Abstract:
A safety circuit for use with a spa system includes a pressure sensor which generates a signal representative of the pressure in the system. The safety circuit provides a constant current to the pressure sensor. A microcontroller is coupled to receive the signal from the sensor and is configured to store a first pressure level. The microprocessor compares the first pressure level with the subsequently measured pressure level and generates a control signal if the comparison indicates a change in pressure which exceeds a predetermined amount. The control signal is sent to a spa control circuit. An electronically controlled switch is coupled to receive the control signal from the microcontroller and turn electrical power to the pump off in response thereto.

Description:
The present invention is a continuation-in-part of the previously filed application entitled SPA PRESSURE SENSING SYSTEM CAPABLE OF ENTRAPMENT DETECTION filed Jul. 15, 1999 and assigned Ser. No. 09/354,932, which application is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to spas and hot tubs and more specifically to control systems and circuits utilized in such spas and hot tubs. 
     2. Description of the Related Art 
     Pools, whirlpool spas, hot tubs and related systems typically include a tub for holding water, a pump for circulating the water and a heater. The pump draws water from the tub through a drain, forces the water through the heater and out through jets into the tub, thereby circulating the water and causing it to be heated by passing it through the heater. 
     When the pump is operating, personal contact with the drain can be dangerous, painful or even fatal. When the body or hair of a person is positioned in close proximity to the drain, the body or hair may completely or partially block the drain, thereby creating a vacuum or entrapment. This can cause entrapment of the person. Many pumps used in such systems, if obstructed, can draw a partial vacuum at the drain that may exert sufficient suction force to prevent a person from pulling free of the drain. Even if the person can pull free of the drain, bruises, welts, or other damage may result. 
     One approach to overcoming this safety hazard has been the use of multiple drains or suction ports and suction covers or grates which are formed to minimize the possibility of hair entanglement and prevent an airtight seal between a person&#39;s body and the drain. However, there are many systems still in use that were installed prior to the recognition of this safety hazard. It can be extremely difficult and expensive to rebuild or retrofit such existing systems to conform to modern safety regulations. Mechanical systems such as vacuum breakers and a Stengil switch can be retrofitted into such systems to give some measure of protection. However, such systems are not particularly sensitive to partial conditions of entrapment such as hair entanglement. 
     In addition, it is the current trend in safety regulations to require that such systems have a flow sensor. One use of flow sensors is to insure that water is flowing through the system and the heater before the heater is activated. Such flow sensors have typically been implemented as an electro-mechanical flow switch consisting of a microswitch activated by a diaphragm in contact with the water. These pressure switches are usually set to an arbitrarily low value, which may be 10 to 20 percent of the actual full pressure of the system in normal operation. Exceeding this low value is used as an indication that the pump is working. However, it is insufficient to detect significant pressure changes such as would be caused by partial entrapment. 
     In addition, it is required to supply electrical power to the pressure sensor and accompanying circuitry. To minimize the possible risk of electrocution, it is desirable to limit the amount of current at locations that could come in contact with water in the spa through a fault or failure. 
     SUMMARY OF THE INVENTION 
     The present invention provides a safety circuit which can send a signal to a control circuit to automatically remove electrical power from a device such as a pump in response to an indication of a change in the pressure in the circulating system. 
     The safety circuit can contain a sensor that generates a signal representative of the pressure generated by the pump. A microcontroller is coupled to receive the signal from the sensor and is configured to store a first level indicative of a signal received from the sensor at a first time. The microcontroller is configured to compare the first level with a second level indicative of a signal received from the sensor at a second time. The microcontroller is configured to generate a control signal when the comparison between the two levels indicates a change in pressure which exceeds a predetermined amount of change. The microcontroller sends the control signal to the spa control circuit. The spa control circuit controls the application of electrical current to the pump. An electrically controlled switch is coupled to receive a signal from the spa control circuit and is configured to control application of electrical power to a device, such as a pump, in response to that signal. 
     In one aspect of the invention the sensor is a pressure sensor which is capable of producing a signal representative of changes in pressure in the spa system. The safety circuit can be used to detect conditions of entrapment or partial entrapment and immediately shut off the pump in the spa when such conditions are detected. 
     In another aspect of the invention, the safety circuit contains a constant current source to limit the electrical current available at locations that could come in contact with water. 
