Block Detection

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Block Detection is a process of detecting if a Section of Track (or block) has rolling stock on it.

Contents 1 Background 1.1 Optical Detector 1.1.1 Light Dependent Resistor 1.1.2 Photo Transistors and IR detector pairs 1.2 Conductive Detection 1.2.1 Voltage Drop 1.2.2 Current Transformer 1.2.3 Hall Effect Current Sensor 1.3 Proximity Detection (Metallic or Ultrasonic) 1.3.1 Reed Sensors 2 Desired Objectives 3 DC Detection 4 DCC Detection 5 See Also

[edit] Background

The term Block Detection refers to the ability to detect if a specific block (section) of track is occupied or not. The reasons for the need to detect occupancy vary widely, but are most commonly use for signaling and collision prevention.

There are a few types of detectors commonly used on tracks:

• Optical Detectors - the use of an optical Transmitter / Receiver pair
• Conductive Detectors - the use of electrical current flowing through the tracks to indicate occupancy

and (less commonly)

• Proximity Detectors - the use of a proximity sensor to detect occupancy

[edit] Optical Detector

Many references can be found to various designs for optical detection, making use of Light Dependent Resistors (LDR) or Photo Transistors and IR detector pairs. Each has its own challenges and applications depending on the track layout, detection requirements, sensitivity and expense.

Advantages:

• Is independent of the operational control system, i.e. DC or DCC control and power on the track.
• Can detect a small area of the block, and if paired, can detect direction of occupancy
• Does not need any resistors or modifications to rolling stock.

Disadvantages

• Can be more complicated to align detector pairs.
• In the case of LDRs can be sensitive to ambient light and require careful shielding.
• Can look between rolling stock and appear to be unoccupied. This can be countered by diagonal placement.

[edit] Light Dependent Resistor

Probably the most common detector used in the past was the LDR or Light Dependent Resistor. This is an electronic device which changes its resistance in proportion to the amount of light to which it is exposed.

A detection circuit deploying a LDR would typically use an incandescent light or LED on one side of the track, and an LDR on the other and detect the interruption of light impacting the LDR, as a locomotive or stock moving or standing on the track in front of the detector.

Advantages:

• Has a natural hysteresis due to its lower switching speed.

Disadvantages

• Can be more complicated to align detector pairs.
• Is sensitive to ambient light and require careful shielding.

For more details see Block Detection:Optical Detection:LDR

[edit] Photo Transistors and IR detector pairs

A more current and popular optical detection solution these days is the IR Detector pair. This circuit makes use of a matched Infrared Transmitter and receiver spanning the track to detect occupancy.

Advantages:

• Has longer range and can be designed to detect over longer distances, even down the track.
• Is not as sensitive to ambient light and thus more reliable that LDRs.

Disadvantages

• Hysteresis must be designed into the detection circuit to cater for the quick switching speed of an IR photo detector, else errors can be detected.

For more details see Block Detection:Optical Detection:IR

[edit] Conductive Detection

Many references can be found to various designs for conductive detection, making use of the detection of any current being consumed on the track block, as an indication that the block is occupied.

Advantages:

• Is independent of the ambient lighting environment around the track.
• Will detect an entire block`s occupancy and can detect direction of occupancy (Block to Block)
• Can detect any rolling stock in the block that is drawing electrical current.

Disadvantages

• Circuit is dependent of the operational control system, i.e. DC or DCC control and power on the track, and is more complicated to design for DCC.
• Track needs to be broken into electrically isolated blocks.
• Needs the addition of a resistor on he insulated axles of rolling stock (like guard Vans) if they are to be detector on the block, which in turn draws more power overall.

[edit] Voltage Drop

The simplest form of circuit uses a Diode (2 Back to Back Diodes for DCC) in the power feed to the block, and measures the Voltage Drop (VD) across the diode to detect current flowing across the tracks, and thus occupancy.

Advantages:

• Simple circuit to design and low cost

Disadvantages

• Diode causes a voltage drop to the track.
• Diode must be able to accommodate short circuit currents !!

For more details see Block Detection:Conductive Detection:VD

[edit] Current Transformer

Design uses the conductor that feeds the power to the track, passing through a Current Transformer (CT) and a detection circuit to measure the current generated in the CT. Design needs to cater for the DCC signal noise but works for both DC and DCC.

Advantages:

• Does not cause any voltage drop to the track.
• Is isolated from the track voltage and current.
• Is more immune to noise than Voltage Drop circuit.

Disadvantages

• Circuit is more expensive that Voltage Drop circuit
• Must be rated for short circuit current but is more tolerant.

For more details see Block Detection:Conductive Detection:CT

[edit] Hall Effect Current Sensor

Design uses the conductor that feeds the power to the track, passing over a Hall Effect sensor (HE) and a detection circuit to measure the sensor response. Design needs to cater for the DCC signal noise but works for both DC and DCC.

Advantages:

• Does not cause any voltage drop to the track.
• Is isolated from the track voltage and current.
• Is more sensitive to current passing through the tracks.

Disadvantages

• Circuit is more expensive that Voltage Drop circuit
• Must be rated for short circuit current but is more tolerant.

For more details see Block Detection:Conductive Detection:HE

Can also be used in some cases to detect the Magnetic Field generated by the Motor in the Locomotive as it passes over the sensor, embedded between the tracks!

[edit] Proximity Detection (Metallic or Ultrasonic)

Design uses a Proximity Sensor (PS) to detect a metallic object passing within a short distance of the sensor. The sensor can be embedded between the tracks and detect the Locomotive or Rolling stock passing over it.

Advantages:

• Is isolated from the track power.
• Detects a larger area with less errors.

Disadvantages

• Circuit is much more expensive than any of the others.
• Metallic Proximity Sensor will only detect the Locomotive, and not plastic rolling stock.
• Ultrasonic sensors are prone to noise from other occupied tracks. ie Not good in a yard.

For more details see Block Detection:Conductive Detection:PS

[edit] Reed Sensors

This is Roco's preferred method of train detection, and they manufacture all necessary components. Design uses reed sensors placed inside or between track sleepers and are triggered by magnets on the train's bottom.

"Advantages:"

• Not affected by light
• Cheap and simple to install
• Reed contacts are available at electronics retailers
• A special train can be exempted from triggering things by not adding a magnet

"Disadvantages:"

• Requires small magnets to be attached to underside of locomotives (available from Roco)
• Reed contacts will melt if large currents are passed through them; requiring a special module or a relay

[edit] Desired Objectives

[edit] DC Detection

[edit] DCC Detection

Ed 00:43, 23 January 2009 (EST)

[edit] See Also

Block detection