GB2616478A

GB2616478A - Graphene Based Linear Encoder

Info

Publication number: GB2616478A
Application number: GB2203424.3A
Authority: GB
Inventors: David Biddulph Phillip
Original assignee: Paragraf Ltd
Current assignee: Paragraf Ltd
Priority date: 2022-03-11
Filing date: 2022-03-11
Publication date: 2023-09-13
Also published as: GB202203424D0

Abstract

A linear encoder comprising a magnetic array (800, Fig. 5) including a plurality of magnetic cells (300, Fig. 5) arranged in a linear series in a first elongate direction, wherein each cell of length L produces a magnetic field in a magnetic field region parallel to the first elongate direction. Encoder has graphene Hall sensor assembly 410 in the magnetic field region, comprising a first graphene Hall sensor 412 in a first plane and configured to provide a first signal indicative of magnetic field experienced by the sensor 412; and a second graphene Hall sensor 414 in a second plane parallel to the first plane and configured to provide a second signal indicative of magnetic field experienced by the second sensor. Graphene Hall sensors 412, 414 are distributed in a second elongate direction that is parallel to the first elongate direction, such that they experience a change in magnetic field in the magnetic field region as a result of relative axial movement between the first and second elongate directions. There is a processor configured to receive the signals and calculate a position of the assembly 410 relative to the magnetic array 800.

Description

Graphene Based Linear Encoder

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 linear encoder comprising: a magnetic array including a plurality of magnetic cells arranged in a linear series in a first elongate direction, wherein each magnetic cell of the plurality of magnetic cells produces a magnetic field in a magnetic field region parallel to the first elongate direction, and wherein each of the plurality of magnetic cells has a length L in a direction parallel to the elongate direction; and a graphene Hall sensor assembly in the magnetic field region, the graphene Hall sensor assembly comprising: a first graphene Hall sensor in a first plane and configured to provide a first signal indicative of magnetic field experienced by the first graphene Hall sensor; and -2 -a second graphene Hall sensor in a second plane parallel to the first plane and configured to provide a second signal indicative of magnetic field experienced by the second graphene Hall sensor; wherein: the first and second graphene Hall sensors are distributed in a second elongate direction that is parallel to the first elongate direction, such that the first and second graphene Hall sensors experience a change in magnetic field in the magnetic field region as a result of relative axial movement between the first and second elongate directions; and wherein the linear encoder further comprises a processor configured to receive the first signal and the second signal and to use the first signal and the second signal to calculate a position of the graphene Hall sensor assembly relative to the magnetic array.

In this way, linear position can be resolved to a high resolution.

The second plane may be spaced apart from the first plane by L (n + 1/2) where n is an integer.

This maximises resolution in a hall sensor comprising two graphene Hall sensors.

The first and second planes may be perpendicular to the first elongate direction.

The first and second graphene Hall sensors may be distributed along a second elongate direction.

The graphene Hall sensor assembly may further comprise a third graphene Hall sensor in a third plane parallel to the first plane and configured to provide a third signal indicative of magnetic field experienced by the third graphene Hall sensor, wherein the processor is configured to receive the third signal for use in calculating the position of the graphene Hall sensor assembly relative to the magnetic array.

In this way, linear position can be determined to a greater resolution. -3 -

The second plane may be spaced apart from the first plane by L (n + %), where n is an integer; and the third plane may be spaced apart from the first plane by L (m -%), where m is an integer.

This maximises resolution in a hall sensor comprising three graphene Hall sensors.

The third plane may be perpendicular to the first elongate direction.

The first, second and third graphene Hall sensors may be distributed along the second elongate direction.

The first, second and, where present, third planes may all be perpendicular to the second elongate direction.

Each magnetic cell may comprises a north pole and a south pole and the magnetic array may comprise an alternating series of north and south poles.

Each magnetic cell may comprise a north pole and a south pole and an order of north pole and south pole may be changeable.

The magnetic array may comprises a Halbach array.

