The present invention generally relates to a position encoder with
a fault indicator. More particularly, the present invention relates to a position
encoder with a fault indicator for use in a switched reluctance drive.
In general, a reluctance machine can be operated as an electric motor
in which torque is produced by the tendency of its movable part to move into a
position where the reluctance of a magnetic circuit is minimized, i.e. the inductance
of the exciting winding is maximized.
In one type of reluctance machine the energisation of the phase windings
occurs at a controlled frequency. These machines can be operated as a motor or
a generator. They are generally referred to as synchronous reluctance motors.
In a second type of reluctance machine, circuitry is provided for detecting the
angular position of the rotor and energizing the phase windings as a function
of the rotor's position. This second type of reluctance machine may also be a motor
or a generator and such machines are generally known as switched reluctance machines.
The present invention is generally applicable to switched reluctance machines,
including switched reluctance machines operating as motors or generators.
Figure 1 shows the principal components of a switched reluctance
drive system 10 for a switched reluctance machine operating as a motor. The input
DC power supply 11 can be either a battery or rectified and filtered AC mains.
The DC voltage provided by the power supply 11 is switched across the phase windings
of the motor 12 by a power converter 13 under the control of the electronic control
unit 14. The switching must be correctly synchronized to the angle of rotation
of the rotor for proper operation of the drive 10. As such, a rotor position detector
15 is typically employed to supply signals corresponding to the angular position
of the rotor. The rotor position detector 15 may also be used to generate a speed
The rotor position detector 15 may take many forms. In some systems,
the rotor position detector 15 can comprise a rotor position transducer that provides
output signals that change state each time the rotor rotates to a position where
a different switching arrangement of the devices in the power converter 13 is required.
In other systems, the rotor position detector 15 can comprise a relative position
encoder that provides a clock pulse (or similar signal) each time the rotor rotates
through a preselected angle.
In systems where the rotor position detector 15 comprises a rotor
position transducer, failure of the rotor position transducer circuitry to properly
provide output signals representative of the angular position of the rotor can
seriously degrade the performance or, in the worst case, render the motor inoperable.
In some circumstances, a controller 14 attempting to control a machine based on
faulty rotor position transducer outputs could potentially damage both the machine
and the remainder of the control circuitry.
The importance of accurate signals from the rotor position detector
15 may be explained by reference to Figures 2 and 3. Figures 2 and 3 explain the
switching of a reluctance machine operating as a motor.
Figure 2 generally shows a rotor pole 20 approaching a stator pole
21 according to arrow 22. As illustrated in Figure 2, a portion of a complete phase
winding 23 is wound around the stator pole 21. As discussed above, when the portion
of the phase winding 23 around stator pole 21 is energised, a force will be exerted
on the rotor tending to pull rotor pole 20 into alignment with stator pole 21.
Figure 3 generally shows the switching circuitry in power converter
13 that controls the energisation of the portion of the phase winding 23 around
stator pole 21. When power switching devices 31 and 32 are switched ON phase winding
23 is coupled to the source of DC power and the phase winding is energised.
In general, the phase winding is energised to effect the rotation
of the rotor as follows: At a first angular position of the rotor (called the turn-ON
angle), the controller 14 provides switching signals to turn ON both switching
devices 31 and 32. When the switching devices 31 and 32 are ON the phase winding
is coupled to the DC bus which causes an increasing magnetic flux to be established
in the motor. It is this magnetic flux pulling on the rotor poles that produces
the motor torque. As the magnetic flux in the machine increases, electric current
flows from the DC supply provided by the DC bus through the switches 31 and 32
and through the phase winding 23. In some controllers, current feedback is employed
and the magnitude of the phase current is controlled by chopping the current by
switching one or both of switching devices 31 and/or 32 on and off rapidly.
