Motor control system for determining a reference d-axis current and a q-axis current

The present invention relates to a control system for a motor, and more particularly to a control system for a motor that determines a reference d-axis current and a reference q-axis current.

The output torque of an interior permanent magnet synchronous motor (IPMSM) may be determined by a voltage command and a phase advance angle. A specific output torque of the IPMSM is determined by first selecting a specific quadrature axis (also referred to as the q-axis) reference current and a direct axis (also referred to as the d-axis) reference current, and then determining the voltage command and the phase advance angle based on the selected quadrature axis reference current and the direct axis reference current.

In one embodiment, a motor control system for determining a reference d-axis current and a reference q-axis current is provided. The motor control system includes a motor having measured motor speed, a DC power source and DC input lines, and a current command controller. The DC power source generates a bridge voltage across the DC input lines. The current command controller is in communication with the motor and the DC input lines. The current command controller is configured to monitor the bridge voltage and a torque reference command. The current command controller is configured to calculate the reference q-axis current based on a torque reference command. The current command controller is configured to calculate the reference d-axis current based on a magnitude of the reference q-axis current.

In another embodiment, a method for determining a reference d-axis current and a reference q-axis current is provided. The method includes monitoring a bridge voltage and a torque reference command sent to a current command controller. The method also includes calculating the reference q-axis current based on the torque reference command. The method also includes calculating the reference d-axis current based on a first region of operation if a magnitude of the reference q-axis current is less than or equal to a predetermined reference q-axis current value, and if a magnitude of a voltage command value is less than the bridge voltage.

These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings.

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a block diagram of a motor control system in accordance with an exemplary embodiment of the invention; and

FIG. 2 is a process flow diagram for determining a reference d-axis current and a reference q-axis current, in accordance with another exemplary embodiment of the invention.

Referring now to the Figures, where the invention will be described with reference to specific embodiments, without limiting same, FIG. 1 is an exemplary block diagram of a motor control system 10 . The motor control system 10 includes a current command controller 20 , a d-axis proportional plus integral gain (PI) controller 22 , a q-axis PI controller 23 , a polar conversion controller 24 , a pulse width modulation (PWM) inverter controller 26 , an inverter 28 , a DC power source 30 , a motor 32 , a position sensor 34 , a speed sensor 36 , a transform controller 38 , an a-axis current amplifier 40 , a b-axis current amplifier 42 , an a-axis analog to digital converter (ADC) 44 , and a b-axis ADC 46 . In one embodiment, the motor 32 may be an interior permanent magnet synchronous motor (IPMSM), however it is to be understood that any type of electric motor that is controlled using phase current may be used as well.

In the embodiment as shown in FIG. 1 , the inverter 28 is connected to the DC power source 30 , where the DC power source 30 may be, for example, a battery. The DC power source 30 may be connected to the inverter 28 by DC input lines 49 . A transducer 51 may be used to monitor a bridge voltage V ecu across the DC input lines 49 . A control signal 53 representing the bridge voltage V ecu may be sent to the current command controller 20 and the PWM inverter controller 26 . In the exemplary embodiment as shown, the inverter 26 transmits three alternating current (AC) phase currents to the motor 32 (e.g., i a , i b , and i c ) by line 50 , line 52 , and line 54 for operation and control of the motor 32 .

For feedback control purposes, the phase currents i a and i b transmitted to the motor 32 by lines 50 and 52 may be detected to determine the instantaneous current flow to the motor 32 . Specifically, a transducer 56 may be used to monitor the phase current i a on the line 50 , and a transducer 58 may be used to monitor the phase current i b on the line 52 . It should be noted that although transducer 56 and transducer 58 are illustrated, only one of the lines 50 or 52 may be monitored to measure either phase current i a or phase current i b . A control signal 60 representing the measured phase current i a may be sent to the a-axis current amplifier 40 from the transducer 56 , and a control signal 62 representing the measured phase current i b may be sent to the b-axis current amplifier 42 from the transducer 58 . An augmented or amplified value of the phase current i a is then sent to the a-axis ADC 44 from the a-axis current amplifier 40 , and an amplified value of the phase current i b 62 is sent to the b-axis ADC 46 from the b-axis current amplifier 42 . The a-axis ADC 44 converts the amplified value of the phase current i a into a digital value 64 . The digital value 64 represent the magnitude of the phase current i a . The b-axis ADC 46 converts the amplified value of the phase current i b into a digital value 66 . The digital value 66 represents the magnitude of the phase current i b .

