Abstract:
The device is applied to the wheel of a motor vehicle and includes a component that is sensitive to the pressure inside the tire and has a conductive, sliding element. When this element reaches the extreme limits of its travel it closes an electrical contact and activates a radio transmitter whose signals, picked up by a receiver located in the passenger compartment, activate a visual and/or acoustic alarm.

Description:
FIELD OF THE INVENTION 
     This invention concerns a device capable of indicating abnormal pressure conditions in tires on a motor vehicle in motion. 
     BACKGROUND OF THE INVENTION 
     It is common knowledge how important it is to check the tire pressure on a motor vehicle. Too high a pressure can in fact cause the tire to burst with disastrous consequences for the passengers and persons in the vicinity of the vehicle. If the pressure is too low it will not only accelerate tire wear but also rapidly raise the temperature and consequently increase the pressure which can cause the tire to burst. Despite the fact that it is of such great importance, the pressure is measured all too infrequently since the vehicle must remain stationary for a certain period of time to allow the tyre to cool down. Various devices have been proposed for monitoring tyre pressure during motion which normally include a pressure sensitive element that activates, when the tire pressure becomes abnormal or dangerous, a transmitter whose signals are picked up by a receiver located in the passenger compartment. 
     These devices, however, are difficult or complicated to produce since they need to be of a reduced size to be fitted to the wheels of any type of vehicle, or else they present sealing problems inside the sensor. 
     SUMMARY OF THE INVENTION 
     These problems have been overcome by the invention that is characterized by the fact that the sensor consists of a case with an axial cavity, divided into two parts; the first of these parts houses a movable element that is permanently in contact with one of the transmitter&#39;s terminals, and is closed by an adjustable ring nut that establishes one of the travel limits of the movable element. The other part is divided into two chambers, (the first communicating with the first part and the second chamber communicating with the inside of the tire) by means of a flexible membrane with edges held with a seal ring between a bush made of conductive material, that defines the travel limit of the movable element, and a second ring nut screwed into a fixed positon in the cavity in the case. Both the bush and the second ring nut have axial cavities that house a first and a second piston respectively; the first piston is held between the membrane and the movable element, and the second piston rests one end against the membrane and has the other end free and exposed to the tire pressure. These pistons keep the movable element half way between the two limit positions as long as the pressure remains within a range of values that has been defined normal. 
     This invention features another advantage in that it includes a receiver located in the passenger compartment which can be connected to a tachograph and/or an on-board computer to permit permanent storage of the defect indications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For further details reference is made to the attached drawings in which: 
     FIG. 1 illustrates the device fitted to a wheel, 
     FIG. 2 is an enlarged view of a detail, 
     FIG. 3 is a cross-section view of the pressure sensor, 
     FIG. 4 is a block diagram of the device&#39;s electronics. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIGS. 1 and 2, it can be seen that the inflation valve 3 of tire 2 on a motor vehicle wheel 1 is connected to a pressure sensor 5 situated in a container 9 possibly secured to wheel 1 with a bracket. 
     Sensor 5 is inserted to act as a switch in the circuit of a radio transmitter 6, activated by the sensor when the tire pressure leaves a pre-established range of values. The transmitter components are mounted on a conventional printed circuit board 50 positioned inside the container with the supply battery 8 and a led 10 which is active when the transmitter is in operation. 
     The signals emitted by the transmitter are sent via an antenna 7, inside tube 4, to a receiving antenna possible located on the vehicle&#39;s cab, and then onto a receiver that activates a visual and/or acoustic alarm and sends a signal to the tachograph and on-board computer, if present. 
     FIG. 3 shows that sensor 5 consists of a hollow body 11 that is basically cylindrical without ends, whose inner surface has a step 12 that divides the axial cavity into two parts with different diameter. The part with the narrowest diameter 13 is closed by an adjustment ring nut 14 that is adjustably screwed on body 11. 
     In this part a disk 15 made of conductive material slides against the action of a spring 16 calibrated in relation to the normal pressure used. One end of this spring rests on side 15a of disk 15 and the other fits in a circular groove 17 in the inner side of ring nut 14. 
     This groove is also the terminating point of a conductor 18 which is connected to the transmitter, and is in permanent contact with disk 15 via spring 16. 
     The ring nut also bears a connecting plug 19 that enters cavity 13 and is connected to a conductor 19a connected to the transmitter. The plug is intended to work in conjunction with a central piece 20 protruding from side 15a of the sliding disk 15. 
