Electric power steering apparatus

Abstract

An electric power steering apparatus of a vector control system converts calculated dq-axes current command values into 3-phase duty command values, driving-controls a 3-phase brushless motor by an inverter of a PWM control, and applies an assist torque to a steering system of a vehicle, wherein compensation signs of 3-phase current model command values in which the dq-axes current command values are converted into a 3-phase current command value model are estimated, wherein a dead time compensation amount is calculated based on an inverter-applying voltage, and wherein dead time compensation is performed by adding dead time compensation values that are 2-phase values converted from 3-phase values in which the compensation signs are multiplied with the dead time compensation amount, to dq-axes voltage command values, or by adding 3-phase dead time compensation values to 3-phase voltage command values.

Claims

1. An electric power steering apparatus of a vector control system that converts dq-axes current command values calculated based on at least a steering torque into 3-phase duty command values, driving-controls a 3-phase brushless motor by an inverter of a pulse width modulation (PWM) control, and applies an assist torque to a steering system of a vehicle, wherein compensation signs of 3-phase current model command values in which said dq-axes current command values are converted into a 3-phase current command value model are estimated, wherein a dead time compensation amount is calculated based on an inverter-applying voltage, and wherein a dead time compensation of said inverter is performed by adding dead time compensation values that are 2-phase values converted from 3-phase values in which said compensation signs are multiplied with said dead time compensation amount, to dq-axes voltage command values.

2. The electric power steering apparatus according to claim 1, wherein a motor rotational angle is used in conversions of said 3-phase current model command values and said dead time compensation amount.

3. The electric power steering apparatus according to claim 2, wherein a relationship between said inverter-applying voltage and said dead time compensation amount is that said dead time compensation amount is a first dead time compensation amount being a constant when said inverter-applying voltage is equal to or lower than a predetermined voltage VR1, said dead time compensation amount is a second dead time compensation amount increasing when said inverter-applying voltage is higher than said predetermined voltage VR1 and is equal to or lower than a predetermined voltage VR2 which is higher than VR1, and said dead time compensation amount is a third dead time compensation amount being a constant when said inverter-applying voltage is higher than said predetermined voltage VR2.

4. An electric power steering apparatus of a vector control system that converts dq-axes current command values calculated based on at least a steering torque into 3-phase voltage command values, calculates duty command values based on said 3-phase voltage command values, driving-controls a 3-phase brushless motor by an inverter of a pulse width modulation (PWM) control, and applies an assist torque to a steering system of a vehicle, wherein compensation signs of 3-phase current model command values in which said dq-axes current command values are converted into a 3-phase current command value model are estimated, wherein a dead time compensation amount is calculated based on an inverter-applying voltage, and wherein a dead time compensation of said inverter is performed by adding 3-phase values that said compensation signs are multiplied with said dead time compensation amount, to said 3-phase voltage command values.

5. The electric power steering apparatus according to claim 4, wherein a motor rotational angle is used in conversions of said 3-phase current model command values and said dead time compensation amount.

6. The electric power steering apparatus according to claim 5, wherein a phase adjustment is performed by changing a phase of said motor rotational angle depending on a motor rotational velocity.

7. The electric power steering apparatus according to claim 6, wherein said 3-phase current model command value is calculated from said dq-axes current command values and a calculation or a table.

8. The electric power steering apparatus according to claim 4, wherein a relationship between said inverter-applying voltage and said dead time compensation amount is that said dead time compensation amount is a first dead time compensation amount being a constant when said inverter-applying voltage is equal to or lower than a predetermined voltage VR1, said dead time compensation amount is a second dead time compensation amount increasing when said inverter-applying voltage is higher than said predetermined voltage VR1 and is equal to or lower than a predetermined voltage VR2 which is higher than VR1, and said dead time compensation amount is a third dead time compensation amount being a constant when said inverter-applying voltage is higher than said predetermined voltage VR2.

