could
 affect the accuracy of the system. These errors can occur due to
 variations in the circuit components, errors in the mechanical presetting
 of the load cell described in the previous paragraph, and due to the use
 of differing lavatory components such as the use of different toilet seats
 and toilet shrouds, which may change the overall weight applied to the
 toilet. To automatically adjust for these errors, the pressure derivation
 circuit automatically generates an adjusted threshold value when the
 toilet system is powered-up. Upon toilet system power-up, the pressure
 derivation circuit receives an initial pressure signal from the pressure
 sensor and stores it as a base pressure value. The pressure derivation
 circuit then adds this base pressure value to the threshold value creating
 the new adjusted threshold value. The base pressure value corresponds to
 the error value for which the system needs to compensate. Thus, when the
 pressure derivation circuit automatically adds the base pressure value to
 the threshold value and creates the adjusted threshold value, these errors
 are eliminated. During operation, the pressure derivation circuit then
 compares the subsequent pressure signals received to the adjusted
 threshold value and generates a standard flush signal if the pressure
 signal is greater than the adjusted threshold value indicating that a
 toilet user has defecated and a standard amount of rinse water is used.
 Advantageously, the system self-adjusts for errors so that it remains
 accurate and requires less servicing and/or replacement over a long period
 of use.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 With reference to FIG. 1, a system 10 for reducing the amount of water
 consumed in the flushing of an aircraft vacuum toilet system, in
 accordance with a preferred embodiment of the invention, is illustrated in
 block diagram form. Although the invention will be described for use with
 an aircraft vacuum toilet system, it should be appreciated that the system
 of the present invention can be used with any type of toilet system.
 As shown in FIG. 1, the system includes a weight sensing device, such as a
 pressure sensor 12, which can be mounted to an aircraft toilet. The
 pressure sensor responds to the force applied by a toilet user sitting
 upon the toilet seat and generates a pressure signal proportional to the
 amount of force. A pressure derivation circuit 14 is preferably connected
 through an amplifier 16 to the pressure sensor to receive the pressure
 signal from the pressure sensor. Upon receipt of the pressure signal, the
 pressure derivation circuit compares the pressure signal to a threshold
 value and generates a standard flush signal if the pressure signal is
 greater than the threshold value thus indicating that the toilet user has
 sat on the toilet and has defecated. Otherwise, the pressure derivation
 circuit does not generate a signal therefore indicating that the toilet
 user has stood and urinated. A flush control unit 18 is connected to the
 pressure derivation circuit and controls the amount of rinse water
 utilized in the flushing of the toilet. When the toilet is flushed and the
 flush control unit has received the standard flush signal, a standard
 amount of rinse water is used to remove the solid waste of the toilet
 user. On the other hand, if the toilet is flushed and the flush control
 unit has not received the standard flush signal, a reduced amount of rinse
 water is used.
 The system 10 provides an important advantage, in that, by differentiating
 when a toilet user has either defecated or urinated, and adjusting the
 amount of water used accordingly, it significantly reduces the amount of
 water required by the toilet system during a typical flight, while
 maintaining current standards of toilet system cleanliness and waste
 system reliability. Thus, by utilizing this system, less water is required
 to be carried by the aircraft and less fuel is needed. This can translate
 into significant cost savings for the aircraft operator. Also, since less
 water is needed for a typical aircraft flight, more space is available in
 the aircraft for passengers and cargo. Additional advantages related to
 other design features are described below.
 With reference to FIG. 2, a typical aircraft vacuum toilet system 20 is
 illustrated. An aircraft toilet system usually includes a toilet 22 having
 a toilet seat 24 and a toilet seat cover 26 hinged to a shroud 30, a
 toilet bowl 27, and a supporting frame 28 for the toilet. Often, aircraft
 toilets include a shroud which surrounds the toilet system and keeps the
 internal components of the toilet system hidden from the view of the
 toilet user. The shroud provides a sanitary enclosure for the toilet
 system and blends to the contours of the lavatory walls. Also, a
 semi-compliant pad 32 is mounted between the shroud and the toilet bowl.
