Background of the invention Most wheel-carried vehicles make use of tires fitted to the wheels. These tires are generally the only means by which the machine makes contact with the ground. It is of importance to the operator to be able to recognise early when one or more tires are loosing pressure. Great costs can be saved if it is possible at an early stage to get a warning regarding a latent puncture, e. g. by being able to monitor the tire pressure in each and every one of the vehicle's tires from the operator's seat. Costs can also be saved when it is not necessary to remove valve caps and manually measure the pressure. Further costs could be saved if tires were not run broken due to low tire pressure.

Systems for monitoring tire pressure from the driver's seat are known from the passenger car industry.

Systems can be furnished e. g. from SmarTire Systems Inc, Richmond, Canada or Nokian Tyres PLC, Nokia, Finland. The prior art within the field of means and methods for monitoring tire pressure includes US20020067285A1, US6499343 and EP1172236A2, among many others. A typical prior art system includes a sensor unit mounted on a rim of a wheel, usually by means of a steel band or belt arranged around the rim. The sensor unit is capable of measuring at least one parameter, usually tire pressure.

The unit typically also comprises a transmission unit capable of transmitting a radio signal representative of

a measured parameter value to a display unit, usually mounted in the driver's cabin. The transmission usually takes place at regular intervals. The sensor unit takes its power from a built-in power source, usually a battery.

A problem with prior art is that the energy provided from the battery is limited, and when said battery is worn out, it is necessary to remove the tire and replace the battery or the entire sensor unit. A solution for at least partly overcoming this problem for passenger cars has been proposed by SmarTire, by adding a motion sensor, e. g. an accelerometer, to the sensor unit. The motion sensor is coupled to a switch and delay circuit such that the sensor is controlled to be active and transmitting only when said passenger car is moving. In order to provide some margin, the prior art system is devised such that the motion sensor must detect a minimum acceleration above a certain defined threshold value, the acceleration being dependent on the speed of rotation of the tire and the radial placement of the sensor unit. This way, normal vibrations occurring in an idle vehicle due to wind etc, will not trigger pressure measurement and/or signal transmission, thereby saving power.

A problem with this type of solution is related to the dependence of the speed of rotation of the tire, namely that for slowly moving vehicles, the threshold level for the sensor unit will not be exceeded. The result is that the system will not trigger any measurements and/or transmissions, and the system will therefore refuse to work. This is typically the case for e. g. forestry machines, but in general to any type of wheel-carried vehicle moving at low speed.

It is a further problem with known systems that they are not applicable to tires with tubes, because the sensor unit generally is attached to the rim, thereby devised to measure the pressure of the surrounding gas.

In a tubeless tire the rim is exposed to the tire

pressure, but in a tire with a tube the pressure stays within the tube. Prior art solutions are therefore not an option for tires with tubes.

Summary of the invention It is therefore an object of the present invention to provide an improved system for monitoring tire parameters in moving tires. In particular, the invention is aimed at providing an improvement over the prior art which is usable for slow moving vehicles, such as forestry machines, and to provide means for reducing power consumption in such monitoring systems.

It is also an object of the present invention to provide means for measuring tire parameters for tires having tubes.

The present invention is based on the inventors realisation that one problem with current systems for reducing energy consumption is that the motion sensor of current systems is not sensitive enough to sense the difference between an idle wheel and a moving one. Tests have also shown that if the signal from the motion sensor is amplified before it is compared to a threshold value, the incidence of false motion signals will increase and so will the energy consumption.

According to the invention, the above-referenced object is fulfilled by increasing the motion of the sensor. This is basically achieved by providing a protective carrier around the sensor unit that permits said unit to freely tumble inside a tubeless tire, or inside a tube of a tire. Tests conducted by the inventor and his team show that the protective carrier and sensor unit function very well as a unit. The protective carrier preferably comprises a piece of expanded plastic or a material with similar shock absorbing qualities, big enough to house the sensor unit inside it and to provide a layer of shock absorbing material around the sensor unit that is thick enough to protect the sensor unit from wear and tear when it is inside the tire. A layer

thickness of 2-4 centimetres, depending on the size and weight of the sensor unit may be suitable. The shape of said carrier is preferably like a solid soap or pillow- like. Other possible shapes include polyhedral, ellipsoidal, and spherical.

