U.S. patent number 3,610,920 [Application Number 04/882,183] was granted by the patent office on 1971-10-05 for apparatus and method for deriving a uniform time warning.
This patent grant is currently assigned to General Signal Corporation. Invention is credited to Klaus H. Frielinghaus.
United States Patent |
3,610,920 |
Frielinghaus |
October 5, 1971 |
APPARATUS AND METHOD FOR DERIVING A UNIFORM TIME WARNING
Abstract
An improved warning system has been provided for activating
crossing signal as a function of train speed. A transmitter coupled
to the rails generates an input signal which is modified by the
shunting of the rails by the railroad vehicle wheels. A receiver
coupled to the transmitter through the rails converts the modified
signal into a receiver signal. The improvement for providing a
uniform warning time includes a detector which periodically samples
the receiver signal and provides an output signal for energizing
the crossing signal when the difference between any successive pair
of samples is greater than a predetermined value indicative of the
uniform warning time. The amplitude of the receiver signal
decreases logarithmically as the train approaches the crossing. The
variation provides a characteristic to the system such that for
increasing vehicle speeds the crossing signal is activated at
increasing vehicle distances from the crossing such that the
warning time is substantially the same for any vehicle speed. The
logarithmic variation is achieved by selective impedance
manipulation of the coupling between the transmitter and
receiver.
Inventors: |
Frielinghaus; Klaus H.
(Rochester, NY) |
Assignee: |
General Signal Corporation
(Rochester, NY)
|
Family
ID: |
25380066 |
Appl.
No.: |
04/882,183 |
Filed: |
December 4, 1969 |
Current U.S.
Class: |
246/128;
246/122R |
Current CPC
Class: |
B61L
29/286 (20130101) |
Current International
Class: |
B61L
29/28 (20060101); B61L 29/00 (20060101); B61l
001/02 () |
Field of
Search: |
;246/125-130,34CT,122 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: La Point; Arthur L.
Assistant Examiner: Libman; George H.
Claims
What I claim is:
1. An improved control system for a railroad crossing warning
providing a uniform warning time having a transmitter for
impressing an alternating current input signal on the rails in the
proximity of the crossing, said signal modified by vehicle
movement, along the rails and a receiver likewise coupled to the
rails proximate to the crossing responsive to the magnitude of said
modified input signal for initiating the warning wherein the
improvement for providing the uniform warning time comprises: means
for sampling said modified signal detection means included in the
receiver responsive to the difference between any two successive
samples of said modified signal for producing a triggering signal
for initiating the warning when said differences achieve the preset
value; and
impedance compensating means coupled to the output and input of the
transmitter and receiver respectively for influencing the variation
of said modified signal such that said modified signal varies
substantially at an exponential rate whereby for various vehicle
speeds the modified signal achieves the preset rate and the warning
is activated at a uniform time prior to the vehicle's arrival at
the crossing.
2. The control system of claim 1 wherein said impedance
compensating means comprises:
an output impedance coupled to the transmitter, and
an input impedance coupled to the receiver, said output and input
impedances ranging in magnitude from 1 to 4 ohms and 2 to 10 ohms
respectively.
3. The control system of claim 2 wherein said impedance
compensating means further includes an impedance in shunt across
the rails disposed on opposite sides of the crossing at the extreme
range of the system for stabilizing the impedance of the rails
under various climatic conditions.
4. An improved control system for activating a railroad crossing
signal and establishing a uniform warning time as a function of
vehicle speed comprising:
a transmitter coupled to the rails proximate to the crossing for
generating an input signal, the amplitude of the input signal
modified by shunting of the rails by vehicle wheels;
a receiver coupled to the rails likewise proximate to the crossing
responsive to the modified signal for producing a receiver signal
representative of said modified signal; wherein the improvement
comprises:
a detector responsive to the receiver signal for sampling the
amplitude of the receiver signal at a predetermined periodic rate;
and
means responsive to the detector producing a first output signal
for activating the crossing signal, when the difference between any
successive pair of periodic samples of one polarity is greater than
a predetermined value indicative of the uniform warning time,
the amplitude of the receiver signal varying approximately as an
exponential function of the distance of the vehicle to the
crossing.
