Device for controlling the idling speed of an engine

An engine comprising a main intake passage having a throttle valve therein. A bypass passage is branched off from the main intake passage located upstream of the throttle valve and is connected to the main intake passage located downstream of the throttle valve. A flow control valve, actuated by a step motor, is arranged in the bypass passage. The step motor comprises a rotor and a stator having exciting coils. When the engine is operating in an idling state, the exciting coils are excited for rotating the step motor in a rotating direction wherein the engine speed approaches a desired idling speed. When the engine speed becomes equal to the desired engine speed and is higher than a predetermined speed, the exciting coil, which was finally excited immediately before the engine speed becomes equal to the desired idling speed, is intermittently excited in order to maintain the step motor stationary.

BACKGROUND OF THE INVENTION 
The present invention relates to a device for controlling the idling speed 
of an internal combustion engine. 
An idling speed control device has been known in which a bypass passage 
branches off from the intake passage of an engine upstream of a throttle 
valve, and connects again to the intake passage downstream of the throttle 
valve. A diaphragm type vacuum operated control valve device is arranged 
in the bypass passage. The diaphragm vacuum chamber of the control valve 
device is connected via a vacuum conduit to the intake passage located 
downstream of the throttle valve. An electromagnetic control valve is 
arranged in the vacuum conduit for controlling the cross-sectional area of 
the vacuum conduit. In this idling speed control device during idling, the 
level of the vacuum produced in the diaphragm vacuum chamber of the 
control valve device is controlled with the electromagnetic control valve 
in accordance with the operating condition of the engine and, in addition, 
the air flow area of the bypass passage is controlled in accordance with a 
change in the level of the vacuum produced in the diaphragm vacuum 
chamber. As a result of this, the amount of air fed into the cylinders of 
the engine from the bypass passage is controlled. However, in such a 
conventional idling speed control device, firstly, in the case wherein a 
vehicle is used in a cold region, the electromagnetic control valve 
becomes frozen and, thus, it is impossible to control the cross-sectional 
area of the vacuum conduit. As a result of this, since it is also 
impossible to control the air flow area of the bypass passage, a problem 
occurs in that it is impossible to control the amount of air fed into the 
cylinders from the bypass passage. Secondly, in a conventional idling 
speed control device, since the diaphragm type vacuum operated control 
valve device is used, the controllable range of the air flow area of the 
bypass passage is very narrow. Therefore, even if the control valve device 
is fully opened, the amount of air necessary to operate the engine at the 
time of fast idling cannot be fed into the cylinders of the engine from 
the bypass passage. Consequently, in a conventional idling speed control 
device, an additional bypass passage is provided in addition to the 
regular bypass passage, and a valve, which is actuated by a bimetallic 
element, is arranged in the additional bypass passage. When the 
temperature of the engine is low, the valve, which is actuated by the 
bimetallic element, opens. As a result of this, since additional air is 
fed into the cylinders of the engine from the additional bypass passage in 
addition to the air fed into the cylinders of the engine from the regular 
bypass passage, the amount of air necessary to operate the engine at the 
time of fast idling can be ensured. As mentioned above, in a conventional 
idling speed control device, since the additional bypass passage and the 
valve, actuated by the bimetallic element, are necessary in addition to 
the regular bypass passage, a problem occurs in that the construction of 
the idling speed control device becomes complicated. In addition, since 
the amount of air fed into the cylinders of the engine is controlled by 
only the expanding and shrinking action of the bimetallic element at the 
time of fast idling, there is a problem in that it is impossible to 
precisely control the amount of air fed into the cylinders of the engine. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a novel device for 
controlling the idling speed, which device is capable of precisely 
controlling the amount of air flowing within the bypass passage at the 
time of idling and maintaining the idling speed of the engine at an 
optimum speed. 
According to the present invention, there is provided a device for 
controlling the idling speed of an engine comprising a main intake 
passage, a throttle valve arranged in the main intake passage, a bypass 
passage branched off from the main intake passage upstream of the throttle 
valve and connected to the main intake passage downstream of the throttle 
valve, and a control valve arranged in the bypass passage, said device 
comprising: a step motor actuating the control valve and comprising a 
stator which has exciting coil means and a plurality of spaced pole pieces 
polarized by the exciting coil means, and a rotor having polarities the 
pitch of which is two times the pitch of the pole pieces; first means for 
detecting the engine speed to produce an output signal indicating the 
engine speed; second means for detecting the operating condition of the 
engine to produce an output signal indicating that the engine is operating 
in an idling state, and; an electronic control unit operated in response 
to the output signal of said first means and the output signal of said 
second means and exciting said exciting coil means for rotating the step 
motor in a rotating direction wherein the engine speed approaches a 
predetermined desired idling speed when the engine is operating in an 
idling state and for maintaining the step motor stationary when the engine 
speed becomes equal to the desired idling speed and when the engine speed 
is higher than a predetermined speed, said electronic control unit 
stopping the exciting operation of said exciting coil means when the 
engine speed becomes equal to the desired idling speed and when the engine 
speed is lower than the predetermined speed. 
