Cruise control apparatus for a vehicle using a control quantity to actuate the throttle and a control quantity integrator to actuate the gear change determiner

A cruise control apparatus for a vehicle can control an automatic transmission such that gear shifting is carried out in accordance with a running resistance of the vehicle while taking account of the slope of a road and the weight of the vehicle, thereby allowing proper and desired speed control. A speed deviation calculator 24 calculates, based on a speed signal and a target speed signal, a deviation between the target speed and the actual vehicle speed and generates a corresponding speed deviation signal. A control quantity calculator 25 calculates a control quantity Tn for controlling a driving force of the vehicle based on the acceleration signal and the speed deviation signal. A throttle valve actuator 11 drives the throttle valve 29 based on the control quantity Tn. A control quantity integrator 27 integrates the control quantity Tn with respect to time based on an output signal from a gear change determiner 26 to thereby simulate a change in the driving force of the vehicle. The gear change determiner 26 determines, based on the acceleration signal, the speed deviation signal and the integrated control quantity, whether the gear ratio of the automatic transmission 28 is to be changed.

BACKGROUND OF THE INVENTION 
The present invention relates to a cruise control apparatus for a vehicle 
which serves to maintain the speed of the vehicle at a constant value in 
an automatic fashion. 
FIG. 9 shows the general arrangement of a conventional cruise control 
apparatus for a vehicle as disclosed in Japanese Patent Laid-Open No. 
58-39311. In this figure, a set switch 1 is adapted to be manipulated by 
the driver of the vehicle to start cruise control. A cancellation switch 2 
is turned on in response to the application of the brakes by the driver to 
cancel the cruise control. A speed sensor 3 senses the speed of the 
vehicle, and comprises a cruciform rotary member 3a which has four 
magnetic poles equally spaced from each other at an interval of 90 degrees 
and which is rotated by an unillustrated transmission through an 
unillustrated metering cable, and a reed switch 3b disposed near the 
rotary member 3a so that it is open or closed each time one of the four 
magnetic poles comes in proximity of the reed switch 3b for generating a 
pulse signal in the form of a series of pulses having a frequency 
proportional to the speed of the vehicle. A power source in the form of a 
battery 4 is connected through a power or main switch 5 to a control unit 
6 including a computing and processing circuit 6a such as a microcomputer, 
so that the control unit 6 is powered by the battery 4 when the main 
switch 5 is turned on. The control unit 6 receives the output signals from 
the set switch 1, the cancellation switch 2 and the speed sensor 3. Based 
on these signals, the control unit 6 performs various operational 
calculations for automatically controlling the speed of the vehicle so as 
to make it equal to a target speed V, and generates various output signals 
for automatic cruise control. A throttle actuator 7 in the form of a 
motor-operated actuator having an unillustrated motor is electrically 
connected to the control unit 6 so that it receives the output signals 
from the control unit 6 for driving a throttle valve 9 in a closing or 
opening direction. The throttle valve 9 is disposed in an intake pipe 8 of 
the engine and it is connected to an unillustrated accelerator pedal 
through an unillustrated cable or the like so that it is caused to rotate 
around a pivot shaft by an accelerator pedal operation of the driver 
through the cable for controlling the amount or flow rate of intake air 
supplied to the engine via the intake pipe 8. The throttle actuator 7 is 
operatively connected to the throttle valve 9 through a link member 7a and 
a connecting rod 7b, so that the link member 7a is caused to rotate around 
its pivot axis under the action of the motor in the throttle actuator 7 to 
drive the throttle valve 9 via the rod 7b. The unillustrated motor of the 
throttle actuator 7 is operatively coupled with the link member 7a through 
an unillustrated electromagnetic clutch which is operated by an 
electromagnetic clutch operating signal from the control unit 6 to control 
the mechanical connection between the motor and the link member 7a. 
