Position sensor elimination technique for the switched reluctance motor drive

Rotor position information for a switched reluctance (SR) motor drive is obtained indirectly in response to the motor phase inductance. An oscillator generates a signal having a time period that is a function of the inductance. The signal is processed by other circuits to obtain proper instants of commutation. In the preferred motor drive, the energized phase windings are isolated from the oscillator, and periodic signals are obtained which have periods indicating the phase inductances of the unenergized phase windings. The periods are compared to threshold values to obtain position indicating signals, and commutating signals are derived from the position indicating signals.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to variable-speed motor drives, and 
more specifically to electronic commutation of a switched reluctance 
motor. In particular, the present invention relates to indirectly 
determining rotor position for electronic commutation in order to 
eliminate the need for a rotor position sensor. 
2. Description of the Related Art 
Variable speed drives of less than 20 kW preferably use brushless dc drives 
to obtain high efficiency and flexible control characteristics. In 
particular, the development of computer aided design tools and efficient 
power semiconductor devices have made the switched reluctance (SR) motor 
especially attractive. 
The SR motor does not require permanent magnets and produces torque by the 
variable reluctance principle. A position sensor, however, is required in 
the SR drive in order to synchronize phase excitation pulses to the rotor 
position. 
The process of deriving correctly phased signals from the rotor shaft 
position and using them to control the timing of switching operation of 
the power semiconductor devices in the drive is called "electronic 
commutation". The speed-torque characteristics of the SR motor can be 
flexibly controlled by changing the switching angles according to speed 
and torque requirements. 
The sensing of rotor position is usually performed by optical or 
Hall-effect sensors. This involves mounting the sensors in close proximity 
to the rotor. Such a position sensor, however, constitutes a substantial 
fraction of the total system cost and tends to reduce the system 
reliability. For the appliance industry and particularly for hermetically 
sealed compressors, the factors of cost and reliability are especially 
important and have lead to consideration of an alternative technique of 
determining rotor position. 
An indirect method of position sensing is described in McMinn, et al., 
"Application of the Sensor Integration Techniques to the Switched 
Reluctance Motor Drive," IEEE Industry Applications Conference Record 
1988, pp. 584-588. In this method, short duration, low level voltage 
pulses are applied to the two unenergized phases of an SR motor and the 
resulting current pulses are measured to obtain an indication of the 
impedances of the unenergized motor phases and an estimation of the rotor 
angle. The circuitry for implementing this method, however, is rather 
complex and relatively expensive. 
SUMMARY OF THE INVENTION 
The primary object of the invention is to provide a reliable and cost 
effective method of indirectly determining the position of a rotor in a 
switched reluctance motor. 
A specific object of the invention is to provide an accurate method of 
determining phase inductance in a switched reluctance motor. 
In accordance with the most basic aspect of the present invention, rotor 
position information for a switched reluctance (SR) motor drive is 
obtained indirectly by sensing the motor phase inductance. The 
frequency-modulated signal is generated by an oscillator connected to 
stator windings of the motor. Other circuits are responsive to the 
frequency-modulated signal to provide proper instants of commutation.

While the invention is susceptible to various modifications and alternative 
forms, specific embodiments thereof have been shown by way of example in 
the drawings and will herein be described in detail. It should be 
understood, however, that it is not intended to limit the invention to the 
particular forms disclosed, but on the contrary, the intention is to cover 
all modifications, equivalents, and alternatives falling within the spirit 
and scope of the invention as defined by the appended claims. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning now to the drawings, there is shown in FIG. 1 a switched reluctance 
(SR) motor 20 having a rotor 21 and a stator 22. As shown, the rotor 21 
has four poles RP1-RP4, and the stator 22 has six poles SP1-SP6. The 
present invention, however, can be used with SR motors having various 
numbers of rotor and stator poles. 
In order to apply a torque to the rotor 21, respective windings W1-W6 are 
wound about the stator poles SP1-SP6. As will be further described below 
in connection with FIG. 2, the windings for pairs of diametrically 
opposite stator poles are wired together, and the pairs are energized by 
respective ones of three phases of current. In addition, the phase 
inductance varies appreciably as a function of rotor position so that when 
a pair of phase windings are energized, a torque is generated tending to 
align the rotor poles with the two stator poles having the energized 
windings. 
To apply a relatively constant torque to the rotor 21 for rotation in 
either a forward or reverse direction, it is necessary to energize the 
phase windings in sequence as a function of the relative angular position 
of the rotor 21 with respect to the stator 22. The rotor position sensing 
has typically been done using optical or Hall sensors mounted in close 
proximity to the rotor 21. This has had an adverse effect upon the 
reliability of the motor and is relatively costly in terms of the cost of 
the sensors and the space occupied by the sensors in the motor assembly. 