A shape resembling a pillow will add to the tumbling behaviour inside the tire more than e. g. a spherical shape. Depending on the sensor, however, also a spherical shape is likely to work, since it will rotate at a higher rotational speed than the tire itself, and thereby cause the acceleration to become higher than a threshold value of an energy saving circuit as described above. The sensor unit is preferably arranged in a cavity in the middle of said piece.

The protective carrier is very easily placed inside a tire in conjunction with the mounting of said tire to a rim. In tubes, the carrier can be placed inside the tube at the manufacturing of said tube, or later. If it is later, a short incision is preferably made in the tube, the carrier with the sensor unit is inserted, and the incision is subsequently sealed, by way of e. g. vulcanisation.

In the above example the carrier may be provided with an outer cover of a suitable material.

Brief description of the drawings Embodiments of the invention will be further described and explained in connection with the enclosed drawings of which Figs. la and b show a top and a side view of a protective carrier, respectively; Fig. 2 shows a cross sectional view from the side of a tubeless tire with a protective carrier ; Fig. 3 shows a cross sectional view from the side of a tire having a tube with a protective carrier; and Fig. 4 schematically illustrates functional elements of a sensor unit for use in a an embodiment according to the invention.

Detailed description of preferred embodiments Fig. la shows a top view of a protective carrier 100 according to an embodiment of the present invention. The carrier 100 comprises a block 101 of a shock-absorbing material such as e. g. expanded PU plastic, rubber, cork or some other material with shock-absorbing material characteristics. In a preferred embodiment block 101 is provided with a cavity 102 for housing a sensor unit (not shown). The cavity 102 can be omitted dependent on the softness and flexibility of the material of the block 101, in that a soft material can adjust itself around said sensor unit. During assembly of the carrier and the sensor unit, the sensor unit is placed in an incision or in a cut or carved cavity 102 in the middle of said carrier 100. The carrier 100 is then sealed by means of glue, adhesive tape or another suitable method. Carrier 100 is preferably given a tumbling friendly shape and rounded edges 104. The shape preferably resembles a solid soap, or pillow. Such shape will add to the tumbling behaviour inside the tire more than e. g. a spherical shape. A tumbling behaviour is likely to give rise to high accelerations, which can trigger the sensor and transmission unit to transmit data. In an alternative embodiment, carrier 100 has a spherical shape, since the diameter of carrier 100 by necessity will be smaller than the diameter of the tire in which it is placed, carrier 100 will rotate at a higher rotational speed than the tire itself, and thereby cause the acceleration to become higher than the threshold value of a power saving circuit, as described above. Fig lb shows a side view of the carrier in fig. la.

Fig. 4 illustrates, by means of a schematic block diagram, an embodiment of a sensor unit 401 for use in the inventive system. It should be understood that the blocks of Fig. 4 represent functional elements, which are not necessarily physically separated. Sensor unit 401 comprises a pressure sensor 403 for measuring gas or

fluid pressure. Sensor 403 typically has an interface to the exterior of sensor unit 401, including a membrane, a pressure-sensitive crystal, or any other means known from the background art. A power source 404 is further included in sensor unit 401. Power source 404 is preferably a battery. Dependent on the type of pressure sensor used, power source 404 may or may not be needed to pick up a pressure measurement value from pressure sensor 403. Passive mechanical or hydraulic solutions may not need electrical power, whereas e. g. a piezo-electrical embodiment would. Sensor unit 401 further includes signal transforming means 402 for transforming pressure values as detected by pressure sensor 403 into a radio transmittable signal. Such signal transforming means 402 preferably includes a microprocessor. A radio transmitter 406 is also included in sensor unit 401, connected to signal transforming means 402. At least signal transforming means 402 and radio transmission means 406 are operated by means of electrical power, which is supplied from power source 404. Sensor unit 401 further comprises means for reducing power consumption, typically a motion sensor 405, in the form of an accelerometer that controls at least transmission through transmission means 406, and optionally also pressure measurement by sensor 403, and signal transformation by signal transforming means 402. In particular, motion sensor 405 is operatively connected to at least transmission means 406 such that no transmission occurs when no motion above a preset threshold level is detected, where said threshold level preferably is an acceleration level.