 Mounted behind the toilet bowl is a valve assembly 34, utilized in the
 flushing of the toilet, and an associated sewer pipe 36 for communicating
 the waste of the toilet bowl to a waste reservoir (not shown).
 The supporting frame 28 mounts the toilet 22 to the bottom floor 38 of the
 aircraft lavatory. The toilet bowl 27 is directly mounted to the
 supporting frame by a set of four prime nut and bolt attachments 40 (only
 two shown). Also, a set of secondary nut and bolt attachments (not shown)
 are located at the rear of the supporting frame providing extra strength
 and stability for the toilet in case of severe tip loads. When the toilet
 seat cover 26 is raised and a toilet user sits down upon the toilet seat
 24, the force is transferred through the toilet seat, the shroud 30, and
 the semi-compliant pad 32 to the toilet bowl and the supporting frame. It
 should be appreciated, that this is merely an illustration of a typical
 aircraft toilet system, for exemplary purposes, and does not constitute
 part of the invention.
 With reference to FIG. 3, a feature of the invention is that the weight
 sensing device, such as the pressure sensor 12, can be easily mounted to
 the existing supporting frame 28 of the toilet 22, to measure the
 deflection of the toilet bowl 27 in response to a toilet user sitting upon
 the toilet seat. This is accomplished by mounting a support structure 43
 to the supporting frame of the toilet beneath the underside of the toilet
 bowl. The support structure includes a rectangularly shaped pressure
 sensor mounting platform 44 and a V-shaped mounting portion 46. The
 V-shaped mounting portion includes a front leg 48 and a back leg 50 which
 extend downwardly from the pressure sensor mounting platform. Preferably,
 the support structure is welded to the supporting frame. The pressure
 sensor is mounted to the pressure sensor mounting platform of the support
 structure.
 In a preferred embodiment, a button-style load cell, containing a strain
 gauge device, is utilized as the pressure sensor 12. Three bolts 52 (only
 two shown) extend through the pressure sensor mounting platform 44 and
 into the body of the load cell firmly securing it to the support structure
 43 and the supporting frame 28. Also, washers 54 may be interposed between
 the bolts and the mounting platform. Load cells of this type are
 well-known in the art. Alternatively, a strain gauge device could be
 directly mounted to the supporting frame. When a toilet user sits upon the
 toilet seat, the force is directly transferred to the toilet bowl 27 and
 to the supporting frame with sufficient deflection to stimulate the load
 cell mounted beneath the toilet bowl, as shown, or a strain gauge device
 directly mounted to the supporting frame. Advantageously, the support
 structure and the load cell are easily mountable to an aircraft toilet and
 no additional retrofitting of the aircraft lavatory is required.
 An adjustment screw 58 engages the load cell 12 to transmit the force of
 the toilet bowl 27 against the load cell and further provides a presetting
 feature for the load cell. An L-shaped bracket 60 is preferably welded to
 the underside of the toilet bowl for adjustably mounting the adjustment
 screw. The adjustment screw is threaded to the bracket so that the
 position of the adjustment screw can be adjusted, by rotating the
 adjustment screw, relative to the load cell. The adjustable positioning of
 the adjustment screw permits the application of a desired amount of
 pressure against the load cell. Additionally, a lock nut 62 is provided
 around the adjustment screw to lock the adjustment screw in the desired
 position.
 When the toilet system is first integrated, the load cell 12 can be preset
 to provide a zero or null response by expanding the adjustment screw 58
 against the load cell until the desired response from the load cell is
 obtained. After the presetting of the load cell is accomplished, the lock
 nut 62 is tightened to secure the adjustment screw against the load cell
 and to retain the load cell in the proper preset condition. This procedure
 advantageously compensates for standard component manufacturing and
 assembly tolerances and eliminates these errors. Once this presetting
 procedure is finished, it does not need to be repeated unless the toilet
 is later disassembled.
