CA2246347A1 - A pneumatic pressure sensor device - Google Patents

Abstract

A pneumatic sensor switch for giving a warning signal if a pneumatic vehicle tire has too low pressure has a gas-filled chamber (11), separated from the inflation space in the tire by a flexible diaphragm (8) cooperating electrically with a metal part (16) in the chamber to form an electric switch open at safe pressures in the tire and closed at dangerous lower pressures. There is an electric circuit with battery (4) and transmitter (5) to transmit warning signals to the outside if said switch is closed. There may be a second chamber (13) with electric switch contact means, filled with gas at a lower pressure and also separated from the inflation space in the tire by a flexible diaphragm (10) so that said contact means are open at atmospheric pressure therein and closed by the diaphragm at higher pressures. The electric switch means of both chambers (11, 13) are connected in series so that a warning signal is only given at pressures in the dangerous range, when both switches are closed, and not at atmospheric and at safe operating pressures in the tire. There may be a roll switch in one of the chambers to interrupt the electric circuit when the device is stationary. The diaphragm (8) may have mechanical contact means or operate such means through compression of the gas in the closed chamber (11). There may be more than one such sensor, operating at mutually different tire pressures. The gas filled chambers make the device take up the same temperature as the inflation space in the tire, so that the device adapts itself adequately to differing temperatures in the tire.

Description

CA 02246347 l998-08-l3 A pneumatic pressure sensor device FIELD OF THE INVENTION

This invention relates to a pneumatic pressure sensor switch device for a pneumatic tire, with means for mounting it in the inflation chamber of the tire.

PRIOR AR~
Such devices are known in different embodiments, in which the pres-sure in the inflation chamber in the tire is compared with a reference pressure by having these pressures act on both sides of a flexible dia-phragm separating both pressures. If the pressure in the tire becomes hazardously low, the reference pressure makes the diaphragm flex as to' close electrical contact means to give a signal to the outside. To this end there is a battery and a radio transmitter taken up in an electric circuit with these contact means. Such signals may be received by a receiver outside the tire, e.g. in the driver's cabin of the vehicle, and there be transformed into an audible and/or visible warning signal.
The reference pressure may be the pressure exerted on the diaphragm by a mechanical spring or by a gas. In the first case (USP 4,048,614 to Shumway) the sensor reacts when the pressure in the tire drops below a predetermined absolute pressure. This is not very accurate in view of mechanical tolerances and because the spring has to be taken up in a chamber, which cannot be hermetically sealed and in which the air pressure varies with the air pressure in the tire. The entire structure is complicated and w lnerable. Means to vary the tension of the spring allow different settings of the pressure, at which an alarm is given.
In the second case, applying a gas to exert the reference pressure (USP 4,890,090 to the applicant) a diaphragm is used, which is slightly pervious to air. Thereby the gas pressures in the tire and in a chamber for the reference pressure, in contact with the diaphragm, tend to equal-ize gradually. As soon as the pressure in the tire drops hazardously, the pressure in the reference rh~mh~r cannot follow this pressure drop imme-diately, so that the diaphragm is flexed to close the electrical contact means, by which a signal is given to warn about a hazardous situation.
This has the advantage that no signal is given during considerable ~I~ENDED S~lEFr i) CA 02246347 l998-08-l3 -pressure differences in the tire, which are only due to normal deviations in operation, such as differences in inflation pressure and pressure differences owing to varying temperatures, e.g. between cold storage of the vehicle during the night and heavy traffic under high load, sllnshin~, etc. during hours of operation. Vehicles stored for extended periods, in which tire pressure drops due to permeability of tire casing, will not set off the alarm.
Another such sensor, shown in US-A-3,810,090 is embodied as a dimple or bu~kl ;ng switch, in which a metal diaphragm buckles outwardly to close an electric circuit if the tire pressure drops too much. The diaphragm forms one wall of a gas filled, hermetically sealed chamber.
There are also such devices trapping air to give the reference pressure in a closed space when a sensor with this space is screwed externally on the valve stem of the tire (USP 5,040,562 to Achterhold).
In USP 4,701,742 (Ruehr) a pressure sensor is given, screwed into and partially outside the rim of a vehicle wheel, having a closed chamber with gas at a reference pressure therein. The chamber is bordered by two diaphragms, one exposed to the pressure in the inflation space of the tire, the ot~er one expose~ to at~osp~eri~ ~r~ssure. T~e first ~iaph~a~
makes contact in an electric circuit if the tire pressure is correct and moves out of contact when this pressure is too low. The other diaphragm makes contact in the electric circuit when the pressure in the reference chamber becomes too low, so that it can be refilled. Temperature compen-sation between the inflation space in the tire and the reference chamber, as explained below, is only partly obtained, the device is not fail safe as small leakage of an O-ring or diaphragm will produce a leaking tire, and its battery will soon be depleted as the electric circuit is closed at correct tire pressures.