The operation of the above-mentioned conventional cruise control apparatus 
will now be described in detail below. When the main switch 5 is turned on 
by the driver, the control unit 6 is powered by the battery 4 and 
processes the output signal from the speed sensor 3. The speed sensor 3 
generates a speed signal comprising a series of pulses having a frequency 
proportional to the speed at which the vehicle is travelling. The control 
unit 6 measures the periods of successive pulses and calculates the 
vehicle speed based thereon. In this state, if the driver manipulates the 
set switch 1, a corresponding signal is sent therefrom to the control unit 
6 which then stores the vehicle speed at that time as a target speed, 
while starting cruise control. Thereafter, the control unit 6 successively 
compares the actual speed of the vehicle successively detected with the 
target speed and generates control signals to the throttle actuator 7 
which is thereby operated to properly control the opening of the throttle 
valve 9 in order to make the vehicle travel at the target speed. 
Specifically, if the actual vehicle speed is lower than the target speed, 
the control unit 6 generates a throttle-opening signal for opening the 
throttle valve 9 by a predetermined quantity, whereas if the vehicle speed 
is higher than the target speed, the control unit 6 generates a 
throttle-closing signal for closing the throttle valve 9 by a 
predetermined quantity. As a result, the vehicle can travel at the 
constant target speed in an automatic fashion without the need of the 
driver's accelerator pedal operation. 
If the driver applies the brakes of the vehicle during such cruise control, 
the cancellation switch 2 is operated, generating a cruise cancellation 
signal to the control unit 6. Upon receipt of the cancellation signal, the 
control unit 6 immediately generates a clutch releasing signal to the 
throttle actuator 7 which then disengages the unillustrated 
electromagnetic clutch. Thereafter, the driver can control the vehicle 
speed at his or her own will by stepping down or up the accelerator pedal 
to manually adjust the opening of the throttle valve 9. 
With the conventional cruise control apparatus as constructed above, the 
control unit 6 successively makes a comparison between the varying actual 
speed of the vehicle and the target speed at predetermined time intervals 
measured by a timer or when a deviation between the actual vehicle speed 
and the target speed exceeds a predetermined value without regard to the 
slope of the road, the weight of the vehicle, etc., so that it generates a 
control output to the throttle actuator 7 for making the actual vehicle 
speed equal to the target speed, whereby the throttle actuator 7 can 
properly control the opening of the throttle valve 9. However, this 
results in the following drawbacks. Namely, when the vehicle is travelling 
on an uphill slope, the transmission can be forced to frequently perform 
shift-up and shift-down operations, or when the vehicle is travelling on a 
downhill slope, the transmission can be controlled to continuously shift 
down, thus preventing proper or necessary speed control. 
SUMMARY OF THE INVENTION 
The present invention is intended to overcome the above-mentioned problems 
encountered with the conventional cruise control apparatus. 
An object of the invention is to provide a novel and improved cruise 
control apparatus for a vehicle which can control a transmission such that 
gear shifting is carried out in accordance with a running resistance of 
the vehicle while taking account of the slope of a road, the weight of the 
vehicle and the like, thereby allowing proper and desired speed control. 
In order to achieve the above object, according to the present invention, 
there is provided a cruise control apparatus for a vehicle comprising: a 
speed sensor for sensing a speed of the vehicle at which the vehicle is 
travelling, and for generating a corresponding speed signal; acceleration 
sensing means for successively sensing an acceleration of the vehicle at 
predetermined intervals based on the output signal from the speed sensor; 
a target speed setter for setting a target speed of the vehicle at which 
the vehicle is to travel; target speed signal generating means for 
generating a target speed signal representative of the target speed; speed 
deviation calculating means for calculating, based on the speed signal and 
the target speed signal, a deviation between the target speed and the 
actual vehicle speed and for generating a corresponding output signal; 
control quantity calculating means for calculating a control quantity for 
controlling a driving force of the vehicle based on the output signals 
from the acceleration sensing means and the speed deviation calculating 
means; throttle valve actuating means for driving a throttle valve in an 
engine of the vehicle based on the output signal from the control quantity 
calculating means; control quantity integrating means for integrating the 
control quantity from the control quantity calculating means with respect 
to time based on an output signal from a gear ratio change determining 
means to thereby simulate a change in a driving force of the vehicle; and 
an automatic transmission operable to perform change its gear ratio in 
response to the output signal from the gear change determining means; 
wherein the gear change determining means determines, based on the output 
signals from the acceleration sensing means, the speed deviation 
calculating means and the control quantity integrating means, whether the 
gear ratio of the automatic transmission is to be changed. 