The transform controller 38 receives as input the digital value 64 from the ADC 44 and the digital value 66 from the ADC 46 . In one embodiment, the transform controller 38 is a three-phase to two-phase transformation controller where measured values for the AC current (e.g., the digital value 64 representing the phase current i a and the digital value 66 representing the phase current i b ) are converted into equivalent measured DC current components, which are a measured d-axis current I dMEASURED and a measured q-axis current I qMEASURED . The measured d-axis current I dMEASURED is sent to a subtractor 70 and the measured q-axis current I qMEASURED is sent to a subtractor 72 .

The current command controller 20 receives as input a torque reference command T e , an angular speed ω m , and the control signal 53 representing the bridge voltage V ecu from the transducer 51 . The torque reference command T e represents a commanded torque value, and may be derived from another controller (not shown), or may correspond to a torque value generated by an operator. The angular speed ω m is measured by the speed sensor 36 . The speed sensor 36 may include, for example, an encoder and a speed calculation circuit for calculating the angular speed of a rotor (not shown) of the motor 32 based on a signal received by the encoder. The current command controller 20 calculates a reference d-axis current I d _ REF and a reference q-axis current I q _ REF based on the torque command T e , the bridge voltage V ecu , and the angular speed ω m , which is described below. The reference d-axis current I d _ REF is sent to the subtractor 70 , and the reference q-axis current I q _ REF is sent to the subtractor 72 .

The subtractor 70 receives the measured d-axis current I dMEASURED and the reference d-axis current I d _ REF . The subtractor 70 determines a d-axis error signal 74 based on the measured d-axis current I dMEASURED and the reference d-axis current I d _ REF . The d-axis error signal 74 represents the error between the measured d-axis current I dMEASURED and the reference d-axis current I d _ REF . The subtractor 72 receives the measured q-axis current I qMEASURED and the reference q-axis current I q _ REF . The subtractor 72 determines a q-axis error signal 76 based on the measured q-axis current I qMEASURED and the reference q-axis current I q _ REF . The q-axis error signal 76 represents the error between the measured q-axis current I qMEASURED and the reference q-axis current I q _ REF .

The d-axis PI controller 22 receives as input the d-axis error signal 74 from the subtractor 70 . The d-axis PI controller 22 calculates a d-axis voltage signal V D . The d-axis voltage signal V D is based on a d-axis proportional gain K P , and a d-axis integral gain K i . Likewise, the q-axis PI controller 23 receives as input the q-axis error signal 76 from the subtractor 72 . The q-axis PI controller 23 calculates a q-axis voltage signal V Q . The q-axis voltage signal V Q is based on a q-axis proportional gain K P , and a q-axis integral gain K i .

The polar conversion controller 24 receives as input the d-axis voltage signal V D from the d-axis PI controller 22 and the q-axis voltage signal V Q from the q-axis PI controller 23 . Based on the inputs, the polar conversion controller 24 determines a voltage command V cmd and a phase advance angle δ. The PWM inverter controller 26 receives as inputs the voltage command V cmd and the phase advance angle δ from the polar conversion controller 24 . The PWM inverter controller 26 also receives a rotor angle value θ r measured by the motor position sensor 34 . In one exemplary embodiment, the PWM inverter controller 26 may include an over-modulation space vector PWM unit to generate three respective duty cycle values D a , D b , and D c . The duty cycle values D a , D b , and D c are used to drive gate drive circuits (not shown) of the inverter 28 that energize phases of the of the motor 32 .

