Position sensing system
The present invention relates to a position sensing system for a switched
reluctance (SR) machine.
Some known SR machines, such as Controlled Power Technologies'
Speedstart ®, incorporate a sensing mechanism comprising a sensor and a magnet.
The stationary sensor senses changes in a magnetic field produced by the magnet,
which is mounted on an end of the rotating shaft of the SR machine.
The sensor has a working range, or tolerance band, defined by an upper and
a lower value of magnetic flux density. If the magnetic flux density is below the
lower value of the tolerance band, the sensor cannot detect it and therefore cannot
function correctly. If the magnetic flux density value exceeds the upper value of the
tolerance band, the sensor signal becomes saturated.
Figure 1 is a graphical example of magnetic decay in flux density vs distance
in accordance with a prior art sensing system. As shown in this graph, magnetic flux
density decreases in a non-linear manner as the sensor is moved away from the
magnet. The magnetic field strength of a particular magnet, in combination with the
working range of the sensor, dictates a function, or operating range band for the
particular sensor/magnet combination.
Whilst phase winding connections can be configured to result in no
electromagnetic field along the centreline of the machine (i.e. the position sensor
axis) in practice a number of factors may result in a non-zero field. These factors
include uneven current sharing in parallel conductors due to localised heating
affecting resistivity, differing lengths of winding connections, and rotor eccentricity.
Known SR machines are formed of components of various materials such as
steel, plastics and aluminium, having differing coefficients of thermal expansion and
magnetic attenuation properties. Since typical operating temperatures of an SR
machine could be in the range of -40 °C to 200 °C, a significant degree of movement
can occur between the sensor and the magnet as a result of thermal expansion of
components. This expansion results in a change in the magnetic field strength
sensed by the sensor, and at extreme temperatures may result in the magnetic flux
density falling outside the working range of the sensor.
Furthermore, the frequent switching of high currents within the SR machine
produces electromagnetic interference. Firing phase currents with a small angular
position error can result in very high current spikes and unpredictable performance.
This is especially true when coils are fired with the rotor is in an unaligned position
without the inductance (or rising inductance) to limit the current level.
For this case, when the coils are fired in the unaligned position the stray
fields are likely to be much greater since the stator teeth will provide flux into a large
air gap and hence the fields will not be fully contained within the steel laminations.
Large currents with a relatively high production of stray fields will cause
exacerbated sensor errors. Since the sensor is in the machine control loop it can be
anticipated that relatively small position errors can ultimately lead to loss of control.
The present invention is aimed at providing a position sensing system for an
SR machine wherein the working range of the sensor is maximised, and wherein the
problems discussed above are at least mitigated.
Accordingly the present invention provides a position sensing system as
claimed in claim 1.
The magnet carrier and magnet shield of the present invention allows a pair
of magnets to be held in a magnet carrier in such a manner so as to concentrate the
magnetic flux within a well formed by the magnet and magnet carrier, in a direction
normal to a central axis (z-axis) of the shaft.
Preferably, the position sensing system further comprises a sensor shield,
which surrounds the sensor element, and which screens the sensor element from
external electric interference caused for example by switching of the SR machine.
The position sensing system may further comprise an external shield which
surrounds the magnets, magnet carrier, sensor element and circuit board.
An embodiment of the invention will now be described by way of example
only and with reference to the figures in which;
Figure 2a is a cross-sectional representation of a magnet carrier in
accordance with the present invention;
Figure 2b is a plan view of a magnet carrier in accordance with the present
invention;
Figure 3 is a representation of an arrangement of a nested magnet carrier,
magnet shield, sensor and sensor shield in accordance with the present invention;
Figure 4 is a detailed view of the magnet carrier, magnet and sensors of
Figure 3;
Figure 5 is a graphical representation of a the axial working range of a prior
art sensing system and the axial working range for the present invention;
Figure 6 is a graphical representation of a the radial working range of a prior
art sensing system and the radial working range for the present invention;
Figure 7 shows the magnetic field strength at the centre of the machine for a
system having no shield;
Figure 8 shows the rotor angular position error of a sensing system having
no shield;
Figure 9 shows the central magnetic field strength of a sensing system
having a shield in accordance with the present invention;
Figure 10 is a graphical representation of the rotor angular position error of a
sensing system with a shield in accordance with the present invention;
and
Figure 11 is a representation of the flux tubes contained within the magnet
carrier well in accordance with the present invention.
Referring to Figure 3, the present invention comprises a position sensing
system for an SR machine having a rotor shaft 4. A central axis of the rotor shaft 4
is indicated by Z-Z in Figure 4.
The sensing system comprises a magnet carrier 6, a sensor element 8, a
sensor shield 10, an external magnet shield 20 and a pair of magnets comprising a
first magnet 12 and a second magnet 14.
The sensor element 8 is mounted on a plinth-like circuit board 16 which is
aligned with the end of the rotor shaft 4.
