Apparatus for external inductance sensing for variable-reluctance motor commutation

An apparatus for controlling commutation of a three-phase variable reluctance machine includes a sensor, a sampling circuit, an inductance waveform generating circuit, a phase advance circuit, a threshold signal generating circuit, and an output comparator. The sensor includes a stack of several laminations which are identical to the motor laminations that form the motor stator, and is mounted at one end thereof. The motor includes a plurality of stator windings forming a plurality of corresponding machine phases. The sensor also includes a plurality of sensor windings electrically isolated from the stator windings. Each sensor winding is associated with a corresponding stator winding. Each sensor winding also has an inductance characteristic that varies according to an actual position of the rotor. The sampling circuit is coupled to the sensor windings for selectably energizing at least one of the windings to generate a plurality of pulses indicative of the current passing through the energized sensor winding. The inductance waveform generating circuit includes a sample and hold device responsive to these pulses and outputs a generally sinusoidal signal indicative of an inductance waveform associated with the sensor winding. The phase advance circuit provides a turn-on signal having a magnitude that is modulated as a function of the motor speed. The threshold generating circuit comprises a switch operated in accordance with machine phase crossover transitions for selecting either the turn-on threshold, or a separately generated turn-off threshold voltage. The threshold voltage is compared with the inductance waveform, and a digital commutation signal is generated for the motor phase corresponding to the sampled sensor winding.

BACKGROUND OF THE INVENTION 
1. Technical Field 
This invention relates generally to a system for controlling a 
variable-reluctance (VR) motor, and more particularly, to an apparatus for 
external inductance sensing for controlling VR motor commutation. 
2. Discussion of The Related Art 
Variable-reluctance (VR), or as they are alternatively known, 
switched-reluctance (SR) machines have been the subject of increased 
investigation due to their many advantages, which makes it suitable for 
use in a wide variety of situations. A VR machine operates on the basis of 
varying reluctance in its several magnetic circuits. In particular, such 
machines are generally doubly salient motors--that is, they have teeth or 
poles on both the stator and the rotor. The stator teeth have windings 
which form machine phases of the motor. In a common configuration, stator 
windings on diametrically opposite poles are connected in series to form 
one machine phase. 
When a stator phase is energized, the closest rotor pole pair is attracted 
towards the stator pole pair having the energized stator winding, thus 
minimizing the reluctance of the magnetic path. By energizing consecutive 
stator windings (i.e., machine phases) in succession, in a cyclical 
fashion, it is possible to develop torque, and thus rotation of the rotor 
in either a clockwise, or counter-clockwise direction. 
As further background, the inductance of a stator winding associated with a 
stator pole pair varies as a function of rotor position. Specifically, the 
inductance varies from a lower level when a rotor pole is unaligned with a 
corresponding stator pole, where it rises to an upper or maximum level 
when the rotor pole and stator pole are in alignment. Thus, when the rotor 
pole rotates and sweeps past a stator pole, the inductance of the stator 
winding varies through lower-upper-lower inductance levels. This 
inductance-versus-rotor position characteristic is particularly relevant 
for controlled operation of the motor. Specifically, current flowing 
through the stator winding must be switched on prior to (i.e., advanced), 
and maintained during the rising inductance period to develop positive 
torque. Since positive phase current during the decreasing inductance 
interval produces a negative or breaking torque, the phase current must be 
switched off before this interval occurs to avoid negative torque. 
Accordingly, rotor position sensing is an integral part of a closed-loop 
variable-reluctance motor drive system so as to appropriately control 
torque generation. 
The prior art has taken two fundamentally opposing approaches in 
determining rotor position: direct methods, and indirect methods. Direct 
methods include the use of direct rotor position sensors, such as optical 
encoders and Hall effect devices, which are commonly used in closed-loop 
motor drives for the purpose of phase current commutation. However, 
sensors of this type increase the cost of the drive system, and are not 
sufficiently rugged, (i.e., are relatively unreliable) in automotive 
applications. 
Indirect methods were investigated, partially, due to the shortcomings of 
the above-mentioned direct techniques. In one indirect method, advanced 
control theory techniques are used, such as an observer-based state 
variable model, to estimate rotor position using operating parameters such 
as phase current, voltage, or inductance of deenerigized stator windings. 
