Method of determining a position of a rotor of a brushless permanent magnet motor
A method of determining a position of a rotor of a brushless permanent magnet motor includes measuring a current value indicative of current flowing through a phase winding of the motor and providing a reference voltage value indicative of a voltage applied to the phase winding of the motor. The method includes calculating a phase of back EMF induced in the phase winding using the measured current value and the reference voltage value, and determining a zero-crossing point of the back EMF induced in the phase winding using the calculated phase of back EMF induced in the phase winding. The method includes generating a rotor position signal based on the determined zero-crossing point.
1. A method of determining a position of a rotor of a brushless permanent magnet motor, the method comprising:
measuring a current value indicative of current flowing through a phase winding of the motor during excitation of the phase winding,
providing a reference voltage value indicative of a voltage applied to the phase winding of the motor during excitation of the phase winding,
calculating a phase of back EMF induced in the phase winding using the measured current value and the reference voltage value,
determining a zero-crossing point of the back EMF induced in the phase winding using the calculated phase of back EMF induced in the phase winding, and
generating a position signal of the rotor based on the determined zero-crossing point, and
wherein the phase of back EMF induced in the phase winding is calculated using the equation:
-
E
p
h
X
∝
I
p
h
X
R
p
h
X
+
(
L
self
phX
-
L
mutual
phX
)
dI
p
h
X
d
t
-
V
p
h
X
;
where E phX is the back EMF induced in the phase winding X, L selfphX is the self-inductance of the phase winding X, L mutualphX is the mutual inductance of the phase winding X with other phase windings of the motor, I phX is the current value indicative of current flowing through the phase winding X, R phX is the resistance of the phase winding X, and V phX is the reference voltage value indicative of the voltage applied to the phase winding X.
2. The method as claimed in claim 1 , wherein determining a zero-crossing point of the back EMF induced in the phase winding comprises utilising any of a calculated phase of back EMF induced in the phase winding, an amplitude representative of the amplitude of back EMF induced in the phase winding, and a frequency representative of the frequency of back EMF induced in the phase winding.
3. The method as claimed in claim 1 , wherein calculating a phase of back EMF induced in the phase winding comprises integrating the equation:
-
E
p
h
X
∝
I
p
h
X
R
p
h
X
+
(
L
self
phX
-
L
mutual
phX
)
dI
p
h
X
d
t
-
V
p
h
X
;
to obtain a relationship representative of integrated back EMF.
4. The method as claimed in claim 3 , wherein calculating a phase of back EMF induced in the phase winding comprises equating integrated back EMF to an integral of a sinusoidal waveform representative of back EMF induced in the phase winding.
5. The method as claimed in claim 3 , wherein calculating a phase of back EMF induced in the phase winding comprises equating a relationship representative of integrated back EMF with an integral of a sinusoidal waveform representative of back EMF induced in the phase winding.
6. The method as claimed in claim 1 , wherein the method comprises utilising a determined zero-crossing point of back EMF induced in the phase winding to calculate an electrical period of the rotor.
7. The method as claimed in claim 1 , wherein the method comprises utilising a determined zero-crossing point of back EMF induced in the phase winding to calculate a speed of the rotor.
8. The method as claimed in claim 1 , wherein the method comprises utilising a determined zero-crossing point of back EMF induced in the phase winding to generate a signal representing continuous position of the rotor.
9. The method as claimed in claim 8 , wherein the method comprises controlling a voltage applied to the phase winding of the motor based on the signal representing continuous position of the rotor.
10. The method as claimed in claim 9 , wherein the method comprises controlling the voltage applied to the phase winding of the motor using space vector pulse width modulation.
11. The method as claimed in claim 10 , wherein the method comprises controlling the voltage using 5-step space vector pulse width modulation.
12. The method as claimed in claim 1 , wherein the method comprises obtaining the reference voltage value from a look-up table based on at least one of a measured DC link voltage and a calculated rotor speed.
13. The method as claimed in claim 1 , wherein the reference voltage value comprises a sinusoidal voltage value.
14. The method as claimed in claim 1 , wherein the method comprises calculating a reference back EMF waveform, and calculating an error offset for the determined zero-crossing point of back EMF induced in the phase winding using the reference back EMF waveform.
15. The method as claimed in claim 14 , wherein the calculated reference back EMF waveform is dependent on the reference voltage value.
16. The method as claimed in claim 1 , wherein the method further comprises controlling the brushless permanent magnet motor to obtain a desired rotor power using a closed feedback loop.
