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-DVRT-TC 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.

Designed to plug directly into MicroStrain's enclosure and power supply motherboard system, DEMOD-DVRT-TC cards are a snap to install. Four and eight channel enclosures are available. Smart MB-STD's are also available, with serial (RS-232) outputs, for direct interface to personal computers. Optional backlit liquid crystal digital displays provide a direct readout in physical units.

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 (part number: DEMOD-DVRT-TC), 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.