5. The improved control system of claim 4 wherein said exponential
variation follows the approximate formula:
E=K 1n (S/S.sub.o
where E is the received signal
K is a constant
S is the measured distance to the crossing
S.sub.o is the value of the premeasured distance to the
crossing
1N (S/S.sub.o is a natural logarithmic expression representing the
exponential variation of the receiver signal.
6. The improved control system of claim 5 wherein the transmitter
comprises:
a signal generator for producing the input signal at a
predetermined frequency, the input signal represented by a
potential across the rails; and
impedance compensating means coupled the signal generator to the
receiver through the rails for influencing the characteristic of
said receiver signal such that the receiver signal varies
exponentially.
7. The improved control system of claim 5 wherein the receiver
comprises:
a filter responsive to said modified signal for eliminating
modified signals of other than predetermined frequency; and
a rectifier responsive to the filtered modified signal for
converting the filtered modified signal into the receiver
signal.
8. The improved control system of claim 5 wherein the detector
comprises:
a pulse generator for producing the period of the detector;
a switching means responsive to the pulse generator for gating the
detector;
an amplifier periodically gated by the switching means for sampling
the receiver signal; and
delay means controlled by the switching means for controlling the
output of the amplifier, the delay means being effective to hold
the sampled signal in the amplifier until the next succeeding gate
of the switching means.
9. The improved control system of claim 8 wherein the means
responsive to the detector for producing the output signal
includes:
a comparator responsive to the output of the detector for producing
a triggering signal, when the difference between the sampled signal
and held signal exceeds the predetermined value;
an output switching circuit having a conductance state responsive
to the triggering signal; and
the output signal is produced in response to the conductance state
of the output switching circuit, the output signal energizing the
warning device when the switching circuit is in said conductance
state.
10. The improved control system of claim 4 including means
responsive to the detector producing a second output signal for
deactivating the crossing signal when a difference between any
successive pair of periodic signals exists at an opposite
polarity.
11. The improved control system of claim 4 including means
responsive to the receiver for activating the crossing signal when
a railroad vehicle is within a minimum predetermined distance from
the crossing.
12. An improved method for predicting when a railroad vehicle will
reach a railroad crossing and energizing a warning device at a
uniform time before such occurrence involving the procedure of:
imposing an input signal on the rails having a peculiar
characteristic;
attenuating the signal by vehicle movement along the rails; wherein
the improvement for producing a uniform warning time comprises the
steps of;
detecting the slope characteristic of the attenuated signal;
comprising the steps of:
measuring successive values of magnitude of the attenuated signal
at a periodic rate;
storing the first of successive values of the attenuated signal
measured; and
summing the stored signal with the next successive measured signal
over the time interval, said summation indicative of the slope
characteristic of the attenuated signal over the period
encountered;
generating a first output signal for energizing the warning when
the slope characteristic of the attenuated signal is one polarity
and reaches the predetermined value; and
selectively adjusting impedance coupling of the input signal and
the detected attenuated signal such that the magnitude of the
attenuated signal varies logarithmically relative to the input
signal, whereby for various vehicle speeds the attenuated signal
reaches the predetermined value at a time substantially at a
uniform time before the vehicle reaches the crossing.
13. The improved warning system of claim 12 wherein detecting the
slope of the attenuated signal comprises the steps of:
a. measuring successive values of magnitude of the attenuated
signal at a periodic rate;
b. storing the first of successive values of the attenuated signal
measured; and
c. summing the stored signal with the next successive measured
signal over the time interval;
said summation indicative of the slope characteristic of said
attenuated signal over the period encountered.
14. The improved warning system of claim 13 wherein the summation
of the stored signal and next successive measured signal further
comprises the steps of:
inverting the polarity stored signal so that the summation yields a
difference between said stored signal and next successive measured
signal.
15. The improved method of claim 12 further comprising:
generating a second output signal for deenergizing the warning when
the slope characteristic of the attenuated signal achieves the
opposite polarity, when said vehicle is past the crossing.
16. The improved method of claim 12 further comprising:
generating a third output signal for activating the signal when the
attenuation of the signal achieves a minimum threshold indicative
of vehicle presence at a minimum predetermined distance to the
crossing.
