Control circuit for switched reluctance motor

A motor control circuit for a switched reluctance motor which includes a rotor and a plurality of stator coils, and a driver circuit for energizing the coils in sequence, includes a current sensing resistor for detecting a signal proportional to the current in the coils and a slope detector for detecting the rate of change of the current signal with respect to time over predetermined time intervals during which the voltage in the coils is dominated by a term representing back EMF. A variable oscillator is responsive to an integrated output of the slope detector for generating a timing signal for the driver circuit. When the voltage across the coils is dominated by the back EMF term, the current should be substantially constant, but when the rotor is out of phase with the driver circuit, the current signal during this time has a substantially linear slope which is either positive or negative depending upon whether the rotor is leading or lagging the driver circuit.

BACKGROUND OF THE PRESENT INVENTION 
The following invention relates to a sensorless control circuit for use 
with a switched reluctance electric motor. 
Switched reluctance stepping motors employ an iron core rotor having a 
plurality of poles and pairs of stator coils aligned on opposite sides of 
the rotor so as to create a magnetic circuit when the poles of the rotor 
are aligned with the oppositely placed coils. The reluctance of the 
magnetic circuit is lowest when a pole pair is aligned directly between a 
pair of opposed coils. By sequentially energizing pairs of coils, the 
rotor is caused to rotate in order to find the position of lowest 
reluctance. 
Control circuits for such motors typically include an external sensor which 
is connected to the rotor to sense rotor position. This signal is used to 
maintain alignment between the coil switching sequence and the rotor 
position. Hall effect sensors have long been employed for this purpose. A 
mechanical sensor is shown in the U.S. Pat. No. 3,601,678, to Abraham, et 
al. In the Abraham patent, a mechanical emitter coupled to the shaft of 
the rotor provides position information for a reader head which in turn 
generates a feedback signal to a control loop which controls motor speed. 
Sensors, however, take up space, are costly, and typically cannot 
withstand the harsh environments for which switched reluctance motors are 
rated. 
Various types of sensorless control circuits exist for DC brushless motors 
which employ permanent magnet rotors. Permanent magnet rotors generate 
back EMF in stator coils which various sinusoidally. Thus, determining the 
phase of the back EMF signal gives an indication of rotor position. An 
example of this type of control circuit is shown in Plunkett, U.S. Pat. 
No. 4,928,043 entitled BACK EMF SAMPLING CIRCUIT FOR A PHASE-LOCKED LOOP 
MOTOR CONTROL which is assigned to the same assignee as the present 
invention. The Plunkett invention discloses a sensing network for sensing 
the back EMF on an unenergized motor winding. This signal represents a 
phase error when compared with an optimum value and drives a voltage 
controlled oscillator which in turn generates timing signals for an 
inverter. However, because switched reluctance motors do not employ 
permanent magnets, there is no back EMF induced in an unenergized coil as 
a direct result of rotor rotation. 
The inductance in the coils does vary as a function of rotor position. When 
a pair of rotor poles become aligned with a pair of stator coils, the 
inductance reaches a maximum. In the past, motor designers have attempted 
to build control loops utilizing some value of sensed inductance. However, 
this requires complex bridge circuitry which tends to have a high level of 
noise associated with it. Saturation and hysterysis effects also 
compromise inductance sensing techniques. 
SUMMARY OF THE PRESENT INVENTION 
The present invention solves the aforementioned problems in a switched 
reluctance stepping motor by providing a current sensing circuit to detect 
a signal proportional to current in the energizing coils of the motor. A 
slope detector determines the rate of change of the signal proportional to 
current over a set of discrete time intervals. The time intervals are 
provided by a window generator which masks the slope detector during 
periods when the current signal is rapidly fluctuating so that sampling 
takes place only when the current signal is either slowly increasing with 
time, slowly decreasing with time or constant. A variable oscillator is 
responsive to an integrated output of the slope detector and generates 
timing pulses for a driver circuit which is coupled to the energizing 
coils. 
This circuit operates on the theory that back EMF is generated by the 
driving current and rotor rotation. A switched reluctance motor does not 
generate back EMF in the sense that a DC motor does; that is, externally 
spinning the rotor of an unenergized switched reluctance motor does not 
produce a voltage on the windings as is seen in a DC permanent magnet 
motor. A switched reluctance motor does, however, convert electrical power 
into mechanical power. Therefore, some generated voltage must exist to 
oppose the current and force the supply to provide power at least equal to 
the mechanical output power. 
