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Description
Designed for ease of use and general versatility, this printed circuit card
provides complete Temperature Compensated conditioning for MicroStrain's DVRT®
sensors. These signal conditioners utilize both alternating current and direct
current excitation of the sensor's inductive bridge to accurately measure
position in the face of temperature gradients, to achieve improved temperature
compensation.
Each card contains all the requisite connectors to facilitate plug-and-play use,
including: active, jumper selectable low pass filters, input supply line
filters with reverse input protection, and output line buffers.
DEMOD-DIN cards provide high level output from a wide variety of bridge type
transducers, including capacitive gap, inductive proximity, electrolytic tilt
sensors. Resistive strain gauges may also be conditioned, with the DEMOD's
sinewave excitation and sychronous demodulator providing excellent noise
rejection and elimination of thermally induced potentials.
How it works
Previous oscillator/demodulator circuits for use with half bridge LVDT's, DVRT's®
and non-contacting variable reluctance/eddy current sensors have relied on the
use of two coils arranged in a differential manner to amplify position and to
cancel temperature effects. However, this method of compensation is only
effective when both coils experience the same temperatures simultaneously. In
practice, thermal gradients commonly occur across sensing coils, resulting in a
difference in the sensing coil's resistance as compared to the reference coil.
The new conditioner overcomes this limitation by injecting both AC and DC
excitation into the coils, and demodulating the AC component of the bridge
signal separately from the DC component. The differential signal produced by
the bridge in response to the DC excitation is amplified and subtracted from
the demodulated (and amplified) AC response. The resulting output is free from
temperature gradient errors. Linear plunger type and non-contacting inductive
sensors are now available in very small diameters (<1 mm), and single coil
sensors possess nearly 1:1 body length-to-stroke ratios, without sacrificing
thermal stability.
To illustrate the effectiveness of the new signal conditioner, we provide two
examples: Example 1) In a medical application, a non-contacting, 1 mm OD DVRT®
sensing coil experienced a temperature input of 37˚C, while the reference
coil remained at room temperature. Traditional AC difference methods
demonstrated a voltage shift of 1.5 volts (30% of full scale). This same
sensor, when used with the new gradient compensation methods, and under
identical test conditions, demonstrated a shift of only .007 volts (0.14% FS).
Example 2) In an automotive application, dual coil subminiature DVRT's® were
exposed to a thermal gradient of 75˚C. The temperature gradient
compensation method improved thermal stability from 4.3% of FS to only 0.125%
of FS.
These new systems will improve performance in the face of thermal gradients,
extreme thermal environments, and where tight space constraints exist.
Applications include advanced metrology, medical devices, and
automotive/robotic control systems.
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