Therefore, the present invention is directed to providing an indirect 
means by which the rotor position can be determined. In particular, the 
present invention involves generating a frequency-modulated signal 
responsive to the phase inductance of the unenergized windings of the 
motor. 
Turning now to FIG. 2, there is shown a schematic diagram of the preferred 
power circuits which selectively energize the SP motor windings W1-W6 in 
response to phase commutating signals .phi..sub.1, .phi..sub.2, 
.phi..sub.3, .phi..sub.1 ', .phi..sub.2 ', .phi..sub.3 '. As shown, the 
windings W1 and W4 are wired in series for the first phase, the windings 
W2 and W5 are wired in series for the second phase, and the windings W3 
and W6 are wired in series for the third phase. The windings for the three 
phases are selectively connected to the positive voltage of a DC power 
supply 26 through respective electronic switches such as power 
field-effect transistors 27, 28 and 29. In a similar fashion, the phase 
windings are selectively connected to the negative voltage of the DC 
supply 26 by respective electronic switches such as power field-effect 
transistors 30, 31 and 32. To use the SR motor in a conventional household 
appliance, for example, the DC supply 26 includes a full-wave bridge 
rectifier for converting the standard 120 VAC household current to direct 
current. 
Preferably two switching devices are used for each phase so that inductive 
energy stored in the winding circuits is recovered when the electronic 
switches shut off the flow of current to the windings. In particular, 
associated with the phase windings are directional diodes 33-38 which 
return power to the power supply 26 when the electronic switches 27-32 are 
turned off. This improves the electrical efficiency of the SR motor. 
Turning now to FIG. 3, there is shown a timing diagram of the commutating 
signals .phi., .phi.'. Preferably the commutating signal .phi. has a duty 
cycle of 331/3 percent, and the commutating signal .phi.' is commutated in 
a similar fashion and also gated by a pulse-width modulated signal. This 
scheme permits the speed of the motor to be controlled by the pulse-width 
modulation. 
The derivation of the commutating signal .phi.' from the commutating signal 
.phi. is shown in FIG. 4. An oscillator 41 generates a periodic signal 
that is fed to a pulse-width modulator 42. The pulse-width modulator 
modulates the duty cycle of the periodic signal in response to a speed 
command signal. A gate 43 gates the commutating signal .phi. with the 
pulse-width modulated signal to provide the commutating signal .phi.'. 
Turning now to FIG. 5, there is shown a schematic diagram of an inductance 
responsive oscillator 45 having a linear inductance-to-time conversion 
characteristic. The oscillator 45 employs two operational amplifiers 46 
and 47 to generate a periodic binary signal V.sub.o having a period T that 
is proportional to the phase inductance L.sub.ph of a respective pair of 
windings W1-W6 on the SR motor (20 in FIG. 1). The operational amplifiers 
46 and 47 have no special requirements, and part No. 741 can be used. The 
circuit 45 is a kind of relaxation oscillator in which the second 
operational amplifier 47 is configured with positive feedback resistors 
R.sub.1 and R.sub.2 to act as a Schmitt trigger, and the first operational 
amplifier 46 is configured to provide a delay proportional to the L/R time 
constant of the phase windings 48 and a resistor R. In addition, the first 
operational amplifier 46 is provided with resistors R.sub.3 and R.sub.4 
which ensure that the DC gain of the first operational amplifier is 
positive, but the AC gain of the first operational amplifier is negative, 
causing the oscillator 45 to self-start immediately under all conditions. 
The values of R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are, for example, 10K 
ohms. 
The time period T of the periodic signal V.sub.o is given by: 
EQU T=K L.sub.ph /(R+R.sub.ph) 
where L.sub.ph is the phase inductance of the SR motor windings, R.sub.ph 
is the phase inductance of the SR motor windings, and K is a constant on 
the order of 1 that is set by the ratio of R3 to R4 and the saturation 
voltage levels of the operational amplifiers 46 and 47. Therefore, by 
selecting the value of R, the frequency of the periodic signal V.sub.o can 
be selected to be relatively high compared to the maximum rotational 
velocity of the rotor 21 to give a rather precise indication of the 
position of the rotor. In this regard, the period T of the periodic signal 
V.sub.o represents a certain minimum time for the oscillator 45 to provide 
a signal indicating the value of phase inductance. The preferred frequency 
of the periodic signal V.sub.o is about 5 to 20 kHz or higher, with higher 
frequencies preferred for operating the SR motor at higher maximum 
rotational velocities. 