The linear encoder may further comprise a tape and the tape may comprise the magnetic array.

The tape may be mounted in a reel to reel arrangement between a first reel and a second reel and the graphene Hall sensor assembly may be located between the first reel and the second reel.

Each magnetic cell may be attributed a magnetic cell number in consecutive order and the processor may store a current magnetic cell number based on a currently adjacent magnetic cell relative to the graphene Hall sensor assembly.

The processor may be configured to count each magnetic cell on passing. -4 -

The plurality of magnetic cells arranged in a linear series may be arranged along an axis.

Alternatively, the plurality of magnetic cells arranged in a linear series may be arranged along an arc, optionally along an ellipse.

In a second aspect of the disclosure there is provided a linear encoder comprising: a magnetic array including a plurality of magnetic cells arranged in a linear series in a first elongate direction, wherein each magnetic cell of the plurality of magnetic cells produces a magnetic field in a magnetic field region parallel to the first elongate direction, and wherein each of the plurality of magnetic cells has a length L in a direction parallel to the first elongate direction; and a graphene Hall sensor assembly in the magnetic field region, the graphene Hall sensor assembly comprising: a first graphene Hall sensor in a first plane and configured to provide a first signal indicative of magnetic field experienced by the first graphene Hall sensor; and a second graphene Hall sensor in a second plane and configured to provide a second signal indicative of magnetic field experienced by the second graphene Hall sensor; wherein: the first and second planes are non-parallel, such that the first and second graphene Hall sensors experience a change in magnetic field in the magnetic field region as a result of relative elongate movement between the magnetic array and the graphene Hall sensor assembly in a direction parallel to the first elongate direction; and wherein the linear encoder further comprises a processor configured to receive the first signal and the second signal and to use the first signal and the second signal to calculate a position of the graphene Hall sensor assembly relative to the magnetic array.

Figures Specific embodiments of the invention will now be described with reference to the accompanying drawings in which: -5 -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 plan view of a graphene sheet (a single transducer element) at different locations (a to e) adjacent and perpendicular to a magnetic cell comprising a magnet; Figure 5 shows a linear encoder comprising a graphene Hall sensor and a magnetic array including a plurality of magnetic cells arranged in a linear series with alternating north and south poles; Figure 6 shows a linear encoder like that of Figure 5 but with two graphene Hall sensors; Figure 7 shows a linear encoder like that of Figures 5 and 6 but with three graphene Hall sensors, Figure 8 shows a linear encoder like that of Figure 7 but with the three graphene Hall sensors arranged in a delta formation; Figure 9 shows a linear encoder wherein the magnetic array comprises a magnetic tape; -6 -Figure 10 shows a linear encoder like that of Figures 5 to 9 wherein the magnetic array comprises a Halbach array; Figure 11 shows a signal obtained from a single graphene Hall sensor of a linear encoder including a magnetic array comprising a sequence of ten magnetic cells; Figure 12 shows signals obtained from each of three graphene Hall sensors of a linear encoder comprising three graphene Hall sensors, such as that shown in Figure 7; Figure 13 shows a schematic view of an electronic circuit for use with a graphene Hall

sensor in accordance with 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 Hall sensor in the direction from top to bottom as shown in the orientation of Figure 1; Figure 17 shows results data for resolvable separation angle versus signal to noise ratio at a number of different sampling times; and Figure 18 shows modelled data for encoder speed versus resolution.

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. -7 -

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 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 -8 -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.

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 Measuring electrical potential in the graphene sheet provides an output that varies based on a. -9 -

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 k0 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 cm' 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 400 comprising a graphene sheet 100 like that of Figure 1 shown in five different locations (labelled a, b, c, d and e). In each of the five locations, the graphene Hall sensor 400 is perpendicular to the north-south axis of a magnetic cell (and orthogonal to the plane of the page). This document adopts the term "magnetic cell" 300 for any north-south magnetic pair, whether a conventional permanent magnet or any other device having the properties of a north-south magnetic pair. For example, a magnetisable tape may be configured to exhibit the properties of a north-south magnetic pair. As such, a conventional magnet is an example of a magnetic cell but the term magnetic cell includes north-south magnetic pairs that may not ordinarily be termed a magnet.

