Traction control system for motor vehicles

In a traction control system for motor vehicles, the design is made such that during traction control operation, detection is made as to whether both the left-hand and the right-hand driving wheel speed have become equal to or higher than a predetermined threshold value, and detection is also made as to whether one of the driving wheel speeds which is increasing and the other one of the driving wheel speeds which is decreasing, have crossed each other; when the results of such detections are both "YES", this is regarded as an indication that a hunting phenomenon is likely to occur, and in response thereto, buildup and reduction of the brake hydraulic pressure is restricted; during the restriction, detection is made as to whether both of the driving wheel speeds have become equal to or lower than the predetermined threshold value, the above-mentioned restriction is removed. In this way, occurrence of a hunting phenomenon is restrained so that the difference between the left-hand and right-hand driving wheel speeds is decreased, thus preventing occurrence of vehicle body vibration.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a traction control system for motor 
vehicles, and more particularly it pertains to such a system wherein a 
slip of driving wheels which tends to be caused when the motor vehicle 
starts or accelerates is detected to control the braking of the driving 
wheels, thereby restraining the slip of the driving wheels. 
2. Description of the Prior Art 
Known in the art is a traction control system for motor vehicles, which is 
arranged, when the motor vehicle runs on a low-friction road surface such 
as a frozen, snow-covered or muddy one, to restrain a slip of driving 
wheel which tends to be caused in an attempt to start or accelerate the 
motor vehicle, thereby enhancing the starting/accelerating performance as 
well as the running stability of the vehicle. With such a conventional 
traction control system, it has been the usual practice that a slip of 
each of the left-hand and right-hand driving wheel is detected; and when 
the extent of the slip exceeds a predetermined value, a brake hydraulic 
pressure applied to a brake device for each driving wheel is increased so 
that the braking of the driving wheels is controlled, while at the same 
time the engine output is also controlled, as disclosed in Japanese Patent 
"Kokai" (Laid-opened) Publication No. 61-85248. 
To control the brake hydraulic pressure to be applied to the driving 
wheels, the following procedures are commonly performed: A plurality of 
threshold levels are set up in relation to the vehicle speed so that a 
plurality of driving wheel speed ranges are established between adjacent 
ones of the aforementioned threshold levels; detection is made as to which 
speed range the current driving wheel speed belongs to, and detection is 
also made as to whether such driving wheel speed is increasing or 
decreasing; and buildup, reduction and holding of the brake hydraulic 
pressure for the driving wheels are controlled on the basis of such 
detections. 
Since the left-hand and right-hand driving wheels are associated with each 
other through a differential gear, however, with such a conventional 
control system that the brake hydraulic pressure is increased or reduced 
in dependence on the aforementioned behavior of the driving wheels, there 
is a tendency that there occurs such a hunting phenomenon that if one of 
the driving wheels is rapidly decelerated by being braked, then the other 
driving wheel is rapidly accelerated so that a great speed difference is 
repeatedly caused to occur between the left-hand and right-hand driving 
wheels during a short period of time, thus causing the vehicle body to be 
vibrated. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a traction 
control system for motor vehicles, which is so designed as to prevent 
occurrence of vehicle body vibration which tends to be caused due to such 
a hunting phenomenon as mentioned above, during traction control 
operation. 
Briefly stated, according to the present invention, during traction control 
operation, detection is made as to whether both the left-hand and the 
right-hand driving wheel speed have become equal to or higher than a 
predetermined threshold value, and detection is also made as to whether 
one of the driving wheel speeds which is increasing and the other of the 
driving wheel speeds which is decreasing, have crossed each other; when 
the results of such detections are both "YES", this is regarded as an 
indication that a hunting phenomenon is likely to occur, and in response 
thereto, buildup and reduction of the brake hydraulic pressure is 
restricted; during the restriction, detection is made as to whether both 
of the driving wheel speeds have become equal to or lower than the 
predetermined threshold value, the above-mentioned restriction is removed. 
In this way, occurrence of a hunting phenomenon is restrained so that the 
difference between the left-hand and right-hand driving wheel speeds is 
decreased, thus preventing occurrence of vehicle body vibration. 
