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Safe braking under pressure.

          
There is no
          room for error in train braking systems and a recently

          developed transducer technology is said to ensure the highest

          levels of safety in such applications. 



          The vast majority of the world's trains are equipped with braking
          systems

          that use compressed air as the force to push blocks onto wheels
          or pads

          on discs. A core component of the air braking system is the pressure
          sensor,

          which determines brake cylinder pressure or even brake pad pressure.

          The reading is sent to an on-board computer or data unit and
          alarms are

          provided in the driver's cab if a unit reveals an unsafe condition.
          During

          maintenance this facility can also be used to download data indicating

          wear, failures or likely failures.



          As there are no margins for error, designers require a high specification
          of

          components in such systems. Long-term stability, reliability
          and the ability

          to withstand vibration are key factors when specifying pressure
          transducers.

          Plasma enhanced Chemical Vapour Deposition (CVD) technology developed

          by Gems Sensors, is said to provide an inherently stable structure
          which is

          less sensitive to thermal changes than silicon based transducers.
          Features

          of the unit include all-stainless steel wetted parts, no internal
          '0' rings or oil

          screens. The high-gauge factor properties of polysilicon-based
          strain gauges

          eliminates adhesives which tend to age, or fluid fills which
          may cause

          unwanted thermal shifts, making the Gems Sensors Series 2200
          C\TD

          transducers suitable for operation in harsh working environments.



          Formed to provide good over pressure capabilities, the stainless
          steel

          diaphragm is high-temperature vacuum brazed to the mechanical
          pressure

          connection, ensuring a rugged design capable of long term repeatability

          and longevity. CVD also ensures less drift, which means the sensor

          requires less recalibration.

          
Enhanced accuracy

          Available with an enhanced accuracy option that provides an optimum

          0.15% static error band and 1.0% thermal error band over a temperature

          range of between -20° and +80°C, the Series 2200 is typically
          used to

          perform accurate load weighing using the air suspension, or hydraulic
          load,

          and pressure adjustment in the control unit. This ensures that
          braking

          systems provide smooth control, regardless of the level of dynamic
          braking

          so the total braking effect matches the demand required.



          In the case of analogue electro-brake equipment, the system makes
          use of

          one air train pipe to carry the main reservoir supply and an
          electrical ring

          circuit (continuity circuit) which must be energised to keep
          the brakes released.

          The equipment comprises an electro-pneumatic service brake, and
          electro-

          pneumatic emergency brake, a spring applied parking brake and
          air supplies

          for other air-operated equipment on the vehicle.



          Both the emergency and service brakes are fully applied automatically
          if the

          electrical continuity circuit is deenergised for any reason.
          Self-maintaining,

          the service brake provides graduated application and release
          under manual

          control of the driver, or under the control of the automatic
          train operation

          equipment.



          All brake applications, whether service or emergency, are regulated
          according

          to the load, so that the brake forces obtained are in proportion
          to the weight

          of the vehicle. The brake control equipment provides an electrical
          signal as

          a demand to the dynamic brake. When the dynamic brake is in operation,

          the air brake is restricted to a complementary value - while
          ensuring the

          overall required brake force is maintained.

          
Converting
          pressure

          The transducer converts the air suspension pressure into an electrical
          signal

          which represents the car weight. This signal is fed into a multiplier
          which

          combines the brake demand signal with the car load signal, then
          passes it

          on to the output amplifier.



          The output amplifier provides continuous blending between the
          friction and

          the dynamic braking system so that regardless of the level of
          dynamic

          braking available, the total braking effort matches the demand.
          During

          propulsion mode, the output from the transducer is switched so
          that the

          load signal is fed to the traction circuit.



          With an operating life in excess of 100 million full-pressure
          reversals, the

          pressure transducers have an input range of up to 400 Bar, and
          a pressure

          limit of two times rated pressure up to 250 Bar or 1.5 times
          rated pressure

          at 400 Bar. Burst pressure is just under 10 times the rated pressure.



          CVD transducers are also used in the brake system's pressure
          control unit.

          Designed to produce a varying pneumatic output corresponding
          to a varying

          electrical input, the system comprises two solenoid valves, a
          pressure

          transducer and an electrical control circuit.



          Commenting in the system, Gems Sensors' product manager, Cohn

          Lussenden, said: "This type of mission-critical application
          requires

          precision technology you can depend on. When it comes to public

          safety, specifiers cannot afford to cut corners, but want a cost-effective

          solution."

          ________________________________________________

          
For more information
          contact:-

          
 

          
April 2002

          



                
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