     These and other features and advantages of the invention will be readily apparent to those skilled in the art from the following detailed description of embodiments of the invention with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a spa employing the invention; 
     FIG. 2 is an exploded perspective view of one embodiment of the safety system; 
     FIG. 3 is a cross sectional view of the device shown in FIG. 2 taken along line  3 - 3 ; 
     FIG. 4 is a detailed circuit diagram of a circuit embodying aspects of the safety system; and 
     FIG. 5 is a flow diagram of the operation of the circuit of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention provides a safety system including a pressure or vacuum sensor and an associated safety circuit, which can be connected with a spa control circuit in a tub, spa, or similar system, which uses a pump to circulate water. Spas, hot tubs, pools and similar systems are generally referred to herein as spas. The spa control circuit implements the normal functions required of a modem digital spa or pool control including pump control, water flow detection and heat control. The safety system rapidly detects conditions that are indicative of entrapment brought about by a person being trapped or partially trapped against the suction of the pump. When the safety system detects entrapment, a signal is sent to the spa control circuit and the pump is immediately shut off. 
     Referring to FIG. 1, the overall configuration of a spa utilizing the present invention will be described. The spa includes a tub  12 , having at its bottom a drain  14 . A suction cover  16  covers the drain  14 . A return pipe  18  couples the drain  14  of the tub  12  to the input of a pump  20 . The output of the pump  20  is coupled to a return jet  22  via an exhaust pipe  24 . The circulating system of the spa of includes the return pipe  18 , the pump  20  and the exhaust pipe  24 . A single jet  22  is shown for ease of description, though most spas employ multiple jets. Similarly, some spas also employ multiple drains. The safety system  50  is connected to the return pipe  18  near the input of pump  20 . 
     A spa control circuit  26  provides electrical power to the pump via electrical line  28 . The spa control circuit  26  receives its electrical power from an alternating current source, such as a typical wall outlet (not shown). The spa control circuit provides electrical power to the safety system  50  via electrical line  52 . The spa control circuit  26  can control various functions of the spa such as lights, a heater and other functions. 
     FIG. 2 illustrates one embodiment of the safety system  50  which includes a lower case  54 , an upper case  56 , a first circuit board  58  and a second circuit board  60 . An RJ type connector  72  is mounted on the top surface of the second circuit board  60 . The connector  72  forms one end of the connection between the safety system  50  and the spa control circuit  26 . An adapter  74  fits over the outer portion of the RJ type connector  72  and mates with an opening in the top surface of the upper case  56 . 
     The lower case  54  can be glued to the upper case  56 . Other ways of attaching the lower case  54  to the upper case  56  can also be used. Preferably, the outer surface of the lower case  54  has two protrusion locks  78 , spaced 180 degrees apart. The two locks  78  slidably fit into two groves  80  in the upper case  56  to securely fasten the lower case  54  to the upper case  56  in a defined relationship. 
     Turning now to FIG. 3, a hollow narrow neck  62  extending outwardly from a first end  63  of the lower case  54  is shown. The narrow neck  62  has threads  64  on the outside to enable the safety system  50  to be screwed into a threaded fitting, such as a reducing tee, in the suction pipe  18  (see FIG.  1 ). Alternately, the narrow neck  62  can have threads on the inside to engage the fitting or it can be smooth and bonded to a fitting on the suction pipe  18  by an adhesive. Near a second end  65  of the lower case  54 , a lower lip  66  is formed on the interior surface of the lower case  54 . The first circuit board  58  is seated on the lower lip  66 . 
     A pressure sensor  70  is mounted on the side of the first circuit board  58  facing the narrow neck  62 . The hollow narrow neck  62  has an opening  67  sized to receive a portion of the pressure sensor  70  so that one end of the pressure sensor  70  is protruding into and in fluid connection with the water in suction pipe  18  (see FIG. 1.) The pressure sensor  70  can be a conventional strain/gage bridge device implemented with piezo resistive material. Such devices are available from manufacturers such as Honeywell, Motorola, and Lucas. For example, Honeywell manufacturers such a sensor identified as model 22PC. Alternatively, a pressure sensor device that produces an electrical output representative of pressure and/or changes in pressure can also be used. 
     A first flexible seal  71 , such as an o-ring, is compressed between the bottom surface of the lower case  54  and the pressure sensor  70  to provide a watertight seal. A second flexible seal  74 , such as an o-ring, is compressed between the first circuit board  58  and the lower lip on the lower case  54 , providing a further watertight seal. An air chamber  75  is formed between the first circuit board  58  and the base of the lower case  54  to collect any water leakage past the first flexible seal  71 , thereby protecting the rest of the safety system  50  from contact with and possible damage from water. 
     The upper case  56  has a fist end with a diameter slightly larger than the diameter of the second end of the, lower case  54  so that the upper case  56  receives a portion of the lower case  54 . A third flexible seal  77 , such as an o-ring, is compressed between first circuit board  58  and a lip  80  on the interior surface of the upper case  56  to form a watertight seal. 
     The second circuit board  60  is housed in the upper case  56 . A four-pin ribbon cable  76  electrically connects the second circuit board  60  with the first circuit board  58 . 
     The ribbon cable  76  provides a flexible connection, so an exact alignment of the first and second circuit boards  58 ,  60  is not required. Alternately, other suitable electrical connectors can be used. 