     The other part of the cavity inside body 11 houses the low pressure contact that closes the transmitter circuit, and the compoents that move disk 15 against the action of spring 16. 
     To be more precise, a bush 21 made of conductive material rests on step 12 and blocks a second conductor 22 connected to the transmitter, against the inner surface of body 11; this bush also has an axial cavity that houses a piston 23. One end of this piston rests on the other side, 15b, of disk 15, and the other end touches one side of a flexible membrane 24 inserted without backlash in the cavity in body 11. The edges of this membrane are pressed against bush 21 by a gasket 25 that has been fitted around the end part of a ring nut 26 (screwed into a fixed position at the end of the cavity in body 11) so that it seals against the walls of body 11. 
     This ring nut is also made of conductive material and blocks the end of a conductor 27 of the transmitter antenna circuit against the inner wall of body 11. Ring nut 26 has an axial cavity 28 that communicates with tube 4 and houses a piston 29, the same as piston 23, one end of which rests against membrane 24 while the other end is free and consequently exposed to the pressure conditions in the tire. 
     One end, 30, of the ring nut juts out of the cavity in body 11, and can be connected to tube 4. This part of the ring nut 30 has a seat 31 for a plug (not illustrated) used during storage to press on piston 29 and keep disk 15 half way between the two contacts, thereby preventing battery discharge. Ring nut 26 establishes electrical continuity between conductor 27 and the antenna wire 7 inside tube 4. 
     Correct functioning of the sensor illustrated in FIG. 3 can be easily seen from the following: for a given optimal inflation pressure, ring nut 14 in body 11 should be screwed in until disk 15 under the opposing pressure of spring 16 and pistons 23, 29 assumes a balanced position in which both its faces are more or less equidistant from bush 21 and plug 19. 
     While the pressure remains within a pre-established range of values, the transmitter circuit remains open, since any movement of disk 15 towards plug 19 or towards bush 21 is not sufficient to bring it into contact with these parts. 
     If the pressure increases excessively, pistons 23, 29 will bring the central protruding part of disk 15 into contact with plug 19 whereupon the transmitter circuit is closed between wires 19a and 18 via plug 19, disk 15 and spring 16. 
     If the pressure decreases excessively, spring 16 will push disk 15 against bush 21, closing the transmitter circuit between wires 18 and 22 via spring 16 disk 15 and bush 21. 
     The drawing clearly shows the simplicity of the sensor&#39;s construction despite the reduced dimensions it must have. In addition, the presence of gasket 25 guarantees a perfect seal in cavity 28 despite the hole for wire 27, thus ensuring piston effectiveness. 
     FIG. 4 illustrates a block diagram of the device electronics. 
     The parts that have already been described are indicated with the same numbers as those used in the previous figures. 
     Sensor 5 is represented as a switch. 
     Transmitter 6 is a conventional frequency modulation transmitter with a radio frequency oscillator 31 connected to antenna 7, controller by the low frequency oscillator 32 that generates the modulation frequency. 
     Receiver 33 contains a frequency modulation tuner 34 connected to antenna 35 with a periodically variable tuning research around a central value which corresponds to the nominal carrier frequency generated by the transmission oscillator 31. 
     If a carrier modulated to the nominal frequency is present in the turning band, tuner 34 emits a signal that is the modulation frequency of the received signal on a low frequency output. 
     This output is connected to a decoder 36 that compares the tuner output signal with the signal generated by a time constant 48. If the two signals (frequence is) are equal, it emits a logic signal that is stored in a memory device 38 (for instance a bistable) whose output is connected to: 
     a luminous indicator e.g. LED 39, 
     an on-board computer 40, if present, 
     a counter 41 that activates an acoustic indicator 43, via a driving circuit 41, until it has counted a pre-established number of clock signals emitted from a generator 47 enabled by the tuner 34, and 
     a recording device 45 of the kind known as a &#34;tachograph&#34; through the conventional driving circuitry 44 of such device. 
     A manual reset pushbutton 46 resets the bistable and deactivates alarm 39 once the driver has identified the defective tire. To do this the driver simply needs to stop the vehicle and inspect the wheels until he finds the one where the warning light 10 is lit. Obviously if the cause of the alarm is not eliminated, tuner 34 reactivates the indicators, tachograph and computer as soon as it recognizes the transmitter frequency in the next &#34;sweep&#34;. To simplify matters, the converter that uses the vehicle&#39;s battery voltage to power the receiver circuitry and the associated connections are not illustrated