Other objects, features and advantages of the present invention will become 
apparent from the ensuing description taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, there is shown a systematic diagram of the traction 
control system according to an embodiment of the present invention, 
wherein a master cylinder 11 including two pressure chambers (not shown) 
is actuated through a brake pedal 10 serving as a brake operating member; 
brake fluid is pumped up by means of a pump 13 from a reservoir 12 
attached to an upper portion of the master cylinder 11 and stored in an 
accumulator under a highly-pressurized condition. 
One of the pressure chambers of the master cylinder 11 is connected to a 
wheel cylinder 21 of a brake device for braking a right-hand front wheel 
20R (driving wheel) through a fluid passage 15 which is provided with a 
gate valve GV and connected to a wheel cylinder 21 of a brake device for 
braking a left-hand rear wheel 22L (non-driving wheel) through a cut valve 
CV. The other pressure chamber of the master cylinder 11 is connected to a 
wheel cylinder of a brake device for braking a left-hand front wheel 20L 
(driving wheel) through a fluid passage 16 which is provided with a gate 
valve GV and connected to a wheel cylinder 21 of a brake device for 
braking a right-hand rear wheel 22R (non-driving wheel) through a cut 
valve CV. Hold valves HV are connected between the fluid passage 15 and 
accumulator 14, and between the fluid passage 16 and the accumulator 14. 
Decay valves DV are connected between the fluid passage 15 and the 
reservoir 12, and between the fluid passage 16 and the reservoir 12. The 
highly pressurized brake fluid accumulated in the accumulator 14 is 
supplied to or interrupted from the wheel cylinders through the 
opening/closing operation of the respective hold valves HV. The brake 
fluid contained in the wheel cylinders 21 is discharged to or interrupted 
from the reservoir 12 through the opening/closing operation of the decay 
valves DV. 
The gate valves GV, cut valves CV, hold valves HV and decay valves DV may 
comprise solenoid valves and are also used to effect antilock control when 
the motor vehicle is braked, and the opening/closing operations of these 
valves are controlled in accordance with commands from a 
microcomputer-based control circuit 23. When the traction control is 
commenced with respect to the left-hand and right-hand driving wheels 20L 
and 20R, the gate valves GV and cut valves CV are closed; the wheel 
cylinders 21 for the driving wheels 20L and 20R are separated from the 
master cylinder 11; and the wheel cylinders 21 for the non-driving wheels 
22L and 22R are separated from the fluid passages 15 and 16. When the gate 
valves GV and cut valves CV are closed, the wheel cylinders for the 
non-driving wheels 22L and 22R are held in such a state that no brake 
fluid pressure is applied thereto. The brake fluid pressure in the wheel 
cylinders 21 for the driving wheels 20L and 20R are increased when the 
hold valves HV are opened and the decay valves DV are closed; it is held 
when the hold valves HV and decay valves DV are closed; and it is reduced 
when the hold valves HV are closed and the decay valves are opened. 
Connected to the control circuit 23 are wheel speed sensors 24L, 24R, 25L 
and 25R for detecting the speeds of the wheels 20L, 20R, 22L and 22R; a 
brake switch 26 for detecting depression of brake pedal 10; and a sensor 
28 for detecting the extent of depression of an accelerator pedal 27. The 
control circuit 23 controls the engine output by driving a throttle 
control mechanism 29 of the engine in response to information derived from 
these sensors, while at the same time controlling the brake fluid pressure 
in the wheel cylinders 21 for the driving wheels 20L and 20R by causing 
the gate valves GV, cut valves CV, hold valves HV and decay valves DV to 
be opened and closed. When the brake switch 26 is operated, the traction 
control is ended immediately. 
FIG. 2 shows the construction of the control circuit 23, and FIGS. 3 and 4 
show the timing charts for the traction control executed by the control 
circuit 23. The speeds of the driving wheels 20L and 20R are detected by 
wheel speed sensors 24L and 24R respectively, and driving wheel speeds VdL 
and VdR are computed in computing circuits 31 and 32. The wheel speeds VdL 
and VdR are inputted to a control logic circuit 30. The speeds of the 
left-hand and right-hand non-driving wheels 22L and 22R are detected by 
wheel speed sensors 25L and 25R respectively, and left-hand and right-hand 
non-driving wheel speeds VnL and VnR are computed in computing circuits 33 
and 34 respectively. The left-hand and right-hand driving wheel speeds VdL 
and VdR are passed to computing circuits 35 and 36 respectively so that 
the acceleration/deceleration dVdL/dt and dVdR/dt of the wheel speeds VdL 
and VdR are inputted to the control logic circuit 30. 
