Vehicular braking system utilizing a hydrodynamic brake and friction brake

A braking system utilizing a hydrodynamic brake has a control circuit that adjust the torque exerted by the hydrodynamic brake to adjust the deceleration of the axle on which the hydrodynamic brake is mounted toward a desired value of deceleration, which desired value may be zero.

BACKGROUND OF THE INVENTION 
The present invention relates to a braking system, intended primarily for 
use in highway vehicles, in which a friction brake is associated in the 
conventional manner with each wheel of the vehicle and a hydrodynamic 
brake in provided on one axle, generally the rear axle, of the vehicle. 
The hydrodynamic brake and the friction brakes may be controlled by means 
of separate respective operating levers, or the hydrodynamic brake may be 
controlled jointly with the friction brakes by means of a single operating 
lever. 
One common problem arises when the braking torque exerted on an axle 
exceeds a certain critical value, the axle locks and the vehicle skids, 
the driver often losing control of it in the process. It is therefore 
highly desirable to prevent such a condition, called "overbraking". 
One well known manner of attacking this problem is to provide an automatic 
retaining device, for instance a hydropneumatic control valve, on the 
friction brakes of the axle on which the hydrodynamic brake is mounted. 
The retaining device reduces the braking torque exerted at any instant by 
the rear friction brakes, by an amount equal to the braking torque exerted 
by the hydrodynamic brake. 
SUMMARY OF THE INVENTION 
According to the present invention, one axle (assumed hereinafter to be the 
rear axle, although any axle might be chosen) is equipped with a 
hydrodynamic brake and may further be equipped with conventional friction 
brakes. Each wheel on the other axles is provided with a friction brake 
only. The deceleration of the rear axle is monitored by a control device 
whenever the hydrodynamic brake is operating, including both those changes 
instead which are due to a gradient in the road and those changes which 
are brought about by operation of any of the friction brakes of the 
vehicle. If the driver increases the force exerted by the friction brakes, 
the control device decreases the braking action of the hydrodynamic brake 
by the amount necessary to maintain the deceleration of the vehicle (or, 
more exactly, that of the rear axle) at a constant value. If the friction 
and the hydrodynamic brakes are jointly controlled, the friction brakes of 
the vehicle axle equipped with the hydrodynamic brake are actuated only 
after the hydrodynamic brake is producing the greatest braking torque 
possible at the speed of rotation in question; however, the friction 
brakes of the other axles are applied even when the braking torque exerted 
by the hydrodynamic brake is small. In this fashion, the braking force is 
distributed as uniformly as possible among the various axles. If the force 
exerted by the friction brakes alone provides a deceleration greater than 
that desired, then the control device completely shuts the hydrodynamic 
brake off. By this means, over-braking of the axle equipped with the 
hydrodynamic brake is made less likely than would be the case with a 
conventional brake system, and in this way the vehicle can be controlled 
better in critical traffic situations (for instance during emergency 
braking), since the braking forces are distributed optimally among all the 
wheels of the vehicle. 
The control device of the invention entails no extra expense, since the 
known braking devices already have a control device which, however, is 
designed to keep not the deceleration but the braking torque constant at a 
set value, and since the automatic retention device is omitted from the 
braking system of the present invention. 
Without the retention device, in the lowest speed range of the vehicle and 
particularly when the vehicle is to be brought to a stop, the friction 
brakes are no longer automatically applied to compensate for the fact that 
the hydrodynamic brake loses its braking force at low speed, and all the 
friction brakes must now be actuated by the driver. Any minor 
inconvenience this may represent, however, is far outweighed by the 
advantages of the present invention. 
There is known (see for example, U.S. Pat. No. 3,917,356) a braking device 
having a control means that is intended to keep the acceleration of the 
axles or of the vehicle at a selected value, and including as well an rpm 
measuring device and a differentiating device connected thereto. Braking 
systems of this known type include no hydrodynamic brake, but only a 
plurality of identical hydraulically-actuatable friction brakes. 
The system of the invention has additional advantages over conventional 
systems. Because the control means regulates the deceleration rather than 
the braking torque, the instantaneous value of the mass of the vehicle is 
automatically also taken into consideration, just as is done by 
load-dependent braking-force regulators; that is, the control means 
increases the hydrodynamic braking torque when the load of the vehicle 
increases, in order to keep the deceleration constant. With increasing 
weight of the vehicle, the traction of the wheels against the road is 
improved and thereby a greater braking moment can be transferred to the 
roadway without the wheels locking. 
