GB2613340A

GB2613340A - Graphene based rotational transducer and torque sensor

Info

Publication number: GB2613340A
Application number: GB2117093.1A
Authority: GB
Inventors: David Biddulph Phillip
Original assignee: Paragraf Ltd
Current assignee: Paragraf Ltd
Priority date: 2021-11-26
Filing date: 2021-11-26
Publication date: 2023-06-07
Also published as: GB202117093D0

Abstract

A transducer for determining rotational movement comprises a rotatable bar 710 and a magnet 722/724 providing a magnetic field extending between first and second sides of the rotatable bar 710. The magnet 722/724 is connected to, and rotates in unison with, a first axial location of the rotatable bar 710. The transducer comprises a graphene hall sensor assembly 750 being rotationally stationary with respect to the rotatable bar 710 and located between the rotatable bar 710 and the magnet 722/724 such that the graphene hall sensor assembly 750 sits in the magnetic field adjacent to the first axial location. The graphene hall sensor assembly 750 may comprise angularly offset graphene hall sensors 754/756/758. A second transducer may be provided at a second axial location of the rotatable bar 710 to form a torque sensor 700. The rotatable bar 710 may be a steering column of a vehicle.

Description

Graphene based rotational transducer and torque sensor

Background

Graphene is an allotrope of carbon consisting of a single layer of atoms arranged in a two-dimensional honeycomb lattice. Charge carrier behaviour within graphene is influenced by the presence of a magnetic field.

As such, if a current flows through graphene in a first direction in the plane of the graphene it is possible to use a magnetic field to influence the current density profile in a second direction in the plane of the graphene, perpendicular to the first direction. When the magnetic field lines are parallel to the plane of the graphene (referred to in this document as a = 0 °), there is no influence on the current density profile. The influence on the current density profile is greatest when the magnetic field lines are orthogonal to the plane of the graphene (referred to as a = 90 °). However, the sensitivity to change is greatest at a = 0 ° and lowest at a = 90 °. To generalise, absolute signal strength is proportional to sin(a) while change in signal strength is proportional to cos(a).

It is known to deploy these properties of graphene in the context of a Hall-effect sensor.

Summary

Against this background, in a first aspect of the disclosure there is provided a transducer for determining rotational movement, the transducer comprising: a rotatable bar having an axis of rotation about which the rotatable bar is configured to rotate; a first magnet providing a first magnetic field extending in a first magnetic field direction from a first side of the rotatable bar to a second side of the rotatable bar located on an opposite side of the axis of rotation from the first side of the rotatable bar, wherein the first magnet is connected to the rotatable bar at a first axial location such that the first magnet rotates in unison with the first axial location of the rotatable bar; a first graphene hall sensor assembly being rotationally stationary with respect to the rotatable bar and located between the rotatable bar and the first magnet such that the -2 -graphene hall sensor assembly sits in the first magnetic field adjacent the first axial location of the rotatable bar; whereby rotation of the rotatable bar relative to the first graphene hall sensor assembly causes the first magnetic field direction to change alignment relative to the first graphene hall sensor assembly, thereby generating a first electrical potential in the first graphene hall sensor assembly indicative of an angle of rotation of the rotatable bar relative to the first graphene hall sensor assembly.

In this way, improved accuracy may be achieved in measuring rotation.

The transducer may comprise a first graphene hall sensor and a second graphene hall sensor, wherein the first and second graphene hall sensors are angularly offset relative to the first magnetic field direction.

In this way, improved accuracy may be achieved in measuring rotation.

The first graphene hall sensor assembly may further comprise a third graphene hall sensor angularly offset from both the first and the second graphene hall sensor relative to the first magnetic field direction.

In this way, further improvements in accuracy may be achieved.

The first graphene hall sensor and the second graphene hall sensor may be angularly offset by 90 °.

In this way, when the first graphene hall sensor is providing its least accurate data, the second graphene hall sensor is providing its most accurate data, and vice versa. Thus the combined output of first and second graphene hall sensors is of sufficient accuracy regardless of angular position.

Where there are first, second and third graphene hall sensors, these may be angularly offset from each of the others of the first, second and third graphene hall sensors by an angle of 120 °. -3 -

In this way, accuracy of the combined output of the three graphene hall sensors may be optimised.

The transducer may further comprise a first carrier at the first axial location that comprises a first carrier radial portion that projects radially from the rotatable bar and a first carrier axial portion that projects parallel to the rotatable bar, wherein the first carrier axial portion comprises the first magnet.

In this way, the first magnet may be held in appropriate position relative to the first magnet.

The first graphene hall sensor may be located in a first volume bounded by rotatable bar, the radial portion and the axial portion.

In this way, the first graphene hall sensor may be held in appropriate position relative to the first magnet.