The gear change determining means generates a down-shift signal for 
down-shifting the automatic transmission when the speed deviation 
.epsilon. is negative and when the absolute value of the speed deviation 
.epsilon. is greater than a first predetermined value .epsilon.a. 
The control quantity integrating means integrates the control quantity Tn 
to provide a current integrated value In using the following formulae in 
dependence upon the sign of the control quantity Tn: 
when the control quantity Tn is positive, 
EQU In=In-1+K3.times..vertline.Tn.vertline., 
where In-1 is a preceding integrated value, and K3 is a constant; 
when the control quantity Tn is negative; 
EQU In=In-1-K4.times..vertline.Tn.vertline., 
where In-1 is a preceding integrated value, and K4 is a constant; and 
when the control quantity Tn is equal to zero, 
EQU In=In-1, 
where In-1 is a preceding integrated value. 
The gear change determining means generates an up-shift signal for 
up-shifting the automatic transmission when the absolute value of the 
speed deviation .epsilon. is less than a second predetermined value 
.epsilon.b and when the absolute value of the acceleration .alpha. is less 
than a predetermined value .alpha. and when the integrated value In is 
less than a predetermined value Ib. 
The above and other objects, features and advantages of the invention will 
be more readily apparent from the following detailed description of a 
preferred embodiment of the invention taken in conjunction with 
accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWINGS 
A preferred embodiment of the present invention will now be described in 
detail while referring to the accompanying drawings. 
Referring to the drawings, FIG. 1 shows in block form the general 
arrangement of a cruise control apparatus for a vehicle constructed in 
accordance with the principles of the present invention, and FIG. 2 shows 
the more detailed construction thereof. 
As shown in FIG. 1, the cruise control apparatus of the invention 
conceptually includes the following elements. Namely, a speed sensor 20 
senses an actual speed of the vehicle at which the vehicle is travelling, 
and generates a corresponding speed signal. An acceleration sensing means 
21 successively senses or calculates an acceleration of the vehicle at 
predetermined intervals based on the output signal from the speed sensor 
20 and generates a corresponding acceleration signal. A target speed 
setter in the form of a set switch 22 is adapted to be operated by the 
driver to set a target speed of the vehicle and generate a corresponding 
output signal. A target speed signal generating means 23 is connected to 
receive the output signal from the target speed setter 22 for generating a 
target speed signal representative of the target speed. A speed deviation 
calculating means 24 calculates, based on the speed signal and the target 
speed signal, a deviation between the target vehicle speed and the actual 
vehicle speed, and generates a corresponding speed deviation signal. A 
control quantity calculating means 25 calculates a control quantity for 
controlling the driving force of the vehicle based on the acceleration 
signal and the speed deviation signal, and generates a corresponding 
control quantity signal. A throttle valve actuating means 11 drives a 
throttle valve 29 of the vehicle engine based on the control quantity 
signal from the control quantity calculating means 25. A control quantity 
integrating means 27 integrates the control quantity signal from the 
control quantity calculating means 25 with respect to time in response to 
an output signal from a gear change determining means 26 to thereby 
simulate a change in the driving force of the vehicle. The gear change 
determining means 26 determines, based on the output signals from the 
acceleration sensing means 21, the speed deviation calculating means 24 
and the control quantity integrating means 25, whether the gear ratio of 
an automatic transmission 28 of the vehicle is to be changed, and controls 
the automatic transmission as a result of this determination. 