Determining the reference d-axis current I d _ REF and the reference q-axis current I q _ REF by the current command controller 20 will now be described. The current command controller 20 determines the reference d-axis current I d _ REF and the reference q-axis current I q _ REF based on the torque reference command T e , the bridge voltage V, and the angular speed ω n , periodically. Specifically, the current command controller 20 determines the reference d-axis current I d _ REF and the reference q-axis current I q _ REF according to either a first region of operation RI, a second region of operation RII, or a third region of operation RIII, which is described below.

The current command controller 20 first determines the reference q-axis current I q _ REF . In one embodiment, the reference q-axis current I q _ REF is based on the torque command T e , may be determined by Equation 1:

I q _ REF =(2 *Te )/(√{square root over (3)} *Ke )  Equation 1

where Ke is the motor constant. The current command controller 20 may then compare a magnitude of the reference q-axis current I q _ REF as calculated in Equation 1 with a predetermined reference q-axis current I q _ REF _ MAX . The predetermined reference q-axis current I q _ REF _ MAX represents a q-axis current for producing a maximum torque output by the motor 32 , and may be determined by Equation 2:

I q _ REF _ MAX =I REF _ MAX *cos(α MAX )  Equation 2

where T REF _ MAX represents a phase current required for producing the maximum torque output by the motor 32 , and α MAX represents a value between the d-axis and the q-axis current required to produce maximum torque output by the motor 32 . If the magnitude of the reference q-axis current I q _ REF is less than or equal to the predetermined reference q-axis current I q _ REF _ MAX , then the reference d-axis current I d _ REF is based on a smoothening function in the first region of operation RI, which is described below. However, if the magnitude of the reference q-axis current I q _ REF is greater than the predetermined reference q-axis current I q _ REF _ MAX , then the d-axis current I d _ REF is determined by the second region of operation RII, and is either set to a reference d-axis maximum current I d _ REF _ MAX (which is described below), or is determined based on the smoothening function.

The smoothening function is used to determine the reference d-axis current I d _ REF for smoothening transitions between various torque-speed operating points of the motor 32 . In one embodiment, the current command controller 20 may first calculate the reference d-axis current I d _ REF by the smoothening function using Equations 3 and 4:

I d _ REF =K 1*(abs(ω m )+ k _ K 2*abs( T e )− k _ K 3)  Equation 3

K 1= ka 2*( T e ) 2 +k _ a 1*( T e )+ k _ a 0  Equation 4

where k_a0, k_a1, k_a2, k_K1, and k_K2 are constants derived from parameters of the motor 32 . Once the reference d-axis current I d _ REF is calculated, the reference d-axis current I d _ REF may be determined using the algorithm:

if I d _ REF <0, then I d _ REF =0

else if I d _ REF >I d _ REF _ MAX , then I d _ REF =I d _ REF _ MAX

where I d _ REF _ MAX is determined by Equation 5 as:

I d _ REF _ MAX =I REF _ MAX *sin(α MAX )  Equation 5

where if the reference d-axis current I d _ REF as determined by Equations 3-4 is less than zero, then the reference d-axis current I d _ REF is set to zero. However, if the reference d-axis current I d _ REF is greater than the reference d-axis maximum current I d _ REF _ MAX , then the reference d-axis current I d _ REF is set to the reference d-axis maximum current I d _ REF _ MAX as determined by Equation 5.

In the first region of operation RI, the reference d-axis current I d _ REF is either the reference d-axis maximum current I d _ REF _ MAX as determined by the smoothening function, or is set to zero. Also, in the first region of operation RI, if the reference d-axis current I d _ REF is a non-zero value, then the current command controller 20 determines the reference q-axis current I q _ REF by Equation 6:

I q _ REF =( T e /(sqrt(3)/2 *Ke*I q _ REF _ MAX +k _ P/ 2*3/2* I d _ REF _ MAX *( L q −L d ))  Equation 6

where k_P is the number of poles of the motor 32 , L q is the q-axis inductance, and L d is the d-axis inductance.