OBJEÇT OF INVENTION

In many cases there is the need to have sensors of such type react as accurately as possible to absolute pressures in the tire, if such pressures become too low, and it is the main object of this invention to provide such sensors with a simple and improved design and reliable operation, as little as possible sensitive to mechanical tolerances and temperature differences.

AMENDED S~lELT

, CA 02246347 l998-08-l3 SUM~ARY OF I~VENTION

Starting from the device known from said US-A-3,810,090, which shows a pneumatic pressure sensor switch device for a pneumatic tire, compris-ing means for mounting said switch device fully inside the inflationchamber of the tire, a sensor having a hermetically sealed closed chamber filled with gas, one of the walls of the chamber being a flexible dia-phragm, of which the surface remote from the chamber is exposed to the pressure in the inflation chamber of the tire, a movable electric switch contact point cooperating with a stationary switch contact point on the chamber, said movable contact point being movable with the diaphragm so that the diaphragm moves said contact point out of contact with said stationary contact point at predetermined pressures in the tire and allows said contact points to make electrical contact at lower pressures in the tire, said sensor being taken up in an electric circuit with means to take up a source of electric energy and a transmitter to give radio signals to the outside of the tire when said circuit is closed.
Such a device is, according to the present invention, characterized in that said closed chamber is filled with gas of a pressure above atmo-spheric pressure and is bordered, in its wall opposite the diaphragm, bya second flexible diaphragm, separating said chamber from a second closed chamber filled with gas, the second diaphragm carrying said movable con-tact point so that, at increasing pressures in the tire, the first said diaphragm compresses the gas in the first said closed chamber so that this gas moves the second diaphragm with its movable contact point in the direction to bring it out of contact with said stationary contact point.
In this way it is simply and reliably obtained that the switch device can be made small and reliable in operation and can be provided with thin walls, mainly of metal, so that the gas in the chamber rapidly takes up the temperature of the air in the inflation space of the tire, thus giving an automatic temperature comr~n~ation for normal variations of the temperature within the tire, as the gas in the chamber will vary in pressure proportional to the variation of pressure by temperature changes in the tire. This will be more fully obtained if the pressure in the chamber is close to normal operating pressures in the tire, although deviations, taking e.g. into account the resiliency of the diaphragm, are possible, depending also on the amount to which the tire pressure is allowed to drop before a warning signal is required. The gas pressure in ~;. icN~ED ~tEET