The north-south axis of the magnetic cell 300 may be termed an elongate direction of the magnetic cell 300.

In location a, the graphene Hall sensor 400 is aligned with the centre of a north pole. In location e the graphene Hall sensor 400 is aligned with the centre of a south pole. Thus, in locations a and e the graphene Hall sensor 400 will experience magnetic field lines that are largely parallel to the plane of the graphene Hall sensor 400. In other words, at locations a and e, a = 0 °. Thus, in locations a and e, the sensitivity of the graphene Hall sensor 400 is -1 0 -at its highest and therefore the graphene Hall sensor 400 is capable of resolving small movements of the graphene Hall sensor 400 orthogonal to the plane of the graphene Hall sensor 400 (left and right in the context of the Figure 4 view).

In location c, the graphene Hall sensor 400 is mid-way between the centre of a north pole and the centre of a south pole. Thus, in location c, the graphene Hall sensor 400 will experience magnetic field lines that are largely orthogonal to the plane of the graphene Hall sensor 400. In other words, at locations c, a = 90 °. Thus, in location c, the sensitivity of the graphene Hall sensor 400 is at its lowest.

In location b, the graphene Hall sensor 400 is mid-way between location a and location c. In location d, the graphene Hall sensor 400 is mid-way between location c and location e. Thus, in locations b and d the graphene Hall sensor 400 will experience magnetic field lines that are approximately 45 ° parallel to the plane of the graphene Hall sensor. In other words, a = 45 ° at locations b and d. Thus, in locations b and d, the sensitivity of the graphene Hall sensor 400 is mid-way between high and low.

It is therefore clear that if one takes a graphene Hall sensor 400 and moves it relative to the magnet between positions a and e, the location of the graphene Hall sensor 400 can be deduced based on the effect of the magnetic field on the current density profile in the graphene Hall sensor 400 (per Figures 2 and 3). Sensitivity of the measurement will be highest at position c and lowest at positions a and a Figure 5 shows a magnetic array 800 comprising a sequence of magnetic cells 300 arranged in a linear series with alternating north and south poles. In the illustrated embodiments, the sequence of magnetic cells 300 is arranged in a linear series along an axis. However, being arranged along an axis is not essential, which is to say that it is not essential that the linear series is in a straight line. The sequence of magnetic cells 300 may be arranged in a linear series along a one-dimensional locus, such as an arc or an ellipse. Thus, in a broad reading, "arranged in a linear series" means arranged along a one-dimensional locus, whether straight or otherwise. While any one-dimensional locus may be possible, a one-dimensional locus defined by a continuous function may be preferable. This is because a discontinuity in a one-dimensional locus may give rise to magnetic fields which are more complicated for the graphene Hall sensors to resolve.

While the magnetic cells 300 are shown in the illustrated embodiments as abutting one another, it is possible that they may be arranged in a linear series with a gap between each magnetic cell.

In the illustrated embodiment of Figure 5, each magnetic cell 300 is a single rectangular magnet with one north pole and one south pole. A graphene Hall sensor 400 (comprising a graphene sheet 100) is mounted in a first plane perpendicular to the elongate direction of the magnetic array 800 and is adjacent to the magnetic array 800 such that it is movable along the path x-y whilst remaining always perpendicular to the north-south direction of the magnetic array 800. In this way, an output signal from the graphene Hall sensor 400 is sufficient to allow the location of the graphene Hall sensor 400 to be determined. The graphene Hall sensor 400 may retain a count of the number of magnetic cells 300 in the magnetic array 800 have been passed.