In many systems, the phase winding remains connected to the DC bus
lines (or connected with chopping if chopping is employed) until the rotor rotates
such that it reaches what is referred to as the rotor "Freewheeling angle" When
the rotor reaches an angular position corresponding to the Freewheeling angle (position
24 in Figure 2) one of the switches, for example 31, is turned OFF. Consequently,
the current flowing through the phase winding will continue to flow, but will now
flow only through one of the switches (in this example 32) and through only one
of the return diodes (in this example 34). During the freewheeling period there
is little voltage differential across the phase winding, and the flux remains
substantially constant. The motor system remains in this freewheeling condition
until the rotor rotates to an angular position known as the "turn-OFF" angle (represented
by position 25 in Figure 2). When the rotor reaches the turn-OFF angle, both switches
31 and 32 are turned-OFF and the current in phase winding begins to flow through
diodes 33 and 34. The diodes 33 and 34 then apply the DC voltage from the DC bus
in the opposite sense, causing the magnetic flux in the machine (and therefore
the phase current) to decrease.
The energisation of the phase windings in a switched reluctance motor
depends heavily on accurately detecting the angular position of the rotor. If the
rotor position detector fails and the controller continues to energize the phase
windings, dangerously high currents could build up in the motor, potentially damaging
the motor and the controller. Moreover, when a drive system fails, it is often
necessary to test various control and motor components to find the failed elements.
It would be beneficial to have an indicator that specifically indicates that the
failure of the drive system was the result of a rotor position detector failure
so that unnecessary testing and debugging is not attempted. While some complicated
rotor position detectors have some fault indicating circuits, such encoders are
relatively expensive and require additional hardware for proper operation. Known
position decoders do not provide a low cost, compact rotor position detector that
provides an indication when the rotor position detector has failed.
It is an object of the invention to overcome the above described
and other disadvantages of known position detectors and provide a relatively inexpensive
rotor position detector that provides an indication when a fault has occurred
without the need for complex or expensive additional circuitry.
The present invention is defined in the independent claims. Preferred
features of the invention are recited in the claims respectively dependent thereon.
The present invention extends to a rotor position detector that provides
a signal indicating a failure of the position detector. In one embodiment of the
present invention, the rotor position detector comprises a plurality of position
sensors and a failure detector that receives position signals from the plurality
of position sensors. The position signals represent the rotor position for a switched
reluctance motor, and the position signals have allowable states that occur when
the rotor position detector is operating properly. If one or more of the plurality
of sensors fails, an illegal state occurs in the position signals from the position
sensors. A similar illegal state can occur if the rotating element of the position
detector is dislodged from its position, is damaged, or if part of the rotating
element becomes detached. The failure detector detects these illegal states and
produces a failure signal upon the occurrence of an illegal state. Accordingly,
the motor controller can respond to the failure signal to stop motor operation
or trigger an alternate positioning scheme.
In an alternative embodiment, the output signals from the position
encoder define output states and the output states are such that there are allowable
sequences of output states that occur when the position encoder is operating properly.
In this embodiment, the sequence of the output states from the position encoder
is monitored and an encoder failure signal is generated whenever an output sequence
occurs that is not one of the allowable sequences.
Other aspects and advantages of the present invention will become
apparent upon reading the following detailed description of exemplary embodiments
and upon reference to the drawings in which:
- Figure 1 shows the principal components of a switched reluctance drive system;
- Figure 2 shows a rotor pole approaching a stator pole and the commutation points
for the portion of the phase winding associated with the stator pole;
- Figure 3 generally shows the switching circuitry in a power converter that
controls the energisation of the portion of the phase winding associated with the
stator pole of Figure 2;
- Figure 4 shows a position encoder using a vane and three position sensors that
can be utilized with one embodiment of the present invention;
- Figure 5 shows failure detection circuitry according to one embodiment of the
- Figure 6 shows a timing diagram for the position encoder of Figure 4 detailing
the operation of one embodiment of the present invention; and
- Figure 7 generally illustrates an alternate failure detection circuit for detecting
illegal output state sequences in accordance with the present invention.
Similar reference characters indicate similar parts throughout the
several views of the drawings.
Illustrative embodiments of the invention are described below as
they might be implemented using the failure detection circuitry of the present
invention to effectively detect the failure of a position detector or a position
encoder of a switched reluctance drive. In the interest of clarity, not all features
of an actual implementation are described in this specification.
