IntroductionRelays and Relay Logic

Train Detection

Relays and Train Detection

Andy Lawrence

1st September 2011

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

An Overview of the Presentation

Overview:

Part I, Relays and Relay Logic: An introduction to relays,circuit diagrams, practical examples.

Part II, Train Detection: Track circuits, axle counters.

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

Part I: Relays and Relay Logic

Part I: Relays and Relay Logic

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

Why are relays interesting???

Prior to the advent of the SSI, interlockings were constructed usingrelay circuitry.

Each interlocking would contain hundreds or even thousands ofinterconnected relays.

In order to be backwards compatible the workings ofthe SSI replicates relay logic.

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

What is a Relay???

A relay consists of a number of contacts and a coil.

A contact consists of two flexible arms

These arms are designed to act as a switch

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

Relays and Coils

Relays are controlled by an electromagnetic coil

Passing a current through the coil moves and arm connected tothe contacts

A relay is said to be energised if current is passing through thecoil and de-energised otherwise.

A back contact is closed when the relay is energised

A front contact is open when the relay is energised.Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

Relays come in a variety of flavours

Here is a small sample of the different types of relay in use:

Neutral A basic relay with no special features

Biased Only operates when the current is following in onedirection

Latched Contains two coils. Once energised the current canbe removed without affecting the contacts. It can bereset by passing a current through the second coil

Slow release or pick-up The pick up or release of the contacts inthese coils are delayed.

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

Relay Circuit Diagrams

Relay circuit diagrams are used for a variety of different activitiesincluding

Design

Checking

Installation

Testing

Maintenance

Each diagram must contain all the relavent information for each ofthese activities.

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

Basic Components of a Relay Circuit Diagram

Coils and Contacts:

Positive Power Supply:

Negative Power Supply:

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

A Simple Example

I will now introduce a small example which satisfies the followingrequirement

Relay R to be energised only when relay A is energised

To start we draw a line with the two types of power supply ateither end

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

We want to control the relay R with the contacts of relay A. So weplace the coil of R at the right handside of the diagram

Then we place the contacts of coil A on the left of the diagram.

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

Conversly...

We can just as easily construct a circuit for the converse statement:

Relay R to be energised only when relay A is de-energised

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

Logical And

Logical functions can also be constructed using relays. In thefollowing we see an example of an And:

The Relay R is to be energised only when relay A AND relay B areboth energised

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

Logical Or

Likewise we can also construct a circuit using relays that functionsas a logical Or:

The Relay R is to be energised only when either relay A OR relay Bis energised

Note the similarity between relay circuits and ladder logic diagrams.

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

Incorporating Buttons

We might want to have some external input into our circuit andthis is accomplished using push buttons.

A button that can be pushed is represented as follows:

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

Push Button Example

In the following we see an example of a push button at use in acircuit:

Relay R is to be energised when a button is pushed and to remainenergised when the button is released

This circuit is can be started in the following way:

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

Push Button Example

In order to get the property that the Relay R remains energised wemust make an addition:

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

A bigger example

Now we can use a relay circuit to control a two aspect signal:

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

Part II: Train Detection

Part II: Train Detection

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

The Problem of Train Detection

One major problem with any signalling system is determiningwhether or not a section of track is clear

There are currently two solutions to this problem

Track circuits

Axle counters

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

Basic Track Circuits

Track circuits form circuit using a power supply and relayWhen a train is present on the track it has one of two affects onthe relay

completes the circuit and energises the coil

provides a path of less resistance and takes current away fromthe coil

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

Track Circuit Example 1

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

Track Circuit Example 1

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

Axle Counters

Axle counters count the number of axles that pass through a pointon the track.

The operate magnetically, which means they work in wetconditions or on a metal deck.

The consist of a transmitter and a reciever.

Andy Lawrence Relays and Train Detection

IntroductionRelays and Relay Logic

Train Detection

A Refreshing Summary

This talk covered two topicsRelays and relay logic:

Relays, Coils and contacts

Basic Circuits

A bigger example (2 Aspect Signal)

Train detection:

Track Circuits

Axle Counters

Andy Lawrence Relays and Train Detection