Switched reluctance machine with toothed-wheel rotor sensor

An apparatus and method for estimating rotor position in and commutating a switched reluctance motor (SRM), using both a flux/current SRM angle estimator and a toothed wheel generating a magnetic pickup. Commutation is based on the flux/current SRM at startup and lower speeds. At higher speeds, when magnetic pickup sensed from the toothed wheel is sufficiently large in amplitude to create frequent reliable interrupts, commutation is based on signals from the toothed wheel magnetic pickup. Phase errors can be compensated by adjusting the angle input to the commutator as a function of estimated speed. Alternatively, the flux/current SRM angle estimator can be run in a background mode to tune the toothed wheel interrupt angle signal at different speeds.

FIELD OF THE INVENTION 
The present invention is related generally to switched reluctance machines, 
and, more particularly, to a rotor position estimation and machine 
commutation using a toothed-wheel rotor sensor in combination with a 
flux/current angle estimator. 
BACKGROUND OF THE INVENTION 
Switched reluctance drives are controlled by switching the phase currents 
on and off in synchronism with the rotor-position. Usually, this 
synchronism is achieved by feeding back the rotor position to the 
controller using a shaft position sensor such as a resolver or an optical 
encoder. Shaft sensors are not always sufficiently robust. Considerable 
work has been done on estimating rotor position in switched reluctance 
drives in order to eliminate the shaft position sensor. The most 
comprehensive of these approaches involves the use of either a 
flux/current map, as described in commonly-assigned U.S. Pat. No. 
5,097,190 to J. P. Lyons and S. R. MacMinn on Mar. 17, 1992, (e.g. lookup 
tables) or a solution of a lumped parameter flux/current model, as 
described in commonly-assigned U.S. Pat. No. 5,107,195 to J. P. Lyons, S. 
R. MacMinn, and M. A Preston on Apr. 21, 1992, to determine rotor 
position. Both non-intrusive techniques derive a position estimate from 
stator flux-linkage and current of the torque producing phases. Machine 
stator flux is estimated by integrating the quantity (v-iR). 
In commonly-assigned U.S. Pat. No. 5,325,026 to J. P. Lyons, S. R. MacMinn, 
and A. K. Pradeep on Jun. 28, 1994, these techniques are extended to low 
rotational speeds where the torque-producing current pulses are of 
sufficient duration to allow for significant error to accumulate in the 
flux integrators. These techniques eliminate the need for the shaft 
position sensor by solving models of the magnetic characteristics of the 
switched reluctance machine (SRM) in real-time. However, this can add a 
significant computational burden to the control processor, particularly 
for the high speed applications encountered with aircraft motors and 
generators. The increased computational requirements can lead to the use 
of expensive digital signal processors or the added expense of an 
additional processor dedicated to the shaft position estimation 
algorithms. 
Commonly-assigned U.S. Pat. No. 5,525,886 to J. P. Lyons and M. A. Preston 
on Jun. 11, 1996 describes a method for estimating rotor position of a 
switched reluctance motor during rotor startup of low speed operation by 
applying a sequence of high-frequency, short-duration electric probing 
pulses to at least two inactive excitation phase windings of a multi-phase 
motor. 
SUMMARY OF THE INVENTION 
Accordingly, it would be desirable to provide a simple, low cost, robust 
sensor to be used to control a switched reluctance machine (SRM) in 
hostile high performance environments, e.g., a flight surface actuator 
servomechanism. It would also be useful to provide a fault-tolerant sensor 
with the addition of redundant magnetic pickups. 
This invention uses a microprocessor to provide the commutation timing 
required by the individual SRM phases. The microprocessor based commutator 
uses a rotor state estimator or observer whose input can come from either 
of two sources: a toothed wheel interrupt signal or a flux/current SRM 
angle estimator. The rotor state estimator receives input signals 
including an electrical angle estimate and a sampling time stamp. 
The SRM commutator takes its input signal from the flux/current SRM angle 
estimator when starting the switched reluctance machine, as the angle 
estimation algorithm is capable of operation to standstill. After the SR 
machine is up to a threshold speed where the signal created in the 
magnetic pickup of the toothed wheel sensor is sufficiently large in 
amplitude (signal amplitude being proportional to rotational speed) to 
create frequent reliable interrupts, the SRM commutator then takes its 
input from the toothed-wheel interrupt timing. 