However, one disadvantage of these types of methods is that they require 
an expensive processing device, such as a microprocessor, to acquire and 
evaluate the numerous samples needed to determine the rotor position. 
Further, performance is generally poor with these methods at the outer 
limits of the motor operating range (i.e., very low speed, and high speed 
conditions). 
Accordingly, there is a need to provide an improved apparatus for 
commutation of a variable-reluctance machine that minimizes or eliminates 
one or more of the problems as set forth above. 
SUMMARY OF THE INVENTION 
The present invention provides an improved apparatus for controlling 
commutation of a variable or switched-reluctance machine using external 
inductance sensing. The apparatus is suitably adapted for use with a VR 
motor that includes a rotor, a stator, and a plurality of stator windings 
forming a plurality of machine phases. In one aspect of the invention, a 
sensor is provided for use with the apparatus, and is formed from a 
plurality of laminations (which may be identical to the motor 
laminations), and includes a plurality of sensor windings independent from 
the stator windings. In particular, each sensor winding is associated with 
a corresponding stator winding and has an inductance characteristic that 
varies according to an actual position of the rotor. 
The apparatus for controlling commutation includes means for energizing at 
least one of the sensor windings and generating in response thereto a 
current indicative signal representative of the current passing through 
the energized sensor winding. The apparatus further includes means for 
generating an inductance indicative signal, using the current indicative 
signal, that is representative of an inductance waveform associated with 
the energized sensor winding. The inductance waveform is a function of the 
actual rotor position. Finally, the control apparatus includes means for 
comparing the inductance waveform with a threshold signal for generating a 
commutation signal to control energization of the stator winding 
corresponding to the energized sensor winding. 
A device in accordance with a preferred embodiment of this invention is 
cost effective and efficiently commutates a VR motor over a wide range of 
speeds (i.e., turn-on and turn-off angles are easily initialized and 
automatically adjusted according to the speed of the motor). This 
represents a significant improvement over prior art methods, which lack 
flexibility and are only capable of commutating a VR motor over a limited 
speed range. 
These and other features and objects of this invention will become apparent 
to one skilled in the art from the following detailed description and 
accompanying drawings illustrating features of this invention by way of 
example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings wherein like reference numerals are used to 
identify identical components in the various views, FIG. 1 shows the major 
mechanical components of a variable-reluctance (VR) electric motor 10, 
sometimes referred to as a switched-reluctance (SR) motor, which includes 
a stator assembly 12, and a rotor assembly 14. 
Although the invention will be described and illustrated in the context of 
a variable-reluctance electric motor 10, it will be appreciated that this 
invention may be used in conjunction with another well-known electric 
motor structures. Stator assembly 12, in a preferred embodiment, comprises 
a first plurality of laminations 16.sub.m. A second plurality of 
laminations 16.sub.s is disposed adjacent the first plurality of 
laminations 16.sub.m to define a sensor 17. The laminations 16 are formed 
using a magnetically permeable material, such as iron. Individual ones of 
laminations 16.sub.m, and 16.sub.s may be identical, and are identical in 
a constructed embodiment. The sensor laminations, however, can be formed 
in other configurations to provide alternative inductance characteristics. 
Although, in the illustrated embodiment, the sensor laminations 16.sub.s 
are mounted on one end of the stator assembly 12, it should be appreciated 
that other configurations are available that would remain within the 
spirit and scope of this invention. 
Stator 12 is generally hollow and cylindrical in shape. A plurality of 
radially inwardly extending poles or teeth 18 are formed on stator 12 (via 
laminations 16) and extend throughout the length thereof. Similarly, poles 
18 are also formed on sensor 17 (via laminations 16.sub.s) and extend 
throughout the length of sensor 17. Poles 18 of stator 12 and sensor 17 
are aligned. Poles 18 are preferably provided in diametrically opposed 
pairs. For purposes of clarity, six (6) poles 18 are provided. It should 
be appreciated, however, that a greater or lesser number of poles 18 may 
be provided in any particular configuration. For example, in a constructed 
embodiment, 6 stator poles (with two teeth per pole for a total of 12 
teeth) are provided. 