 Thus, after proper presetting, when a toilet user sits upon the toilet seat
 the force is transmitted through the toilet bowl 27 to the adjustment
 screw 58 and against the load cell 12 such that the load cell measures the
 amount of force applied to toilet seat. The load cell generates a pressure
 signal proportional to this amount of force and transmits this pressure
 signal through a shielded wire 66 to the rest of the system 10,
 illustrated in FIG. 1, which preferably is an electronic circuit that is
 housed in a control box (not shown) at the side of the toilet. As will be
 discussed in more detail below, the system 10 determines whether or not a
 toilet user has sat upon the toilet seat and uses this information to
 control the delivery of either a standard amount of rinse water or a
 reduced amount of rinse water to the toilet.
 As shown in FIG. 1, which is a block diagram of an electronic circuit for
 accomplishing this task, the pressure signal is first received by an
 amplifier 16 which amplifies the pressure signal for receipt by the
 pressure derivation circuit 14. Upon receipt of the pressure signal, the
 pressure derivation circuit compares the pressure signal to a threshold
 value, calculated to be analogous to approximately 31 pounds of weight
 upon the toilet seat, and generates a standard flush signal for receipt by
 the flush control unit 18 if the pressure signal is greater than the
 threshold value. The standard flush signal indicates to the flush control
 unit that a toilet user has sat on the toilet seat and has defecated and
 that a standard amount of water should be used. On the other hand, if no
 pressure signal is received by the pressure derivation circuit, or a
 pressure signal is received that compares lower than the threshold value,
 the pressure derivation circuit does not generate a signal. The absence of
 the standard flush signal indicates to the flush control unit that the
 toilet user has stood and urinated and that a reduced amount of water
 should be used.
 A feature of the pressure derivation circuit 14 is that, when the toilet
 system is powered-up, the pressure derivation circuit automatically
 adjusts for any errors that could affect the accuracy of the system 10.
 These errors can occur due to variations in the circuit components, such
 as amplifier offset, errors in the mechanical presetting of the load cell,
 and due to the use of differing lavatory system components such as the use
 of different toilet seats and toilet shrouds, which may change the overall
 weight applied to the toilet. To automatically adjust for these errors,
 the pressure derivation circuit automatically generates an adjusted
 threshold value when the toilet system is powered-up.
 Referring to FIG. 4, upon toilet system power-up, the pressure derivation
 circuit 14 receives an initial pressure signal 70 from the pressure sensor
 12. The pressure derivation circuit reads the initial pressure signal and
 stores it as a base pressure value 72. The pressure derivation circuit
 then adds the base pressure value to the threshold value 73 to create the
 adjusted threshold value 74. As previously described, the threshold value
 corresponds to an amount of weight, approximately 31 pounds, applied to
 the toilet seat, sufficient to determine that a toilet user has sat on the
 toilet seat. The base pressure value corresponds to any additional weight
 or errors that the system needs to compensate for. Thus, when the pressure
 derivation circuit automatically adds the base pressure value to the
 threshold value and creates the adjusted threshold value, these additional
 weights and errors are accounted for. During operation, the pressure
 derivation circuit then compares the subsequent pressure signals received
 to the adjusted threshold value and generates a standard flush signal if
 the pressure signal is greater than the adjusted threshold value
 indicating that a toilet user has defecated and a standard amount of rinse
 water should be used. Advantageously, the system self-adjusts for internal
 errors and differing toilet system components so that it remains accurate
 and requires less servicing and/or replacement over a long period of use.