The voltage and torque equations for a switched reluctance motor operating 
in the linear region (no saturation) are as follows: 
EQU V=i.multidot.R+d(L.multidot.i)/dt 
EQU T=1/2.multidot.(dL/d.theta..multidot.i).multidot.i 
The induction component in the voltage equation is written with both the 
inductance and current inside the derivative because both components vary 
in time. The voltage equation can be expanded as: 
EQU V=i.multidot.R+di/dt.multidot.L+(dL/d.theta..multidot.i).multidot..OMEGA. 
where .OMEGA.=d.theta./dt. The term 
(dl/d.theta..multidot.i).multidot..OMEGA. is a term which represents back 
EMF. This is the voltage generated by the energy conversion process. The 
term inside the parentheses also appears in the torque equation but is 
multiplied by one-half. Thus, only half of the energy converted due to the 
change of inductance with position is converted as mechanical work. The 
other half is stored in internal magnetic fields. 
In order to obtain maximum torque and efficiency, the current pulse should 
be aligned with the peak of the back EMF term. This occurs when 
dl/d.theta. is at a maximum. When a coil is energized, current begins to 
rise quickly, the voltage-current relationship is dominated by the di/dt*L 
term. The current will soon level off and, because di/dt is nearly zero, 
the voltage-current relationship is dominated by the back EMF term. If the 
current pulse and back EMF is aligned, the slope of the current will be 
zero. If the current pulse is leading the back EMF then the dl/d.theta. 
term will be increasing toward its maximum. The current will decrease 
during this period in order to keep the product dl/d.theta.*i constant. If 
the current pulse is lagging the back EMF, then the dl/d.theta. term will 
be past its maximum and decreasing. The current will increase during this 
period. The circuit of the invention senses the current in the coils and 
detects the slope of the current pulse with respect to time during the 
time intervals that the back EMF dominates the voltage-current 
relationship. The resulting signal is proportional to the phase error 
between the rotor position and drive waveforms. 
In one embodiment of the invention a differentiator is used as a slope 
detector. Since only the peak portions of each phase are dominated by the 
back EMF term, the invention utilizes a window generator to mask the slope 
detector during transitional periods where one coil is turned off and the 
next coil is turned on. This is to isolate the slope detector during times 
when the current changes rapidly due to switching transitions from one 
coil to the next. 
In a second embodiment, the slope detector includes a switched inverting 
amplifier that divides the current peak into two equal portions, inverts 
one portion and compares the relative amplitudes of each. The comparator 
output should be zero when the motor is in phase but produces a net 
positive or negative error signal when out of phase. 
In a third embodiment, the slope detector, window generator, proportional 
integral controller (PI controller), VCO, and sequencer may be implemented 
in a digital processor. For the slope detector, the current is digitally 
sampled at the beginning of the window and again at the end. The phase 
error signal is generated as the arithmetic difference between these two 
values. 
A principal object of this invention is to provide a sensorless control 
circuit for a switched reluctance motor. 
Another object of the invention is to provide an essentially noise immune 
control loop for a switched reluctance motor using a minimum number of 
components. 
A further object of the invention is to provide a control loop which senses 
the slope of the currents in the motor coils at a time when back EMF is at 
a maximum to generate a phase error signal indicating whether the motor 
rotation is in phase with, leading or lagging the driving current pulses. 
The foregoing and other objectives, features, and advantages of the 
invention will be more readily understood upon consideration of the 
following detailed description of the invention, taken in conjunction with 
the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
A switched reluctance motor 10 includes a rotor 12 constructed of 
magnetically permeable material such as iron. The rotor 12 is surrounded 
by a plurality of stator poles 14 wrapped with wire coils 16. There are 
three sets of poles 14 labeled A, B and C with each set containing two 
mutually opposed poles. The coils 16 of the pole pairs 14A, 14B and 14C 
are energized by a driver circuit 18 (refer to FIG. 2). The driver 18 is 
controlled by a sequencer 20 which is in turn driven by a voltage 
controlled oscillator 22. A counter 24 coupled to the sequencer 20 
provides an output indicative of rotor position. The sequencer 20 provides 
timing pulses for the driver 18 which in turn drives the motor 10 with a 
three phase driving voltage. 
A feedback line 26 connects the output of the voltage controlled oscillator 
(VCO) 22 with a window generator 28. The purpose of the window generator 
28 is to mask certain portions of the output (or the input) of a slope 
detector 30 which senses the slope of signals proportional to current 
which are sensed through a current sensing resistor 32. The current signal 
from resistor 32 forms an input to the slope detector 30, and the slope 
detector output forms a first input to an AND gate 34. The other input to 
the gate 34 is the output of the window generator 28. The output of the 
gate 34 is coupled to inverting amplifier 36 which is in turn connected to 
a PI controller 38. The output of the PI controller 38 forms the input to 
the VCO 22. The coils 16 for stator poles A, B and C are connected to a 
common node 40 to which current sensing resistor 32 is also connected. The 
current signal at node 40 is thus the sum of the currents flowing in the 
coils 16 at any point in time. 
The output of the PI controller 38 will adjust the phase of the VCO 22 
until the phase error signal is zero. This occurs when the rotor rotation 
and the driver 18 are in phase. In certain applications it is desirable to 
control the driver 18 to lead the motor 10 at high speeds. This allows the 
current to rise to a higher level before the back EMF dominates the 
voltage-current relationship. This produces greater output torque and 
power at the cost of a small decrease in efficiency. 