The present invention may be more fully understood from the description of 
a preferred embodiment of the invention set forth below, together with the 
accompanying drawings.

DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring to FIG. 1, 1 designates an engine body, 2 a surge tank, 3 an 
intake duct, 4 a throttle valve and 5 an air flow meter. The inside of the 
intake duct 3 is connected to the atmosphere via the air flow meter 5 and 
an air cleaner (not shown). The surge tank 2, which is common to all the 
cylinders of the engine, has a plurality of branch pipes 6, each being 
connected to the corresponding cylinder of the engine. A fuel injector 7 
is provided for each cylinder and mounted on the corresponding branch pipe 
6. In addition, a flow control valve device 8 is mounted on the surge tank 
2. As illustrated in FIG. 2, the flow control valve device 8 comprises a 
motor housing 10 of a step motor 9, a motor housing end plate 11 and a 
valve housing 12. The motor housing 10, the end plate 11 and the valve 
housing 12 are interconnected to each other by means of bolts 13. As 
illustrated in FIGS. 1 and 2, a flange 14 is formed in one piece on the 
valve housing 12 and fixed onto the outer wall of the surge tank 2. A 
valve chamber 15 is formed in the valve housing 12 and connected via a 
bypass pipe 16, fixed onto the valve housing 12, to the inside of the 
intake duct 3, which is located upstream of the throttle valve 4. In 
addition, a hollow cylindrical projection 17, projecting into the surge 
tank 2, is formed in one piece on the side wall of the flange 14, and a 
cylindrical air outflow bore 18 is formed in the hollow cylindrical 
projection 17. An annular groove 19a is formed on the inner end of the air 
outflow bore 18, and a valve seat 19 is fitted into the annular groove 
19a. 
As illustrated in FIG. 2, the step motor 9 comprises a valve shaft 20, a 
rotor 21 coaxially arranged with the valve shaft 20, and a pair of stators 
22, 23, each being stationarily arranged in the motor housing 10 and 
spaced from the cylindrical outer wall of the rotor 21 by a slight 
distance. The end portion of the valve shaft 20 is supported by a hollow 
cylindrical bearing 24 made of a sintered metal and fixed onto the motor 
housing 10, and the intermediate portion of the valve shaft 20 is 
supported by a hollow cylindrical bearing 25 made of a sintered metal and 
fixed onto the end plate 11. A first stop pin 26, which abuts against the 
rotor 21 when the valve shaft 20 reaches the most advanced position, is 
fixed onto the valve shaft 20, and a second stop pin 27, which abuts 
against the rotor 21 when the valve shaft 20 reaches the most retracting 
position, is fixed onto the valve shaft 20. In addition, an axially 
extending slot 28, into which the first stop pin 26 is able to enter, is 
formed in the bearing 24. External screw threads 29 are formed on the 
outer circumferential wall of the valve shaft 20, which is located within 
the motor housing 10. The external screw threads 29 extend towards the 
right in FIG. 2 from the left end of the valve shaft 20 and terminate at a 
position wherein the valve shaft 20 passes through the second stop pin 27 
by a slight distance. In addition, an axially extending flat portion 30, 
which extends towards the right in FIG. 2 from a position near the 
terminating positin of the external screw threads 29, is formed on the 
outer circumferential wall of the valve shaft 20. As illustrated in FIG. 
3, the inner wall of the shaft bearing hole of the bearing 25 comprises a 
cylindrical wall portion 31 and a flat wall portion 32 which have a 
complementary shape relative to the outer circumferential wall of the 
valve shaft 20. Consequently, the valve shaft 20 is supported by the 
bearing 25 so that the valve shaft 20 cannot be rotated, but is able to 
slide in the axial direction. In addition, as illustrated in FIG. 3, an 
outwardly projecting arm 33 is fromed in one piece on the outer 
circumferential wall of the bearing 25, and a bearing receiving hole 34 
(FIG. 2), having a contour shape which is the same as that of the bearing 
25, is formed on the inner wall of the end plate 11. Consequently, when 
the bearing 25 is fitted into the bearing receiving hole 34, as 
illustrated in FIG. 2, the bearing 25 is non-rotatably supported by the 
end plate 11. A valve head 36, having a substantially conical shaped outer 
wall 35, is secured onto the tip of the valve shaft 20 by means of a nut 
37, and an annular air flow passage 38 is formed between the valve seat 19 
and the conical outer wall 35 of the valve head 36. In addition, a 
compression spring 39 is inserted between the valve head 36 and the end 
plate 11 in the valve chamber 15. 