If over-braking should occur for any reason, for instance on ice, and the 
wheels start to skid, the control means notes this immediately and reduces 
the braking moment accordingly. The immediate operation of the control 
means is assured by the fact that the information responsive to which the 
control means acts, i.e. the deceleration of the vehicle, is obtained by 
differentiating the speed of rotation of the rear axle of the vehicle (or 
of the universal shaft driving it) and not, for instance, by an instrument 
that records changes in inertia. 
It is necessary to protect the hydrodynamic brake from excessive mechanical 
strain. Conventional hydrodynamic vehicular braking systems measure the 
fluid pressure inside the hydrodynamic brake, which is at least 
approximately equal to the hydrodynamic braking torque (such a system is 
described in German Pat. No. 2 408 876), or the braking torque may be 
measured directly by means of a torque connection fastened to the stator 
(as, for instance, in German Unexamined Application for Pat. No. 2 239 
008). With such systems, it is easy to limit the fluid pressure 
established in the housing of the hydrodynamic brake and the mechanical 
load exerted on its structural parts to a permissible amount. With the 
braking system of the invention, these customary methods cannot be used, 
since now, as already mentioned, the control means no longer monitors the 
braking torque but the deceleration of the braked vehicle axle. In order 
to protect the hydrodynamic brake, a maxium allowable pressure for the 
brake fluid is established, and means is included to prevent the pressure 
from exceeding that limit. 
It is preferable to use, instead of conventional mechanical or hydraulic 
control means generally used in hydrodynamic brakes, an electronic control 
means, such as that taught in the aforementioned U.S. Pat. No. 3,917,356. 
In this case it is self-evident that in order to measure the deceleration 
an electrical measuring device is provided and in order to provide a 
reference signal representing the desired value an electrical reference 
signal generator is provided. As pressure switches, electric switches are 
preferred; furthermore it is preferable to control the amount of fluid in 
the hydrodynamic brake, to provide solenoid valves controlled by the 
control means. In this way the various data and operating commands can be 
processed easily and simply. Furthermore, the hysteresis which is 
unavoidable in known controllers can be prevented or at least 
substantially reduced. 
Another advantage which is obtainable by the invention resides in the fact 
that, when desirable, a "zero deceleration" order can be given to the 
reference signal generator. In this way, the speed of the vehicle can, if 
desired, be maintained at a selected constant value. This is particularly 
advantageous when the vehicle is traveling down a long hill. It has been 
found in this connection that a constant speed can under certain 
circumstances (for instance in the case of a non-uniform gradient) be 
maintained only for a short time. It is therefore desirable to provide a 
separate control means for use when constant speed is desired. 
The primary object of the present invention is to provide an improved 
braking system that will avert over-braking of the vehicle axle equipped 
with the hydrodynamic brake (generally the rear axle) more reliably, and 
yet no more expensively, than can be done conventionally. 
It is a further object of the present invention to provide a brake system 
that will unlock the wheels of the vehicle if overbraking does occur. 
It is another object to provide a braking system that will adjust the 
braking torque according to the vehicle's load. 
It is still another object to provide a braking system that can be used to 
maintain the vehicle's speed constant at a selected value. 
It is yet another object to provide a braking system that will achieve the 
above objects without subjecting the structural components of the 
hydrodynamic brake to excessive strain.

DETAILED DESCRIPTION OF THE INVENTION 
In FIG. 1, the front axle of a highway vehicle is shown at 31 and the rear 
axle at 32. There are also shown a drive unit with transmission 30 and 
drive shaft 30a, a hydrodynamic brake 1 arranged on the drive shaft 30a, a 
universal joint shaft 2, and a differential 3. The mechanical brake system 
includes two brake cylinders 33 on the front axle 31, two brake cylinders 
34 on the rear axle 32, and a foot-pedal valve 37 which is connected with 
the brake cylinders via pressure-fluid conduits 35 and 36. 
To regulate the hydrodynamic brake 1 an electronic control means 7 is 
provided. An electric speed measuring device 4 arranged on the universal 
shaft 2 is connected via a frequency voltage converter 28 and a line 28' 
to a differentiating unit 27, which is connected to the control means 7. 
