DESCRIPTION
Title of the Invention: CONTROL DEVICE FOR ELECTRIC POWER STEERING
APPARATUS
Technical Field
The present invention relates to a control device for an
electric power steering apparatus that provides a steering system
of a vehicle with a steering assist force generated by a motor,
and in particular to a control device for an electric power steering
apparatus that improves dead time compensation of an inverter
for a motor drive depending on a steering status and an inverter
temperature.
Background Art
An electric power steering apparatus that energizes a
steering apparatus of a vehicle by using a rotational torque of
a motor as an assist torque, applies a driving force of the motor
as the assist torque to a steering shaft or a rack shaft by means
of a transmission mechanism such as gears or a belt through a
reduction mechanism. And then, in order to supply a current to
the motor so that the motor generates a desired torque, an inverter
is used in a motor drive circuit.
A general configuration of a conventional electric power
steering apparatus will be described with reference to FIG.1.
As shown in FIG.1, a column shaft (a steering shaft) 2 connected
to a steering wheel (handle) 1, is connected to steered wheels
3
8L and 8R through reduction gears 3, universal joints 4a and 4b,
a rack and pinion mechanism 5, and tie rods 6a and 6b, further
via hub units 7a and 7b. Further, the column shaft 2 is provided
with a torque sensor 10 for detecting a steering torque of the
steering wheel 1, and a motor 20 for assisting the steering force
of the steering wheel 1 is connected to the column shaft 2 through
the reduction gears 3. Electric power is supplied to a control
unit 100 for controlling the electric power steering apparatus
from a battery 13, and an ignition key signal is inputted into
the control unit 100 through an ignition key 11. The control
unit 100 calculates a current command value of an assist (steering
assist) command based on a steering torque T detected by the torque
sensor 10 and a velocity V detected by a velocity sensor 12, and
controls a current supplied to the motor 20 based on a voltage
command value E obtained by performing compensation and so on
with respect to the current command value in a current control
section. Furthermore, it is also possible to receive the velocity
V from a CAN (Controller Area Network) and so on.
The control unit 100 mainly comprises a CPU (or an MPU
or an MCU), and general functions performed by programs within
the CPU are shown in FIG.2.
Functions and operations of the control unit 100 will be
described with reference to FIG.2. As shown in FIG.2, the steering
torque T detected by the torque sensor 10 and the velocity V detected
by the velocity sensor 12 are inputted into a current command
value calculating section 101. The current command value
calculating section 101 decides a current command value Iref1
4
that is the desired value of the current supplied to the motor
20 based on the steering torque T and the velocity V and by means
of an assist map and so on. The current command value Iref1 is
added in an addition section 102A and then the added value is
inputted into a current limiting section 103 as a current command
value Iref2. A current command value Iref3 that is limited the
maximum current, is inputted into a subtraction section 102B,
and a deviation Iref4 (=Iref3-Im) between the current command
value Iref3 and a motor current value Im that is fed back, is
calculated. The deviation Iref4 is inputted into a PI control
section 104 serving as the current control section. The voltage
command value E that characteristic improvement is performed in
the PI control section 104, is inputed into a PWM control section
105. Furthermore, the motor 20 is PWM-driven through an inverter
106 serving as a drive section. The current value Im of the motor
20 is detected by a motor current detector 107 and is fed back
to the subtraction section 102B. In general, the inverter 106
uses EFTs as switching elements and is comprised of a bridge circuit
of FETs.
Further, a compensation signal CM from a compensation
section 110 is added in the addition section 102A, and the
compensation of the system is performed by the addition of the
compensation signal CM so as to improve a convergence, an inertia
characteristic and so on. The compensation section 110 adds a
self-aligning torque (SAT) 113 and an inertia 112 in an addition
section 114, further adds the result of addition performed in
the addition section 114 and a convergence 111 in an addition
5
section 115, and then outputs the result of addition performed
in the addition section 115 as the compensation signal CM.
In the case that the motor 20 is a 3-phase brushless motor,
details of the PWM control section 105 and the inverter 106 become
a configuration such as shown in FIG.3. That is, the PWM control
section 105 comprises a duty calculating section 105A that
calculates PWM duty command values D1 ~ D6 of three phases according
to a given expression based on the voltage command value E, dead
time sections 105C1 ~ 105C3 that set a dead time with respect
to the PWM duty command values D4 ~ D6 respectively, and a gate
driving section 105B that drives each gate of FET1 ~ FET3 by the
PWM duty command values D1 ~ D3 and simultaneously switches on/off
after driving each gate of FET4 ~ FET6 by PWM duty command values
D4d ~ D6d that the dead time from the dead time sections 105C1
~ 105C3 is set respectively. The inverter 106 comprises a
three-phase bridge having top and bottom arms comprised of FET1
and FET4, top and bottom arms comprised of FET2 and FET5, and
top and bottom arms comprised of FET3 and FET6, and drives the
motor 20 by being switched ON/OFF based on the PWM duty command
values D1 ~ D3 and D4d ~ D6d.