Description
BACKGROUND OF THE INVENTION
This invention relates to highway grade-crossing protection and in
particular to an improved system for providing a uniform warning
time at the crossing for trains of various speeds.
Highway crossing protection systems present problems which require
study in the area of fail-safe operation, economy of installation
and maintenance and efficiency of operation. The parties involved
in the decision as to when and where to install such crossing
protection are concerned with the amount of equipment necessary for
the proper operation of the system and balancing of certain
criteria including the safety provided by a grade-crossing
protector and the cost of installation. Systems of this type must
also be efficient in operation; that is, they must not only operate
properly all the time but reduce to a minumum the delay to cross
traffic. Generally, the way to prevent long delays at a crossing is
to provide s uniform warning time; that is, for any vehicle within
a foreseeable range the crossing signal will be activated a given
number of seconds before railroad vehicle reaches the crossing so
that the cross traffic will have a chance to clear the tracks and
the highway will be blocked for a minimum amount of time. Some
systems use multiple sections of track and roughly compute the
warning time by using timing sections and the like. However, the
installation and maintenance of such systems is quite expensive and
the protection afforded in terms of its accuracy is not necessarily
justified. Some systems have a section of track insulated from the
main line which is shunted as soon as the train enters the section
and a device responsive to the shunt activates the signal
immediately. Such a system works well for fast-moving trains
because track section must be long enough to provide an adequate
warning time. However, slow trains activating the signal upon
entering a long section produce an inordinately extended warning
time. In such a case, the traffic at the crossing is delayed for an
unnecessary amount of time and motorists who may be waiting at the
crossing will attempt, no doubt, to cross against the prohibition
of the signal. This also occurs when a train stops within a track
circuit and starts up again. The purpose of the signal, therefore,
is defeated if it is disobeyed. Numerous accidents have occurred
where motorists disregard such signals because of lack of
patience.
Other systems in use compute the distance that the train is from
the crossing by measuring the impedance across the rails as the
train moves towards the crossing. A change in impedance occurs
because the wheels shunting the rails move towards the crossing.
The rails are treated as analogous to a transmission line. This
type of system has proven effective because it can provide a
generally uniform warning time and is compatible with present track
systems using coded information. Such a system does not necessarily
require insulated track circuit sections and therefore the
necessity of providing AC bypass of such insulated sections is
obviated reducing substantially the cost of installation. The
system itself can be located entirely at the crossing with
connections to the rails simply installed. The advantages therefore
in terms of ease of installation, maintenance and accuracy are
quite important.
Certain problems however have been observed in the application of
such a system. For example, the complexity of the circuits involved
has had to be increased in order to reduce some problems associated
with noise and effective range of the system as a whole.
Systems employing apparatus for determining the uniform warning
time operate generally on the principle that the instantaneous
position of the shunt at any time defines the impedance of the
tracks. This value of impedance changes as the train moves toward
the crossing. The rate of change of impedance is correlated to the
velocity of the train and this information is operated upon by an
analog computer to calculate the point in time to activate the
warning. A signal is impressed upon the rails and for each position
of the train with respect to the crossing a different amplitude
signal is received by a receiver. The signal indicative of the
position of the train (i.e. impedance of the rails) is then
differentiated yielding a signal representative of the velocity. If
additional information is necessary, that is, acceleration, the
velocity signal is differentiated to give this value. The signals
obtained from the various steps of differentiation are operated
upon to produce a solution to the equation of a linearly moving
object. This information is then compared with a preset value which
is correlated to the uniform warning time. The prediction equation
is solved with the uniform time inserted, and the warning device is
activated upon satisfaction of the equation. Such an apparatus,
however, requires a number of amplifier circuits to achieve the
results required.
Another problem which bears more explanation is that involving
track circuit length. It has been observed that as the length of
track circuit increases, the reflected signal from a shunt across
the rails reaches a maximum at some point and then begins to
decrease as the shunt is moved farther away from the crossing. It
is undesirable for this to occur because the effective range of the
system is drastically reduced. It is quite necessary for the
reflected signal to reach its maximum at a distance from the
crossing long enough to accommodate rapidly moving trains.