The latter thus receives as actual value of the deceleration (also known 
as the "control value") an electric signal proportional to the 
deceleration of the universal shaft 2. By means of a slide resistor 6 
which can be adjusted by a hand lever 5, an electric signal representing 
the desired value of the deceleration is generated; it is fed via a 
resistance-voltage converter 24 to the control means 7. In the drawing, 
the hand lever 5 is shown in position A (brake 1 disengaged). Upon the 
turning of the hand lever 5 in the direction indicated by the arrow P, it 
comes first of all into the position O, in which it is held by a detent 5a 
and in which "zero deceleration" is ordered, i.e. the speed of travel is 
to be maintained constant at the value it has at the moment the switch is 
moved to position O. From position O, the lever can be shifted 
continuously up into the position I, whereby various deceleration values 
other than zero are selected. Current is supplied to the control means via 
line 40. 
The hydrodynamic brake 1 has a rotor impeller 1a and a housing 1c with a 
stator impeller 1b. The conduit system for the working fluid of the brake 
1 comprises an outlet line 41, an inlet line 42, a cooler 43, a filling 
and emptying line 44, and a hydraulic accumulator 13. The air pressure in 
the accumulator 13 determines, in a known manner, the amount of fluid in, 
and thus the braking action of, the fluid brake 1. The air side of the 
hydraulic accumulator 13 is connected to a pressure source 12 via a line 
11b/11a in which there is an electromagnetically actuable on-off valve 11, 
which is closed in its rest position. From line 11b, a discharge line 15a 
branches off to another electromagnetically actuable on-off valve 15, 
which is open in its rest position. 
The control means 7 has two outputs, one output 8 for a signal to increase 
braking torque and an output 9 for a signal to reduce braking torque. 
Output 8 is connected to one input of an AND gate 10 whose output is 
connected via another AND gate 45, to the solenoid of valve 11. Output 9 
is connected to one input of a NOR gate 14, the output of which is 
connected via and AND gate 46 to the solenoid of valve 15. 
The control means 7 compares the control value (i.e. the measured actual 
value) with the reference value of the deceleration, and the appropriate 
signal "increase braking torque" or "decrease braking torque" appears on 
the output 8 or the output 9, respectively. If the actual torque of the 
fluid brake 1 is too low to obtain the preselected deceleration, a signal 
to increase torque will appear at the output 8, until the preselected 
deceleration of the vehicle is reached, when the signal disappears from 
output 8. If the actual value of the deceleration is higher than the 
desired value, then a signal appears at output 9. A signal at output 8 
will, in the proper circumstances (described below), activate the solenoid 
to open valve 11, admitting more fluid into the fluid brake 1 and 
increasing the fluid braking torque. A signal at output 9, on the other 
hand, blocks NOR gate 14 from putting out a signal, so that the solenoid 
of valve 15 cannot operate and valve 15 must be in its rest position, 
open, thus draining fluid from the hydrodynamic brake 1, as long as output 
9 carries a signal. 
In order to monitor the pressure in the fluid brake 1, pressure switches 16 
and 17 are provided. Pressure switch 16 monitors the pressure in the inlet 
line 42 of the brake 1, and pressure switch 17 that in the housing 1c. 
Their electric outputs are respectively connected to the inputs of an OR 
gate 18, the output of which is inverted and input into the second input 
of AND gate 10. 
Pressure switch 16 generates a signal when the liquid pressure in the inlet 
line 42 reaches a certain value, at which the fluid brake 1 is full to 
capacity of operating liquid. The purpose of this signal is to prevent 
further increase of the pressure in the hydraulic accumulator 13 and 
unnecessary consumption of control energy. 
Pressure switch 17 generates a signal when the maximum tolerable liquid 
pressure is reached in the braking housing 1c. In this case also, a 
further increase of the pressure in the hydraulic accumulator 13 is 
prevented by a signal from OR gate 18. Thus, if the pressure in either the 
fluid brake 1 itself or its inlet line 42 is excessive, the resulting 
signal from OR gate 18, inverted and input to AND gate 10, prevents the 
opening of valve 11, even when the control means 7 is generating a signal 
on output 8 to increase the fluid braking torque. This is necessary to 
avert the danger that the fluid brake 1 would produce an intolerably high 
braking moment or that parts of the brake housing 1c would be subjected to 
excessive stresses. 
