The present invention relates to a washing machine, particularly a dishwashing machine, provided with an improved water supply control system.
It is known from U.S. Pat. No. 3,294,110 to provide a device for controlling the level of a liquid supplied to a tub. Such device includes a first chamber adapted to receive liquid (water) from a supply main and communicate via an overflow wall with a second chamber cooperating with a pressure switch adapted to control a solenoid valve for controlling the supply of water from the water main. The first chamber also communicates with the tub through an intake of a discharge pump. During operation, the water from the water main flows into the tub into the first chamber to fill the first chamber up to the level of the overflow wall, whereupon the water overflows into the second chamber, whereupon the pressure switch is abruptly actuated to close the supply solenoid valve. This control device is of simple construction and is reliable in operation. However, its use in a washing machine, particularly in a dishwashing machine, nevertheless is unsatisfactory because it is designed to function with the respective machine in an inoperative state, i.e. when the circulation pump is not operating. This means that the control device, contrary to what would be desirable, is not capable of stopping the water supply to the tub as soon as the minimum water level required for proper operation of the circulation pump has been achieved. Particularly, this control device does not take into account the various parameters such as the charge of dishes or the like, the amount of water in circulation, the characteristics of the pump and of the piping within the machine, which factors together determine the optimum level of water to be charged into the tub.
However, with the increasing concern about the conservation of resources, it becomes fundamentally important to reduce the amount of water supplied to a washing machine for each operating cycle to the absolute minimum required for effective operation. Accordingly, it has been proposed, as described in European Patent Application No. EP-A-0,118,719, to provide a dishwashing machine with a water supply control system of the dynamic type capable of taking into account all of the above mentioned parameters and factors. Particularly, this control system comprises a water supply valve controlled by a program control unit so as to open during a water supply phase during which the circulation pump of the machine also is in operation. A transducer generates an output signal proportional to the output pressure of the pump. The output signal of the transducer is applied to a control means responsive to variations of the output signal for controlling the valve when the magnitude of these variations drops below a predetermined value. While this control system is highly accurate in operation, it requires relatively complicated control and actuating components, particularly electronic components, which it would be desirable to eliminate.