As indicated with reference to Figs la and lb, sensor unit 401 is placed inside or attached to a protective carrier 100, which in turn is freely located in the tire of a vehicle. A receiver unit 407 is placed exterior of the tire, and comprises means for receiving radio signals transmitted from transmission means 406, and means for decoding a measured parameter value which

has been transformed into the received signal by transforming means 402. This way, a tire parameter value measured by sensor 403 is transferred to the exterior of the tire, preferably to a vehicle to which the tire is attached. Receiver unit 407 may e. g. be positioned close to the tire, or centrally in the cockpit of the vehicle.

Furthermore, one and the same receiver unit 407 may be used for plural tires, i. e. to their respective sensor units 401, or alternatively one receiver unit 407 is used for one single sensor unit 401.

Fig. 2 shows a side view of a tire 201 attached to a wheel rim 205. The inner space 203 of tire 201, usually filled with pressurised air, is provided with a freely moveable protective carrier 100 having a sensor unit 401 inside, such as the sensor unit described in conjunction with Fig. 4. Said sensor unit 401 provides means for measuring air pressure, transforming pressure values representing said pressure to a radio transmittable signal, and transmitting said signal. Sensor unit 401 also comprises a power source, e. g. a battery for powering the transmitter and other electronics. The unit further comprises means for reducing power consumption, typically a motion sensor in the form of an accelerometer that controls transmission and/or measurements such that no transmission and/or measuring is triggered when no motion above a pre-set acceleration occurs. At least for slowly rotating wheels, the acceleration which sensor unit 401 is subjected to when the combined carrier 100 and sensor unit 401 tumbles inside tire 203 as the wheel turns, is higher than what would have been the case if the sensor unit had been firmly attached to rim 205 of the wheel. This way, a threshold acceleration level may be overcome for the motion sensor 405 even if the acceleration at rim 205 is less than the threshold level.

Another advantage with the present invention is indicated in Fig. 2. Assume that the vehicle is moving in the direction of the arrow A, then the periphery of the

tire moves according to arrow B. Carrier 100 follows the inside of the tire during the first quarter of a revolution, or less, before it drops or tumbles down to the lowest portion of the tire again. Most of the time the carrier will be at a position between zero and 90 degrees towards the rear part of the tire, counted from lowest position. This is an advantage since tires of forestry machines sometimes travel over stones 210 and other objects that compress the tire all the way to the rim, and such a compression always begins at the leading side of the tire. The present invention therefore prevents, by the effect described above, the carrier 100 from being damaged between the rim 205 and a compressed part 220 of the tire 201, since carrier 100 will normally be located at the trailing side of the tire, as illustrated.

Fig. 3 shows a side view of wheel with a tire 201 having an internal tube 208. Said tube 208 is shown in cross section where two sections occur, one inner section 207 close to rim 205 and one outer section 206 close to the inner surface of the tread of tire 201. As in Fig. 2 the protective carrier 100 is shown in the right lower part of the air-filled tire, and in the embodiment of Fig. 3, the combined carrier 100 and sensor unit 401 is also freely located inside tube 208. The problem of prior art not being able to measure the pressure in tires with tubes is overcome with a system according to Fig. 3.

In the foregoing, the principles of the present invention have been described with reference to preferred embodiments, as well as the merits and advantages of the invention. It should be noted, though, that variations of the specific embodiments are also included in the invention as such. For one thing, the sensor unit may be used for monitoring other tire parameters than pressure, such as e. g. temperature or humidity. The scope of the invention is defined by the appended claims.