This feature can be implemented by adding velocity feed forward. The input 
to the VCO 22 is indicative of motor velocity. It can be fed forward 
through a gain element K.sub.ff and subtracted from the phase error 
signal. The gain element could be a simple resistor. At low speeds, this 
term is negligible and the driver will be in phase with the rotor 
rotation. At high speeds, the velocity feed forward becomes significant 
and the PI controller 38 will adjust the phase of the VCO 22 until the 
difference between the phase error and the velocity feed forward term is 
zero. Thus, the driver will lead the motor which produces the required 
negative phase error signal. 
Waveform diagrams shown in FIGS. 5, 6 and 7 illustrate the operation of the 
invention, for the cases phase alignment, phase lag and phase lead 
respectively. A clock pulse train generated by the sequencer 20 causes the 
driver 18 to produce driving voltages V.sub.a, V.sub.b and V.sub.c. These 
voltage pulses are impressed upon the coils sequentially to form current 
pulses I.sub.a0, I.sub.b0, and I.sub.c0 where a, b and c stand for the 
three motor coil pairs. The next waveform represents the sum of the 
current pulses at node 40. It is this signal which is sensed by the 
current sensing resistor 32 and provided as an input to the slope detector 
30. The next waveform in FIG. 5 represents the derivative with respect to 
time of the current sum signal and is the output of the slope detector 30. 
The window generator 28 generates pulses which mask the derivative signal 
during times when it is rapidly fluctuating due to transitions from one 
coil to the next by the driver 18. When the window generator signal is 
slow the output of AND gate 34 is blocked. The result is shown in the 
bottom-most line of FIG. 5 where the output is essentially zero because 
the driving voltage and the back EMF are in phase. It should be recalled 
that the signal generated by back EMF is dependent upon rotor position. 
When the rotor position and the driving voltage pulses are out of phase, 
the output of gate 34 is of the form shown in the bottommost waveform of 
FIG. 6 or FIG. 7. The positive or negative pulses representing either the 
lagging or leading of the rotor position with respect to the driver pulses 
are inverted in amplifier 36 and integrated by PI controller 38. The 
result is an analog voltage that controls the frequency of the VCO 22. 
The preferred form of the slope detector is shown in FIG. 3. The current 
sum signal from resistor 32 is applied to a differentiator circuit 30 
comprising resistor R1 connected in series with capacitor C1 which is in 
turn connected in series with amplifier U8A. The output of amplifier U8A 
is coupled through resistor R13 to gate circuit 34 comprising buffer 
amplifier U9A and switch S2. The switch S2 is connected in parallel with 
the positive input to amplifier U9A. This input is grounded whenever 
switch S2 is closed. Switch S2 is closed when the window signal provided 
by window generator 28 is low. This periodically masks the output of the 
slope detector 30 and the portions of its output that are passed to the PI 
controller 38 are those where the slope is determined by the back EMF. 
Nonlinearities in the slope signal which occur around transitions in the 
driving signal from one coil to the next are eliminated. 
A second form of the slope detector is shown in FIG. 4. According to this 
embodiment of the invention, the detection of the slope of the current sum 
signal takes place after masking by the window generator. Thus, to 
reconcile the embodiment of FIG. 4 with the schematic diagram of FIG. 2 
the connections between gate 34 and slope detector 30 would be reversed. 
Referring to FIG. 4 the current sum signal is applied through a resistor 
R27 to an amplifier U11A which functions as a buffer. This signal is 
grounded by switch SW1 which is driven by a signal from the window 
generator. The pulsed output of the buffer amplifier U11A is applied to an 
inverting amplifier U11B. The positive input to amplifier U11B is 
connected through a switch SW2 to ground. The switch SW2 is controlled by 
a gate U5C which includes a clock input which operates at the frequency of 
the VCO and a signal derived from the window generator circuit which 
operates at twice the frequency of the VCO. This causes switch SW2 to 
effectively divide each pulse from the output of amplifier U11A into 
inverted and noninverted portions. When the rotor position and the motor 
driver are in phase alignment the positive and negative portions of each 
detected slope pulse are the same but when the signal either leads or lags 
the rotor position the portions become unequal, thus generating a phase 
error signal which is integrated to become an analog control voltage for 
the VCO. 
Alternative structures for the various components disclosed herein may be 
used without departing from the spirit of the invention. Other sensing 
methods may be employed to sense a signal proportional to current, and 
other slope detection methods may also be used to determine the phase 
error between the driver pulses and rotor position. 
The terms and expressions which have been employed in the foregoing 
specification are used therein as terms of description and not of 
limitation, and there is no intention, in the use of such terms and 
expressions, of excluding equivalents of the features shown and described 
or portions thereof, it being recognized that the scope of the invention 
is defined and limited only by the claims which follow.