Page 19 - Prosig Catalogue 2005

HARDWARE PRODUCTS
WHAT ARE DB, NOISE FLOOR & DYNAMIC RANGE?

bonded on to beam structure that deforms as weight or mass is applied to replacing sensors.
the load cell. In most cases strain gauges are used in a Wheatstone bridge
configuration as this offers the maximum sensitivity and temperature From the users point of view, when using a TEDS transducer, upon
compensation. Two of the four gauges are usually used for temperature connection certain important fields are automatically uploaded from the
compensation. transducer microprocessor and can then be used in the test setup matrix.
This can be very useful in a test environment. Furthermore, when used
There are several ways in which load cells can operate internally: bending, in conjunction with a transducer database, all the fields in the test setup
shear, compression or tension. All of these types are based on strain matrix will be automatically filled in upon transducer connection.
gauges. Training & Support
CAN-bus
Less common are the hydraulic load cell and the pneumatic load cell.
The hydraulic load cell, as the name implies is based on a fluid under Whilst not strictly a measurement transducer in its own right the CAN-
pressure. As the pressure or bus is having an increasing impact on engineering measurement across a
weight on the cell changes wide range of applications.
a diaphragm is moved. This The Controller Area Network or CAN-bus for short, is a multicast shared
type of load cell is more serial bus standard, originally developed in the 1980s by Robert Bosch
commonly used in situations GmbH. The bus is designed for use when connecting electronic control
where temperature can be units. CAN was specifically designed to be robust in noisy electromagnetic
highly dynamic. Hydraulic environments and can utilize a differential balanced line like RS-485. It
load cells can be almost as can be even more robust against noise if twisted pair wire is used.
accurate as the strain gauge
based type. Pneumatic load Although initially created for the automotive market, it is now used in
cells operate on the force many embedded control applications that may be subject to high levels of
balance principle; they external noise. The use of the CAN bus continues to grow in all automotive Condition Monitoring
use multiple dampener chambers to provide higher accuracy than the sectors, and even in the aerospace sector.
hydraulic load cell. They would normally be used in situations where the Conclusion
mass being weighed is small, and as they are not based on fluid they can
be used in clean room environments. Additionally, they have a very high All of the transducers and measurement systems in this article are
tolerance of variation in temperature. supported by the P8000 hardware. However, there are many other
types of transducers not discussed here that can be used with the
Load cells are often used in automotive brake testing and development; P8000. If you have other requirements please contact Prosig to discuss.
the force applied to the pedal is compared to the force generated at
the disk or drum, for example. The assist braking system can then be
optimized for the expected use. It is possible to adjust the system so that What are dB, Noise Floor
for a given force on the pedal an appropriate force is generated at the
brake pad to optimize the vehicle deceleration. Load cells are also used
in such diverse areas as engine dynamometry, suspension spring testing, & Dynamic Range?
production line batch weighing and production line connector insertion Software
force monitoring. Most engineers are probably familiar with or have come across the decibel
or dB as a unit of measurement. Its most common use is in the field of
Thermocouples acoustics where it is used to quantify sound levels. However, as will be
A thermocouple is a sensor for measuring temperature. It normally explained in this article, it is also useful for a wide variety of measurements
consists of two dissimilar materials joined at one end that produce a in other fields such as electronics and communications.
small voltage at a given temperature proportional to the difference in One particular use of dB is to quantify the dynamic range and accuracy of
temperature of the two materials. an analogue to digital conversion system. This applies to Prosig’s P8000
Thermocouples are among the easiest temperature sensors to use, and range of data acquisition hardware where the noise floor, dynamic range
are used heavily in industry. They are based on the Seebeck effect that and resolution are all specified in terms of dB.
occurs in electrical conductors that undergo a temperature change along Decibel (dB)
their length.
The decibel is a logarithmic unit of measurement that expresses the
Thermocouples are available in different combinations of metals. The four magnitude of a physical quantity relative to a reference level. Since Hardware
most common types are the J, K, T and E types. Each has a different it expresses a ratio of two quantities having the same units, it is a
temperature range and is intended for use in a different environment. dimensionless unit.
Thermocouples are often used for temperature measurement of corrosive Definition
liquids or gasses, usually at high pressures. Thermocouples are used
extensively in the steel and iron production industries; they are used to A decibel is used for the measurement of power or intensity. The
monitor temperatures through the manufacturing process. Because of mathematical definition is the ratio (L) of a power value (P ) to a reference
1
their low cost they are suitable for extreme environments where they power level (P ) and in decibels is given by:
0
often have to be replaced. They are a versatile transducer type and
are probably the one used in most fields, from aerospace to cryogenic L = 10Log ( P /P )
db
10
0
1
applications. When considering amplitude levels, A, the power is usually estimated to
be proportional to the square of the amplitude and so the following can
TEDS - Transducer Electronic Data Sheets
be used:
Transducer Electronic Data Sheet or TEDS for short, is a set of electronic L = 10Log ( A /A ) or System Packages
2
2
data in a standardized format defined within the IEEE-P1451.4 standard. db 10 1 0
This data specifies what type of sensor is present, describes its interface, L = 20Log ( A /A )
and gives technical information such as sensitivity, reference frequency, db 10 1 0
polarity and so on. Since the decibel is a logarithmic quantity it is especially good at
representing values that range from very small to very large numbers.
TEDS offers large benefits in that it simplifies troubleshooting, greatly The logarithmic scale approximately matches the human perception of
reduces costs and removes the need for recalibration when changing or both sound and light.
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