9. An electric power steering apparatus of a vector control system that converts dq-axes current command values calculated based on at least a steering torque into 3-phase duty command values, driving-controls a 3-phase brushless motor by an inverter of a pulse width modulation (PWM) control, and applies an assist torque to a steering system of a vehicle, comprising: an inverter-applying voltage sensitive-compensation amount calculating section to calculate an each phase dead time compensation amount based on an inverter-applying voltage; a 3-phase current command value model to calculate a 3-phase current model command values based on said dq-axes current command values; a phase current compensation-sign estimating section to estimate compensation signs of said 3-phase current model command values; and a dead time compensation value outputting section to output dead time compensation values by multiplying said compensation signs with said each phase dead time compensation amount and by converting said multiplied values into dq-axes, wherein a dead time compensation of said inverter is performed by adding said dead time compensation values to dq-axes voltage command values.

10. The electric power steering apparatus according to claim 9, wherein said dead time compensation value outputting section comprises: a multiplying section to multiply said compensation signs with said dead time compensation amount; and a 3-phase alternating current (AC)/dq-axes converting section to convert 3-phase outputs of said multiplying section into said dead time compensation values on dq-axes.

11. An electric power steering apparatus of a vector control system that converts dq-axes current command values calculated based on at least a steering torque into 3-phase voltage command values, calculates duty command values based on said 3-phase voltage command values, driving-controls a 3-phase brushless motor by an inverter of a pulse width modulation (PWM) control, and applies an assist torque to a steering system of a vehicle, comprising: an inverter-applying voltage sensitive-compensation amount calculating section to calculate an each phase dead time compensation amount based on an inverter-applying voltage; a 3-phase current command value model to calculate a 3-phase current model command values based on said dq-axes current command values; a phase current compensation-sign estimating section to estimate compensation signs of said 3-phase current model command values; and a dead time compensation value outputting section to output 3-phase dead time compensation values in which said compensation signs are multiplied with said each phase dead time compensation amount, wherein a dead time compensation of said inverter is performed by adding said dead time compensation values to said 3-phase voltage command values.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the accompanying drawings:

(2) FIG. 1 is a configuration diagram showing a general outline of an electric power steering apparatus;

(3) FIG. 2 is a block diagram showing a configuration example of a control unit (ECU) of the electric power steering apparatus;

(4) FIG. 3 is a block diagram showing a configuration example of a vector control system;

(5) FIG. 4 is a wiring diagram showing a configuration example of a general inverter;

(6) FIG. 5 is a block diagram showing a configuration example (the first embodiment) of the present invention;

(7) FIG. 6 is a characteristic diagram showing an example of a characteristic of a phase adjusting section;

(8) FIG. 7 is a block diagram showing a configuration example of an inverter-applying voltage sensitive-compensation amount calculating section;

(9) FIG. 8 is a characteristic diagram showing a characteristic example of the inverter-applying voltage sensitive-compensation amount calculating section;

(10) FIG. 9 is a waveform chart showing an example of an output waveform of a 3-phase current command value model;

(11) FIGS. 10A and 10B are waveform charts showing operation examples of a phase current compensation-sign estimating section;

(12) FIG. 11 is a block diagram showing a configuration example of a spatial vector modulating section;

(13) FIG. 12 is a diagram showing an operation example of the spatial vector modulating section;

(14) FIG. 13 is a diagram showing an operation example of the spatial vector modulating section;

(15) FIG. 14 is a timing chart showing an operation example of the spatial vector modulating section;

(16) FIG. 15 is a waveform chart showing an effect of the spatial vector modulation;

(17) FIG. 16 is a waveform chart showing an effect of the present invention (the first embodiment);

(18) FIG. 17 is a waveform chart showing an effect of the present invention (the first embodiment);

(19) FIG. 18 is a block diagram showing a configuration example (the second embodiment) of the present invention;

(20) FIG. 19 is a waveform chart showing an effect of the present invention (the second embodiment); and

(21) FIG. 20 is a waveform chart showing an effect of the present invention (the second embodiment).