FIG. 2 shows a more concrete construction of the cruise control apparatus 
of the invention. Specifically, the operations of the acceleration sensing 
means 21, the target speed signal generating means 23, the speed deviation 
calculating means 24, the control quantity calculating means 25, the 
control quantity integrating means 27 and the gear change determining 
means 26 can be performed by a control unit 10 in the form of a 
microcomputer which comprises an input interface 10a, a memory 10b 
including a read only memory (ROM) for storing instruction and control 
programs, and a randos access memory (RAM) for temporarily storing data, a 
central processing unit (CPU) 10c which executes the programs stored in 
the ROM while reading out data stored in the RAM and writing data into the 
RAM, and an output interface 10d, as is well known in the art. The control 
unit 10 is powered by a battery 40 through a power or switch 50. The input 
interface 10a of the control unit 10 is connected to a cancellation switch 
19 for cancelling cruise control, the speed sensor 20 and the set switch 
22. The cancellation switch 19 and the set switch 22 are the same as the 
cancellation switch 2 and the set switch 1 of FIG. 9. The output interface 
10d of the control unit 10 is connected to the throttle valve driving 
means 11 and to a gear changing solenoid 16 through a solenoid driving 
circuit 15. The gear changing solenoid 16 is driven to change the gear 
ratio of the automatic transmission 28 through the solenoid driving 
circuit 15 in accordance with an output from the output interface 10d of 
the control unit 10. The solenoid driving circuit 15 and the gear changing 
solenoid 16 are incorporated in the automatic transmission 28. 
As shown in FIG. 2, the throttle valve driving means 11 comprises a 
diaphragm-type actuator 14 for controlling the opening of the throttle 
valve 29 disposed in an intake pipe 28 of the vehicle engine, and a pair 
of first and second electromagnetic valves 12, 13 for controlling the 
actuator 14 based on control signals y1, y2 from the control unit 10. 
Specifically, the actuator 14 includes a housing 14a having a hollow 
interior space thereof divided into a pair of first and second chambers 
14b, 14c by a diaphragm member 14d on the opposite sides thereof. The 
first chamber 14b can be placed in fluid communication on one hand with an 
unillustrated vacuum source through a vacuum tube 12b under the action of 
the first electromagnetic valve 12 interposed therein, and on the other 
hand, it can be placed into fluid communication with the ambient 
atmosphere through an air pipe 13b under the action of the second 
electromagnetic valve 13 interposed therein. The first electromagnetic 
valve 12 is controlled to open or close by a first control signal y1 from 
the output interface 10d of the control unit 10, and the second 
electromagnetic valve 13 is controlled to open or close by a second 
control signal y2 from the output interface 10d of the control unit 10. 
The diaphragm member 14d is operatively connected through a link mechanism 
14f with the throttle valve 29 in the intake pipe 28, so that the throttle 
valve 29 is caused to move in a valve-closing or opening direction in 
accordance with the movement of the diaphragm 14d through the link 
mechanism 14f. A biasing spring 14e is disposed in the housing 14a under 
compression between one side wall of the housing 14a and a central portion 
of the diaphragm 14d for biasing the diaphragm member 14d in a direction 
to close the throttle valve 29. 
With the above arrangement of the throttle valve driving means 11, when the 
first control signal y1 is at a low level, the first electromagnetic valve 
12 takes a closed position in which the first chamber 14b is out of 
communication with the vacuum source, whereas when the first control 
signal y1 is at a high level, the first electromagnetic valve 12 takes an 
open position in which the first chamber 14b is placed into communication 
with the vacuum source. On the other hand, when the second control signal 
y2 is at a low level, the second electromagnetic valve 13 takes an open 
position in which the first chamber 14b is placed into communication with 
the ambient atmosphere, whereas when the second control signal y2 is at a 
high level, the second electromagnetic valve 13 takes a closed position in 
which communication between the first chamber 14b and the ambient 
atmosphere is blocked. As a result, the throttle valve driving means 11 
has three operational modes including an acceleration mode, a deceleration 
mode and a hold mode, as shown in Table I below. 
