STC SmartScanIS, User Manual

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14 January 2021, SmartScanIS User’s Guide for Union Pacific, EUD-2016026-00 Rev.2
A firmware-controlled feature of the comparator circuit allows adjustment of the threshold
voltage based on the speed of the train. Trains traveling less than 20 mph (32 kph) use the
620-millivolt threshold setting for maximum sensitivity. However, faster trains will cause the
transducers to generate a higher signal voltage, which allows the system to increase the
threshold voltage. This improves the signal-to-noise ratio resulting in better signal validation.
Processor-A (on the Processor board) internally generates interrupts on the positive going and
negative going edges of each transducer pulse coming from the comparator. The positive
edge of each comparator pulse starts a timer. The timer stops with the detection of a negative
going edge. The width of each pulse is a determining factor of signal validity. The
Minimum
Transducer Pulse Width option on the Equipment submenu lets you set the minimum pulse
width. Five-milliseconds is the default. Rejected pulses don't increment the transducer
counters.
In normal operation, the gating transducers fire in sequence, first one then the other. With
24-inch (61-centimeter) gating-transducer spacing, a wheel passes over both gating
transducers before the next wheel passes over a gating transducer. Therefore, two
consecutive pulses on the same gating transducer aren't physically possible. However, in the
unlikely event that a gating transducer is ever caused to fire by another source, such as
electrical interference from a locomotive's traction motor, a spike-counter for the gating
transducer that misfired will be incremented by one. To increment the axle counter, one pulse
from each gating transducer must occur in sequence.
10.6 Axle Scanning
When the train first enters the site, the SmartScanIS begins reading the scanners for a heat
signal when the wheel is at the center of the first gating transducer. Thereafter, it starts
reading the scanners 20 inches (50.8 centimeters) before the center of the first gating
transducer. (See
Appendix G - Predictive Gate Scanning for more information about this
20-inch (50.8-centimeter) pregate scanning.) The system continues reading until the wheel is
at the center of the second gating transducer. This is the period that the axle is in view of the
scanner. A negative going edge of a transducer signal is an indication that the wheel is in the
center of that transducer.
The firmware uses Dynamic Scan Rate sampling between the centers of the two transducers
(that is, between the gate), which means that heat readings taken every 1/2 inch
(1.27 centimeters) of wheel movement are used to detect alarms, regardless of the speed of
the train. As a result, the system takes 48 evenly spaced samples as the wheel rolls through
the 24-inch (61-centimeter) detection zone that is established by the distance between the
gating transducers.