Here, the reason for setting the dead times by the dead
time sections 105C1 ~ 105C3 is the following.
Every the top and bottom arms that comprise the inverter
106, for example, FET1 and FET4 alternately repeat ON/OFF, in
the same way, FET2 and FET5 alternately repeat ON/OFF, and also
FET3 and FET6 alternately repeat ON/OFF. However, FET is not
an ideal switch and requires a turn on time Ton and a turn off
6
time Toff without instantly performing ON/OFF as instructed by
gate signals. As a result, for example, when an ON-instruction
for FET1 and an OFF-instruction for FET4 are issued at the same
time, FET1 and FET4 become ON at the same time and there is a
problem that the top and bottom arms short. Therefore, in order
not to generate a flow-through current by turning FET1 and FET4
on at the same time, in the case of giving an OFF-signal to the
gate drive section 105B, by giving an ON-signal to the gate drive
section 105B after the elapse of a given time called the dead
time in the dead time section 105C1 without giving an ON-signal
to the gate drive section 105B immediately, short of the top and
bottom arms comprised of FET1 and FET4 can be prevented. In the
same way, this is applied to other FET2 ~ FET6 as well.
However, existence of the above dead time becomes a cause
that causes problems such as insufficient torque and torque ripple
for control of the electric power steering apparatus.
At first, the dead time, the turn on time and the turn
off time will be described with reference to FIG.4. The duty
command value D1 (D4) from the duty calculating section 105A shown
in FIG.4(A), is set as an ON/OFF-signal with respect to FET1 and
FET4. However actually, a gate signal K1 shown in FIG.4(B) is
given to FET1, and a gate signal K2 shown in FIG.4(C) is given
to FET4. That is, with respect to both of the gate signals K1
and K2, a dead time Td is ensured. A terminal voltage comprised
of FET1 and FET4 is set as Van shown in FIG.4(D). Even the ON-signal
based on the gate signal K1 is given, FET1 turns on after the
elapse of the turn on time Ton without performing ON immediately.
7
Further, even the OFF-signal is given, FET1 turns off after the
elapse of the turn off time Toff without performing OFF immediately.
In addition, “Vdc” is a power-supply voltage (a voltage of the
battery 13) of the inverter 106. Therefore, a total delay time
Ttot is indicated by the following Expression 1.
(Expression 1)
Ttot = Td + Ton - Toff
Next, influences on the electric power steering apparatus
by the dead time Td will be described.
Firstly, an influence on the voltage is as follows. As
shown in FIG.4, with respect to the ideal gate signals (D1, D4),
the actual gate signals K1 and K2 become signals that are different
from the ideal gate signals due to the influence of the dead time
Td. As a result, although voltage distortion occurs, in the case
that the direction of the motor current Im is positive (i.e. in
the case that the direction of the current flows from the power
supply to the motor), that distortion voltage ΔV becomes the
following Expression 2, and in the case that the direction of
the motor current Im is negative (i.e. in the case that the direction
of the current flows from the motor to the power supply), that
distortion voltage ΔV becomes the following Expression 3.
(Expression 2)
-ΔV = -(Ttot/Ts)·(Vdc/2)
where “Ts” is an inverse number (Ts = 1/fs) of a PWM frequency
8
fs in the case of PWM-controlling the inverter 106.
(Expression 3)
ΔV = (Ttot/Ts)·(Vdc/2)
By representing the above Expressions 2 and 3 in one
expression, the following Expression 4 can be obtained.
(Expression 4)
ΔV = -sign(Im)·(Ttot/Ts)·(Vdc/2)
where sign(Im) represents the polarity of the motor current
Im.
It is derived from the above Expression 4 that when the
PWM frequency fs is high and the power-supply voltage Vdc is large,
as the distortion voltage ΔV is high, the influence of the dead
time Td greatly appears.
