Optical Pressure Sensor

An optical pressure sensor including an optical radiation source and an optical guide that may be optically coupled to the radiation source and may be configured to obtain a total internal reflection condition. The optical guide may define an interface wall. The sensor may also include an element elastically deformable and transparent to optical radiation that has a face facing said interface wall and configured so that a pressure exerted on the deformable element changes a contact area with the interface wall so that the optical guide assumes a frustrated total internal reflection condition with emission of an output optical radiation towards the first face of the deformable element dependent on the exerted pressure. The sensor may further include a photoresistor optically coupled to the second face of the deformable element and configured to provide an electrical signal dependent on the output optical radiation.

TECHNICAL FIELD

The present invention relates to optical pressure sensors, and in particular to sensors that operate based on the phenomenon of frustrated total internal reflection.

STATE OF THE ART

As known, Total Internal Reflection (TIR) indicates the complete reflection of electromagnetic radiation within a material, in the presence of an interface with another material. The phenomenon occurs if the angle of incidence is greater than a certain limiting angle, called the critical angle, depending on the refractive indices of the two interfaced materials, according to Snell's law.

Although in a TIR condition the radiation is basically completely reflected, there is a part of the electromagnetic field that crosses the interface. This field does not propagate but decays very quickly with distance from the interface.

However, in the presence of another material within this distance, with a refractive index at least equal to that of the first material, some of the radiation may propagate again and the reflection will not be complete. In this case we speak of frustrated total internal reflection (in English, Frustrated TIR).

The paper S. Zhu, A. Yu, D. Hawley, and R. Roy “Frustrated total internal reflection: a demonstration and review”, American Journal of Physics, vol 54, no. 7, pp. 601-607, 1986, provides a theoretical description of the FTIR phenomenon.

The paper A. Lavatelli, A. Zanoni, E. Zappa, A. Cigada, “On the Design of Force Sensors Based on Frustrated Total Internal Reflection,” IEEE Transactions on Instrumentation and Measurements, vol. 68, no. 10, pp 4065-4074, 2019, describes among others, an experiment documenting the behavior of the FTIR phenomenon at the micromechanical level and proposes a Greenwood-Williamson (GW) model as a tool to predict the response of an FTIR-based pressure sensor.

U.S. Pat. No. 9,880,653 describes a pressure-sensitive tactile system consisting of a transparent sheet, with a light source and detector arranged along perimeter of the transparent sheet.

SUMMARY OF THE INVENTION

The present invention addresses the problem of providing an optical pressure sensor, based on the FTIR phenomenon, which is an alternative to known ones and which, in particular, is not limited to the measurement of pressures exerted on large surfaces but allows its miniaturization.

The present invention relates to an optical pressure sensor as defined by independent claim1and particular embodiments thereof, as defined by dependent claims2-14

DETAILED DESCRIPTION

In this description, similar or identical items or components will be referred to in the figures by the same identifying symbol.

FIG.1shows an example of an optical pressure sensor100operating according to the principle of frustrated total internal reflection.

The optical pressure sensor100(hereinafter, for brevity, “optical sensor”) includes at least one optical radiation source1, an optical guide2, at least one elastically deformable element3, and at least one photoresistor4.

In accordance with the particular embodiment described herein, the sensor100is such that it generates an electrical signal dependent on the pressure exerted by the hand of a user.

According to the example described herein, there are two optical radiation sources1each configured to operate at wavelengths that may be chosen depending on the application.

In particular, each optical source1is realized by means of a corresponding LED (Light Emitting Diode), housed in a corresponding adapter6. The adapter6, in addition to acting as a housing, allows the electrical connection between an electrical power source and the corresponding LED1. In accordance with an example, the LED1emits at a wavelength of blue. It is not excluded that the optical source1may emit in frequency bands outside of the visible range such as, for example, infrared or ultraviolet.

According to a preferred form of implementation, the optical guide2is a solid body, for example cylindrical in shape. In particular, the two optical sources1are coupled to the ends of the cylindrical optical guide2. For example, the optical guide2is immersed in air.

The material with which the optical guide2is made is chosen so that a total internal reflection condition can be obtained for the radiation emitted by the sources1. The total internal reflection condition is also obtained by appropriately designing the inclination of the radiation beam emitted by the optical sources1with respect to the axis of the optical guide2.

