Motor-driven power steering control apparatus

The motor-driven power steering control apparatus of the invention detects overload of a motor according to the calculation of coil resistance and temperature detection based thereon. The coil resistance is calculated according to terminal voltage or effective voltage based on duty ratio, and counter-electromotive force, and the driving current. Thereby overload of the motor can be detected without delay.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a motor-driven power steering control apparatus 
assisting steering force by an electric motor. More particularly, the 
invention relates to a motor-driven power steering control apparatus for 
detecting overload of the electric motor without delay. 
2. Description of Related Art 
When a vehicle such as a car is stopped or running at a low speed, an 
enormous amount of power is required in order to operate the steering 
wheel. A power steering apparatus, assisting steering force with an 
electric motor in order to reduce steering force required for a driver, is 
well known. (Japanese Patent Application Laid-Open No. 60-35663) 
The aforementioned electric motor is provided in a narrow space in an 
engine compartment in which heat is generated. The ambient temperature 
thereof is very high by calorification of an engine. 
As explained above, the electric motor assisting steering force is 
frequently exposed to high temperature, as the working environment is not 
ideal. In such a working environment, in order not to burn the electric 
motor, the one whose output and application temperature has some 
tollerance may be used. But in that case, the electric motor itself is 
enlarged to require a wide fixing space, so it is impossible to fix, and 
results in great expense. Therefore, in the case where a small-sized 
electric motor is used, it is considered that current supply time to the 
motor and current value thereof are calculated and overload thereof is 
detected and the electric motor is protected. But, in this case, there is 
a problem that a difference of the detected overload occurs between 
initial driving and driving after a specified time of the electric motor. 
On the other hand, independently of the above case, it is considered that 
a temperature sensor for detecting the temperature of the electric motor 
is provided thereon, however, in this case, there is a problem that the 
overload state cannot be detected immediately and properly because of a 
delay of operation of the temperature sensor. 
SUMMARY OF THE INVENTION 
This invention has been devised in consideration of the above 
circumstances, and the primary object thereof is to provide a motor-driven 
power steering control apparatus for detecting overload state of an 
electric motor immediately and accurately by calculating a coil resistance 
on the basis of driving current, counter-electromotive force, and terminal 
voltage of the electric motor, thereby detecting temperature of the 
electric motor without delay. 
Another object of the present invention is to provide a motor-driven power 
steering control apparatus which can be applied to a small-sized electric 
motor, protecting the electric motor from overload by reducing the driving 
current of the electric motor when the overload state is detected. 
Still another object of the invention is to provide a motor-driven power 
steering control apparatus capable of detecting overload of an electric 
motor without being effected due to a change in battery voltage by 
calculating coil resistance on the basis of driving current, 
counter-electromotive force, and effective voltage based on duty ratio, 
thereby detecting the temperature of the electric motor. 
The above and further objects and features of the invention will more fully 
be apparent from the following detailed description with accompanying 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the following, the present invention is to be described in detail 
referring to drawings showing the embodiments. 
FIG. 1 is a block diagram of the main parts of a motor-driven power 
steering control apparatus related to the invention. A positive electrode 
of a DC power supply 1 which grounds the negative electrode thereof and 
consists of battery, is connected to a PWM (pulse width modulation) 
control unit 3 for controlling driving current of an electric motor to be 
described later, through a protection switch 2 which opens in the case 
where abnormal current flows. To this PWM control unit 3, a small-sized 
electric motor 6 for assisting steering force is connected. The electric 
motor 6 is driven by a PWM-controlled voltage of required duty ratio. 