In the second region of operation RII, the reference d-axis current I d _ REF is either determined by Equation 7 below, or is determined by the smoothening function. Equation 7 determines the reference d-axis current I d _ REF :

I d _ REF =( T e −sqrt(3)/2 *Ke*I q _ REF _ MAX )/( k _ P/ 2*3/2 *I qREF _ MAX *( L q −L d ))  Equation 7

Specifically, the current command controller 20 selects the greatest value between the reference d-axis current I d _ REF as determined by Equation 7 and the reference d-axis current I d _ REF as determined by the smoothening function, and sets the reference d-axis current I d _ REF to the greatest value. Also, in the second region of operation RII, the current command controller 20 determines the reference q-axis current I q _ REF by Equation 6 as described above.

The current command controller 20 determines the voltage command V cmd . In one embodiment, the voltage command V cmd is determined by Equations 8-12:

V QFF =R*I q _ REF *√{square root over (3)}* Ke*ω m −2* X d *I d _ REF   Equation 8

V DFF =R*I d _ REF +2 *X q *I d _ REF   Equation 9

X q =( k _ P/ 2)*ω m *I q   Equation 10

X d =( k _ P/ 2)*ω m *L d   Equation 11

V cmd =√{square root over ( V QFF 2 +V DFF 2 )}  Equation 12

where V QFF is the feed forward q-axis voltage, V DFF is the feed forward d-axis voltage, R is resistance per phase of the motor 32 , X d is a d-axis reactance, and X q is a q-axis reactance.

Once the voltage command V cmd is determined, the current command controller 20 compares the magnitude of the voltage command V cmd with the bridge voltage V ecu , and determines if the reference d-axis current I d _ REF and the reference q-axis current I q _ REF are determined by the third region of operation RIII instead of the first region of operation RI or the second region of operation RII. Specifically, referring to Table 1 below, if the magnitude of the voltage command V cmd is greater than or equal to the bridge voltage V, then the third region of operation RIII is selected.

According to Table 1, if the magnitude of the reference q-axis current I q _ REF is less than or equal to the predetermined reference q-axis current I q _ REF _ MAX , and if the magnitude of the voltage command V cmd is less than the bridge voltage V ecu , then the first region of operation RI is selected. If the magnitude of the reference q-axis current I q _ REF is greater than the predetermined reference q-axis current i q _ REF _ MAX , and if the magnitude of the voltage command V cmd is less than the bridge voltage V ecu , then the second region of operation RII is selected.

If the third region of operation RIII is selected, either the smoothening function as described above or the target voltage circle calculation may be used to determine the reference d-axis current I d _ REF . Specifically, the current command controller 20 determines the reference d-axis current I d _ REF based on the target voltage circle approach by first determining Equations 13-17:

Z q _ sqr =R 2 +X q 2   Equation 13

Z d _ sqr =R 2 +X d 2   Equation 14

B Dax =I q _ REF *R *( X q −X d )−2* Ke/ √{square root over (3)}*ω m *X d   Equation 15

C Dax =I q _ REF 2 *Z q _ sqr +Ke/ √{square root over (3)}*ω m *R*I qref +( Ke/ √{square root over (3)}*ω m ) 2 −V ecu 2 /3  Equation 16

S Dax =B Dax 2 −4* Z d _ sqr *C Dax   Equation 17

where Z q _ sqr is a q-axis impedance of the motor 32 squared, Z d _ sqr is a d-axis impedance squared, B Dax , C Dax and S Dax are variables based on the target voltage circle, I q _ REF is determined using Equation 1 above, and V CMD =V ecu . The reference d-axis current I d _ REF may then be determined by the following d-axis target voltage circle algorithm:

If S Dax >=0, then I d _ REF =(− B Dax −√{square root over ( S Dax )})/(2* Z d _ sqr )

Else

I d _ REF (− B Dax )/(2 *Z d _ sqr )

where the reference d-axis current I d _ REF is calculated based on the variable S Dax . The current command controller 20 then determines the greatest value between the reference d-axis current I d _ REF as determined by the smoothening function above and the reference d-axis current I d _ REF as determined by the target voltage circle approach, and sets the reference d-axis current I d _ REF to the greatest value.

