Sensors help make ticket machines vandal proof

A strain gauge system from measurement technology specialists HBM UK is helping Deutsche Bahn (German Railways) protect its new touch-screen ticket machines from vandalism.

A strain gauge system from measurement technology specialists HBM UK is helping Deutsche Bahn (German Railways) protect its new touch-screen ticket machines from vandalism.

The new machines have a vandal-proof touch screen, formed by placing a pane of unbreakable safety glass in front of a normal computer monitor.

The use of such glass is normal in this type of application and it is the robustness of the HBM force sensor - developed by the company's specialist OEM sensor division - that is the key to allowing the ticket machine to continue working even after suffering repeated violent attacks.

The pane of unbreakable glass is housed in a rectangular frame, at each corner of which is an HBM DBB 01 force sensor, an aluminium double bending beam connected to HBM foil strain gauges.

The double bending beam type of force sensor makes it easy to arrange overload protection.

Using a metal construction for the sensing element means that the sensor can withstand a peak force 500 percent greater than the nominal load, allowing the ticket machine to continue operating even after repeated violent impacts from, say, a baseball bat.

This compares with competing ceramic and silicon sensors, which can only cope with peak forces up to 150 percent of the nominal.

Aluminium is used because it is an easy material from which to manufacture bending beams and is also much cheaper than stainless steel.

Another feature of the HBM system is its use of strain gauges based on foil strain gauge technology.

With this type of strain gauge, the strain induced by a force depends only on the magnitude of the force and not on its frequency, meaning that static forces and pressures can be easily measured.

People who are not familiar with touch screens often push carefully and lightly on the screen.

This small static force is easily detected by foil strain gauges, but not by the piezo electric devices used in other touch screen monitors.

Ticket machines are often situated beside the tracks and one main benefit of static working sensors is that vibrations caused by passing trains do not produce a signal, unlike piezo electric sensors, which will react to these vibrations.

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