EP0118719A1

EP0118719A1 - Water inlet control system for a dishwasher

Info

Publication number: EP0118719A1
Application number: EP84101016A
Authority: EP
Inventors: Claudio Milocco
Original assignee: Industrie Zanussi SpA
Current assignee: Industrie Zanussi SpA
Priority date: 1983-02-14
Filing date: 1984-02-01
Publication date: 1984-09-19
Also published as: IT8345703A1; ATE21209T1; DE3460404D1; EP0118719B1; IT8345703A0; IT1174941B

Abstract

A water inlet control system for an automatic dishwater comprising a water inlet valve (6) controleld by a program unit (7) for opening during a charging phase, in the course of which a circulation pump (9) is also operated. A transducer (13) generates an output signal proportional to the outlet pressure of the pump.

The output signal of the transducer is applied to control means (16) adapted to respond to variations of the output signal for causing the valve (6) to close when the amplitude of these variations decreases below a predetermined value.

Description

The present invention relates to an automatic dishwasner, and more particularly to a water inlet control system for such dishwasher. Known from GB Patent No. 1,595,556 is a water inlet control system for a dishwasher of the type in which the water circulation pump is operated during the phase during which the machine is being charged with tap water. An electro-mechanical transducer is operative to measure the outlet pressure of the pump and to deenergize a water inlet electrovalve when the outlet pressure exceeds a predetermined value. In theory, this value is calculated so as to correspond to the saturation condition of said circulation pump, and thus to the minimum amount of water required for optimum operation of the pump.
As generally acknowledged, the increasing emphasis on energy saving underlines the importance of minimizing the amount of water supplied to a dishwasher for each cycle of operation thereof, particularly in the case of dishwashers for domestic use. In this respect, the solution proposed by the cited GB Patent 1,595,556, which takes into account all of the main parameters determining the minimum amount of water required for optimum.operation, constitutes a considerable improvement over conventional water inlet control systems employed in dishwashers, such as float-or pressostat-controlled systems (inaccurate and rather unreliable) or systems controleld by an optical level sensor (such systems being very delicate and expensive and incapable of taking into account the functional charact--eristics of the circulation pump and variations of the amount of dishes or the like charged into the dishwasher). On the other hand, the practical implementation of this solution offers certain difficulties, since it is based on the sensing of an absolute pressure value by means of a transducer which, while being of sophisticated construction, may be conducive to.functional tolerances and operative inaccuracies depending on manufacturing tolerances of the hydraulic circuits of the machine, such inaccuracies being unacceptable, however, for this specific application. Under limit conditions, the control system may for some reason not respond to the predetermined pressure value for deenergizing the water inlet electrovalve, with resultant flooding of the location at which the dishwasher is installed. In the opposite case, the circulation pump may never attain its saturation condition. It is therefore an object of the invention to provide a water inlet control system for a dishwasher capable of ensuring accurate and reliable operatoin independently of the tolerances of its principal components while taking into account all of the fundamental parameters determining the minimum amount of water required for optimum operation of the dishwasher.
According to the invention, this object is attained by a water inlet control system for an automatic dishwasher, comprising a water inlet valve controlled by a program unit so as to open during at least one charging phase, and a water circulation pump controlled by said program unit to operate at least during the last part of the charging phase, a transducer being provided to generate an output signal proportional to the outlet pressure of said pump. This system is basically characterized in that the output signal of the transducer is applied to control means responsive to variations of said output signal for causing the inlet valve to be closed when the-amplitude of the variations decreases below a predetermined value.
Further characteristics and advantages of the invention will become more clearly evident from the following description of an exemplary embodiment with reference to the accompanying drawings, wherein:

fig. 1 shows a diagrammatic view of a dishwasher provided with a water inlet control system according to the invention,
fig. 2 shows a block circuit diagram of a preferred embodiment of a water inlet control system according to the invention, and
fig. 3 shows a diagram of the development of the outlet pressure of a water circulation pump employed in the dishwasher of fig. 1.