As illustrated in FIG. 2, the rotor 21 comprises a hollow cylinderical 
inner body 40 made of a synthetic resin, a hollow cylindrical intermediate 
body 41 made of a metallic material and rigidly fitted onto the outer 
circumferential wall of the hollow cylindrical inner body 40, and a hollow 
cylindrical outer body 42 made of a permanent magnet and fixed onto the 
outer circumferential wall of the hollow cylindrical intermediate body 41 
by using an adhesive. As will be hereinafter described, an N pole and S 
pole are alternately formed on the outer circumferential wall of the 
hollow cylindrical outer body 42 made of a permanent magnet along the 
circmferential direction of the outer circumferenctial wall of the hollow 
cylindrical outer body 42. As illustrated in FIG. 2, one end of the hollow 
cylindrical intermediate body 41 is supported by the inner race 44 of a 
ball bearing 43 which is supported by the motor housing 10, and the other 
end of the hollow cylindrical intermediate body 41 is supported by the 
inner race 46 of a ball bearing 45 which is supported by the end plate 11. 
Consequently, the rotor 21 is rotatably supported by a pair of ball 
bearings 43 and 45. Internal screw threads 47, which are in engagement 
with the external screw threads 29 of the valve shaft 20, are formed on 
the inner wall of the central bore of the hollow cylindrical inner body 
40. Therefore, when the rotor 21 rotates, the valve shaft 20 is caused to 
move in the axial direction. 
The stators 22 and 23, which are stationarily arranged in the motor housing 
10, have the same construction and, therefore, the construction of only 
the stator 22 will be hereinafter described with reference to FIGS. 4 
through 7. Referring to FIGS. 4 through 7, the stator 22 comprises a pair 
of stator core members 51 and 52, and a stator coil 53. The stator core 
member 51 comprises an annular side wall portion 54, an outer cylindrical 
portion 55, and eight pole pieces 56 extending perpendicular to the 
annular side wall portion 54 from the inner periphery of the annular side 
wall portion 54. The pole pieces 56 have a substantially triangular shape, 
and each of the pole pieces 56 is spaced from the adjacent pole piece 56 
by the same angular distance. On the other hand, the stator core member 52 
comprises an annular side wall portion 57 and eight pole pieces 58 
extending perpendicular to the annular side wall portion 57 from the inner 
periphery of the annular side wall portion 57. The pole pieces 58 have a 
substantially triangular shape, and each of the pole pieces 58 is spaced 
from the adjacent pole piece 58 by the same angular distance. The stator 
core members 51 and 52 are assembled so that each of the pole pieces 56 is 
spaced from the adjacent pole piece 58 by the same angular distance, as 
illustrated in FIGS. 6 and 7. When the stator core members 51 and 52 are 
assembled, the stator core members 51 and 52 construct a stator core. When 
an electric current is fed into the stator coil 53 and flows within the 
stator coil 53 in the direction illustrated by the arrow A in FIG. 7, a 
magnetic field, the direction of which is as illustrated by the arrow B in 
FIG. 6, generates around the stator coil 53. As a result of this, the S 
poles are produced in the pole pieces 56 and, at the same time, the N 
poles are produced in the pole pieces 58. Consequently, it will be 
understood that an N pole and an S pole are alternately formed on the 
inner circumferential wall of the stator 22. On the other hand, if an 
electric current flows within the stator coil 22 in the direction which is 
opposite to that illustrated by the arrow A in FIG. 7, the N poles are 
produced in the pole pieces 56 and, at the same time, the S poles are 
produced in the pole pieces 58. 