MODE FOR CARRYING OUT THE INVENTION

(22) In order to resolve a problem that a current distortion and a torque ripple occur due to an influence of a dead time of an inverter in a control unit (ECU) and a steering sound is louder, the present invention compensates the dead time by converting dead time compensation values into 3-phase current model command values based on dq-axes current command values, estimating compensation signs, calculating a dead time compensation amount of the inverter calculated from an inverter-applying voltage, and converting the dead time compensation values compensated by the estimated compensation signs into 2-phase values and adding (feedforwarding) the 2-phase values to a voltage command values on the dq-axes (the first embodiment), or adding (feedforwarding) the dead time compensation values compensated by the estimated compensation signs to the 3-phase voltage command values (the second embodiment). Thereby, without the tuning operation, the dead time of the inverter is compensated on the dq-axes or the 3-phase alternative current (AC), and improvements in the distortion of the current waveform and the responsibility of the current control can be achieved.

(23) Embodiments according to the present invention will be described with reference to the drawings.

(24) FIG. 5 shows a whole configuration example (the first embodiment) of the present invention corresponding to FIG. 3, and there is provided a dead time compensating section 200 to calculate dead time compensation values v.sub.d* and v.sub.q* on the dq-axes. A d-axis current command value i.sub.d*, a q-axis current command value i.sub.q*, a motor rotational angle and a motor rotational number are inputted into the dead time compensating section 200. An inverter-applying voltage VR applied to the inverter 161 is also inputted into the dead time compensating section 200.

(25) The d-axis current command value i.sub.d* and the q-axis current command value i.sub.q* in which the maximum values of steering assist command values calculated at a steering assist command value calculating section (not shown) are limited, are inputted into subtracting sections 131d and 131q, respectively. A current deviation i.sub.d* between the d-axis current command value i.sub.d* and a feedback current i.sub.d is calculated at the subtracting section 131d, and a current deviation i.sub.q* between the q-axis current command value i.sub.q* and a feedback current i.sub.q is calculated at the subtracting sections 131q. The calculated current deviation i.sub.d* is inputted into a PI-control section 120d and the calculated current deviation i.sub.q* is inputted into a PI-control section 120q. A d-axis voltage command value v.sub.d and a q-axis voltage command value v.sub.q which are PI-controlled are respectively inputted into adding sections 121d and 121q, and are respectively added to the dead time compensation values v.sub.d* and v.sub.q* from the dead time compensating section 200 as described below. The d-axis voltage command value v.sub.d and the q-axis voltage command value v.sub.q are compensated. The respective compensated voltage values are respectively inputted into a subtracting section 141d and an adding section 141q. A voltage v.sub.d1* from a d-q non-interference control section 140 is subtraction-inputted into the subtracting section 141d. A voltage command value v.sub.d.sup.** that is a difference between the compensated voltage and the voltage v.sub.d1* is obtained. A voltage v.sub.q1* from the d-q non-interference control section 140 is inputted into the adding section 141q. A voltage command value v.sub.q** that is an added result is obtained. The voltage command values v.sub.d** and v.sub.q** that the dead time is compensated are inputted into a spatial vector modulating section 300 that converts the 2-phase voltage command values on the dq-axes into the 3-phase voltage command values whose components are a U-phase, a V-phase and a W-phase, and superimposes a third-harmonic wave. The 3-phase voltage command values V.sub.u*, V.sub.v* and V.sub.w* which are vector-modulated at the spatial vector modulating section 300 are inputted into a PWM-control section 160. As described above, a motor 100 is driving-controlled via the PWM-control section 160 and the inverter 161.

(26) Next, the dead time compensating section 200 will be described.

(27) The dead time compensating section 200 comprises an adding section 201, a multiplying section 202, an inverter-applying voltage sensitive-compensation amount calculating section 210, a 3-phase current command value model 220, a phase current compensation-sign estimating section 221, a phase adjusting section 230 and a 3-phase AC/dq-axes converting section 240. As well, the multiplying section 202 and the 3-phase AC/dq-axes converting section 240 constitute a dead time compensation-value outputting section. The motor rotational angle is inputted into the adding section 201 and the motor rotational number is inputted into the phase adjusting section 230. The inverter-applying voltage VR is inputted into the inverter-applying voltage sensitive-compensation amount calculating section 210. A motor rotational angle m that is calculated and is phase-adjusted at the adding section 201 is inputted into the 3-phase current command value model 220.