Although the influence of the dead time Td with respect
to the voltage distortion is described as above, even with respect
to the current or the torque, there are undesirable influences
caused by the dead time Td. With respect to current distortion,
when the current changes from positive to negative or from negative
to positive, the dead time Td causes a zero clamping phenomenon
(i.e. a phenomenon that the current sticks to the vicinity of
zero). This is because, since a load (the motor) is inductance,
there is a trend that voltage drop caused by the dead time Td
keeps the current at zero.
9
Further, the influence of the dead time Td with respect
to the torque, appears in an insufficient output torque and an
increase in torque ripple. That is, the current distortion
generates a low order harmonics, and that is conducive to the
increase in the torque ripple. Moreover, since the actual current
that is affected by the dead time Td, becomes smaller than the
ideal current, the lack of output torque occurs.
In order to prevent such an undesirable influence of the
dead time Td, various measures (so called “dead time
compensation”) are considered. The basic concept is to
compensate the distortion voltage ΔV shown in the above Expression
4. Therefore, the compensating expression 4 is to correct by
means of a dead time correction value (voltage) Δu shown in the
following Expression 5.
(Expression 5)
Δu = sign(Im)·(Ttot/Ts)·(Vdc/2)
In the dead time compensation, there is a problem that
it is impossible to accurately detect the polarity sign(Im) of
the current Im. When measuring the polarity of the current Im,
noises of the PWM control and the above-described zero clamping
phenomenon of the current make it difficult to accurately measure
the polarity of the current Im.
Furthermore, in the electric power steering apparatus,
in a straight running, with respect to characteristics of the
vicinity of a steering neutral position, a fine control such as
10
repeating a steering reverse with a weak current is required
constantly. In particular, since it is a straight running state,
for example, in running at a high speed, road vibration being
transmitted to the steering wheel is small, thus unstable elements
of the assist easily transmit as vibrations.
The List of Prior Art Documents
Patent Documents
Patent Document 1: Japanese Patent Application Laid-Open
No.2006-199140
Patent Document 2: Japanese Patent Application Laid-Open
No. H11-27951
Patent Document 3: Japanese Patent Application Laid-Open
No.2009-5485
Summary of the Invention
Problems to be Solved by the Invention
As a means for solving the above-described problems, a
control device for an electric power steering apparatus disclosed
in Japanese Patent Application Laid-Open No.2006-199140 (Patent
Document 1), is proposed. In this control device, by calculating
compensation amount of the dead time compensation and the sign
of the current based on the current and steering conditions and
adding to the voltage command value, in accordance with various
steering conditions and loading statuses, an optimal dead time
compensation value is set from the view of steering feeling.
With respect to setting a dead time of an inverter, although
11
a predetermined value is generally set in a CPU (such as a
microcomputer), a value of the actual dead time varies with a
temperature change of the switching element (FET). However, in
the device of Patent Document 1, since correcting the dead time
compensation amount based on steering conditions does not consider
the temperature change of the switching element, in the case that
the actual dead time changed with the temperature change, the
dead time becomes disaccording with the dead time compensation
amount, thus there is a possibility that the current distortion
occurs without being able to perform a suitable compensation and
the torque ripple gets worse. In particular, outside the vicinity
of the steering neutral position, although wanting to accord the
dead time with the dead time compensation amount and improve the
responsibility of the steering, when using a compensation amount
that is set at an ordinary temperature, for example, under a
high-temperature environment, there are characteristics that the
compensation amount becomes overmuch, thus there is a possibility
that the current distortion occurs and the torque ripple occurs
easily.
Further, although an inverter control apparatus disclosed
in Japanese Patent Application Laid-Open No. H11-27951 (Patent
Document 2), corrects the dead time compensation amount based
on a thermistor temperature, since the inverter control apparatus
does not relate to an electric power steering apparatus, it is
not completely considered to correct the dead time compensation
amount based on steering conditions. Thus, it is impossible to
apply to the electric power steering apparatus.
12
Moreover, although a dead time correction method disclosed
in Japanese Patent Application Laid-Open No.2009-5485 (Patent
Document 3), corrects a dead time set value itself based on a
temperature detected by a temperature change detection means,
this dead time correction method is a method for suppressing an
increase in an apparatus temperature, and switches the width of
the dead time depending on a temperature change with a flow-through
current at the time of an ON/OFF-switching of top and bottom arms
of a motor drive circuit. Therefore, this dead time correction
method is not a method that considers the environment of a vehicle,
and does not become a substantive solution of the torque ripple.
