Gain adjustment to improve torque linearity in a field weakening region
View Patent ↗Systems and methods are disclosed to improve torque linearity of an electric machine when operating in a field-weakening region. The systems and methods adjust the q-axis and the d-axis components of the stator current commands of the electric machine using a flux weakening control loop and a torque linearity control loop such that torque linearity is maintained when the machine operates in a field weakening region of operation.
1. A control system for an electric machine, the control system comprising:
a current regulator module designed to generate a first voltage command signal, and a second voltage command signal;
a field weakening module designed to generate a first adjusting current command signal based on the first voltage command signal and the second voltage command signal; and
a torque linearity module designed to generate a second adjusting current command signal based on the first adjusting current command signal, a torque command signal and a first limited current command signal, wherein the torque linearity module further comprises:
a derivative gain module having a derivative gain that is computed based on the torque command signal and the first limited current command signal, wherein the first adjusting current command signal is multiplied by the derivative gain to generate a raw second adjusting current command signal; and
a limiter module, coupled to the gain module, and designed to generate the second adjusting current command signal by limiting the raw second adjusting current command signal within limits set by the limiter module.
2. The system according to claim 1 , wherein the derivative gain of the derivative gain module is a function of the torque command signal and a first adjusted current command signal.
3. The system according to claim 2 , wherein the derivative gain of the derivative gain module is computed based on the equation:
ⅆ
I
Q
ⅆ
I
D
=
4
3
P
×
T
*
×
(
L
Q
-
L
D
)
[
λ
+
I
D
**
*
×
(
L
D
-
L
Q
)
]
2
,
wherein P is the number of poles of the electric machine, T* is the torque command signal, wherein I D *** is the first limited current command signal, wherein L D and L Q are a first inductance and a second inductance of the electric machine, respectively, and wherein λ is a flux linkage constant.
4. The system according to claim 1 , further comprising:
a current command look-up table module that receives the torque command signal, a rotor angular velocity, and a DC-link voltage, and that generates a first current command signal and a second current command signal.
5. The system according to claim 4 , further comprising:
a first adder designed to generate a first adjusted current command signal based on the first current command signal and the first adjusting current command signal; and
a second adder designed to generate a second adjusted current command signal based on a second current command signal and the second adjusting current command signal.
6. The system according to claim 5 , further comprising:
a phase current limiter module that receives a DC-link voltage, and a rotor angular velocity of the electric machine, and that limits the first adjusted current command signal to generate the first limited current command signal that is provided to the current regulator module, and limits the second adjusted current command signal to generate a second limited current command signal that is provided to the current regulator module.
7. A control system for an alternating current (AC) electric machine, the control system comprising:
a current regulator module designed to generate a d-axis voltage command signal, and a q-axis voltage command signal based on a feedback d-axis current signal, a feedback q-axis current signal, a limited d-axis current command signal, and a limited q-axis current command signal;
a field weakening module designed to generate a d-axis adjusting current command signal based on the d-axis voltage command signal and the q-axis voltage command signal;
a first adder designed to generate an adjusted d-axis current command signal based on a d-axis current command signal and the d-axis adjusting current command signal;
a torque linearity module designed to generate a q-axis adjusting current command signal based on the d-axis adjusting current command signal, a torque command signal and the adjusted d-axis current command signal; and
a second adder designed to generate an adjusted q-axis current command signal based on a q-axis current command signal and the q-axis adjusting current command signal.
8. The system according to claim 7 , wherein the torque linearity module further comprises:
a derivative gain module having a derivative gain that is computed based on the torque command signal and the limited d-axis current command signal, wherein the d-axis adjusting current command signal is multiplied by the derivative gain to generate a raw q-axis adjusting current command signal; and
a limiter module, coupled to the gain module, and designed to generate the q-axis adjusting current command signal by limiting the raw q-axis adjusting current command signal within limits set by the limiter module.
9. The system according to claim 8 , wherein the derivative gain of the derivative gain module is a function of the torque command signal and the adjusted d-axis current command signal.
10. The system according to claim 9 , wherein the derivative gain of the derivative gain module is computed based on the equation:
ⅆ
I
Q
ⅆ
I
D
=
4
3
P
×
T
*
×
(
L
Q
-
L
D
)
[
λ
+
I
D
**
*
×
(
L
D
-
L
Q
)
]
2
,
wherein P is the number of poles of the electric machine, T* is the torque command signal, wherein I D *** is the first limited current command signal, wherein L D and L Q are a first inductance and a second inductance of the electric machine, respectively, and wherein λ is a flux linkage constant.
11. The system according to claim 7 , further comprising:
a current command look-up table module that receives the torque command signal, a rotor angular velocity, and a DC-link voltage, and that generates the d-axis current command signal and the q-axis current command signal.
12. The system according to claim 7 , further comprising:
a phase current limiter module that receives a DC-link voltage, and a rotor angular velocity of the electric machine, and that limits the adjusted d-axis current command signal to generate the limited d-axis current command signal, and limits the adjusted q-axis current command signal to generate the limited q-axis current command signal.
13. A method for controlling an electric machine, the method comprising:
generating a first voltage command signal, and a second voltage command signal based on a first feedback current signal, a second feedback current signal, a first limited current command signal, and a second limited current command signal;
generating a first adjusting current command signal based on the first voltage command signal and the second voltage command signal; and
generating a second adjusting current command signal based on the first adjusting current command signal, a torque command signal and a first adjusted current command signal.
14. The method according to claim 13 , wherein the step of generating a second adjusting current command signal, comprises:
computing a derivative gain based on the torque command signal and the first limited current command signal;
determining a product of the first adjusting current command signal and the derivative gain to generate a raw second adjusting current command signal; and
limiting the raw second adjusting current command signal to generate the second adjusting current command signal.
15. The method according to claim 14 , wherein the derivative gain of the derivative gain module is computed based on the equation:
ⅆ
I
Q
ⅆ
I
D
=
4
3
P
×
T
*
×
(
L
Q
-
L
D
)
[
λ
+
I
D
**
*
×
(
L
D
-
L
Q
)
]
2
,
wherein P is the number of poles of the electric machine, T* is the torque command signal, wherein I D *** is the first limited current command signal, wherein L D and L Q are a first inductance and a second inductance of the electric machine, respectively, and wherein λ is a flux linkage constant.
16. The method according to claim 13 , further comprising:
receiving the torque command signal, a rotor angular velocity, and a DC-link voltage; and
generating, based on the torque command signal, the rotor angular velocity, and the DC-link voltage, a first current command signal and a second current command signal.
17. The method according to claim 16 , further comprising:
adding the first current command signal to the first adjusting current command signal to generate the first adjusted current command signal; and
adding the second current command signal to the second adjusting current command signal to generate a second adjusted current command signal.
18. The method according to claim 17 , further comprising:
receiving the DC-link voltage, and the rotor angular velocity of the electric machine;
limiting the first adjusted current command signal based on the DC-link voltage, and the rotor angular velocity to generate the first limited current command signal; and
limiting the second adjusted current command signal based on the DC-link voltage, and the rotor angular velocity to generate the second limited current command signal.