Inside of the PWM control unit 3, a current detecting unit (not shown) 
consisting of, for example, a shunt resistance is provided for detecting a 
driving current of the electric motor 6. The PWM control unit 3 transmit a 
driving current signal S.sub.IM related to the driving current detected by 
the current detecting unit to a negative input terminal 7a of a 
differential amplifier 7 and to a coil resistance calculating unit 5. The 
differential amplifier 7 transmits a terminal voltage average value signal 
S.sub.VMA, which is the output thereof, related to a terminal voltage 
average value of the electric motor 6 to the PWM control unit 3, and coil 
resistance calculating unit 5. And a DC voltage signal S.sub.VD related to 
the voltage of DC power supply 1 is given to the coil resistance 
calculating unit 5. 
Input to a steering control unit 8 in which PID-control is performed is a 
torque signal S.sub.T related to torque which acts upon a steering axis 
(not shown) due to steering operation, a vehicle speed signal S.sub.S 
related to driving speed of a vehicle, and a steering angle signal S.sub.H 
related to rotational quantity of aforesaid steering axis respectively. 
The steering control unit 8 compensates the given torque signal S.sub.T 
according to the vehicle speed signal S.sub.S and the steering angle 
signal S.sub.H, and outputs a reference current signal S.sub.S.sub.IO 
related to the compensated torque signal S.sub.T and gives it to an upper 
limit circuit 9. 
The upper-limit circuit 9 is capable of reducing the upper-limit value of 
the given reference current signal S.sub.IO according to the output of the 
coil resistance calculating unit 5. The reference current signal S.sub.IO 
output from the upper-limit circuit 9 is input to positive input terminal 
7b of the aforesaid differential amplifier 7 and the aforesaid coil 
resistance calculating unit 5. In addition, to the coil resistance 
calculating unit 5, a rotary signal S.sub.M related to the detected number 
of revolutions of the electric motor 6 is given, and the output of the 
coil resistance calculating unit 5 is given to aforesaid upper-limit 
circuit 9. The coil resistance calculating unit 5 is capable of 
calculating a real terminal voltage V.sub.M of the electric motor by 
compensating the terminal voltage average value signal S.sub.VMA according 
to a voltage variation of the DC power supply 1 based on the given DC 
voltage signal S.sub.VD and the terminal voltage average value signal 
S.sub.VMA. The coil resistance calculating unit 5 is also capable of 
writing into a memory (not shown) inside the coil resistance calculating 
unit 5 the calculated terminal voltage V.sub.M, driving the current 
I.sub.M of the electric motor 6 due to aforesaid driving current signal 
S.sub.IM, and the number of revolutions N.sub.M of the electric motor 6 
due to the rotary signal S.sub.M respectively. 
Next, the operation of the motor-driven power steering control apparatus 
constructed as described above will be explained referring to FIG. 1 and 
FIG. 2. FIG. 2 is a flow chart showing the calculation procedure of the 
coil resistance calculating unit 5. 
When the vehicle runs, a vehicle speed sensor (not shown) detects the 
vehicle speed thereof, the vehicle speed signal S.sub.S being given to the 
steering control unit 8. In the case where the steering wheel (not shown) 
is operated, torque acted upon the steering axis is detected by a torque 
sensor (not shown). The torque signal S.sub.T and the steering angle 
signal S.sub.H related to the rotation quantity of the steering axis are 
given to the steering control unit 8. Thereby, the steering control unit 8 
compensates the given torque signal according to the vehicle speed signal 
S.sub.S and the steering angle signal S.sub.H. In the case, for example, 
where the vehicle speed or the rotation quantity of the steering axis is 
more than the specified value, the reference current signal S.sub.IO 
related to the torque signal S.sub.T is not output. And the reference 
current signal S.sub.IO for driving the electric motor 6 which assists 
steering force is given to the positive input terminal 7b of the 
differential amplifier 7 and the coil resistance calculating unit 5 
through the upper-limit circuit 9. To the coil resistance calculating unit 
5, the reference current signal S.sub.IO is given, and in the case where 
the signal is within the predetermined range, the signal to the 