The present invention involves a failure detector for a position
encoder that uses a plurality of position sensors. The failure detector receives
position signals from the plurality of position sensors. The position signals
represent the rotor position of an electric machine (e.g., a switched reluctance
machine), and the position signals have allowable states that occur if the position
sensors are operating properly. If one or more of the plurality of sensors fail,
an illegal state occurs in the position signals from the position sensors. An
illegal state will also occur if the rotating element of the position encoder is
damaged (e.g., if it is dislodged from its position or loses a piece). The failure
detector produces a failure signal upon the occurrence of an illegal state. Accordingly,
the controller can respond to the failure signal to stop motor operation or trigger
an alternative positioning scheme.
Figure 4 shows one type of position encoder that can be utilized
with the failure detector circuit of the present invention. The position encoder
includes a rotatable element comprising a vane 40 shown with 8 equally spaced
light blocking parts 42a-h and 8 equally spaced light transmissive parts 44a-h.
In this embodiment, the vane 40 is mounted on the rotor shaft of the machine.
In this way, the vane 40 reflects the angular position of the rotor. The position
encoder further includes 3 slotted optical sensors 46a-c, which are mounted 15
degrees apart on a stationary member.
The sensors 46a-c include a light source that provides a light beam
that impinges upon a light detector when a light transmissive portion of the vane
fills the sensor (i.e., when the sensor is near a space region of the vane). When
the light from the light source impinges on the detector, the sensor 44 produces
a digital output signal at a first logic level, e.g., logic 0. When a light blocking
portion of vane 40 (i.e., one of portions 42a-h) fills one of sensors 46a-c (i.e.,
when the sensor is near a mark region of the vane), it will block the light and
no light will impinge on the detector for that sensor. When there is no light impinging
on the detector, the sensor will produce a digital output signal of a second logic
level, e.g., logic 1. In general, the light transmissive portions of the vane that
cause the sensors to produce logic 0 signals may be referred to as the "space"
regions of the vane and the light inhibiting portions may be referred to as the
"mark" regions of the vane.
In accordance with the teachings of the present invention, the sensors
46a-c are positioned such that the outputs from the sensors define an output state
and there are certain output states of the sensors 46a-c that will never occur
when the sensors are operating properly and the rotating vane is undamaged and
properly positioned. For example, in the embodiment of Figure 4, the angular span
of the mark and space regions of the vane define an angular distance of 22.5°.
As indicated in the figure, the angular distance between each of the sensors is
15° (less than the angular span of the mark and space regions) and the angular
distance between the outermost sensors is 30° (greater than the angular extent
of the mark and space regions).
Because of the relationship between the angular extent of the mark
and space regions of the vane and the positioning of the sensors, there are certain
sensor output states that cannot occur when the rotor position detector is operating
properly. For example, in the embodiment of Figure 4, the output states (or output
patterns) from the sensors 46a-c when operating properly can be: 101, 001, 011,
010, 110 and 100. When operating properly, however, the outputs from the sensors
can never be in the state or pattern 111 because both the mark and space regions
of the vane have an angular spread less than the 30° angular distance between the
outermost sensors 46a and 46c. In the same manner, the angular distance of the
mark and space regions and the positioning of the sensors precludes an output state
or pattern of 000 when the rotor position detector is operating properly.
In accordance with one embodiment of the present invention, rotor
position encoding defects, including failures of the sensors 46a-46c, are detected
and indicated by monitoring the outputs from the sensors and producing a fault
signal whenever either of the two illegal output states 111 or 000 occurs.
