Technical field

The invention relates to the field of force measurement, and more specifically a method for wireless measurement of a force or a torque acting on an object using an mechanically vibrating member mechanically attached under strain to said object to have a mechanical resonance frequency which varies with the deflection said object and means therefore.

Prior Art

EP 1 281 056 Bl discloses a method for detecting a measuring quantity of an object involving irradiation of a mechanical resonance element with a microwave signal. The mechanical resonance element is part of a sensor, and it is connected with said object in such a way that when the object e.g. is deflected the strain of the resonance element will be affected with a resulting change of its mechanical resonance vibration movement, which will be registered using of a microwave transceiver emitting microwaves towards and receiving amplitude modulated microwaves from the mechanical resonance element.

US 8,276,451 B2 discloses a device for improved response when measuring vibration frequency of a vibrating object. This device for measuring of a vibration frequency of a mechanically vibrating object, such as a string or rod, of an electrically conducting material, comprises a micro wave transmitter for directing microwaves towards the vibrating object and a microwave receiver for receiving said microwaves amplitude modulated by said frequency of vibration, wherein a stationary member of an electrically conducting material is arranged adjacent a vibration maximum of said vibrating string, in order to increase said amplitude modulation.

It is to be noted that, in the embodiments of said two patent publications, the vibrating member is believed to be radiating the reflected microwave field, which implies that the vibrating member needs to be conductive and in fact acts as an antenna. As is stated in the above mentioned documents, the principal function of this reflection is not yet fully known.

The fact that the vibrating member acts as the antenna implies that it has to be mounted in such a way that it is not shielded by the object to be measured, which will cause problems and negative consequences, as will be discussed further below. Accordingly, there is a need for an improved method and means for detecting a force or a torque acting on an object in order to obtain better design freedom with minimum interference with the object to be measured.

Summary of the Invention The invention is based on the principle that an vibrating member is attached to an object which in use is subjected to a force or a torque causing a deformation/deflection which according to the invention is measured with a method as defined in claim 1 and with the aid of a device having the characterizing features defined in the characterizing part of claim 9.

Developments and embodiments of the invention are defined in the sub claims. According to embodiments of the invention a very compact passive device can be designed, which device will have a mechanical resonance frequency influenced upon by the deflection of the object and having an electrical resonance which is determined by the characteristics of the electronic components in the circuitry. By tuning the electronic circuitry the device will form a detectable signal, when an antenna thereof is put in a microwave field. The

mechanically vibrating member does not need to be in the microwave field leading to very good design freedom.

According to an embodiment of the invention more than one unit comprising a vibrating member being part of a variable capacitor and antenna element can be supervised in the microwave field by one transceiver unit. Each unit can be identified by means of different mechanical vibration frequencies and/or different radio frequencies.

The vibrating member can be tuned to different oscillating frequencies by means of changing the weight, by changing the length, and/or by changing the pre-applied strain to the vibrating member.

The individual radio frequency at which each of the units operates, can be tuned my means of inductors and capacitors being parts of the circuit.

By combining the above mentioned methods of identifying and tuning oscillating units according to the invention, several units can be read simultaneously using only one microwave transceiver. The field of application of the invention is vast. It can be used to measure a force or a torque exerted on e.g. parts of a bicycle such as a pedal, crank arm or crank shaft, or on a body part of a car to measure deflections in different directions when said body part is subjected to strain. A fundamental difference from the prior art discussed above is that, while according to the prior art it is the oscillating/vibrating member that is radiating the reflected microwave field, which implies that the vibrating member needs to be conductive and act as an antenna, the vibrating member according to the invention does not need to be conducting, it only has to have a conductive surface forming part of a capacitor. This conductive surface can be placed on an element attached to the vibrating member or on the vibrating member itself, and the vibrating member can of course also be conductive as such. However, the vibrating member does not act as an antenna. This fact makes it possible to embed the vibrating member in a conductive object, such as a shaft or a beam. The capacitor is connected to the other components of the circuit by means of electrical conductors. The antenna is the only component that needs to be exposed to the microwave field, and as it can be in the form of a patch antenna separated from the rest of the device, there are no design restrictions.

Brief description of the Drawings

The invention will described in further detail below with reference to the accompanying drawings on which Fig. 1 shows the equivalent schematics for the force measurement device according to the invention,

Fig. 2 shows one example of a mechanical design for the force measurement device according to the invention,

Fig. 3 shows a schematic view of an example of separating two force measurement devices according to the invention while using only one microwave transceiver,

Fig. 4 illustrates the possibility to vary the mechanical frequency of the force

measurement device according to the invention, and

Fig. 5 shows one example of a block diagram for a microwave transceiver to be used with the force measurement device according to the invention. Detailed description of Embodiments of the Invention

The invention will now be described with reference to the accompanying drawings, which shows embodiments of the invention, but which should not be regarded as limitations, only as examples of the implementation of the invention. Fig.1 shows a scheme for the force measurement device according to an embodiment of the invention. An vibrating member, e.g. a string or a beam, is connected under strain to an object to be measured. The strain is such that the string is always stretched even when the object is exposed to compression. A conductive surface is arranged on the vibrating member. This conductive surface constitutes a first part of a capacitor referenced Q in Fig. 1, the second part of the capacitor being a conductive area of a stationary dielectric element of the force measurement device according to the invention. The vibrating member may be a string of metal or a non-conducting material, such as a plastic material, e.g. aramid. Also a carbon fiber material can be used.