upper-limit circuit 9 is not output. Therefore, the differential amplifier 
7 compares the driving current signal S.sub.IM given to the negative input 
terminal 7a with the reference current signal S.sub.IO, the output signal 
related to the difference, that is, the terminal voltage average value 
signal S.sub.VMA of the electric motor 6 being given to the PWM control 
unit 3. This allows DC voltage V.sub.D of the DC power supply 1 to be PWM 
controlled, and the PWM controlled voltage of the required duty ratio is 
supplied to the electric motor 6. The electric motor 6 generates 
rotational force corresponding to the reference current signal S.sub.IO, 
thereby assisting steering force. In addition, when the reference current 
signal S.sub.IO is more than the predetermined value, the coil resistance 
calculating unit 5 gives the output to the upper-limit circuit 9, thereby 
reducing the upper-limit value of the reference current signal S.sub.IO to 
reduce the driving current of the electric motor 6. And in the case where 
the steering operation is stopped, the torque acting upon the steering 
axis is extinguished, such that the output of the steering control unit 8 
is extinguished, and the driving of the electric motor 6 is stopped. When 
the electric motor 6 is driven, the terminal voltage average value signal 
S.sub.VMA, the driving current signal S.sub.IM, the voltage signal 
S.sub.VD and the rotary signal S.sub.M are respectively given to the coil 
resistance calculating unit 5. Thereby, prior to the calculating operation 
of the coil resistance, the coil resistance calculating unit 5 calculates 
the terminal voltage V.sub.M of the electric motor 6 corresponding to 
voltage variation of the DC power supply 1 according to the DC voltage 
signal S.sub.VD and the terminal voltage average signal S.sub.VMA. After 
that, according to the flow shown in FIG. 2, the coil resistance 
calculating unit 5 reads the driving current I.sub.M due to the driving 
current signal S.sub.IM and writes it into the memory (not shown) in the 
coil resistance calculating unit 5 (S1), along with the terminal voltage 
V.sub.M previously calculated (S2), and the number of revolutions N.sub.M 
due to the rotary signal S.sub.M (S3). Then, referring to the number of 
revolutions N.sub.M and the DC voltage V.sub.D due to the DC voltage 
signal S.sub.VD, the coil resistance calculating unit 5 calculates 
counter-electromotive force V.sub.G induced at the coil of the electric 
motor in the case where the electric motor 6 is driven (S4). Then, 
according to the terminal voltage V.sub.M, counter-electromotive force 
V.sub.G, and driving current I.sub.M, it calculates the expression of 
(V.sub.M -V.sub.G) / I.sub.M to calculate coil resistance R.sub.M of the 
electric motor 6 (S5). Next, it calculates the temperature of the electric 
motor 6 according to the coil resistance R.sub.M so calculated as above 
and the temperature coefficient of the coil resistance (S6). And in the 
case where the calculated temperature of the electric motor 6 reaches the 
predetermined value, the coil resistance calculating unit 5 generates 
output to be given to the upper-limit circuit 9. Then, the upper limit 
circuit 9 reduces the upper-limit value of the reference current signal 
S.sub.IO given from the steering control unit 8. The reference current 
signal S.sub.IO is given to the differential amplifier 7, the terminal 
voltage average value signal S.sub.VMA, being the output thereof, being 
reduced. Thereby PWM control is executed to reduce the driving current 
I.sub.M of the electric motor 6, and the electric motor 6 whose 
temperature is too high is protected from overload. 
In calculating the temperature of the electric motor, the 
counter-electromotive force V.sub.G induced by driving the electric motor 
6 is subtracted from the terminal voltage V.sub.M of the electric motor 6. 
Then the subtracted terminal voltage is divided by the driving current 
I.sub.M to calculate coil resistance R.sub.M, thereby calculating coil 
temperature of the electric motor 6. Therefore, the temperature of the 
electric motor 6 can be calculated accurately without delay. Accordingly, 
the electric motor 6 can be protected from overload immediately and 
accurately, and a small-sized electric motor can be applied to the 
motor-driven power steering, the reliability of the protection for the 
electric motor being improved. 