The axial portion may be cylindrical and coaxial with the rotatable bar such that the first volume has an annular cross section In this way, the first graphene hall sensor may be held in appropriate position relative to the first magnet.

The first magnetic may comprise a plurality of first magnets.

In this way, the first magnetic field may be of appropriate strength in relation to the first graphene hall sensor.

In a second aspect of the disclosure, there is provided a torque sensor comprising: a first transducer in accordance with the transducer of any preceding claim; and a second transducer comprising: a second magnet providing a second magnetic field extending in a second magnetic field direction from a third side of the rotatable bar to a fourth side of the rotatable bar located on an opposite side of the axis of rotation from the third side of the rotatable bar, wherein the second magnet is connected to the rotatable bar at a second axial location -4 -such that the second magnet rotates in unison with the second axial location of the rotatable bar; a second graphene hall sensor assembly being rotationally stationary with respect to the rotatable bar and located between the rotatable bar and the second magnet such that the second graphene hall sensor assembly sits in the second magnetic field; whereby rotation of the rotatable bar relative to the second graphene hall sensor assembly causes the second magnetic field direction to change alignment relative to the second graphene hall sensor assembly, thereby generating a second electrical potential in the second graphene hall sensor assembly indicative of an angle of rotation of the rotatable bar relative to the second graphene hall sensor assembly; such that torque within the rotatable bar between the first axial location and the second axial location is proportional to a difference between the first electrical potential and the second electrical potential.

In this way, improved accuracy may be achieved in measuring torque.

Optionally, the torque sensor, in a neutral position of the rotatable bar in which the rotational bar experiences zero torque: the first side of the rotatable bar is aligned with and axially offset from the third side of the rotatable bar; and the second side of the rotatable bar is aligned with and axially offset from the fourth side of the rotatable bar.

In this way, the first and second transducers may be matched.

The second graphene hall sensor assembly may comprise a fourth graphene hall sensor and a fifth graphene hall sensor, wherein the fourth and fifth graphene hall sensors are angularly offset relative to the second magnetic field direction.

In this way, accuracy of the second graphene hall sensor may be improved.

The second graphene hall sensor assembly may further comprise a sixth graphene hall sensor angularly offset from both the fourth and the fifth graphene hall sensor relative to the second magnetic field direction. -5 -

In this way, accuracy of the second graphene hall sensor may be improved.

The fourth graphene hall sensor and the fifth graphene hall sensor are angularly offset by 90 °.

Each one of the fourth, fifth and sixth graphene hall sensors may be angularly offset from each of the others of the fourth, fifth and sixth graphene hall sensors by an angle of 120 °.

The torque sensor may further comprise a second carrier at a second axial location that comprises a second carrier radial portion that projects radially from the rotatable bar and a second carrier axial portion that projects parallel to the rotatable bar, wherein the second carrier axial portion comprises the second magnet.

In this way, the second graphene hall sensor may be held in appropriate position relative to the second magnet.

The second graphene hall sensor may be located in a second volume bounded by rotatable bar, the second carrier radial portion and the second carrier axial portion.

The second carrier axial portion is cylindrical and coaxial with the rotatable bar such that the second volume has an annular cross section.

In another aspect of the disclosure there is provided a vehicle comprising a steering system including a steering column and a torque sensor of any of claims 10 to 18, wherein the steering column comprises the rotatable bar.

In this way, an improved steering system for a vehicle is provided. -6 -

Figures Specific embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 provides a schematic view that shows the distribution of current flowing in a graphene sheet that is not subject to a magnetic field; Figure 2 provides a schematic view that shows the distribution of current flowing in the graphene sheet of Figure 1 when subject to a magnetic field orthogonal to the plane of the graphene sheet (which is the angle at which the influence of the magnetic field on current density profile is greatest but the sensitivity to change is least); Figure 3a shows a schematic view of a graphene sheet at 90° to the direction of the magnetic field, at 45° to the direction of the magnetic field, and parallel to the direction of

the magnetic field;

Figure 3b shows a plot of (i) a function of measured electric potential versus angle and (ii) absolute change in the function of measured electric potential versus angle of graphene

sheet relative to the magnetic field;

