The Benefits of DIGITAL SENSORS.
The reliability and accuracy of analogue pressure sensors is
well documented, but the question arises that if analogue and
thin film technologies are so reliable, why are there so many
digital offerings becoming available?
Switching to digital alternatives can offer many advantages,
particularly where networked communications are desirable.
Implementation of bus lines gives a reduction in wiring costs
as there is no need to wire each sensor back to the evaluation
unit - a typical reduction is around 30%. The use of bus
compatible sensors also allows for longer cables and cables
of varying length to be employed from the sensors to the data
acquisition system and gives the ability to have several sensors
transmitting along one cable, instead of one cable per sensor.
It is possible to instantly unplug or plug sensors into the system,
without affecting other components. Sensors can be removed
and replaced with freshly calibrated sensors with no programming
changes to the system itself. Calibration information is stored
within the sensor, eliminating the need to transport the system
along with the sensor for calibration.
Digital technology also offers the ability to monitor and report
the sensors 'health' by monitoring the bridge integrity and
provides enhanced measurement accuracy. Demand for bus
compatible products comes mostly from automotive and marine
customers who want the convenience of a networked solution
in areas where they were using a series of sensors.
"Specifiers wanted to have several sensors operating along one
cable, to be able to identify a fault in the line and locate exactly
where it is. The digital solution allowed them to plug in sensors
or replace them without an interruption to the overall system,"
explains Cohn Lussenden, from Gems Sensors. "The high transfer
rate and high transmitter reliability, as well as self diagnostics,
was an attraction and we wanted to respond to this demand.
" As the company embarked on the development of a digital
sensor, a decision had to be made over which protocol it would
In the automotive and marine industries there is already
widespread acceptance of the CANbus protocol - a serial bus
system suited to interconnecting smart devices to build smart
systems or sub-systems. One of the main benefits of using
digital transmitters with a CANbus interface is the incorporation
of a micro-controller which enables decentralised pre-processing
and filtering signals, ultimately reducing the amount of information
that needs to be processed on the level of measuring and control
Real-time processing needs
In real-time processing, the urgency of messages to be exchanged
over the network can differ. A rapidly changing dimension has
to be transmitted more frequently and therefore with fewer delays
than other dimensions. The priority at which a message is
transmitted with less urgent messages is specified by the identifier
of the message concerned. The priorities are laid down during
system design and cannot be changed dynamically. Transmission
requests are handled in the order of message importance for the
system as a whole.
Data transmission is also performed safely and without errors since
data modifications, caused by electromagnetic interferences, are
detected by the CAN inherent error monitor. If an error occurs,
the message is retransmitted.
Originally developed for use in automotive applications, CANbus
has been adopted by the industry as the standard network
technology - due to its fast response and high reliability for
applications as demanding as control of anti-lock brakes and
air-bags. The automotive industry also uses CAN as the in-vehicle
network for engine management and body electronics. Currently,
demand from motor manufacturers on test cell applications, which
require the system flexibility of addressable pressure transducers
for the real time monitoring of a range of parameters.
In boats, ships and vessels, CAN-based networks are used as
embedded networks in sub-systems and as integration networks
connecting sub-systems. Research also suggests that the European
marine equipment and systems industry is increasingly turning
to CAN networks.
For machine control, CAN is used as an embedded network
within industries such as textile, printing, injection moulding
and packaging - while CANopen is the main protocol. Other
industries for embedded control are railway applications.
Research suggests that CANopen will have a market share
of about 50% of networked railway control systems within
5 years. Here, pressure transducers are widely used to ensure
effective control of pneumatic braking systems.
Ultimately, one of the distinguishing features of CANopen is
its support for data exchange at the supervisory control level
as well as accommodating the integration of very small sensors
and actuators on the same physical network. This avoids the
unnecessary expense of gateways linking sensor systems
with higher communication networks and makes CANopen
attractive to OEMs.
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