The speeds VnL and VnR of the left-hand and right-hand non-driving wheels 
22L and 22R are provided to a select-high circuit 37 so that the higher 
one of the wheel speeds VnL and VnR is selected and computed to provide a 
computed vehicle speed Vv which is approximate to the real vehicle speed; 
and the computed vehicle speed Vv is provided to an adding circuit 38 in 
which a first predetermined value Vk is added to the computed vehicle 
speed Vv to provide a first threshold value Vt1 (=Vv+Vk) as shown in FIG. 
3; a second predetermined value Vk' is added to the first threshold value 
Vt1 to provide a second threshold value Vt2 (=Vv+Vk+Vk'); and a third 
predetermined value Vk" is added to the second threshold value Vt2 to 
provide a third threshold value Vt3 (=Vv+Vk+Vk'+Vk"). Further, another 
threshold value Vt0, which serves as control ending threshold, is 
established between the computed vehicle speed Vv and the first threshold 
value Vt1 by adding a fourth predetermined value Vk'" to Vv (Vt/=Vv+Vk'"). 
The threshold values Vt0 to Vt3 are inputted to the control logic circuit 
30. In response to the various signals mentioned above, the control 
circuit 23 drives a throttle control mechanism 29 to control the throttle 
opening degree when the motor vehicle starts or accelerates, while at the 
same time controlling the opening/closing operations of the hold valves 
HV, decay valves DV and so forth, thereby controlling the brake fluid 
pressure at wheel cylinders 21 for left-hand and right-hand driving wheels 
20L and 20R so that traction control is performed as shown in FIG. 3. 
Description will now be made of an example of traction control system 
performed by the control circuit 23, by referring to FIGS. 3 and 4 showing 
timing charts for brake fluid pressure control at each status, and FIG. 5 
showing a flow chart for control routine. In this example of traction 
control, brake fluid pressure control is effected independently with 
respect to the left-hand and right-hand driving wheels. In the ensuing 
explanation, any particular one of the left-hand and right-hand driving 
wheel speeds VdL and VdR is represented by Vw. 
[STATUS 0] 
This is status at which no traction control is effected. Before the control 
is started, the wheel speed Vw is lower than the first threshold value Vt1 
(Vw&lt;Vt1); and the control is started after a lapse of time period t from 
the time point when the wheel speed Vw becomes lower than the threshold 
value Vt0. In this case, both the hold valves HV and decay valves DV are 
closed; both the bate valves GV and cut valves CV are opened; and the 
brake fluid pressure is in a state of maximum reduction. 
[STATUS 1: PRELIMINARY PRESSURE BUILDUP] 
As the wheel speed Vw is increased, the result of judgment made at step S1 
becomes "YES" at a time point A when the wheel speed Vw becomes equal to 
the first threshold value Vt1, and the process proceeds to step S1 so that 
control is started. At the time point A, the gate valves GV and cut valves 
CV are closed, and the hold valves HV are opened for a preset time T to 
effect preliminary buildup of the brake fluid pressure. 
At the next step S3, judgment is made as to whether or not the wheel speed 
Vw is increasing. The status remains unchanged until the wheel speed Vw 
reaches the second threshold value Vt2. Thus, the process proceeds to step 
S5 while the condition at the status 1 is maintained. The process now 
proceeds to step S6. Such a procedure that the process proceeds to step S6 
via the steps S4 and S5, is always repeated for each subsequent status. 
The step S6 constitutes one of the control ending conditions. In this 
case, since the condition that Vw&lt;Vt0 is not prevailing, the process 
returns to the step S3 and proceeds to the step S4. 
[STATUS 2: PRESSURE BUILDUP] 
When the wheel speed Vw as increasing reaches the second threshold value 
Vt2 at a time point B, the result of the judgment made at the step S4 
becomes "YES", and needless to say, the result of judgment at step S7 
(Vw.gtoreq.Vt2?) also becomes "YES"; thus the process proceeds to step S8 
at which buildup of the brake fluid pressure is started. The hold valves 
HV are opened and closed with ON-OFF intervals which are preset in 
accordance with the acceleration dVw/dt which occurs when the wheel speed 
Vw becomes equal to the second threshold value Vt2 (Vw=Vt2), as a result 
of which the brake fluid pressure builds up step-wise. 