TABLE I 
______________________________________ 
Operational Nodes 
y1 y2 Throttle Valve 
______________________________________ 
Acceleration High High Open 
Deceleration Low Low Close 
hold Low High Constant 
______________________________________ 
As can be seen from Table I above, when the first and second control 
signals y1 and y2 are both at the high level, the throttle valve driving 
means 11 takes the acceleration mode in which the first and second 
electromagnetic valves 12, 13 are moved to the open and closed positions, 
respectively, so that the first chamber 14b in the housing 14a is placed 
in communication with the vacuum source alone. As a result, the diaphragm 
14d is moved to the left in FIG. 2 against the biasing spring 14e under 
the action of vacuum in the first chamber 14b, whereby the throttle valve 
29 is rotated through the link mechanism 14f in the opening direction, 
thus accelerating the vehicle. On the other hand, when the first and 
second control signals y1, y2 are both at the low level, the throttle 
valve driving means 11 takes the deceleration mode in which the first 
electromagnetic valve 12 is moved to the closed position and the second 
electromagnetic valve 13 is moved to the open position, so that the first 
chamber 14b is placed in communication with the atmosphere alone. 
Consequently, the diaphragm 14d is caused to move to the right in FIG. 2 
under the action of the biasing spring 14e, whereby the throttle valve 29 
is rotated in the closing direction through the link mechanism 14d, thus 
decelerating the vehicle. Further, when the control signals y1, y2 are at 
the low and high levels, respectively, the throttle valve driving means 11 
takes the hold mode in which the first and second electromagnetic valves 
12, 13 are both moved to their closed positions. Accordingly, the first 
chamber 14b is out of communication with both the atmosphere and the 
vacuum source, and hence the diaphragm member 14d is held at its constant 
position. Thus, the opening of the throttle valve 29 is also held 
constant. 
The operation of this embodiment will be described below while referring to 
FIGS. 3 through 8. First, when the main switch 50 is turned on by the 
driver, the control unit or microcomputer 10 is powered by the battery 40 
and begins to execute a main routine as illustrated in FIGS. 3 and 4. On 
the other hand, during travel of the vehicle, the speed sensor 20 
generates a speed signal comprising a series of successive pulses or a 
pulse train with a frequency proportional to the speed of the vehicle. 
When the speed signal is input to the input interface 10a of the 
microcomputer 10, the microcomputer 10 executes an interrupt routine as 
shown in FIG. 5. That is, upon input of each pulse in the speed signal, as 
shown in FIG. 6, the microcomputer 10 carries out the processing of the 
interrupt routine as follows. First in Step S40, the microcomputer 10 
reads out from an unillustrated timer incorporated therein the point in 
time tn at which a current pulse in the speed signal rises. Then in Step 
S41, a difference or period .DELTA.t between the current time tn and the 
preceding time tn=1 (tn-tn-1) is calculated. Thereafter, the interrupt 
routine ends and a return is carried out to the main routine of FIGS. 3 
and 4. 
Turning now to the main routine of FIG. 3, first in Step S1, the 
microcomputer 10 is initialized, and then in Step S2, the current speed Vn 
of the vehicle at time tn is calculated from the most recent or latest 
pulse period .DELTA.t obtained in the above-mentioned current interrupt 
routine of FIG. 5 as follows: 
EQU Vn=g/.DELTA.t 
where g is a speed conversion coefficient. 
Subsequently, when the driver manipulates the set switch 22 to start cruise 
control, an input signal in the form of a cruise start signal is input 
from the set switch 22 to the input interface 10a of the microcomputer 10 
in Step S3. In Step S4, it is determined whether the signal input to the 
microcomputer 10 is from the set switch 22. If so, then in Step S5, the 
current speed Vn of the vehicle calculated in the above manner is set as a 
target vehicle speed Vm. 
Subsequently in Step S6, a flag for an automatic cruise control (ACC), in 
which the vehicle can travel at a constant speed (e.g., greater than about 
40 km/h and less than about 100 km/h) desired and instructed by the 
driver in an automatic fashion without the need of the driver's 
accelerator pedal operation, is set up or made into a high level, and in 
Step S7, cruise control processing is carried out. On the other hand, if 
in Step S4 the input signal to the microcomputer 10 is not from the set 
switch 22, then in Step S8, it is determined whether the cancellation 
switch 19 is on or off. If the cancellation switch 19 is off, the control 
program goes to Step S9 wherein it is further determined whether the ACC 
flag is at the high level. If so, the control program goes to Step S7. If, 
however, the cancellation switch 19 is on in Step S8, or if the ACC flag 
is not set up (i.e., not at the high level) in Step S9, the program goes 
to Step S10 where the ACC flag is reset or made into a low level, and then 
in Step S11 the cancellation processing for cancelling the cruise control 
is performed. Thereafter, a return is carried out to Step S2. 
