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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a control apparatus for an automatic
transmission of a motor vehicle in which the optimum running gear, and
hence the correct control oil pressure in the transmission, are determined
in accordance with signals representing the engine load and vehicle speed.
In such a control apparatus, the engine load and vehicle speed are
represented by electric signals. The optimum running gear and
corresponding optimum control oil pressure for various running conditions
are determined in response to these signals by an electronic arithmetic
circuit. The electronic arithmetic circuit drives solenoid valves which
control an oil pressure clutch or oil brake of the transmission mechanism
so as to smoothly effect gear changes in accordance with the input signals
representing the engine load and vehicle speed.
To provide the signals representing the engine load and vehicle speed,
typically, devices such as a potentiometer, which produces an electric
signal in proportion to the amount of opening of the engine throttle, and
a magnetic pickup, which produces a voltage or frequency signal indicative
of the rate of rotation of the output shaft of the transmission, are
employed. These signals are processed by the electronic arithmetic circuit
so as to effect shifting towards lower gears as the amount of the throttle
opening increases.
However, if a fault occurs in the speed sensor such that, for instance, its
output suddenly falls to zero when the vehicle is operated at a high
speed, the transmission will suddenly be shifted to first gear. This is
dangerous in that, not only does the engine overrev, but skidding may
occur. To overcome this drawback, it has been the practice to detect such
a fault and to lock the transmission in an intermediate gear is which the
stability is relatively high. However, in the conventional system, this
safety measure was effected even if the fault in the output of the speed
sensor was temporary, such as may be due to vibration, shock, or
electrical surges. Accordingly, the safety circuit tended to be actuated
in situations in which such was not really necessary.
SUMMARY OF THE INVENTION
In view of the above, it is a primary object of the present invention to
provide a control apparatus for an automatic transmission for a motor
vehicle which is substantially insensitive to temporary abnormalities in
the outputs of sensors which provide signals representing the operating
state of the vehicle, such as the speed of the vehicle and the throttle
opening.
A further object of the present invention is to provide such a control
apparatus in which, when an abnormality in one of the sensor outputs
continues over a predetermined period of time, the transmission is set to
a safe gear, and further gear change operations are inhibited, thereby to
prevent dangerous conditions from occurring.
In accordance with the above and other objects, the present invention
provides a control apparatus for an automatic transmission of a motor
vehicle including a gear change timing arithmetic circuit which determines
the times of gear change on the basis of at least a signal representing
the load on the engine of the vehicle and the speed of the vehicle. A
fault detecting circuit detects the existence of a fault in the load and
speed signals. Also, rate-of-change detecting circuits detect variations
in the load and speed signals and produce output signals in response
thereto when the rate of change in the load and speed signals exceeds
predetermined threshold values. When the latter signals are present, gear
change operations are inhibited for a predetermined period of time, and
operations performed in response to the outputs of the fault detecting
circuit are delayed for that period of time.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing the arrangement of a control apparatus
for an automatic transmission constructed in accordance with the present
invention;
FIG. 2 is a more detailed diagram of the arrangement of FIG. 1;
FIG. 3 is a graph showing transmission characteristics and is used for
explaining the operation of the control device of the present invention;
and
FIG. 4 is a flowchart used for explaining the operation of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the drawings, a preferred embodiment of a safety control
device for an automatic transmission constructed in accordance with the
teachings of the present invention will now be described. In FIG. 1, which
is a block diagram showing the arrangement of a preferred embodiment of
the inventive control device, reference numeral 1 indicates a throttle
opening sensor which produces an electrical signal having a magnitude
corresponding to the amount of opening of the throttle of the engine. The
output of the throttle opening sensor 1 is applied to a gear change timing
arithmetic circuit 3, a fault detecting circuit 4, and a rate-of-change
detecting circuit 6.
Further in FIG. 1, reference numeral 2 designates a speed sensor which
produces a voltage or frequency signal having a voltage or frequency in
proportion to the rotational rate of an output shaft of the transmission
of the vehicle, that is, corresponding to the speed of the vehicle. The
output of the vehicle speed detecting circuit 2 is applied to
corresponding inputs of the gear change timing arithmetic circuit 3, the
fault detecting circuit 4, and a second rate-of-change detecting circuit
7.