Figure 4 shows a schematic view of a graphene sheet (a single transducer element) rotatably mounted about an axis that allows rotation of the graphene sheet so as to include within its rotational scope both a position in which it sits parallel to the magnetic field lines and a position in which it sits perpendicular to the magnetic field lines; Figure 5 shows a schematic view of an embodiment of a transducer comprising a graphene sheet arrangement including a pair of graphene sheets 104, 106, with a fixed offset of 90 etherebetween; Figure 6 shows the embodiment of Figure 5 with the magnet and associated elements rotated relative to the graphene sheet arrangement; -7 -Figure 7 shows an alternative embodiment to that shown in Figure 5 in which the graphene sheet arrangement comprises three graphene sheets with a fixed offset of 120 ° between each graphene sheet and its neighbours; Figure 8 shows the embodiment of Figure 7 with the magnet and associated elements rotated relative to the graphene sheet arrangement; Figure 9 shows a first set of (rotating) components of a torque sensor in accordance with the present disclosure wherein the top of Figure 9 shows a side view while the bottom shows a cross sectional view through the line A-A; Figure 10 shows a second set of (non-rotating) components of a torque sensor in accordance with the present disclosure wherein the top of Figure 10 shows a side view while the bottom shows a cross sectional view through the line A-A; Figure 11 shows the first and second sets of components of the torque sensor of Figures 9 and 10 in combination; Figure 12 shows the embodiment of Figure 11 with the magnet and associated elements rotated relative to the graphene sheet arrangement; Figure 13 shows a schematic view of an electronic circuit for use with a graphene sheet in accordance with the transducers of the disclosure; Figure 14 shows a schematic view of an alternative electronic circuit to the one shown in Figure 13; Figure 15 shows a schematic view of a further alternative electronic circuit to the ones shown in Figures 13 and 14; Figure 16 shows a current drive circuit for driving a current in the graphene sheet in the direction from top to bottom as shown in the orientation of Figure 1; -8 -Figure 17 shows for the graphene sheet arrangement of Figure 7 a logarithmic plot of the minimum resolvable angle for each of three graphene sheets versus the angle of each graphene sheet relative to the magnetic field lines; and Figure 18 shows how a stacked arrangement of graphene sheet arrangements, each offset along the axis of rotation, which may be provided as part of a transducer in accordance with the disclosure.

Detailed description

The term "graphene sheet", as used herein, refers to one or more layers of graphene. Accordingly, the term graphene sheet may refer to a monolayer of graphene or to multilayer graphene (which may be termed a graphene layer structure') arranged in a laminar structure. Thus, graphene sheet refers to a graphene layer structure having from 1 to 10 monolayers of graphene. A monolayer of graphene on a substrate may be preferred The term "transducer element", as used herein, refers to the deployment of a graphene sheet to convert between electrical and kinetic energy and/or between kinetic energy and electrical energy.

The term "graphene sheet arrangement", as used herein, refers to a plurality of graphene sheets, each either monolayer or multilayer, wherein each graphene sheet in the graphene sheet arrangement is arranged in a different orientation relative to the other graphene sheets in the graphene sheet arrangement.

The term "transducer", as used herein, refers to a device comprising one or more transducer elements.

Figure 1 shows a graphene sheet 100 having a cross-shape comprising a wide portion 140 located between a first narrow portion 130 and a second narrow portion 150. The graphene sheet 100 is shown with a current flow 200 through it from top 110 to bottom 120 of the graphene sheet 100. (Directional references such as top, bottom, left and right are not absolute and refer only when viewing in the orientation shown in Figure 1.) In the absence of a magnetic field, current flow 200 is evenly distributed from left to right of the -9 -graphene sheet, in the orientation of Figure 1. This even distribution is represented by the current flow lines 210 evenly distributed from left to right. (Note that the current flow lines 210 are particularly schematic.) The cross-shape of the graphene sheet 100 means that the current flow lines 210 become more distributed as the current passes through the wide portion 140 and less distributed as the current passes through the first and second narrow portions 130, 150. Both in the narrow portions 130, 150 and the wide portion 140, the current flow lines 210 are evenly distributed across the width of the graphene sheet. While the phenomena described are particularly apparent in a cross-shaped graphene sheet, a cross-shape is not essential.

Figure 2 shows the same graphene sheet 100 as Figure 1 with a current flowing through it from top to bottom of the graphene sheet. (As with Figure 1, directional references such as top, bottom, left and right are not absolute and refer only when viewing in the orientation shown in Figure 2.) A magnetic field is present such that magnetic field lines are orthogonal to the plane of the graphene sheet 100. The effect of the magnetic field is to alter the charge distribution between left and right sides of the graphene sheet 100. In particular, the current density shifts to the left in the orientation of Figure 2. This uneven distribution is represented by current flow lines 212 unevenly distributed. (Note again that the current flow lines 212 are particularly schematic.) A schematic plot of the charge density profile is plotted beneath the graphene sheet. While the phenomena described are particularly apparent in a cross-shaped graphene sheet, a cross-shape is not essential.

These properties described in relation to Figures 1 and 2 can be deployed as a transducer comprising a graphene sheet 100 configured to receive a current flow whereby a charge density of the current flow is influenced by a magnetic field.

In particular, the relative angle between the magnetic field lines and the plane of the graphene sheet can be detected by measuring the electrical potential resulting from uneven charge density distribution.

With reference to Figures 3a and 3b, where the magnetic field lines are perfectly orthogonal to the plane of the graphene sheet (a = 90 °), the effect on current density profile is greatest (that is, the electrical potential is greatest) but the sensitivity to change in angle as the graphene moves out of perfect orthogonality (that is, the rate of change of electrical potential) is least.

