GB2185110A - Temperature sensing apparatus for indicating icy road conditions - Google Patents

Abstract

Apparatus for detecting and indicating the risk of ice on a road surface comprises a temperature sensitive element 11 adapted to be mounted in a position where it is exposed to the air flow past a motor vehicle when the motor vehicle is in operation. The sensor 11 is connected in a bridge circuit 12 the voltage across which is supplied to an amplifier 22 acting as a primary comparator and the output from which is supplied to three comparators 24,25,16. Two of the comparators 24,25 receive the output signal from the amplifier 22 on their inverting and non-inverting inputs respectively, the other inputs being supplied by a potential divider setting the two comparators to different voltage levels. Outputs from the comparators 24,25 are supplied to illuminable indicators 20,21 the first of which is energised and illuminated below a first threshold value e.g. 5 DEG C, determined by the comparator 16, and above a second threshold value in the region of 0 DEG C, and the other of which is extinguished above a third threshold value e.g. 2 DEG C, between the first and second threshold values, and illuminated below the said third threshold value. <IMAGE>

Description

SPECIFICATION A temperature sensitive device The present invention relates to a temperature sensitive device, and particularly to a device sensitive to ambienttemperatures and operable to provide an indication of the attainment of a critical temperature or temperature range.

Such devices are being provided more commonly on motor vehicles, the critical temperature range being between 2"C and 0 C at which ice may form on the road. Such devices are loosely termed "ice detectors" although in factthey operate merely to de tect ambient conditions at which the likelihood ofthe formation of ice is sufficiently high for there to be a serious possibility of the presence of ice on the road, rather than actually acting to detect such presence.

However,a warningto a driverthattheambientcon- ditions have become such that the presence of ice on the road is possible or likely can alert him or her to this danger and thereby ensure that greater care than normal is taken in conditions where the friction ofthe road surface may be in doubt.

Known such ice detectors include atemperature sensitive element which is positioned as close to the road surface as reasonably practicable in order to detect the ambienttemperature. Because the motor vehicle may be travelling ata high speed or low speed attempts have been made, when fitting pre viously known ice detectors, to avoid the so-called "wind chill factor" caused bythevarying wind speeds which would be experienced by a tem perature sensor, and this has been achieved by en capsulating the temperature sensor in a body of such stantial thermal mass, such as the plastics material bumper or bumperfilling.This has been considered to offerthe advantage of positioning the sensor at the forward end ofthevehicleto give an earlywarn ing that the temperature ofthe ambient air is app- roaching zero degrees C, whilst at the same time avoiding the variations which would result from the sensor experiencing a different wind speed when the vehicle is travelling at different speeds, which itwas thoughtwouldcausethesensortogivedifferent, and inaccurate, readings due to variations in the wind chill factor at different speeds.

Unfortunately by mounting the temperature sensor in the bumper or other component of re latively high thermal mass the sensitivity ofthe equi pment is severely reduced and fluctuations in tem perature over a relatively short period, which can be experienced by a vehicle when travelling due to the fact that it can pass from a region of relatively warm airto a region of cold orfreezing airwithin a matter of minutes or less, particularly when travelling ata high speed, have not given rise to an accurate indica tion of the change in an adequately shorttime so that the vehicle may be in fact travelling in conditions with ice on the road for a relatively long period, in the region of tens of minutes, before the indicatorwill give an indication ofthis.

Known ice detectors also suffer from various other disadvantages; the circuits themselves must be cal ibrated carefully upon manufacture before they can be fitted to a vehicle in order to ensure thatthey operate over the critical range between 0 C and 2"C, and because oftemperature drift which can occur in service a periodic re-calibration of the circuit is advisable in orderto ensure that it functions in the correct range during its working life. The necessityforfactory calibration on manufacture obviously increases the cost of the item by introducing n additional operation which the manufacturer must perform before the circuit can be installed.The requirement for periodic re-calibration as a "maintenance" operation obviously adds to the cost of maintenance ofthevehicle by introducing an additional maintenance operation which must be performed periodically.

Finally, known ice detectors are susceptible to damage duetothevoltagefluctuations which area notorious feature of automotive electrical circuits.

