GB2105184A - Control of windscreen wipers - Google Patents

Abstract

In a control circuit for a windscreen wiper mechanism which includes means for sensing the presence of liquid on the exterior of the windscreen and actuating means responsive to the sensing means to operate the windscreen wiper mechanism when liquid is present, the sensing means is an electrically capacitive means located internally of the windscreen in a region of the latter swept by the wiper mechanism.

Description

SPECIFICATION Control of windscreen wipers The present invention relates to a control circuit for a window wiper mechanism and more particularly to a control circuitfordetectingthepresenceofliquidon a window and in response thereto activating the window wiper mechanism.

Automatic windscreen wiper control circuits are known, but hitherto the means for sensing rain on the windscreen has always been located externally thereof. This leads to the gradual damage of the rubber wiper blades by the external sensor, scratching and damagetothe sensor by dust, grit and stones,the necessity of feeding leads through the windscreen and of fixing the sensor with weather-proof adhesive.

It is an object of the present invention to obviate or mitigate these disadvantages by locating the sensor internally of the windscreen.

According to the present invention there is provided a control system for a window wiping mechanism, comprising means for sensing the presence of liquid on the exterior of a window to be wiped and means responsive to said sensing means to activate thewindowwiping mechanism when liquid is sensed thereon, characterised in that said sensing means comprises electrically capacitive means located orfor location internally of the window and in a position corresponding to that swept by the wiping mechanism, the electrical capacitance of which varies when liquid is present on the exterior of the window.

The invention also includes within its scope a windscreen carrying suitable sensors, the control circuit in combination with the windscreen wiping mechanism, a vehicle including such a control circuit and kit of parts for constructing such a circuit and connecting it to a vehicle.

Embodiments of the invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which: Figure 1 shows a first embodiment of a control circuit involving detection of liquid by capacitance increase above a predetermined level; Figure 2 shows a second embodiment of a control circuit involving detection of liquid by abrupt capacitance change; Figure 3 is a front elevational view of a sensor for use in the above circuits; and Figure 4 is a cross-sectional view taken along the line IV-IV of Figure 3.

Referring now to the drawings, a conventional self-parking windscreen wiper assembly is connected atW and is controlled by the switching of relay 1. The relay 1 can, of course, be replaced by a solid state device, e.g. a thyristor, having a similar function. If necessary of course, a switch can be provided to by-pass the relay 1 and operate the windscreen wip ers normally.

In a first embodiment ofthe invention, a sensor 2, in the form of a variable capacitor, forms part ofthe time constant of a first oscillator 3 which has a fixed resis tance 4. A reference capacitorS forms part ofthetime constant of a second oscillator 6 which has a variable resistance 7. This variable resistance 7can be set so that the frequency of the second oscillator 6 is just below that of the first oscillator 3. When rain falls on the windscreen, the value of variable capacitance 2 increases, thereby slowing down the first oscillator 3 to a frequency below that of the second oscillator, whereupon a D-type flip-flop 8 connected to the output of each oscillator changes over and actuates the relay 1.

Connection 9 is a trigger from the windscreen wipers, whereby the circuit can be reset when the wipers have parked. In this respect most wiper mechanisms short circuit the motor when the wipers are parked to prevent any overshoot of the wipers and this short circuiting of the motor can be used to provide the necessary signal on the trigger line 9.

The first and second oscillators 5 and 6, are identical, and therefore behave similarly at different temperatures and voltages so that the unit is operable over a wide range of conditions.

The variable resistor 7 can be used to vary the oscillating frequency of the second oscillator 6 and thereby vary the amount of water on the windscreen needed to operate the system.

When the windscreen wipers have cleared the rain from the windscreen, the value ofthe variable capacitance 2 decreases, the first oscillator frequency increases and the flip-flop 8 changes back thereby deactivating the relay.

In a second embodiment of the invention, there is again provided a sensor 2, in the form of a variable capacitor, which forms part ofthe time constant of an oscillator 10. When rain falls on the windscreen, the frequency of the oscillator 10 changes abruptly and an output is given by FM detector demodulator circuit 11. This output pulse triggers a mono-stable multivibrator 12 and activates the relay. Once the windscreen wipers have cleared the windscreen of rain, the relay again becomes deactivated.

Self-parking wiper mechanisms are well suited to work with these control circuits. As an alternative, the relay (or solid state device) could control a wiper mechanism with "single wipe" facility by just making momentary contact. When the single wipe is complete, the circuit becomes reset.