Each of the poles 18 may have a generally rectangular shape, when taken in 
cross-section. The radially innermost surfaces of poles 18 are slightly 
curved so as to define an inner diameter to define bore 20. Bore 20 is 
adapted in size to receive rotor assembly 14. 
Rotor assembly 14, when assembled (see FIG. 2) is coaxially supported 
within stator 12/sensor 17 for relative rotational movement by 
conventional means. For purposes of illustration only, rotor assembly 14 
may be supported by conventional bearings (not illustrated) mounted in 
conventional end bells (not shown) secured to the longitudinal ends of 
stator 12/sensor 17. Rotor assembly 14 includes a generally cylindrical 
shaft 22, and rotor 24. Shaft 22 may be, as illustrated, hollow. Rotor 24 
is secured to shaft 22 for rotation therewith. For example, rotor 24 may 
be secured to shaft 22 by means of spline (not shown), or other 
conventional means well-known in the art. Thus, it should be appreciated 
that shaft 22, and rotor 24 rotate together as a unit. 
Rotor 24 includes a plurality of poles 26 formed on an outer surface 
thereof. Each pole 26 extends radially outwardly from the outer surface 
thereof and is formed having a generally rectangular shape, taken in 
cross-section. Rotor poles 26 extend longitudinally throughout the entire 
length of the outer surface of rotor 24. The radially outermost surfaces 
of rotor poles 26 are curved so as to define an outer diameter, adapted in 
size to be received within the inner diameter defining bore 20. That is, 
the outer diameter formed by the poles 26 is slightly smaller than the 
inner diameter defined by the radially innermost curved surfaces of stator 
poles 28. Rotor poles 26 are also preferably provided in diametrically 
opposed pairs. Four (4) rotor poles 26 are provided on the illustrated 
rotor assembly 14. However, it should be appreciated that a greater or 
lesser number of rotor poles 23 may be provided. For example, in a 
constructed embodiment, a 14 tooth rotor is provided. For VR motors, in 
general, the number of rotor poles 26 differs from the number of stator 
poles 18. Rotor 24, including poles 26, may be formed from a magnetically 
permeable material, such as iron. 
Referring now to FIG. 2, a diagrammatic view of a cross-section through an 
assembled motor 10 is illustrated. In particular, as referred to above, 
poles 18 occur in pairs: i.e., A A', B B', and C C'. The rotor poles 26 
also appear in pairs. Stator windings 28 (shown only on stator pole A for 
clarity) of diametrically opposite poles (e.g., A and A') associated with 
stator 12, are connected in series to form one machine phase. Thus, the 
windings 28 on poles A, and A' are referred to as "machine phase A" of VR 
motor 10. In the illustrated example, VR motor 10 also has machine phase 
B, and a machine phase C. Each of these three machine phases may be 
energized individually, which, when done in a controlled manner, provides 
for rotation of rotor 24. Although a three-phase machine is described and 
illustrated, a machine having more phases is contemplated as falling 
within the present invention. 
In a like manner to that illustrated in FIG. 2, sensor 17, as defined by 
laminations 16.sub.s, also has a plurality of sensor windings associated 
with each pole 18. In the preferred embodiment, sensor windings (shown 
schematically in FIG. 5 as winding 48.sub.i) associated with diametrically 
opposite poles are connected in series to form a single electrical sensor 
winding. Thus, in addition to the winding 28 for pole pair A A' formed by 
laminations 16.sub.m, a sensor winding is also provided for pole pair A A' 
formed by laminations 16.sub.s. That is, pole pairs A A', B B', and C C' 
formed by the plurality of laminations 16.sub.s, are each wound with a 
respective sensor winding, which corresponds to the stator windings 28 
associated with pole pairs formed by laminations 16.sub.m. The stator 
windings and the sensor windings are electrically isolated. Each one of 
the plurality of sensor windings 48 has an inductance characteristic that 
varies according to the actual position of rotor 24. Likewise, each stator 
winding also has an inductance characteristic that varies according to the 
actual position of rotor 24. Since rotor 24, as installed, is adjacent to 
both pluralities of laminations 16.sub.m and 16.sub.s and corresponding 
windings, the inductance characteristic of the sensor winding provides a 
good model of the inductance characteristic of the corresponding stator 
winding. Accordingly, the sensor winding can be used to control 
energization thereof. 