-10 -By contrast, where the magnetic field lines are parallel to the plane of the graphene sheet (a = 0 °), there is no effect on the current density profile (that is, there is no electrical potential generated) but the sensitivity to change in angle as the graphene moves out of perfect parallelism (that is, the rate of change of electrical potential) is greatest.

To generalise, absolute signal strength is proportional to sin(a). However, change in signal strength is proportional to cos(a). Thus signal strength is maximum when a = 90 and zero when a = 90 °. But change in signal strength with angle is minimum when a = 90 ° and maximum when a = 0 °. Therefore, the transducer element is at its most sensitive at a = 0 °.

Measuring electrical potential in the graphene sheet provides an output that varies based on a.

Sensor signal (T) in Figure 3b is determined relative to the sensitivity of the sensor. This may be V/T, or V/AT where A is the current drive, or V/VT (output volts per Tesla field strength).

The applicant has determined that it is possible to resolve the magnetic strength down to e-5 T RMS. This was shown for a temperature of 295 K, a magnetic field strength of 1 T, a bias current of 10 pA, a total resistance of 10 ku and using a graphene sheet in a package having dimensions of approximately 10 mm x 10 mm x 3 mm with a sensitive area of 0.03 cm2 and a sensitivity of 1,000 V/AT.

Thus, if the rotation frequency is less than electronics noise Bandwidth frequency, the sensitivity of the device enables the detection of rotations as small as 0.05 °. In other words, it is possible to detect a change in angle between a = 0 ° and a = 0.05 °. By reducing the total resistance of the graphene (determined by the purity of the graphene) the minimum detectable rotation can be reduced.

Figure 4 shows a graphene hall sensor comprising a graphene sheet 100 like that of Figure 1 mounted on an axle 180 (having an axis of rotation X) so as to allow rotation of the graphene sheet 100 between a plane that is parallel to the magnetic field lines 350 (a = 0 °) and a plane that is perpendicular to the magnetic field lines 350 (a = 0 °). In this way, measuring the effect on current density in the graphene sheet 100 (that is, electrical potential across the graphene sheet) enables angle to be inferred. Sensitivity to change in current density (and hence to change in angle) is greatest nearest to a = 0 °.

Figure 5 shows a transducer 400 comprising a graphene sheet arrangement 101 including a pair of graphene sheets 104, 106, with a fixed offset of 90 lherebetween, and a magnet 300 which provides magnetic field lines 350 and which is rotatable relative to the graphene sheet arrangement 101 about axle 380. In this arrangement, when an appropriate current is provided and an effect of the magnetic field on current density is measured, each of the two graphene sheets 104, 106 serves as a graphene hall sensor. The north (N) and south (S) poles of the magnet 300 are identified, but could be reversed without making any other changes.

By providing a pair of graphene sheets 104, 106 at a 90 °offset, at a rotational position of the magnet where one graphene sheet has zero sensitivity, the other graphene sheet has maximum sensitivity. In this way, the sensitivity profile of the combination of two graphene hall sensors provides high sensitivity at any angular position of the magnet.

Figure 6 shows the transducer 400 of Figure 5 with the magnet 300 rotated relative to Figure 5. From this, it is clear that rotation of the magnet influences current density in both graphene sheets 104, 106 of the graphene sheet arrangement 101, to a degree that can be determined in accordance with the relationships plotted in Figure 3b when deployed as graphene hall sensors.

Figure 7 shows a transducer 600 comprising a graphene sheet arrangement 102 comprising three graphene sheets 104, 106, 108, with a fixed offset of 120 ° between each graphene sheet and its neighbours, and the magnet 300 which is rotatable relative to the graphene sheet arrangement 102. (Note that while the internal angle between each graphene sheet and its neighbour is 60 °, the external angle is 120 °.) By providing three graphene sheets 104, 106 at 120 °offsets, the sensitivity profile of the combination provides an even higher sensitivity than the embodiment having two graphene sheets, for any angular position of the magnet. In this way, the sensitivity profile of the combination of three graphene hall sensors provides high sensitivity at any angular position of the magnet.

-12 -Figure 8 shows the transducer 600 of Figure 7 with the magnet 300 rotated relative to Figure 7. From this, it is clear that rotation of the magnet influences current density in all three graphene sheets 104, 106, 108 of the graphene sheet arrangement 102, to a degree that can be determined in accordance with the relationships plotted in Figure 3b.

Figure 9 shows a first (rotating) set of components of a torque sensor 700 in accordance with the present disclosure. The top part of Figure 9 shows a side view while the bottom part of Figure 9 shows a cross sectional view through the line A-A.