Phase errors can be introduced by the change in pickup signal amplitude 
with speed. This can be compensated by adjusting the angle input to the 
SRM rotor state observer as a function of estimated speed. Alternatively, 
the flux/current SRM angle estimator can be run in a background mode to 
tune the toothed wheel interrupt angle at different speeds.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
FIG. 1 is a block diagram illustrating the present invention. A 
microprocessor 101 is used to provide the commutation timing required by 
the individual SRM phases. A microprocessor-based commutator 102 uses a 
rotor state estimator or observer 103 whose input arrives via a 
multiplexer (MUX) 104 from either of two sources: an interrupt timer 105 
providing a toothed wheel interrupt signal based on magnetic pickup 
readings taken from a toothed wheel rotor 106, or from a flux/current SRM 
angle estimator 107 which in turn receives its input from a sample and 
hold (S&H) circuit 108, the circuit sampling the phase flux-linkages and 
current (.PSI.,I).sub.a,b,c 109 from each of the three phases a,b,c. The 
rotor state estimator 103 receives as input signals an electrical angle 
estimate and a sampling time stamp, and provides as output signals to a 
commutation timer 109 state estimates of angular position .theta., 
velocity co and acceleration .alpha.. The commutation timer, in turn, 
generates appropriate commutation timing signals 110 controlling the three 
phases a,b,c. The state estimator 103, the commutation timer 109, and the 
generation of timing for the three machine phases are described in 
above-referenced U.S. Pat. No. 5,325,026. Appropriate control logic 111 is 
also included within the microprocessor 101. 
The toothed wheel rotor 106 and interrupt timer 105 are optimally organized 
to have one tooth per rotor pole so that one interrupt signal is generated 
by the timer 105 and received by the state estimator 103 via the MUX 104 
per electrical cycle of the SRM. This arrangement creates a fixed 
electrical measurement angle so that the new information for the state 
estimator 103 is contained in the time of interrupt. 
The flux/current SRM angle estimator 107 uses sampled values of phase 
flux-linkages and currents .PSI., I as inputs to a model based algorithm 
to determine an estimate of the rotor angle, as described in 
above-reference U.S. Pat. Nos. 5,325,026, 5,097,190 and 5,107,195. The 
input signal to the rotor state observer 103 via the MUX 104 includes both 
the estimated angle and the sampling time stamp. This sampling can occur 
either synchronous or asynchronous to SRM rotation. Synchronous sampling 
will generally yield the best accuracy results; however, asynchronous 
sampling can be more useful at low rotational speeds. 
When starting the SR machine, the flux/current SRM angle estimator 107 will 
provide an input signal to the SRM commutator 102 because the angle 
estimation algorithm is capable of operation at standstill. After the SR 
machine is up to a threshold speed where the signal created in the 
magnetic pickup sensed from the toothed wheel rotor 106 is sufficiently 
large in amplitude (signal amplitude being proportional to rotational 
speed) to create frequent reliable interrupts, control logic 111 will 
switch MUX 104 over such that the SRM commutator 102 will then be 
controlled by the toothed wheel interrupt timer 105. 
Phase error can be introduced in several different manners. One type of 
phase error results from installation variability (such as mechanical 
alignment or electrical alignment) and is independent of speed. A second 
type of phase error is dependent on speed and results from eddy-currents 
produced by most conventional tooth sensors from magnetic or fixed delays 
in the sensors and electronics signal paths which become more significant 
with increasing speed. Both types of phase errors can be compensated in 
the present invention in either a foreground mode or a background mode. 
Phase errors which are dependent on speed can be compensated by adjusting 
the angle input to the SRM rotor state observer 103 as a function of 
estimated speed. A motor including this type of phase compensation will 
not be dependent on speed and thus will provide an advantage over other 
approaches which would have to perform a compensation as a function of 
speed. 
Phase errors which are dependent on installation variability can be 
compensated by running the flux/current SRM angle estimator 107 to tune 
the toothed wheel interrupt angle generated by the interrupt timer 105 at 
different speeds A motor including this type of phase compensation will be 
easier to install and service than motors that do not compensate for 
installation variability because mechanical and electrical alignment will 
not be critical. 
Saliency in both the rotor and stator of a switched reluctance motor causes 
the SRM to have an airgap of varying effective area. Thus, the phase 
inductance seen from the terminals of the stator phase windings is a 
strong function of rotor position. The current in one phase winding of a 
switched reluctance motor and the flux linked by that winding are related 
by the winding inductance through the relationship .PSI.=L i . The flux 
current methods for SRM rotor position estimation exploit the inherent 
magnetic characteristics of the SRM flux path to infer the rotor angular 
position. 