Referring now to FIG. 3, an apparatus 30 for controlling commutation of 
variable-reluctance electric motor 10 is illustrated, and includes 
external inductance sensing and commutation logic 32, power gate drive 
circuitry 34, converter circuitry 36, and VR motor 10. External inductance 
sensing and commutation logic 32 uses external inductance sensor 17 to 
monitor the rotor position of VR motor 10. Block 32 makes use of the 
variation in incremental phase inductance in the sensor windings of VR 
motor 10 to sense rotor position for purposes of generating machine phase 
commutation signals 40.sub.1, . . . 40.sub.n, where n indicates the number 
of phases of the machine being controlled. 
Gate drive circuitry 34 is provided for interfacing logic level commutation 
signals 40.sub.i generated by block 32 with the power-level signals 
required of converter 36. Gate drive circuitry 34 is conventional, and 
well-known in the art. 
Converter 36 is responsive to the power-level drive signals from gate drive 
circuitry 34 to energize selected machine phases (i.e., stator windings 
28) in accordance with predetermined control criteria. Converter 36 may 
take any one of the plurality of well-known configurations known in the 
art. In connection with this energization function, FIG. 3 shows stator 
winding energizing paths 44a, and 44b. External inductance sensing and 
commutation generation logic 32 energizes sensor windings 48.sub.i (see 
FIG. 5) by way of sensor winding energizing paths 46a, and 46b, as also 
shown in FIG. 3. 
Referring now to FIG. 4, in the illustrated embodiment, external inductance 
sensing and commutation generation logic 32 includes machine phase A logic 
50, machine phase B logic 52, and machine phase C logic 54, wherein 
communication between each of the logic blocks 50, 52, and 54 is 
accomplished by way of communication lines CL.sub.ab, CL.sub.bc, and 
CL.sub.ac. Phase A logic 50 generates a commutation signal for machine 
phase A of motor 10, indicated at .PHI.A, phase B logic 52 generates a 
commutation signal for machine phase B, indicated at .PHI.B, and phase C 
logic 54 generates a commutation logic for machine phase C of motor 10, 
indicated at .PHI.C. Signals .PHI.A, .PHI.B, and .PHI.C, collectively 
define commutation signals 40.sub.i, as shown in FIG. 3. It should be 
understood, as detailed above, that although a three-phase motor/sensor 
structure is described for purposes of simplicity, any number of phases 
could be utilized and would fall within the spirit and scope of this 
invention. In particular, it should be understood that a block, similar to 
blocks 50, 52, and 54, is needed in the illustrative embodiment for each 
phase of external inductance sensor 17. 
Referring now to FIG. 5, a simplified schematic and block diagram view is 
shown illustrating, in greater detail, phase A logic block 50 of FIG. 4. 
Each one of the logic blocks 52, and 54 also substantially comprise the 
circuit of FIG. 5, with appropriate changes to the input signals to be 
described in detail below. Circuitry 50 includes an energizing circuit 56, 
inductance indicative signal generating circuit 58, a phase advance 
circuit 60, a threshold signal generating circuit 62, and a comparing 
circuit 64. 
Means or circuit 56 is provided for energizing a sensor winding 48.sub.i 
and generating in response thereto a chopped, analog signal representative 
of an electrical current passing through the energized sensor winding. 
Energizing circuit 56 includes an astable multivibrator 66, a switching 
means, such as metal-oxide-semiconductor field effect transistor (MOSFET) 
Q, sense resistor R.sub.s, dissipation resistor R.sub.D, and diode D. 
Multivibrator 66 is used to drive MOSFET Q and is provided for chopping 
the current passing through the winding of phase A (.PHI.A) of sensor 17, 
at a frequency substantially higher than the motor commutation frequency. 