For example, considering LED sources1operating at wavelengths of blue, the cylindrical optical guide2is realizable in plexiglass, a material having a refractive index n1with respect to air of about 1.5. The optical guide2defines an outer wall7, which corresponds to an interface surface with the surrounding air.

Alternatively to the cylindrically shaped Plexiglas guide, other types of optical guides could also be used. Preferably, the optical guide2can be made of a material having the following properties:(A) Transparency, i.e., a non-zero transmissivity, preferably, greater than 70%;B) Stiffness: the material of the optical guide2does not deform when subjected to the pressure to be measured;C) Refractive index greater than that of air (or the fluid in which the sensor100is immersed).

Note that a material such as, for example, glass is suitable for use in making the optical guide2.

With reference to the shape of the optical guide2, it should be noted that in order not to lose the total internal reflection condition, the optical guide2has a shape that has no edges except at the end portions. Therefore, the most suitable shapes are cylinder shapes or its deformations such as, for example, prisms with an elliptical base even irregular.

The optical guide2can also have a non-rectilinear axis: small curvatures with respect to the length are permissible, as they do not damage the total internal reflection condition.

With reference to the elastically deformable element3, it should be noted that it is deformable under the action of a pressure and that it brings into contact with the optical guide2and regains its initial shape when this pressure ends. The elastically deformable element3has an elastic modulus appropriate for the type of application. Considering the case in which the pressure exerted by a user's hands is measured, the elastically deformable element3has an elastic modulus such that it is deformable for manually exerted pressures.

Furthermore, the elastically deformable element3is such that it is transparent to optical radiation emitted by the optical source1. By the term “transparent” is meant a non-zero transmissivity, preferably, greater than 70%.

In accordance with the described example, the elastically deformable element3is made of silicone rubber and has a refractive index with respect to air n2of about 1.4.

As will be made clear later, the elastically deformable element3is such that, in the presence of a pressure exerted on the element itself, it changes a respective contact area with the outer wall7of the optical guide2, so as to originate a condition of frustrated total internal reflection. In this condition, there occurs the emission from the optical guide2of an optical output radiation which passes through the elastically deformable element3.

In particular, as can be seen inFIG.1, the sensor100described comprises two elastically deformable elements3which are substantially equal to each other. According to one example, each of said elastically deformable elements3has a hemispherical shape and identifies a first face8(in the example, a spherical cap) facing the wall7of the optical guide2and an opposite second (flat) face, not shown in the figures.

Each elastically deformable element3is associated with a corresponding photoresistor4which is attached to the second face of the element3itself, by means of an adhesive layer. Advantageously, the deformable element3covers an input optical port of the photoresistor4so that said photoresistor can be invested by the radiation escaping from the optical guide2and not by that of the surrounding environment.

It should be noted that, as already described, the optical guide2, in any of its embodiments, is such that it is rigid, i.e., non-deformable due to the pressures exerted on it by the elastically deformable element3. With respect to non-deformability, conventional optical fibers are not preferred for use in the sensor100.

For example, considering the application relating to manually exerted pressure, the elastically deformable element3has a diameter of between 8.00 and 5.00 mm and a total height of between 2.00 and 5.00 mm. The diameter is chosen so that the elastically deformable element3can cover the entire photoresistor4, and its height is such as to result in a wide range of pressure detection.

As known, the photoresistor is an electronic component whose resistance is dependent (in particular, inversely proportional) to the amount of optical radiation that strikes it. In particular, the value of its resistance decreases as the intensity of the radiation that strikes it increases so that the electric current flowing through this component is proportional to the intensity of the radiation.

The photoresistor4is arranged to receive the output optical radiation escaping from the optical guide2when frustrated total internal reflection conditions occur.

The photoresistors4and associated elastically deformable elements3may be encompassed in a support structure so as to provide a load cell on which hand pressure may be exerted and, accordingly, be moved closer to or away from the optical guide2. An example of such a load cell will be described later.

Each photoresistor4is electrically connected between a first electrical terminal9and a second electrical terminal10, to which corresponding electrical conductors are connected. The photoresistor4is such that it provides the electrical terminals9and10with an electrical signal (typically a voltage) that is dependent on the optical radiation that has hit the photoresistor.