FIG. 8 illustrates the case wherein the stator 22 and the stator 23 are 
arranged in tandem, as illustrated in FIG. 2. In FIG. 8, similar 
components of the stator 23 are indicated with the same reference numerals 
used in the stator 22. As illustrated in FIG. 8, assuming that the 
distance between the pole piece 56 of the stator 22 and the adjacent pole 
piece 58 of the stator 22 is indicated by l, each of the pole pieces 56 of 
the stator 23 is offset by l/2 from the pole piece 56 of the stator 22, 
which is arranged nearest to the pole piece 56 of the stator 23. That is, 
assuming that the distance d between the adjacent pole pieces 56 of the 
stator 23 is one pitch, each of the pole pieces 56 of the stator 23 is 
offset by a 1/4 pitch from the pole piece 56 of the stator 22, which is 
arranged nearest to the pole piece 56 of the stator 23. On the other hand, 
as illustrated in FIG. 9, the N pole and the S pole are alternately formed 
on the outer circumferential wall of the hollow cylindrical outer body 42 
of the rotor 21 along the circumferential direction of the outer 
circumferential wall of the hollow cylindrical outer body 42, and the 
distance between the N pole and the S pole, which are arranged adjacent to 
each other, is equal to the distance between the pole piece 56 and the 
pole piece 58 of the stator 22 or 23, which are arranged adjacent to each 
other. 
Turning to FIG. 1, the step motor 9 is connected to an electronic control 
unit 61 via a step motor drive circuit 60. In addition, a vehicle speed 
sensor 62, a cooling water temperature sensor 63, an engine speed sensor 
64, a throttle switch 65, and a neutral switch 66 of the automatic 
transmission (not shown) are connected to the electronic control unit 61. 
The vehicle speed sensor 62 comprises, for example, a rotary permanent 
magnet 67 arranged in the speed meter (not shown) and rotated by the speed 
meter cable (not shown), and a reed switch 68 actuated by the rotary 
permanent magnet 67. A pulse signal, having a frequency which is 
proportional to the vehicle speed, is input into the electronic control 
unit 61 from the vehicle speed sensor 62. The cooling water temperature 
sensor 63 is provided for detecting the cooling water of the engine, and a 
signal, representing the temperature of the cooling water, is input into 
the electronic control unit 61 from the cooling water temperature sensor 
63. The engine speed sensor 64 comprises a rotor 70 rotating in a 
distributor 69 in synchronization with the rotation of the crank shaft 
(not shown), and an electromagnetic pick-up 71 arranged to face the saw 
tooth shaped outer periphery of the rotor 70. A pulse is input into the 
electronic control unit 61 from the engine speed sensor 64 everytime the 
crank shaft rotates at a predetermined angle. The throttle switch 65 is 
operated by the rotating motion of the throttle valve 4 and turned to the 
ON position when the throttle valve 4 is fully closed. The operation 
signal of the throttle switch 65 is input into the electronic control unit 
61. The neutral switch 66 is provided for detecting whether the automatic 
transmission is in the drive range D or in the neutral range N, and the 
detecting signal of the neutral switch 66 is input into the electronic 
control unit 61. 
FIG. 10 illustrates the step motor drive circuit 60 and the electronic 
control unit 61. Referring to FIG. 10, the electronic control unit 61 is 
constructed as a digital computer and comprises a microprocessor (MPU) 80 
executing the arithemtic and logic processing, a random-access memory 
(RAM) 81, a read-only memory (ROM) 82 storing a predetermined control 
program and an arithmetic constant therein, an input port 83 and an output 
port 84 are interconnected to each other via a bidirectional bus 85. In 
addition, the electronic control unit 61 comprises a clock generator 86 
generating various clock signals, and a back-up RAM 88 connected to the 
MPU 80 via a bus 87. This back-up RAM 88 is connected to a power source 
89. Furthermore, the electronic control unit 61 comprises a counter 90, 
and the vehicle speed sensor 62 is connected to the input port 83 via the 
counter 90. The number of output pulses, issued from the vehicle speed 
sensor 62, is counted for a fixed time period in the counter 90 by the 
clock signal of the clock generator 86, and the binary coded count value, 
which is proportional to the vehicle speed, is input into the MPU 80 via 
the input port 83 and the bus 85 from the counter 90. In addition, the 
electronic control unit 61 comprises an A-D converter 91, and the cooling 
water temperature sensor 63 is connected to the input port 83 via the A-D 
converter 91. The cooling water temperature sensor 63 comprises, for 
example, a thermistor element and produces an output voltage which is 
proportional to the temperature of the cooling water of the engine. The 
output voltage of the cooling water temperature sensor 63 is converted to 
the corresponding binary code in the A-D converter 91, and the binary code 
is input into the MPU 80 via the input port 83 and the bus 85. The output 
signals of the engine speed sensor 64, the throttle switch 65 and the 
neutral switch 66 are input into the MPU 80 via the input port 83 and the 
bus 85. In the MPU 80, the time interval of the output pulses issuing from 
the engine speed sensor 64 is calculated, and the engine speed is 
calculated from the time interval. On the other hand, the output terminals 
of the output port 84 are connected to the corresponding input terminals 
of the latch 92, and the output terminals of the latch 92 are connected to 
the step motor drive circuit 60. Step motor drive data, obtained in the 
MPU 80, is written in the output port 84, and the step motor drive data is 
retained in the latch 92 for a fixed time period by the clock signal of 
the clock generator 86. 