The present invention has been developed in view of the
above-described circumstances, and an object of the present
invention is to provide a control device for an electric power
steering apparatus that suppresses disaccord between the actual
dead time and the dead time compensation value by correcting the
dead time compensation value based on the temperature of the
switching element (the inverter) and reduces the distortion of
the motor current and the occurrence of the torque ripple, and
simultaneously reduces the occurrence of noises by performing
a dead time compensation corresponding to the steering conditions
and constantly obtains good steering performances even under an
environment from a low temperature to a high temperature.
Means for Solving the Problems
The present invention relates to a control device for an
electric power steering apparatus that controls a motor providing
13
a steering mechanism with a steering assist force by means of
an inverter based on a current command value calculated based
on a steering torque generated in a steering shaft and a voltage
command value from a current control section inputting said current
command value, the above-described object of the present invention
is achieved by that comprising: a dead time characteristic section
that calculates a dead time characteristic value based on said
current command value; a steering status determining section that
determines a steering status of a steering wheel; a gain section
that varies a gain of said dead time characteristic value in
accordance with a determination of said steering status
determining section; a polarity determining section that switches
polarity determining methods in accordance with said
determination of said steering status determining section and
simultaneously determines a polarity based on a detected current
of said motor, said current command value, or a model current
based on said current command value; a temperature sensor that
detects a temperature of said inverter; a dead time temperature
correction value calculating section that calculates a dead time
temperature correction value corresponding to said temperature;
and a calculation processing section that calculates and processes
said dead time temperature correction value with respect to a
dead time compensation value with polarity that is determined
by said polarity determining section based on an output of said
gain section and outputs a dead time compensation value, wherein
a dead time of said inverter is compensated by adding said dead
time compensation value to said voltage command value.
14
Further, the above-described object of the present
invention is more effectively achieved by that wherein at a time
of a high temperature of said inverter, decreasing said dead time
compensation value, and at a time of a low temperature of said
inverter, increasing said dead time compensation value; or wherein
said dead time temperature correction value calculating section
is comprised of a temperature correction limit value calculating
section that performs calculation of a temperature correction
limit value, and said calculation processing section is comprised
of a temperature-sensitive limiter; or wherein said dead time
temperature correction value calculating section is comprised
of a temperature correction subtraction value calculating section
that performs calculation of a temperature correction subtraction
value, and said calculation processing section is comprised of
a subtraction section; or wherein said dead time temperature
correction value calculating section is comprised of a temperature
correction gain calculating section that performs calculation
of a temperature correction gain, and said calculation processing
section is comprised of a multiplication section.
Effects of the Invention
According to a control device for an electric power steering
apparatus of the present invention, since performing the dead
time compensation with respect to the voltage command value by
means of a dead time compensation value considering the temperature
of the inverter, being different from the dead time compensation
based on a measured current including noises, it is possible to
15
provide a high performance control device for an electric power
steering apparatus that distortions of the motor voltage and the
motor current are small and furthermore regardless of the
temperature change, constantly performs the dead time
compensation with small torque ripple.
Further, since performing the dead time compensation that
also considers a motor current change corresponding to a steering
status, being different from the dead time compensation based
on only a fixed value, it is possible that distortions of the
motor voltage and the motor current are small, and it is possible
to perform the dead time compensation with the small torque ripple
depending on the steering status.
Brief Description of the Drawings
In the accompanying drawings:
FIG.1 is a diagram illustrating a configuration example
of a general electric power steering apparatus;
FIG.2 is a block diagram showing an example of a control
unit;
FIG.3 is a wiring diagram showing a configuration example
of a PWM control section and an inverter;
FIG.4 shows time charts that illustrate relationships among
a dead time, a turn on time and a turn off time;
FIG.5 shows a low temperature characteristic diagram and
a high temperature characteristic diagram that illustrate
examples of temperature variation of a dead band of a switching
element;
16
FIG.6 is a characteristic diagram showing one example of
temperature variation of the width of a dead band of a switching
element;
FIG.7 shows characteristic diagrams (in the case that
compensation is insufficient, in the case that compensation is
adequate, and in the case that compensation is overmuch) that
illustrate characteristics at the point of current zero crossing
caused by dead time compensation;
FIG.8 is a block diagram showing a configuration example
of the present invention;
FIG.9 is a block diagram showing a configuration example
(a first embodiment) of a dead time compensation section;
FIG.10 is a characteristic diagram showing one example
of dead time characteristics;
FIG.11 is a characteristic diagram showing a characteristic
example of a temperature correction limit value calculating
section;
FIG.12 is a characteristic diagram showing a characteristic
example of a temperature-sensitive limiter;
FIG.13 is a diagram illustrating a turning/returning
determination of a steering wheel;
FIG.14 is a block diagram showing a configuration example
(a second embodiment) of the dead time compensation section;
FIG.15 is a characteristic diagram showing a characteristic
example of a temperature correction subtraction value calculating
section;
FIG.16 is a characteristic diagram showing one example
17
of compensation value variations with temperature;
FIG.17 is a block diagram showing a configuration example
(a third embodiment) of the dead time compensation section;
FIG.18 is a characteristic diagram showing a characteristic
example of a temperature correction gain calculating section;
and
FIG.19 is a characteristic diagram showing one example
of compensation value variations with temperature.