After the reference d-axis current I d _ REF is determined (using either the smoothening function or the target voltage circle approach), the current command controller 20 re-calculates the reference q-axis current I q _ REF using Equation 6 above, where the d-axis current I d _ REF as determined by the target voltage circle is used for I d _ REF _ MAX . The current command controller 20 then determines the smallest value between the reference q-axis current I q _ REF as determined by Equation 6 and the reference q-axis current I q _ REF as determined using Equation 1 as described above. The current command controller 20 then sets the reference q-axis current I q _ REF to the smallest value.

Once the reference d-axis current I d _ REF and the reference q-axis current I q _ REF are determined, the current command controller 20 re-calculates the voltage command V cmd using Equations 8-12 above. The current command controller 20 also determines a peak current I PEAK _ CURRENT according to Equation 18:

I PEAK _ CURRENT =√{square root over ( I q _ REF 2 +I d _ REF 2 )}  Equation 18

Once the voltage command V cmd is re-calculated and the peak current I PEAK _ CURRENT is determined, the current command controller 20 determines if either the magnitude of the voltage command V cmd is greater than or equal to the bridge voltage V ecu (V cmd ≧V ecu ), or if a maximum current value Max_Current is greater than the peak current I PEAK _ CURRENT (Max_Current>I PEAK _ CURRENT ). The variable Max_Current is a predetermined value stored in memory. If either condition (e.g., V cmd ≧V ecu or Max_Current>I PEAK _ CURRENT ) is met, then the current command controller 20 may re-calculate the reference q-axis current I q _ REF using the target voltage circle approach, which is described below. However, if neither condition is met, then the current command controller 20 may re-calculate the feed forward q-axis voltage V QFF and the feed forward d-axis voltage V DFF according to Equations 8-9 above and save the feed forward q-axis voltage V QFF and the feed forward d-axis voltage V DFF in memory.

The current command controller 20 may re-calculate the reference q-axis current I q _ REF based on the target voltage circle approach as described in Equations 19-21, where:

B Qax =I Q _ REF *R *( X q −X d )+2 *Ke/ √{square root over (3)}*ω m *R   Equation 19

C Qax =I d _ REF 2 *Z d _ sqr −Ke/ √{square root over (3)}*ω m *X d *I dref +( Ke/ √{square root over (3)}*ω m ) 2 −V ecu 2 /3  Equation 20

S Qax =B Qax 2 −4* Z q _ sqr *C Qax   Equation 21

where I d _ REF is determined using Equations 13-17 above, and V cmd =V ecu . The reference q-axis current I q _ REF may then be determined using the following q-axis target voltage circle algorithm:

If S Qax >=0, then I q _ REF =(− B Qax −sqrt( S Qax ))/(2* Z q _ sqr )

Else

I q _ REF =(− B Qax )/(2 *Z q _ sqr )

Once the reference q-axis current I q _ REF is determined using the q-axis current voltage circle algorithm, the current command controller 20 may then re-calculate the feed forward q-axis voltage V QFF and the feed forward d-axis voltage V DFF according to Equations 6-7 above, and save the feed forward q-axis voltage V QFF and the feed forward d-axis voltage V DFF in memory.