In order to further reduce delays, means must be incorporated for
detecting when a train stops before the crossing. In addition, as
soon as the last car passes the crossing, the signals must be
extinguished and as the system of the present invention does not
require insulated joints at the crossing, a means responsive to the
departing train must be included.
It is therefore the intention of this disclosure to provide a
system which is economical in its required apparatus.
It is another object to provide a system which will operate
effectively over long track circuit lengths.
It is yet another object of the invention to provide a system which
will yield a substantially uniform warning time.
It is another object of the invention to provide a simple apparatus
for the calculation of the uniform warning time.
The foregoing and other objects and advantages of the invention
will become apparent from the following description when read in
conjunction with the accompanying drawings, in which:
FIG. 1 is a block diagram of the preferred embodiment of the
invention.
FIGS. 2, 3 and 4 A-D are curves to be used in the description of
the preferred embodiment and the theory of the operation of the
invention.
FIG. 5 is a partial block, partial schematic diagram of the
preferred embodiment of the invention.
There has been provided an improved system for activating a
railroad crossing signal as a function of train speed. The system
includes a transmitter coupled to the rails for impressing an
alternating current input signal thereon in the vicinity of the
crossing. The input signal is modified by the movement of railroad
vehicles along the rails by attenuating the input signal as the
vehicles approach the crossing. A receiver is coupled to the rails
near the crossing and is responsive to the magnitude of the
modified signal. The receiver initiates a triggering signal for
energizing the crossing at a time when the rate of variation of the
modified signal achieves a preset value. The improvement for
providing the uniform warning time includes, a detector in the
receiver which responds to the rate of variation of the modified
signal and produces the triggering signal when the rate of
variation reaches the preset value. Impedance-compensating means,
couples the output and input of the transmitter and receiver
respectively in order to influence the variation of the modified
signal so that it varies substantially at an exponential rate. At
various vehicle speeds the modified signal achieves the preset rate
and the warning is activated at a uniform time prior to the
vehicle's arriving at the crossing.
There has also been provided a method of deriving a uniform warning
time wherein a signal imposed on the rails and modified by railroad
vehicle movement thereon is sampled at a periodic rate for
detecting the slope characteristic thereof. When the slope achieves
a preset rate, a warning signal is activated. Selective impedance
adjustment coupling the impressed signal and the sampled signal
constrains variation of the impressed signal to a substantially
logarithmic rate so that for various vehicle speeds the signal
reaches the preset slope at a uniform time before the vehicle
reaches the crossing.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The invention described herein is intended to provide a system
wherein a highway crossing warning system is activated a fixed
number of seconds before the train enters the crossing regardless
of speed. In case the train stops before it enters the crossing, it
releases the crossing to vehicular traffic, but when the train
starts up again, the highway crossing warning system is again
initiated a fixed number of seconds before the train enters the
crossing. As the end of the train clears the crossing, the crossing
signal is extinguished. Should the train stop and back up on the
highway, the crossing signal is again activated a fixed number of
seconds before the rear of the train enters the crossing. This
fixed warning time remains substantially constant regardless of
train direction or speed and can be adjusted to a number of
values.
It is also the intent of this description to disclose a method of
deriving a uniform time warning by selective manipulation of
coupling impedances.
The basic equipment for the scheme is shown in FIG. 1. It includes
a track transmitter 20 with a designated value of source impedance
ZS connected to the rails 10 and a track receiver 21 with a desired
value receives input Z.sub.in coupled to the transmitter 20 through
the rails 20. As the train (represented by shunting wheels 11)
approaches the crossing, the receiver voltages E.sub.R follows the
curve shown in FIG. 2. This curve has a constantly changing slope
which may be determined in this application as the difference
between any two successive values of the received voltages E.sub.R
over respective values of distance for a constant measuring period.
This slope increases as the train approaches the crossing. The
receiver has a slope detector 31 built into it and it detects a
certain value of slope or greater for activating output generator
41. This particular slope is the predetermined value indicative of
the uniform warning time. Generator 41 triggers warning signal 51
when the proper signals are received. High-speed trains are
detected when the train is a considerable distance from the
crossing; while slow-moving trains are not detected until they are
relatively close to the crossing.