 Referring again to FIG. 1, the flush control unit 18 controls the amount of
 water utilized in the flushing of the toilet. The flush control unit is
 connected to the pressure derivation circuit 14 for receipt or non-receipt
 of the standard flush signal. When the toilet is flushed and the flush
 control unit has received the standard flush signal from the pressure
 derivation circuit, indicating that the toilet user has defecated, a
 standard amount of water is used, approximately eight fluid ounces, to
 ensure removal of solid waste from the toilet bowl. On the other hand, if
 the toilet is flushed and the flush control unit has not received the
 standard flush signal, a reduced amount of water is used, approximately
 three fluid ounces, since the toilet user has stood and urinated. Because
 the system detects whether the occupant of a toilet has sat on the toilet
 seat and uses that information to control either a shorter or longer
 period of flow of rinse water, this allows for a significant reduction in
 average water consumption during the flight of the aircraft. Specifically,
 when the toilet is flushed, and no one has sat on the toilet seat, only
 three fluid ounces of water is used, which is five fluid ounces less than
 the standard eight fluid ounces of water used when a toilet user sits on
 the toilet. Additional water savings are accrued from common compulsions
 such as flushing prior to use.
 With reference to FIG. 5, a specific circuit embodiment 75 of the system 10
 for the reduced water consumption vacuum toilet of FIG. 1 will now be
 described. However, it should be appreciated that this specific circuit
 embodiment is only exemplary, and a multitude of different circuits could
 be used. The pressure sensor 12 is a button-style load cell which houses a
 temperature compensated strain gauge device 76. Load cells and the use of
 load cells are well-known in the art. The particular load cell used in
 this specific circuit embodiment is a load cell developed by Transducer
 Techniques, Inc., and is designated as part number LBO-500. This type of
 load cell is rated as a 500 lb. unit and was selected for its durability
 and commercial availability. It has a response of 2 mVN full-scale or 20
 .mu.V/lb @ 5 V. Thus, the load cell with a 5 V input produces an output of
 20 .mu.V for every pound of force the load cell is subjected to. It should
 be noted that the circuit components used in this specific circuit
 embodiment 75, to be described as follows, are well known in the art and
 are commercially available.
 This specific circuit embodiment 75 is powered by the flush control unit 18
 which supplies 28 V of direct current (DC) to a standard isolated DC/DC
 converter 77. The DC/DC converter, in turn, produces a bi-polar output of
 +/-12 V direct current to the amplifier 22. The amplifier and the
 transistor 78 produce a precision 5 V direct current to excite the strain
 gauge device 76 contained within the load cell 12. When no force is
 present upon the load cell, the strain gauge is balanced and no difference
 exists between the positive and negative output nodes 80 of the strain
 gauge. However, when a toilet user sits upon the toilet seat and the
 toilet bowl deflects downwards such that the adjustment screw applies a
 force against the load cell, the resistance of the strain gauge device
 becomes unbalanced proportional to this applied force and produces a 20
 .mu.V/lb response, the pressure signal, along line 82 to the input pin 84
 of the amplifier.
 The gain of the amplifier 22 is set by the resistor 86 such that the output
 of the amplifier, the amplified pressure signal, along line 88, is
 approximately 0.04 V/lb. The gain of the amplifier is set so that the
 amplified pressure signal along line 88 will increase approximately 1.25 V
 when a force of approximately 31 lbs. or more is detected upon the toilet
 seat. The amplified pressure signal along line 88 is received by the
 pressure derivation circuit 14 at the input pin 90.
 The pressure derivation circuit 14 compares the amplified pressure signal
 to the adjusted threshold value and activates the output pin 92 of the
 pressure derivation circuit to generate a standard flush signal along line
 94 if the amplified pressure signal is greater than the adjusted threshold
 value. On the other hand, if the amplified pressure signal compares lower
 than the adjusted threshold value, the output pin 92 is deactivated and no
 signal is sent along line 94. As previously discussed, the pressure
 derivation circuit receives an initial pressure signal when the toilet
 system is powered-up, and stores the initial pressure signal as a base
 pressure value. The pressure derivation circuit adds the base pressure
 value to the threshold value to create an adjusted threshold value. The
 pressure derivation circuit then compares subsequent pressure signals to
 the adjusted threshold value and generates a standard flush signal if the
 subsequent pressure signal is greater than the adjusted threshold value.