CA 02246347 l998-08-l3 the closed chamber is and should be the main parameter and contributor to the trip point at which a signal is given.
The closed chamber protects the electric contacts against fouling.
PrePerably, this chamber is filled with an inert gas such as nitrogen or argon.
As therein the first diaphragm moves the second one by compressing - the gas between them, no mechanical means such as a spring moved mechani-cally by the first diaphragm are necessary, while the electrical contact means can be mounted entirely inside the sensor, fully protected against moisture and dirt. Moreover, there is the least possible influence of changes of temperature in the tire on the correct result as the closed chamber between the diaphragms will rapidly take up the temperature inside the tire so that its gas pressure changes in the same direction and about to the same amount as the air pressure in the tire. This mea~s that a warning signal can be given if the tire pressure has a predeter-mined pressure difference below the desired and safe pressure even if this pressure changes with temperature.
It may, however, be preferred to embody the invention in another way, by having a movable spring contact in the first closed chamber, moved by the first diaphragm to open and close the electric circuit.
In electrical series connection with the contact points of the first diaphragm and first closed chamber there may be a set of contact points operated by a further flexible diaphragm, bordering a further closed chamber with lower gas pressure, said diaphragm being also exposed on one side to the pressure in the inflation chamber of the tire so that, at atmospheric pressures therein, or with the sensor outside the wheel, such as during storing or shipping, manufacture or assembly, the electric circuit is interrupted to avoid depletion of the battery and unnecessary alarm signals.
There may also be, instead of or supplementary to such a further diaphragm and chamber, a roll switch interrupting the electric circuit when the device is not rotating and is closed when the roll switch body is moved by centrifugal force to cause electrical contact to be estab-l;~h~d between this body and another part of the circuit in the device.
If the roll switch body is provided in one of the closed gas chambers, it will also be protected against corrosion, dirt and moisture. Roll switches are known as such, but the invention proposes to mount a roll switch body and the concerning electrical contacts within a closed gas fn ~T

CA 02246347 l998-08-l3 chamber for such protection.
The invention will now be explained in more detail with reference to the attached drawings, in which:
Fig. 1 is a section through a sensor according to this invention in a preferred embodiment;
Fig. 2 is a section through a combination of two such sensors, oper-_ ating in more than one pressure region;
Fig. 3 is a section through a sensor in another embodiment;
Fig. 4 is a section through a sensor of a different type useful for understanding the invention;
Fig. 5 is a section through a sensor in an embodiment somewhat dif-ferent from the one of Fig. 4, but also useful for underst~n~;ng the invention;
Fig. 6 gives an axial view of the spring body of Figs 4 and 5;
Fig. 7 shows in vertical section and partly diagrammatically a sensor device with part of wheel rim and fastening means; and Fig. 8 shows a section and view along the line VIII-VIII in Fig. 7.
In Fig. 1 the sensor is built into a multiple-layer board, which may be a printed-circuit-board, with layers 1, 2 and 3. These boards are only shown in the zone of the sensor, but may extend considerably to one or more or all sides with respect to the drawing, so as to take up further electrical parts, leads, etc. and to provide mounting means for the sensor on a vehicle wheel, such as by being provided with a belt slung around the wheel and buckled thereto, or by being taken up in a casing glued, welded or soldered to the wheel, or in any other suitable way. So, this board carries a battery 4, a transmitter 5 and electric leads 6 and other electric parts deemed necessary or preferable. Vide also Figs 7 and 8.
The sensor itself, indicated by 7, has three thin gastight flexible diaphragms 8, 9 and 10, preferably of metal. So, they are electrically conductive. They may be made from another material, conductive or not, and in the latter case there should be separate means, metal strips or coatings to give the required electrical conductivity. Between diaphragms 8 and 9 there is a gastight first chamber 11, filled with nitrogen at a pressure somewhat near the normal inflation pressure (operating pressure) in the tire. Below diaphragm 9 and bordered at its bottom by pc-board-layer 3 is a second gastight chamber 12 filled with nitrogen at a pressure about the same as the normal inflation pressure in the tire.
Vide below for details of these pressures.