FIG. 2 shows the relation of receiver voltage E.sub.R vs. S the
distance to the crossing. It should be pointed out here that the
curve of FIG. 2 has been normalized to a curve in the first
quadrant and that the values A and B are absolute values to be
compared as explained later in this disclosure with respect to the
comparator polarities. It is necessary at this point to determine
the slope by arithmetic differentiation as follows. The slope is
generally defined as a change in the ordinate over the change in
the abscissa value or
.DELTA.E/.DELTA.S where m is the slope (1)
m=B-A/S.sub.2 -S.sub.1 (2)
The rate which the train crosses the crossing determines the
magnitude of the difference between A and B over a set period of
time. If the train moves swiftly, the difference of B and A is
large with respect to the distance. On the other hand, if the speed
is halved as shown further in FIG. 2, then the slope represented in
equation (2) is not achieved until the vehicle reaches a distance
to the crossing which is half of 2500 feet or, as shown, 1250 feet.
In accordance with this explanation, it can be seen that:
m=.DELTA.E/.DELTA.S= .DELTA.E'/.DELTA.S' (3)
when
(B-A)/(S.sub. 2 -S.sub.1)= (B'-A')/(S.sub. 2 '-S.sub.1 ') (4)
If the speed of the vehicle for the left side of the equation is
two times the speed of the vehicle for the right side of the
equation. If a minimum slope detection is set, it is possible to
determine from the curve the distance from the crossing at which
the signal is actuated for each train speed.
The FIGS. 4A-4D show curves which represent the variations in
warning time for different values of receiver impedance. By correct
proportioning of the transmitter source impedance and the receiver
input impedance, an almost uniform warning time can be achieved for
trains over a wide speed variation. For the sake of example, a
uniform time of 25 seconds is shown for these curves represented by
the dotted line in each figure.
FIG. 4A shows curves representing high receiver impedance
characteristics. At low track ballast R.sub.b =5 ohms per 1000
feet, the curve approaches the uniform warning time. However, it is
somewhat below the time and would not be reliable in terms of
safety. A high track ballast R.sub.b =300 ohms per 1000 feet
produces a characteristic curve which deviates greatly from the
uniform time.
FIG. 4B shows the characteristics, for low receiver impedance.
While the uniform warning time at low track ballast is within a
useful range at high track ballast, again the curve shows a large
deviation from what would be considered a uniform warning time.
FIG. 4C shows the optimum receiver impedance for this
configuration. However, as can be seen from the curve, when track
ballast is high, the warning time tends again to be somewhat away
from the uniform warning time. In order to alleviate this problem,
the track circuit of the system is artificially loaded to reduce
the effect of high track ballast on the system. This loading is
represented by impedances Za in FIGS. 1 and 5 at a distance of
approximately 2500 feet from the crossing. The curves in FIG. 4D
illustrate the relatively small deviation from the uniform time for
extreme values of track ballast resistance. The correct
proportioning of these impedance values assists in the shaping of
the curve in FIG. 2 such that for a substantial range of values the
curve varies logarithmically with respect to the distance from the
crossing. At distances below 375 feet from the crossing, the
logarithmic variation breaks down so that a special circuit has
been incorporated into the system for detecting speeds below 15
feet per second. This circuit gives at least a 25 second warning
for low speed trains within 375 feet from the crossing. However,
this type of situation is or would be rare on the applications
called for since warning system is essentially designed for use
where relatively rapidly moving trains with varying speeds are
prevalent.
FIG. 5 shows a partial block diagram of the system incorporated
into the invention. Transmitter 20 with proper source impedance
generates an input signal which is impressed upon the rails 10 at
the highway crossing. Receiver 21 coupled to the rails at the
highway crossing receives the input signal. However, if a train is
on the tracks as represented by the wheels and axle 11, then the
input signal is modified to a value proportional to the distance S
of the wheels 11 from the crossing HC. The modified input signal is
received by band-pass filter 22 which eliminates signals other than
at a carrier frequency F1. This modified signal is transformed by
step-up transformer 23 and rectified by full-wave rectifier
consisting of diodes 24 and 25. The diodes 24 and 25 are reverse
bias so that the polarity of the modified signal must be negative
to pass. Resistor 26 and parallel connected capacitors 27 and 28
smooth out the DC rectified signal and provide a receiver signal at
point A. The amplitude of the receiver signal represents the
distance to the crossing.
