GB2157902A - Windscreen wiper motor protection circuit - Google Patents

Abstract

A motor protection circuit for a windscreen wiper motor incorporating a limit switch which is actuated periodically in use whenever a wiper blade driven by the motor is at a parking position includes a timer (R2 C1) which is reset whenever the limit switch (11) is actuated but runs whenever the limit switch is not actuated whilst the motor is energised. The timer controls an interrupter (Q1, 12, 12a) to disconnect the motor from its supply if the limit switch remains unactuated for a predetermined time. A further timer (R10, C2) operates the interrupter if the limit switch remains actuated for a predetermined time while the motor is energised. <IMAGE>

Description

SPECIFICATION Windscreen wiper motor protection circuit This invention relates to a windscreen wiper motor protection circuit intended to interrupt the current path through the motor if the motor stalls.

Generally speaking, windscreen wiper motors are built to withstand stall current for a short time, but it is desirable to ensure that stall current is not allowed to flow for too long, such as can occur if the windscreen wiper blade becomes stuck to the windscreen by frost or otherwise. It has been found, however, that simple detection of the current level is not satisfactory, since the stall current and the current in normal high load conditions differ very little. It is accordingly an object of the invention to provide a protection circuit which detects stall without detecting motor current directly.

In accordance with the invention there is provided a motor protection circuit for a windscreen wiper motor incorporating a limit switch which, during running of the motor is activated periodically when the windscreen wiper driven by the motor is at a park position, the circuit comprising timer means connected to said limit switch so as to be reset on actuation of the limit switch, motor circuit interrupter means connected to the timer means so as to be actuated thereby to interrupt motor current whenever a predetermined interval passes without the timer means having been reset, and latch means for latching said interrupter means in its actuated condition.

Preferably the circuit includes a further timer means connected to said limit switch so as to be reset thereby whenever the limit switch is not actuated, said further timer means also being connected to a terminal of the motor so that said further timer means only runs when an operating voltage is applied to said terminal, the interrupter means being connected to said further timer means so as to be actuated thereby when the operating voltage is applied to said motor terminal, but said limit switch remains actuated for a predetermined time.

An example of the invention is shown diagrammatically in the accompanying drawing.

The motor M has a fast brush F, a slow brush S and a grounded brush G and the application of operating voltages to the brushes S, F is controlled by a conventional wiper control circuit 10. The motor incorporates, as is also conventional, a limit switch 11 which has a common terminal C, a grounded contact P which is closed when the windscreen wiper blade is at one end of its travel (i.e. at a park position), and a contact R which is at supply voltage whenever the windscreen wiper blade is out of the park position. Since the above described arrangement is very well known in the art no further description thereof will be given herein and the operation of the motor and wiper control in its various well known, slow, fast, intermittent and flick wipe modes will also be taken as generally understood by those versed in the art.

The protection circuit of the present invention operates by detecting the state of the limit switch 11 and is not sensitive in any way to the motor current. To this end, the common contact C of the limit switch is connected to the input of a first timer means which includes a capacitor C, one terminal of which is grounded and the other of which is connected via two resistors R, and R2 in series to the common contact C. A zener diode ZD, connects the junction of these resistors R1, R2 to ground and a diode D, is connected in parallel with the resistor R2 with its cathode connected to the junction of the resistors R1, R2. A further resistor R3 connects the common contact C to ground.The arrangement described above is such that, whenever the windscreen wiper blade is out of its park position, the supply voltage is applied via the limit switch through the resistor R, to the zener diode ZD,. Capacitor C1 charges relatively slowly through the resistor R2, which is of high ohmic value compared with resistor R,. Whenever the windscreen wiper blade is in its park position, the capacitor C, discharges relatively quickly through diode D1, resistor R1 and the limit switch 11. Thus each actuation of the limit switch as the wiper blade reaches its park position on each stroke, causes resetting of the timer means.