The torque sensor 700 comprises a torsion bar 710 including a first carrier 720 at a first axial location of the torsion bar 710 such that the first carrier 720 rotates in unison with the first axial location of the torsion bar 710. The first carrier 720 comprises a magnet or magnets 732, 734 that give rise to a first magnetic field at the first axial location in a radial direction from a first side of the torsion bar 710 to a second, opposite, side of the torsion bar 710. The first carrier 720 may be part of the torsion bar 710 or mounted to the torsion bar 710. Either way, the first carrier 720 is configured to rotate in unison with the torsion bar 710.

Similarly, the torsion bar 710 includes a second carrier 730 at a second axial location of the torsion bar 710 such that the second carrier 730 rotates in unison with the second axial location of the torsion bar 710. The second carrier 730 comprises a magnet or magnets 732, 734 that give rise to a second magnetic field in a radial direction from the first side of the torsion bar 710 to the second side of the torsion bar 710. The second carrier 730 may be part of the torsion bar 710 or mounted to the torsion bar 710. Either way, the second carrier 730 is configured to rotate in unison with the torsion bar 710.

In an event that the torsion bar 710 is under torsion, this results in a rotational offset between the rotational position of the first carrier and the rotational position of the second carrier. In a first alternative, the rotational offset may be because the first carrier will rotate in a first direction (e.g. clockwise) and the second carrier will also rotate in the first direction (e.g. clockwise), but the extent of rotation of the first carrier will be different to the extent of rotation of the second carrier. In a second alternative, the rotational offset may arise because the first carrier 720 will rotate in the first direction (e.g. clockwise) and the second carrier 730 will rotate in a second, opposite direction (e.g. anti-clockwise). In a third -13 -alternative, the rotational offset may arise because the first carrier will rotate in the first direction (e.g. clockwise) and the second carrier will remain rotationally stationary. In any of these scenarios, the result is a rotational offset between the rotational position of the first carrier and the rotational position of the second carrier.

Since the torque sensor uses angular measurements to determine torque, the torque sensor can also provide those angular measurements as an output. This may avoid the need for separate sensors to measure angle and torque.

Figure 10 shows a second (non-rotating) set of parts of a torque sensor 700 in accordance with the present disclosure. The top part of Figure 10 shows a side view while the bottom part of Figure 10 shows a cross sectional view through the line A-A.

While the first set of components (shown in Figure 9) is configured to rotate, the second set of components (shown in Figure 10) is configured to remain rotationally stationary.

Referring to Figure 10, the torque sensor 700 further comprises a support 740 on which is mounted, at a first end, a first graphene sheet arrangement 750 and, at a second end, a second graphene sheet arrangement 760. The first graphene sheet arrangement 750 comprises first, second and third graphene sheets 754, 756, 758 (only two of which are visible in the top half of Figure 10) arranged in the same way as the graphene sheet arrangement 102 of Figure 7. The second graphene sheet arrangement 760 comprises a first, second and third graphene sheets 764, 766, 768 also arranged in the same way as the graphene sheet arrangement 102 of Figure 7.

Figure 11 shows the components of Figures 9 and 10 combined in one view such that the relative locations of the components are clear. In particular, the first pair of magnets 722, 724 provides the first magnetic field extending in a first magnetic field direction (vertically between 722 and 724). The second pair of magnets 732, 734 provides the second magnetic field extending in a second magnetic field direction (vertically between 732 and 734). When the torsion bar 710 is subject to no torsion, the first and second magnetic field directions are parallel but axially offset relative to the torsion bar 710.

-14 -The first graphene sheet arrangement 750 is located radially within the first pair of magnets 722, 724 such that the first graphene sheet arrangement 750 is located within the first magnetic field regardless of the rotational position of the first carrier 720.

The second graphene sheet arrangement 760 is located radially within the second pair of magnets 732, 734 such that the second graphene sheet arrangement 760 is located within the second magnetic field regardless of the rotational position of the second carrier 730.

Figure 12 shows the components of Figure 11 wherein the first graphene sheet arrangement 750 is rotated relative to the first carrier 720. In this way, the outputs of the first, second and third graphene sheets 754, 756, 758 are different from when at the position shown in Figure 10. In this way, the rotational angle of the second graphene sheet arrangement 760 is determined to a high degree of accuracy (as explained in more detail below).

Torsional movement of the torsion bar 710 (which is to say a differential in rotational movement between different axial positions along the torsion bar 710) results in a differential in outputs from the first graphene sheet arrangement 750 relative to the second graphene sheet arrangement 760.

By facilitating the collection of highly accurate rotational positions of the first and second carriers 720, 730 (which is to say the first and second magnetic fields) relative to the first and second graphene sheet arrangements 750, 760, respectively, a highly accurate measure of rotational difference is achieved, which translates to a highly accurate measure of torsion in the torsion bar 710.