     FIG. 4 illustrates a schematic depiction of an embodiment of a safety circuit  51  that can be located on the second circuit board  60  of FIG.  2 . The safety circuit  51  includes a voltage regulator  100 , a microcontroller  82 , a constant current source  86 , and a differential amplifier  96 . 
     The connector  72  (see FIG. 2) can be an RJ11 connector. An input voltage, typically 12-20 volts-DC, is applied to the safety circuit  51  through input terminals  81 ,  84  on the RJ11 connector  72 . 
     The input voltage across input terminals  81 ,  84  on the RJ11 connector  72  is applied to the voltage regulator  100 . Operational amplifier  103  in cooperation with a Zener diode (D 3 )  102  and a resistor (R 15 )  104  cooperate to form the voltage regulator  100 . The voltage regulator  100  produces a constant, regulated 5-volt DC output appropriate for use with microcontrollers. The voltage regulator  100  can include one of the four operational amplifiers of a quad operational amplifier LM324. A filtering capacitor (C 3 )  106  cooperates with the voltage regulator  100  in providing a well-regulated 5-volt DC output. The capacitance of the capacitor  106  can be 220 micro-farads. Diode (D 2 )  108  is placed between the outputs of input terminals  81 ,  84  to provide reverse voltage protection. 
     The 5-volt DC power is supplied to the microcontroller  82 . The microcontroller  82  can be a microcontroller model 12C671 8-byte microcontroller from Microchip Technology, Inc. or any other suitable commercially available microcontroller or microprocessor. 
     The input voltage across input terminals  81 ,  84  on the RJ11 connector  72  is also applied to the constant current source  86  that produces a constant current of, for example, 490 microamperes. Other suitable constant current levels can be used, but a constant current of less than 500 microamperes is highly desirable to minimize the risk of electrocution should the first circuit board  58  come in contact with water from the spa. The constant current source  86  can be a LM334 or similar device. 
     The constant current of, for example, 490 microamperes is applied to the pressure sensor  70  through input pin  94  and ground through input pin  92  across the 4-pin ribbon cable  76 . The differential voltage across the outputs  91 ,  93  of the pressure sensor  70  are supplied to an instrumentation differential amplifier  96 . An output signal  98  from the differential amplifier  96  is supplied to the microcontroller  82 . The output signal  98  of the pressure sensor  70  is a differential resistance change that is approximately linearly proportional to the pressure force (or vacuum force) of the water pressure applied to the pressure sensor  70 . 
     The differential amplifier  96  can be implemented using three of the operational amplifiers of an integrated circuit quad operational amplifier. A quad operational amplifier such as LM324, which is manufactured by National Semiconductor, among others, can be used for this purpose. [Bill, can you add more info on how the differential amplifier works, i.e. how the output is filtered. What is the purpose of the resistors R 7 , R 8 , R 9 , R 10 . What is the purpose of Capacitor C 2 ] 
     The output signal  98  is clamped to no higher than 5.1 volts by diode (D 1 )  101  placed in a line connecting the output signal with the output of the voltage regulator.  100  to protect the microcontroller  82  from spikes from the differential amplifier  96 . The microcontroller  82  receives the output signal  98 . 
     The microcontroller  82  provides a control signal to the spa control circuit  26  through a transistor (Q 1 )  110 . The transistor  110  electrically isolates the microcontroller  82  from the spa control circuit  26 . The transistor  110  operates like a switch and allows current to flow to the spa control circuit output terminal  82  of the RJ11 connector  72  when the microcontroller  82  applies a logic high signal to the transistor  110 . The microcontroller  82  applies a logic high signal when no entrapment problem is detected. When an entrapment problem is detected, a logic low signal is sent, the transistor  110  no longer allows current to flow to the spa control circuit  26  and the spa control circuit  26  shuts off the pump  20 . 
     Describing the operation of the safety system  50  in the spa system, when the pump  20  is operating, water is drawn in through the drain  14 , travels through the suction pipe  18  where it enters the pump  20 . The pump  20  pushes the water through the exhaust pipe  24  and out through the jet  22  back into the tub  12 . In addition, the spa may include a heater, electrical lights and other enhancements known to those of skill in the art. Those elements are not represented in FIG. 1 for ease of description. 
     The spa control circuit  26  controls the application of electrical power to the pump  20 . An on/off switch  40  can be activated by a user to turn the pump on. Before providing electrical power to the pump  20 , the spa control circuit  26  first determines if the water level in the tub is sufficiently high to cover the jet  22 . The water level is detected using circuitry not shown. 
     After water is detected in the tub, the spa control circuit  26  applies electrical power to the pump  20 . The pump then begins pushing water through the system which increases the water pressure on the outlet side  42  of the pump  20  at the same time decreasing the pressure (increasing the vacuum level) on the inlet side  44  of the pump. 