As described above, according to the present invention, at either the 
initial driving or after the driving, the overload of the electric motor 
can be detected accurately and without delay. Accordingly, a small-sized 
electric motor used for the motor-driven power steering can be realized 
which is capable of protecting the electric motor from overload properly. 
Next, an explanation will be given of another embodiment. In the aforesaid 
embodiment, an explanation is given of the motor-driven power steering 
control apparatus which is capable of using a small-sized electric motor 
by detecting respectively driving current, terminal voltage, and number of 
revolutions of the electric motor for assisting steering force, and 
thereby calculating coil resistance of the electric motor to calculate 
temperature of the electric motor 6. By reducing driving current of the 
electric motor when the calculated temperature reaches the predetermined 
value, the electric motor is protected from overload. In the aforesaid 
embodiment, however, sometimes overload cannot be detected accurately in 
the case where the terminal voltage of the electric motor is effected by 
the battery voltage. In this embodiment, in place of the terminal voltage, 
overload is detected by effective voltage. Overload is detected without 
being effected by the variation of the battery voltage. 
FIG. 3 is a block diagram of the main parts of the motor-driven power 
steering control apparatus of another embodiment. The positive voltage of 
the battery 1 whose negative electrode is grounded is given to the PWM 
(pulse width modulation) control unit 3 for controlling the driving 
current of the electric motor to be described later and the battery 
voltage detecting unit 4 through protective switch 2 which opens in the 
case where abnormal current flows. The DC voltage signal S.sub.B related 
to the terminal voltage of the battery detected by the battery voltage 
detecting unit 4 is given to the coil resistance calculating unit 5 for 
calculating the coil resistance of the electric motor. At PWM control unit 
3, a small-sized electric motor 6 for assisting steering force is 
connected. The electric motor 6 is driven by PWM controlled voltage due to 
a required duty ratio. And the PWM signal of the PWM control unit 3 is 
also given to a duty ratio detecting unit 10. The duty ratio signal 
S.sub.D detected by the duty ratio detecting unit 10 is given to the coil 
resistance calculating unit 5. Inside of this PWM control unit 3, an 
electric current detecting unit (not shown) consisting of, for example, a 
shunt resistance is provided for detecting a driving current of the 
electric motor 6. The driving current signal S.sub.IM related to the 
driving current detected by the electric current detecting unit is given 
respectively to the coil resistance calculating unit 5 and the negative 
input terminal--of the differential amplifier 7. 
Input to the steering control unit 8 is the torque signal S.sub.T related 
to torque acted upon the steering axis (not shown) by steering operation, 
the vehicle speed signal S.sub.S related to driving speed of the vehicle, 
the steering angle signal S.sub.H related to the rotation quantity of 
aforesaid steering axis and the rotary signal S.sub.M related to the 
rotation speed of the electric motor 6 respectively. The rotary signal 
S.sub.M is given to the coil resistance calculating unit 5. The steering 
control unit 8 compensates the given torque signal S.sub.T by the vehicle 
speed signal S.sub.S and the steering angle signal S.sub.H. The reference 
current signal S.sub.IO, related to the compensated torque signal S.sub.T, 
is output to the upper-limit circuit 9. 
The upper-limit circuit 9 is capable of reducing the upper-limit value of 
the reference current signal S.sub.IO given thereto, according to the 
output signal S.sub.R of the coil resistance calculating unit 5. The 
reference current signal S.sub.IO output from the upper-limit circuit 9 is 
given to the positive input terminal + of the aforesaid differential 
amplifier 7 and the coil resistance calculating unit 5. The output signal 
S.sub.R of the coil resistance calculating unit 5 is given to the 
upper-limit circuit 9. The coil resistance calculating unit 5 does not 
output the output signal S.sub.R in the case where the given reference 
current signal S.sub.IO is below the predetermined value. The coil 
resistance calculating unit 5 is so constructed as to calculate effective 
voltage V.sub.E of the electric motor 6 according to the DC voltage signal 
S.sub.B given to the calculating unit 5 and the duty ratio signal S.sub.D 
of the PWM control unit 3. The calculated effective voltage V.sub.E, the 
driving current I.sub.M of the electric motor 6 due to the driving current 
signal S.sub.IM, and the number of revolutions N.sub.M of the electric 
motor 6 due to the rotary signal S.sub.M are respectively stored in 
memories (not shown) inside the coil resistance calculating unit 5. 