Turning now to FIG. 6, there is shown an alternative inductance responsive 
oscillator circuit 50 that employs a precision Schmitt trigger including 
resistors R.sub.5, R.sub.6, R.sub.7 for setting upper and lower voltage 
thresholds, threshold comparators 51 and 52, a set-reset flip-flop 53, and 
an output driver 54. Such a precision Schmitt trigger is easily fabricated 
as an integrated circuit and a similar circuit is used in the conventional 
type 555 timer integrated circuit. 
To provide a signal indicating the phase inductance L.sub.ph ', the 
windings 55 are part of a low-pass filter providing feedback from an 
output terminal 56 to an input terminal 57 of the precision Schmitt 
trigger. The low-pass filter includes a shunt resistance R' which also 
sets the frequency of oscillation such that the period is proportional to 
the phase inductance L.sub.ph ' according to: 
EQU T'=K' L.sub.ph '/(R'+R.sub.ph ') 
where K' is a constant on the order of 1 that is set by the thresholds of 
the precision Schmitt trigger. A capacitor C can also be included in the 
low-pass filter to reject noise pulses from the switching of the power 
circuits. 
For the sake of economy, it is desirable for the inductance responsive 
oscillator 45 or 50 to be constructed using relatively low voltage 
circuits. For many applications, however, it is desirable to drive the 
motor windings W1-W6 with much higher voltages. In this case, the motor 
windings W1-W6 should not be connected directly to the inductance 
responsive oscillators. One way to solve this problem is to use a 
potential transformer across each phase winding to isolate the inductance 
responsive oscillator from the power circuits. The use of such a potential 
transformer, however, changes considerably the phase inductance that the 
oscillator is responsive to. If the potential transformer has a ratio of 
N:1, then the effective inductance sensed by the oscillator will be 
L.sub.eq +L.sub.ph /N.sup.2, where L.sub.eq is the transformer equivalent 
inductance referred to the low voltage winding. Therefore, the presence of 
the transformer equivalent inductance reduces the accuracy of the 
oscillator's response to the phase inductance. 
Turning to FIG. 7, there is shown an alternative method of isolating the 
inductance responsive oscillators from the phase windings. In this case, a 
SR motor 60 has inductance responsive windings W.sub.s that are separate 
from the power windings W. In particular, each inductance responsive 
winding W.sub.s includes windings 61 about the base of a respective stator 
pole SP, and the power windings include windings 62 wound over the 
inductance sensing windings 61 about the stator pole. The inductance 
responsive windings 61 include a relatively small number of turns compared 
to the power windings 62, so that the voltages induced on the inductance 
responsive windings 61 are relatively small when the power windings 62 are 
energized. 
Turning now to FIG. 8, there is shown another method of isolating an 
inductance responsive oscillator 71 when its respective motor windings 72 
are energized. In this case, a comparator 74 has its inputs connected via 
a resistive voltage divider 75 across the windings 73. The comparator 74 
senses whether the phase windings 73 are energized. If the phase windings 
are energized, then the comparator activates an analog switch 76 to 
disconnect the inductance responsive oscillator 71 from its respective 
phase windings 73. Alternatively, the analog switch 76 could be controlled 
by the control circuitry that energizes the phase winding 73, but that 
control circuitry should also include timing circuitry that activates the 
analog switch to reconnect the inductance responsive oscillator 71 to the 
phase windings 73 only after a certain delay time during which the voltage 
across the phase windings can decay to a relatively low level. 
In order to derive the commutating signals .phi., .phi.' from the periodic 
signals from the inductance responsive oscillators, the periods of the 
periodic signals are compared to threshold values to obtain position 
indicating signals. As shown in FIG. 9, this can be done by using a 
frequency-to-voltage converter 81 and a threshold comparator 82. The 
frequency-to-voltage converter 81, for example, is an integrated circuit 
such as Part No. TSC 9400CJ. This integrated circuit 81 operates in 
connection with a reference capacitor 82, an integrating resistor 83, an 
integrating capacitor 84, and resistors 85, 86, 87 and 88. The output 
voltage V.sub.o " of the frequency-to-voltage converter is determined 
according to: 
EQU V.sub.o "=(V.sub.ref C.sub.ref R.sub.int)f.sub.in 
where V.sub.ref is a reference voltage supplied to pin 7 of the integrated 
circuit 81, C.sub.ref is the value of the reference capacitor 82, and 
R.sub.int is the value of the integrating resistor R.sub.int. 