Additional protection against these dangers is provided by the connection 
of the output of pressure switch 17 to the second input of the NOR gate 
14, the other input of which is the "decrease braking torque" signal of 
output 9 of control means 7. A signal from pressure switch 17 thus 
prevents the NOR gate 14 from producing an output signal, with the result 
that the solenoid of valve 15 cannot operate and the valve 15 opens to 
vent fluid from the hydrodynamic brake 1. 
An additional assurance against too great an increase of the liquid 
pressure in the fluid brake 1 can also be provided. A third pressure 
switch 19, which is closed in its position of rest, is connected 
hydraulically to the brake housing 1c. The switch point of this pressure 
switch 19 is higher than that of pressure switch 17, so that the former 
only opens when a disturbance occurs which results in an intolerable 
increase of the liquid pressure in the brake housing 1c. The pressure 
switch 19 also has an electrical input, which is connected via a switch 21 
arranged on the reference signal generator 5, 6 to the supply voltage 
(line 40). The output of the pressure switch 19, which is closed in its 
position of rest, is connected with one input of an AND gate 29, the 
output of which is connected to the solenoid of a third 
electromagnetically actuable on/off valve 20. This value is in a second 
vent line 20a connected to vent line 15a, and serves to drain the fluid 
brake 1 quickly, and thus lower the fluid braking torque quickly, when the 
hydrodynamic brake system is deactivated. When the lever 5 is moved to 
activate the braking system of the invention, switch 21 closed, and as 
long as switch 19 is not opened by excessive pressure in the fluid brake 
1, a signal is input to AND gate 29 from switch 19. As long as this in the 
case, and the other input to AND gate 29 is in the proper state (which 
occurs in circumstances described below), the solenoid of valve 20 
operates to hold valve 20 closed. When the pressure in the fluid brake 1 
becomes intolerably high, or the braking system is deactivated by the 
movement of control lever 5 to position A, the signal from pressure switch 
19 to AND gate 29 ceases, and the AND gate can no longer keep the solenoid 
of valve 20 in operation, with the result that the valve 20 is allowed to 
return to its (open) rest position, thus draining the fluid from the fluid 
brake system. 
Thus, the arrangement of pressure switch 19, valve 20, etc., described 
above serves two functions. The "safety" pressure switch 19 protects the 
fluid brake 1 from overload in the event of a possible disturbance, for 
instance jamming of one of the valves 11 or 15 or failure of the control 
means 7. Also, upon disconnection of the fluid brake 1 a rapid decrease in 
the braking torque is assured. In this way a definite saving in fuel can 
be achieved, since the vehicle frequently is accelerated immediately after 
the deactivation of the fluid brake. If the brake did not empty itself 
sufficiently fast in this case, the drive unit 30 would have to work 
temporarily against the remaining braking moment, with consequent increase 
in the consumption of fuel. 
It is frequently desirable for the fluid brake 1 to empty automatically 
when the vehicle is stationary so that the shaft packings are not under 
unnecessary pressure. For this purpose, the following measure is employed. 
A zero-voltage indicator 39 is connected as a limit-value transmitter to 
the line 28', which conducts a signal proportional to the speed of 
rotation of the universal shaft 2. The zero-voltage indicator sends a 
signal to the AND gates 29, 45 and 46 whenever the speed of rotation, and 
thus the speed of travel, are above a certain limiting value, for instance 
a vehicular speed of 5 km/hr. When the vehicle is traveling below this 
speed, provided that the brake 1 has been previously activated, 
zero-voltage indicator 39 puts out no signal. This blocks AND gates 29, 45 
and 46 from producing a signal, and the solenoids of valves 11, 15 and 20 
are deactivated, allowing the three valves to return to their rest 
positions, which are closed, open and open, respectively. The result is 
the rapid draining of fluid from the fluid brake 1. AND gates 45 and 46 
prevent the control means 7 from actuating valves 11 and 15 when the 
vehicle is stationary. 
During braking, valve 20 remains closed, and adjustment of the braking 
torque is effected exclusively by means of valves 11 and 15. Due to the 
presence of the so-called rapid-discharge valve 20, valve 15 can be of 
smaller dimensions, making it more suitable for the control processes. 