The first magnet 12 and the second magnet 14 are arranged in a
"horseshoe" arrangement, i.e. such that the magnetic flux follows a path similar to
that of a horseshoe magnet. The first magnet 12, second magnet 14 and magnet
carrier 6 form a well 18, within which the sensor element 8 and circuit board 16 are
positioned.
The sensor shield 10, which is circular in cross-section, surrounds the sensor
element 8, and acts to encompasses the magnetic flux from the first and second
magnets 12, 14 and also to shield the sensor element 8 from any stray interference,
for example that caused by electrical switching of the SR machine.
The external magnet shield 20 surrounds the first magnet 12, the second
magnet 14, the magnet carrier 6, the sensor element 8 and the circuit board 16.
The arrangement of the first and second magnets 12, 14 in the magnet
carrier 6 effectively form a 'horseshoe' magnet whereby the primary magnetic flux
tubes produced by the magnets 12, 14 flow across the internal air space formed
within the well 18.
The combination of the magnets 12, 14, magnet carrier 6, sensor shield 10
and sensor element 8 has been shown to result in a significant increase in the
functional range of the sensing system over a wide range of temperatures. For
example, the graph of Figure 5 shows the axial sensitivity, i.e. magnetic flux density
against distance from the sensor, for a prior art sensing system (indicated as
"current spec range"), and for a sensing system in accordance with the present
invention. The sensing system of this graph has a working range of 4.8mm
compared to 2mm for the prior art system. The graph shows the effect of the shield
in reducing the effect of stray magnetic fields close to the sensor element 8.
Figure 6 shows the radial working ranges of a prior art sensing system
(indicated as "current spec range") and of a sensing system in accordance with the
current invention.
Figure 7 shows the magnetic flux density at the centre of a 3 phase, 12
stator pole, 8 rotor pole SR machine resulting from the phase currents being fired as
the machine rotates at 400 rpm. The results are for a prior art system. In this
instance flux density Bz represents the flux density at the sensor element 8 along
the central, z-axis of the SR machine (corresponding to the z-axis indicated in
respect of the current invention in Figure 4). Flux density Bnormal is the integral of
all of the radial flux fields around the sensor element 8.
Figure 8 shows the position error of a prior art sensing system. Position
error calculations can be performed for different levels of magnetic flux density at
the sensor element site (signal noise ratio study). For the lowest value of sensor
signal, the position error can be 0.8 degrees, which may cause feedback errors and
ultimately loss of control.
Figure 9 shows the central magnetic field of a sensing system with sensor
shields and magnets in accordance with the present invention. It can be seen that
this arrangement reduces the magnetic fields from the phase firing events at the
centre of the SR machine to negligible levels.
Figure 10 shows the position error of a sensing system in accordance with
the present invention. The position error can be 0.012 degrees which is unlikely to
cause any significant feedback errors and is within the geometric tolerance of the
sensor element alignment etc.
In the given examples, the maximum likely position error for an SR machine
having a sensing system without a sensor shield is 0.8 degrees. The maximum
magnitude of the magnetic field at the site of the sensor element 8 is approximately
0.2mT.
The maximum likely position error for a machine having a sensing system in
accordance with the present invention is 0.012 degrees. The maximum magnitude
of the field at the sensor element site is 3mT, which is smaller than the earth's
magnetic field (which is between 30mT and 60mT) .
Accordingly it can be seen that the sensing system of the present invention
reduces the magnitude of magnetic flux density at the site of the sensor element 8
and the likely position error by a factor of approximately 70.
The rotor position error without the sensor shield 8 is likely to cause error
feedback in the system, and ultimately uncontrolled operation of the SR machine. It
must be noted however that other factors may affect the magnetic field at the central
position and may compound the uncontrolled operation of the SR machine.
The sensor shield 10 and magnet arrangement of the present invention
therefore provides a safeguard against offset rotor and stray magnetic fields from
the electronics of an SR machine, which are very difficult to control in volume
production, and which may compound the central magnetic field problems.
Figure 11 shows the effect of the present invention on concentrating
magnetic flux tubes 22 in a direction normal to an axis of the shaft of the SR
machine.

WE CLAIMS:-
A position sensing system for a switched reluctance machine, the system
comprising magnets, a magnet carrier, and a sensor element, wherein the
magnet carrier is mounted upon an end of a rotatable shaft of the switched
reluctance machine, and wherein the sensor element is mounted on a
circuit board and sits in a well formed by the magnets and magnet carrier in
a manner whereby magnetic flux produced by the magnets is concentrated
within the well in a direction normal to an axis of the shaft of the switched
reluctance machine.
A position sensing system claimed in claim 1 wherein the magnets are
arranged such that the magnetic flux follows the path of a horseshoe
magnet.
A position sensing system as claimed in claim 1 or claim 2 further
comprising a sensor shield which surrounds the sensor element, thereby
encompassing magnetic flux produced by the magnets and also shielding
the sensor element from electric interference.
A position sensing system as claimed in any one of the preceding claims
further comprising an external shield which surrounds magnets, the magnet
carrier, the sensor element and the circuit board.
A position sensing system substantially as hereinbefore described and with
reference to the accompanying Figures 2 to 11.