Malfunction detector for acceleration sensor

An apparatus detects a malfunction of a plurality of acceleration sensors used in an antilock control system. The device has a plurality of state changeover devices corresponding to the respective acceleration sensors for changing their outputs from 0 to 1 if the outputs of the respective acceleration sensors change from low deceleration states to high deceleration states, and from 1 to 0 if their outputs change from high to low deceleration states. The apparatus also includes a reset signal output device for producing a reset signal if the outputs of all the state changeover devices are 1 and if the outputs of all of the state changeover devices change from 1 to 0 thereafter. A lamp actuating device is provided for turning a warning lamp on if the output of at least one of the state changeover devices is 1 and for turning the lamp off if there is a reset signal from the reset signal output device.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a malfunction detector for an acceleration sensor 
provided in an antilock control system of a vehicle. 
2. Description of the Related Art 
The deceleration of a vehicle while its wheels are in a locked state 
represents the frictional coefficient of the road surface (hereinafter 
denoted .mu.), provided the road surface is uniform. Thus, by providing an 
antilock control system with an acceleration sensor, it becomes possible 
to judge whether the vehicle is travelling on a road surface having a 
high-.mu. value or one having a low-.mu. value. This in turn makes it 
possible to change over the control mode in such a manner so as to 
increase the locking detecting sensitivity or to increase the sensitivity 
for detecting the recovery from a locked state while the vehicle is on a 
low-.mu. road, thus reducing the brake pressure to a greater degree, and 
to lower such a detecting sensitivity while on a high-.mu. road, thus 
reducing the brake pressure to a lesser degree. 
In addition to the above method, various proposals have been made to change 
the processing level of antilock control, depending upon the ON and OFF 
positions of an acceleration sensor. For example, it was proposed to 
re-increase the brake pressure after pressure reduction while on a 
high-.mu. road and slowly while on a low-.mu. road, or to control the rate 
of change of the estimated vehicle speed. 
Such acceleration sensors come in various structures. FIG. 2 shows on 
example which includes a switch body 1 and a glass tube 2 fixedly mounted 
in the switch body 1 at an angle .beta. with respect to the mounting 
surface M. If a deceleration larger than tan .beta. acts while the vehicle 
is moving in the direction of the depicted arrow (i.e. parallel to the 
mounting surface), mercury 3 in the glass tube 2 moves toward electrodes 
4, electrically connecting the electrodes 4 together. While the electrodes 
4 are electrically connected, a deceleration which is larger than a 
predetermined level is present. If the electrodes 4 are not so connected, 
then no such deceleration is present. 
In contrast, electrodes may be provided at the lower end of the glass tube 
2 so that an electrical connection will be cut off while a deceleration 
which is larger than a predetermined level is present. Another type of 
acceleration sensor has rolling elements adapted to roll when subjected to 
deceleration to actuate a limit switch. 
If such acceleration sensors should fail, it becomes impossible to change 
the control level of the antilock control system. This poses a problem in 
that the braking distance increases. 
To prevent this problem, it was proposed to provide a plurality of 
acceleration sensors and to carry out control for a high-.mu. road 
according to one of two methods: (1) if one of the acceleration sensors 
indicates a high deceleration, or (2) only if all of the acceleration 
sensors show a high deceleration. 
However, the first method has a drawback in that if the mass in one of the 
acceleration sensors should get stuck at the high deceleration side, the 
antilock control performance on a low-.mu. road will drop because the 
control mode is fixed to that for a high-.mu. road. 
Further, the second method has a drawback in that if the mass in any one of 
the sensors should get stuck at the low deceleration side, the braking 
distance tends to increase while on a high-.mu. road, because the control 
mode is fixed to one for a low-.mu. road. 
Further, even if no mass becomes stuck within the sensors, if the 
sensitivity of the acceleration sensors should shift for some reason, the 
same problems as described above will result. 
It is an ordinary practice to provide an antilock device with a plurality 
of acceleration sensors, if provided at all, for safety's sake. In one 
conventional method for testing whether or not these acceleration sensors 
are working normally, it is checked whether the acceleration sensor 
outputs have changed over after inclining the vehicle by an angle at which 
the outputs are supposed to change over. 