~?.;ENDED ~EE~

=

W O 97/29919 PCT~B97/00122 Below pc-board-layer 3 the~e is the third diaphragm lO confining with this layer 3 a third gastight chamber 13 at a pressure slightly above at~ospheric pressure.
The diaphragms 8, 9 and lO are so thin, that their inherent elasticity does not play an important role in their functioning.
The electric leads 6 are e}ectrically connected to diaphragm 8 and conducting layer 14, and by one or more local connections 15 through pc-layer l to annular contact zone 16. Diaphragm 9 makes electrical contact with a metal layer 17 around it, which has one or more local connections 18 through layer 2 to a metal layer l9 on pc-board-layer 3, from which a central electrical connection 20 passes through layer 3 to terminate in a flat contact point (zone) 21 on the bottom of layer 3. Diaphragm lO is electrically connected to the leads 6 at the opposite end of the electric circuit.
It will thus be clear that an electric current can only flow to transmitter 5 if diaphragm 9 contacts contact zone 16 and if diaphragm lO
contacts contact zone Zl simultaneously.
At atmospheric ambient pressure, the diaphragms 8, 3 and lO are in the position shown, so that no signal is given as no current can flow because diaphragm lO does not contact zone 21.
If the outside pressure rises, diaphragm lO is pushed upwardly as seen in Fig. l. until it touches contact zone 21, which will take place at a pressure. depending on the gas pressure in chamber 13, e.g. at 5 psig. The electric circuit is now closed (as diaphragm 9 contacts zone 16~, so that transmitter 5 transmits a warning signal.
As the pressure in the tire rises, diaphragm 8 is further depressed, which raises the gas pressure in the relatively small closed chamber ll between diaphragms 8 and 9 until diaphragm 9 bends downwardly to break contact with contact zone 16. If the normal operating pressure is e.g. 30 psig: this e.g. takes place at 27 psig pressure in the tire.
- Further rising of the pressure in the tire depresses diaphragm 8 further, without influence on the electric conditions.
This means that, at such a normal tire pressure of 30 psig, no electric current flows and no signal is given at this and at higher pressures.
As soon as the tire pressure drops to 27 psig or less, a signal is given to warn that the tire pressure is too low.