The switching arrangement described also serves to disconnect the 
hydrodynamic brake 1 rapidly in the event that the rear axle 32 locks. If 
during normal braking the actual value of the deceleration is too great, 
this is counteracted, as already mentioned above, by the control means 7 
by means of valve 15. However, if the wheels of the rear axle are locked, 
the locking is eliminated in the fastest possible manner by means of 
elements 39, 29, 45 and 46, which drain the fluid brake 1 rapidly, as 
explained above. 
Since the hydrodynamic brake 1 normally transmits its braking torque to the 
road via the universal shaft 2, the differential 3, the rear axle 32 
(frequently with additional step-down transmissions) and the wheels, these 
parts are correspondingly placed under a load during braking. These parts 
are designed to transmit the engine power. The braking power, however, 
often amounts to several times the engine power. It is therefore very 
desirable to limit the hydrodynamic braking as a function of the speed of 
travel. For this purpose, the line 24' between resistance-voltage 
converter 24 and control means 7 is connected via a switch 23, a diode 
circuit 25, and a limit-value generator 26, with the line 28' between 
converter 28 and differentiator 27. The limit-value generator 26 produces 
a limit voltage dependent on the voltage produced by the frequency-voltage 
converter 28 (which is a measure of the speed of rotation of the universal 
shaft 2 and thus of the speed of travel). The limit voltage goes from the 
limit value generator 26 to the diode circuit 25. As long as switch 23 is 
open, the reference signal determined by means of the sliding resistor 6 
is fed unchanged into the control means 7. However, if switch 23 is 
closed, then the diode circuit 25 ensures that the reference voltage 
generated by the converter 24 does not exceed the limit voltage. 
With the circuit described above one limits the reference signal voltage in 
a way dependent on the instantaneous speed of the vehicle. In this way, a 
limitation of the braking power is obtained. The arrangement described 
will be sufficient for many practical purposes. It has the advantage that 
it can be produced simply. 
If the accuracy of the arrangement described above is not sufficient, it is 
necessary to feed to the limit value generator 26 a signal depending on 
the actual torque of the fluid brake 1. This signal could have the form of 
a voltage produced by a torque measurement shaft or by a pressure 
measurement capsule that measures the pressure of the operating liquid in 
the housing 1c of the brake 1, which pressure is approximately 
proportional to the braking torque. 
In the embodiment shown by way of example, the control means 7 controls the 
setting devices of the fluid brake 1, and therefore valves 11 and 15, by 
means of digital signals. As a result, the air pressure in the lines 11b 
and 15a changes stepwise. Nevertheless, the braking action of the brake 1 
is changed substantially continuously since the hydraulic accumulator 13 
acts as a dampening element. The advantage of this method of control is 
that valves 11 and 15 can be simple on-off valves. 
The case is different when the amount of fluid in the fluid brake 1 is to 
be controlled by a control valve which is arranged in a line for the 
working fluid, without employing a hydraulic accumulator. In this case it 
is advisable to use as a control valve a valve whose movable valve member 
can assume any desired number of intermediate positions, for example, a 
so-called servovalve or a proportional valve. The control means must in 
this case control the control valve by means of an analog electrical 
signal, i.e. a continuously variable electric signal. This method can, of 
course, also be used or controlling a fluid brake pneumatically through a 
hydraulic accumulator. 
A portion of another emobodiment of the invention is shown in FIG. 2. In 
this figure there are only shown those parts of the control means of the 
braking system which are necessary for an understanding of this 
embodiment. 
An additional electronic control means 47 is provided whose control value 
(actual value of the deceleration) input is connected to the line 28' and 
therefore with the output of the frequency-voltage converter 28 which 
forms part of the electric speed measuring device 4. Thus the speed of 
rotation of the universal shaft 2, and thus the speed of travel, is 
introduced into the control means 47 as the control value. 
In order for it not to be necessary, for instance, to provide an additional 
actual-value measuring device for the additional control means 47, the 
reference signal which is introduced into the second control means 47 is 
formed in the following fashion by means of the existing reference signal 
generator 5, 6. A so-called sample-and-hold storage 51 is provided. The 
instantaneous speed is continuously fed to the second control means 47 
from the frequency-to-voltage converter 28. Furthermore, a zero-voltage 
indicator 56 is connected to the lines 6' connecting the adjustable 
sliding resistor 6 to the converter 24. The output of the indicator 56 is 
connected to a second input of the sample-and-hold storage 51. When the 
resistance set on the sliding resistor 6 reaches a certain low value, 
indicated on the scale by the position 0, the zero-voltage indicator 56 
generates a signal which is equivalent to the command "zero deceleration". 