It will be appreciated that other circuits responsive to changes in capacitance to provide an output signal to trigger a relay or its equivalent may be used in preference to the above. For example, a self stabilising phase lock loop circuit may be used to provide an output. Alternatively, a capacitance bridge circuit may be used, where the sensor capacitance is compared to a varactor diode capacitance whose control voltage is derived from a self balancing feed back loop. As a further alternative a high voltage charge may be applied to the sensor which voltage is monitored by a high input impedance circuit, so that any change in the capacitance of the sensor causes a change in the voltage across the sensor which is detected by the high input impedance circuit.

In Figure 3, there is shown in more detail one possible geometry for the sensor 2 which comprises a number of flat plates connected to form an interlocking finger type arrangement. These plates 13 and 14 are so arranged thatthe capacitance between them is low until a water droplet 16 falls on the exterior of the windscreen 15. The plates may if desired by made of tin oxide, indium oxide, or other such conductive coatings which are transparent. They may be set into the windscreen 15 or, as shown, be coated or adhered onto the surface thereon.

The above control circuit has many advantages over a conventional intermittent wiper control since the intermittent action is automatically adjusted to the rate at which the windscreen is being fouled by rain. Also if the windscreen is splashed with water by e.g. a passing lorry, the system immediately operates to clearthe windscreen and automatically stops itself, thereby maintaining the driver's vision without his being distracted by searching for a switch.

It will be appreciated that the frequency of the oscillator can be selected to provide a relatively large change in frequency of oscillation for a relatively small change in the capacitance of the sensor.

It will be manifest that the variable resistor7 can be replaced by a variable capacitor.

Finally, it will be understood that where the capacitive sensor is employed to change the output frequency of an oscillator any conventional means responsive to changes in frequency may be employed to detect or indicate said changes.

Claims (20)

1. A control system for a window wiping mechanism, comprising means for sensing the presence of liquid on the exterior of a window to be wiped and means responsive to said sensing means to activate the window wiping mechanism when liquid is sensed thereon, characterised in that said sensing means comprises electrically capacitive means located or for location internally of the window and in a position corresponding to that swept by the wiping mechanism, the electrical capacitance of which varies when liquid is present on the exterior of the window.

2. A control system according to claim 1, characterised in that said electrically capacitive means comprises electrically condutive material.

3. A control system according to claim 2, characterised in that the electrically conductive material is embedded in the window material.

4. A control system according to claim 2, characterised in that the electrically conductive material is on the interior surface of the window.

5. A control system according to claim 3 or 4, characterised in that the electrically conducting material is transparent.

6. A control system according to anyone of claims 2to 5 characterised in that the electrically conducting material is configured to minimise interdigitated capacitance plates side-by-side.

7. A control system according to anyone of claims 2to 6, characterised in that the electrically conducting material is on or for only a portion of the area corresponding to that swept by the wiping mechanism.

8. A control system according to any preceding claim, characterised in that the means responsive to said sensing means comprises electrical signal producing means whose output varies according to variations in the capacitance of said electrically capacitive means.

9. A control system according to claim 8, characterised in that the electrical signal producing means comprises a variable frequency signal source in which said electrically capacitive means comprises part of a time constant setting means.

10. A control system according to claim 9, characterised in that the variable frequency signal source comprises an oscillator whose output is sensed for variation from an expected or normal frequency corresponding to absence of any or sufficient liquid.

11. A control system according to claim 10 characterised in that means for sensing frequency variation serves to operate the wiping mechanism.

12. Acontrol system according to claim 9, characterised in that the variable frequency signal producing source is an oscillator whose output is compared with the output from a further preset oscillator.

13. A control system according to claim 12, characterised in that means are provided for detecting when the first-mentioned oscillator frequency output is higher than that of the further preset oscillator and for activating the wiping mechanism accordingly.

14. A control system according to claim 12 or claim 13, wherein the preset oscillator is frequencyadjustable to determine the amount of liquid required to be on the window in order to operate the wiping mechanism,

15. A control system according to claim 8, characterised in that the said electrical signal producing means comprises a high input impedance circuit connected to the electrically capacitive means to monitor changes in a voltage applied thereto as a result of variations in the capacitance of the capacitive means and to provide an output signal accordingly.

16. A control system according to any preceding claim, characterised in that the wiping mechanism is of an automatic return-to-rest type.

17. A control system according to claim 16, characterised in that each stroke of the wiping mechanism is determined by the control system.

18. A control system for a window wiping mechanism arranged and adapted to operate substantially as herein described with reference to the drawings.

19. A vehicle fitted with a control system according to any preceding claim for its windscreen and/or rear window and/or lamp(s).

20. Kit-of-parts constituting a control system according to any preceding claim for fitting to a vehicle.