System for measuring the position of an electric motor

Details System for measuring the position of an electric motor System for measuring the position of an electric motor

2002-04-12

2004-03-09

Duda, Rina I. (Department: 2837)

Electricity: motive power systems

Automatic and/or with time-delay means

Terminal voltage or counter-electromotive force of...

C318S500000, C318S254100, C318S132000, C318S434000, C388S907200, C388S928100, C324S207130, C324S207190

Reexamination Certificate

active

06703805

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to electronic monitoring of electric motor position by utilizing a bridge amplifier circuit to measure the ratio of impedances between two motor windings, or legs of a single winding. The measurements made can control commutation or provide position and velocity feedback to a control system.
REFERENCES CITED
U.S. Patent Documents
U.S. Pat. No. 3,931,553 (January/1976) to Stich et al.
U.S. Pat. No. 4,005,347 (January/1977) to Erdman
U.S. Pat. No. 4,027,212 (May/1977) to Studer
U.S. Pat. No. 4,092,572 (May/1978) to Murata
U.S. Pat. No. 4,495,450 (January/1985) to Tokizaki et al.
U.S. Pat. No. 4,654,566 (March/1987) to Erdman
U.S. Pat. No. 4,746,844 (May/1988) to MacKelvie et al.
U.S. Pat. No. 4,758,768 (July/1988) to Hendricks et al.
U.S. Pat. No. 4,882,524 (November/1989) to Lee
U.S. Pat. No. 5,191,270 (March/1993) to McCormack
U.S. Pat. No. 5,192,900 (March/1993) to Ueki
U.S. Pat. No. 5,304,902 (April/1994) to Ueki
U.S. Pat. No. 5,327,053 (July/1994) to Mann et al.
U.S. Pat. No. 5,350,987 (September/1994) to Ueki
U.S. Pat. No. 5,751,125 (May/1998) to Weiss
U.S. Pat. No. 5,821,713 (October/1998) to Holling et al.
U.S. Pat. No. 5,864,217 (January/1999) to Lyons et al.
U.S. Pat. No. 5,990,642 (November/1999) to Park
U.S. Pat. No. 6,169,354 (February/2001) to Springer et al.
U.S. Pat. No. 6,288,514 (September/2001) to Direnzo et al.
U.S. Pat. No. 6,304,045 (October/2001) to Muszynski
Other Publications
Conference Record of the IEEE Industry Applications Meeting
(1999, p. 143), “Review of Sensorless Methods for Brushless DC”
IEEE Transactions on Industry Applications, Volume
28
Issue
1, (January/February 1992. p. 120), “Brushless DC motor control without position and speed sensors”
IEEE Transactions on Industry Applications, Volume
30
Issue
1, (p 85, January/February 1994, p. 85), “New modulation encoding techniques for indirect rotor position sensing in switched reluctance motors”
Design News
, (Apr. 8, 1991), “Brushless DC Motors Yield Design Payoffs”
Conference Record of the IEEE Industry Applications Meeting
(1990, p. 443), “An approach to Position Sensorless Drive for Brushless DC Motors”
IEEE Transactions on Industry Applications, V
37
Issue
1, “Eliminating Starting Hesitation for Reliable Sensorless Control of Switched Reluctance Motors”
Conference Record of the IEEE Industry Applications Meeting
(1999, p 151), “Sensorless Brushless DC Control Using A Current Waveform Anomaly”
Conference Record of the IEEE Industry Applications Meeting
(1997), “Initial Rotor Angle Detection of a Non-Salient Pole Permanent Magnet Synchronous Machine”
BACKGROUND OF THE INVENTION
Many types of electrical motors are known. All electrical motors have a stator and a moving component. In rotary motors the moving component is called a “rotor”. In linear motors the moving component is typically called a “slider”. This invention applies to all electrical DC motors, including linear motors. For simplicity, the term “rotor” is used here to refer to the moving component of all motors, and it is understood that the term “rotor” also comprises “sliders”.
FIG. 1
illustrates one type of electric motor. At the center of the motor is the rotor
1
which is the moving part of the motor. The rotor contains eight permanent magnets
2
arranged as shown so that a sequence of alternating North and South magnetic poles are exposed along the outer rim. In this drawing the rotor is shown to be rotating in a counterclockwise direction.
Surrounding the rotor is the stator
3
, which is stationary. The stator is made up of twelve electromagnets
4
, divided up into three phases A, B, and C. All four electromagnets of phase A are driven together by the same electrical signal, and likewise for phases B and C. The apparatus to drive the three phases of electrical current is outside the motor and not shown in FIG.