For each SRM phase the stator flux-linkage is estimated by 
.PSI.=.intg.V-IR, where V is the applied phase voltage, I the phase 
current, and R the winding resistance. Then, given estimated V and 
measured I, the rotor position relative to alignment for each of the SRM 
phases can be obtained from the magnetic characteristic as illustrated in 
FIG. 2, which depicts rotor angle .theta.(.degree.) (in electrical 
degrees) versus phase excitation F (A-t), wherein (A-t) denotes 
(Ampere-turns), for different values of phase flux (.PSI.).sub.i, over i 
motor phases generally, where i=a,b,c for the particular three-phase 
embodiment illustrated herein, as described in above-referenced U.S. Pat. 
Nos. 5,097,190 and 5,107,195. This non-intrusive method monitors the 
normal torque-producing voltage and current waveforms in order to infer 
the rotor position. Additional logic then chooses the best available 
relative angle measurement and subsequently translates the relative rotor 
angle obtained from the magnetic characteristic into the absolute rotor 
angle for commutation control of the SRM. 
This flux-current map technique for determination of SRM rotor angle, 
however, utilizes a single-phase magnetic characteristic as an underlying 
model. This model assumes that only the sensing phase is conducting 
current or that mutual coupling effects between conducting phases are 
negligible. For many applications neither of these assumptions is valid. 
FIG. 3, therefore, presents a lumped-parameter model for a 3-phase switched 
reluctance machine which does account for mutual coupling between phases. 
In order to predict the rotor angle using this multi-phase model it is 
necessary to sample all phase currents and flux-linkage estimates 
simultaneously and then solve the reluctance mesh equations to isolate the 
gap-tip reluctance terms (R.sub.gt (.theta.,.phi.)).sub.i. The gap-tip 
reluctance function, at a known rotor flux level flux, can then be 
inverted to yield the relative angle to alignment .theta..sub.i for each 
of the stator poles. This lumped-parameter model is described in 
above-referenced U.S. Pat. Nos. 5,107,195 and 5,525,886. 
The inverse gap-tip reluctance function will most commonly be stored as a 
two-dimensional characteristic as in FIG. 4, which depicts rotor angle 
.theta. from alignment versus reluctance (A-t/Wb) for different values of 
flux .PHI.. The optimal absolute rotor position estimate is again obtained 
via post-processing logic. 
Both the single and multi-phase flux current methods utilize a voltage 
integrator to estimate the stator flux-linkage. This estimation technique 
is particularly effective in the switched reluctance machine because the 
flux returns to zero at each cycle, allowing the integrator to be reset 
and thus preventing large error accumulation. However, the integrator 
accumulates offset errors in the measurement path, thereby limiting the 
low-frequency performance of these position estimator techniques, which 
effectively precludes their use at very low machine speeds. 
For low speed position estimation utilizing the Flux/Current Method, high 
frequency voltage pulses are applied to inactive machine phases in order 
to circumvent the low frequency limitations of the flux-linkage 
integrators. The voltage pulses are designed to be long enough in duration 
to ensure measurable current and to allow high-frequency eddy currents to 
decay down, while also being short enough in duration to prevent 
integrator windup and prevent significant torque production by the phases 
being probed. The on-to-off duty cycle ratio of the pulses can be chosen 
to ensure that the current and flux will decay to zero prior to an ensuing 
test pulse, thus allowing the flux integrators to be periodically reset to 
a known value (i.e., zero). 
This high frequency probing technique can be made very accurate in that for 
short duration, high-voltage pulses the IR drop term is essentially 
negligible, thus minimizing the influence of stator resistance variations 
and the integrator offset error accumulation. 
To obtain initialization data for the rotor, SRM control logic 111 can 
either initially locate the rotor (assuming the machine is stopped) or 
begin to track a rotor that is already rotating (e.g. an SR generator). 
This can be achieved by applying a sequence of synchronized high frequency 
test pulses to all motor phases. At the end of the voltage-on cycle the 
flux-linkage and current for each of the phases is sampled in response to 
a command from control logic 111 and the rotor position calculated using 
either the single or multi-phase flux-current technique, as described 
earlier. A short sequence of such test pulses is sufficient to establish 
the initial operating state of the SRM (i.e. rotor position and velocity). 