(28) In a case that the dead time compensation timing is hastened or is delayed in response to the motor rotational number co, the phase adjusting section 230 has a function to calculate the adjustment angle depending on the motor rotational number co. The phase adjusting section 230 has a characteristic as shown in FIG. 6 in a case of a lead angle control. The calculated phase adjustment angle is inputted into the adding section 201 and is added to the motor rotational angle . The phase-adjusted motor rotational angle m (=+) that is an added result of the adding section 201 is inputted into the 3-phase current command value model 220 and the 3-phase AC/dq-axes converting section 240.

(29) After detecting a motor electric angle and then calculating the duty command values, a time delay whose time is several tens of microseconds to one hundred microseconds is existed until actually reflecting the PWM-signals. Since the motor is rotating during the delay time, a phase shift between the motor electric angle in the calculation and the motor electric angle in the reflection is generated. In order to compensate this phase shift, the lead angle is performed depending on the motor rotational number co and the phase is adjusted.

(30) Since the optimal dead time compensation amount is changed depending on the inverter-applying voltage VR, the dead time compensation amount DTC is calculated depending on the inverter-applying voltage VR and is changeable in the present invention. The inverter-applying voltage sensitive-compensation amount calculating section 210 that inputs the inverter-applying voltage VR and outputs the dead time compensation amount DTC has a configuration as shown in FIG. 7. The maximum value of an absolute value of the inverter-applying voltage VR is limited at an input limiting section 211. The inverter-applying voltage VR.sub.1 whose maximum value is limited is inputted into an inverter-applying voltage/dead time compensation-amount converting table 212.

(31) The inverter-applying voltage/dead time compensation-amount converting table 212 has a characteristic, for example, as shown in FIG. 8. That is, the outputted dead time compensation amount DTC is a constant value DTC1 when the inverter-applying voltage VR.sub.1 is less than a predetermined inverter-applying voltage VR1. The outputted dead time compensation amount DTC linearly (or nonlinearly) increases when the inverter-applying voltage VR.sub.1 is the predetermined inverter-applying voltage VR1 or more and is less than a predetermined inverter-applying voltage VR2 (>VR1). The outputted dead time compensation amount DTC is a constant value DTC2 when the inverter-applying voltage VR.sub.1 is the predetermined inverter-applying voltage VR2 or more.

(32) The d-axis current command value i.sub.d* and the q-axis current command value i.sub.q* are inputted into the 3-phase current command value model 220 with the motor rotational angle m. The 3-phase current command value model 220 calculates the sinusoidal 3-phase current model command values Icm that respective phases deviate 120 [deg] as shown in FIG. 9, by a calculation or using a table (refer to following Expressions 1 and 2). The 3-phase current model command values Icm are different depending on the motor type. The d-axis current command value i.sub.ref_d and the q-axis current command value i.sub.ref_q are converted into the 3-phase current command values (U-phase, V-phase and W-phase) using the motor electric angle e. The above relationship is represented by the below Expression 1.

(33) $\begin{array}{cc}\left[\begin{array}{c}{i}_{\mathrm{ref\_u}}\\ i_{\mathrm{ref\_v}}\\ i_{\mathrm{ref\_w}}\end{array}\right]=\left[\begin{array}{cc}\cos \left({}_e\right)& \sin \left({}_e\right)\\ \cos \left({}_e-\frac{2}{3}\right)& \sin \left({}_e-\frac{2}{3}\right)\\ \cos \left({}_e+\frac{2}{3}\right)& \sin \left({}_e+\frac{2}{3}\right)\end{array}\right]\left[\begin{array}{c}{i}_{\mathrm{ref\_d}}\\ i_{\mathrm{ref\_q}}\end{array}\right]& \left[\mathrm{Expression}1\right]\end{array}$

(34) The respective current command values are calculated from the above Expression 1, and the U-phase current command value model i.sub.ref_u, the V-phase current command value model i.sub.ref_v and the W-phase current command value model i.sub.ref_w are represented by the below Expression 2.