It should be noted that the use of vane 40 with light transmissive
space regions and light blocking mark regions, and light detecting sensors 46a-c
is exemplary only. The present invention is applicable generally to all forms
of position detectors that use a plurality of sensors that have certain output
states that will not occur during normal operation. For example, the present invention
is applicable to position detectors utilizing a vane including magnetic mark regions
and non-magnetic space regions where the sensors that detect the mark and space
regions are Hall-effect devices. Similarly, the vane could comprise teeth of ferromagnetic
material and the sensors could each be a form of reluctance sensor. Other means
of deriving the digital signals include regions of capacitance or inductance that
vary and a suitable sensor to detect the changes. Also, light reflectance variations
instead of regions of varying light transmissivity could be used. The present
invention is also applicable to position detectors using a number of sensors different
from that discussed above in connection with Figure 4.
In general, the present invention may be beneficially applied to
position detectors that produce digital output position signals where only one
bit of the output changes for each change in the state of the rotor. In other
words, the present invention is particularly adapted to position detectors that
produce position signals in a Gray code.
Further, the present invention is applicable to position detectors
utilizing mark and space regions, mark to space ratios, and numbers of sensors
different from that illustrated in Figure 4.
It should be further noted that the present invention requires only
that there be one or more illegal states that will not occur when the rotor position
detector is operating properly. For example, if there are N sensors, each producing
either a logic high or low signal, there must be less than 2N allowable
output states such that there is at least one illegal state. Occurrence of the
illegal state indicates failure of one or more sensors or the rotating vane.
Figure 5 shows one embodiment of a failure detector circuit 50 in
accordance with the present invention. The failure detector circuit 50 receives
as its inputs the outputs from the three position sensors 46a-46c of Figure 4.
The outputs from the three sensors 46a-46c are provided as inputs to triple-input
NOR gate 52 and triple-input AND gate 54. The output of triple-input NOR gate
52 will be logic high only when all three inputs to NOR gate 52 are all logic low.
Accordingly, the output of NOR gate 52 will be high when the illegal output state
000 occurs. In a similar manner, the output of AND gate 54 will be logic high
only when its three inputs are logic high. Accordingly, the output of AND gate
54 will be logic high whenever the illegal output state 111 occurs.
The outputs from NOR gate 52 and gate 54 are applied as inputs to
OR gate 56 such that the output of OR gate 56 will be logic high whenever an illegal
state occurs. Accordingly, a logic high output from OR gate 56 signals an error
in one of the sensors 46a-c or a problem with the rotating vane 40. In the embodiment
of Figure 5, after the error signal at the output of OR gate 56 occurs, it is
stored in fault latch 58, thereby maintaining the failure indication from the failure
detector 50. Controller circuitry (not shown) can monitor the output of the failure
detector 50 or the output of the fault latch 58 to determine when a failure has
occurred. Upon the occurrence of a failure, the controller circuitry can stop operation
of the drive, switch to an auxiliary positioning scheme or perform some type of
Figure 6 generally illustrates the operation of the sensors 46a-c
and the failure detector 50. In general, the top three waveforms of Figure 6 illustrate
exemplary outputs from sensors 46a-c as the vane 40 rotates past the sensors 46a-c
during operation of the machine. The lower waveform of Figure 6 represents the
failure output which, in the example of Figure 5, is the output of fault latch
Referring to Figure 6, for the normal operating states, the failure
detector 50 produces a low logic sensor failure that indicates normal operation.
If a position sensor 46 fails, for example sensor 46a, at point 60 and produces
an illegal state (e.g., 000) the failure detector 50 will produce a logic high
output indicating that a sensor failure has occurred. This high output will be
latched into the fault latch 58 and the output of the fault latch 58 will remain
high until the fault latch 58 is reset.
Although the embodiment of Figure 5 utilizes discrete logic gates
to detect the illegal states indicating a failure, embodiments are envisioned where
the failure detector 50 comprises an integrated digital circuit chip, such as
an Application Specific Integrated Circuit (ASIC) or a microprocessor, which determines
whether the position signals 46a-c are indicating normal or faulty operation of
the position sensors.
In an alternative embodiment of the present invention the sequence
of the output signals provided by the position encoder are monitored and an encoder
failure is indicated whenever an illegal sequence of output states occurs. For
example, in the embodiment of Figure 4, the output state 100 will never follow
the output state 011 when the position encoder is operating properly. Similarly,
the output state of 001 will not follow the output state of 011 when the encoder
is operating properly. Thus, the occurrence of either of the output sequences
011-100 or 011-001 indicates an error or failure of the encoder. This method of
error detection can detect an error in the encoder, even if each individual output
state is a legal state.