The present invention seeks to provide a temperature sensitive circuit, usable as an ice detector, which will not be subject to the same disadvantages as known prior art detectors, which will have a very rapid response to changes in temperature, and which will provide a simple indication ofthe approach and attainment of the critical temperature to which the circuit is set.

To achieve this the present invention provides, in a first aspect of the invention, apparatus for detecting the risk of ice on a road surface bysensingtheair temperature at a position spaced above the road surface, in which a temperature sensitive element of the apparatus is mounted on a vehicle in such a position as to experience the airflow past the vehicle so as to operate with a flow of air over it when the vehicle is moving. This is in direct contradiction to the previous ideas on the positioning of such sensors, and is based on the realisation that the so-clled wind chill factor is one which only affects bodies which can be considered as sources of heat, such as the human body.Atemperature sensor operating in a region close to 0 C has a heat content which is very low and consequently there is no manifestation of the wind chill factor in such circumstances since the wind passing over the body cannot remove heat which is not thereto startwith.

The present invention also comprehends a motor vehicle having apparatus for providing a visual or audible indication of the attainment of a critical ambient temperature, including a temperature sensitive element mounted on a vehicle in a position such as to be exposed to the airflow past the vehicle when the vehicle is in movement.

A motorvehicle incorporating temperaturesensit- ive apparatus as defined hereinabove may include a thermistor as the temperature sensitive element, in which case the thermistor is preferably sheathed in a metal cladding or casing. This cladding orcasing does not constitute a thermal mass, but merely serves as a mechanical and physical protection for the thermistor and serves to transmit temperature fluctuations in the surrounding air by acting as a good thermal contact between the thermistor and the air.

The thermistor conveniently forms part of a wheat stone bridge circuit, the voltage across which varies as a result of the variation in the resistance ofthe thermistor with changes in temperature. The voltage across the wheatstone bridge circuit may be used via a set of comparators to control two illuminable indicators a first ofwhich is illuminated between first and second temperatures, and a second ofwhich is illuminated below a third temperature, which lies between the said first and second temperatures so that afirstofthe indicators is illuminated betweenthe first and third temperatures, both illuminable indicators are illuminated between the second and third temperature, and the second illuminable indicator is illuminated below the third temperature.For use as an ice detector the first temperature may be 5"C, the second temperature 0 C and the third temperature 2"C whereby to provide, with a simple indicator arrangement, an indication ofthe ambienttem perature when itis close to or very close to zero so that a drivercantell ata glancewhattheambient temperature conditions are without requiring to focus on the indicator for any length of time thereby taking his attention away from the road.

One embodiment ofthe invention will be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure Iisa blockschematicdiagram illustrating the apparatus ofthe present invention; and Figure2 is a circuit diagram illustrating the appar atus in more detail.

Referring nowto Figure 1,the blockschematicdia- gram illustrated shows a thermistor 11 which is con nected into a bridge circuit 12 the output from which is fed to a first comparator 13 which feeds three com parators 14,15 and 16. Thefirstcomparator 13 is supplied with voltage from a positive voltage source 17 via a voltage regulator 18, and this voltage is app lied across the comparator 13to a ground line 19.

The second, third and fourth comparators 14,15 and 16 actto control two illuminable indicators 20 and 21 which, in this embodiment, are differentiable bythe colour of emitted light, the indicator 20 emitting green light and the indicator 21 emitting red light The settings ofthe second, third and fourth comparators, 14, 15 and 16 respectively determine the operation ofthe circuit. These comparators are fed with a varying output signal from the comparator 13 in dependence on the voltage across the bridge 12 which is varied with a variation in temperature due to the varying resistance of the thermistor 11.The fourth comparator 16 is an "enable" comparator which is set at a voltage corresponding to that which appears at the output of the comparator 13 when the temperature sensed by the thermistor is in the region of 5"C. Above this temperature the comparator 16 maintains the display inactivated, and below this temperature the comparator 16 allows the display to be active.

The third comparator 15 is set to a voltage cor- responding to that which is at the output of the com parator 13 when the thermistor 11 is at 20Cand above 2"Cthe comparator 15 switched off whilst below a critical threshold in the region of 2"C it is switched on.

The second comparator 14 is set to a reference voltage which corresponds to that appearing atthe output ofthe comparator 13 when thethermistorex- periences a temperature in the regjon of 0 C. Above O"C the comparator 14isswitched on and below O"C the comparator 14 is switched off.

