Specifications

50 Control and Automation Solutions Guide
Pressure Sensors and Weigh Scales (Force Sensing)
Overview
The need to detect and measure
pressure and weight is a very common
requirement for modern industrial
control and system monitoring. Pressure
measurement is especially important,
as it is also used indirectly to measure
ow, altitude, and other properties.
Pressure-and weight-measurement
devices can be regarded as “force
sensors,” since force is the property
that aects the transducers’ outputs.
The applications for force sensors are
vast and range from vacuum gauges to
heavy machinery weigh scales, industrial
hydraulic equipment, and manifold
absolute pressure (MAP) sensors for
internal-combustion engines. Each
application has its own diverse needs for
precision, accuracy, robustness, and cost.
Although there are several methods
and technologies for measuring
force, the most commonly used
measurement element is the strain
gauge. The two most common types
of strain gauges are the metal-foil type
used in a variety of weight/pressure
sensors, and the semiconductor-based
piezoresistive transducers, widely used
to measure pressure. Compared to
metal-foil transducers, piezoresistive
transducers are more sensitive and
linear, but have large temperature
dependence and large initial osets.
In principle, all strain gauges react to
an applied force by varying a resistance
value. Therefore, in the presence of
electrical excitation they eectively
convert a pressure or weight to an
electrical signal. Usually one, two, or
four of these active resistive elements
(strain gauges) are arranged in a
Wheatstone bridge conguration
(sometimes called a load cell) to
produce a dierential output voltage
in response to pressure or weight.
Engineers can design a sensor module
that meets the unique requirements
of diverse force-sensing applications.
A successful design would include
the suitable sensing element
for the physical property and an
appropriately designed signal chain.
Block diagram of the signal chain in a force-sense application. For a list of Maxim’s recommended pressure-sensor solutions, please go to: www.maxim-ic.com/psi.
EXCITATION
μC
TRANSDUCER /
BRIDGE
AFE FILTER ADC DIGITAL DOMAIN
ADC
Reference
DAC
OP AMPS
REF
TEMP
SENSORS
PGA
IA
FILTERS
REFERENCE
LEVEL
TRANSLATOR
ISOLATION
ADC
OP AMPS
DIGITAL
POTENTIOMETERS
PRECISION
RESISTORS
INTEGRATED SIGNAL CONDITIONER
= MAXIM SOLUTION