The present invention relates to a wiper control device which controls a wiper for an automobile. Conventionally, a wiper control device in which a wiper motor is controlled by a solid state switching element is known. For example, a xe2x80x9cwiper control devicexe2x80x9d which controls a wiper motor using two field effect transistors (FETs) is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 5-58250. The structure of this wiper control device is shown in FIG. 1.
This wiper control device comprises a wiper motor 12 included in a wiper motor circuit 11, and a wiper switch circuit 17. When a high speed sweep mode (HI) is set, the wiper switch circuit 17 connects a power source +B to the high speed operation terminal of the wiper motor 12. When an intermittent sweep mode (INT) is selected, a FET drive circuit 20 in a control circuit 16, which operates in response to instructions from the wiper switch circuit 17, operates a first FET 21, and controls the wiper motor 12 so as to realize intermittent sweeping. When the high speed sweep mode is set, a current due to back electromotive force is outputted to the low speed operation terminal of the wiper motor 12. Then the flow of current into the first FET21 is prevented by diodes D1 and D2 for preventing reverse flow, and the first FET21 is prevented from breaking.
However, the wiper control device disclosed in the aforementioned publication has the following problems. Namely, when a second FET 22 is turned on and braking operation is carried out, a voltage drop arises due to a diode D3 which is inserted downstream (at the source side) of the second FET 22, and the braking efficiency deteriorates.
Further, if the first FET 21 is turned off in a state where the wiper motor 12 is locked, the extra large current which has flowed until then (the current differs in accordance with the wiper motor, but is on the order of 5 to 30A) is rapidly cut off. Therefore, a back electromotive force V (=xe2x88x92Ldi/dt) due to an inductance L of the wiper motor 12 arises, and is expressed as a surge. As the result, there is the possibility that the diode D3 inserted downstream of the second FET 22 and the first FET 21 will cause avalanche breakage.
Further, because the diodes D1 and D2 are inserted on the path of the main current from the plus terminal of a battery 14 via the first FET 21 to the wiper motor 12, a voltage drop arises and the driving voltage of the wiper motor 12 falls. During low speed rotation when the state of the wiper switch circuit 17 is the intermittent sweep mode (INT) and the low speed sweep mode (LO), the motor current always flows in the diodes D1 and D2, and much of heat in correspondence with the voltage drop at the two diodes D1 and D2 is generated, and therefore a heat dissipating plate, a blower mechanism, and the like are needed. Moreover, large-sized diodes D1 and D2 for electric power are needed in order to impart sufficient current capacity. In the similar way, the current supply to the wiper motor at the time of the low speed continuous operation is carried out by the first FET 21, then the first FET 21 must be made to have a current capacity and a heat dissipating ability such that the FET can withstand continuous current, and the device becomes larger.
The object of the present invention is to solve the various problems which were described above, and to provide a wiper control device in which a solid state switching element is used, which is compact and highly dependable, and in which movements are reliable.
A first aspect of the present invention provides a wiper control device which comprises a wiper motor for moving a wiper, a switch which outputs a signal which inverts at each single sweep of the wiper, a combination switch instructing movement of the wiper, a first solid state switching element which drives the wiper motor, a controller which turns the first solid state switching element on and off intermittently on the basis of the signal from the switch in a case in which intermittent wiper movement is instructed by the combination switch, and a second solid state switching element which is connected in series to the first solid state switching element and is turned on by the power source voltage.
In accordance with a second aspect of the present invention, there is provided a third solid state switching element, which is connected in series to the second solid state switching element and which is for braking the wiper motor, and the controller, after turning the first solid state switching element off, turns the third solid state switching element on, thereby forming a closed circuit which is for making reverse current flow to the wiper motor so as to brake the wiper motor.
In accordance with a third aspect of the present invention, in a case in which the signal from the switch is not switched for a predetermined time, the controller turns the first solid state switching element off.
In accordance with a fourth aspect of the present invention, the wiper motor further comprises a high speed operation terminal which is for rotating at high speed and a low speed operation terminal which is for rotating at low speed, and in a case in which high speed movement is instructed by the combination switch, the high speed operation terminal of the wiper motor is directly connected to the power source and driven, and in a case in which low speed movement is instructed by the combination switch, the low speed operation terminal of the wiper motor is directly connected to the power source and driven.
In accordance with a fifth aspect of the present invention, the wiper motor comprises one power source terminal which is for rotating at a predetermined speed, and in a case in which a rotation movement is instructed by the combination switch, the power source terminal of the wiper motor is directly connected to the power source and driven.
In accordance with a sixth aspect of the present invention, the first solid state switching element is provided between the wiper motor and a power source, or between the wiper motor and ground.
A seventh aspect of the present invention further comprises a second wiper motor which is for moving a second wiper, a second switch which outputs a signal which inverts at each single sweep of the second wiper, a second combination switch which instructs movement of the second wiper, and a relay which drives the second wiper motor, and the controller controls the relay in response to the instruction of the second combination switch and drives the second wiper motor.