FIELD OF THE INVENTION

Embodiments of the present invention relate generally to methods and systems for monitoring magnetic objects. For instance, the present invention relates to methods and systems for determining the stir rate of magnetic stir bars and subsequently detecting anomalies in the stir rate.

BACKGROUND OF THE INVENTION

Accurately monitoring rotating magnetic objects presents a number of practical difficulties that apply to a wide variety of technical fields. Visual methods typically suffer from inaccuracy and rely upon line-of-sight, which is difficult when the magnetic object is rotating in opaque media. One such example is the use of a magnetic stir bar to stir liquids in a laboratory. A magnetic stir bar, sometimes referred to as a magnetic flea, is a piece of laboratory equipment that used to stir a liquid-containing medium, such as a chemical reaction . In order to maintain a consistent temperature throughout a liquid medium under heat, or to ensure even mixing, a magnetic stir bar is placed in the bottom of the vessel and subjected to a rotational magnetic field by a stir plate or similar device located beneath the vessel. When operating as desired, the stir bar will rotate around its centroid, in the X-Y plane (relative to the stir plate's magnetic field), at the target stir rate determined by the stir plate.

Due to inconsistencies in the stir plate's magnetic field, the design of the stir bar, the influence of the material in the vessel, environmental conditions such as solid matter suspended in the stirrer liquid and other unknown factors, the stir bar can deviate from this desired behaviour. This is most often characterised by an elliptical rotation or seemingly random movement, but can also manifest as a slow rotation or even no rotation in highly viscous materials. This behaviour may or may not rectify itself given enough time. However, during the period of anomalous movement, the stir rate of the liquid in the vessel is non-uniform . This non-uniform stir rate can cause a different outcome to the chemistry being performed, such as lower yield, failure of a chemical reaction or the chemical degradation of valuable starting materials. For instance, a biphasic chemical reaction can only proceed effectively if stirring is sufficient to intimately mix the immiscible phases. When abnormal stirring occurs and is observed, the speed of the stir plate can be adjusted to stabilise the stir bar before returning it to the original speed, e.g. by turning off the stir plate, allowing the stir bar to come to rest and reinitiating stirring. However, detecting and rectifying abnormal stirring in this way can only occur if a chemist is present, something which is unfeasible when the experiment duration is days rather than hours. Thus, a chemist may be unaware of the anomalous stir bar behaviour, as it can both occur and stabilise before they return to the lab. This has implications for the repeatability of the experiment, as the anomalous stir rate may result in an outcome which cannot be reproduced, resulting in wasted time and resources. Conversely, one may choose not to repeat the experiment as they believe it has failed due to the unobserved anomalous stir rate, which could produce a viable result under normal conditions.

SUMMARY OF THE INVENTION

A first aspect of the present invention provides a method for determining the rotational speed of a magnetic object comprising the steps of: performing a first magnetic field measurement at a first time; performing a second magnetic field measurement at a second time; performing a third magnetic field measurement at a third time; determining, based on the first, second and third magnetic field measurements, a centre point of a magnetic field associated with the magnetic object; and determining, based on the centre point and at least two of the first, second and third times, the rotational speed of the magnetic object.

The step of determining the rotational speed of the magnetic object may comprise calculating vectors between the centre point and at least two of the first, second and third magnetic field measurements respectively; determining an angle between the at least two calculated vectors; and determining, based on the angle and the respective of the first, second and third times, the rotational speed of the magnetic bar. The method may further comprise comparing, prior to determining the centre point of the magnetic field, the first, second and third magnetic field measurements; determining, based on the comparison, whether the magnetic bar is rotating; and if it is determined that the magnetic bar is not rotating, outputting a signal indicating that the magnetic stir object is not rotating.

The method may further comprise outputting the determined rotational speed of the magnetic object. In this manner, a user may be updated as to a live rotational speed of the magnetic object.

The magnetic field measurement may be a three-dimensional magnetic field measurement. In using a three-dimensional measurement, the measurement device may be placed anywhere within the magnetic field.