In Step S12, a speed deviation .epsilon.n between the target vehicle speed 
Vm and the current vehicle speed Vn actually measured is calculated, and 
in Step S13, a current acceleration .alpha.n of the vehicle is calculated 
from the current vehicle speed Vn and the preceding vehicle speed Vn-1 as 
follows: 
EQU .alpha.n=(Vn-Vn-1)/to 
where to is the period between the current pulse and the preceding pulse in 
the speed signal from the speed sensor 20. In Step S14, a current control 
quantity Tn for adjusting the opening of the throttle valve 9 is 
calculated as follows: 
EQU Tn=K1.times..epsilon.n-K2.times..alpha.n (1) 
where K1 is a coefficient for obtaining the control quantity Tn from the 
speed deviation .epsilon.n, and K2 is a coefficient for obtaining the 
control quantity Tn from the acceleration .alpha.. 
In Step S15, it is determined whether the control quantity Tn is greater 
than Zero. If so (i.e., Tn&gt;0), then in Step S16, the microcomputer 10 
generates an acceleration output (i.e., the first and second control 
signals y1, y2 are both made into the high levels) whereby the throttle 
valve driving means 11 is operated to move the throttle valve 28 in the 
opening direction to thereby accelerate the vehicle. If not (i.e., 
Tn.ltoreq.0), however, then in Step S17, it is determined whether the 
control quantity Tn is equal to zero. If Tn=0, the program goes to Step 
S18 where the microcomputer 10 generates a hold output (i.e., the first 
control signal y1 is made into the low level and the second control signal 
y2 is made into the high level) so that the throttle valve driving means 
11 holds the opening of the throttle valve 29 at a constant level. If 
Tn&lt;0, and the program goes to Step S18, the microcomputer 10 generates a 
deceleration output (i.e., the first and second control signals y1, y2 are 
both made into the low level) whereby the throttle valve driving means 11 
is operated to move the throttle valve 29 in the closing direction, 
decelerating the vehicle. 
Subsequently in Step S20, it is determined whether the output signal 
supplied from the microcomputer 10 to the solenoid driving circuit 15 is 
at a high level (i.e., whether the transmission is in a high gear). If so, 
then in Step S21, it is further determined whether the speed deviation 
.epsilon. is less than zero. If .epsilon.&lt;0, it is further determined in 
Step S22 whether the absolute value of the speed deviation .epsilon. is 
greater than a predetermined value .epsilon.a (e.g., 8 km/h) at time t2, 
as shown in FIG. 7. If so (.vertline..epsilon..vertline.&gt;.epsilon.a), then 
in Step S23, an integration value In, which will be described later in 
detail, is initially set, at time t2, to an initial value Ia corresponding 
to a predetermined opening of the throttle valve 29 below which the 
transmission is down-shifted. Thereafter in Step S24, the output signal 
from the microcomputer 10 to the solenoid driving circuit 15 is set to a 
low level for down-shifting the transmission, so that in Step S25, the 
solenoid driving circuit 15 generates a down-shifting signal to the gear 
changing solenoid 16. If, however, the answer in Step S21 or S22 is 
negative, a return is performed to Step S2. 
If in Step 20 it is determined that the transmission is in a low gear, then 
in Step S26, it is further determined, based on the control quantity Tn, 
whether the microcomputer 10 generates an acceleration output. If the 
answer to this question is positive (i.e., Tn&gt;0), then in Step S27, the 
control quantity Tn, which controls the driving power of the vehicle or 
the output power of the engine, is integrated to provide a current 
integrated value In, which corresponds to the opening of the throttle 
valve 29, using the following formula: 
EQU In=In-1+K3.times..vertline.Tn.vertline. (2) 
where K3 is a constant, and In-1 is the preceding integrated value. 
