APPLICATION STORY
 

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."

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April 2002

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Contact Mike Coope - Tel: +44(0) 1902-700973 or Email: sensland@aol.com