A second aspect of the present invention provides a device for determining the rotational speed of a magnetic object, the device comprising : a magnetometer, arranged to measure a magnetic field; a processor, in communication with the magnetometer; and a memory, in communication with the magnetometer and the processor, wherein the device is arranged to: perform a first magnetic field measurement at a first time; perform a second magnetic field measurement at a second time; perform a third magnetic field measurement at a third time; determine, based on the first, second and third magnetic field measurements, a centre point of a magnetic field associated with the magnetic object; and determine, based on the centre point and at least two of the first, second and third times, the rotational speed of the magnetic object.

In determining the rotational speed of the magnetic object, the device may be further arranged to calculate vectors between the centre point and at least two of the first, second and third magnetic field measurements respectively; determine an angle between the at least two calculated vectors; and determine, based on the angle and the respective of the first, second and third times, the rotational speed of the magnetic object.

The device may be further arranged to compare, prior to determining the centre point of the magnetic field, the first, second and third magnetic field measurements; and determine, based on the comparison, whether the magnetic object is rotating.

The device may be further arranged to output the determined rotational speed of the magnetic object.

The magnetic field measurement may be a three-dimensional magnetic field measurement.

A third aspect of the present invention provides a system comprising one or more processors; and at least one computer-readable storage medium, the computer- readable storage medium storing instructions that, when executed, cause the one or more processors to perform a method according to the above.

A fourth aspect of the present invention provides a method for detecting an anomalous stir rate of a magnetic stir object, comprising the steps of: receiving data relating to a sinusoidally variable stir rate of the magnetic stir object; determining a first absolute value, the first absolute value being defined by the difference between two consecutive maxima of the stir rate; determining a second absolute value, the second absolute value being defined by the difference between two consecutive minima of the stir rate; aggregating the first absolute value and the second absolute value to form a combined value; scaling the combined value to form a scaled value; and determining whether the scaled value is greater than a predetermined threshold value.

The method may further comprise filtering, before determining the first absolute value, the received data in order to remove noise. The filtering may comprise using Kalman filtering.

The method may further comprise, if it is determined that the scaled value is greater than the predetermined threshold, outputting a signal indicating an anomalous stir rate.

The step of scaling may comprise dividing the combined value by a moving average of the stir rate. The method may further comprise, if it is determined that the scaled value is greater than the predetermined threshold, performing the method steps again in order to obtain a first scaled value and a second scaled value respectively; and comparing the first scaled value with the second scaled value; and determining, based on the comparison, whether the stir rate is anomalous.

The method may further comprise, if it is determined that the scaled value is greater than the predetermined threshold value, the method further comprises the steps of: sending a first request signal, the first request signal being arranged to cause a magnetic field input to transition from a first target stir rate to a second magnetic target stir rate; determining whether the stir rate is anomalous; and if it is determined that the stir rate is not anomalous, sending a second request signal, the second request signal being arranged to cause the second magnetic field input to transition to the first magnetic field input.

The first magnetic field input may have a higher, or lower rotational speed than the second magnetic field input, preferably the first magnetic field input has a higher rotational speed than the second magnetic field input.

A fifth aspect of the invention comprises a system for detecting an anomalous stir rate of a magnetic stir object, the system comprising : one or more processors; and at least one computer-readable storage medium, the computer-readable storage medium storing instructions that, when executed, cause the one or more processors to perform a method according to the above.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and benefits of embodiments of the present invention will become apparent from a consideration of the following description and

accompanying drawings, in which :

FIGURE 1 shows a system in accordance with the present invention;

FIGURE 2 shows a schematic of a device in accordance with the present invention;

FIGURE 3 shows a flow diagram of a method of determining a stir rate in accordance with the present invention;

FIGURE 4 shows a flow diagram of a method of determining anomalies in the stir rate in accordance with the present invention;

FIGURE 5 shows an example data set of stir rate detection under normal stir bar conditions;

FIGURE 6 shows an example data set comprising various stir bar conditions; and

FIGURE 7 shows an example data set of frequency changes in a rotating magnetic field.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present invention considers that a source magnetic field which induces rotation in a target magnetic object can be measured and these measurements used to determine whether the target object is rotating at the same rate as the source field. The target magnetic object must have a magnetic field, but the strength and properties of this field are unknown under real-world conditions, as these are influenced by unknown variables such as the size of the object, its magnetic strength, distance to source magnetic field and measurement device, vessel material etc. Therefore, the rotation of the target object is inferred by observations of the measured magnetic field, which is comprised of the interaction between source and target magnetic fields. Under desired operating conditions, both the source and target objects will be rotating at the same speed, with the latter influenced solely by the former. This causes measurements to appear generally constant, with minimal noise, allowing the rotational speed of both objects to be determined. Conversely, if these fields are not in alignment, it can be deduced that the target magnetic object is not rotating at the desired source speed.