More in detail, the electrical conductors connect the electrical terminals9and10to an electronic processing circuit (not shown inFIG.1) configured to provide a readout signal to the photoresistor4and receive the electrical signal (i.e., an electrical voltage) dependent on the optical radiation that has hit the photoresistor4. Further, the electronic circuit is such that it processes this electrical signal received from the photoresistor4and measures an electrical parameter (the voltage drop across the photoresistor4) that depends on the pressure exerted. The electronic circuit may further associate with this measured electrical parameter (e.g., after an analog-to-digital conversion) an information of interest for the specific application of the sensor100.

Such information of interest may be the same value of the pressure exerted on the sensor100or the value of another quantity that is associated with the pressure exerted on the load cell of the sensor100according to a predetermined mode of correlation between such pressure and the information of interest. As will be clarified by a subsequent example, said quantity of interest may be, non-limitingly, a force exerted by the hand, a speed or an angle indicative of a direction of travel or an inclination required for a vehicle maneuvered by means of the sensor100.

Such electronic circuitry may be integrated into the same container in which the components shown inFIG.1are housed, or may be external to the container.

It should be noted that, advantageously, a calibration is performed for each combination of photoresistor4and radiation source1so that the sensor can operate correctly in the range of pressure values of interest, generating an absolute measurement.

Note that, preferably, the sensor100is configured so that each elastically deformable element3is in contact with the optical guide2even in the absence of exerted pressure and thus in the condition of total internal reflection.

In the operation of the sensor100, when a user exerts manual pressure on the load cell and thus on the hexastatically deformable elements3these change their relative contact area with the wall7of the optical guide2depending on the exerted pressure.

With the contact of the wall7of the guide2with the elastically deformable elements3, there is a switch from a total internal reflection (TIR) to a total frustrated internal reflection (FTIR) situation and thus a transmitted radiation is generated which escapes from the optical guide2and, passing through the elastically deformable elements3, affects the photoresistors4. For each elastically deformable element3, the portion of radiation transmitted externally to the optical guide2is proportional to the contact area between the external part7of the optical guide2and said element3. This contact area increases with increasing hand pressure exerted by the user as the elastically deformable element3is increasingly pressed against the outer wall7.

The photoresistors4vary their resistance as a function of the radiation passing through them. The electronic circuit connected to the photoresistors4can trace the voltage drop across the photoresistor4or the resistance of the photoresistor and then after appropriate processing provide the information of interest for the particular application.

When the hand pressure ceases, the load cell and thus each elastically deformable element3moves away from the outer wall7of the optical guide2, reducing the contact area until the initial condition of total internal reflection is re-established.

The following description relates to a particular form of sensor actuation100that takes the form of a control stick200for a helicopter or helicopter driving simulator.FIG.2shows, in addition to the control stick200, a chair300on which a pilot can sit while holding the control stick200with one hand. Applicant has constructed and tested a device corresponding to the control bar200described herein.

According to the example, the control bar200is suitable for cyclic control. Cyclic control famously involves controlling the roll (tilt to the right or left) and pitch (tilt forward or backward) angles of the aircraft.FIG.9also shows a collective control bar400that may be implemented by employing one or more sensors100.

As is well known, collective control controls the lift generated by the main rotor of the aircraft. In a level flight, when the helicopter is tilted forward, the collective control also serves to adjust the forward speed. The pilot can hold the collective control bar400with one hand and the cyclic control bar200with the other hand.

The control bar200(FIGS.2and3) includes an outer casing201that is, preferably, curved and hollow so as to accommodate the components described below.

For example, the outer casing201is obtained by 3D printing. In addition, the control bar200is provided with a control head210on which control buttons or further sensors are mounted, also of a conventional type, which are not represented.

According to the embodiment considered, the control bar200includes four load cells11, each including a support structure12(FIG.7) configured to support two elastically deformable elements3and two related photoresistors4.

The support structure12includes a support block13(FIG.5), such as a body having a substantially rectangular base provided with two housings14(e.g., cylindrical in shape) for relative photoresistors4upon which corresponding elastically deformable elements3are attached.

In addition, the support structure12is provided with a fastening frame15(FIGS.7-8) that mechanically couples to the support block13so as to hold the photoresistors4and associated elastically deformable elements3in place.