With particular reference to fig. 1, a dishwasher comprises an inlet conduit 4 for feeding tap water to a washing tub 5. In a per se known manner, inlet conduit 4 is provided with an electromagnetic inlet valve 6 controlled by a program unit 7 of the dishwasher via a normally open breaker switch 17, for instance an electronic interruptor. Inlet conduit 4 is additionally provided with a hydraulic safety device including a free flow section 8.
The end portion of inlet conduit 4 communicates with a collecting well 10 of the dishwasher preferably in the manner described in applicant's European Patent Application No. 80102829.1/1980. A water circulation pump 9 is also controlled by program unit 7 for drawign water from collecting well 10 and pressure-feeding it to at least one spray rotor 11.
Collecting well 10 and circulation pump 9 are preferably disposed as described in applicant's Italian Patent Application No. 45715/79, corresponding to GB Patent 2,049,406.
The outlet conduit of pump 9 is connected to the control inlet 14 of a pressure transducer 13 through a brnach conduit 12. Pressure transducer 13 may for example be a semiconductor pressure transducer sold by Motorola Inc. under the designation MPX 100. As generally known, a transducer of this type is adapted to generate at its output 15 a DC voltage signal the magnitude of which is proportional to the hydraulic pressure applied to inlet 14. According to the invention, output 15 of transducer 13 controls a control circuit 16 the output of which is employed for controlling breaker switch 17.
A preferred embodiment of control circuit 16 is shown in fig. 2. As shown therein, the voltage appearing at the output 15 of transducer 13 is amplified by an amplifier 18 and passed through a high-pass filter 19 for controlling a peak detector 20, the output of which is applied to a threshold value stage 21. Stage 21 is of a per se known type adapted to generate a DC output voltage only when the amplitude of a signal applied to its input excedds a predetermined value for at least a certain period of time. The output of stage 21 is connected to a first input of an "OR" gate circuit 22, the output of which controls breaker switch 17. In particular, breaker switch 17 is operated to close by a voltage appearing at the output of gate circuit 22.
The output of amplifier 18 is appied, in addition to high-pass filter 19, to a second threshold value stage 23 substantially similar to stage 21, but having an inverting output. The two threshold value stages 21 and 23 may for instance be formed by respective sections of an integrated circuit LM 324 made by RCA. For the sake of simplicity, the various connections for the power supply to the various components are not shown in figs. 1 and 2. The present invention is substantially based on the finding that, depending on various structural and functional properties of the hydraulic circuit of the dishwasher familiar to those# skilled in the art, the initial operation of circulating pump 9 develops in a particular manner as shown in fig. 3 after pump 9 is started for operation at least during the last part of the charging phase, during which electrovalve 6 is open for admitting water to the washing tub (as described for instance in the cited GB Patent 1,595,556). After an initial period to - t1, the water supplied to well 10 attains a level at which pump 9 starts to draw in water, whereupon its mean output pressure (represented by curve 24) rises to attain a rated value P1 at instant t2. During this period the operation of pump 9 is "erratic", because the water it draws in is intermixed with air. As the water level in tub 5 rises, the amount of the aspirated air diminishes, until a minimum water level for saturation of the pump is attained, so that the operation of the pump is stabilized at the rated pressure. Also during this period, the instantaneous output pressure of pump 9 develops in the manner indicated by curve 25 in fig. 3. Starting from point t1, curve 25 shows decreasing oscillations until point t2 is attained, at which the output pressure of pump 9 attaines the rated value P1, and the amplitude of the oscillations has decreased to a minimum. On the base of these considerations it is possible to determine the minimum water level to be admitted to the dishwasher, i.e. the minimum water level required and sufficient for ensuring proper washing action, by measuring the pressure variations at the outlet of the pump and closing the inlet electrovalve 6 as soon as the amplitude of these variations decreases below a predetermined value,i.e. as soon as the saturated operative condition of pump 9 is attained.
Considering the operation of the control system according to the invention, it is noted that the voltage at the output 15 of transducer 13 develops in a manner corresponding to curve 25 in fig. 3, which may be represented by a continuous component corresponding to curve 24 in fig. 3, with an attenuated oscillatory component superimposed thereon.
At the beginning of a charging phase for admitting water to the tub of the dishwasher, the program unit 7 causes the entire system to be energized and controls the opening of electrovalve 6 by actuating braker switch 17 to the closed condition. The output of Transducer 13 is at zero voltage, as is also the input of threshold value stage 23. This causes the latter to generate an output voltage which is applied to OR gate 22 for actuating breaker switch 17 to its closed state, as already stated, independently of the output s.ignal of stage 21.