Systems and methods for determining the rotational speed of a magnetic object are described in detail below and are shown in Figures 1 to 7. The systems and methods are described by reference to an exemplary embodiment of the present invention, which concerns magnetic stir bar stir rates and, subsequently, anomalies in said stir rates. Although the exemplary embodiment detailed below relates to measuring the stir rate of a magnetic stir bar being driven by a magnetic stir plate, the skilled person will appreciate that these methods apply to measuring the rotational speed of any magnetic object.

With reference to Figure 1, there is provided a measurement device 1 arranged to measure magnetic field variations in three dimensions. The measurement device

1 is located proximate to a magnetic stir bar 2 and a magnetic stir plate 3.

In general, during an experiment, the magnetic stir bar 2 is located within a vessel 4 or some other form a receptacle capable of storing liquids. For example, the vessel 4 can be a single- or multi-neck round bottom flask made from materials such as regular or borosilicate glass. The vessel 4 is located above the magnetic stir plate 3, such that the magnetic stir plate 3 is able to cause the magnetic stir bar 2 to rotate about a central point in, for example, an X-Y plane.

During use, the measurement device 1 is arranged to intermittently record magnetic field measurements in three dimensions. These measurements are stored and processed in order to determine the stir rate of the magnetic stir bar

2 and, further, to detect whether there are any anomalies in the stir rate. The stir rate and/or anomalies are sent to an external device 5, such as a computer, to be presented to a user.

Referring now to Figure 2, there is shown a schematic of the components of the measurement device 1. In order to perform magnetic field measurements in three dimensions, the measurement device 1 comprises a magnetometer 6, arranged to measure a magnetic field. Measurements by the magnetometer 6 are stored in a memory 7. The measurements can also be processed in a processor 8. Finally, in order to communicate with the external device 5, the measurement device 1 comprises a receiver 9 and a transmitter 10 for transmitting information through wired or wireless means.

Referring now to Figure 3, there is shown a flow diagram detailing a method for determining the stir rate of a magnetic stir bar 2 in accordance with an exemplary embodiment.

First, at step 301, a determination is made as to the maximum stir rate capable of being measured by the measurement device 1. This determination is based on the frequency at which measurements are taken by the measurement device 1. Using the Nyquist Theorem, it is possible to determine the maximum stir rate which can be measured. Of course, if the actual stir rate is higher than can be measured based on a pre-set frequency of measurements, the frequency at which measurement are taken can be increased, by the user or otherwise.

Once the maximum stir rate has been determined, at step 302, the measurement device 1 makes three measurements of the magnetic field using the magnetometer 6. Each of the three measurements measures the magnetic field in three dimensions. As such, the measurement device 1 measures three 3- dimensional magnetic field vectors m _a , mb, m _c . Each of the magnetic field measurements also has a corresponding timestamp t _a , tb, t _c , associated with it.

For the purpose of further calculations, the three magnetic field measurements m _a , mb, me are treated as co-ordinates located in a three-dimensional space. At step 303, the measurement device 1 determines whether the three co ordinates nria, mb, m _c are the same. If this is the case, then it can be inferred that the magnetic stir bar 2 is not rotating. The measurement device 1 then outputs this determination, at step 308, to the external device 5 for informing the user.

If, at step 303, it is determined that the three co-ordinates m _a , mb, m _c are not the same, the method moves on to step 304, where the measurement device 1 determines the centre of rotation of the magnetic field generated by the interaction of the stir plate 3 and the stir bar 2.

Starting with the three three-dimensional co-ordinates m _a , mb, m _c , the determination at step 304 may be performed in a number of ways. There is described herein one exemplary way of determining the centre of rotation of the magnetic field, which is chosen based on its low computational requirements. However, it is to be understood that other methods of calculating the centre of rotation may be used.