This alternative embodiment may be implemented through the use of
a look-up table that, for each output state, has stored within it the allowable
adjacent (or next) state or states. When the encoder's output changes from a first
output state to a second output state, the second output state is compared to the
allowable next state(s) for the first output state. If the second output state
does not match the allowable next output state(s) an encoder failure signal is
generated indicating a position encoder error.
Figure 7 generally illustrates one example of an alternate failure
detection circuit for detecting illegal output state sequences in accordance with
the embodiment of the present invention. In Figure 7, the current output state
from the encoder appears across a data bus 70. Data bus 70 is coupled to the input
of delay latch 71. Delay latch 71 is clocked by a circuit (not shown) that generates
a clock pulse upon each change in the output state of the position encoder. The
construction of a circuit for generating a clock pulse upon a change in the output
state is within the level of ordinary skill in the art and is not discussed herein.
The output of the delay latch 71 represents the delayed output state of the position
encoder (i.e., the previous output state with respect to the current output state).
The previous output state is provided as an input to look-up table 73 via bus
Look-up table 73 has stored within it the allowable next output state
(or states) for the previous output states. In response to a legal output state
at its input, the look-up table 73 provides at its output the allowable next output
state(s) for the previous output state. In the embodiment of Figure 7, there is
only one allowable next output state for each legal output state, although embodiments
are envisioned wherein there are more than one allowable next output states.
The allowable next output state signal from look-up table 73 is provided
via data bus 74 to one input of a digital comparator 75. The other input to digital
comparator 75 is the current output state, which is provided by data bus 70. The
digital comparator 75 compares the current output state with the allowable next
output state for the previous output state and generates a fault signal at its
output (bus 76) whenever the current output state does not match the allowable
next state for the previous output state. The fault signal from comparator 75
may be handled by the motor system in the same manner discussed above for the error
signal from OR gate 56 of Figure 5. In some applications, it may be necessary
to clock comparator 75 such that the comparison occurs only after the output of
look-up table 73 has settled in response to the previous change in the output
As indicated above, in Figure 7 there is only one allowable next
state for each legal output state. In applications where there are more than one
allowable next output states for each legal output state, additional comparators
may be used. The outputs of the additional comparators may be combined via logic
circuitry to produce an encoder failure signal when the current encoder output
state does not match any of the allowable next output states for the previous state.
While Figure 7 illustrates the use of discrete circuitry, the alternate
embodiment may be implemented through the use of a properly programmed microprocessor,
a microcontroller, an ASIC or the like. Moreover, although not shown in Figure
7, the circuitry for detecting illegal output state sequences may be combined
with the previously discussed circuitry for detecting illegal output states.
Although the invention has been described in terms of rotary machines,
the skilled person will be aware that the same principles of operation can be applied
to a linear position encoder to equal effect. For example, the skilled person
will be aware that a reluctance machine (as with other types of electric machine)
can be constructed as a linear motor. The moving member of a linear motor is referred
to in the art as a rotor. The term "rotor" used herein is intended to embrace the
moving member of a linear motor as well.
The principles of the present invention, which have been disclosed
by way of the above examples and discussion, can be implemented using various circuit
types and arrangements. The failure detector can be implemented using a variety
of logic components, devices and configurations depending on the position encoder
implementation and the desired performance characteristics. Moreover, the encoder
and position sensor detector can be used with a reluctance machine having rotor
or stator poles different in number from those illustrated herein. Further, the
present invention is applicable to inverted machines (i.e., machines where the
rotor rotates outside of the stator) and to any position encoder, such as position
encoders for brushless DC motors or other commutated motors. Those skilled in
the art will readily recognize that these and various other modifications and changes
may be made to the present invention without strictly following the exemplary
application illustrated and described herein and without departing from the true
spirit and scope of the present invention, which is set forth in the following