As already mentioned, the positional sensitivity of the graphene Hall sensor 400 varies sinusoidally relative to the position in the field lines, with peak sensitivity occurring where the magnetic field lines are orthogonal to the plane of the graphene Hall sensor 400 and with minimum sensitivity occurring where the magnetic field lines are parallel to the plan of the graphene Hall sensor 400. Thus, when the graphene Hall sensor 400 is mid-way between north and south poles the sensitivity is greatest and when the graphene Hall sensor 400 is central relative to a pole the sensitivity is least.

In order to improve sensitivity, a graphene Hall sensor assembly may be provided in which there are a plurality of graphene Hall sensors that move in unison and are offset by any distance other than the length of a magnetic cell 300. Expressing this in terms of angles, if a magnet (comprising one north pole and one south pole) is 360 °, then a graphene Hall sensor assembly comprising two graphene Hall sensors offset by any angle other than 360 ° will improve the sensitivity relative to one graphene Hall sensor operating alone.

Figure 6 shows a graphene hall sensor assembly 410 comprising two graphene Hall sensors 412, 414. The first graphene Hall sensor 412 sits in a first plane perpendicular to the elongate direction of the magnetic array 800. The second graphene Hall sensor 414 sits in a second plane parallel to the first plane. Thus, the first graphene Hall sensor 412 and the second graphene Hall sensor 414 are mutually parallel. The first graphene Hall sensor 412 is offset from the second graphene Hall sensor 414 in a second elongate -12 -direction parallel to the first elongate direction. The offset is 90 ° since this provides maximum improvement in sensitivity for a graphene Hall sensor assembly comprising two graphene Hall sensors. This is because, as is clear from Figure 3b, where one of the graphene Hall sensors is most sensitive the other is least sensitive, and vice versa.

Figure 7 shows a graphene Hall sensor assembly 420 comprising three graphene Hall sensors 422, 424, 426. The third graphene Hall sensor 426 sits in a third plane that is parallel to the first and second planes (of the first and second graphene Hall sensors 422, 424). The offset between each graphene Hall sensor and its adjacent graphene Hall sensor is 120 °. This is because this offset provides the optimum combined sensitivity for a graphene Hall sensor assembly 420 comprising three graphene Hall sensors.

In the examples of Figures 6 and 7, the first, second and (where present) third graphene Hall sensors are aligned with one another in the sense that they are each the same distance from the magnetic array. This is not essential. The first, second and (where present) third graphene Hall sensors may be at different distances from the magnetic array 800 in a direction perpendicular to the elongate direction of the magnetic array 800.

Figure 8 shows a graphene Hall sensor assembly 430 comprising three graphene Hall sensors 432, 434, 436. The three graphene Hall sensors 432, 434, 436 are arranged in a delta formation such that the first second and third planes are non-parallel. In the Figure 8 arrangement, the first, second and third planes are distributed by 120 °relative to each other (rather than distributed by 120 ° linearly relative to the magnetic array 800). Again, this is because 120 ° provides the optimum combined sensitivity for a graphene Hall sensor assembly 420 comprising three graphene Hall sensors.

In an alternative to the arrangement of Figure 8, the three graphene Hall sensors may be arranged in a star formation rather than a delta formation. The angular distribution of the three graphene Hall sensors may remain as 120 °.

In the examples of Figures 5, 6, 7 and 8, the graphene Hall sensor 400 or the graphene hall sensor assembly 410, 420, 430 is shown to move relative to the magnetic array 800. Of course, it may be that the magnetic array 800 moves relative to the graphene Hall sensor 400 or the graphene hall sensor assembly 410, 420, 430. Alternatively, it may be -13 -that both the magnetic array 800 and the graphene Hall sensor 400 or the graphene hall sensor assembly 410, 420, 430 are movable.

Figure 9 shows an arrangement in which the graphene Hall sensor assembly 420 is stationary and the magnetic array 800 moves relative to the graphene Hall sensor assembly 420. In this embodiment, the magnetic array 800 comprises a tape which is configured to run from reel to reel.

