Relays are utilized for a variety of applications. In most appliances, several different types of relays (switches) are necessary to insure that the variety of switching circuitry in the appliance operates properly. For example, in a dishwasher, the heater and motor loads require a switch rated at 3-7 amperes, while the detergent and rinse load actuator requires a switch rated at less than 1 ampere of load current. Similarly, in a refrigerator, the compressor may present a 12 ampere load, the defrost heater may present a 5 ampere resistive load, the ice maker motor and heater load may draw 1-2 amperes, and the ice dispenser motor may draw less than 1 ampere of load current.
In conventional appliances, each of the loads or switches would require a separate electromechanical or solid state relay device. Accordingly, in such an appliance, a variety of different relays would be required.
Since these relays are bulky components, they are not easily inserted in the control board of the appliance by automated machinery, leading to higher assembly costs. The use of a variety of relay types also reduces the number of any individual type of relay used in the appliance, and thus increases the unit cost and the inventory necessary at the factory and at various service locations. Each of the individual relays further requires a protective cap or cover, which increeases the overall appliance cost.
There are many types of relays that can be utilized in an appliance for the various switching functions located therein. In the past, the relays have been either electro-mechanical or solid state devices. Electromechanical devices present many disadvantages including large size and weight, high power consumption, and lack of reliability. When used in an appliance such as a refrigerator or a washing machine, the sheer size and complexity of the appliance greatly exaggerates these disadvantages.
Solid state devices, while much smaller in size and requiring less power than electomechanical relays, present the disadvantages of fragility in most types of real-world operating conditions. This fragility gives a relay system implemented with solid state devices a potentially high failure rate, making them difficult and expensive to maintain.
It is known in the art to use piezoelectric relays. Relays of this type are particularly well suited to use in appliances or the like. In general, the physical size of a piezoelectric relay is much smaller than the size of an equivalent electromechanical relay. It is known that a number of individual relays can be constructed from a common piezoelectric bimorph structure. A detailed description of known bimorph structures follows.
A bimorph structure typically consists of two elongated strips of piezoelectric material bonded to a center conducting strip. The outer surfaces of the two elongated strips, which are isolated from the center conductor, are covered with a conducting material to form outer electrodes. Each of the elongated strips is polarized such that the application of an electric field across the narrow dimension of the strip results in a change in the length of the strip. In previous relays which employ this type of actuator, the electric field is appied to the two strips in the bimorph structure such that one of the two strips is shortened while the other of the two strips is lengthened.
The application of these electronic fields results in a deflection of the bimorph in a direction perpendicular to the axis of the elongated strips. This deflection is typically utilized to make or break an electrical circuit by causing one contact on the bimorph to touch or move away from a second contact.
In previously known piezoelectric relays, the relays are made by cutting gaps in a bimorph element such that each outer electrode of each finger is isolated from the corresponding outer electrode of the neighboring finger. Each of the fingers is similarly driven by circuitry contained in a chip or other integrated circuit device. However, in previously known piezoelectric relay modules, each relay has the same load rating. Thus, previous piezoelectric relay modules have not been utilized for providing relays capable of handling a variety of electrical loads.
Accordingly, what is needed is a relay module that can be utilized in an appliance or the like to handle the different rated loads therein. What is also needed is a relay module that is easily adaptable to different load configurations. Finally, what is needed is a module that can be customized for a particular load configuration.
Broadly, it is an object of the present invention to provide a piezoelectric relay that can control multiple loads within a single device.
It is another object of the present invention to provide a piezoelectric relay that is less costly than previously known relays to control such devices.
It is a further object of the present invention to provide a piezoelectric relay that permits multiple load control in which all of the different loads can be controlled in a single modular design, thereby reducing the overall cost of the relay.