The input frequency f.sub.in is provided by the output V.sub.o of the 
inductance responsive oscillator 45 of FIG. 5. The input frequency 
f.sub.in is converted to a voltage level on an output line 89. Therefore, 
the voltage on the output line 89 is inversely proportional to phase 
inductance. To determine when the phase inductance reaches a predetermined 
value, the comparator 82 compares the voltage V.sub.o " on the converter 
output 89 to a reference voltage V.sub.t selected by a potentiometer 90 
and a voltage divider including resistors 91 and 92. 
Turning now to FIG. 10, there is shown an alternative circuit using digital 
logic for generating signals S and S' for indicating when the phase 
inductance exceeds respective first and second levels. The circuit 100 
receives, for example, the periodic signal from the inductance responsive 
oscillator 50 of FIG. 6 and compares its frequency f.sub.in to preselected 
submultiples of the frequency of a high speed clock (CLOCK). A first delay 
flip-flop 101 synchronizes the periodic signal to the high speed clock. 
Then a second delay flip-flop 102 and a NOR gate 103 detect the presence 
of a low-to-high transition in the periodic signal. When such a transition 
occurs, the gate 103 resets a binary counter 104 that is clocked by the 
high speed clock. 
To detect when the frequency f.sub.in ' is less than a first preselected 
frequency, a delay flip-flop 105 is clocked by the reset pulse from the 
gate 103 and receives the Q.sub.n output of the nth binary stage of the 
counter 104. Therefore, the delay flip-flop 105 generates a signal S which 
is a logic low when the frequency f.sub.in ' is greater than the frequency 
of the clock divided by 2.sup.n, and it is a logic high when the frequency 
f.sub.in ' is less than the frequency of the clock divided by 2.sup.n. In 
a similar fashion, a second flip-flop 106 is clocked by the reset pulse 
and receives the output of a NAND gate 107 which combines the output of 
the nth and cth stages of the binary counter 104. Therefore, the signal S' 
is a logic low when the frequency f.sub.in ' is greater than the clock 
frequency divided by (2.sup.n +2.sup.c), and the signal S' is a logic high 
when the frequency f.sub.in ' is less than the frequency of the clock 
divided by (2.sup.n +2.sup.c). 
Turning now to FIG. 11, there is shown a diagram illustrating the desired 
relationships between the signals S and S' for each of the three phases of 
the SR motor (20 in FIG. 1) as a function of the angle of the rotor with 
respect to the stator. These relationships result from the circuit of FIG. 
10, for example, by selection of the clock frequency, the number n of 
stages in the binary counter, and the number c of stages which selects the 
difference between the threshold levels of the signals S and S'. In 
particular, the leading edges of the signals S define the instant of time 
when the phase windings are energized and deenergized in sequence during 
continuous rotation of the rotor 21 of the SR motor 20. For efficiency, it 
is desired that the phase windings for a respective set of opposite poles 
be commutated to begin deenergization of the windings slightly prior to 
the alignment of the poles. This defines the interval t.sub.x in FIG. 11. 
The phase windings are energized in sequence during the intervals denoted 
1, 2, and 3 between the leading edges of the signals S as shown in FIG. 
11. This relationship between the signals S, however, makes it somewhat 
difficult to start the motor from a rest position since the initial rotor 
position cannot be determined with sufficient certainty in the case when 
all of the S signals happen to be high. Therefore, it is desirable to 
provide a second position indicating signal S' for each of the three 
phases. Once this is done, the initial starting and running of the SR 
motor can be readily performed by control logic responsive to the signals 
S and S'. 
Turning to FIG. 12, there is shown a state diagram of control logic for 
starting and running the SR motor in a single direction in response to the 
position indicating signals S and S'. The control logic has four states, 
including an initial state 0, and states 1, 2 and 3 through which the 
control logic cycles when the motor is running. When the control logic is 
in state 1, the first phase windings W1 and W4 are energized; in state 2, 
the second phase windings W2 and W5 are energized; and in state 3, the 
third phase windings W3 and W6 are energized. The initial state 0 is 
reached in response to a reset signal R which is generated when the motor 
is initially turned on and whenever the motor is found to be in a "stall" 
condition. From the initial state 0, either state 1, 2 or 3 is reached 
depending upon the logic states of the position indicating signals S and 
S' for all three phases. Once states 1, 2 and 3 are reached, however, the 
next state in cyclic order 1, 2, 3, 1, 2, 3 etc. is reached depending upon 
the logic states of the signals S and S' for the unenergized phases, and 
only when an enable signal E indicates that these unenergized phases have 
been unenergized for a predetermined amount of time sufficient to 
guarantee that the inductance responsive oscillator will properly indicate 
phase inductance and hence rotor position. 