In an exemplary embodiment for determining the centre of rotation of the magnetic field, the following calculation is used. First, a vector between the first co-ordinate and the second co-ordinate is calculated:

Then a vector between the first co-ordinate and the third co-ordinate is calculated : ui ^ ^c = jn ^c — m ^a (E2)

The two vectors are then squared, and the elements are summed to derive scalars vl, v2. This is the equivalent of calculating the dot product of each vector by itself. A third scalar v3 is then calculated as the dot product between m ^&b and m ^&c \ Additional scalars b, ki, k2 are then used in a further equation to derive the centre point of the three magnetometer readings, where 'c' is analogous to a co-ordinate defining the centre of rotation of a stir plate magnet inside the stir plate 3, the stir plate magnet being the source of the magnetic field of the stir plate 3. k _t = b * v ₂ * (v _± — v ₃ ) (E7) k ₂ = b * v _± * ( ₂ — v ₃ ) (E8) c = m ^a + k- n ^&b + k ₂ m ^&c (E9)

Once the centre of rotation of the magnetic stir bar 2 has been calculated at step 304, at step 305, vectors are then calculated between the centre of rotation c and two of the three co-ordinates m _a , mb, m _c : va = ^m a— c (E10)

v _b = m _b — c (El l)

At step 306, the angle Q between the two vectors v _a and Vb is calculated:

At step 307, using the angle Q between the vectors v _a , Vb and the corresponding timestamps t _a , tb, the stir rate w is then calculated :

Finally, at step 308, the stir rate w is output to the external device 5. In outputting the stir rate, it is meant that the stir rate w is then stored at the external device 5 and/or merely presented to the user (e.g. on a visual display). In essence, outputting may be any form of moving the data.

As the stir rate is calculated by measuring the rotation of the magnetic field, the above method provides a highly accurate reading of the actual stir rate of the stir bar 2, under normal operating conditions. In contrast, a user of prior art systems typically has to rely on the stir plate 3 itself to indicate the stir rate of the stir bar 2. This represents a significant drawback of the prior art because many traditional stir plates only enable the user to set a target stir rate using a rotating analogue dial. Not only are rotating analogue dials inaccurate, they merely indicate a target stir rate, not the actual stir rate, as calculated above.

Figure 4 shows a flow diagram detailing a method for determining whether the stir rate w calculated above is anomalous. As discussed above, the stir rate w is calculated based on a magnetic field measurement which is measured along three axes simultaneously.

Under normal, stable conditions, a series of measurements in each of the three measured axes will be flat or become sinusoidal over time, within an acceptable tolerance. This is largely due to the way in which the stir plate 3 works. The stir plate 3 comprises a controller which uses a feedback loop in order to adjust the speed of a motor of the stir plate 3 in real time, in order to drive the stir plate magnet. This leads to variations in the magnetic field as the controller attempts to maintain its magnetic field rotation at a target stir rate. However, on top of the variation caused by the stir plate controller, the distance between the measurement device 1 and the magnetic stir bar 2 causes an accentuation of the variation. As such, the amplitude of the variations measured by the measurement device 1 appear larger than they actually are. Of course, these variations can be minimised with high quality sensors, reductions in the distance between objects etc.

Anomalous behaviour occurs when the stir bar 2 does not rotate uniformly around its centroid, or rotates at a faster/slower speed than the stir plate's magnet, in turn causing the rotation of the liquid to be non-uniform. Due to this non-uniform movement of the stir bar 2 in the vessel 4, the measured magnetic field on at least one of the axes measured will also become non-uniform which, in turn, will cause the calculated stir rate signal to be non-sinusoidal over time. In order to determine that there is an anomaly in the stir rate, the method detailed in Figure 4 is performed.

At a first step 401, the external device 5 queries the measurement device 1 to determine whether there is any new data available for processing.

If no new data is available, the external device 5 waits for a predetermined amount of time before querying the measurement device 1 again. The frequency of data becoming available is assumed to be sufficient for detection of anomalous stir rates. This can be set depending upon the target stir rate of magnetic stir bar 2.

If new data is available, at step 402, the stir rate data is sent by the measurement device 1 to the external device 5.

As sensor data often contains large amounts of noise, at step 403, the stir rate data is filtered using a low-pass filter, such as a moving average or a Kalman filter. This filtering removes noise associated with the signal that would otherwise make it difficult to determine whether the signal is anomalous, such as recurring transient negative signals. The resulting signal is assumed to retain the underlying pattern of normal or anomalous behaviour of the interaction between the stir plate's magnet and the magnetic stir bar 2.