With these settings the operation of the device is as follows: Assuming thetemperature initially starts at 6"C and falls to 0 C during the time period covered bythe following explanation,the display indicators 20 and 21 will initially be in the extinguished state sincethe fourth comparator 16 will be in the,"off" condition because the voltage appearing atthe output of the comparator 13 will be higherthanthatappearing at the output of the comparator 13 when the thermistor 11 experiences a temperature in the region of 5"C. At this initial temperature of 6"C the comparator 1 4will be on (this being switched on at all times above from 0 C) butthe indicator 20 will not be illuminated because ofthe "enable" inhibition imposed by the comparator 16. The comparator 15, on the other hand, will be switched off since this does not come on until the temperature has fallen to the vicinity of 2"C.

As the tem perature falls to 50C the threshold set by the comparator 16 will be reached and the display comprising the indicator 20 and 21 will be enabled and, immediately, the green indicator 20 will be illuminated since the comparator 1A is already in the "on" condition and it only requires the commutation ofthecomparatorl6tocompletetheenablement.

This indicates to the driverthatthte temperature is approaching zero and is in the 2" and 5" range. As the temperature falls this condition remainsuntilthe temperature of 2"C is experienced bythethermistor 11, at which time the voltage appearing at the output of the comparator 1 3 will match the reference voltage of the third comparator 15 and this will thus switch on to cause illumination ofthe red indicator 21. The illumination of both the red and the green indicators 20 and 21 will inform the driver that the temperature has now reached close to the critical zero degrees and is within the range O"C to 20C.Since a temperature difference of up to 2"C may occur in winter between the ground temperature and the air temperature (which latter is detected by the sensor) the illumination of the red indicator 21 provides the driver with an early warning that care must betaken due to the possibility of the formation of ice on the road.

Finally, when the critical threshold temperature of 0 C is detected by the thermistor 11 the voltage appearing atthe output ofthe comparator 13 will match that set by the reference voltage of the second comparatorl4andthiscomparatorwillthusswitch offturning off the green indicator 20 leaving onlythe red indicator 21 illuminated indicating that the likelihood that ice may form on the road has increased and the driver is alerted by the presence of the single red illuminated display indicator 21 that now very great care must be taken.

Figure 2 illustrates the circuit described in block diagram form hereinabove in more detail.

The reference numerals used for identification purposes in Figure 1 will be usedforthe same componants or assemblies in Figure 2. Here, the wheatstone bridge generally indicated 12 has the thermistor 11 in one branch and three balancing resistors R1 0, R1 1, R12 in a conventional configuration forawheat- stone bridge. The voltage across the wheatstone bridge, taken from the junction between the resistor Rl0andthermistorll,and betweenthetwore- sistors R11 and R12 are fed to the inverting and non inverting inputs of an operational amplifier 22 for ming part ofthe first comparator 13, which further includes a resistor R9 providing negative feedback.

The output from the operational amplifier 22 is supplied to the inverting input of a furtheroperational amplifier 16 which constitutes the fourth comparator described hereinabove, and also to the opposite inputs of two other operational amplifiers 24, which form part of the second and third comparators 14, 15 in combination with biasing resistors R1, R2, R3 and R4 connected in series between the supply voltage line 26 and the ground line 19. The output from the operational amplifier 24 is supplied via a resistor R5 to a green light emitting diode con- stituting the indicator 20 and the output from the operational amplifier 25 is supplied via a resistor R6 to a red light emitting diode constituting the indicator 21.

The cathodes ofthe two light emitting diodes 20,21 are connected together and to the output of the amplifier 16.

The biasing ofthetwo amplifiers 24,25 by theresistors R1, R2, R3 and R4 determines their switching points, which are inverse with respect to one another since the junction between the resistors R1 and R2 is connected to the non inverting input ofthe amplifier 24 whilst the junction between the resistors R3 and R4 is connected to the inverting input of the amplifier 25. The voltage from the amplifier 22 is supplied to the inverting input of the amplifier 24 and the non inverting input ofthe amplifier 25.

Finally, the biasing of the fourth comparator constituted by the amplifier 16 is achieved via two resistors R13 and R14 connected again between the supply line 26 and the ground line 19, and connected in parallel with the capacitor C3 between the non inverting input ofthe amplifier 16 and the ground line 19.