In Fig. 2 an illustrating embodiment of the invention is shown. The illustrated device has a base 1 with wedge-like protrusions 2, 3, which could be locked into corresponding recesses in an object to be measured. The base is elongate and defines an open space 4, over which a string or beam 5 is arranged under strain. On top of the base there is a dielectric element 6 which delimits the open space of the base. The base may be made of metal or plastic.

According to a preferred embodiment of the invention, the device according to the invention is slightly thicker in the middle, i.e. the base is slightly curved upwards at the respective end and will thus be bent when mounted in said recesses in an object to be measured, resulting in a pre-straining of the vibrating member.

In this embodiment, an element 7 is attached to the vibrating string/beam, which element has a conductive upper surface 8. On the lower side of the dielectric element 6 there is at least one conductive surface 9 opposite to the conductive surface 8 of the element attached to the string/beam 5. The two conductive surfaces 8, 9 form a capacitor with a varying capacitance, comprised in an electronic circuit. This circuit, which further includes resistors, inductors and antenna means, forms a passive oscillator with the electric resonance frequency determined by the values of the resistors, inductors and capacitors in the circuit. Accordingly, a very compact passive device is accomplished according to an embodiment of the invention, having a mechanical resonance frequency depending on the object's deflection or strain and an electrical resonance frequency depending on the electronic components in the circuitry. The circuitry/conductive surface 9 is connected to an antenna 10, which could be situated on top of the dielectric element 6, or could be a patch antenna placed anywhere but connected to the stationary portion of the capacitor by wires. There may be two stationary capacitor portions 9 and a single conductive surface 8 on the string and arranged opposite to the two stationary capacitor portions as shown in Fig. 2.

The antenna can be a dipole or any type of antenna. It is important to note that the vibrating string belongs to a mechanical vibration system and does not need to be in the microwave field. This offers the possibility to integrate the string into an object with only a waveguide and antenna on the surface of the object or elsewhere.

According to the invention the mechanical vibration system is separated from the radio frequency components in such a way that they can be trimmed separately without interference from each other.

The trimming components are placed on the stationary dielectric element. By introducing these trimming components two kinds of channel separation can be chosen, when there are several gauge elements in the microwave field of one and the same transceiver.

Of course several mechanical designs can be made for this principle, and as illustrated in Fig. 2, one is to place the vibrating member 5 under the dielectric element 6.

As is illustrated in Fig. 3, there can be different elements operating at different frequencies, cf. e.g. channel separation of any type of radio system. This opens up the possibility to have a large number of gauge elements connected to each transceiver unit, e.g. supervising a large system of moving parts in a machine, etc.

Within each such radio frequency, the mechanical vibration frequency can be made different by introducing different weights on the string, as is exemplified in Fig. 4, by varying the strain of each vibrating member, and/or by varying the length of each vibrating member.

The microwave field used can vary in frequency. The prototypes under development use 24 GHz frequency. The use of such high frequencies gives a number of advantages such as small assembly on the measuring object, low risk of interference and standard components can be used. In Fig. 5 a block diagram for the transceiver TRX 024 06 is illustrated, which could be used with the present invention. As is illustrated in Fig. 5, the transceiver unit has two separated signal paths:

1. Transmitting path sending a distinct signal at a certain frequency. The signal is

amplified depending on the range requirements. The path has an antenna.

2. Receiving path acts an antenna connected to a LNA feeding the signal to a mixer to retrieve the vibration signal.

The method used according to the invention is so sensitive that the self-induced vibration of the string, i.e. vibrations picked up from the environment, is enough to read a signal from a distance of more than one meter.

There are a number of important considerations to make regarding the mechanical design of the force measurement device according to the invention:

The mechanically vibrating member needs to be either directly connected to the deformed object or connected via a mechanical transferring system, - The member needs to be strained so that it will always oscillate even if the object is compressed,

The design shall be such that the first harmonic is separated in frequency in relation to other harmonic modes.

The antenna 10 can be made very thin such as a patch antenna residing about 1 mm from the ground plane of the shaft, being a very slim design.

In summary, the resonance string gauge RSG according to the invention gives the following unique properties:

- Wireless measurement of deflection (force acting on the object)

- High accuracy in the measurement - Inherently stable technology regarding external factors such as vibration, temperature and humidity Small passive gauge element on the measured object.

The RSG according to the invention has multiple application areas. The technology is used to create a direction changing force meter for road cyclists. All current force meters use strain gauges or in some rare cases piezoelectric sensing elements. This results in quite expensive devices that are not very stable.

With the RSG according to embodiment of the invention, a stable sensing technology is achieved.

The passive gauge element (oscillating member) may have the size of about 1 x 2 x 15 mm. These dimensions make it possible to place the element on almost all the components of e.g. a bike transferring force from the cyclist to the rear wheel of the bike. Possible locations are in the pedal, in the crank arm, in the crankshaft, in the front chain wheel, the chain or the spindle attached to the front chain wheel.

It is obvious that bicycle use is only one, very illustrative example of uses of embodiments of the present invention, which can be used on most objects subjected to a deformation force, objects that are rotating and/or moving or not.