While the torque sensor 700 of Figures 9 to 12 comprises six graphene hall sensors (corresponding to the six graphene sheets 754, 756, 758, 764, 766, 768), a torque sensor 700 in accordance with the present disclosure is not limited to comprising six graphene hall sensors, three proximate each end of the torsion rod. For example, where only modest torsion is expected, the arrangement may comprise only two graphene hall sensors, one proximate each end of the torsion rod. In another example, there may be four graphene hall sensors, two proximate each end of the torsion rod. In an example requiring extremely high accuracy, a torque sensor 700 may comprise more than six graphene hall sensors.

-15 -The term "graphene hall sensor assembly" may be used to as a numerically ambiguous term for any number of at least one graphene hall sensors located proximate each end of the torsion rod. Hence, the embodiment of Figures 9 to 12 comprises two graphene hall sensor assemblies, each comprising three graphene hall sensors.

Figure 13 shows an arrangement of hardware 500 for driving and acquiring an output from a graphene sheet 100 such as that shown in Figures 1 and 2. In a transducer comprising graphene sheet arrangements having more than one graphene sheet, such as that shown in Figures 5 and 7, the hardware may be duplicated for each graphene sheet.

The hardware 500 comprises a transducer driver and amplifier circuit 510. The hardware may be deployed in tandem with data acquisition software (not shown).

The transducer driver and amplifier circuit 510 comprises a supply 512 used to drive the graphene sheet 100 by generating a current flow in one orientation of the graphene sheet (referencing Figure 1, this current flows from top to bottom in the illustrated orientation).

The transducer driver and amplifier circuit 510 circuit comprises an amplifier 514 and an amplifier supply 516 which is used to drive the amplifier 514. The amplifier circuit 510 may further comprise a ground connection 518 that facilitates removal of an offset.

One or more filters, pre-amplifiers and analogue to digital converters may also be provided (not shown) after the transducer and amplifier circuit 510 and before the PC data acquisition software A data acquisition system (not shown) may comprise a PC or other processor configured to receive an output from the sensor drive and output of the amplifier circuit 510. The data acquisition system may comprise an analogue digital converter 524.

In the arrangement shown in Figure 13, there is AC coupling between the graphene sheet 100 and the amplifier 514.

In an alternative arrangement, shown in Figure 14, there may be DC coupling in place of AC coupling.

-16 -Figure 15 shows the arrangements of Figures 13 and 14 and also includes a sensor offset cancellation functionality 511.

Figure 16 shows a possible arrangement for the supply 512 comprising a transistor 612 and a potentiometer 610 for offset cancellation. Other constant current supply implementations are possible.

Figure 17 shows the minimum angle resolvable for each of three graphene sheets (three transducer elements) in a transducer such as that shown in Figure 7 (or from either one of the graphene sheet arrangements shown in Figures 11 and 12).

In the nomenclature of Figure 17, an overall transducer angle of a = 0 is when ai = 0 az = 120 ° and az = 240 °.

By using the combination of three signals (one from each transducer element) it is possible to calculate with a considerably degree of confidence the rotational position of the transducer between a = 0 ° and a = 360 °.

Resolving the overall transducer angle may involve a number of steps.

In a first aspect of the process, a transducer element angle reading (ai, az, az) is determined for each transducer element (that is graphene sheet 104, 106, 108). The output for each transducer element is a voltage proportional to the sine (-1.0 to 1.0) of the angle (a) of the magnetic field to the plane of the transducer element. This results in a two-fold ambiguity since there is not a one to one mapping of angle of the transducer element to the output voltage. There are two possible angles for each output voltage since, for one full period of a sine wave, the same voltage arises at two different angles.

However, the ambiguity can be resolved by considering the output from all three transducer elements. While, in the illustrated arrangements, there are three transducer elements, it would equally be possible to resolve the ambiguity with only two transducer elements.

For a transducer comprising three transducer elements, each separated by 120 degrees (such as that shown in Figures 6 and 7), the steps are: -17 -a/ Use arcsine function to calculate the principle and secondary angles for the each transducer element (transducer element angle), b/ Remove the offset (0, 120 and 240 degrees) from the principle and secondary transducer element angles to arrive at the transducer overall angle; c/ Use the modulo operator to restrict the principle and secondary angles for each transducer element to the range 0 to 360 degrees; d/ Determine the correct transducer overall angle for each transducer element for each angle by considering when transducer element responses are expected to be the same due to the fixed angles between the transducer elements.

In a second aspect of the process, it is an object to determine a best transducer overall derived angle (a) by using a combination of the angles from each transducer element.

The transducer element output (V_out) is proportional to the sine of the transducer element angle (an) to the magnetic field. Setting the maximum output for each device to 1 V and assuming a uniform magnetic field of 1 T, then V_out=sin(a). The change in voltage with a change in Theta is dV_out=cos(a)"da.