Next, explanation will be given on the operation of the motor-driven power 
steering control apparatus so constructed as above referring to FIG. 4. 
FIG. 4 is a flow chart showing the calculating procedure of the coil 
resistance calculating unit 5. 
When a vehicle runs, a vehicle speed sensor (not shown) detects the vehicle 
speed, and the vehicle speed signal S.sub.S thereof is given to the 
steering control unit 8. When a steering wheel (not shown) is operated, 
torque acted upon the steering axis is detected by the torque sensor (not 
shown), and the torque signal S.sub.T thereof and the steering angle 
signal S.sub.H related to the rotation quantity of the steering axis are 
given to the steering control unit 8. Thereby, the steering control unit 8 
compensates the given torque signal S.sub.T according to the vehicle speed 
signal S.sub.S and the steering angle signal S.sub.H. For example, in the 
case where the vehicle speed or the rotation quantity of the steering axis 
is more than predetermined value, the reference current signal S.sub.IO 
related to the torque signal S.sub.T is not output. And the reference 
current signal S.sub.IO for driving the electric motor 6 assisting the 
steering force is given to the positive input terminal + of the 
differential amplifier 7 and the coil resistance calculating unit 5 
through the upper-limit circuit 9. To the coil resistance calculating unit 
5, the reference current signal S.sub.IO is input, and in the case where 
the signal is below the predetermined value, it does not output the output 
signal S.sub.R to the upper-limit circuit 9. Therefore, the differential 
amplifier 7 compares the driving current signal S.sub.IM given to the 
negative input terminal--thereof with the reference current signal 
S.sub.IO, and gives the output signal related to the difference, that is, 
the terminal voltage average value signal S.sub.VMA of the electric motor 
6 to the PWM control unit 3. Then the voltage V.sub.B of the battery 1, 
given to the PWM control unit 3, is PWM controlled, and the voltage with 
the required duty ratio is given to the electric motor 6, and the electric 
motor 6 generates the turning force corresponding to the reference current 
signal S.sub. IO and assists the steering force. In addition, when the 
reference current signal S.sub.IO is more the predetermined value, the 
coil resistance calculating unit 5 gives the output signal S.sub.R to the 
upper-limit circuit 9 to reduce the upper-limit value of the reference 
current signal S.sub.IO, thereby reducing the driving current I.sub.M of 
the electric motor 6. In the case where the steering operation is stopped, 
the torque acting upon the steering axis is extinguished causing the 
reference current signal S.sub.IO outputted from the steering control unit 
8 to be extinguished and the driving of the electric motor 6 to be 
stopped. By the way, in the case where the electric motor 6 is driven, the 
duty ratio signal S.sub.D for PWM controlling, the driving current signal 
S.sub.IM, the DC voltage signal S.sub.B and the rotary signal S.sub.M are 
respectively given to the coil resistance calculating unit 5. According to 
the flow shown in FIG. 4, the battery voltage V.sub.B is read and written 
into the memory (not shown) in the coil resistance calculating unit 5 
(S1). Next the duty ratio D of the battery voltage V.sub.B for PWM 
controlling (S2) is read. According to the battery voltage V.sub.B and the 
duty ratio D, the effective voltage V.sub.E of the electric motor 6 is 
calculated by V.sub.B .times.D (S3). Next, according to the rotary signal 
S.sub.M, the number of revolutions N.sub.M of the electric motor 6 is read 
and written into the memory (S4), and according to the number of 
revolutions N.sub.M and voltage generation coefficient K.sub.M previously 
stored in the coil resistance calculating unit 5, the 
counter-electromotive force V.sub.G of the electric motor 6 is calculated 