Turning now to FIG. 13, there is shown a schematic diagram of control logic 
which operates according to the state diagram in FIG. 12. As should be 
evident from FIG. 13, it would be possible to integrate all of the control 
logic together with the inductance responsive oscillators 53 and frequency 
discriminators 100 on a relatively small integrated circuit. 
The four states of the control logic are defined by a two-stage register 
110 clocked by a high-speed clock 111. The outputs of this register are 
decoded by a decoder 112 to provide the commutating signals .phi..sub.1, 
.phi..sub.2 and .phi..sub.3 for power circuits 113. The power circuits 113 
drive the SR motor 60. The position of the rotor in the SR motor 60 is 
determined by an oscillator 53 and a discriminator 100 for each of the 
three phases. The position indicating signals S and S' from each 
discriminator are fed to present state/next state logic 114 which 
determines the next state of the register 110. The present state/next 
state logic is defined by the following truth table: 
TABLE I 
______________________________________ 
PRESENT STATE/NEXT STATE LOGIC 
D.sub.1 
D.sub.0 
Q.sub.1 
Q.sub.0 
E S'.sub.3 
S.sub.3 
S'.sub.2 
S.sub.2 
S'.sub.1 
S.sub.1 
______________________________________ 
0 1 0 0 d 1 d d d d 1 
0 1 0 0 d d d d 0 d 1 
1 0 0 0 d d d d 1 1 d 
1 0 0 0 d d 0 d 1 d d 
1 1 0 0 d d 1 1 d d d 
1 1 0 0 d d 1 d d d 0 
1 0 0 1 1 0 d d 1 d d 
1 1 1 0 1 d 1 d d 0 d 
0 1 1 1 1 d d 0 d d 1 
______________________________________ 
The register 110 is initially put in the zero state by a reset signal R. 
This reset signal is generated upon a power on condition by a resistor 
115, a capacitor 116, a Schmitt trigger invertor 117 and a OR gate 118. 
The reset signal is also generated when a stall condition is detected by a 
binary counter 119. The binary counter 119 is clocked by the clock 111 and 
during normal operation, it is reset by the Schmitt trigger invertor 117 
or periodically when the register 110 changes its state, as detected by 
exclusive OR gates 120, 121 and an OR gate 122. The binary counter has a 
sufficient number m of stages to define a predetermined time interval 
sufficiently long so that the binary counter 119 will be reset during that 
time interval unless the motor 60 is stalled. 
The present state/next state logic 114 uses an enable signal E. This enable 
signal is provided by another binary counter 123 which is also reset by 
any transition in the state of the register 110. The counter 123 counts 
for a predetermined delay following the transition of the register 110. 
The predetermined delay is indicated by a logic low output of the dth 
stage of the counter 123. The enable signal is fed back through an 
invertor 124 to a gate 125 which gates the clock to the counter 123 in 
order to hold the enable signal high until the next transition in the 
state of the register 110. 
In view of the above, there has been described a reliable and 
cost-effective method of indirectly determining the position of a rotor in 
a switched reluctance motor. The method is reliable because phase 
inductance is a precisely defined function of rotor position, and the 
ratio of maximum to minimum phase inductance in a SR motor is usually 
three or greater. By determining the phase inductance with an oscillator 
circuit, the inductance determination can be relatively immune to the 
energizing of other phase windings. In particular, accurate position 
indicating signals are obtained by measuring the periods of the periodic 
signals from the inductance responsive oscillators when the respective 
phase windings are not energized. In addition, the inductance responsive 
oscillators, frequency discriminators, and control logic for generating 
the commutating signals can be provided on an integrated circuit which 
makes the method of the present invention cost effective. 
It should be apparent to persons of ordinary skill that the preferred 
embodiment of the invention is susceptible to various modifications. More 
complex controls could be used in connection with the inductance 
responsive oscillators to more precisely indicate the position of the 
rotor. This additional rotor position information could be used, for 
example, to adjust the speed and torque characteristics of the SR motor by 
advancing or retarding the commutating signals. The control logic, for 
example, could include logic for reversing the direction of the motor or 
for providing a decelerating as well as an accelerating torque on the 
rotor. Moreover, a microprocessor or microcontroller could be used for 
generating the commutating signals in response to phase inductance 
information. The microprocessor or microcontroller, for example, could use 
an analog-to-digital converter in connection with the frequency-to-voltage 
converter of FIG. 9, or could use a multi-bit register strobed to receive 
the value of the binary counter 104 in FIG. 10 in response to the reset 
signal from the gate 103, to obtain a numerical value of the phase 
inductance. From the numerical value of the phase inductances of the 
unenergized phases, the microprocessor or microcontroller could calculate 
the phase angle of the rotor in the SR motor.