Detector 31 is used to sample the values of the receiver signal at
periodic intervals. During each interval, the signal is sampled and
held for a fixed amount of time. This sample is then compared with
the next succeeding sample. When the first sample exceeds the
second by a certain amount as determined by comparator 42, then a
signal is generated which causes the railroad crossing to be
activated.
The receiver signal at point A is fed into amplifier 34 through
resistor R1 and field-effect transistor (FET) switch 33. Pulse
generator 32 gates the FET switch and allows the FET 33 to turn on
amplifier 34 when the pulse generator is in an ON condition. During
this ON time, a sample of A is taken and fed into amplifier 34 to
be held on capacitor C1. Resistors R1 and R2 set the proportions
for the amplification of amplifier 34. The signal A is then held in
amplifier 34 for the OFF period of a pulse generator 32. At the
next succeeding ON time of generator 32, the output of amplifier 34
at signal A is transmitted to B as a positive magnitude signal
equal but opposed in polarity to its original value. Again, a
sample is taken at point A. This sample is then compared with the
value of B through the voltage divider R3 and R4 at point C. R3 and
R4 are equal in value and so the value at C represents the
difference between the voltages A and B.
Output generator 41 is used to detect when C is of a certain value
and provides a triggering signal such that the railroad crossing 51
is activated. Comparator 42 is responsive to the signal at C and is
biased by the time warning adjustment 40 to a preset value
representative of the desired uniform warning time. When the signal
at A becomes less than the signal at B by the value set in time
adjustment 40, then comparator 42 generates an output indicative of
the fact that the certain set slope of the track voltage has been
exceeded. The slope may be determined in accordance with the
previous definition as the difference in voltage between B and A
per the OFF period of the pulse generator 32. If the frequency of
the pulse generator is relatively high, the slope calculation
approaches the accuracy of a direction differentiation.
In the previous discussion of the slope, relative to vehicle speed,
it was shown that vehicle speed is measured only incidently in that
differences in the distance are determined as a function of
received voltage E.sub.R (i.e. for each value of E.sub.R there
exists a corresponding distance S where the shunt 11 exists). If
values of E.sub.R are sampled at a fixed rate, then the speed of
the vehicle is implicit in the differences between the two
successive samples. This must be qualified by the assumption that
the curve which E.sub.R follows is defined. If, for example, the
received voltage varied in accordance with equation of a straight
line, then the actual differences of A and B could be directly
correlated to the speed. However, the nature of the track circuit
is not so convenient, as may be seen by the various curves of FIG.
4. If, therefore, E.sub.R is caused to vary approximately as a
logarithmic curve shown in FIG. 3, then the speed of the vehicle
may be determined indirectly as a function of the differences in
the received voltage E.sub.R over the differences in distances from
the shunt to the crossing over a defined interval of time. This is
because the change in E.sub.R is not a constant. In this respect,
it should be noted that this agrees with equation (2) because the
variation in the curve in FIG. 2 must be correlated with a specific
distance S.sub.1 and S.sub.2 that the shunt 11 is from the
crossing. In the linear equation, on the other hand, S.sub.1 and
S.sub.2 have no net effect on the equation, because the value of
E.sub.R is directly correlated to distance of the vehicle from the
crossing.
An example of a numerical approximation method of arriving at
distances at which the warning must be activated is described by
the following example with reference to the curves E vs. S FIG. 2.
A train travelling at 100 feet per second would require 25 seconds
to reach the crossing from a distance of 2500 feet. The slope of
the curve R.sub.b =5 ohms per 1000 feet at 2500 feet for a 1 second
interval equals one-fourth. If the velocity value is reduced to 50
feet per second, then the slope on the same curve would be equal to
one-eighth at 2500 feet. However, the slope of the curve at 1250
feet is approximately one-fourth. This means that if the slope
detection is set for example at one-fourth, then the signal 51 at
the crossing HC would be activated at a distance of 2500 feet from
the crossing at a train speed of 100 FPS and in the vicinity of
1250 feet from the crossing at 50 FPS. For various speeds, the
slope can be picked off the curve knowing what uniform warning time
is desired and the measuring period. It is true that the faster the
detecting rate is, the more accurate will be the detection of the
slope for various speeds. However, what has been shown by way of
example to illustrate the way in which the curve of FIG. 2 varies
with respect to the distance to the crossing and the velocity of
the vehicles approaching the crossing. For high track ballast
resistance situations as illustrated by the curve R.sub.b =300 ohms
per 1000 feet in FIG. 2, the warning time is increased as shown in
FIGS. 4A-4D. Low track ballast resistance provides a response
yielding the shortest warning time and the system for safety has
been designed with reference to low track ballast resistance
values.