In other words, by this signal the command is given that the speed of 
travel is to be maintained constant. The signal actuates the 
sample-and-hold storage 51, which now stores the speed of travel present 
at the time of the appearance of the command "zero deceleration". This 
stored speed value is output by the sample-and-hold storage 51, as long as 
the command "zero deceleration" is maintained, as a reference signal to 
the second control means 47. 
The control means 7 shown in FIG. 2, which serves to maintain the 
deceleration constant, has, as in the embodiment of FIG. 1, an output 8 
for the signal "increase braking torque" and an output 9 for the signal 
"reduce braking torque". Similarly, the second control means 47 which 
holds the speed of travel constant has an output 48 for the signal 
"increase braking torque" and an output 49 for the signal "reduce braking 
torque". As in FIG. 1, the output of AND gate 10 is connected via the line 
10' with AND gate 45, not shown in FIG. 2, and the output of NOR gate 14 
is connected via the line 14' with AND gate 46, not shown in FIG. 2. 
In this embodiment, however, the outputs 8 and 9 of the control means 7 in 
FIG. 2 are no longer connected directly with the logic elements 10 and 14. 
Rather, a switching device comprising the logic elements 52-55, 58 and 59, 
as long as "zero deceleration" is ordered, makes the logic elements 10 and 
14 responsive to the outputs 48 and 49 of the second control means 47 and 
unresponsive to the output of the first control means 7. Each of outputs 
8, 9 and 48, 49 is connected to one input of an AND gate 53, 52, 55 or 54, 
respectively. The other input of each of gates 52-55 is connected to the 
output of the zero-voltage indicator 56, the signal from which is 
inverted, however, before entering AND gates 52 and 53. The outputs of AND 
gates 53 and 55 are connected to the input of OR gate 58, the output of 
which goes to one input of AND gate 10, corresponding to the direct 
connection of output 8 alone to AND gate 10 in the embodiment of FIG. 1. 
Similarly, the output of AND gates 52 and 54 are connected to the two 
inputs of OR gate 59, the output of which is connected to one input of AND 
gate 14, corresponding to the direct connection of output 9 to the AND 
gate 14 in the embodiment of FIG. 1. 
Therefore as long as a deceleration greater than zero is ordered by the 
reference signal generator 5, 6 and no signal is, accordingly, being 
output by zero-voltage indicator 50, signals appearing at output 8 or 9 
will activate AND gates 52 or 53, respectively, which will provide a 
signal, via the OR gate 58 or 59, to the logic element 10 or 14, 
respectively. In this case, the braking device acts precisely in the same 
ways as in FIG. 1, i.e. the fluid brake 1 is automatically controlled in 
such a manner that a selected value of deceleration is maintained. On the 
other hand, upon the command "zero deceleration", the zero-voltage 
indicator 56 produces a signal which, inverted, deactivates AND gates 52 
and 53, so that signals from the first control means 7 are blocked, and 
permits AND gates 54 and 55 to be activated, respectively, by a signal 
from output 49 or 48 of the second control means 47. Signals from the 
second control means 47 will now be transmitted by AND gate 54 or 55 and 
OR gate 59 or 58 to element 14 or 10, to control the hydrodynamic brake 
pressure. Accordingly the amount of fluid in the hydrodynamic brake 1 
(FIG. 1) is now controlled so that the speed of the vehicle is maintained 
constant. If the reference signal generator 5, 6 is returned to a 
higher-resistance setting, above 0, the system resumes maintaining a 
constant deceleration rather than a constant speed. 
The preferred embodiments described above may be varied in a number of 
ways. For example, means can be included in the braking system of the 
invention to reduce the fluid pressure if the temperature in the fluid 
brake 1 exceeds a certain level. Particularly, a temperature switch means 
can be provided for reducing the amount of fluid in the hydrodynamic brake 
when the temperature inside the housing of the brake exceeds a given 
temperature. 
Although the invention has been described with reference to two preferred 
embodiments, many variations and modifications thereof will now be 
apparent to one skilled in the art, and it is preferred that the scope of 
the invention be limited not by the details of the embodiments described 
above, but only by the terms of the appended claims.