It is possible to provide-the sensor device with more than one of such sensors 7 with, in the chambers ll and 12. mutually different gas pressures so that one sensor will give a warning signal at a higher pressure than the other one. The sensors may have separate electrical systems with leads 6 to their own transmitter, which separate transmitters may emanate different signals, e.g. of different frequency, for each sensor. The receiver may thus give a signal showing which sensor activated the transmitter, so in which pressure range the tire is. There may e.g. be three sensors giving signals one at e.g. 27 psig tire pressure or lower, one at 24 and one at 20 psig. The sensors may, however, have a combined electrical system, in which they are taken up in parallel, with e.g. one transmitter only, adapted to give different S7 gnAl .~ depending on which sensor gives the alarm, e.g. signals of different frequency.
At the highest pressure the warning signal may e.g. be transformed in the receiver to a visual signal only, whereas at a lower pressure the second and possibly third warning signals are audible.
Another possibility is adapted to the fact that the same tires may operate at different inflation pressures. So, the tire may be inflated to a higher pressure if more load has to be taken up, also e.g. when wheels and tires are switched from front to rear of the vehicle. It may then be that the same tire at some moment has a normal operating pressure of 40 psig, whereas at another time this inflation pressure is and should be 30 psig. It should then be avoided that in the 40 psig-mode the sensor gives a constant warning signal because the pressure is 30 psig, so well below the 40 psig. This is according to the invention obtained by applying two such sensors in the way as shown in Fig. 2. This device allows a warning signal to be given in a pressure range below the highest desired operat-ing pressure but above the lower operating pressure. E.g. a warning signal may be given at tire pressures between 33 and 37 psig and between 27 and 5 psig only.
The two sensors 7 and 7' are mounted in the same laminated pc-board l, 2, 3. The lefthand sensor 7 is entirely the same as the one of Fig. l apart from electrically conductive coatings instead of the local connec-tions l~ and 18. This means, assuming the gas pressures in the closed ~ chambers ll, 12 and 13 to be the same as described above for Fig. l, that through sensor 7 no current is allowed to pass at atmospheric outside pressures and at outside pressures above 27 psig, but electric current W O 97/29919 PCTnB97/00122 passes and can ~enerate a warnlng signal in the transmitter at pressures in the tire of 27 psig and lower, down to 5 psig.
The sensor 7' may have a common connection to the battery, etc. with sensor 7, as its diaphragm lO' is electrically connected to diaphragm lO
of sensor 7. All the other diaphragms and conductive parts of both sen-sors are separated. The gas pressure in chamber ll' may e.g. be about 40 psig, chamber 12' may be at a pressure of about the same height and chamber 13' may be at say 33 psig in the condition shown and at atmo-spheric pressure on the outside. If the tire pressure is 40 psig or higher, diaphragm 8' will keep diaphragm 9' out of contact with its stationary contact zone on pc-board-layer l. At pressures of say 37 psig ~nd lower, diaphragm 8' will bulge out and will cause diaphragm 9' to take up the position shown so as to make contact. Diaphragm lO' will be pressed against its stationary contact zone at higher pressures until the outside pressure drops to e.g. 33 psig and lower, at which pressure the gas in chamber 13' pushes diaphrag~ lO' down out of contact with its stationary contact zone.
This means that a signal is given at tire pressures between 33 and 37 psig and at pressures between 5 and 27 psig. No signal is given between 27 and 33 psig pressure in the tire. For the higher operating pressures the lefthand sensor 7 will not give a signal until the tire pressure will have dropped far too much, when the other sensor 7' will already have given the alarm. So, there is no undue loss of electric energy and there is a selective warning depending on operating pressure.
Sensor 7 may be connected at its top at 8 to another transmitter or other means in the same transmitter than sensor 7' by its top conductive part 8', to distinguish between the two alarms at the receiver side.
In all cases, the gas in the sensor chambers ll, 12, ll' and 12' and ~3' easily takes up the same temperature as the air in the tire by the good conductivity of the metal diaphragms. This means that the pressures therein will rise and fall due to temperature differences to about the same amount as the pressure in the tire. This good temperature compensa-tion has the advantage. that a signal is given i~ the tire pressure is a certain amount below what is the normal operating pressure in the tire, notwithst~n~;ng the fact that this operating pressure may vary rather considerably.
~ or chamber 13 the pressure is too low and thus too different from the normal pressure in the tire to give considerable temperature compen-CA 02246347 l998-08-l3 W O 97/29919 PCT~B97/~0122 sation in this sense, but this is not necessary for this chamber and the pressure at which diaphragm 10 contacts zone 21 may well remain about at the same low value such as at 5 psig.
As to the gas pressures in the sensor, there may be a ~as filling passage through part of the layers 1, 2, 3 into the closed ~h~h~r5. In manufacturing the sensors, the pressure at which it has to give a signal is exerted on diaphragms 8 (and 8') and 10 (and 10') in a suitable apparatus taking up the sensor. Chamber 11 is now filled to the somewhat higher normal operating pressure of the tire. Chamber 11 is now sealed and chamber 12 is filled with gas under pressure until diaphragm 9 makes contact with zone 16, after which the gas passage to chamber 12 is sealed. It is thus certain that the "trip-point", the outside pressure, at which a signal is given, is at the desired value.
Chamber 13 does not give a problem as it is filled with gas at a pressure only slightly above atmospheric and not very critical.