The polarity connection on comparator 42 makes it only sensitive to
negative values of slope, i.e. values of A less than values of B as
the train approaches the crossing. With this arrangement,
comparator 42 starts generating an output pulse for every period of
the pulse generator, a uniform time before the train enters the
crossing regardless of the train speed. This output pulse is
transmitted to a one-shot multivibrator 43 which turns on in
response to the pulse. The one-shot produces a pulsing signal which
is transformed through transformer 44 and rectified to a full-wave
DC signal through diodes 45. The rectified signal energizes relay
46 opening back contact 47 which deenergizes relay 49 through
closed front contact 48. The deenergization relay 49 drops back
contact 50 energizing the crossing signal 51. When the train passes
the highway crossing, comparator 42 sensitive only to negative
values of slope will stop producing output pulses and the one-shot
43 ceases to be triggered consequently dropping relay 46. However,
the crossing will not be turned off because contact 48 has at this
time been deenergized by another circuit as will be explained later
on. Comparator 52 is sensitive to positive values of slope as seen
from its polarity connections.
As soon as the train passes the crossing HC, the values of A and B
change and B becomes less positive than A indicating a positive
slope. This positive slope is detected by comparator 52 sensitive
only to positive slope because of biasing resistor 58 which
generates a signal to one-shot multivibrator 53 which produces a
pulsed output which is transformed by transformer 54, rectified by
full-wave bridge including diodes 55 for energizing relay 56
picking up contact 57 and energizing relay 49, which picks up
contact 50 and deenergizes the signal.
As was previously noted, for trains moving below a specified
minimum speed in the order of 15 miles per hour, low-speed warning
time adjustment 61 is provided. Adjustment 61 is arranged to
deenergize relay 65 when the train is within 375 feet of the
crossing. This distance is used for convenience and may vary
between 300 and 400 feet. This is accomplished by tapping the
secondary of the transformer 23 which provides a voltage which is
rectified by the full-wave bridge including diodes 63. The signal
produced at the output of the bridge is conducted to relay 65 which
remains energized. Resistor 64 is used to adjust for a minimum
current in the relay 65. As the train approaches the crossing, the
signal produced at the transformer 23 is constantly decreasing and
the resistor 64 is adjusted to drop relay 65 when the signal
produced at the secondary of the transformer is proportional to a
distance of 375 feet from the crossing. This assures the trains
travelling below a certain speed in the order of 15 miles per hour
will be detected in order to produce a signal at the crossing of at
least the uniform warning time. However, since relay 65 remains
deenergized even when the train is within 375 feet of the crossing
in either direction, the comparator network incorporating
comparator 52 which detects positive slopes for trains moving away
from the crossing, one-shot 53, transformer 54, rectifier 55 and
relay 56 is used so that when the train passes the crossing HC, the
signal will be immediately extinguished. The use of comparator 42,
comparator 52, and low-speed warning time adjustment 61 provides a
system which keeps delays to highway traffic to a minimum.
In this connection, however, it should be noted that a train
stopping before the crossing within 375 feet of the crossing will
cause the system to maintain the crossing signal 51 on. Similarly
if the rear of the train passes the crossing and the last car is
within 375 feet of the crossing and stops the signal 51 will remain
energized. Any movement of the train away from the crossing after
the last car passes the crossing HC, will provide sufficient
signals to the system for maintaining relay 56 energized and even a
very slowly moving train which has passed the crossing will not
cause a signal actuation.
With reference to FIG. 2, a method of achieving a uniform warning
time will be explained with respect to the curve representing the
receiver voltage at the output of the receiver 21. This voltage
varies substantially logarithmically over a range from 4000 feet to
above 375 feet from the crossing. This logarithmic variation
provides a characteristic which aids in the development of a simple
uniform warning time system.