On the other hand, if the answer in Step S26 is negative (i.e., 
Tn.ltoreq.0), then in Step S28 it is determined, based on the control 
quantity Tn, whether the output signal from the microcomputer 10 is a 
deceleration signal. If the answer to this question is positive (i.e., 
Tn&lt;0), then in Step S29, the control quantity Tn is integrated to provide 
a current integrated value In using the following formula: 
EQU In=In-1-K4.times..vertline.Tn.vertline. (3) 
where K4 is a constant. 
If it is determined in Step S28 that the output signal from the 
microcomputer 10 is not a deceleration signal but a hold signal (i.e., 
Tn=0), then in Step S30, a current integrated value In is given by the 
following equation: 
EQU In=In-1 (4) 
The integrated value In thus obtained is successively stored in the memory 
10b in the microcomputer 10 which comprises, for example, an 8-bits 
memory. 
In Step S31, it is further determined whether the memory 10b of the 
microcomputer 10 storing the integrated value In overflows during the 
integration (i.e., whether the integrated value In exceeds a predetermined 
value such as, for example, 255). If so, in Step S32, the integrated value 
In is clipped to an appropriate value. For example, the integrated value 
In is clipped to 255 when the memory overflows during acceleration, 
whereas it is clipped to zero when the memory underflows (i.e., overflows 
in the decreasing sense) during deceleration. If, however, there is no 
overflow or underflow in Step S31, the program jumps to Step S33 while 
skipping Step S32. 
Subsequently, in Steps S33 and S34, it is determined whether smooth cruise 
control is performed during the time when the transmission has 
down-shifted down to increase the driving force of the vehicle. That is, 
in Step S33, it is determined whether the absolute value of the speed 
deviation .epsilon. is less than a predetermined value .epsilon.b which is 
equal to or less than the predetermined value .epsilon.a. If so (i.e., 
.vertline..alpha..vertline.&lt;.alpha.a), then in Step S35, it is determined 
whether the absolute value of the acceleration .alpha. is less than a 
predetermined value .alpha.a. If the speed deviation .epsilon. and the 
acceleration .alpha. satisfy the above conditions (i.e., 
.vertline..epsilon..vertline.&lt;.epsilon.b and 
.vertline..alpha..vertline.&lt;.alpha.a), the program goes to Step S35 where 
it is determined whether the integrated value In corresponding to the 
throttle opening is less than a predetermined value Ib which is less than 
the initial integration value Ia. If so (In&lt;Ib), then in Step S36, the 
output signal from the microcomputer 10 to the solenoid driving circuit 15 
is set to a high level for up-shifting the transmission. As a result, in 
Step S37, the solenoid driving circuit 15 generates an up-shift signal to 
the gear changing solenoid 16. If, however, the answer in Step S33, S34 or 
S35 is negative, a return is performed to Step S2. 
As described above, in order to make the actual vehicle speed Vn equal to 
the target speed Vm, the control quantity Tn for controlling the driving 
force of the vehicle is integrated to simulate or estimate the opening of 
the throttle valve 29. When the absolute value of the speed deviation 
.epsilon. exceeds the predetermined value .epsilon.a, the transmission is 
down-shifted, and the integrated value In of the control quantity Tn 
corresponding to the throttle opening at that time is set to an initial 
value Ia, whereas when the speed deviation .epsilon. and the acceleration 
.alpha. decrease below the predetermined values .epsilon.b, .alpha.a, 
respectively, and when the integrated value In corresponding to the 
opening of the throttle valve 29 becomes less than the predetermined value 
Ib, the microcomputer 10 generates an up-shift signal for up-shifting the 
gear of the transmission. With this arrangement, gear change such as 
up-shifting or down-shifting of the transmission can be effected in 
accordance with the running resistance of the vehicle, thus enabling more 
precise and proper speed control during the vehicle is travelling under 
cruise control.