Sensor 7' of Fig. 2 is made in the same way, but with the higher pressure, e.g. of 40 psig, in chamber 11' and a pressure of e.g. 33 psig in chamber 13'. The filling of ~h~mher 12' also takes place up to a pressure, at which a signal is given. In this case, the pressure of filling of chamber 13' is more critical than for chamber 13.
The movements of the diaphragms as described can in fact be limited to even parts of a milli~eter, This makes it essy not to rely on inherent elasticity of the diaphragms, which can be very thin and very flexible, so that the trip points are only negligibly affected by the diaphragms 2~ and are overridingly determined by gas pressures in the closed chambers.
Fig. 3 shows a sensor much resembling the sensor of Fig. 1, with essentially the same parts 1 to 9, 11 and 12, but with conductive layers 1, 2 and 3, so that conducting parts 15-20 are superfluous. Fig. 3 shows a different embodiment of the means to interrupt the electric circuit when the device is not in operation. The chamber 13 and its flexible diaphragm 10 of Fig. 1 are here omitted and replaced by a roll switch 22, consisting of an annular metal body, vertically (as seen in Fig. 3) movable by centrifugal force and restrained and urged downwardly by a metal spring 23, 24 being star-shaped and having a central circular part 23, riveted to metal layer 3, and three radially extPn~;ng prongs 24 urging body 22 to its lowest position. If the device is mounted in the inflation chamber of the wheel so that body 22 will move outwardly ~upwardly as seen in Fig. 3) by centrifugal force if the wheel rotates at W O g7/29919 PCT~B97/00122 at least a certain speed, body 22 will then contact diaphragm 9 with its top edge and thus close the electric circuit between layer 3 and diaphragm 9. If the wheel is stationary, ~ody 22 is kept in a lower position by prongs 24 so as not to contact diaphragm 9, so that this electric circuit is interrupted. This is particularly helpful to avoid nnec~Sary alarm signals generated and thus depletion of the battery for e.g. trucks stored without moving during longer periods of time.
Means may be applied to give an electric signal to the outside as soon as roll switch 22 contacts diaphragm 9 for the first time after such a period of inactivity, to show to the driver that the system is potentially active ~even if the tire pressure is correct), after which no such signal is transmitted.
Diaphragm lO and chamber 13 of Fig. l are now superfluous, as roll switch 22 avoius any closing of the electric circuit if the wheel does not rotate. In some cases it may, however, be desired to maintain this diaphragm lO and cham~er 13, e.g. for one of the sensors of Fig. 2, in order to operate at a certain desired pressure, and in such a case such a ~h~m~er 13 and diaphragm lO may easily be mounted to the bottom of body 3 in Fig. 3, as will immediately be clear.
The sensor o~ Fig. 4 is essentially circular in shape. It has an annular bo~y 2~ of electrically non-conductive and gas-impermeable materi~l such as Perspe~ or P1~X;g1AC~ a transparent resin of polymethylmetacrylate.
A chamber 26 of metal such as a copper alloy or a stainless steel is pressed with a gas-tight fit into the annular body 25. At both sides of body 25 a ~langed ring 27, 28 of metal is tightly connected thereto and secured to the flange of each ring as by soldering or welding is an electrically conductive diaphra~m 8, lO. Diaphragm lO is, also in unloaded condition, curved so as to be concave at the outside. The opposite ~other) diaphragm 8 is curved oppositely, i.e. so as to be convex at the outside.
The chamber 26 has in the center of its bottom, as seen in Fig. 4, a somewhat dome-shaped part 29, acting as an electric contact cooperating with diaphragm lO. It is iUst out of contact with diaphragm lO at atmo-spheric ambien~ pressures. In its top surface, as seen in Fig. 4, thischamber 26 has an opening 30. Between body 25 and diaphragm 8 a spring contact body 31 is provided, vide also Fig. 5. This has an outer annulus 32, fixed between body 25, ring 27 and diaPhragm 8 so as to be in W O 97129919 PCT~B97/00122 electric contact therewith, and a spring contact arm 33 bent out of the plane of annulus 32 in a way which is clearly shown in Fig. 4 and so as to have an elbow 34 positioned close to the center of diaphragm 8, vide also Fig. 6. This contact 31 is of a resilient metal. e.g. as usual for small disk or leaf springs. In its unloaded condition the arm 33 contacts the edge of opening 30 of chamber 26 at 35.
In this way there are formed two spaces 36 and 37 in this sensor, both gas tight. Space 36 is filled with a gas of low pressure, which may range from vacuum to somewhat above atmospheric, but preferably its pressure is about atmospheric.
Preferably it is filled with an inert gas such as nitrogen or argon.
This has the advantage of avoiding corrosion of the electric contact parts. With some pressure in it, it tends less to leakage into it than full vacuum would do.
Space 37 is filled with gas, preferably the same gas as space 36, at a much higher pressure, in the range of the normal inflation pressure in the tire.
The chamber 26 may have an upstanding edge part 38 to avoid diaphragm 8 from deforming permanently and from pushing arm 33 too deeply 20 into chamber 26 by being pushed down too much if the tire would be inflated to too high a pressure.
~iaphragm lO is electrically connected to the electric circuit again including a battery 4 and a radio transmitter 5 and is electrically connected to diaphragm 8.
Fig. 5 shows such a sensor in a somewhat different embodiment. The chamber 26 differs from the one of Fig. 4 in that it is more shallow, its bottom having a pin-shaped part 40, the bottom of which replaces the dome-shaped part Z9 of Fig. 4. This part 40 is surrounded by and securely housed in the transparent mass of body 25 at 41 as shown, in which body 30 25 space 36 is recessed.
Fig. 6 is a view of spring contact 31 as described above and as applied both in Fig. 4 and in Fig. 5.
The operation of the sensor device of Fig. 4 and 6 is as follows.
Even with the battery installed and at atmospheric ambient pressure, e.g.
in storage or on a vehicle wheel with the tire removed or not inflated, diaphragm lO will not contact dome-shaped part 29 of chamber 26, so that no electric current can flow from battery 4 to casing 26, diaphragm 8 and transmitter 5.