In a further embodiment, which is a variation on the Figure 9 arrangement, the magnetic array 800 may comprise a re-magnetisable tape wherein each magnetic cell 300 (N S or S N) may be magnetically reversed (N S S N or S N N 5). In this way, the magnetisable tape may be deployed as a memory device readable by the graphene Hall sensor assembly 420.

In a further embodiment, illustrated in Figure 10, the magnetic array may be a Halbach array.

Figure 11 shows a signal (not to scale) obtained from one graphene Hall sensor in a linear encoder comprising a magnetic array consisting of 10 north-south magnetic cells. The peak signal derived from the first north and the last south in the magnetic array is significantly larger than that of the other norths/souths in the array since, in the case of the first and the last, there is no adjacent south/north contributing to the magnetic field. This behaviour may be used to identify a starting position of the graphene Hall sensor (e.g. a magnetic cell number one) and/or an ending position of the graphene Hall sensor. This may be useful for calibrating the linear encoder or for triggering an end indicator.

Figure 12 shows a signal obtained from each of three graphene Hall sensors in a linear encoder such as that shown in Figure 7 (but ignoring edge effects explained in relation to Figure 11). From this it is clear that, for any location of the graphene Hall sensor assembly, at least one of the three graphene Hall sensors is within 60 ° of its peak sensitivity. Cross referring with Figure 3b shows that this means the minimum sensitivity (when one graphene Hall sensor is at its least sensitive position) the sensitivity of the other two is still very high on the logarithmic scale.

-14 -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 for use in a graphene Hall sensor 400 of the present disclosure. In a linear encoder comprising more than one graphene Hall sensor, such as those shown in Figures 6 and 10, the hardware 500 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 Hall sensor 400 by generating a current flow in one orientation of the graphene sheet 100 (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.

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

-15 -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 a plot of results data for a linear encoder in accordance with the disclosure. Each line shows a data for a different sampling time. Regardless of sampling time, signal to noise ratio is linear resolving angles down to 0.1 ° (wherein 360 ° represents the total length of a magnetic cell). For longer sample times, the signal to noise ratio remains linear all the way down to angular resolutions as low as 0.05 °.

Figure 18 shows a plot of modelled data for encoder speed versus resolution. From this it is clear that resolutions of the order of individual microns are possible for low encoder speeds of microns per second. Even at encoder speeds of 1,000 km per second, the resolution is still of the order of tenths of millimetres.

Linear encoders in accordance with the disclosure may be used not only to determine high resolution positional information but also to derive speed and acceleration.

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.

Linear encoders have applications in a wide range of technologies, and across a wide range of different scales from microns to meters. Applications include: metrology instruments, motion systems, printers, high precision machining tools, coordinate-measuring machines, laser scanners, callipers, gear measurement, tension measurement, digital read outs, conveyors, elevators, valves/flow monitoring, and servo-controlled motion systems -e.g. robotics, machine tools, pick-and-place PCB assembly equipment, semiconductor handling and test equipment, wire bonders, printers, and digital presses.