According to a particular embodiment, the fastening frame15comprises three separate modules which when assembled adequately secure the two photoresistors4to the corresponding support block13. The fixing frame15identifies a concave wall16with holes from which the two elastically deformable elements3emerge. On a wall17of the support block13, opposite to the concave wall16, there are holes18that allow electrical conductors to access the terminals9and10of each photoresistor4.

As depicted inFIG.4, the four load cells11are arranged within the enclosure201such that two of the four load cells11cooperate with an optical guide2, while two other load cells11cooperate with another optical guide2. In particular, the concave wall16accommodates part of the waveguide2allowing contact with the elastically deformable elements3.

In particular, the housing201includes a lower portion202and an upper portion203. According to the described embodiment, the lower portion202houses a relative optical guide2and two load cells11arranged on opposite sides from the optical guide itself. For example, one of the two load cells11is arranged on the left side, while the other load cell11is placed on the right side of the housing201(where in defining left and right refers to the viewpoint of the pilot sitting in the chair300).

According to this implementation form, the upper portion203houses another optical guide2and two other load cells11arranged on opposite sides of the optical guide itself. For example, one of the two load cells11is disposed in an anterior position (i.e., on a side of the housing201facing the pilot) while the other load cell11is disposed in a posterior position (i.e., on a side of the housing201facing in a direction opposite to where the pilot is sitting in the chair300).

In accordance with the foregoing, the load cells11placed on the sides of the lower portion202of the enclosure201provide for controlling the lateral inclinations of the helicopter, while the load cells11arranged in the upper portion203provide for controlling the roll and pitch angles.

The enclosure201provides, at each load cell11, an opening204(FIG.3) from which the pilot's hand can act on the wall17of the load cell itself.

In addition, the four load cells11are housed in the openings204so that, upon exerting pressure on them, they can make a linear movement of approaching the corresponding optical guide2and upon ceasing to exert such pressure, regain their initial position. According to one example, such apertures204may be employed to insert load cells11.

The housing201may comprise locking means arranged to prevent lateral displacement of the load cells11or release of the load cells11from the apertures204, for example, such locking means may be formed with suitable form-fitting couplings and/or comprise elements that hinder release of the load cells11(e.g., plates held in place by magnets).

In particular, the magnitude of the displacements of the load cells11is limited to linear movement toward the optical guide2associated with deformation of only the elastically deformable elements3.

Note that the arrangement of the four load cells11within the housing201described above allows the hand and fingers of the driver to act fully on the control bar200.

Further, it should be noted that advantageously, each of the load cells11has a sufficiently large size to allow for a slight change in hand position, which can occur when the pilot, for example, presses a button on the cloche head without loss of sensitivity for the load cells. In other words, the hand movements on the control bar200required to press a button, for example, are not sufficient to move out of the area of competence of the load cells11. Thus, under normal use, the pilot can make all normally necessary movements without the load cell losing contact with the hand.

According to the form of implementation described herein of the bar200, the optical waveguide2is a cylindrical plexiglass rod having a circular cross section having a diameter of 12 mm. The light sources1are two blue LEDs having a diameter of 5 mm, held in place by 3D printed adapters6.

The choice of the blue color for the LEDs1is the result of a test performed by the Applicant using five different colors of LEDs (white, green, yellow, blue, red) to determine to which color the photoresistor4was most sensitive, in particular, in terms of minimum detectable brightness and sensitivity to changes in brightness.

The following describes how to determine the critical angle, i.e., the limiting angle to obtain the total internal reflection condition, with reference to example materials.From Snell's law we obtain:

sin⁡(αr)sin⁡(αi)=n1n2where:αris the angle of the reflected radiation beam, which is set to 90° to find the limiting angle;αiis the angle of incidence of the radiation beam to be determined;n1/n2=1.5 is the refractive index of the plexiglass with respect to air;With these conditions, the critical angle obtained is αi-cr=42°.

It should also be noted that even if a total internal reflection is not realized, the functionality of the sensor100is not compromised because the load cells11make a measurement of a relative value and the zero value can be set at each initialization.

With regard to the choice of photoresistor4, it should be noted that a photoresistor having a suitably wide operating range for this application is the GM5549 type.

FIG.9schematically shows an example of an electronic circuit500external to the control bar200and connected thereto by conductors501extending within a support lever502supporting the control bar itself.