Mode for Carrying Out the Invention
A dead time that is given for preventing a flow-through
current of an inverter comprised of switching elements (such as
FETs, IGBTs, TRIACs and so on), is generated as a characteristic
distortion (a dead band DB) of an output current with respect
to a duty command value at the time of zero ampere cross, for
example as shown in FIG.5(A). However, when this dead band DB
is set as a dead band in the case of a low temperature (for example
0 ℃) as shown in FIG.5(A), in the case that the temperature changes
and becomes a high temperature (for example 40 ℃), the dead band
becomes narrow as shown in FIG.5(B) (the dead band DB’ ( < DB)).
In general, due to the characteristics of the switching elements,
the width of the dead band widens when the temperature becomes
low, and the width of the dead band narrows when the temperature
becomes high. Temperature characteristic of the width of the
dead band is shown in FIG.6 in the case of FETs configuring the
inverter. That is, when “t” represents the temperature of the
18
inverter (FETs), “C” represents a temperature coefficient, and
DB0 represents the width of the dead band at 0 ℃, the width of
the actual dead band DB can be represented by the following
Expression 6.
(Expression 6)
DB = -C·t + DB0
Here, the dead time compensation is to apply a compensation
voltage with a timing of zero ampere cross, and is to eliminate
the characteristic distortions (DB, DB’) of the output current
that are shown in FIG.5(A) and FIG.5(B). That is, by setting
the width of the actual dead band DB expressed in Expression 6
as the dead time compensation amount, it is possible to obtain
a characteristic without a current distortion shown in FIG.7(B).
However, when correcting the dead time compensation amount by
the steering conditions only, since the width of the actual dead
band DB varies with the temperature, at the time of the temperature
decline, as shown in FIG.7(A), the compensation becomes
insufficient, on the other hand, at the time of the temperature
rise, as shown in FIG.7(C), the compensation becomes overmuch.
The present invention performs the calculation of the dead
time compensation value in accordance with the temperature of
the inverter and the steering status such as turning, returning
or release of the steering wheel, and simultaneously performs
the dead time compensation with respect to the voltage command
value of the inverter that drives the motor. As a result, even
19
the temperature varies (-40 ~ 80 ℃), it is possible that distortions
of the motor voltage and the motor current are constantly small,
and it is possible to realize a high-performance dead time
compensation with the small torque ripple.
Hereinafter, embodiments of the present invention will
be described with reference to the accompanying drawings.
FIG.8 shows a configuration example of the present
invention corresponding to FIG.2. As shown in FIG.8, the present
invention is provided with a dead time compensation section 200
that calculates a dead time compensation value Δu and compensates
a dead band that appears in an actual current of the inverter
106, and simultaneously provided with a temperature sensor 300
that detects a temperature t of the inverter 106. Further, a
rotation sensor 301 such as a resolver is attached to the motor
20, and the present invention is also provided with a rotation
angle detection section 302 for detecting a rotation angle θ from
an output signal of the rotation sensor 301 and an angular velocity
detection section 303 for detecting a motor angular velocity ω
from the rotation angle θ. The steering torque T, the velocity
V, the rotation angle θ, the angular velocity ω, the current command
value Iref2 and the temperature t are respectively inputted into
the dead time compensation section 200. The dead time
compensation section 200 calculates the dead time compensation
value Δu, and the calculated dead time compensation value Δu is
added to the voltage command value E in an addition section 201.
A voltage command value E’( = E + Δu) that is obtained by the
addition at the addition section 201, is inputted into the PWM
20
control section 105 and PWM-controlled, and drives the motor 20
by the inverter 106. As the input to the dead time compensation
section 200, it is possible to use the voltage command value E
in placing of the current command value Iref2.
Next, a configuration example (a first embodiment) of the
dead time compensation section 200 will be described with
reference to FIG.9.