However, this method suffers a drawback in that it requires a plurality of 
audio-visual means for indicating the output states of the respective 
acceleration sensors. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a device which can 
easily detect malfunctions of an acceleration sensor. 
In accordance with the present invention, there is provided a malfunction 
detector for acceleration sensors which includes a testing device for 
testing the acceleration sensors, a changeover device for selectively 
supplying the output signals from the acceleration sensors to a 
processing/lock state detecting device or to the testing device, and a 
warning device connected to the testing device. The testing device is 
adapted, upon receipt of the output signals from the acceleration sensors, 
to change the position of the acceleration sensors from their normal 
position to a position for a high deceleration to check whether or not all 
of the acceleration sensors provide high-deceleration signals and then to 
move the acceleration sensors back to the normal position to check whether 
or not the output signals from all of the acceleration sensors change from 
high-deceleration signals to low-deceleration signals and to output the 
results to the warning device. 
According to the present invention, the acceleration sensors are tilted to 
artificially create a state where a high deceleration is detected and a 
state where a low deceleration is detected to check whether or not the 
output signals from all the acceleration sensors correspond to the 
abovementioned state. This makes it possible to accurately determine 
whether or not the acceleration sensors are working normally. 
According to this invention, there is provided a test mode changeover 
device and the acceleration sensor testing device. During the test mode, a 
high deceleration state and a low deceleration state are created 
artificially to securely check whether or not the acceleration sensors are 
working normally and to activate the warning lamp when appropriate.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, an electronic control unit 10 carries out various 
calculations and judgments based on signals from wheel speed sensors S1-S4 
(only S1 is shown) associated respectively with the vehicle wheels and 
outputs control signals to a fluid pressure circuit. 
The AC voltage signals from the wheel speed sensor S1 are converted into 
pulses and fed from a wheel speed detecting device 11 (which counts the 
pulses) to a processing/lock state detecting device 12 as wheel speed 
signals. Based on the deceleration and estimated vehicle speed calculated 
in the device 12, it is determined whether or not the deceleration has 
decreased below a reference value or whether the slip speed (the 
difference between the estimated vehicle speed and the wheel speed) has 
increased above a reference point. If such a locking tendency is detected, 
a pressure reduction command is output to a solenoid actuating circuit 13. 
In response, the circuit 13 energizes solenoids SOL1 and SOL2. Thus, a 
pressure control valve 21 will be moved (to the lefthand side of FIG. 1), 
shutting off the fluid pressure circuit extending from a master cylinder 
23 to a wheel cylinder 24, and a pressure control valve 22 will move 
(upwardly in FIG. 1), opening communication between the wheel cylinder 24 
and a reservoir 25. At the same time, a motor M is started by a motor 
actuating circuit 14 to return the brake fluid in the reservoir 25 into an 
accumulator 26 and the master cylinder 23 by means of a pump P. The brake 
pressure will thus drop. 
When the wheel speed begins to increase again and the deceleration or the 
slip speed exceeds a reference value, it is judged that the locking 
tendency has disappeared. Thus, the lock state detecting device 12 outputs 
a pressure increase command to the solenoid actuating circuit 13, which in 
response deenergizes the solenoids SOL1 and SOL2 to move the pressure 
control valves 21 and 22 back to their original positions shown in FIG. 1. 
Thus, the fluid pressure source and the fluid pressure circuit of the 
wheel cylinder 24 will communicate with each other and the brake pressure 
will rise. 
When pressure hold command is given to interrupt the pressure reduction or 
pressure increase command, the solenoid SOL1 is energized and the solenoid 
SOL2 is deenergized. Thus, the pressure control valve 21 moves (to the 
lefthand side of FIG. 1), shutting off the fluid pressure circuit, whereas 
the pressure control valve 22 remains in the position shown in FIG. 1. 
Thus, fluid pressure is sealed in the wheel cylinder 24 and the brake 
pressure is kept constant. In FIG. 1, numeral 27 designates a bypass 
valve. 