In particular, in a constructed embodiment, the maximum motor commutation 
frequency is 900 Hz, while multivibrator 66 generates an output signal at 
node 68, which is connected to the base terminal of MOSFET Q, to chop the 
current at a frequency of approximately 35 kHz. Resistor R.sub.d and diode 
D are series-connected and are used to quickly dissipate the energy in 
coil or winding 48.sub.i so that the chopping/sampling frequency of 
circuits 56, 58 can be relatively high. Sense resistor R.sub.s is 
connected to the source terminal of FET Q for providing an output voltage 
on node 70 that is proportional to the magnitude of the current pulses. In 
effect, circuit 56 is a sampling circuit coupled to the sensor windings 
48.sub.i for selectively energizing at least one of the sensor windings to 
generate a plurality of current pulses wherein the current pulses are 
converted to a corresponding series of voltage pulses representative of 
the current passing through the energized sensor winding. 
Means or circuit 58 is responsive to the chopped current signal and is 
provided for generating an inductance indicative signal representative of 
an inductance waveform associated with the energized sensor winding. 
Circuit 58 includes an amplifier 72, a delay circuit 74, and a sample and 
hold circuit 76. The amplifier 72 includes an input coupled to receive the 
chopped current signal appearing at node 70. The amplifier 72 then 
amplifies the chopped signal in a manner well-known in the art. The sample 
and hold circuit 76 is also a conventional structure, and is fed the 
output of amplifier 72. The sample and hold circuit 76 is operable to 
sample the amplifier 72 output at predetermined times according to a 
sampling signal produced by delay circuit 74. The sample and hold circuit 
76 generates an inductance indicative signal associated with machine phase 
A, and which is indicated in the Figures at V.sub.A. Delay circuit 74 
functions to delay the multivibrator 66 output appearing at node 68 by a 
predetermined amount in order to generate the sampling signal. The amount 
of delay is such that the sampling by sample and hold circuit 76 occurs 
just before multivibrator 66 turns off MOSFET Q. Thus, the amount of delay 
introduced by circuit 74 is, in one respect, a function of the operating 
frequency of multivibrator 66. For example, as the frequency of operation 
of multivibrator 66 goes up, the chopping period decreases, and therefore 
so does the delay amount to maintain sampling just before turn-off. Delay 
circuit 74 is conventional in the art. 
Phase advance circuit 60 includes a logic circuit 78 having a first 
exclusive-or (XOR) gate 80, and second exclusive-or (XOR) gate 82, a 
frequency-to-voltage converter 84, and a summing amplifier 86. 
For three-phase motor 10, as in the illustrated embodiment, three (3) 
circuits 50, 52, and 54 operate simultaneously to generate commutation 
signals .PHI.A, .PHI.B, and .PHI.C for machine phases (i.e., stator 
windings 28) A, B, and C of motor 10, respectively. Accordingly, these 
commutation signals are simultaneously available for use between each of 
the blocks 50, 52, and 54 (see also FIG. 4). The present invention makes 
use of these signals to calculate a phase advance based on rotor speed. 
Specifically, logic circuit 78 is responsive to predetermined commutation 
signals associated with the plurality of machine phases of the motor for 
generating a first motor speed signal at the output of gate 82 whose 
frequency corresponds to the rotational velocity of the rotor 24. This 
should be apparent to one of ordinary skill in the art. 
Frequency-to-voltage converter 86 is then operative to convert the first 
motor speed signal to another, second motor speed signal whose magnitude 
(i.e., DC output) also corresponds to the rotational velocity of the rotor 
24. Summing amplifier 86 adds the output of frequency-to-voltage converter 
84, with a preselected advance signal V.sub.ON ', to generate an the 
energizing threshold signal V.sub.on. The preselected advance signal is a 
fixed DC signal used to set the voltage threshold (which corresponds to a 
rotor position in electrical degrees) level at which commutation of 
machine phase A occurs. Thus, the output V.sub.on is the base threshold 
level V.sub.ON ', modulated as a function of the motor rotor speed. 
Threshold signal generating circuit 62 includes a comparator 88, and an 
analog switch 90. The comparator 88 is responsive to the inductance 
indicative signal associated with machine phase B, V.sub.B, as well as the 
inductance indicative signal associated with machine phase C, V.sub.C, for 
generating a switch signal. The switch signal controls switch 90 to select 
one of either the energizing threshold signals V.sub.on, or deenergizing 
threshold signal V.sub.off. The switch signal changes state when V.sub.B 
and V.sub.C crossover, and is thus indicative of a machine phase crossover 
point. 
