Device for measuring the distance travelled and the speed of a rail vehicle

A device for measuring the distance travelled by and the speed of a rail vehicle fitted with wheel-coupled distance-pulse generators is disclosed in which both distance and speed are calculated from the time distance between successive distance pulses, and in which the change in wheel diameter due to wheel wear is already taken into account in the measurement of the time distance between the distance pulses (distance-pulse intervals). To measure the distance-pulse intervals, clock pulses of reduced repetition rate are counted which are obtained by dividing basic clock pulses of high repetition rate in an adjustment divider (T1). The wheel diameter is taken into account by changing the division ratio of the adjustable divider by means of a coding switch (S1).

The present invention relates to an improved distance and speed-measuring 
device. 
A prior art such device is described in an article by H. Uebel and U. 
Drager published in "Eisenbahntechnische Rundschau" 32 (1983), No. 1/2, 
pp. 63 to 66. 
The article describes the structure and operation of a distance- and 
speed-measuring system in which the previously used, rather inaccurate 
speed measurement involving the evaluation of the frequency of distance 
pulses from wheel-pulse generators is replaced by a more accurate period 
measurement. 
The period measurement is performed in the above-cited prior art device by 
a microcomputer whose internal timer is triggered by the distance pulses 
from the wheel-pulse generator. On each interrupt caused by the trailing 
edge of a distance pulse, the count of the timer is read out and the timer 
is reset. 
At slow speeds of the wheel, the period may become so long that the count 
capacity of the timer is not sufficient. In that case, an additional 
counter is necessary for registering the overflows of the timer. The 
calculation of speed from the counts of the timer is done by the computer. 
The wheel diameter required for the calculation, which may be up to 7% 
below an initial value due to wheel wear, is entered into the computer 
through a special input device. 
The object of the present invention is to provide a distance- and 
speed-measuring device of the above kind which makes it possible to allow 
for wheel wear already during the period measurement and avoids errors 
caused by variations in the instants of the timer read-out operations. 
Compared with the conventional device, the distance- and speed-measuring 
device in accordance with the invention has the advantage that, if the 
wheel diameter is precisely set, the number of pulses counted during a 
distance-pulse interval is the same for the same speed. The wheel diameter 
can be entered by means of a simple coding switch, and the need for a 
special entry into the computer, which is otherwise required prior to each 
train movement, is eliminated. Furthermore, for constant (interrogation) 
intervals, the sum of the changes of the two alternately used counters is 
independent of speed and only a function of the division ratio. This 
allows the evaluation computer to calculate the frequency of the clock 
pulses of reduced repetition rate and to perform a reverse calculation to 
determine the preset wheel wear; this can be used for checking purposes. 
The presently preferred embodiment of the invention permits the measurement 
interval to be extended to two or more distance-pulse intervals in a 
simple manner. The device can thus be very well adapted to different 
distance-pulse generators commonly used on rail vehicles.

In FIG. 1, a clock generator TG provides clock pulses of a high repetition 
rate which are divided down in a following first divider T1. Via a coding 
switch S1, the division ratio can be changed so that the frequency of the 
pulses appearing at the divider output changes by up to 7%. For example, 
the division ratio can be reduced in steps from 100:1 to 93:1. 
The pulse sequence appearing at the divider output is applied to two NOR 
gates UG1, UG2, which are controlled by distance pulses from a 
distance-pulse generator WG (not shown). The distance pulses are first 
synchronized with the clock pulses of reduced repetition rate by means of 
a D flip-flop FF and then applied simultaneously to the two NOR gates via 
a second divider T2 and a switch S2; the lead to one of the NOR gates, in 
this case the NOR gate UG1, contains an inverter IN. 
The outputs of the NOR gates are coupled to counters Z1 and Z2, whose 
outputs are read by a computer R (not shown). This circuit ensures that, 
upon arrival of a distance pulse (which inhibits one of the gates and 
enables the other), the clock pulses of reduced repetition rate cannot be 
registered by both counters by mistake. 
FIG. 2 is in the form of a pulse diagram. It shows schematically the pulse 
sequences at the output of the clock generator TG (FIG. 2a) and at the 
output of the divider T1 (FIG. 2b), the sequence of distance pulses (FIG. 
2c), the sequence of distance-pulse intervals (FIG. 2d), and possible read 
and reset times for the counters Z1 and Z2 (FIG. 2e). 
The clock pulses shown in FIG. 2a, which have a repetition rate on the 
order of 1 MHz and may also be provided, for example, by the clock 
generator of the computer R evaluating the counts, are divided by 93 . . . 
100 in the divider T1, and are then available as count pulses (FIG. 2b) 
for the period measurement. 
The period is determined by the time distance between the pulses of the 
pulse train from the distance-pulse generator (distance-pulse intervals), 
which is shown in FIG. 2c. These distance pulses, which are shown in FIG. 
2c, are synchronized with the clock pulses of reduced repetition rate by 
the flip-flop FF. The distance-pulse intervals (FIG. 2d) are available at 
the output of the divider T2. 
As the gates UG1 and UG2 are enabled alternately, the pulses arriving 
during successive distance-pulse intervals are counted alternately either 
by the counter Z1 or by the counter Z2. While the counter Z2 is counting, 
the count of the counter Z1 determined during the previous distance-pulse 
interval remains stored, so that it can be read by the computer during the 
whole time that the counter Z2 is counting. The counter Z1 can be reset 
either by the computer immediately after read-out of the count or, as 
shown in FIG. 2e, upon switch-over of the NOR gates UG1 and UG2 by the 
next distance pulse. Conversely, the count of the counter Z2 remains 
stored during the pulse interval in which count pulses are applied to the 
counter Z1. 
Since the number of count pulses has already been adjusted to compensate 
for possible changes in the wheel diameter, speed can now be calculated 
using a preset wheel diameter. It is not necessary to enter the 
instantaneous wheel diameter again. Distance is determined by integrating 
the calculated speed over time. If distance were measured simply by 
counting the distance pulses, the result would be inaccurate, because 
wheel wear would not be taken into account. 
By means of the divider T2, the distance-pulse interval and, thus, the 
period can be doubled. This doubling must also be communicated to the 
computer evaluating the counts. The greater time basis increases 
measurement accuracy, since unavoidable digitization jumps between 
successive counts are no longer of such great consequence. In addition, 
distance-pulse generators with different distance increments 
(distance-pulse separation) can be adapted to the evaluation computer 
without additional circuitry.