(35) $\begin{array}{cc}{i}_{\mathrm{ref\_u}}=i_{\mathrm{ref\_d}}.Math.\cos \left({}_e\right)+i_{\mathrm{ref\_q}}.Math.\sin \left({}_e\right)& \left[\mathrm{Expression}2\right]\\ i_{\mathrm{ref\_v}}=i_{\mathrm{ref\_d}}.Math.\cos \left({}_e-\frac{2}{3}\right)+i_{\mathrm{ref\_q}}.Math.\sin \left({}_e-\frac{2}{3}\right)& \\ i_{\mathrm{ref\_w}}=i_{\mathrm{ref\_d}}.Math.\cos \left({}_e+\frac{2}{3}\right)+i_{\mathrm{ref\_q}}.Math.\sin \left({}_e+\frac{2}{3}\right)& \end{array}$

(36) The table may be stored in an electrically erasable and programmable read-only memory (EEPROM) or may be loaded to a random access memory (RAM). In using the Expression 2, the values of sine are stored in the table. The values of cos may be calculated by offsetting the input to 90 and other sine function terms may be calculated by offsetting the input to 120. In a case that the ROM capacity is sufficiently large or the command value model is complicated (for example, a pseudo rectangular wave motor), the values of the overall Expression 2 are stored in the table.

(37) The 3-phase current model command values Icm are inputted into the phase current compensation-sign estimating section 221. The phase current compensation-sign estimating section 221 outputs compensation signs SN, which have a positive value (+1) or a negative value (1) and indicates a hysteresis characteristic shown in FIGS. 10A and 10B, against the inputted 3-phase current model command values Icm. The compensation signs SN are estimated based on zero-cross points of the 3-phase current model command values Icm as a reference. In order to suppress the chattering, the compensation signs SN have the hysteresis characteristic. The estimated compensation signs SN are inputted into the multiplying section 202. The positive and negative thresholds of the hysteresis characteristic are appropriately changeable.

(38) In a case that the signs of the dead time compensation values are simply determined from the current signs of the phase current command value model, the chattering is occurred in the low load. For example, when the handle is slightly steered to the left or the right near the on-center, the torque ripple is occurred. In order to improve this problem, the hysteresis is adopted in the sign judgement (0.25 [A] in FIG. 10A). The current signs are held except for a case that the signs are changed beyond the set current value, and the chattering is suppressed.

(39) The dead time compensation amount DTC from the inverter-applying voltage sensitive-compensation amount calculating section 210 is inputted into the multiplying section 202. The multiplying section 202 outputs the dead time compensation amounts DTCa (=DTCSN) that the compensation signs SN are multiplied with the dead time compensation amount DTC. The dead time compensation amounts DTCa are inputted into the 3-phase AC/dq-axes converting section 240. The 3-phase AC/dq-axes converting section 240 outputs the 2-phase dead time compensation values v.sub.d* and v.sub.q* in synchronization with the motor rotational angle m. The dead time compensation value v.sub.d* and v.sub.q* are added to the voltage command values v.sub.d and v.sub.q, respectively, and the dead time compensation of the inverter 161 is performed.

(40) In the present invention, the dq-axes current command values are converted into the 3-phase current model command values, and the compensation signs are estimated. The dead time compensation amount of the inverter calculated from the inverter-applying voltage VR is calculated, and the voltage command values on the dq-axes are feedforward-compensated by the dead time compensation values calculated from the estimated compensation signs. The 3-phase current model command values are used in the compensation signs of the dead time, and the dead time compensation amount is calculated from the inverter-applying voltage VR. The compensation values are changeable so that the magnitudes and directions of the compensation values are optimal depending on the magnitude of the current command values (i.sub.d* and i.sub.q*) and the magnitude of the inverter-applying voltage VR.