The minimum resolvable signal for each transducer element (V_noise) depends on the electrical noise floor (which will depend on sampling bandwidth). Setting the signal to noise to 1, V_outN_noise=1=cos(a)*da/V_noise. So the minimum resolvable angle will be da=V_noise/cos(a). In other words, there is a large change of signal for small rotations, when the angle to the field is very small. At high angles (perpendicular) there is no change in signal for a small rotation.

Given the first aspect of the process, it is conceptually straightforward to obtain a highly accurate measure of the transducer overall angle (a) by determining the weighted average of the three transducer element angles (al. az, az). A natural weighting is 1/ da-2.

So the normalised weighting for transducer element 1 is: Weight_Theta_1=(cos Theta1)**2/( (cos Theta1)**2+(cos Theta2)**2+(cos Theta3)**2).

The denominator appears to be a constant.

-18 -The limit on the minimum resolvable angle is determined by the electronic noise floor of the electronics.

Varying the noise floor in the calculator changes the expected minimum resolvable angle.

For a relative noise floor of 0.00001, the minimum resolvable angle is 0.0006 degrees.

(This assumes a very large electronic sampling time.) Noise and sampling time can be selected for any particular application in order to arrive at an appropriate level of precision for the application.

While the disclosure has focused on transducers having either a pair of transducer elements (Figure 5) or a trio of transducer elements (Figure 7), the disclosure is not limited to such arrangements. In particular, embodiments having different numbers of transducer elements (including one transducer element) also fall within the scope of the disclosure. Where there is more than one transducer element, it is necessary for increased resolution for each transducer element to have a different angle to the magnetic field lines.

Otherwise, the contribution of additional transducer elements does not add to the sensitivity of the combination. In an arrangement of two transducer elements it may be, for example, that the first and second transducer elements are offset by 90 ° such that when one is providing minimum sensitivity, the other is providing maximum sensitivity. However, other angle offsets are contemplated and would fall within the scope of the disclosure.

In embodiments comprising more than one transducer element, there is no requirement for the transducer elements to be axially in line with one another. Indeed, they may be axially offset from one another.

Further sensitivity could be achieved by providing more than three transducer elements in a magnetic field. For example, Figure 18 discloses a specific arrangement of multiple transducer elements stacked in a direction parallel to the axis of rotation. In the Figure 18 arrangement, each level in the stack comprises three transducer elements (that is three graphene sheets 104, 106, 108) in a delta formation around the axis of rotation of the axle 180. There is an offset angle between the delta formation in each level in the stack relative to the delta formation in each other level in the stack. In this way, the precision of the transducer may be increased further since there are a total of nine transducer elements, each at a different angle. This arrangement of nine transducer elements may be reproduced at both ends of the torsion bar 710.

-19 -The stacking arrangement does not necessarily require a delta arrangement of transducer elements at each level in the stack. Indeed, there may be only one transducer element at each level in the stack. Or there may be more than one transducer element at each level in the stack, for example there may be two or three transducer elements at each level in the stack distributed about the axis.

While the description discusses the use of graphene, the same arrangements and principles could be adopted using other two-dimensional materials that avoid the planar effects of three-dimensional materials. If you made a hall sensor out of other 2D materials, the same principles would apply (i.e. lack of planar effect due to 2D material). In this context, the term 2D means that the material needs to be flat in three dimensions.

Examples of other two-dimensional materials that might be substituted for graphene include silicene, germanene and borophene.

One example of a potential application for the torque sensor as set out here is for detecting torque on the steering column of a vehicle.

One example of a potential application for a transducer for determining rotational movement may be a seismometer. In this example, a transducer for determining rotational movement in accordance with the disclosure may be suspended in space such that seismic movement (e.g. an earthquake) would alter the magnetic field, thus triggering detection of rotational movement indicative of the strength of the seismic movement.

Another example of a potential application for the transducer for determining rotational movement may be a pick up for an electric guitar or other stringed musical instrument that uses a steel (ferromagnetic) string. In such an application, the rotatable bar is the string of the instrument. A magnet produces a magnetic field in the vicinity of the steel string.

When struck, the steel string vibrates and alters the magnetic field. The graphene hall sensor is arranged such that the 20 plane of the graphene is in line with the field lines of the magnet. Thus the graphene hall sensor provides a sensitive output indicative of the vibration in the steel string.

