The invention relates to a structure for an energy collector, according to the preamble to Claim 1.

The invention also relates to a method for collecting energy.

In many wireless devices, the availability of energy has become a central concern. Piezo- based solutions have been developed for objects that contain vibration, but these solutions have low durability. MEMSs have been seen as a promising solution, but their ability to collect energy is poor, due to their small size.

Solutions based on RFID technology are also known, but the equipment assembly in them is expensive. Solutions based on induction are also known from automobile tyres, but their durability has been low in environments that impose a very high mechanical load. In the measurement of the pressure of automobile tyres, batteries have also been used in connection with the pressure sensor and its transmitter, but this solution has the drawback of requiring the repeated replacement of the batteries. Finnish patent application 20105929 discloses a solution, in which a pin is used, which is made to vibrate when the tyre flexes as it impacts the ground. In this solution, one end of the pin is vulcanized onto the inner surface of the outer carcass of the tyre. In the end of the pin there is either an MEMS acceleration sensor or a piezo-element, which can collect energy from acceleration. When the tyre of a car meets the surface of the road, a sudden change in angle of several tens of degrees, which depends on the tyre pressure, takes place in the tyre. The change in angle causes the pin to flex and to start to vibrate. Periodic acceleration acts on the MEMS or piezo-collector in the end of the pin, until attenuation destroys the vibration. This solution requires a difficult procedural stage in the manufacture of the tyre and there is no guarantee as to the reliability of the solution, as the operating environment is extremely demanding. As the sensor is on the outer circumference of the tyre, the pin is unavoidably subject to even major impacts. In the generally known prior art, there are problems with both durability and the efficiency of the energy collection, especially when collecting impulse-type energy, where there is a real danger of the collector becoming saturated. The present invention presents a device and method for collecting energy, which is particularly well suited to the tyres of vehicles, typically a car.

More specifically, the energy collector according to the invention is characterized by what is stated in the characterizing portion of Claim 1.

The method according to the invention is, for its part, characterized by what is stated in the characterizing portion of Claim 10.

Considerable advantages are gained by means of the invention.

The invention offers a cheap and maintenance-free pressure sensor, for example for an automobile tyre. Calculation shows that using the arrangements makes it possible to obtain easily more than 1-mW power, which is up to even ten times more than the power required by the electronics. One advantageous application of the method is the monitoring of a vehicle's tyre pressures, but of course it can also be used for other applications. The present method concentrates on a spring storing the energy initially, but of course one can also envisage the energy being initially stored in mass through acceleration. This of course opens the method to several applications. The operating principle functions not only in tyres, but also, for example, in the suspension between a combustion engine and a chassis, in shoes, and in roller-track mats, so that it can be used, for example, in connection with the tracks of skidoos, to monitor the condition of the rollers, or the properties of the snow.

In the following, the invention is examined with the aid of embodiments according to the accompanying figures, in which

Figure 1 shows a side view of one energy collector according to the invention, before installation.

Figure 2 shows the energy collector of Figure 1 installed in a tyre, Figure 3 shows a schematic side view of a second energy collector according to the invention, and particularly the tool for installing it,

Figure 4 shows a photograph of test equipment for an energy collector according to the invention, Figure 5 shows graphically the voltage produced by the energy collector, as a function of time, and

Figure 6 shows an alternative way to support the energy collector.

In the present application, the following terms, for instance, will be used in connection with the reference numbers: 1 valve body

2 air gap

3 electronics

4 rotation mechanism

5 support structure 6 antenna

7 MEMS

8 Mass

9 Oscillator/pendulum = spring 10 + mass 8 + MEMS 7

10 Spring 11 Piezo-membrane

12 Support head, contact with tyre 14 Tyre

15 Installation tube

20 Energy collector = oscillator 9 + support structure 5

21 Free end of oscillator

23 Support corresponding to rim

24 Part of loudspeaker corresponding to tyre

25 Support point

26 Pendulum

1 length of oscillator 9

Generally, the invention is intended to produce electrical energy for an electric circuit 3, which is connected to the tyre 14, or its rim 13, of a vehicle. The electric circuit 3 typically comprises a measurement device for measuring pressure, as well as a transmitter component for sending the measurement data wirelessly to a receiver typically elsewhere in the vehicle.