However, if the rotor comprises one tooth per pole as in a preferred 
embodiment, its direction of rotation is known, and it is spinning 
sufficiently quickly, the toothed wheel interrupts can be immediately used 
to establish and initialize the rotor observer position and velocity. 
Once the rotor is initially located, the SRM is started through a sequence 
of torque producing current pulses applied sequentially to the appropriate 
machine phases. For the initial few cycles the rotor speed and hence 
stator electrical frequency are too low to rely on the flux integrators 
for these torque producing phases. In order to locate the rotor as it 
begins to spin, the non-torque producing phases can be probed with 
synchronized high-frequency voltage pulses. At the end of the voltage-on 
cycle the flux-linkage and current for each of the probed phases is 
sampled and the rotor position inferred using either the single or a 
modified multi-phase flux-current technique. The multiphase technique can 
be modified in order to ignore the unknown flux-linkage corresponding to 
the torque-producing phase(s)--this is done by assuming that the unknown 
flux-linkage is zero thus effectively reducing the multi-phase coupled 
reluctance model (FIG. 3) to a single phase reluctance model. 
FIG. 5 is an oscilloscope trace of a 3 phase SRM starting using this 
technique, in the manner discussed in the above-referenced U.S. Pat. Nos. 
5,525,886 and 5,097,190. There is an initialization sequence of twenty 
probing pulses on all three phases, at the conclusion of which control 
logic 111 initiates a starting sequence by energizing phase B. The SRM 
accelerates while the high frequency probing pulses continue on the two 
non-torque producing phases. In this trial the probe voltage pulses are 
200 vdc, 100 .mu.s on and 300 .mu.s off. This mode of operation continues 
until the rotor reaches a sufficient transition speed where the integrator 
errors are tolerable, whereupon the intrusive probing is stopped. If the 
SRM drive operates continuously at low speeds as in a servomechanism 
application, controller logic 111 should be capable of initiating these 
probing pulses whenever rotor speed falls below the transition speed. 
FIG. 6 illustrates both the motoring torque-producing regions and 
sensing/probing regions superimposed on an idealized inductance vs. rotor 
angle characteristic for a 3 phase SRM. During starting, one phase at a 
time is excited with 120 (electrical) degrees for torque production. The 
remaining two phases are available for probing pulses. Each of the optimal 
sensing regions are thus 120 degrees in width centered at 90 degrees 
relative to phase alignment. The three contiguous regions provide 
continuous sensing opportunities throughout the electrical cycle as 
described in the above-referenced U.S. Pat. Nos. 5,525,886 and 5,097,190. 
Commutation control of an electronically commutated machine is achieved 
with shaft position information available at discrete time instants, which 
are not, in general, the required commutation times. Such commutation 
means are used for microprocessor-based commutation schemes, in 
particular, for machine commutation schemes based on inferred shaft 
position information. 
The filter and state observer 103 provide estimates of the mechanical 
states of the rotating machine i.e. angular position, velocity and/or 
acceleration. In one embodiment, a Kalman filter of variable gain is used 
for estimating the mechanical states. In another embodiment, a sliding 
mode observer is used for estimating the mechanical states. In a preferred 
embodiment, shown in FIG. 7, a combination of a sliding mode observer and 
a constant gain Kalman filter is used to obtain the mechanical state 
estimates--the Kalman filter with small constant gains provides smooth 
tracking of a steadily rotating machine while the sliding mode observer 
provides fast acquisition during transient conditions, as described in 
above-referenced U.S. Pat. No. 5,325,026. 
The mechanical state estimates are translated into machine phase 
commutation signals. In one embodiment, depicted in FIG. 8 and described 
in above-referenced U.S. Pat. No. 5,325,026, a phase-locked-loop, closed 
in software, forces a hardware counter to track and interpolate between 
the discrete time position estimates. This counter's output emulates the 
output of an R/D converter and can be directly used in a variety of 
hardware commutation schemes, as described in above-referenced U.S. Pat. 
No. 5,325,026. 
In a preferred embodiment, FIG. 9, commutation control logic 111 software 
translates the desired commutation angles and mechanical state estimates 
into commutation event times. These commutation event times are then 
loaded into hardware counters which will trigger actual phase commutation 
upon expiration, as described in U.S. Pat. No. 5,325,026. 
While only certain preferred features of the invention have been 
illustrated and described, many modifications and changes will occur to 
those skilled in the art. It is, therefore, to be understood that the 
appended claims are intended to cover all such modifications and changes 
as fall within the true spirit of the invention.