[STATUS 3: PRESSURE BUILDUP] 
The wheel speed Vw increases further, and reaches a third threshold value 
Vt3 at a time point C. At step S9, judgment is made as to whether or not a 
high peak of the wheel speed Vw is reached. In this case, the result of 
judgment at step S9 is "NO", and hence the process returns to the step S3 
via the steps S5 and S6, and again to steps S8 via the steps S4 and S7, so 
that the pressure buildup continues. The hold valves HV are opened and 
closed with ON-OFF intervals which are set up in accordance with the 
acceleration dVw/dt which occurs when the wheel speed Vw becomes equal to 
the third threshold value (Vw=Vt3), so that the brake fluid pressure 
further builds up step-wise. 
[STATUS 4: PRESSURE HOLDING] 
At a time point D when a high peak of the wheel speed Vw is reached, the 
result of judgment at step S9 becomes "YES", and thus the process proceeds 
to step S10 where the hold valves HV are closed so that the brake fluid 
pressure prevailing at the time point D is held. The process returns to 
the step S3 via the step S6. At and after the time point D, the wheel 
speed Vw is changed from increase to decrease so that the result of 
judgment at the step S3 becomes "NO", and thus the process flow of FIG. 5 
shifts to right-hand side step S11 where like at the steps S4, judgment is 
made as to whether or not the status has changed. Unless the status has 
changed, the process repeatedly takes the passage S5 - S6 - S3. 
[STATUS 5: PRESSURE REDUCTION] 
When the wheel speed as decreasing becomes lower than the third threshold 
value Vt3 at a time point E, the result of judgment at step S11 becomes 
"YES", and thus the process proceeds to step S12 so that reduction of the 
brake fluid pressure is started. The decay valves DV are opened and closed 
with ON-OFF intervals which are preset in accordance with the deceleration 
dVw/dt which occurs when the wheel speed Vw becomes equal to the third 
threshold value Vt3 (Vw=Vt3), so that the brake fluid pressure is reduced 
step-wise. At the next step S13, judgment is made as to whether or not a 
low peak of the wheel speed Vw is reached. In this case, the result of 
judgment at step S13 is "NO", and thus the process proceeds to the step S5 
so that the reduction of the brake fluid pressure is continued. 
[STATUS 6: PRESSURE REDUCTION] 
The wheel speed Vw further drops, and reaches the second threshold value 
Vt2 at a time point F; thus the result of judgment at step S11 becomes 
"YES" again, and the process proceeds to step S12. The decay valves DV are 
opened and closed with ON-OFF intervals which are preset in accordance 
with the deceleration dVw/dt which occurs when the wheel speed Vw becomes 
equal to the second threshold value Vt2 (Vw=Vt2), so that the brake fluid 
pressure is further reduced. 
[STATUS 7: PRESSURE REDUCTION] 
The wheel speed Vw further decreases, and reaches the first threshold value 
Vt1 at a time point G; thus the result of judgment at step S11 becomes 
"YES" again, and the process proceeds to step S12. The decay valves are 
opened and closed with ON-OFF intervals which are preset in accordance 
with the deceleration dVw/dt which occurs when the wheel speed Vw becomes 
equal to the first threshold value Vt1 (Vw=Vt1), so that the brake fluid 
pressure is further reduced. A low peak of the wheel speed Vw is reached 
at a time point H, and the result of judgment at step S13 becomes "YES"; 
thus the process proceeds to step S14 where judgment is made as to whether 
nor not the wheel speed Vw lies in the region between the first threshold 
value Vt1 and the third threshold value Vt3. In this case, since the wheel 
speed Vw is lower than the first threshold value Vt1 (Vw&lt;Vt1), the result 
of judgment at step S14 is "NO". In this way, the reduction of the brake 
fluid pressure is further maintained so that the wheel speed Vw is changed 
from increase to decrease, and thus the control flow shifts to the step 
S4. 