     During normal operation, the microcontroller  82  checks the vacuum at the input side of the pump  20  very frequently, for example, dozens of times per second. The sensed pressure is compared against the baseline originally acquired and stored. If a decrease in pressure of more than a pre-determined amount from the baseline occurs for example, 20%, and lasts for more than a pre-determined time, for example, 0.1 seconds, the microcontroller  82  sends a signal to the spa control circuit  26 , which shuts off power to the pump  20 . Alternatively, any two or more measurements or indications of the pressure separated in time can be compared to determine whether there has been a change in pressure. If the change in pressure exceeds a predetermined amount, the safety system  50  sends a signal to the spa control circuit  26 , which shuts off power to the pump  20 . Of course, one skilled in the art could assemble numerous variations of specific circuits to carry out these functions. 
     Referring now to FIG. 5, operation of the safety circuit  51  depicted in FIG. 4 will be described. Operation of the spa control circuit  26  can be controlled by software or firmware running on the spa control circuit. The software can be stored on a suitable storage device such as ROM or RAM or other computer memory and can be in the form of a software module. 
     When the pump  20  is turned on and begins pushing the water through the spa system, water pressure is increased on the outlet side of  42  of the pump  20  while the pressure level on the inlet side  44  of the pump  20  decreases, represented by block  158 . 
     A predetermined time after the pump is turned on, such as 2 seconds, the microcontroller  82  acquires the pressure level at that time from the pressure sensor  70 , via the differential amplifier  96 . The microcontroller  82  stores that initial or first pressure level, for example, in the microcontroller&#39;s random access memory (RAM), for use as a baseline for future reference as is represented by block  160 . This initial pressure level can be different for each spa system in which the safety circuit  51  is utilized. The differences in initial pressure levels can be because of differences between spas, for example in the diameter and length of their plumbing, the horsepower-rating of pump motors, variations in pump design, the amount of the restriction in the jet plumbing, etc. 
     Storing the baseline pressure level provides an important self-calibration function. This capability allows the safety circuit  51  to be used with different pumps, plumbing arrangements, tubs, etc., because the safety circuit  51  does not require a preset calibration. In addition, this allows the safety circuit  51  to adapt to long-term changes in the overall performance of the spa system such as decreased pump output which can occur as filters become clogged during normal operation. 
     After the baseline pressure level has been acquired, the microprocessor  82  periodically reads the current pressure level via the pressure sensor  70 , for example, two to 500 times per second. The current pressure level is compared to the baseline pressure level previously stored as represented by block  162 . Alternatively, the microcontroller can compare any two pressure level readings separated in time. The microcontroller determines whether there has been a decrease in the pressure level below the baseline as represented by block  164 . A decrease of or in excess of a predetermined amount, such as a 20% decrease below the stored baseline, can be used as an indication that an entrapment has occurred. A percentage change or an absolute change can be used. 
     When such a decrease in pressure is detected, the microcontroller immediately shuts off the pump  20  as represented by block  166 . The microcontroller  82  sends a signal to the spa control circuit  26  to shuts off the pump  20  by sending a logic-LOW signal to the transistor  110 . 
     In addition to selecting a predetermined decrease in pressure, a time requirement can also be included. The microcontroller  82  can use both the detection of a pressure level in excess of the predetermined decrease level and the duration of the decrease in the pressure for determining when to shut off the pump. For example, the microcontroller  82  can be programmed to ignore decreases in the pressure which have a duration shorter than 0.1 seconds. If the decrease in the pressure does not exceed the predetermined decrease and/or does not exceed a predetermined time interval, the microcontroller  82  then continues to regularly read and compare the current vacuum level. 
     Therefore, the safety circuit  51  provides a safety feature of turning off the pump  20  upon the detection of entrapment and/or complete or partial blocking of the drain  14  of the spa system. In addition, the safety circuit  51  can be utilized with many different pumps, plumbing configurations and types of spas because it is self-calibrating upon start-up. It is therefore very convenient for the retrofitting of older installed spa systems. 
     Though the foregoing embodiment has been described with regard to detecting changes in pressure (increases in vacuum level) on the inlet side of the pump, the system can also be implemented based upon changes in pressure at the output  42  of pump  20 . However, there may be a slight delay between a decrease in pressure on the inlet side of the pump and the corresponding decrease in pressure on the outlet side of the pump. As was note above, various sensors for detecting different measurements or indications which relate to or can be correlated with the pressure in the spa system can also be used. In addition, the foregoing embodiment has been described with regard to controlling a pump. However, the same flow detection and control of a device such as a pump in accordance with the flow detection can also be applied to the control of other spa devices such as a heater and can be used to control multiple devices such as a pump and a heater. 
     The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description. All changes and variations which come within the meaning and range of equivalency of the claims are to be embraced within their scope.