Voltage supply to the circuit described here inabove may be made via voltage regulators of the type sold underthe identification numbers LM2931 CT made by National Semiconductor or UPC261 OH made by NEC, each connected in a suit able respective circu it to provide a constant ten volt output from the varying nominally 12 volt input of the automotive electrical circuit.

All the resistors, including the thermistor in the temperature sensing circuit are selected to have close tolerances of no worse than plus or minus 1% so the precision of the thermistor in operation is within plus or minus 0.2"C overthe temperature range O"C to +70"C. This corresponds to a resistance tolerance of iO.4% over this temperature range.

Moreover, by selecting such high precision resistors the circuit can be manufactured with the nominal values determined without requiring calibration or setting up in the factory, so that the subsequentoper ations after the circuit has been made and before it can be sold are eliminated.

An ice detector unit having technically superior operating characteristics, great accuracy and high reliability is thus provided by the device of the present invention. Further, the indicator scheme described hereinabove provides a safe and quickly read indication of the approach ofdanger conditions by first showing a green lightto indicatethatthetem- perature is low but not dangerous, then a red and green lighttoshowthatcaution mustbetaken,and finally a red light alone to show that dangerous conditions exist, all of which indications can be seen at a glance without requiring study of the display to evaluatethe indication.

Moreover, by positioning the thermistor in the free flowofair,forexampleunderthewing ofthevehicle and awayfrom the thermal influence of the heat of the engine, a very rapid response to temperature changes is obtained because of the low thermal mass of the thermistor and its casing within a metal tube, for example a stainless steel tube.As discussed hereinabove the anticipated wind chill factor does not in practice affect an unheated body to any extent within the tolerances of its operation and consequentlythe provision of a motorvehiclewith an ice detector having a temperature sensitive element in the free flow of air past the vehicle constitutes a radical departure from known techniques where the tem perature sensitive element of the ice detector has previously been shielded and encapsulated in components such as the front bumper where they have inevitably been influenced by the thermal mass of the vehicle itself. The provision of ,a high accuracy voltage regulator, together with the use of close tol erance components, particularly the precision thermistor avoids many of the disadvantages inherent in conventional detectors which have to be calibrated at the factory, which were subject to drift and consequently unreliable, and needed recalibration at intervals to maintain credibility of the display.

Claims (11)

1. Apparatus for detecting and indicating the risk of ice on a road surface by sensing the airtemperatureata position spaced abovethe road surface, in which a temperature sensitive elementofthe apparatus is adapted to be mounted on a vehicle in a position such as to be exposed to the airflow pastthe vehicle so as to operate with a flow of air over it when the vehicle is moving.

2. Apparatus as claimed in Claim 1, in which the temperature sensitive element is a thermistor sheathed in a metal cladding.

3. Apparatus as claimed in Claim 2, in which the thermistor is connected as part of a bridge circuitthe balance of which is influenced by variations in the resistance of the thermistor upon changes in the temperature thereof.

4. Apparatus as claimed in any preceding Claim, having a display comprising two illuminable indicators one of which is energised when thetemperature sensitive element experiences a temperature between a first and a second temperature threshold and the other of which is energised when the tem perature sensitive element experiences a tem perature less than a third temperature threshold, the said third temperature threshold lying between the said first and second temperature threshold.

5. Apparatus as claimed in Claim 4, in which a temperature sensitive variable resistor is connected in a wheatstone bridge circuit the voltage across which varies in dependence on the temperatures of the said variable resistor.

6. Apparatus as claimed in Claim 5, in which there are provided a set of comparator devices one input to which is supplied with a signal dependent on the voltage across the said wheatstone bridge circuit and another inputto which is a reference signal det ermining asaidthresholdvalue.

7. Apparatus as claimed in any of Claims 4 to 6, in which the said firstthreshold temperature is in the region of +5"C.

8. Apparatus as claimed in any ofClaims4to 7, in which the said second threshold value is substanti ally 0 C.

9. Apparatus as claimed in any of Claims 4 to 8, in which the said third threshold value is in the region of +2"C.

10. Apparatus for detecting and indicating the risk of ice on a road, substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

11. Amotorvehiclehaving apparatusfordetecting and indicating the risk of ice on a road as claimed in any preceding Claim.