Originally designed by Postfield Systems in the late 1980s to control the broadcast of Public Address Announcements, the Train Mass Detector (TMD) is now used extensively on Driver Only Operation CCTV (DOO) Systems to extend the life of the TV monitors. The monitors last longer as they only need to be on whilst the train is in the station.

What is a Train Mass Detector and how does it work?

The TMD transmits a series of fixed-frequency ultrasonic pulses in the general area where a train is expected to stop. When a train is present, some of the sound waves are reflected and picked up by an ultrasonic receiver. The output of the receiver is used to control a relay, the contacts of which allow the associated monitors to present the camera view of the train to the driver whilst it is in the station.

The TMD enables the receiver to reject echoes that are either too close to the transmitter or distant objects such as trains on a far track.

The strength of a returned echo is dependent on several factors, such as:

• overall surface area of the target facing the sensor
  
• attitude or orientation of the surface
  
• distance from the sensor
  
• nature or texture of the surface
  
• ambient high frequency noise
  
• heat haze in extreme weather conditions

Received echoes must be present for a continuous 'on-time' period of approximately 4-6 seconds before the relay trigger circuits function; this is to prevent short events from falsely triggering the unit.

The TMD has been designed so that, in the event of a mains failure, the relay will 'fail safe', allowing pictures to be displayed. It has a high temperature override circuit that switches the monitors on should the operating temperature reach levels likely to inhibit the performance of the unit. Normal operation resumes automatically when the temperature returns to lower levels.

The TMD is housed in a sealed die-cast box providing environmental protection to IP65.

Other Applications

The TMD can also be used to control either visual displays or audio announcements when the train is actually at the platform.

Train Mass Detector

Related Links

Driver Only Operation

Train Mass Detector

Bailey Concepts

BrightEye



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