Claims

-16 -CLAIMS: 1. A linear encoder comprising: a magnetic array including a plurality of magnetic cells arranged in a linear series in a first elongate direction, wherein each magnetic cell of the plurality of magnetic cells produces a magnetic field in a magnetic field region parallel to the first elongate direction, and wherein each of the plurality of magnetic cells has a length L in a direction parallel to the elongate direction; and a graphene Hall sensor assembly in the magnetic field region, the graphene Hall sensor assembly comprising: a first graphene Hall sensor in a first plane and configured to provide a first signal indicative of magnetic field experienced by the first graphene Hall sensor; and a second graphene Hall sensor in a second plane parallel to the first plane and configured to provide a second signal indicative of magnetic field experienced by the second graphene Hall sensor; wherein: the first and second graphene Hall sensors are distributed in a second elongate direction that is parallel to the first elongate direction, such that the first and second graphene Hall sensors experience a change in magnetic field in the magnetic field region as a result of relative axial movement between the first and second elongate directions; and wherein the linear encoder further comprises a processor configured to receive the first signal and the second signal and to use the first signal and the second signal to calculate a position of the graphene Hall sensor assembly relative to the magnetic array.
2. The linear encoder of claim 1 wherein the second plane is spaced apart from the first plane by L (n + 1/2) where n is an integer.
3. The linear encoder of any preceding claim wherein the first and second planes are perpendicular to the first elongate direction.
4. The linear encoder of any preceding claim wherein the first and second graphene Hall sensors are distributed along a second elongate direction.
-17 - 5. The linear encoder of claim 1 wherein the graphene Hall sensor assembly further comprises a third graphene Hall sensor in a third plane parallel to the first plane and configured to provide a third signal indicative of magnetic field experienced by the third graphene Hall sensor, wherein the processor is configured to receive the third signal for use in calculating the position of the graphene Hall sensor assembly relative to the magnetic array.
6. The linear encoder of claim 5 wherein: the second plane is spaced apart from the first plane by L (n + 1/3), where n is an integer; the third plane is spaced apart from the first plane by L (m -%), where m is an integer.
7. The linear encoder of claim 5 or claim 6 wherein the third plane is perpendicular to the first elongate direction.
8. The linear encoder of any of claims 5 to 7 wherein the first, second and third graphene Hall sensors are distributed along the second elongate direction.
9. The linear encoder of any preceding claim wherein the first, second and, where present, third planes are all perpendicular to the second elongate direction.
10. The linear encoder of any preceding claim wherein each magnetic cell comprises a north pole and a south pole and wherein the magnetic array comprises an alternating series of north and south poles.
11. The linear encoder of any of claims 1 to 9 wherein each magnetic cell comprises a north pole and a south pole and wherein an order of north pole and south pole is changeable.
12. The linear encoder of any preceding claim wherein the magnetic array comprises a Halbach array.
13. The linear encoder of any preceding claim further comprising a tape and wherein the tape comprises the magnetic array.
-18 - 14. The linear encoder of claim 13 wherein the tape is mounted in a reel to reel arrangement between a first reel and a second reel and wherein the graphene Hall sensor assembly is located between the first reel and the second reel.
15. The linear encoder of any preceding claim wherein each magnetic cell is attributed a magnetic cell number in consecutive order and wherein the processor stores a current magnetic cell number based on a currently adjacent magnetic cell relative to the graphene Hall sensor assembly.
16. The linear encoder of any preceding claim wherein the processor is configured to count each magnetic cell on passing.
17. The linear encoder of any preceding claim wherein the plurality of magnetic cells arranged in a linear series are arranged along an axis.
18. The linear encoder of any of claims 1 to 16 wherein the plurality of magnetic cells arranged in a linear series are arranged along an arc, optionally along an ellipse.
19. A linear encoder comprising: a magnetic array including a plurality of magnetic cells arranged in a linear series in a first elongate direction, wherein each magnetic cell of the plurality of magnetic cells produces a magnetic field in a magnetic field region parallel to the first elongate direction, and wherein each of the plurality of magnetic cells has a length L in a direction parallel to the first elongate direction; and a graphene Hall sensor assembly in the magnetic field region, the graphene Hall sensor assembly comprising: a first graphene Hall sensor in a first plane and configured to provide a first signal indicative of magnetic field experienced by the first graphene Hall sensor; and a second graphene Hall sensor in a second plane and configured to provide a second signal indicative of magnetic field experienced by the second graphene Hall sensor; wherein: the first and second planes are non-parallel, -19 -such that the first and second graphene Hall sensors experience a change in magnetic field in the magnetic field region as a result of relative elongate movement between the magnetic array and the graphene Hall sensor assembly in a direction parallel to the first elongate direction; and wherein the linear encoder further comprises a processor configured to receive the first signal and the second signal and to use the first signal and the second signal to calculate a position of the graphene Hall sensor assembly relative to the magnetic array.