The timer means controls a motor circuit interrupter which includes a relay 12 having a normally closed contact 1 2a. The relay winding is connected at one end to the supply and at the other end to the collector of an npn transistor Q,, the emitter of which is grounded. The base of transistor Q, is connected to the cathode of a diode D2, the anode of which is connected to the junction of two resistors R4, R5 connected in series across the capacitor C,. The contact 1 28 controls the connection of the supply to the wiper control 10, so that if capacitor C, charges sufficiently for transistor Q, to turn on and energise the relay 12, the supply to the wiper control is interrupted and the motor circuit is broken.

The values of the circuit components are so chosen that this occurs after a predetermined interval of charging of capacitor C, (without resetting) which is significantly longer than the longest normal return stroke of the wiper blade likely to be encountered in normal use.

It will be appreciated that interruption of the supply to the motor will leave contact R floating, so that capacitor C, will discharge slowly through resistors R1, R2 and R3 in series. To prevent relay 12 dropping out after a time, a latching circuit is provided which includes a pnp transistor Q2 which has its emitter connected to the supply via a diode D3, and its base grounded via a resistor R6 and connected by another diode D4 to the contact 1 2a. The collector of the transistor Q2 is connected by a resistor R7 and a diode D5 in series to the base of transistor Q1. Thus transistor Q, is normally kept off by the diode D4, but if relay 12 is energised transistor Q2 turns on and provides the required latching action.

Whilst the circuit described above provides protection against a stall occurring when the wiper blade is out of its park position it does not provide protection against a stall when the blade is in the park position. Such additional protection is provided by another timer means connected to the common contact C. This further timer means includes an npn transistor Q3 having its base connected to common contact C by a resistor R8 The emitter of the transistor Q, is grounded and its collector is connected to one terminal of a capacitor C2 the other capacitor of which is grounded. A resistor R9 is connected across the capacitor C2 to provide a relatively slow discharge path therefor.The first-mentioned terminal of the capacitor C2 is connected by two resistors R,O, R11 in series to the fast brush F of the motor, and a zener diode ZD2 connects the junction of these resistors R,o, R11 to ground. The transistor Q3 is on whenever the windscreen wiper blade is out of its park position and holds capacitor C2 discharged. If, while the wiper blade is in its park position, the motor is energised, the operating voltage which appears at the fast brush, causes capacitor C2 to charge. A resistor R12 and a diode D6 in series connects the first-mentioned terminal of capacitor C2 to the base of the transistor Q1 so as to turn the latter on when the capacitor C2 has charged sufficiently, i.e. after a predetermined time duration of stall with the limit switch actuated.

Claims (6)

1. A motor protection circuit for a windscreen wiper motor incorporating a limit switch which, during running of the motor is activated periodically when the windscreen wiper driven by the motor is at a park position, the circuit comprising timer means connected to said limit switch so as to be reset on actuation of the limit switch, motor circuit interrupter means connected to the timer means so as to be actuated thereby to interrupt motor current whenever a predetermined interval passes without the timer means having been reset, and latch means for latching said interrupter means in its actuated condition.

2. A circuit as claimed in claim 1 including a further timer means connected to said limit switch so as to be reset thereby whenever the limit switch is not actuated, said further timer means also being connected to a terminal of the motor so that said further timer means runs only when an operating voltage is applied to said terminal, the interrupter means being connected to said further timer means so as to be actuated thereby when the operating voltage is applied to said motor terminal, but said limit switch remains actuated for a predetermined time.

3. A circuit as claimed in claim 2 in which said interrupted means comprises a relay having a winding connected in series with a semiconductor switch device controlled by said timer means and said further timer means, and a contact connecting the motor to a supply.

4. A circuit as claimed in claim 3 in which each timer means includes a capacitor, a resistor through which said capacitor charges when the timer means is not being held reset and a diode connecting said capacitor to a control electrode of the semiconductor switch device.

5. A circuit as claimed in claim 4 in which said latch means comprises a further diode connected to said control electrode and to said contact.

6. A motor protection circuit for a windscreen wiper motor substantially as hereinbefore described with reference to the accompanying drawings.