(41) Next, the spatial vector modulation will be described. As shown in FIG. 11, the spatial vector modulating section 300 may have a function that converts the 2-phase voltages v.sub.d** and v.sub.q** in the d-q spaces into the 3-phase voltages V.sub.ua, V.sub.va and V.sub.wa, and superimposes the third harmonic wave on the 3-phase voltages V.sub.ua, V.sub.va and V.sub.wa. For example, the method of the spatial vector modulation that the applicant proposes in Japanese Unexamined Patent Publication No. 2017-70066, WO2017/098840 (Japanese Patent Application No. 2015-239898) and the like may be used.

(42) That is, the spatial vector modulation has a function that performs a following coordinate transformation based on the voltage command values v.sub.d** and v.sub.q** in the d-q spaces, the motor rotational angle and sector number n (#1 to #6), and controls the rotation of the motor by supplying switching patterns S1 to S6 to the motor. The switching patterns S1 to S6 are corresponding to the sectors #1 to #6, and control turning-ON/turning-OFF of the switching devices (the upper-arm Q1, Q3 and Q5, and the lower-arm Q2, Q4 and Q6) of the inverter with the bridge configuration. With reference to the coordinate transformation, in the spatial vector modulation, the voltage command values v.sub.d** and v.sub.q** perform the coordinate transformation to the voltage vectors V and V in the - coordinate system based on a below Expression 3. A relationship between the coordinate axes that are used in this coordinate transformation and the motor rotational angle is shown in FIG. 12.

(43) $\begin{array}{cc}\left[\begin{array}{c}V\\ V\end{array}\right]=\left[\begin{array}{cc}\cos & -\sin \\ \sin & \cos \end{array}\right]\left[\begin{array}{c}{v}_d^{**}\\ v_q^{**}\end{array}\right]& \left[\mathrm{Expression}3\right]\end{array}$

(44) A relationship shown in an Expression 4 between a target voltage vector in the d-q coordinate system and a target voltage vector in the - coordinate system is existed. The absolute value of the target voltage vector is conserved.
|V|={square root over ((v.sub.d**).sup.2+(v.sub.q**).sup.2)}={square root over (V.sup.2+V.sup.2)}

(45) In the switching pattern of the spatial vector control, the output voltage is defined by using eight discrete reference voltage vectors V0 to V7 (non-zero voltage vectors V1 to V6 that the phase differs every /3 [rad] and zero voltage vectors V0 and V7) that are shown in the spatial vector diagram of FIG. 13, depending on the switching patterns S1 to S6 of the switching devices (the FETs) (Q1 to Q6). The selection of these reference output voltage vectors V0 to V7 and the occurring time are controlled. By using six regions sandwiched between adjacent reference output voltage vectors, the space vector can be divided into the six sectors #1 to #6, and the target voltage vector V is belong to any one of the sectors #1 to #6, and can be assigned to the sector number. The rotational angle in the - coordinate system of the target voltage vector V can determine which sector that is separated into a regular hexagon in the - spaces, as shown in FIG. 13, is existed in the target voltage vector V that is a synthetic vector of V and V. The rotational angle is determined by a sum of the rotational angle of the motor and a phase obtained from the relationship of the voltage command values v.sub.d** and v.sub.q** in the d-q coordinate system (=+).

(46) FIG. 14 shows a basic timing chart that the switching pulse width and the timing in the turning-ON/turning-OFF signals S1 to S6 to the switching devices (the FETs) are determined in order to output the target voltage vector from the inverter by a digital control by means of the switching patterns S1, S3 and S5 of the inverter in the spatial vector control. The spatial vector modulation performs the calculation and the like in every defined sampling period Ts, and outputs the respective pulse widths and the timings in the switching patterns S1 to S6 to which the calculation result is transformed in the next sampling period Ts.