The conditions and timing for interrupting the pressure reduction command 
with the pressure hold command may be determined in any desired manner. 
For example, a pressure hold command may be given when the duration of the 
pressure reduction command reaches a predetermined point or when the wheel 
deceleration exceeds a predetermined threshold value. 
Similarly, the conditions and timing for alternately giving pressure 
increase commands and pressure hold commands may be determined in various 
manners. Generally, pressure hold commands are given at predetermined time 
intervals by use of a pulse generator, for example. 
The judgment as to whether or not a locking tendency has appeared or 
disappeared may be made based on the deceleration, slip speed or any other 
index. In FIG. 1, one fluid pressure circuit for controlling one wheel 
cylinder 24 is shown coupled to the control device. However, it is to be 
understood that the control device is used to control three channels for 
the front left wheel, front right wheel and both rear wheels, or four 
channels for the respective four wheels. 
The ON/OFF signals of acceleration sensors 15 and 15' are supplied to an 
acceleration sensor detecting device 16 and then through a test mode 
changeover device 17 to a level changeover device 18 for antilock 
processing. The level changeover device 18 sets the control mode to a 
high-.mu. road control mode if either of the acceleration sensors 15 and 
15' indicates a high deceleration (ON signal), and to a low-.mu. road 
control mode if both sensors 15 and 15' show a low deceleration (OFF 
signal). Alternately, the control mode may be switched to the high-.mu. 
road control mode if both of the sensors 15 and 15' indicate a high 
deceleration, and to the low-.mu. road control mode if either of the 
sensors 15 and 15' indicates a low deceleration. 
Thus, the processing level changeover device 18 serves to change the 
processing level of the processing/lock state detecting device 12 
depending upon whether the vehicle is travelling on a high-.mu. road or a 
low-.mu. road. 
While on a high-.mu. road, it is common to raise the reference value 
(threshold value) of the deceleration or slip speed for detecting the 
locking state or to carry out pressure re-increase soon after pressure 
reduction. 
On the other hand, while on a low-.mu. road, the threshold value for 
detecting the locking state is lowered to raise the detecting sensitivity, 
whereas the threshold value for detecting the tendency to recover from a 
locked state is raised to carry out a pressure re-increase slowly after 
the locking tendency has complete disappeared. 
By selectively setting a manual switch or a test terminal, the test mode 
changeover device 17 establishes a non-test mode in which signals from the 
acceleration sensor detecting device 16 are applied to the processing 
level changeover device 18, or a test mode in which the signals from the 
acceleration sensor detecting device 16 are applied to an acceleration 
sensor testing device 19. 
The testing device 19 comprises state changeover devices 30 and 30' to 
which the signals from the acceleration sensors 15 and 15' are applied, 
respectively, and a reset signal output device 31. Suppose that the 
initial setting of the acceleration sensors 15 and 15' is such that their 
output will change from low deceleration states to high ones when the 
inclination of the vehicle is between .THETA. and .THETA.' (FIG. 3). If 
the outputs of both the acceleration sensors 15 and 15' change from low to 
high deceleration states when the vehicle is tilted from its horizontal 
state and its angle of inclination reaches between .THETA. and .THETA.', 
the outputs of both the state changeover devices 30 and 30' will change 
from 0 to 1 (FIG. 4). This activates a lamp actuating device 32, thus 
turning a warning lamp WL on. Then if the outputs of both the acceleration 
sensors 15 and 15' change normally from high to low deceleration states 
when the vehicle inclination is returned to below .THETA.', the outputs of 
both the state changeover devices 30 and 30' change from 1 to 0. This 
causes the reset signal output device 31 to produce a reset signal which 
deactivates the lamp actuating device 32, thus turning the lamp WL off. If 
the lamp WL does not turn on when the vehicle is tilted until its 
inclination angle reaches between .THETA. and .THETA.' or if the lamp WL 
does not turn off when the vehicle inclination returns to below .THETA.', 
it is judged that at least one of the acceleration sensors is not 
functioning normally. Thus, according to this invention, malfunction of 
the acceleration sensors can be detected easily with a single warning 
lamp. Three or more acceleration sensors may be provided.