The electronic circuit500includes a first measurement and processing module503connected to the photoresistors4and configured to perform, as already mentioned, the measurement of voltage drop or resistance and generate (after an analog-to-digital conversion) a signal S1representative of the quantities of interest. The first module503is, for example, connected to one or all of the load cells11with which the control bar200is equipped.

The first module503may be configured to evaluate, for each load cell11, the overall voltage drop across the two photoresistors4electrically connected in series, or it may be configured to evaluate the voltage drop across the individual photoresistor4of each cell. It is possible that the first module503is configured to average the pressure values obtained from each photoresistor4of a load cell11.

The signal S1representative of the quantity of interest (such as, for example, the roll or pitch angle), after any amplification performed by an amplifier G, is provided to an electronic output module504that generates appropriate control signals Sc to be provided to actuators or other components of a driving simulator. The amplifier G may be integrated output electronics module54.

In accordance with the described example, the electronic circuit500further includes a second measurement and processing module505connected to other devices that may be provided to the control head210associated with the control bar200. For example, signals provided to the second measurement and processing module505may be obtained from additional sensors that conventionally detect a tilt or orientation assumed by the control bar200.

A third measurement and processing module506may be provided that receives signals from other control devices integrated into the control head210, for example, load cell calibration knobs. The electronic circuit500may be implemented by a microcontroller also capable of providing the appropriate power supply to the LEDs1.

Note that in the event that part of the functional blocks of the electronic circuit500is integrated within the control bar200(or within the same housing as the sensor100), it is possible that the measured, pressure-dependent quantities exerted on the load cells11may be transmitted wirelessly to an external processing module.

Note that the control bar200described above is only one possible example of an implementation of the sensor100, which can be applied in different fields. Indeed, the sensor100can be used in any process or activity involving human-machine coupling and can be placed in an input device of a machine of various types to detect human interaction with it.

Possible examples of such input devices are: the steering wheel of a car, the input sticks of a bulldozer and/or any other construction vehicle, the input sticks of aircraft or rotorcraft, power tools.

With regard to the detectable quantities, the sensor100allows the acquisition of a real-time dataset and information relating, in addition to the pressure exerted on a surface, also to a local force and a direction of the applied force. The determination of the direction is done, for example, by differential reading of the load on the photoresistors4of a single load cell11and comparison with the others.

Sensor100is particularly advantageous for this type of application because it makes it possible to detect hand activity on the device by distinguishing unwanted from voluntary actions and allowing unwanted actions to be corrected. In fact, the sensor100makes it possible, for example, to detect a loss of grip or warn the user of an incorrect hand position. This is achieved by detecting activity (i.e., pressure exerted) at the point of contact between the hand and the control input (according to the example, the back of the load cell11, as shown inFIG.8), avoiding the effect of kinematic chains and being able to detect even actions that are not normally detectable. For example, small displacements that fall within a control dead zone but could be symptomatic of fatigue, tension, or general loss of control of the command can be revealed. It is also possible to distinguish a command resulting from accidental impact with the control bar200from an intentional command based on the point of application on the control bar itself, since what is detected is the actual control action and not its effect on the input device.

In addition, sensor100allows for a real-time dataset that does not interfere with the user's lived experience: because of the sensor's operating principle, its displacement is almost negligible.

Another area of application for the sensor100is in improving the training and muscle recovery experience.

Sensor100can be integrated into training and physiotherapy devices to collect information on improved exercise execution in terms of accuracy, speed, force exerted and regularity of movement, while also providing a time history and quantification of the improvement.

Note that this scope is not limited to hand exercises, but is easily extendable to training and recovery of muscles such as the pelvic floor or others. This is also due to the fact that no wearable sensing parts are required, and all components necessary for sensing muscle activity can be arranged within the training device.

The sensor of the present invention is particularly advantageous because the sensor can be completely integrated into the device of interest without altering the user experience.

Furthermore, because in the sensor100the activity is monitored right in the area of pressure application, and not after a possible kinematic chain, a more accurate and reliable measurement is achieved.

Note that each photoresistor4has only two leads carrying the output signal, and this output signal does not necessarily require conditioning operations. In addition, the sensor100does not present any problems related to signal timing even in the situation where multiple load cells11are working together.

Furthermore, due to the use of optical components, the measurement is not altered by changes in electromagnetic fields or temperature variations.

FIGURE NUMBER LEGEND