The energy collector 20 according to the invention consists of, for example, according to Figures 1 and 2, electronics 3 connected to a valve body 1 attached to a rim 13, and a mechanical structure connected to this, which typically comprises a rotating mechanism 4 of some kind, by means of which the energy collector 20 is made to rotate or bend in the operating position, in such a way that it is supported at its support end 12 on the elastic tyre 14. In practice, the energy collector 20 is thus supported between the valve body I and the tyre 14 by means of a support structure 5, which component can also be used as an antenna 6 for forwarding measurement data. A the other end of the support structure, which is next to the valve body, there is a rotation mechanism 4 and at the other end a support end 12, which is supported on the inner surface of the tyre 14. An oscillator 9 is formed of a spring 10 connected to the support end 12, the opposite free end 21 of which comprises, if necessary of an additional mass 8 and possibly a MEMS element 7 for the collection of energy. Alternatively, a piezo-element 11 could also have been formed on the spring 10 for the collection of energy. Thus, the free end 21 of the oscillator moves freely relative to the support structure 5, the factor attenuating the oscillation being the spring's 10 own mechanical attenuation, the air resistance, and the electrical loading. Mechanically, the free movement of the free end 21 can be implemented, for example, by making a hole corresponding to the mass 8 in the support structure 5, or alternatively by forming a slot suitable for the mass 8 for the support structure 5. The oscillator structure 9 is thus typically supported between a repeatedly deforming or moving structure such as a tyre 14 and a rigid structure such as a rim 13, particularly the body 1 of a valve permanently connected to the rim, supported on this. For the invention, it is sufficient if the energy collector 20 is supported between two structures moving repeatedly relative to each other, such as a tyre 14 and a rim 1, 13.

Thus, the energy collector 20 is based on the valve body 1 incorporating an elongated oscillator structure 9, which flexes in the installation stage, in such a way that the support end 12 of the structure rests against the tyre 14. The part of the tyre 14, which comes into contact with the ground, 'bulges' in such a way that the support head 12 moves by a distance determined by the bulging. Of course, the distance moved depends essentially on how far the support head 12 is from the edge of the rim 13. Energy arises when the free head 21 of the oscillator 9 moves in step with the rotational speed of the tyre. The motion excites the mechanical resonator 9 attached to the support point 12 in the tyre 14. In Figure 1, the structure is in the installation stage and Figure 2 shows a situation in which the energy collector has been installed for use in an automobile tyre.

The mechanical construction and maximum power of the energy collector

Assume that the movement at the support point 12 of the tyre 14 is Δχ, in which case the displacement of the mass 8 of a pendulum 9 with a sufficiently low specific frequency will also be the same. The specific frequency of the resonator 9 should be less than the frequency corresponding to the time constant of the pulse. As a consequence, the free end 21 of the pendulum 9 will begin to swing at this amplitude (peak value) and in such a way that the energy will have halved after the Q period. The open Q value depends mainly on the attenuation of the air, but if it is sufficiently large, the electric loading will determine the quality factor. The energy stored in the pendulum 9 is simply E =J/2 *mAx ²

If the pulse has a length of 2Π/10 when the Q value is 20 or less, we will collect the entire energy after a single rotation of the tyre. First of all, calculate the maximum power at a speed of 50 km/h, assuming that we make as large a mass as possible, however in such a way that the structure will fit through the valve opening. The size of the mass can be 2 mm x 4 mm x 10 mm; so that the mass can be 10 mg. Depending on the length 1 of the oscillator 9, the deformation movement of the tyre could 5 mm. These values give us energy of 250 mJ. The pulse repeats at 1/25 Hz at 50 km/h with a 30-cm radius wheel, so that the power we get is 25 Hz x 250 mJ = 6 W. In practice, this means that we can set the length 1 of the oscillator 9 in such a way that it is very close to the rim 13 (movement in the order of 1 mm) and on the other hand we can use a small and light mass of 1 g. Of course, the small mass may require the spring 10 to be made longer or slacker by folding, so that its specific frequency will be made sufficiently small. 1 mm and 1 g give 24 m W and if one percent of this is extracted we receive 250 μ\Υ, which is sufficient in a pressure- sensor application. In the first demonstration device, we received a power of 200 μ\¥, when the mass was 2 mg and the movement about 1 mm.