On the other hand, in FIG. 8, the stator coil 53 of the stator 22 is wound 
in the direction which is the same as the winding direction of the stator 
coil 53 of the stator 23. In FIG. 10, the winding start terminals of the 
stator coils 53 of the stators 22 and 23 are indicated by S.sub.1 and 
S.sub.2, respectively, and the winding end terminals of the stator coils 
53 of the stators 22 and 23 are indicated by E.sub.1 and E.sub.2, 
respectively. In addition, in FIG. 10, the intermediate taps of the stator 
coils 53 of the stators 22 and 23 are indicated by M.sub.1 and M.sub.2, 
respectively. In the stator 22, the stator coil 53, located between the 
winding start terminal S.sub.1 and the intermediate tap M.sub.1, 
constructs a first phase exciting coil I, and the stator coil 53, located 
between the winding end terminal E.sub.1 and the intermediate tap M.sub.1, 
constructs a second phase exciting coil II. In addition, in the stator 23, 
the stator coil 53, located between the winding start terminal S.sub.2 and 
the intermediate terminal M.sub.2, constructs a third phase exciting coil 
III, and the stator coil 53, located between the winding end terminal 
E.sub.2 and the intermediate tap M.sub.2, constructs a fourth phase 
exciting coil IV. As illustrated in FIG. 10, the step motor drive circuit 
60 comprises four transistors Tr.sub.1, Tr.sub.2, Tr.sub.3 and Tr.sub.4, 
and the winding start terminals S.sub.1 and S.sub.2 and the winding end 
terminals E.sub.1 and E.sub.2 are connected to the collectors of the 
transistor Tr.sub.1, Tr.sub.2, Tr.sub.3 and Tr.sub.4, respectively. In 
addition, the intermediate taps M.sub.1 and M.sub.2 are grounded via the 
power source 89. The collectors of the transistor Tr.sub.1, Tr.sub.2, 
Tr.sub.3 and Tr.sub.4 are connected to the power source 89 via 
corresponding diodes D.sub.1, D.sub.2, D.sub.3 and D.sub.4 for absorbing a 
surge current and via a resistor R, and the emitters of the transistor 
Tr.sub.1, Tr.sub.2, Tr.sub.3 and Tr.sub.4 are grounded. In addition, the 
bases of the transistors Tr.sub.1, Tr.sub.2, Tr.sub.3 and Tr.sub.4 are 
connected to the corresponding output terminals of the latch 92. 
As mentioned above, in the MPU 80, the engine speed is calculted on the 
basis of the output pulses of the engine speed sensor 64. On the other 
hand, a function, representing a desired relationship between, for 
example, the temperature of the cooling water of the engine and the engine 
idling speed, and a function, representing a desired relationship between 
the range of the automatic transmission and the engine idling speed, are 
stored in the ROM 82 in the form of a formula or a data table. In the MPU 
80, the rotating direction of the step motor 9, which is necessary to 
equalize the engine speed to a predetermined engine idling speed, is 
determined from the above-mentioned function and the engine speed at which 
the engine is now driven and, in addition, the step motor drive data, 
which is necessary to rotate the step motor 9 in a stepping manner in the 
above-mentioned rotating direction, is obtained. Then, the step motor 
drive data is written in the output port 84. This writing operation of the 
step motor drive data is executed, for example, every 8 msec, and the step 
motor drive data, written in the output port 84, is retained in the latch 
92 for 8 msec. For example, four bits drive data "1000" is input to the 
output port 84 from the MPU 80 and, if the output terminals of the latch 
92, which are connected to the transistors Tr.sub.1, Tr.sub.2, Tr.sub.3 
and Tr.sub.4, are indicated by I, II, III and IV respectively, the output 
signals "1", "0", "0" and "0" are produced at the output terminals I, II, 
III and IV of the latch 92, respectively, for 8 msec. FIG. 11 illustrates 
output signals produced at the output terminals I, II, III, IV of the 
latch 92. From FIG. 11, it will be understood that, during the time period 
from the time t.sub.1 to the time t.sub.2, the output signals "1", "0", 
"0" and "0" are produced at the output terminals I, II, III and IV of the 
latch 92, respectively. When the output signal, produced at the output 
terminal I of the latch 92, becomes "1", since the transistor Tr.sub.1 is 
turned to the ON condition, the first phase exciting coil I is excited. 