A mathematical derivation of the approximate expression used in
this method is shown below.
From FIG. 2, the curve E (receiver voltage) Vs. S (distance from
the crossing)
The rate of change E with respect to time ##SPC1## dE= Kds/S (12)
integrating E= K 1n (S/S.sub.o (13)
The formula (13) provides an equation which if plotted would yield
a curve which varies logarithmically. The variation provides a clue
as to what would be a useful measuring device in uniform
time-warning devices. The curves in FIG. 2 do not vary
logarithmically over the complete range of values of distance to
the crossing but for a sufficiently useful interval it does. The
upper curve is arrived at when the track ballast resistance R.sub.b
is high in the order of 300 ohms per 1000 feet for example on hot
dry days. The lower curve exists when the track ballast resistance
R.sub.b is in the order of 5 ohms per 1000 feet as may exist after
a rain storm.
The curve in FIG. 3 is a true logarithmic curve becoming zero on
the ordinate when the abscissa is S.sub.0, i.e.
Ke s/s.sub.o =E where (14)
K is a constant
Let K=1 then
E=1n (S/S.sub.o and (15)
Therefore when (S/S.sub.o )=1; E=0
However the curves of FIG. 2 approximate this condition between S=
375 and S= 4000 feet is sufficient for the purposes of the
application. The variation of the track voltage E vs. S the
distance S to the crossing was adjusted to approximately follow the
variation according to equation (13) by manipulating the carrier
frequency, input and output impedance of the receiver and
transmitter respectively and the track-loading impedances. By
mathematical calculation and selective empirical adjustment, the
following are a range of values indicative of the absolute value of
impedances which proved useful in arriving at the desired curve of
FIG. 2.
S.sub.c =200- 500 Hz.
Receiver input impedance = 2-10 ohms
Transmitter output impedance = 1-4 ohms
Track-loading impedance = 1-5 ohms
The above impedances are represented as absolute values of complex
impedances as determined for the range of frequencies and
variations in the track circuit impedance as a function of distance
to go.
The use of the amplifier 34 and its associated circuitry for
deriving the uniform warning time provides a system which is simple
in its concept and efficient in its operation. The system utilizes
curve-shaping techniques to provide a signal variation which is
directly proportional to the uniform warning time such that
complicated circuitry is not necessary for deriving the velocity
function of the received voltage. Instead of differentiating values
indicative of the distance to the crossing to obtain a velocity
function and then solving for the uniform warning time, the system
uses an approximate arithmetic method arriving at substantially the
same result with a simple and efficient design. One of the key
factors in the operation of the system, however, is the matching
impedances to optimum values of the input and outputs of the
transmitter and receiver respectively. It is to be noted in this
connection that the matching of impedances mentioned above is not
necessarily the matching of impedances in the conventional sense
whereby maximum power is transmitted by the exact matching of input
and output impedances, but is a matching to cause the received
voltage characteristic to follow the approximate logarithmic curve
shown in FIG. 2. This is why the impedance listing above shows that
the receiver, transmitter and track-loading values are
substantially different from each other. It is called the matching
of impedances in this disclosure because it achieves the purpose of
shaping the response curve of E.sub.R to the desired
characteristic. However, in literal electrical engineering
terminology, it is miss-match of impedances and not a conventional
means for transferring a signal. In addition, the choice of
frequency is also important for this system. For frequencies higher
than an order of 500 Hz., a standing wave pattern is set up because
a set of rails reveal characteristics similar to that of
transmission line for the distance involved in track circuits of
this invention. If a higher frequency is used, the curve would not
vary logarithmically and the results desired would therefore not
occur. For frequencies higher than 500 Hz. the voltage-distance
curve, at low track ballast conditions rises, becomes flat and
finally reverses and goes down again as the shunt moves away from
the feed point at distances less than the minimum required warning
distance of 2500- 3000 feet. With this type of voltage-distance
curve response, the described uniform time warning system would
generate erroneously short warning times for fast trains.
Having described an apparatus and method for deriving a uniform
time warning it is to be understood that various modifications and
alterations may be made to the specific embodiment shown without
departing from the spirit or scope of the invention.
* * * * *