by V.sub.G =N.sub.M .times.K.sub.M (S5). Next, according to the driving 
current signal S.sub.IM, the driving current I.sub.M of the electric motor 
6 is read and written into the memory of the coil resistance calculating 
unit 5 (S6). According to the driving current I.sub.M, the effective 
voltage V.sub.E of the electric motor 6 and the counter-electromotive 
force V.sub.G of the electric motor 6, the coil resistance R.sub.M of the 
electric motor 6 is calculated by R.sub.M = (V.sub.E -V.sub.G) / I.sub.M 
(S7). Next, according to the calculated coil resistance R.sub.M of the 
electric motor 6, the coil resistance R.sub.0 at the temperature t.sub.0 
previously memorized at the coil resistance calculating unit 5, and the 
temperature coefficient .alpha..sub.0 of the coil resistance at the 
temperature T.sub.0, the temperature t.sub.M of the electric motor 6 is 
calculated from t.sub.M =t.sub.0 +(1/.alpha..sub.0){(R.sub.M 
-R.sub.0)/R.sub.0 } (S8). And in the case where the calculated temperature 
t.sub.M of the electric motor reaches the predetermined value, that is, 
the electric motor 6 is in the state of overload, the coil resistance 
calculating unit 5 outputs the output signal S.sub.R to be given to the 
upper-limit circuit 9. The upper-limit circuit 9 reduces the upper-limit 
value of the reference current signal S.sub.IO given from the steering 
control unit 8. The reference current signal S.sub.IO is given to the 
differential amplifier 7 and the terminal voltage average value signal 
S.sub.VMA, which is the output thereof, is reduced, causing the battery 
voltage V.sub.B being PWM controlled to reduce the driving current I.sub.M 
of the electric motor 6. The steering force is assisted to protect the 
electric motor 6 whose temperature has risen from overload. 
In calculating the temperature of the electric motor, the effective voltage 
V.sub.E of the electric motor is calculated according to the battery 
voltage V.sub.B of the battery for driving the electric motor and the duty 
ratio D of the voltage age to be given to the electric motor for PWM 
controlling. The value obtained by subtracting the counter-electromotive 
force V.sub.G of the electric motor from the calculated effective voltage 
V.sub.E, is divided by the driving current I.sub.M of the electric motor 
to calculate the coil resistance R.sub.M of the electric motor and the 
temperature of the electric motor. Therefore, the calculation of the 
temperature t.sub.M of the electric motor is not effected by the voltage 
variation of the battery, and can be calculated immediately. Accordingly, 
overload of the electric motor assisting the steering force can be 
detected properly regardless of the voltage variation of the battery, 
thereby protecting the electric motor. Therefore, a small-sized electric 
motor can be used to the motor-driven power steering, and the reliability 
for protecting the electric motor can be improved. 
As explained above, according to this embodiment, even if voltage variation 
of the DC supply which drives the electric motor assisting the steering 
force is created, the apparatus is not effected, and the overload of the 
electric motor can be detected accurately and immediately to control the 
driving current of the electric motor. Thereby, a small-sized electric 
motor can be used which is always surely protected from overload. 
Accordingly, a motor-driven power steering control apparatus which has high 
reliability and is inexpensive can be obtained. 
As this invention may be embodied in several forms without departing from 
the spirit of essential characteristics thereof, the present embodiment is 
illustrative and not restrictive. The scope of the invention is defined by 
the appended claims rather than by the description preceding them, and all 
changes that fall within the metes and bounds of the claims, or 
equivalence of such metes and bounds thereof are therefore intended to be 
embraced by the claims.