Then, at the time t.sub.2 in FIG. 11, if it is determined in the MPU 80 
that the step motor 9 should be moved by one step in the direction wherein 
the valve head 36 (FIG. 2) opens, the step motor drive data "1100" is 
written in the output port 84. As a result of this, as illustrated in FIG. 
11, during the time period from the time t.sub.2 to the time t.sub.3, the 
output signals "1", "1", "0" and "0" are produced at the output terminals 
I, II, III and IV of the latch 92, respectively. Consequently, at this 
time, the transistor Tr.sub.2 is also turned to the ON condition and, 
thus, the second phase exciting coil II is excited. As in the same manner 
as desribed above, during the time period from the time t.sub.3 to the 
time t.sub.4 in FIG. 11, since the output signals "0", "1", "1" and "0" 
are produced at the output terminals I, II, III and IV of the latch 92, 
respectively, the second phase exciting coil II and the third phase 
exciting coil III are excited and, during the time period from the time 
t.sub.4 to the time t.sub.5 in FIG. 11, since the output signals "0", "0", 
"1" and "1" are produced at the output terminals I, II, III and IV of the 
latch 92, respectively, the third phase exciting coil III and the fourth 
phase exciting coil IV are excited. From FIG. 11, it will be understood 
that the time duration necessary for the production of the output signals 
produced at the output terminals I, II, III, IV of the latch 92, that is, 
the length of time necessary to produce the exciting pulses applied to the 
exciting coils I, II, III, IV is the same, and that each length of time 
necessary to produce the exciting pulses applied to the adjacent two phase 
exciting coils overlaps by one half, as is shown in FIG. 11. An exciting 
system, in which the time periods of production of the exciting pulses 
applied to the adjacent two phase exciting coils are overlapped by one 
half, is called a two-phase exciting system. 
FIG. 12 illustrates a schematic developed view of the outer circumferential 
surface of the hollow cylindrical outer body 42 of the rotor 21 and the 
pole pieces 56, 58 of the stators 22, 23. FIG. 12 (a) illustrates the case 
wherein only the first phase exciting coil I is excited, as illustrated in 
FIG. 11 between the time t.sub.1 and the time t.sub.2. At this time, the 
polarity of the pole pieces 56 of the stator 22 is N, and the polarity of 
the pole pieces 58 of the stator 22 is S. Contrary to this, the polarity 
does not appear on the pole pieces 56, 58 of the stator 23. Consequently, 
at this time, the rotor 21 remains stopped at a position wherein each of 
the pole pieces 56 of the stator 22 faces the corresponding S pole of the 
hollow cylindrical outer body 42, and each of the pole pieces 58 of the 
stator 22 faces the corresponding N pole of the hollow cylindrical outer 
body 42. When the second phase exciting coil II is excited, as illustrated 
between the time t.sub.2 and the time t.sub.3 in FIG. 11, since the flow 
direction of the current in the secondary phase exciting coil II is the 
same as that of the current in the first phase exciting coil I, the 
polarity of the pole pieces 56 of the stator 23 becomes N, and the 
polarity of the pole pieces 58 of the stator 23 becomes S, as illustrated 
in FIG. 12 (b). Consequently, at this time, the hollow cylindrical outer 
body 42 moves to a position wherein each of the S poles of the hollow 
cylindrical outer body 42 is located between the corresponding pole pieces 
56 of the stator 22 and the corresponding pole pieces 56 of the stator 23, 
and each of the N poles of the hollow cylindrical outer body 42 is located 
between the corresponding pole pieces 58 of the stator 22 and the 
corresponding pole pieces 58 of the stator 23. Therefore, assuming that 
the distance between the adjacent two pole pieces 56 of the stator 22 is 
one pitch, as mentioned previously, the hollow cylindrical outer body 42 
moves by a 1/8 pitch towards the right in FIG. 12 from a position 
illustrated in FIG. 12 (a) to a position illustrated in FIG. 12 (b). 
After this, when the third phase exciting coil III is excited, as 
illustrated between the time t.sub.3 and the time t.sub.4 in FIG. 11, 
since the flow direction of the current in the third phase exciting coil 
III is opposite to that of the current in the first phase exciting coil I, 
the polarity of the pole pieces 56 of the stator 22 becomes S, and the 
polarity of the pole pieces 58 of the stator 22 becomes N as illustrated 
in FIG. 12 (c). As a result of this, the hollow cylindrical outer body 42 
moves by a 1/4 pitch towards the right in FIG. 12, from a position 
illustrated in FIG. 12 (b) to a position illustrated in FIG. 12 (c). In 
the same manner as described above, when the fourth phase exciting coil IV 
is excited, as illustrated between the time t.sub.4 and the time t.sub.5 
in FIG. 11, the hollow cylindrical outer body 42 moves by a 1/4 pitch 
towards the right in FIG. 12, from a position illustrated in FIG. 12 (c) 
to a position illustrated in FIG. 12 (d). After this, during the time 
period from the time t.sub.5 to the time t.sub.6, only the fourth phase 
exciting coil IV is excited and, thus, the polarity does not appear on the 
pole pieces 56, 58 of the stator 22 as illustrated in FIG. 12 (e). 