1
. This example motor would be termed a three-phase, eight-pole, brushless DC motor.
The principle of operation of the motor is shown in
FIG. 1
a
, which is an expanded view of the bottom right quadrant of FIG.
1
. The other three quadrants work identically to the one shown and are omitted from the drawing for clarity. At the moment shown, phase A is “off” (not being driven with current), thus phase A winding has no magnetic field around it. Phase B is driven with current such that a magnetic North pole is induced on the side next to the rotor. Similarly, phase C is driven in series with B to induce a South pole next to the rotor.
Two of the rotor magnets
5
,
6
are shown. At this moment, rotor magnet
5
is positioned between the phase B electromagnet and the phase C electromagnet. The South pole on the phase C electromagnet attracts the North pole on magnet
5
, while the North pole on the phase B electromagnet repels North pole on magnet
5
. These magnetic forces both act to push magnet
5
to the right, thereby imparting counterclockwise torque to the rotor. This force makes the motor rotate. At this time, rotor magnet
6
is positioned near the phase A electromagnet that is switched off, so there is little or no magnetic force on magnet
6
.
As the rotor moves counterclockwise, the magnet
5
moves to the right until it reaches a position adjacent to electromagnet C, where the magnetic forces from windings B and C are no longer effective to push it along. To maintain motor torque at this rotor position, the pattern of currents through the phase electromagnets must be changed. Phase C will then be switched off, and phase A will be driven such that it generates a South pole next to the rotor. At this time, rotor magnet
6
is positioned between electromagnet A and B so magnet
6
is subject to forces pushing it to the right, similarly to the situation with magnet
5
earlier. Since magnet
5
is now next to phase electromagnet C and phase C is switched off, there will be no effective force on magnet
5
in this position.
As the rotor moves still further, a new rotor magnet will move into this quadrant, this one with a North pole exposed at the rim. Following that, another magnet with a South pole will move in. After 90 degrees of rotation, the situation will again appear the same as it did at the start, with a South pole on the rotor adjacent to phase winding A. During this 90 degree rotation, the phase currents will have been switched six times to keep the magnetic field applied appropriately for the rotor magnet positions. This sequence of six states is termed “360 electrical degrees” and is repeated for each pole pair, so we see that for an eight-pole motor, 360 electrical degrees correspond to 90 mechanical degrees of rotation. The process of switching phases to correspond to rotor position is called “commutation”.
FIG. 1
b
illustrates the commutation sequence for the above example, showing the correspondence of rotor position to phase winding state. Phase A begins in the “off” position and phase B is not being driven, as described previously. The sequence of driving phase A, B, C is depicted showing the magnetic poles induced (North or South). The movement of the rotor is also shown relative to the phase A starting position.
The commutation process described above is a simple switching process sometimes called “trapezoidal drive” of a motor. It is sometimes desirable to use a more complex driving method where the motor windings are driven with an arbitrary analog waveform such as a sinusoid, rather than simply being switched on and off. This is often termed “sinusoidal drive”.
The example motor in
FIGS. 1
,
1
a
represents one common configuration for a rotary motor. Many other configurations are in common use. For example, the rotor magnets may be made up of electromagnets instead of permanent magnets; in this case the stator may or may not use permanent magnets. Other possible configurations include having the stationary stator inside the rotor or alongside it in the axial direction. The disclosed invention can be applied in all these configurations.
FIG. 1
c
illustrates an example of a linear motor. It has a three-phase stator
8
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