Claims

-20 -CLAIMS1. A transducer for determining rotational movement, the transducer comprising: a rotatable bar having an axis of rotation about which the rotatable bar is configured to rotate; a first magnet providing a first magnetic field extending in a first magnetic field direction from a first side of the rotatable bar to a second side of the rotatable bar located on an opposite side of the axis of rotation from the first side of the rotatable bar, wherein the first magnet is connected to the rotatable bar at a first axial location such that the first magnet rotates in unison with the first axial location of the rotatable bar; a first graphene hall sensor assembly being rotationally stationary with respect to the rotatable bar and located between the rotatable bar and the first magnet such that the graphene hall sensor assembly sits in the first magnetic field adjacent the first axial location of the rotatable bar; whereby rotation of the rotatable bar relative to the first graphene hall sensor assembly causes the first magnetic field direction to change alignment relative to the first graphene hall sensor assembly, thereby generating a first electrical potential in the first graphene hall sensor assembly indicative of an angle of rotation of the rotatable bar relative to the first graphene hall sensor assembly.
2. The transducer of claim 1 wherein the first graphene hall sensor assembly comprises a first graphene hall sensor and a second graphene hall sensor, wherein the first and second graphene hall sensors are angularly offset relative to the first magnetic field direction.
3. The transducer of claim 2 wherein the first graphene hall sensor assembly further comprises a third graphene hall sensor angularly offset from both the first and the second graphene hall sensor relative to the first magnetic field direction.
4. The transducer of claim 2 wherein the first graphene hall sensor and the second graphene hall sensor are angularly offset by 90 °.
5. The transducer of claim 3 wherein each one of the first, second and third graphene hall sensors is angularly offset from each of the others of the first, second and third graphene hall sensors by an angle of 120 °. -21 -
6. The transducer of any preceding claim further comprising a first carrier at the first axial location that comprises a first carrier radial portion that projects radially from the rotatable bar and a first carrier axial portion that projects parallel to the rotatable bar, wherein the first carrier axial portion comprises the first magnet.
7. The transducer of claim 6 wherein the first graphene hall sensor is located in a first volume bounded by rotatable bar, the radial portion and the axial portion.
8. The transducer of claim 6 or claim 7 wherein the axial portion is cylindrical and coaxial with the rotatable bar such that the first volume has an annular cross section.
9. The transducer of any preceding claim wherein the first magnetic comprises a plurality of first magnets. 15
10. A torque sensor comprising: a first transducer in accordance with the transducer of any preceding claim-and a second transducer comprising: a second magnet providing a second magnetic field extending in a second magnetic field direction from a third side of the rotatable bar to a fourth side of the rotatable bar located on an opposite side of the axis of rotation from the third side of the rotatable bar, wherein the second magnet is connected to the rotatable bar at a second axial location such that the second magnet rotates in unison with the second axial location of the rotatable bar; a second graphene hall sensor assembly being rotationally stationary with respect to the rotatable bar and located between the rotatable bar and the second magnet such that the second graphene hall sensor assembly sits in the second magnetic field; whereby rotation of the rotatable bar relative to the second graphene hall sensor assembly causes the second magnetic field direction to change alignment relative to the second graphene hall sensor assembly, thereby generating a second electrical potential in the second graphene hall sensor assembly indicative of an angle of rotation of the rotatable bar relative to the second graphene hall sensor assembly; such that torque within the rotatable bar between the first axial location and the second axial location is proportional to a difference between the first electrical potential and the second electrical potential.-22 -
11. The torque sensor of claim 10 wherein, in a neutral position of the rotatable bar in which the rotational bar experiences zero torque: the first side of the rotatable bar is aligned with and axially offset from the third side of the rotatable bar; and the second side of the rotatable bar is aligned with and axially offset from the fourth side of the rotatable bar.
12. The torque sensor of claim 10 or claim 11 wherein the second graphene hall sensor assembly comprises a fourth graphene hall sensor and a fifth graphene hall sensor, wherein the fourth and fifth graphene hall sensors are angularly offset relative to the second magnetic field direction.
13. The torque sensor of claim 12 wherein the second graphene hall sensor assembly further comprises a sixth graphene hall sensor angularly offset from both the fourth and the fifth graphene hall sensor relative to the second magnetic field direction.
14. The torque sensor of claim 12 wherein the fourth graphene hall sensor and the fifth graphene hall sensor are angularly offset by 90 °.
15. The torque sensor of claim 13 wherein each one of the fourth, fifth and sixth graphene hall sensors is angularly offset from each of the others of the fourth, fifth and sixth graphene hall sensors by an angle of 120 °.
16. The torque sensor of any of claims 10 to 15 further comprising a second carrier at a second axial location that comprises a second carrier radial portion that projects radially from the rotatable bar and a second carrier axial portion that projects parallel to the rotatable bar, wherein the second carrier axial portion comprises the second magnet.
17. The torque sensor of claim 16 wherein the second graphene hall sensor is located in a second volume bounded by rotatable bar, the second carrier radial portion and the second carrier axial portion.-23 -
18. The torque sensor of claim 16 or claim 17 wherein the second carrier axial portion is cylindrical and coaxial with the rotatable bar such that the second volume has an annular cross section.
19. A vehicle comprising a steering system including a steering column and a torque sensor of any of claims 10 to 18, wherein the steering column comprises the rotatable bar.