GB2108304A - A device for signalling an excess temperature of a rotating member of a running motor vehicle - Google Patents

Abstract

The device consists of a detector, for instance a bimetallic element, which is switched on when reaching a given temperature and is connected in the circuit of a small radio transmitter mounted on the rotating member and actuated upon the detector switching, and of a fixed equipment, mounted on the vehicle and comprising a radio receiver connected to an optical and/or acoustical alarm device. According to a variant, the signalling generated by the detector is transmitted by exploiting the mutual induction between a fixed and a moving circuit. <IMAGE>

Description

SPECIFICATION A device for signalling an excess temperature of a rotating member of a running motor vehicle The present invention relates to a device for signal ing that the temperature of a rotating member in a running motor vehicle has exceeded a predetermined value. In the practice the function of the device according to the invention is to monitor the temperature of the tyres and/or of the braking system.

It is known that the tyres of a running motor vehicle, become heated because of the friction between them and the road paving.

Under normal conditions such heating is kept within acceptable limits, especialiy if the tyre pressure lies in a suitable range. Yet too high tyre temperatures can be reached because of a too low pressure' (for instance due to bad operation of the inner-tube valve, small punctures, wearing of the inner tube and so on) or of the heat received from outside, for instance because of the overheating of the braking system or of the exposure to the sun for a long time. Consequently the tread may unstick, with consequent possible tyre burst which causes very serious dangers.

Generally the tyre vulcanization temperature, during the manufacture, is of about 140 - 1 550C and the working temperature is deemed to be dangerous when it exceeds about 1 25 C.

A similar need for monitoring the temperature is encountered for the braking system which, in case of protracted braking, tends to become overheated.

Generally temperatures of the friction material up to 250"C, and for very short periods up to 500 - 550"C, are deemed acceptable. In this case two dangers occur: first, the braking effect decreases as the temperature increases and eventually it becomes insufficient to slow down or to stop the vehicle with very serious consequences, and, second, the developed heat is transmitted to the adjacent tyre, as already said.

In both cases the temperature of a rotating member is to be monitored during the run and an optical and/or acoustical signalling is to be transmitted to a position convenient for the driver, to alert him when a dangerous situation occurs.

The invention aims to provide a device carrying out said signalling.

The device comprises one or more temperature sensors or detectors connected with a transmission system rotating with the member to be monitored and actuating said alarm device.

The temperature detector may be a passive member, for instance a bimetallic element or a magnet which becomes demagnetized when the Curie point is exceeded, or the like.

The state of the temperature detector is transmitted by a suitable device using electromagnetic waves, for instance by a small radio transmitter. The required power is very small as a range of some ten meters is sufficient.

Generally it may be stated that the transmission method as well as the frequency (taking into account the frequency bands allotted by the international authorities) are indifferent.

In the practical embodiment the particular working conditions are to be taken into account, namely the mechanical stresses due to the rotation of the member (tyre or braking means) carrying the radio transmitter, and the thermal stresses. These problems are to be faced by circuital, technological and mechanical solutions.

From the circuitry standpoint, an extremely simple, realiable and cheap circuit, with very reduced size and consumption, is to be designed. Therefore it is convenient to generate a single frequency: the information will consist in the presence or absence thereof.

The convenience of signalling the anomalous condition by the presence of a frequency is also due to the consequent battery saving as the battery must operate only when a sensor detects the anomalous condition.

The frequency may be generated by an oscillator like those employed for generating the carrier in a conventional radio transmitter.

A good frequency stability is to be ensured to such a simple circuit to reduce to a minimum also the radio receiver complexity and to avoid the risk that a transmission is not received because transmitter and receiver are not tuned.

Due to the already mentioned environmental conditions, the frequency stability is to be obtained without resorting to a quartz oscillator, which is fragile and expensive.

It is possible to achieve a good stability by manufacturing the circuit as a thick film hybrid circuit which ensures an extraordinary mechanical robustness and a good heat distribution among its components and moreover allows a functional adjustment of said components and consequently a perfect tuning of the generated frequency.

From the circuit standpoint moreover it is conve nientto have a controlled sweep of the frequency generated about the tuning value. Two aims are thus achieved: no doubt the receiver will be tuned on one of the frequency values concerned by the sweep, so that failure to receive a transmission is not possible and, second, the received signal will have a pulsing behaviour and, because of its regularity it will be easily detectable, by the driver even in the presence of possible noises.

To obtain the oscillator frequency sweep it is sufficient to associate the oscillator with a ramp generator, i.e. a saw-tooth voltage generator.

