Grade crossing predictor
Encyclopedia
A grade crossing predictor is an electronic device which is connected to the rails of a railroad track, and activates the crossing's warning devices
(lights, bells, gates, etc.) at a consistent interval prior to the arrival of a train at a grade crossing .
in the late 1950s at the request of the Southern Pacific Company (the Southern Pacific Railroad
, now merged into the Union Pacific Railroad
), the design goal of the grade crossing predictor was to provide a consistent warning time for trains approaching a grade crossing.
Previously, the circuits used for activating a crossing's warning devices were very simple, and activated them whenever a train came within a fixed distance (hundreds or thousands of feet) of the crossing. This method required that the crossing be designed to accommodate a train approaching at the track speed limit, which leads to a longer warning times for trains approaching the crossing at lower speeds. Very slow trains could have many minutes of warning time, thus delaying highway traffic unnecessarily.
can be viewed as a single-turn inductor
shaped like a hairpin. As the train approaches the feedpoint, the area enclosed by the inductor diminishes, thus reducing the inductance
.
This inductance can be measured by connecting a constant-current alternating current
source to the rails, and measuring the voltage which results. By Ohm's Law
, the voltage measured will be proportional to the impedance. The absolute magnitude of this voltage and its rate of change can then be used to compute the amount of time remaining before the train arrives at the crossing, assuming it is running at a constant speed.
The crossing's warning devices are activated as soon as the computed time until the train reaches the crossing reaches some programmed threshold. The earliest grade crossing predictors used analog computer
s to perform this calculation, but modern equipment uses digital microprocessor
s.
A predictor circuit in the middle of nowhere is usually terminated with a dead short across the rails at the outer ends. This assumes that there are no ordinary track circuits for block signalling purposes.
Two predictor circuits may overlap, with tuned circuits used for one predictor to jump over the over. The tuned loops would be a dead short for one predictor, and a open circuit for the other.
Grade crossing signals
Grade Crossing Signals are the warning devices for road vehicles at railroad grade crossings. The basic signal consists of two sets of flashing red lights, one on the front and one on the back, a sign that says Railroad Crossing, and a bell. At crossings that typically get trains going at about...
(lights, bells, gates, etc.) at a consistent interval prior to the arrival of a train at a grade crossing .
History
First developed in concept by the Stanford Research InstituteSRI International
SRI International , founded as Stanford Research Institute, is one of the world's largest contract research institutes. Based in Menlo Park, California, the trustees of Stanford University established it in 1946 as a center of innovation to support economic development in the region. It was later...
in the late 1950s at the request of the Southern Pacific Company (the Southern Pacific Railroad
Southern Pacific Railroad
The Southern Pacific Transportation Company , earlier Southern Pacific Railroad and Southern Pacific Company, and usually simply called the Southern Pacific or Espee, was an American railroad....
, now merged into the Union Pacific Railroad
Union Pacific Railroad
The Union Pacific Railroad , headquartered in Omaha, Nebraska, is the largest railroad network in the United States. James R. Young is president, CEO and Chairman....
), the design goal of the grade crossing predictor was to provide a consistent warning time for trains approaching a grade crossing.
Previously, the circuits used for activating a crossing's warning devices were very simple, and activated them whenever a train came within a fixed distance (hundreds or thousands of feet) of the crossing. This method required that the crossing be designed to accommodate a train approaching at the track speed limit, which leads to a longer warning times for trains approaching the crossing at lower speeds. Very slow trains could have many minutes of warning time, thus delaying highway traffic unnecessarily.
Technology
All grade crossing predictors rely on the changes in the electrical characteristics of the rails that occur as a train approaches the point at which the predictor is connected to the rails (the feedpoint). A railroad track occupied by a train or other electrical shuntShunt
Shunt may refer to:* Shunt - a hole or passage allowing fluid to move from one part of the body to another* Shunt - a device allowing electrical current to pass around a point in a circuit...
can be viewed as a single-turn inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
shaped like a hairpin. As the train approaches the feedpoint, the area enclosed by the inductor diminishes, thus reducing the inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
.
This inductance can be measured by connecting a constant-current alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
source to the rails, and measuring the voltage which results. By Ohm's Law
Ohm's law
Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points...
, the voltage measured will be proportional to the impedance. The absolute magnitude of this voltage and its rate of change can then be used to compute the amount of time remaining before the train arrives at the crossing, assuming it is running at a constant speed.
The crossing's warning devices are activated as soon as the computed time until the train reaches the crossing reaches some programmed threshold. The earliest grade crossing predictors used analog computer
Analog computer
An analog computer is a form of computer that uses the continuously-changeable aspects of physical phenomena such as electrical, mechanical, or hydraulic quantities to model the problem being solved...
s to perform this calculation, but modern equipment uses digital microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
s.
Implementation
A predictor includes a short island track which just covers the width of the level crossing.A predictor circuit in the middle of nowhere is usually terminated with a dead short across the rails at the outer ends. This assumes that there are no ordinary track circuits for block signalling purposes.
Two predictor circuits may overlap, with tuned circuits used for one predictor to jump over the over. The tuned loops would be a dead short for one predictor, and a open circuit for the other.