[STATUS (1): PRESSURE HOLDING] 
If the wheel speed increases up to the first threshold value Vt1 at a time 
point I, then the result of judgment at the step S4 becomes "YES", and the 
process proceeds to the step S7. In this case, since the wheel speed Vw is 
lower than the second threshold value Vt2 (Vw&lt;Vt2), the result of judgment 
at the step S7 is "NO" so that the process proceeds to step S10 and the 
brake fluid pressure prevailing at the time point I is held. This brake 
fluid pressure holding state is maintained until the status is changed. 
When the wheel speed Vw reaches the second threshold value Vt2, pressure 
buildup is started; and when the wheel speed Vw becomes lower than the 
first threshold value Vt1, pressure reduction is started. 
[STATUS (0): PRESSURE REDUCTION] 
In a manner similar to that mentioned above, at a time point J when the 
wheel speed Vw reaches the second threshold Vt2, pressure buildup is 
started; at a time point K when a high peak of the wheel speed Vw occurs, 
the brake fluid pressure is held; at a time point L when the wheel speed 
Vw drops down to the second threshold value Vt2, pressure reduction is 
started; at a time point M when the wheel speed further drops down to the 
first threshold value Vt1, status 7 occurs again. At a time point N, the 
wheel speed Vw reaches the control ending threshold value Vt0. At this 
point of time, the result of judgment at the step S6 becomes "YES" for the 
first time, and thus the process proceeds to step S15 where judgment is 
made as to whether or not preset time t in second has elapsed from the 
time point when the wheel speed Vw became lower than Vt0. The result of 
judgment at the step S15 is "NO" until the preset time t in second elapses 
from the time point N, and the control process returns to step S3. The 
pressure reduction at th status (0) is effected with the decay valves 
being closed. 
At a time point Q when the wheel speed Vw becomes higher than the first 
threshold value Vt1, the status (1) occurs again wherein the brake fluid 
pressure is held. More specifically, at this status (1), the result of 
judgment at step S7 is "NO", and thus the process proceeds to step S10 so 
that the brake fluid pressure is held. Status 7 occurs at a time point R 
when the wheel speed Vw becomes lower than the first threshold value Vt1, 
so that the brake fluid pressure is reduced. At a time point S when the 
wheel speed Vw becomes lower than the control ending threshold value Vt0, 
the status (0) occurs again so that the result of judgment at the step S6 
becomes "YES", and the process proceeds to step S15 where judgment is made 
as to whether or not the preset time t has elapsed. At a time point T when 
the preset time t elapses from the time point S, the result of judgment at 
step S15 becomes "YES" so that the control is ended, and the control 
process returns to the status 0, and the gate valves GV and cut valves CV 
are opened. 
FIG. 4 shows the case where a low peak of the wheel speed Vw is decreasing 
is reached when the wheel speed Vw lies in the region between the first 
threshold value Vt1 and the third threshold value Vt3 while the brake 
fluid pressure is being reduced. In this case, the result of judgment at 
the step S16 where at time points X and Y when a low peak of the wheel 
speed Vw occurs, the brake fluid pressure is held. Such a state that the 
brake fluid pressure is held, is maintained until status change occurs, 
thus preventing occurrence of excessive pressure buildup or reduction. The 
control of the brake fluid pressure is effected in such a manner that at a 
time point when the wheel speed Vw as increasing becomes higher than the 
second threshold value Vt2 or third threshold value Vt3 for the first 
time, buildup of the brake fluid pressure is started and at a time point 
when the wheel speed Vw as decreasing becomes lower than the second 
threshold value Vt2 or first threshold value Vt1 for the first time, 
reduction of the brake fluid pressure is started. 
What has been mentioned above relates to an ordinary status control mode. 
According to the present invention, such control as illustrated in FIGS. 6 
and 7 are also effected with a view to restraining occurrence of a hunting 
phenomenon. FIG. 6 is a timing chart illustrating an example that 
occurrence of a hunting phenomenon is restrained by controlling the brake 
fluid pressure for each of the left-hand and right-hand driving wheels, 
and FIG. 7 is a flow chart for anti-hunting control routine. 