The current command value Iref2 from the addition section
102A, is inputted into a steering status determining section 210
and simultaneously inputted into a dead time characteristic
section (a calculating section) 211. A dead time characteristic
value Dt from the dead time characteristic section (the
calculating section) 211, is inputted into a gain section 212.
The dead time characteristic section (the calculating section)
211, outputs the dead time characteristic value Dt having a dead
time characteristic of a characteristic shown in FIG.10 with
respect to the current command value Iref2. Further, a polarity
determining section 213 is to determine the polarity of the input
signal by characteristics with hysteresis, and the detected motor
current Im, the current command value Iref2 or a model current
based on the current command value Iref1 is inputted into the
polarity determining section 213. Based on a steering status
signal ST1 from the steering status determining section 210, the
polarity determining section 213 changes a hysteresis width. By
converting the current command value Iref1 by the transfer
function of the following Expression 7, the model current can
be obtained.
21
(Expression 7)
MR(s) = 1/(1 + Tc·s)
where, Tc = 1/(2π·fc) holds, and “fc” is a cutoff frequency
of the current control loop.
A linear delay function represented by the above Expression
7 is a model function of the current control loop that is derived
from a transfer function 1/(R + s·L) representing the motor 20
based on the PI control section 104, the PWM control section 105,
the inverter 106 and the motor current detector 107.
Here, the actual motor current Im greatly includes noises,
and this makes it difficult to perform a polarity determination
in the vicinity of zero current. Therefore, if generating the
model current of the motor 20 based on the current command value
Iref1 without noises via a linear delay circuit by not using the
actual motor current Im, and then determining the polarity based
on the model current, it becomes more effective.
The steering status determining section 210 comprises a
steering wheel’s release determining function and a
turning/returning determining function, and the motor angular
velocity ω, the steering torque T, the velocity V, the motor
rotation angle θ and the current command value Iref2 are
respectively inputted into the steering status determining
section 210. In the case that the steering status determining
section 210 determines that the steering wheel is released, the
steering status signal ST1 is inputted into the polarity
22
determining section 213. On the other hand, in the case that
the steering status determining section 210 determines that the
steering wheel is turned or returned, a steering status signal
ST2 is inputted into the gain section 212. The polarity sign(Pi)
determined by the polarity determining section 213, is inputted
into a multiplication section 214, and then multiplied by a
gain-adjusted dead time characteristic value Dta from the gain
section 212. A dead time characteristic value with polarity Dtb
that is the result “sign(Pi)·Dta” of the multiplication performed
in the multiplication section 214, is inputted into a
temperature-sensitive limiter 216 as a calculation processing
section that outputs the dead time compensation value Δu. The
temperature t from the temperature sensor 300, is inputted into
a temperature correction limit value calculating section 215 as
a dead time temperature correction value calculating section,
and calculates a temperature correction limit value tr as a dead
time temperature correction value by a characteristic shown in
such as FIG.11. The calculated temperature correction limit
value tr, is inputted into the temperature-sensitive limiter 216,
and then the temperature-sensitive limiter 216 outputs the dead
time compensation value Δu that is obtained by limiting a top
and a bottom of the dead time characteristic value Dtb with polarity
in accordance with a characteristic shown in FIG.12.
In addition, the steering wheel’s release determining
function of the steering status determining section 210, outputs
the steering status signal ST1 when determining a release of the
steering wheel (with a driver’s hands off the steering wheel)
23
that the steering wheel does not rotate and the steering assist
is not performed based on the velocity V, the motor angular velocity
ω and the current command value Iref2. On the other hand, the
turning/returning determining function of the steering status
determining section 210, determines a turning in the case that
the motor angular velocity ω and the steering torque T are the
same direction, determines a returning in the case that the motor
angular velocity ω and the steering torque T are different in
the direction, as shown in FIG.13, based on the motor angular
velocity ω and the steering torque T, and then outputs the steering
status signal ST2.
In such a configuration, the operation will be described.
The detected current Im of the motor 20, the current command
value Iref2, or the model current based on the current command
value Iref1 is inputted into the polarity determining section
213 with the steering status signal ST1, and its polarity is
determined. The sign(Pi) that is the output of the polarity
determining section 213, is outputted in a form of (+1) or (-1)
as shown in the following Expression 8.
As described above, due to the noises or the like, it is
very difficult to measure the actual motor current and the actual
inverter current, and accurately determine the polarity. However,
if using the model current and determining its polarity, the
detection of the polarity becomes easy.