17. The method as claimed in claim 1 , wherein the brushless permanent magnet motor comprises a plurality of phase windings, and the method comprises measuring the current value indicative of current flowing through only one phase winding of the motor, providing the reference voltage value indicative of a voltage applied to the only one phase winding of the motor, calculating the phase of back EMF induced in the only phase winding using the measured current value and the reference voltage value, determining the zero-crossing point of the back EMF induced in the only one phase winding using the calculated phase of back EMF induced in the phase winding, and generating the rotor position signal based on the determined zero-crossing point.
18. The method as claimed in claim 1 , wherein the brushless permanent magnet motor comprises a plurality of phase windings, and the method comprises measuring a plurality of current values each indicative of current flowing through a respective phase winding of the motor, providing a plurality of reference voltage values each indicative of a voltage applied to the respective phase winding of the motor, calculating a phase of back EMF induced in each phase winding using the respective measured current value and the respective reference voltage value, determining a zero-crossing point of the back EMF induced in each phase winding using the calculated phase of back EMF induced in the phase winding, and generating a rotor position signal based on the determined zero-crossing points.
19. The method as claimed in claim 1 , wherein the method comprises determining an initial back emf zero-crossing point and an initial rotor speed in an initialisation step, and generating the rotor position signal based on the initial back emf zero-crossing point and the initial rotor speed.
20. The method as claimed in claim 19 , wherein determining the initial back emf zero-crossing point comprises turning off all power switches of the brushless permanent magnet motor, monitoring a phase voltage of the phase winding of the motor, and utilising a zero-crossing point of the phase voltage as the initial back emf zero-crossing point.
21. The method as claimed in claim 20 , wherein determining the initial back emf zero-crossing point comprises turning off all power switches of the brushless permanent magnet motor, monitoring the phase voltage of the phase winding of the motor to determine at least two zero-crossing points of the phase voltage, and utilising the most recent zero-crossing point of the phase voltage as the initial back emf zero-crossing point.
22. The method as claimed in claim 21 , wherein determining the initial rotor speed comprises utilising the most recent zero-crossing point of the phase voltage and a previously determined zero-crossing point of the phase voltage, and calculating a time period between the most recent zero-crossing point of the phase voltage and the previously determined zero-crossing point of the phase voltage.
23. A brushless permanent magnet motor comprising a controller configured to:
obtain a current value indicative of current flowing through a phase winding of the motor during excitation of the phase winding,
obtain a reference voltage value indicative of a voltage applied to the phase winding of the motor during excitation of the phase winding,
calculate a phase of back EMF induced in the phase winding using the measured current value and the reference voltage value,
determine a zero-crossing point of the back EMF induced in the phase winding using the calculated phase of back EMF induced in the phase winding, and
generate a position signal of the rotor based on the determined zero-crossing point, and
wherein the phase of back EMF induced in the phase winding is calculated using the equation:
-
E
p
h
X
∝
I
p
h
X
R
p
h
X
+
(
L
s
e
l
f
p
h
X
-
L
mutualphX
)
dI
p
h
X
dt
-
V
phX
;
where E phX is the back EMF induced in the phase winding X, L selfphX is the self-inductance of the phase winding X, L mutualphX is the mutual inductance of the phase winding X with other phase windings of the motor, I phX is the current value indicative of current flowing through the phase winding X, R phX is the resistance of the phase winding X, and V phX is the reference voltage value indicative of the voltage applied to the phase winding X.
24. A data carrier comprising machine readable instructions for the operation of a controller of a brushless permanent magnet motor to:
obtain a current value indicative of current flowing through a phase winding of the motor during excitation of the phase winding,
obtain a reference voltage value indicative of a voltage applied to the phase winding of the motor during excitation of the phase winding,
calculate a phase of back EMF induced in the phase winding using the measured current value and the reference voltage value,
determine a zero-crossing point of the back EMF induced in the phase winding using the calculated phase of back EMF induced in the phase winding, and
generate a position signal of the rotor based on the determined zero-crossing point, and
wherein the phase of back EMF induced in the phase winding is calculated using the equation:
-
E
p
h
X
∝
I
p
h
X
R
p
h
X
+
(
L
s
e
l
f
p
h
X
-
L
mutualphX
)
dI
p
h
X
dt
-
V
phX
;
where E phX is the back EMF induced in the phase winding X, L selfphX is the self-inductance of the phase winding X, L mutualphX is the mutual inductance of the phase winding X with other phase windings of the motor, I phX is the current value indicative of current flowing through the phase winding X, R phX is the resistance of the phase winding X, and V phX is the reference voltage value indicative of the voltage applied to the phase winding X.