At the beginning (or during the last part) of the charging phase, program unit 7 also causes circulation pump 9 to be energized so as to initiate its operation at instant t1.
As a result, transducer 13 generates the above described osciallting output voltage, which is amplified by amplifier 18 and applied to threshold value stage 23, the output voltage of which drops to zero as the respective threshold value is attained. During this period between instants t1 and t2, breaker switch 17 nevertheless 'remains closed as a result of the output signal of amplifier 18 being applied to high-pass filter 19, causing the latter to supply to peak detector 20 only an alternating voltage component representing the variations of the outlet pressure of pump 9 independently of its absolute value. Peak detector 20 supplies threshold value stage 21 with a continuous voltage proportional to the peak value of the above mentioned alternating component. During the period t1 - t2, this voltage exceeds the threshold value of stage 21, so that the latter generates an output voltage which is applied to gate circuit 22 for maintaining breaker switch 17 in the closed state.
Solenoid valve 6 therefore continues to admit water to washing tub 5 until, at instant t2, a water level is attained which is sufficient for saturated operation of Pump 9. At the same time, and as explained above, the input voltage of threshold value stage 21 drops to below its threshold value (which by the way may be varied as required by suitably dimensioning its components), so that the output voltage of stage 21 drops to zero.
The output voltage of the other threshold value stage 23 is also zero, because the voltage applied to its input (corresponding to the absolute value of the outlet pressure of pump 9) continues to be above the threshold value of stage 23. As a result, the output of OR gate circuit 22 cummutates to zero voltage, causing breaker switch 17 to open, and solenoid valve 6 to close accordingly.
In accordance with the object of the invention, the charging phase for the admittance of water to the dishwasher is terminated as soon as the minimum water level resquired for proper operation of pump 9 is attained. This is accomplished by means of a control system which takes into account practically all of the parameters (amount of dishes and the like, characteristics of the pump and of the hydraulic circuit of the machine etc.) required for determining the optimum water level. It is further of fundamental importance that the control system according to the invention, being based on the detection of variations of the pump outlet pressure as well as of the absolute value thereof, is substantially not susceptible to malfunction caused for instance by tolerances of transducer 13 or thermal deformations which might affect its functions, and completely independent of tolerances of the hydraulic circuit of the machine. The described water inlet control system may of course be modified in various manners.
It is to be particularly noted, however, that the above discussed considerations lead to the conclusion that the system according to the invention functions properly to control the control means 16 on the base of any physical value corresponding to the outlet pressure of the circulation pump. Such physical value, developing analogously to the curve 25 in fig. 3, may for instance be the current or the active potential absorbed by the electric motor of pump 9, or the speed of rotation of the motor, or even the speed of rotation of spray rotor 11 (employing for instance a device of the type described in European Patent Application No. 81107639.7). In any of these cases the transducer 13 employed will of course have to be of a type adapted to the specific concept as will be realized by those skilled in the art.

Claims

1. A water inlet control system for an automatic dishwasher, comprising a water inlet valve controlled by a program unit so as to open during at least one charging phase, and a water circulation pump controlled by said program unit to operate at least during the last part of the charging phase, a transducer being provided to generate an output signal proportional to the oulet pressure of said pump, characterized in that the output signal (15) of said transducer (13) is applied to control means (16) responsive to variations of said output signal for causing said-inlet valve (6) to be closed when the amplitude of said variations decreases below a predetermined value.

2. A water inlet control system according to claim 1, characterized in that said control means (16) comprise a first threshold value stage (21) the input of which is controlled by the output signal of said pressure transducer (13) through a high-pass filter (19), the output of said threshold value stage being operative to control said inlet valve (6).

3- A water inlet control system according to claim 2, characterized in that the output of said first threshold value stage (21) is connected to a first input of an OR gate circuit (22)-having a second input connected to the input of said high-pass filter (19) through a second threshold value stage (23), the output of said gate circuit being operative to control a breaker switch (17) disposed in the electric energizing circuit of said inlet valve (6).