Means or circuit 64 is provided for comparing the inductance indicative 
signal V.sub.A with the threshold signal V.sub.T for generating a 
commutation signal .PHI.A to control energization of the stator winding 
corresponding to the energized sensor winding. Comparing circuit 64 
includes an analog comparator 92 for activating the commutation signal 
.PHI.A associated with machine phase A when V.sub.A is greater than the 
threshold signal V.sub.T. Thus, circuit 50, shown in FIG. 5, outputs a 
digital commutation signal .PHI.A. It should be understood that the 
foregoing description assumes operation in one direction only--A, B, C, A, 
B, C, etc. 
FIG. 6 illustrates a timing diagram of the relationships between signals 
V.sub.A, V.sub.B, and V.sub.C, relative to the threshold signal V.sub.T. 
The threshold signal V.sub.T allows for independent control of the turn-on 
and turn-off angles of the machine phase by using analog switch 90 to 
toggle between two references according to the rotor position (as derived 
by external sensing incremental phase inductance). The turn-on threshold 
(V.sub.on) is proportional to the speed of the motor rotor 24 (through the 
use of frequency-to-voltage converter 84 and summing amplifier 86) in 
order to provide automatic phase advance. Sampling circuitry will work 
down to, and including, zero speed and will provide accurate commutation 
signals over a wide range of speeds. 
In particular, as shown in FIG. 6, several of the inductance indicative 
signals V.sub.A, V.sub.B and V.sub.C are shown. These signals are 
generally sinusoidal and are representative of the inductance waveform of 
the corresponding sensor winding. As such, these signals are also 
representative of the rotor position. For example, in the graph motor 
phase A (i.e., stator winding 28 for A A') is energized between point 94 
(V.sub.A &gt;V.sub.T), and point 96. Further, as rotor speed varies, V.sub.on 
also varies accordingly. Thus, by adjusting V.sub.on, by way of phase 
advance circuit 60, the intersection of V.sub.A, and V.sub.on will be 
advanced or retarded relative to the position shown in FIG. 6. 
As also can be seen, the threshold voltage V.sub.T changes from its 
energizing value V.sub.on, to its deenergizing value V.sub.off at a point 
where the traces of V.sub.B, and V.sub.C intersect for crossover (i.e., at 
approximately 90.degree.). Likewise, a similar transition occurs at 
270.degree.. 
It should be appreciated, that since blocks 50, 52, and 54 (see FIG. 4) are 
operating simultaneously, all of the motor machine phases (i.e., 3 in the 
illustrated embodiment) can be controlled simultaneously to accomplish 
complete control of the rotation of rotor 24. 
A control apparatus in accordance with the present invention accomplishes 
phase commutation by the use of an external inductance sensor 17, which 
monitors the rotor position of VR motor 10. The circuit makes use of the 
variation in incremental phase inductance in the motor to sense rotor 
position for the purpose of phase commutation. The sensor comprises a 
plurality of laminations 16.sub.s, which are identical to the motor 
laminations 16.sub.m, and, in one embodiment, is mounted on one end of the 
motor. The circuit accomplishes commutation by simultaneously chopping 
each of the phases in the sensor at current levels which are a small 
fraction of the motor current, and monitoring the resulting current peaks 
of the pulses after fixed intervals of time. All of the sampling and 
signal processing is accomplished with analog circuitry to eliminate the 
need for an expensive microprocessor. The sensing circuitry outputs a 
digital signal for each of the motor phases for direct control of the 
phase current commutation. Further, since the sensor is external (i.e., 
not part of the stator windings), the circuitry 50 can be kept distinct 
and independent from the circuitry to energize the stator windings. This 
feature permits optimization of the circuitry 50, which would otherwise 
have to be incorporated into the stator winding energization circuitry. 
The preceding description is exemplary rather than limiting in nature. A 
preferred embodiment of this invention has been disclosed to enable one 
skilled in the art to practice the invention. Variations and modifications 
are possible without departing from the purview and spirit of this 
invention; the scope of which is limited only by the appended claims.