Energy collection from the pendulum

Perhaps the clearest and simplest way to collect energy is to surface the pendulum with a piezo-membrane. This can be done by laminating a plastic or composite-type membrane on the surface of the pendulum's spring. Another way is to place a ceramic (or plastic, or composite) piezo crystal on the mass 8. In this case, it is natural for the piezo itself to form the pendulum's mass 8. A MEMS type energy collector 7, which is capacitive, can also be attached to the mass.

Magnetic harvesters also exist, but these are difficult to miniaturize sufficiently. The energy collector 20 is otherwise completely similar to collectors that have been developed years ago to collect energy from acceleration, which work by collecting energy from pulses.

Construction and installation The collector can be integrated as part of the valve body, so that when a tyre is changed the collector valve too is changed. Figures 1 and 2 show such a construction. In this construction, the structure can be made to bend against the tyre, either a) by using a tool 15 in connection with a pre-stressed energy collector. The tool 15 can be a tube 15 according to Figure 3, with the aid of which the energy-collector structure 20, which is pre-stressed to an acute angle, is straightened to an obtuse angle inside the installation tube 15. The installation tube 15 is pushed into the rim 13. When the installation tube 15 is pulled out, the energy-collector structure 20 turns to its original shape, as shown by the broken line. Thus, in other words, when the energy-collector structure 20 is released, it returns to its acute-angled basic shape (broken line) and the energy collector 20 bends automatically against the tyre 14 at its support point 12, or b) by making the spring of the rotation mechanism 4 bi-stable, the pressure or flow arising when filling the tyre releases the spring against the tyre. The pressure is easy, because the force is very great and it is easy to make the arrangement in such a way that after the rotation the air can enter the tyre, or c) we make a rod inside the structure, which holds the pin straight: after the rod is removed the structure bends towards the tyre. Such a rod can be used, for example, instead of the tube 15 of Figure 3.

We can also make the valve body independent of the structure. In this solution, there is a collar in the collector, which 'clicks' against the rim. The structure can be of soft metal or rubber, so as to obtain good tightness. A solution metallized in one way or another is good, because we thus obtain a good electrical contact with the metal for the antenna. There is a shield tube around the collector, which is detached once the structure is inside the rim. The detaching of the shield tube releases the structure to turn against the tyre. Of course, there are several different alternative ways to place the structure inside the rim, but the final solution can only be chosen once experience has been gained with the different methods. What is most important of course is that the collector is made very cheap and the installation rapid and reliable.

Increasing effect Basically, the maximum effect is limited, so that the movement of the mass cannot exceed a magnitude that will fit to oscillate close to the tyre 14. However, the following methods are available for increasing the effect: a) we can, however, obtain more power using a small movement and a small mass 8 by making an additional spring between the mass and the fixed support structure. This arrangement leads to series and parallel resonance, and, in the parallel resonance, the amplitude is multiplied by Q, i.e. in practice it is tens of times greater than in the basic construction, b) we make the attachment to the tyre 14 loose, so that the accelerating motion lifts the end of the pin off the tyre 14 and, when it impacts the tyre 14 again, creates a greater acceleration and a greater local flexing, c) we make a structure, in which a small movement causes the mass 8 to strike the support structure, which also increases acceleration and makes the flexing of the spring greater, or d) we can shape the spring 10 in such a way that in a specific area it has a sharper bend, which, in the case of a piezo-surfaced spring 10, increases the voltage. However, in this solution it should be noted that the internal impedance of the piezo will increase.