At steps 404 and 405, the filtered stir rate data is analysed to determine signal peaks (maxima) and signal troughs (minima) in the sinusoidal data respectively. Then, at steps 406 and 407, the difference in absolute magnitude between consecutive peaks and consecutive troughs is determined. The principle here is that a stable stir bar 2 will generate peaks and troughs with little difference in magnitude between consecutive pairs, over a single sinusoidal wave (i.e. the signal should be sinusoidal at a steady frequency for a given stir plate speed), or more generally, that a stable stir bar 2 will rotate at the same speed as the stir plate, causing a stir rate with little noise to be generated. The absolute magnitude difference between consecutive peaks and consecutive troughs may be referred to as first and second absolute values respectively. At step 408, the two signals determined in steps 406 and 407 are aggregated or combined into one signal, for example, by adding them together or by taking the maximum of the two signals. This produces a single value, per complete cycle, which represents the absolute variation in the height of consecutive peaks and troughs. For non-anomalous behaviour, this value should be near zero and constant across consecutive waveforms.

The combined value calculated in step 408 represents an absolute difference in RPM, which, for a given severity of anomaly, will be small at low stir plate speeds and large at high stir plate speeds. In order to render the combined value suitable for comparing to a threshold, it must be scaled, using a scaling factor.

At step 409, the scaling factor is determined, based on the received stir rate data. The scaling factor is produced by filtering the received data to remove as much of the sinusoidal pattern as is practical. The resultant data effectively represents the average RPM of the stir bar 2 or, more generally, the overall trend of the stir rate. This data can then be used as the scaling factor.

At step 410, the combined value is scaled, using the scaling factor, to produce a scaled value of the total difference between consecutive peaks and consecutive troughs in the received data. The scaling is performed by simply dividing the combined value obtained at step 408 by the scaling factor derived at step 409. The scaled value represents an anomaly score, which directly indicates whether an anomaly has occurred.

At step 411, the scaled value (or anomaly score) is compared to a threshold value, in order to determine whether there is an anomaly in the stir rate.

At step 412, preferably in parallel to steps 404 to 411, the unfiltered sensor data received at step 402 is analysed to determine whether the stir rate is in a stage of manual change. For example, if a user has manually controlled the magnetic stir plate 3 so as to increase the stir rate, this could be interpreted as anomalous data. As such, it is preferred that any manual changes in the stir rate are not reported as anomalous. The unfiltered data received in step 402 contains the magnetic field data of the magnetic stir plate 3 and, as such, can be used to interpret whether the user has intended the magnetic field to vary.

In order to determine whether the stir rate is in a stage of manual change, at step 412, the dominant frequency of the magnetic field variations is detected over time and a cumulative change is determined over a sliding window of values. As the change over a range of samples will be gradual, if a threshold is exceeded and it is determined to be part of a manual change phase, both the exceeding value and a set number of preceding samples will be considered to be part of the 'manual change' phase and have their associated anomaly scores suppressed.

In an alternative embodiment, the stir plate 3 indicates to the external device 5 when it has received a manual input relating to a change in the stir rate. In this manner, the external device can manually suppress any seemingly anomalous data for an appropriate period of time. However, this requires that the stir plate 3 is capable of communicating with the external device 5, which may not be desirable and may increase the cost of the system.

At step 413, if it is determined that the scaled value (or anomaly score) exceeds the threshold value, the method further comprises determining whether the anomaly is part of a corresponding manual change phase. If the anomaly is part of a manual change phase, the method starts again at step 401.

However, if the anomaly is not part of a manual change phase, the user is informed of an anomaly at step 414. Informing the user may take the form of outputting the anomaly to a record for later evaluation. Alternatively, the user may be informed using a prompt at the external device 5.

In some circumstances, the measurement device 1 may incorrectly determine the rotational speed of the magnetic stir bar 2. If this occurs, the external device 5 may incorrectly determine that an anomaly has occurred. In this regard, the external device 5 can be further arranged to compare consecutive anomaly scores in order to determine whether an anomaly occurred, or whether it was an error in the measurement of the rotational speed. The external device 5, upon determining that an anomaly has occurred, may further be arranged to automatically correct the stir bar's stir rate. For example, upon detecting an anomaly, the external device 5 can output a signal to the stir plate 3 in order to temporarily reduce the target stir rate from a user-defined level to a lower level. By reducing the target stir rate, the anomaly has a decreased likelihood of persisting, such that the stir bar 2 stir rate matches the target stir rate. Once the anomaly has been resolved, the external device 5 can send another signal to the stir plate 3 requesting that the target stir rate be returned to the user-defined level.

