Application
Notes >> Technical Notes
Estimating the
Frequency Response of Variable Reluctance Pressure Sensors in Gas
Summary
Frequency response is defined as the ability of a measurement system
to accurately reflect dynamic pressure changes. The measurement system
consists of a pressure sensor and its associated electronics and plumbing.
Each component of the system affects the dynamic frequency response.
This paper will describe the factors affecting the frequency response
of variable reluctance transducers, and provide test data that can be
used to estimate the frequency response of a variable reluctance transducer
used to measure dynamic pressures.
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Introduction
The ability of a pressure sensor to respond accurately to rapid pressure
changes is a function of three variables: the mechanical response of
the sensor itself, the frequency response of the sensor electronics,
and the natural frequency of the plumbing that brings the pressure waveform
to the transducer. The mechanical response of the sensor depends on
the construction of the sensing element. The electronics connected to
a pressure transducer will most likely include damping, or a low-pass
filter on the output stage that may even be the most limiting factor
in system response. The tubing that leads up to the transducer from
the pressure source will also have a resonant frequency that will limit
the usable response of the pressure measuring system. Each of these
factors must be considered in order to arrive at a good estimate of
the accurate response of the pressure measurement system.
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Sensor Mechanical Response
Almost all pressure sensing technologies rely on a pressure sensing
diaphragm to transmit the dynamic pressure waveform to the electro-mechanical
element of the pressure sensor. For sensing technologies other than
variable reluctance, the sensing diaphragm is connected via linkages
or other mechanical means to a strain gage, piezoelectric, capacitive,
or some other electrical sensing element. The stiffness of the sensing
diaphragm and the associated linkages create a mechanical spring-mass
system whose natural frequency is usually specified by the manufacturer.
If the sensor is under-damped, amplification and also dynamic error,
of the incoming waveform occurs. If the sensor is over-damped, the incoming
pressure waveform is attenuated. In either case, pressure measurement
at or near the natural frequency of the sensor will result in undesirable
distortion of the dynamic signal.
Some pressure transmitters
used in the process industry have an over-pressure protection scheme
whereby the input pressure is transmitted to the sensing element hydraulically.
Such a system is highly over-damped and comparitively slow; response
times on the order of 0.1 seconds are common. Process transmitters are
often further damped electronically because a smooth pressure signal
is better for process control applications. These instruments are not
suitable for dynamic pressure sensing.
For variable reluctance
pressure sensors, the only mechanical part that moves in response to
pressure is the sensing diaphragm, and the total displacement over a
full scale pressure excursion is less than 2 thousandths of an inch.
There are no mechanical linkages or hydraulics inside the sensor to
slow down the sensing element. The position of the diaphragm is measured
inductively, and this is how the sensed pressure is converted to an
electrical signal. The natural frequency of variable reluctance transducer
is a function of range, and as shown in the graph. Because the sensing
diaphragm is in contact with the gas being measured, the sensor is typically
over-damped at its natural frequency.
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