FIG. 2 is an exemplary process flow diagram illustrating a method 200 for determining the reference d-axis current I d _ REF and the reference q-axis current I q _ REF . Referring now to both FIGS. 1-2 , the method 200 may begin at block 202 , where the current command controller 20 receives as input the torque reference command T e , the angular speed ω m , and the bridge voltage V ecu . Method 200 may then proceed to block 204 .

In block 204 , the current command controller 20 determines the reference q-axis current I q _ REF . In one embodiment, the reference q-axis current I q _ REF may be determined by Equation 1 as described above. Method 200 may then proceed to block 206 .

In block 206 , the current command controller 20 determines if the magnitude of the reference q-axis current I q _ REF is greater than the predetermined reference q-axis current I q _ REF _ MAX . If the answer is no, method 200 may then proceed to block 208 , where the reference d-axis current I q _ REF is based on the smoothening function as described above. However, if the answer is yes, method 200 may then proceed to block 210 , where the reference d-axis current I d _ REF is determined based on the smoothening function, or by Equation 7 above (whatever approach yields the greatest value). Method 200 may proceed to block 212 from both blocks 208 and 210 .

In block 212 , the current command controller 20 determines if the magnitude of the voltage command V cmd is greater than or equal to than the bridge voltage V ecu . If the answer is yes, then method 200 may proceed to block 214 . If the answer is no, then either the first region of operation RI or the second region of operation RII is selected, and method 200 may proceed to block 224 .

In block 214 , the third region of operation ME is selected. Method 200 may then proceed to block 216 .

In block 216 , the current command controller 20 determines the d-axis current I d _ REF . Specifically, the current command controller 20 determines the greatest value between the reference d-axis current I d _ REF as determined by the smoothening function above (in either block 208 or 210 ) and the reference d-axis current I d _ REF as determined by the target voltage circle approach, and sets the reference d-axis current I d _ REF to the greatest value. Once the reference d-axis current I d _ REF is determined, the current command controller 20 re-calculates the reference q-axis current I q _ REF using Equation 6 above, and determines the smallest value between the reference q-axis current I q _ REF as determined by Equation 6 and the reference q-axis current I q _ REF as determined in block 204 above (using Equation 1). The current command controller 20 then sets the reference q-axis current I q _ REF to the smallest value. Method 200 may then proceed to block 218 .

In block 218 , the current command controller 20 re-calculates the voltage command V cmd using Equations 6-10 as described above. The current command controller 20 also determines the peak current I PEAK _ CURRENT according to Equation 16 as described above. Method 200 may then proceed to block 220 .

In block 220 , the current command controller 20 determines if either the magnitude of the voltage command V cmd is greater than or equal to the bridge voltage V ecu (V cmd ≧V ee ), or if a maximum current value Max_Current is greater than the peak current I PEAK _ CURRENT (Max_Current>I PEAK _ CURRENT ). If either condition (e.g., V cmd ≧V ecu or Max_Current>I PEAK _ CURRENT ) is met, then method 200 may proceed to block 222 . However, if neither condition is met, then method 200 may then proceed to block 224 .

In block 222 , the current command controller 20 may re-calculate the reference q-axis current I q _ REF . Specifically, the current command controller 20 determines the q-axis current I q _ REF using the target voltage circle approach as described in Equations 19-21. Method 200 may then proceed to block 224 .

In block 224 , the current command controller 20 may re-calculate the feed forward q-axis voltage V QFF and the feed forward d-axis voltage V DFF according to Equations 8-9 above, and save the feed forward q-axis voltage V QFF and the feed forward d-axis voltage V DFF in memory. Method 200 may then repeat, or terminate.

The current command controller 20 as described above determines the reference d-axis current I d _ REF and the reference q-axis current I q _ REF according to either the first region of operation RI, the second region of operation RII, or the third region of operation RIII based on based on the torque command T e , the bridge voltage V ecu , and the angular speed ω m . The reference d-axis current I d _ REF as determined by the current command controller 20 generally compensates for lost torque caused by sensor lag as well as any other limitations with the approach for controlling the motor 32 .

While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description.