W ~ 97/29919 PCT~B97/00122 If the tire is inflated, diaphragm lO will be depressed to contact part 29 of chamber 26, so that current flows via spring-contact 31, 32 (at 35) to diaphragm 8 and so to the transmitter 5. This will thus transmit signals to a well-known receiver outside the tire, e.g. in the driver's cabin, where it will give an audible and/or visual signal to the driver.
On further inflating the tire to a safe operating pressure, diaphragm 8 will be further depressed until it contacts the top 34 of the bent-out spring contact arm 33 to depress it so that contact at 35 between arm 33 and casing 26 around opening 30 is broken. The transmitter 5 is thus deactivated and no signal is given to the driver.
This means that, at safe operating pressures, there is contact between diaphragm lO and part 29 of casing 26, but no signal is given as spring contact 33 is out of contact with casing 26 at 35, that no signal is given at atmospheric pressure because contact between diaphragm lO and part 29 is broken, and that a signal is given at intermediate pressures in the tire, which are at a hazardous low value.
For the sensor of Fig. 5 the operation is the same, but ~oLeover the spring contact 33 will reach the bottom of chamber 26, which here is rather shallow, if the tire is inflated too much, well above normal operating pressures. Thus the electric circuit will be closed at too high pressures in the tire, causing a warning signal to be sent by the transmitter 5.
Figs 7 and 8 show one way of mounting of such sensors to the re-mainder of the switch device and of this device to a vehicle wheel. InFig. 7, 42 is part of a rim of a vehicle wheel. The sensor switch device 43 includes a housing 44 of plastic material, having at each end a bracket 45 with a slot, through which can pass a thin steel strap 46 so as to keep housing 44 fixedly mounted to the wheel. The strap 46 has the usual means to be kept closed and under some tension, such as a suitable buckle not shown.
In the housing 44 there is mounted a PC-board 47 carrying in the usual way the electric parts and printed electric leads of the device.
These include the sensor 7, battery 4 and transmitter ,, the leads between them not being shown. Fig. 8 shows that there are two such sensors 7 side by side. They may be mounted on the PC-board 47 by ma~ing H-shaped holes therein, so that the sensors can easily be connected both me~h~nically to the PC-board as electrically to the relevant electric leads and other parts thereof. The two sensors 7 may be identical in shape and dimensions, but filled with gas in their chambers 37 of mutu-ally different pressures, so that e.g. a visible signal is generated at - too low but not yet dangerous pressures in the tire, whereas the other sensor causes an audible signal to be given at a lower pressure in the tire, which is more dangerous. There may even be three of such sensors reacting to different pressures in the tire and they may be embodied and combined in the way as shown and described in and with reference to Fig.
2.
Instead of a PC-board there may be another carrying surface with electric leads and parts mounted thereon in other ~nown ways.
In a way known as such there may be a receiver in or at the driver's cabin with means to give an audible and/or visible signal if the trans-mitter 5 emits signals representing a warning about the pressure condi-tion in the tire.