Although the method above, at steps 404 to 407, describes that the method is performed by measuring the differences between consecutive peaks (maxima) and consecutive troughs (minima) in the measured signal, it is to be understood that this is a preferred embodiment. Of course, the method may be performed by measuring the difference in consecutive peaks only, or consecutive troughs only. However, in measuring only peaks, or only troughs, particular anomalies may not be detected and, therefore, such a method is less preferable.

Figure 5 shows an example data set showing stir rate detection at several different rotational speeds, but in which the stir bar 2 is exhibiting no anomalous behaviour. The solid line represents the measured stir rate, determined by the measurement device 1. The dashed line represents the true stir rate, as shown by the stir plate 3. The stir rate values are defined using a 50 point moving average of the stir rate readings determined by the measurement device 1, although this value can change dependent upon application and sensor quality. At higher rotational speeds, the sinusoidal variation in the average stir rate increases in amplitude. However, the difference between consecutive peaks and consecutive troughs is largely minimal, indicating no anomalies.

Figure 6 shows an example data set comprising stir rates of 400 rpm, 600 rpm and 800 rpm. The 'blobs' along the data points indicate points where the external device 5 has output an anomalous record. At 400 rpm, the absolute difference between consecutive peaks and between consecutive troughs is largely minimal, causing low anomaly scores to be generated.

At 600 rpm, the stir bar first exhibits seemingly random movement about the vessel 4. Non-stationary waves with extreme differences in height result in large anomaly scores. The stir bar then stabilises completely, resulting in low anomaly scores which do not exceed the specified threshold.

At 800 rpm, the stir bar again first behaves anomalously, then only partially stabilises into an elliptical rotation (i.e. not rotating around its centre, but rather rotating around one end of the stir bar). Although the partially stabilised waveform is generally stationary, in that it is sinusoidally repeating, the absolute height difference between consecutive peaks and troughs varies. As such, several points are considered anomalous.

Manual changes to the stir rate and the associated detection of these are also shown in the image. In line with the method described above, these manual changes in stir rate are not recorded as anomalous.

Figure 7 shows how manual speed changes are detected by monitoring the dominant frequency of the magnetometer readings, using the same data shown in Figure 6. The dashed line shows the cumulative change of the dominant frequency of the magnetic field (which is assumed to be primarily generated by the rotating magnet of the stir plate 3). The solid line shows the rolling difference of the absolute cumulative change in frequency.

It is to be appreciated that signals transmitted by the measurement device 2, stir plate 3 or external device 5 may be electrical, or alternatively, there may be a transmitter and a receiver arrangement, such that the information may be sent via Bluetooth ®, RF signal, WiFi ® or any other type of wireless transmission means.

The skilled person will also realise that steps of various above-described methods can be performed by programmed computers. Accordingly the above-mentioned embodiments should be understood to cover storage devices containing machine- executable or computer-executable instructions to perform some or all of the steps of the above-described methods. The embodiments are also intended to cover computers programmed to perform the steps of the above-described methods.

The functionality of the elements shown in the Figures can be provided using either dedicated hardware and/or software. The expressions "processing", "processing means" and "processing module" can include, but is not limited to, any of digital signal processor (DSPs) hardware, network processors, application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), read only memories (ROMs) for storing software, random access memories (RAMs), and non-volatile storage.

The above embodiments describe one way of implementing the present invention. It will be appreciated that modifications of the features of the above embodiments are possible within the scope of the independent claims. The methods described herein may be applied to any kind of magnetically rotating object. The features of the magnetic stir bar 2 and magnetic stir plate 3 described herein are for example only and should not be seen as limiting to the claimed invention. For example, the method may be applied to measuring the rotational output of the magnetic stir plate 3 only. Alternatively, the method may be applied to measuring the speed of rotation of any object with a rotating magnetic field.

Features of the present invention are defined in the appended claims. While particular combinations of features have been presented in the claims, it will be appreciated that other combinations, such as those provided above, may be used.