The gear change timing arithmetic circuit 3 determines the optimum times
for changing gears in accordance with the output signals from the throttle
opening sensor 1 and the vehicle speed detecting circuit 2 in accordance
with predetermined characteristics, an example of which is shown in FIG.
3. As the curves of FIG. 3 show, as the throttle is opened further, the
tendency to shift to lower gears is increased.
The fault detecting circuit 4 detects an open circuit or a short circuit to
detect faults accordingly. The output of the fault detecting circuit 4 is
applied to a change gear holding circuit 5, as is the output of the change
gear timing arithmetic circuit 3. The change gear holding circuit 5 locks
the transmission in the safest gear, generally, in the highest gear or the
next highest gear, when the fault detecting circuit 4 has indicated a
fault.
Operating in response to the output signals from the throttle opening
sensor 1 and vehicle speed detecting circuit 2, the rate-of-change
detecting circuits 6 and 7 produce output signals in an active state when
the rates of change of the respective input signals exceed respective
predetermined levels. The outputs of the rate-of-change detecting circuits
6 and 7 are ORed together with an OR gate 8, the output of which is
applied to the trigger input of a timer 9. When the output of the OR gate
8 is in the active state indicating that one of the rate-of-change
detecting circuits 6 and 7 has detected the occurrence of a rate of change
of one of the outputs of the throttle opening sensor 1 or vehicle speed
detecting circuit 2 exceeding the predetermined level, the timer 9 is
actuated. The output of the timer 9 is applied to the change gear holding
circuit 5 to cause the latter to prevent the transmission from being
shifted to another gear, regardless of the state of the outputs of the
gear change timing arithmetic circuit 3, until the timer 9 has completed
its timing operation.
Reference numerals 10 and 11 designate solenoid valves provided in an oil
pressure control circuit (not shown) of the transmission. The solenoid
valves 10 and 11 are operated in accordance with the output signals from
the gear change holding circuit 5 such that, for example, the transmission
is set to the fourth gear when only the solenoid valve 11 is actuated, the
third gear when neither of the solenoid valves 10 and 11 is actuated, the
second gear when only the solenoid valve 10 is actuated, and the first
gear when the both of the solenoid valves 10 and 11 are actuated.
Referring now to FIG. 2, the detailed construction of the various elements
depicted in FIG. 2 will be described.
The throttle opening sensor 1 is implemented with a potentiometer, which
produces a voltage output in proportion to the amount of the throttle
opening, and an analog-to-digital converter, which converts the output
voltage from the potentiometer to a digital value.
The vehicle speed sensor 2 is constituted by a pulse generator, which
produces pulses at a rate determined by the rate of rotation of the output
shaft of the transmission, and a counter, which counts the pulses from the
pulse generator to produce a digital output signal representative of the
speed of the vehicle.
The gear change timing arithmetic circuit 3 includes a memory composed of
an up-shift characteristic memory in which is stored data representing the
solid-line up-shift characteristic of FIG. 3 correlating the throttle
opening to speed, and a down-shift characteristic memory in which is
stored data representing the broken-line shift-up characteristic of FIG.
3. Based upon the detected throttle opening and the current gear position,
a desired vehicle speed and shift characteristic are read out of the
memory. The read-out desired vehicle speed is compared with the actual
vehicle speed with an up-shift comparator and a down-shift comparator. As
a result of these comparison operations, an UP instruction is issued if,
as indicated by the output of the up-shift comparator, the actual vehicle
speed is greater than the desired speed, and a DOWN instruction is issued
if, as indicated by the output of the down-shift comparator, the actual
vehicle speed is less than the desired speed. Otherwise, an instruction to
hold the current gear position is issued. A gear change signal generating
circuit provides logic outputs for controlling the solenoid valves 10 and
11 in the manner described above in accordance with the outputs of the
up-shift comparator and the down-shift comparator.