Consequently, at this time, the hollow cylindrical outer body 42 moves by 
a 1/8 pitch towards the right in FIG. 12, from a position illustrated in 
FIG. 12 (d) to a position illustrated in FIG. 12 (e), so that each of the 
pole pieces 56 of the stator 23 faces the corresponding N pole of the 
hollow cylindrical outer body 42, and each of the pole pieces 58 of the 
stator 23 faces the corresponding S pole of the hollow cylindrical body 
42. Then, at that time t.sub.6 in FIG. 11, the step motor drive data 
"0000" is written in the output port 84 and, thus, since all the output 
signals, produced at the output terminals I, II, III, IV of the latch 92, 
become "0", the exciting operation of all the exciting coils I, II, III, 
IV is stopped. At this time, as illustrated in FIG. 12 (e), each of the 
pole pieces 56 of the stator 23 faces the corresponding N pole of the 
hollow cylindrical outer body 42, and each of the pole pieces 58 of the 
stator 23 faces the corresponding S pole of the hollow cylindrical outer 
body 42. Consequently, the hollow cylindrical outer body 42 is 
stationarily maintained at a position illustrated in FIG. 12 (e) due to 
the attracting forces of the N pole and the S pole of the hollow 
cylindrical outer body 42, which forces act on the pole pieces 56 and the 
pole pieces 58 of the stator 23, respectively. In addition, exciting data, 
indicating that the fourth phase exciting coil IV is excited before the 
hollow cylindrical outer body 42 is stationarily maintained, as mentioned 
above, is stored in a predetermined address in the RAM 81. 
At the time t.sub.7 in FIG. 11, in the case wherein it is determined in the 
MPU 80 that the step motor 9 should be moved by one step in the direction 
wherein the valve head 36 (FIG. 2) opens, exciting data, indicating the 
phase of the exciting coil which was finally excited, is read out from the 
RAM 81 and, if the phase of the exciting coil which was finally excited is 
the fourth phase, the step motor drive data "0001" is initially written in 
the output port 84. Consequently, only the fourth phase exciting coil IV 
is excited, as illustrated between the time t.sub.7 and the time t.sub.8 
in FIG. 11. At this time, since the hollo cylindrical outer body 42 is 
located in a position illustrated in FIG. 12 (e), the hollow cylindrical 
outer body 42 remains stationary. After this, when the third phase 
exciting coil III is excited, as illustrated between the time t.sub.8 and 
the time t.sub.9, the polarities, as illustrated in FIG. 12 (d) appear on 
the pole pieces 56, 58 of the stators 22, 23 and, thus, the hollow 
cylindrical outer body 42 moves by a 1/8 towards the left in FIG. 12, from 
a position illustrated in FIG. 12 (e) to a position illustrated in FIG. 12 
(d). 
As illustrated between the time t.sub.1 and the time t.sub.6 in FIG. 11, 
when the exciting coils I, II, III, IV are successively excited from the 
first phase exciting coil I to the fourth phase exciting coil IV, the 
hollow cylindrical outer body 42 of the rotor 21 moves relative to the 
stators 22, 23 and, accordingly, the rotor 21 rotates in one direction. 
When the rotor 21 rotates, since the external screw threads 29 of the 
valve shaft 20 are in engagement with the internal screw threads 47 of the 
hollow cylindrical inner body 40, as illustrated in FIG. 2, the valve 
shaft 20 is caused to move in one direction, for example, towards the left 
in FIG. 2. As a result of this, since the cross-sectional area of the 
annular air flow passage 38 formed between the valve head 36 and the valve 
seat 19 is increased, in FIG. 1, the amount of air fed via the bypass pipe 
16 into the surge tank 2 from the intake duct 3, located upstream of the 
throttle valve 4, is increased. Contrary to this, during the time period 
between the time t.sub.7 and the time t.sub.10, since the valve shaft 20 
is caused to move towards the right in FIG. 2, the cross-sectional area of 
the annular air flow passage 38 formed between the valve head 36 and the 
valve seat 19 is reduced. 