During the period from a time point when brake fluid pressure control in 
the ordinary mode is started (step S21) to a time point U.sub.1, only the 
wheel speed VdR of the left-hand and right-hand driving wheel speeds VdL 
and VdR is higher than the first threshold value Vt1; thus the result of 
judgment made at step S22 as to whether or not both of the wheel speeds 
VdR and VdL are higher than the first threshold value Vt1, becomes "NO" so 
that the process returns to step S21 again and the control is performed in 
the ordinary status mode. From the time point U.sub.1 onward, both the 
speed speeds VdR and VdL become higher than the first threshold value Vt1, 
and thus the result of judgment at step S22 becomes "YES" so that the 
process proceeds to step S23 where judgment is made as to whether or not a 
restriction mode for pressure buildup/reduction is set up. In this case, 
the result of judgment at step S23 is "NO", and thus the process proceeds 
to step S24 where judgment is made as to whether or not the wheel speeds 
VdR and VdL as increasing and decreasing or vice versa have crossed each 
other. When the wheel speeds VdR and VdL do not cross each other, the 
result of such judgment is "NO", so that the process returns to step S21 
again. When a time point U.sub.2 in FIG. 6 is reached, one of the wheel 
speeds, VdL, which is increasing, and the other one of the wheel speeds, 
VdR, which is decreasing, cross each other, and thus the result of 
judgment at step S24 becomes "YES" for the first time, so that the 
restriction mode for pressure buildup/reduction is set up (step S25). In 
this way, the brake fluid pressure is controlled in such a manner that 
pressure buildup is effected only at status 3 and pressure reduction is 
effected only at status 5. 
In the ordinary status control mode, the brake fluid pressure is controlled 
such that preliminary pressure buildup is effected for the first cycle and 
the brake fluid pressure is held for the second and succeeding cycles; the 
brake fluid pressure is increased at status 2 and 3; the pressure is held 
at status 4; and the pressure is reduced at status 5, 6 and 7. When the 
restriction mode for pressure buildup/reduction is set up at step S25, the 
brake fluid pressure is controlled in such a manner as to be increased 
only at status 3, reduced only at status (step S26) and held at all the 
other status than the above. When both the wheel speeds VdR and VdL do not 
become lower than the first threshold value Vt1, the result of judgment at 
step S27 is "NO", the processing at step S26 is continued. At the time 
point U.sub.2, the wheel speed VdR is at status 6, and the wheel speed VdL 
is at status 1; thus the control is effected so that the brake fluid 
pressure prevailing at the time point U.sub.2 is held. 
After the time point U.sub.2, the result of judgment made at step S23 in 
FIG. 7 as to whether or not the restriction mode for pressure 
buildup/reduction becomes "YES", and thus the process proceeds directly to 
step S26 so that the brake fluid pressure prevailing at the time point 
U.sub.2 is held with respect to both the wheel speeds VdR and VdL. With 
respect to the wheel speed VdL, at a time point U.sub.3, status 3 occurs 
so that the brake fluid pressure is increased; at a time point U.sub.4, 
the brake fluid pressure is held; at a time point U.sub.5, status 5 occurs 
so that the brake fluid pressure is reduced; and from a time point U.sub.6 
onward, the brake fluid pressure is held. 
In this way, both the wheel speeds VdR and VdL become lower than the 
threshold value Vt1 at a time point U.sub.7 as a result of the 
above-mentioned control of the brake fluid pressure having been effected 
with respect to the left-hand and right-hand driving wheels 20L and 20R. 
Consequently, the result of judgment at step S27 in FIG. 7 becomes "YES", 
so that the process returns to step S21 and the brake fluid pressure 
control now is effected in the ordinary mode. 
As will be appreciated from the foregoing discussion, according to the 
above-mentioned embodiment of this invention, when it is judged that both 
of the left-hand and right-hand driving wheel speeds VdR and VdL exceeds 
the first threshold value Vt1 and that one (VdL for example) of the 
driving wheel speeds, which is increasing, and the one (VdR for example) 
of the driving wheel speeds, which is decreasing, have crossed each other, 
brake fluid pressure buildup/reduction is restricted such that the wheel 
speeds VdR and VdL are increased only at status 3 and reduced at only 
status 5. In this way, occurrence of a hunting phenomenon is restrained so 
that the difference between the left-hand and right-hand driving wheel 
speeds is decreased to such an extent that occurrence of vehicle body 
vibration is prevented. 
While the present invention has been illustrated and described with respect 
to some specific embodiments thereof, it is to be understood that the 
present invention is by no means limited thereto but encompasses all 
changes and modifications which will become possible within the scope of 
the appended claims.