The rate-of-change detecting circuits 6 and 7 include circuits which
produce output signals having a magnitude in proportion to the amount of
change in the outputs of their respective sensors, and a level detecting
circuit which determines whether the level of the rate-of-change signal
exceeds a predetermined level.
The fault detecting circuit 4 includes a pair of level detecting circuits
which detect whether the output levels of the respective sensors have
reached abnormal levels in either the positive or negative direction, that
is, due to a short-circuit or open-circuit condition.
The timer 9 is constituted by a counter which is reset by the output of the
OR gate 8. The output of the OR gate 8 also is used to set an RS
flip-flop. The counter counts the clock pulses of a constant-frequency
pulse signal provided by an oscillator. When the count value held by the
counter reaches a predetermined level, it resets an RS flip-flop, which
then produces an H-level (high-level) output signal during the subsequent
operating period of the counter.
The gear change holding circuit 5 includes two D-type flip-flops triggered
by the output of the RS flip-flop of the timer 9. Specifically, the D-type
flip-flops are clocked on the leading edge of the output signal from the
RS flip-flop of the timer 9 to store the data signals then being provided
thereto by the gear change signal generating circuit of the gear change
timing arithmetic circuit 3. The outputs of the two D-type flip-flops are
applied to a gating circuit composed of four AND gates through D, two OR
gates and an inverter to actuate the solenoid valves 10 and 11 in the
manner described above. Specifically, if no fault has been detected, the
outputs of the gear change signal generating circuit 3 are passed through
the AND gates A and C and the OR gates to the solenoid valves 10 and 11
because the output from the inverter is then at the H level due to the
absence of a fault. On the other hand, when the fault detecting circuit 4
has detected the presence of a fault, and hence when its output is at the
H level, the output of the inverter will be an L-level (low-level) signal,
thereby connecting the outputs of the D-type flip-flops through the
respective AND gates B and D and the OR gates to the solenoid valves 10
and 11, thereby applying to the solenoid valves 10 and 11 the outputs from
the gear change signal generating circuit which were previously stored in
the D-type flip-flops by operation of the timer 9.
Referring now to FIG. 3, the operation of the control apparatus of the
invention will now be described in more detail.
Assuming now that the vehicle is running in the state corresponding to
point A in FIG. 3 wherein the throttle is open to some extent and the
vehicle is running in a steady-state condition, the fourth (highest) gear
is engaged; that is, the solenoid valves 10 and 11 will be in the
nonactuated and actuated states, respectively. In this condition, if, for
instance, the output of the vehicle speed detecting circuit 2 suddenly
falls to zero due to an open-circuit fault, the operating point designated
by the outputs of the sensor 1 and speed detecting circuit 2 shifts to the
point B. Accordingly, the gear change timing arithmetic circuit 3 produces
an output which would indicate that the transmission should immediately
change the engaged gear to the first (lowest) gear. However, is this were
done, the vehicle would likely go into a skid.
With the invention, however, this does not happen. As indicated by the
flowchart of FIG. 4, when such a fault condition is detected, the outputs
of the throttle opening sensor 1 and vehicle speed detecting circuit 2 are
detected by the rate-of-change detecting circuits 6 and 7, respectively,
at a step 101 and compared with the predetermined comparison values in a
step 102. As a result of the comparison, the time 9 is actuated through
the OR gate 8. Hence, regardless of the instruction generated by the gear
change timing arithmetic circuit 3, in a step 103, the transmission is
held in the present gear by the gear change holding circuit 5 for the time
counted out by the timer 9. In a step 104, it is determined whether or not
the fault condition has continued beyond the predetermined period of time
counted out by the timer 9. If the fault condition has continued, the gear
change holding circuit 5 is actuated by the fault signal produced by the
fault detecting circuit 4 to thereby change the transmission gear setting
to the safest gear, here assumed to be the third gear. Further gear
changes are thereafter inhibited, as indicated at a step 105 in the
flowchart of FIG. 4.
This completes the description of the preferred embodiments of the
invention. Although preferred embodiments have been described, numerous
modifications and alterations thereto are believed to be evident to one of
ordinary skilled in the art without departing from the spirit and scope of
the invention.
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