Experimental result

We performed a test, in which the construction according to Figures 1 and 2 was studied by using a piezo-membrane as an electricity generator. The motion of the tyre was simulated by moving a loudspeaker membrane. The pulse was shaped in such a way that it corresponded more or less to a realistic situation. The tests were made using two different masses (with specific frequencies of 27 and 45 Hz). The result of the test is shown in Figure 4. With one piezo-membrane and a movement of about 1 mm, we obtained an effect of 100 μW by simulating a situation in which a car travels at 50 km/h. The figure shows that the loudspeaker was not attached sufficiently tightly, so that oscillation of the loudspeaker appears at the start of the voltage pulse. The support structure was also not sufficiently stiff, so that part of the flexing was directed to it. The test does, however, show that by using realistic parameters we can easily obtain an effect of more than 100 W and when optimized an effect of at least more than 1 mW. In the figure, component 23 thus corresponds functionally to the rim 13 of Figures 1 and 2, and the loudspeaker element 24, for its part, to the tyre.

Thus, the energy harvester typically comprises a MEMS element 7, the equivalent circuit of which is a capacitor. In practice, the capacitance of this capacitor is formed by a fixed electrode and a rod oscillator located close to it, which formed the capacitor's second electrode. If necessary, a bias voltage (polarization voltage) is formed between the two electrodes, when the oscillating beam will produce alternating electric energy between its terminals. The bias voltage can be formed with the aid of a battery, a piezo-element, or a so-called build-in phenomenon, when a voltage will arise in the vacuum between the two different materials. This electric energy is rectified and fed to an electronics circuit 3, which comprises measurement electronics with a sensor and transmission electronics, to which a transmission antenna 6 is connected. The sensor is typically a pressure sensor.

The element 7 can also be replaced with a piezo-element 1 1 such as a piezo-membrane, which can be located on the surface of the spring 10 or around it.

In practice, the spring 10 begins to oscillate at its specific frequency. A normal MEMS energy collector 5 is set at the end of the spring 10. The pressure sensor too can also be at the free end 21 of the spring 10, i.e. the oscillator, in order to increase the mass. If the Q value of the oscillator 9 is sufficiently high (often as much as 1000), the energy will collect through purely the MEMS 7 in an electrical form. In the MEMS 7, there should additionally be, for example, a piezo-element for creating a bias voltage, allowing the MEMS 7 to act as an energy collector. If, however, the gap in the MEMS 7 is very small, the piezo can be eliminated, because often the so-called build-in voltage will create a voltage over the gap. A battery can also be used to create the bias voltage. The battery will have a very long-life, because in practice the bias voltage will not load the battery at all.

In the present application, the term oscillator 9 refers to an elongated rod-like structure, the maximum diameter of which is clearly less than its length.

Alternative mathematical analysis

If the flexing of the tyre 14 is sufficiently rapid relative to the specific frequency of the oscillator 9, this will rotate by the amount of the angle <p , which depends essentially on the vehicle's weight and tyre pressure. In terms of the mass at the free end 21 of the oscillator 9, this means a displacement x _A . In this case, the energy stored in the oscillator 9 will be in which k is the spring constant of the pin, ω its specific frequency, and m its mass.

The displacement is expressed with the aid of the length I and angle φ of the oscillator 9, which depict the change in shape when the tyre comes into contact with the surface of the road. We can give the displacement in the form χ _Δ = C tancp

The angle φ can be expressed with the aid of the track 2A, and the radius r of the tyre . h

φ = arcsin—

r i.e. we obtain

We can now give the total energy in the form

Assume that the travel from the tyre having a round shape to a flat one takes place once the wheel has moved by the amount of the angle φ _ιη . From this we can calculate the time relating to this displacement.