As mentioned above, in the step motor 9 illustrated in FIG. 2, the rotor 21 
is stationarily maintained by stopping the exciting operation of all the 
exciting coils I, II, III and IV. However, in the present invention, since 
the step motor 9 is directly attached to the surge tank 2, the vibration, 
caused by the rotation of the engine, is directly transferred to the step 
motor 9. Consequently, in the case wherein the step motor 9 is so designed 
that it is stationarily maintained by stopping the exciting operation of 
all the coils I, II, III and IV, when the engine speed is increased and, 
accordingly, the vibration of the engine becomes strong, there is a danger 
that the rotor 21 rotates due to the vibration. In order to eliminate such 
a danger, in the present invention, when the engine speed is increased 
beyond a predetermined speed, the exciting coil, which was finally excited 
before the rotor 21 was stationarily maintained, is repeatedly excited at 
a predetermined time period. 
FIG. 13 illustrates a flow chart of the operation which is excuted for 
maintaining the rotor 9 stationary. Referring to FIG. 13, step 100 means 
that the routine is processed by sequential interruptions which are 
executed periodically at predetermined times. This interruption is 
executed every 4 msec. In stage 101, the logic of the flag f8MS is 
inverted from "1" to "0" or from "0" to "1" every time the routine goes to 
stage 101. Consequently, in stage 101, pulses, having a time period of 8 
msec, are produced due to the interruption, as illustrated in FIG. 14. 
Then, in FIG. 13, the routine goes to stage 102. In stage 102, from the 
flag fRUN, it is determined whether the step motor 9 is now driven 
(fRUN=1) or is not now driven (fRUN=0). This flag fRUN is set or reset by 
the program (not shown) for executing the step motor drive processing. If 
the motor 9 is now driven, since the logic of the flag fRUN is "1", the 
processing cycle is completed. Contrary to this, if the step motor 9 
remains stopped, since the logic of the flag fRUN is "0", the routine goes 
to stage 103. In stage 103, it is determined whether the logic of the flag 
f8MS is "1" or "0" and, if the logic of the flag f8MS is "0", the routine 
goes to stage 104. In stage 104, it is determined whether the number of 
revolution per minute of the engine Ne is not lower than 3600 r.p.m. If 
the number of revolution per minute of the engine Ne is not lower than 
3600 r.p.m., the routine goes to stage 105. In stage 105, exciting data, 
indicating the phase of the exciting coil which was finally excited, is 
read out from the RAM 81, and exciting data, for exciting only the 
exciting coil which was finally excited, is written in the output port 84. 
On the other hand, if it is determined in stage 103 that the logic of the 
flag f8MS is "1", the routine goes to stage 106. In addition, if it is 
determined in stage 104 that the number of revolutions per minute of the 
engine Ne is lower than 3600 r.p.m, the routine also goes to stage 106. In 
stage 106, data for stopping the exciting operation of all the exciting 
coils is written in the output port 84. 
FIG. 14 illustrates a time chart indicating a change in the logic of the 
flag f8MS and the flag fRUN, and indicating the exciting operation of the 
exciting coil which was finally excited. In FIG. 14, I indicates the 
exciting operation of the exciting coil which was finally excited when the 
number of revolutions per minute of the engine Ne is lower than 3600 
r.p.m, and II indicates the exciting operation of the exciting coil which 
was finally excited when the number of revolutions per minute of the 
engine Ne is not lower than 3600 r.p.m. From FIG. 14, it will be 
understood that, when the number of revolutions per minute of the engine 
Ne is not lower than 3600 r.p.m, the exciting coil, which was finally 
excited, is excited for 4 msec every 8 msec. 
According to the present invention, it is possible to precisely control the 
amount of air flowing within the bypass pipe by using a step motor. In 
addition, by adopting the two-phase exciting system, it is possible to 
increase the driving power of the step motor. Furthermore, when the engine 
is operating at a high speed, that is, when the step motor is subjected to 
a strong vibration, the rotor of the step motor is stationarily maintained 
by repeatedly exciting the exciting coil which was finally excited. 
Consequently, since the rotor of the step motor is completely prevented 
from rotating, it is possible to maintain the idling speed of the engine 
at a predetermined speed. In addition, since the exciting coils are not 
excited when the step motor remains stopped and when the engine is 
operating at a low speed, the comsumption of electric power is small, and 
it is possible to prevent the electronic control unit from overheating. 
While the invention has been described by reference to a specific 
embodiment chosen for purposes of illustration, it should be apparent that 
numerous modifications could be made thereto by those skilled in the art 
without departing from the basic concept and scope of the invention.