~4 0 ..,

Claims (15)

1. A pneumatic pressure sensor switch device for a pneumatic tire, comprising means (45, 46) for mounting said switch device fully inside the inflation chamber of the tire, a sensor (7) having a hermetically sealed closed chamber (11) filled with gas, one of the walls of the chamber being a flexible diaphragm (8), of which the surface remote from the chamber is exposed to the pressure in the inflation chamber of the tire, a movable electric switch contact point (9) cooperating with a stationary switch contact point (16) on the chamber, said movable contact point being movable with the diaphragm (8) so that the diaphragm moves said contact point out of contact with said stationary contact point at predetermined pressures in the tire and allows said contact points to make electrical contact at lower pressures in the tire, said sensor being taken up in an electric circuit with means to take up a source of electric energy (4) and a transmitter (5) to give radio signals to the outside of the tire when said circuit is closed, characterized in that said closed chamber (11) is filled with gas of a pressure above atmospheric pressure and is bordered, in its wall opposite the diaphragm (8), by a second flexible diaphragm (9), separating said chamber (11) from a second closed chamber (12) filled with gas, the second diaphragm (9) carrying said movable contact point so that, at increasing pressures in the tire, the first said diaphragm (8) compresses the gas in the first said closed chamber (11) so that this gas moves the second diaphragm (9) with its movable contact point in the direction to bring it out of contact with said stationary contact point (16).

2. A device according to claim 1, in which the stationary contact point is provided in the said first closed chamber (11).

3. A device according to claim 2, in which the stationary contact point is provided on a wall (1) separating the said two diaphragms (8, 9), which wall has an opening for gas to connect the adjoining gas spaces bordering the diaphragms (8, 9) to form together the said first closed chamber (11).

4. A device according to any of claims 1, 2 and 3, in which, at atmospheric pressure in the inflation chamber of the tire, the gas pressure in the said first closed chamber (11) is somewhat lower than the pressure in the said second closed chamber (12).

5. A device according to any of the preceding claims, in which there is a further closed chamber (13, 36), separated by a flexible diaphragm (10) from the inflation space of the tire and filled with gas at a pressure below the pressure in the other closed chamber(s) (11, 12, 26), electrical contact means carried by said diaphragm and a stationary part (29, 40) of said further closed chamber (13, 36), said means being closed when said diaphragm is pressed into said chamber (13, 36) by pressures in the inflation space in the tire above atmospheric pressures, these contact means being taken up in electrical series connection with the contact points of said other closed chamber(s) in the said circuit including the transmitter.

6. A device according to claim 5, in which a common wall (29) separates said further closed chamber (36) from said other, first said closed chamber(s) (26), with its flexible diaphragm (10) bordering said combined device in the end surface thereof opposite the surface of the first said diaphragm (8).

7. A device according to claim 5 or 6, in which the gas pressure in said further closed chamber (36) is slightly above atmospheric pressure and much below normal inflating pressures of the tire.

8. A device according to any of the preceding claims, in which a roll switch (22) is mounted in one of the closed chambers, comprising a movable body, at least in part electrically conductive and taken up in the electric circuit, and a spring urging this body to a position out of contact with another contact point in the device, so that, at rest position of said body, said circuit is interrupted, and at a position reached by said body (22) against the action of said spring (23, 24) and obtained by centrifugal force when the device is rotated with the wheel said body is moved into contact with said other contact point, so that said circuit is closed.

9. A device according to claim 8, in which said roll switch body is provided in said second closed chamber (12), the said other contact point being formed by said second diaphragm (9).

10. A device according to any of the preceding claims, having two such sensors (7, 7'), having mutually different gas pressures in their said first chambers (11, 11') and taken up in an electric circuit in parallel so as to close it and transmit signals to the outside at different pressures in the inflation space of the tire.

11. A device according to claims 5 and 10, in which said further closed chambers (13, 13') of the two sensors have mutually different gas pressures, this chamber (13) of one sensor having a pressure well below the lowest allowable tire pressure and said chamber (13') of the other sensor (7') having a pressure above the pressure in the first-said closed chamber (11) of the said one sensor (7).

12. A device according to any of the preceding claims, in which the said diaphragms themselves are electrically conductive so as to constitute said movable contact points.

13. A device according to any of the preceding claims, in which the gas-filled chambers are filled with an inert gas.

14. A device according to any of the preceding claims, in which the gas-filled chambers are filled with nitrogen.

15. A device according to any of the preceding claims, in which the or each sensor is mounted on a printed circuit board also carrying the other parts of the electric circuit of the device.