Because, in the change stage, the mass should remain stationary, so that the rotation of the surface of the tyre will be converted into spring energy, we obtain the condition for the mechanical resonance frequency of the pin. ω≤2π -^- r<Pm use this limit-value frequency, we obtain the maximum energy in the form

Energy is obtained twice from a rotation, so that the iterative frequency is v a_

TO" We can now express the mean effect in the form

If we assume that the mass m = 0.002 kg, the speed v _o = 50 km/h, the pin's length t - 0.01 m, the wheel's radius r - 0.30 m and the total length of the track 2/? = 0.1 m and the angle of deflection φ _η = 0.03 rad, we obtain an effect of P = 0.4 W. Of course, in practice the effect will be less, because we cannot dimension the pin's resonance frequency to be too high. On the other hand, the MEMS element 7 will not necessarily be able to collect the entire energy stored in the oscillator 9. However, the quality factor of an oscillator 9 of this type can be as high as 1000, so that the MEMS element 7 loads the oscillation in such a way that the quality factor is determined by the energy collection of the MEMS. If we envisage the specific frequency being dimensioned to be about 10 times smaller than the maximum frequency, the effect obtained will be about 4 mW. The pressure-sensoring of an automobile tyre typically requires effect of about 0. 1 mW, so the method will be adequate for this application.

The main application of the invention is the collection of energy from a tyre. The method can, of course also be used in all applications in which there is vibration or the deflection of a structure. In an automobile tyre, the pressure can also be measured with the aid of the Q value of the pin 10, which is directly proportional to the pressure in the tyre 14. Unfortunately, moisture also affects the result. The oscillation of the oscillator 9 may perhaps have to be limited, so that it will last. The advantage of the method is, of course, that as such the deflection does not depend on the car's speed, as acceleration does. The invention permits a such a solution provided acceleration does not appear directly in the movement of the oscillator 9, in which a durable structure is obtained, which at the same time is able to collect energy at low speeds too. Durability can be improved by having the oscillator 9 touch the rubber lightly so that the attenuation will act on a large movement. The maximum amplitude can also be attenuated using this method. If the spring is stiff, the mass will follow the deflection, in which case less torque will be directed to the pin, thus improving the pin' s durability. Of course, it is essential for no attempt to be made to collect more energy than necessary from the oscillator 9, in order to maximize durability.

One embodiment of the invention is an energy-collector structure integrated in an automobile tyre. Because one basic principle of the invention is that the energy collector 20 is supported between two structures 14 and 1, 13 moving repeatedly relative to each other, one typical embodiment is to support the energy collector between an automobile rim 13 and a flexible tyre component 14. A second alternative according to the invention is to support the energy collector 20, for example, between a vehicle's chassis and a flexibly mounted engine, to product electrical energy for electronics in the energy collector, which can act as an independent wireless temperature transmitter and send engine-compartment temperature data. The invention can also be used in connection with crawler tracks, for example, between the adjacent plates of a track. The invention is also suitable for collecting energy in connection with repeatedly moving pivots, or other similar devices.

The invention can be used, for instance, in a shoe, between the rigid sole structure and the elastic part of the shoe.

The invention is also suitable for collecting energy in hot conditions, in which, for example, the thermal load on electronics can be reduced with the aid of a ceramic arm. In the device according to the invention a mechanical convertor can also be used, by means of which the low amplitude of the primary movement can be increased to a movement of the desired magnitude.

Figure 6 shows a solution, in which the oscillator 9 is supported on a rim 13 with an additional support point 25, when the deformation Δχ of the tyre 14 is made to increase by a lever effect at the free end of the oscillator 9 to a greater value Ay. In this way, the sensitivity of the structure 20 can be increased. The movement can be further increased by means of a pendulum 26 located at the end of the oscillator 9.

In the present application, the term supporting the energy collector 20 refers to both a fixed, unified structure such as the valve-body connection 1 of the energy collector and also to, for example, the support from the contact point 12 to the tyre 14 taking place by a spring force.

The term two separate structures refers to the structures presented in the description portion, such as a valve body 1 and tyre 14 connected to a rim, or alternatively the engine of an automobile and the chassis of the automobile, which are thus independent parts of a mechanical structure that are separate, but nevertheless possibly connected to each other. The separate structures are typically, but not necessarily, formed from different materials.