Taking into account the installation conditions, a traditional antenna must be dispensed with.

From the mechanical standpoint, the maximum reliability will be achieved through the manufacture as a thick-film hybrid circuit as well as by embedding in a resin both the transmitting circuit and its supply battery.

The system will be packaged into two resins, namely an inner silicone resin, with shock-absorbing properties, and an outer, fluorocarbon resin, with a great resistance to atmospheric agents and shocks.

By manufacturing as a single block the transmitter and its supply battery, the greatest safety is achieved in the electric contacts, which are exposed neither to atmospheric agents nor to tamperings nor are affected by mounting errors.

According to a variant embodiment the transmission system may be based on the mutual induction between a primary winding carried by the fixed vehicle structure, and a secondary winding carried by the rotating member and located in such a position as to face the primary winding for a short interval at each revolution of the rotating member.

Other features of the invention will become apparent from the following description, with reference to the accompanying drawings in which: Figure 1 schematically shows in axial section a vehicle wheel on which the device according to the invention is mounted; Figure 2 is the block diagram of said device; qFigure 3 is the circuit diagram of the oscillator; Figure 4 is the diagram of the saw-tooth voltage generator; Figure 5 is the block diagram of the device in a variant embodiment operating by mutual induction; Figure 6 is a preferred embodiment of said variant.

Figure 1 shows the tyre 1 and the brake-drum 2 of a motor vehicle wheel, carrying a temperature detector 3 on the brake-drum and one or more detectors 4 on the tyre.

Figure 2 shows the general diagram of the device, consisting of a temperature detector Ra galvanically connected with a transmitter Ti whose output is connected via radio with a receiver Ri comprising an optical and/or acoustical alarm device G.

Figure 3 shows the diagram of the oscillator circuit, in which the collector of an NPN transistor 5 is connected with LC circuit 6,7 in turn connected with supply 8. The base of transistor 5 is connected to a voltage divider 9, 10 inserted between supply 8 and ground 11. A capacitor 12 acting as a filter is connected in parallel with resistor 10.

The emitter of transistor 5 is connected to ground 11 through a resistor 13 and a capacitor 14 in parallel, and is coupled to an inductor 15 acting as antenna.

A capacitor 16 connects collector capacitor 7 and emitter capacitor 14.

The operation of the circuit of Figure 3 is as follows: when the circuit is closed between supply 8 and ground 11, supply 8 charges capacitor 7 till the inhibition of transistor 5, which causes the capacitor discharge; then the cycle starts again.

The inductance of inductor 6 and the capacitance of capacitor 7 determine the oscillation frequency.

Switches 17, 18 and others, if any, in parallel therewith, schematize the temperature detector.

When the detector is a bimetallic element, it acts as an open switch as long as the temperature is below the predetermined threshold value, and acts as a closed switch when the threshold is exceeded.

Therefore the oscillator is not actuated as long as the temperature is below the predetermined threshold and is actuated, i.e. it transmits, when the temperature is above the threshold.

When there are several switches in parallel, as shown by switches 17, 18, the oscillator begins operating as soon as one of the switches closes.

A diode 28 with protection resistor 29 is inserted in parallel in the supply circuit; said diode emits light when the oscillator is operating.

The circuit described generates a single frequency, controlled by adjusting the capacitance of capacitor 7.

For the reasons set forth, it is preferred to generate a frequency cyclically varying between a maximum and a minimum value, close to each other. This is obtainable by a circuit as shown in Figure 4, which modifies the bias of the base of transistor 5 and consequently the switching instant of the transistor, and therefore the generated frequency.

In said Figure 4, the collector of PNP transistor 19 is connected to supply 8 through resistor 20 and to ground 11 through capacitor 21; the emitter of the transistor 19 is connected to the base of NPN transistor 22 and to ground through capacitor 23; the base is connected to the collector of transistor 22, to the supply through resistor 24 and to the base of transistor 5 through the series of capacitor 25 and inductor 26. Resistor 27 grounds the collector of transistor 22.

The operation is as follows: transistor 19 acts as an oscillator whereas transistor 22 acts as a switch to short-circuit the voltage generated by the oscillator when said voltage is decreasing.

In the practice the whole group may be replaced by an integrated circuit having equivalent functions; such a circuit is sold by several manufacturers and is known under the denomination NE 555.

As to the receiver, it may be any conventional and commercially available frequency modulation receiver; a rectifier is connected to the high frequency output to allow the lighting of a warning light and/or the operation of a acoustical alarm.