(47) The spatial vector modulation generates the switching patterns S1 to S6 depending on the sector number that is obtained based on the target voltage vector V. In FIG. 14, in a case of the sector number #1 (n=1), one example of the switching patterns S1 to S6 of the inverter is shown. The signals S1, S3 and S5 show the gate signals of the switching devices Q1, Q3 and Q5 that are corresponding to the upper-arm. The horizontal axis denotes a time, and the sampling period Ts is corresponding to the switching period and is divided into eight periods, T0/4, T1/2, T2/2, T0/4, T0/4, T2/2, T1/2 and T0/4. The periods T1 and T2 are the time depending on the sector number n and the rotational angle .

(48) In a case that the spatial vector modulation is not performed, the dead time compensation of the present invention is applied on the dq-axes, and the dead time compensation value waveform (the U-phase waveform) that the dq-axes/3-phase convertion is performed to only the dead time compensation value is shown in a waveform represented by a dashed line of FIG. 15 that the third-order component is removed. The same phenomena are exhibited in the V-phase and the W-phase. By applying the spatial vector modulation instead of the dq-axes/3-phase conversion, the third-harmonic can be superimposed on the 3-phase signals, the third-order component that is removed by the 3-phase conversion can be compensated, and the ideal dead time compensation waveform that is shown in a solid line of FIG. 15 can be generated.

(49) FIGS. 16 and 17 are simulation results showing an effect of the present invention (the first embodiment). FIG. 16 shows the U-phase current, the d-axis current and the q-axis current in a case that the dead time compensation is not performed. By applying the dead time compensation of the present invention, in the high-speed steering state, the improvements in the waveform distortion of the phase currents and the dq-axes currents as shown in FIG. 17 (the ripple is reduced in the dq-axes current waveforms and the phase currents whose waveforms are almost sinusoidal are obtained) can be confirmed. The torque ripple in the steering maneuver and the steering sound are improved.

(50) As well, FIGS. 16 and 17 typically show the U-phase current.

(51) Next, the second embodiment that the compensation is performed by adding (feedforwarding) the dead time compensation values based on the estimated compensation signs to the 3-phase voltage command values will be described with reference to FIG. 18 corresponding to FIG. 5.

(52) In the second embodiment as shown in FIG. 18, the dead time compensating section 200A does not include the 3-phase AC/dq-axes converting section. Thus, the adding sections 121d and 121q are not included. In the second embodiment, in order to add (feedforward) the dead time compensation values based on the estimated compensation signs to the 3-phase voltage command values, adding sections 163U, 163V and 163W are disposed between the spatial vector modulating section 300 and the PWM-control section 160. The 3-phase dead time compensation values DTCa (DTCau, DTCav and DTCaw), which the compensation signs SN are multiplied with the dead time compensation amount DTC, are inputted into the adding sections 163U, 163V and 163W, respectively. The voltage command values V.sub.u**, V.sub.v** and V.sub.w**, which the dead time compensation values are added and the dead time compensation is performed, are inputted into the PWM-control section 160. The subsequent control operation is the same as described above.

(53) The inverter-applying voltage sensitive-compensation amount calculating section 210, the 3-phase current command value model 220, the phase current compensation-sign estimating section 221 and the phase adjusting section 230 have the same characteristics and the same operations as the first embodiment. The effect of the second embodiment is shown in FIGS. 19 and 20. As well, FIGS. 19 and 20 typically show the U-phase current.

EXPLANATION OF REFERENCE NUMERALS

(54) 1 handle (steering wheel) 2 column shaft (steering shaft, handle shaft) 10 torque sensor 12 vehicle speed sensor 13 battery 20, 100 motor 30 control unit (ECU) 31 steering assist command value calculating section 35, 203, 204 PI-control section 36, 160 PWM-control section 37, 161 inverter 110 angle detecting section 130 3-phase/dq-axes converting section 140 d-q non-interference control section 200, 200A dead time compensating section 210 inverter-applying voltage sensitive-compensation amount calculating section 220 3-phase current command value model 221 phase current compensation-sign estimating section 230 phase adjusting section 240 3-phase AC/dq-axes converting section 300 spatial vector modulating section 301 2-phase/3-phase converting section 302 third-harmonic superimposition section