(Expression 8)
sign(Pi) = (+1) or (-1)
24
Moreover, the polarity determining section 213 is a
polarity determination with the hysteresis, and sets a hysteresis
width of the polarity determination in accordance with the steering
status signal ST1 in the case of determining the release of the
steering wheel (with a driver’s hands off the steering wheel)
as follows.
(Expression 9)
During release of the steering wheel (ST1=1):
the hysteresis width is large
During steering of the steering wheel (ST1=0):
the hysteresis width is small
In the case that the deflection of the current command
exceeds the hysteresis width of the dead time compensation, the
output direction of the dead time compensation switches from
positive to negative or from negative to positive, this causes
self-excited vibrations by a closed loop including the torque
control so as to become a noisy sound. This is a problem that
may occurs in such a situation that the command value varies
centering around almost zero ampere due to disturbances. Since
the command value becomes equal to or more than a certain value
in the steering status, the steering status does not cause the
self-excited vibrations. Therefore, in the release status that
determination of the current command value is difficult, in order
to eliminate sensitivity for a variation in the command value,
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increasing the hysteresis width. By contrast, the steering
status causes a delay of the dead time compensation and generates
the torque ripple, decreasing the hysteresis width during the
steering of the steering wheel.
Further, the steering wheel’s release determining function
within the steering status determining section 210, inputs the
velocity V, the motor angular velocity ω and the current command
value Iref2, and outputs a status determining signal ST=1 when
the following Expression 10 holds.
(Expression 10)
0 < velocity V < given value α, and
motor angular velocity ω < given value β, and
|current command value Iref2| < given value γ, and
steering torque T < given value T0, or
rotation angle θ < given value θ0
Moreover, the given value α is a velocity that sounds caused
by self-excited vibrations can be ignored, the given value β
is a small value that the noises are not detected, and the given
value γ is a small value that the noises are not detected.
Further, the dead time characteristic value Dt from the
dead time characteristic section 211 is inputted into the gain
section 212, and is gain-adjusted in accordance with the steering
status signal ST2 from the turning/returning determining function
26
within the steering status determining section 210. The
determination of the turning/returning is determined as shown
in FIG.13, since the correction is required during the turning,
the gain of the gain section 212 is set to “1” in accordance with
the steering status signal ST2, and since the correction is not
required during the returning, the gain of the gain section 212
is set to “0” or a small value in accordance with the steering
status signal ST2.
In this way, the dead time characteristic value Dta that
is gain-adjusted in accordance with the steering status signal
ST2 from the turning/returning determining function, is
polarity-assigned in accordance with the polarity (positive or
negative) from the polarity determining section 213 in the
multiplication section 214 and inputted into the
temperature-sensitive limiter 216. Then, the
temperature-sensitive limiter 216 outputs the dead time
compensation value Δu based on the characteristic shown in FIG.12
in accordance with the temperature correction limit value tr from
the temperature correction limit value calculating section 215.
The dead time compensation value Δu calculated in such a way,
is added to the voltage command value E that is the output of
the PI control section 104 shown in FIG.2, in the addition section
201. The purpose of adding the temperature-sensitive dead time
compensation value Δu to the voltage command value E, is to add
the compensation value Δu improving the voltage and the current
distortions and the torque ripple that are caused by the dead
time for preventing top/bottom arm short to a basic control
27
indicated by the voltage command value E so as to control.
Next, another configuration example (a second embodiment)
of the dead time compensation section 200 will be described with
reference to FIG.14 corresponding to FIG.9. With respect to the
configurations that are the same as FIG.9, the identical reference
numerals are given without adding the explanations.
This embodiment is provided with a temperature correction
subtraction value calculating section 220 that calculates a
temperature correction subtraction value ts as the dead time
temperature correction value in accordance with the temperature
t from the temperature sensor 300 as the dead time temperature
correction value calculating section, and simultaneously is
provided with a subtraction section 221 that subtracts the
temperature correction subtraction value ts from the dead time
characteristic value Dtb with the polarity from the multiplication
section 214 as the calculation processing section. The
subtraction section 221 subtracts the temperature correction
subtraction value ts from the dead time characteristic value Dtb
with the polarity and outputs a temperature-sensitive dead time
compensation value Δu1. A relationship between the temperature
t and the temperature correction subtraction value ts in the
temperature correction subtraction value calculating section 220
is a solid line or a dashed line shown in FIG.15. By subtracting
the temperature correction subtraction value ts from the dead
time characteristic value Dtb with the polarity in the subtraction
section 221, it is possible to perform a temperature correction
shown in FIG.16. A solid line of FIG.16 is a characteristic of
28
the present invention, and a dashed line of FIG.16 is a
characteristic in the case of not performing a temperature
correction.