If more points are monitored, the problem arises of identifying the faulty wheel (or braking system), as all transmitters operate at the same frequency.

When the oscillator is operating, LED 28 protected by resistor 29 is actuated because of the closing of switch 17, 18... Therefore the driver, noticing the alarm, stops the vehicle-and looks at the different LEDs 28 of the installation to see which is or are lighted.

In the variant embodiment of Figure 5 and 6, the device comprises a detector Ra(S) formed by the secondary winding of a transformer, coupled with a primary winding Ti(P) actuating a number of alarms G1, G2, G3...Gn. The device is shown in greater detail in Figure 6.

The device consists in the whole of three eiements: the first is the secondary winding 30 of a transformer, closed on one or more switches 17, 18.

This first element is mounted on the rotating member to be monitored and while rotating, at each revolution it passes in front of primary winding 31 which is part of the second element and has one end connected to an a.c. generator 32 and the other end connected to resistor 33. The circuit of generator 32 comprises an opto-coupler 34 whose receiving side is connected, on the emitter side, with the base of NPN transistor 35 and, on the collector side, with the collector of transistor 35 in turn connected to supply 36 through resistor 37. The emitter of transistor 35 is connected with the base of transistor 38 and through resistor 39 with the base of transistor 35 itself. The base of NPN transistor 38 is connected to ground 40 through resistor 41, the emitter is directly grounded and the collector is connected through resistor 42 to supply 36 and directly to output 43.

The operation of the device is as follows: the closing of switches 17,18 causes current variations in the primary circuit which are transmitted by opto-coupler 34 to transistor circuit based on transistor 35 (current loop). No signal will be present at output 43 as long as switches 17, 18 are open, whereas when they are closed a signal will be obtained whose frequency is proportional to the rotation speed of the rotating member and which will be used to actuate the alarm.

It is self evident that the function of temperaturesensitive switch may also be carried out by a small magnet which attracts an iron plate thereby keeping a circuit open, whereas the plate is no longer attracted when the magnet temperature exceeds the Curie point.

The invention has been described and shown with reference to two preferred embodiments given by way of examples, some variant being hinted; other variants and modifications are however possible without departing from the scope of the invention defined in the appended claims.

Claims (13)

1. Device for signalling an excess temperature of a rotating member of a running motorvehicle, characterized in that it comprises one or more temperature detectors, connected to a transmission system which rotates together with the member to be monitored and actuates an optical and/or acoustical alarm device located in use in a fixed position on the vehicle.

2. Device according to claim 1, characterized in that said rotating transmission system is a radio transmitter comprising a radio oscillator mounted on the rotating member and said alarm device is actuated by a radio receiver.

3. Device according to claim 1, characterized in that said rotating transmission system is the secondary winding of a transformer of which the primary winding is fixed, said secondary and primary windings becoming faced for a short time interval at each revolution of the rotating member.

4. Device according to claim 2, characterized in that said radio oscillator is realized as a superactive generator by using a thick film hybrid technique on a ceramic support with partly discrete and partly integrated circuit elements.

5. Device according to claim 2 or4, characterized in that said radio transmission system, including a small supply battery, is packaged into a first, silicone based resin and into a second, fluorocarbon resin.

6. Device according to claim 2,4 or S, characte- rized in that said radio transmitter generates only one carrier wave.

7. Device according to claim 6, characterized in that said radio transmitter does not comprise a conventional antenna, the inductor of the oscillating circuit operating as an antenna.

8. Device according to claim 7, characterized in that said radio transmitter comprises a transistor as oscillator, a saw-tooth voltage being applied to the transistor base for varying the bias thereof and cyclically varying the frequency generated within two close limits.

9. Device according to claim 3, characterized in that the supply for the primary winding of said transformer is provided by an a.c. generator the circuit of which comprises an opto-coupler which transmits to the circuit output the current variations caused by the secondary passing in front of the primary winding.

10. Device according to claim 1, characterized in that said temperature detector is a bimetallic element connected as a switch in the alarm generating circuit.

11. Device according to claim 1, characterized in that said temperature detector is a magnetic element which undegoes a state change and actuates a switch whenever it passes through the Curie point.

12. Device according to any preceding claim, characterized in that the signalling circuit comprises one or more light emitting diodes which, when lighted, indicate(s) the group signalling that the temperature is above the predetermined threshold.

13. Device for signalling an excess temperature in a rotating member of a motor vehicle substantially as herein disclosed with reference to the accompanying drawings.