 The standard flush signal along line 94 is connected to the flush control
 unit 18 through an optical-isolator 96 which is useful for preventing
 electromagnetic interference. The flush control unit includes a latch 98
 to receive the standard flush signal from the optical-isolator and a rinse
 timing controller 100 connected to the latch. The rinse timing controller
 controls the rinse valve which opens and closes to deliver water to the
 toilet. When the toilet is flushed with the latch in receipt of the
 standard flush signal from the optical-isolator, a standard rinse duration
 is used to deliver a standard amount of water to the toilet and the latch
 is reset at the end of the flush cycle. However, when the toilet is
 flushed and the latch has not received the standard flush signal from the
 optical-isolator, the rinse timing controller reduces the amount of water
 used. Specifically, the rinse timing controller closes the rinse valve
 more quickly than when standard rinse duration is used so that a reduced
 amount of water is delivered to the toilet.
 The operation of the specific circuit embodiment 75, constructed as
 described above, proceeds as follows. Referring again to FIGS. 2 and 3,
 when a toilet user sits upon the toilet seat 24, the toilet bowl 27
 deflects downwardly such that the adjustment screw 58 transfers the force
 against the load cell 12. With reference to FIG. 5, the strain gauge
 device 76 housed within the load cell 12 becomes unbalanced due to the
 compression and/or elongation of its resistors and produces a pressure
 signal proportional to the applied force along line 82 to the input pin 84
 of the amplifier 22. The amplifier amplifies the pressure signal creating
 an amplified pressure signal and applies this amplified pressure signal
 along line 88 to the input pin 90 of the pressure derivation circuit 14.
 The pressure derivation circuit then compares this amplified pressure
 signal to the adjusted threshold value. If the amplified pressure signal
 is greater than the adjusted threshold value, the output pin 92 is
 activated and a standard flush signal along line 94 is transmitted through
 the optical-isolator 96 to the latch 98 of the flush control unit 18. When
 the toilet is flushed with the latch in receipt of the standard flush
 signal, a standard amount of water is used and the latch is reset at the
 end of the flush sequence.
 On the other hand, if a toilet user utilizes the toilet without sitting on
 the toilet seat 24, the toilet bowl 27 does not deflect downwardly and the
 load cell 12 does not generate a pressure signal. Thus, the pressure
 derivation circuit 14 does not generate a standard flush signal and the
 latch 98 of the flush control unit 18 does not receive a standard flush
 signal. When the toilet is flushed with the latch not in receipt of the
 standard flush signal, the rinse timing controller 100 reduces the amount
 of water used.
 The use of this system provides an important advantage over standard
 aircraft toilets, in that, by differentiating when a toilet user has
 either defecated or urinated, and adjusting the amount of water used
 accordingly, a significant amount of water is saved. Thus, by utilizing
 the system of the present invention less water is required to be carried
 by the aircraft and consequently less fuel is needed. This results in
 significant cost savings for the aircraft operator. Also, since less water
 is needed for a given aircraft flight, more space is available in the
 aircraft to be used for passengers and cargo.
 Although the invention has been principally described thus far with
 reference to its system aspects, the invention embraces a sequence of
 steps constituting a novel method intended to achieve the described
 results. Specifically, the method comprises the steps of: mounting the
 pressure sensor to the toilet such that the pressure sensor is responsive
 to a force applied to the toilet bowl and generates a pressure signal
 proportional to the amount of force; comparing the pressure signal to a
 threshold value utilizing the pressure derivation circuit and generating a
 standard flush signal if the pressure signal is greater than the threshold
 value; and controlling the amount of water utilized in the flushing of the
 toilet with the flush control unit such that when the toilet is flushed
 and the standard flush signal has been received by the flush control unit,
 a standard amount of water is used, otherwise, a reduced amount of water
 is used.
 While the invention has been described with reference to its preferred
 embodiment, it will be appreciated by those skilled in this art that
 variations may be made without departing from the precise structure or
 method disclosed herein which, nonetheless, embody the invention defined
 by the appended claims. For example, although the invention has been
 described for use with an aircraft vacuum toilet system, it should be
 appreciated that the system of the present invention can be used with any
 type of toilet system, such as for use with a toilet system of a train.