Moreover, another configuration example (a third
embodiment) of the dead time compensation section 200 will be
described with reference to FIG.17 corresponding to FIG.9. With
respect to the configurations that are the same as FIG.9, the
identical reference numerals are given without adding the
explanations.
This embodiment is provided with a temperature correction
gain calculating section 230 that calculates a temperature
correction gain tg as the dead time compensation value in
accordance with the temperature t from the temperature sensor
300 as the dead time compensation value calculating section, and
simultaneously is provided with a multiplication section 231 that
multiplies the dead time characteristic value Dtb with the
polarity from the multiplication section 214 by the temperature
correction gain tg as the calculation processing section. The
multiplication section 231 multiplies the dead time
characteristic value Dtb with the polarity by the temperature
correction gain tg and outputs a temperature-sensitive dead time
compensation value Δu2. FIG.18 shows a relationship between the
temperature t and the temperature correction gain tg in the
temperature correction gain calculating section 230. By
multiplying the dead time characteristic value Dtb with the
polarity by the temperature correction gain tg in the
multiplication section 231, it is possible to perform a
29
temperature correction shown in FIG.19. A solid line of FIG.19
is a characteristic of the present invention, and a dashed line
of FIG.19 is a characteristic in the case of not performing a
temperature correction.
Moreover, although the above are descriptions about a
three-phase motor, in the same way, it is possible to apply the
present invention to other motor such as a two-phase motor.
Explanation of Reference Numerals
1 steering wheel
2 column shaft (steering shaft)
10 torque sensor
12 velocity sensor
20 motor
100 control unit
110 compensation section
200 dead time compensation section
210 steering status determining section
211 dead time characteristic section (calculating section)
212 gain section
213 polarity determining section
215 temperature correction limit value calculating section
216 temperature-sensitive limiter
220 temperature correction subtraction value calculating
section
230 temperature correction gain calculating section
300 temperature sensor
30
301 rotation sensor
302 rotation angle detection section
303 angular velocity detection section

We Claim:
1. A control device for an electric power steering apparatus
that controls a motor providing a steering mechanism with a
steering assist force by means of an inverter based on current
command value calculated based on a steering torque generated
in a steering shaft and a voltage command value from a current
control section inputting said current command value, comprising:
a dead time characteristic section that calculates a dead
time characteristic value based on said current command value;
a steering status determining section that determines a
steering status of a steering wheel;
a gain section that varies a gain of said dead time
characteristic value in accordance with a determination of said
steering status determining section;
a polarity determining section that switches polarity
determining methods in accordance with said determination of said
steering status determining section and simultaneously
determines a polarity based on a detected current of said motor,
said current command value, or a model current based on said current
command value;
a temperature sensor that detects a temperature of said
inverter;
a dead time temperature correction value calculating
section that calculates a dead time temperature correction value
corresponding to said temperature; and
a calculation processing section that calculates and
32
processes said dead time temperature correction value with respect
to a dead time compensation value with polarity that is determined
by said polarity determining section based on an output of said
gain section and outputs a dead time compensation value,
wherein a dead time of said inverter is compensated by
adding said dead time compensation value to said voltage command
value.
2. A control device for an electric power steering apparatus
according to Claim 1, wherein
at a time of a high temperature of said inverter, decreasing
said dead time compensation value, and at a time of a low temperature
of said inverter, increasing said dead time compensation value.
3. A control device for an electric power steering apparatus
according to Claim 1 or 2, wherein
said dead time temperature correction value calculating
section is comprised of a temperature correction limit value
calculating section that performs calculation of a temperature
correction limit value, and
said calculation processing section is comprised of a
temperature-sensitive limiter.
4. A control device for an electric power steering apparatus
according to Claim 1 or 2, wherein
said dead time temperature correction value calculating
section is comprised of a temperature correction subtraction value
calculating section that performs calculation of a temperature
33
correction subtraction value, and
said calculation processing section is comprised of a
subtraction section.
5. A control device for an electric power steering apparatus
according to Claim 1 or 2, wherein
said dead time temperature correction value calculating
section is comprised of a temperature correction gain calculating
section that performs calculation of a temperature correction
gain, and
said calculation processing section is comprised of a
multiplication section.