The Linear Voltage Differential Transformer (LVDT) sensor is a position sensor that contains two transformers with a common primary and a movable ferromagnetic core. The sensor's position is typically obtained by applying a sine wave signal to the primary coil and demodulating and decoding the amplitude of the signal across the two secondary coils connected is series opposition. The SSP1492 (Product Brief) takes a different approach. The SSP1492 can sense the position of the ferromagnetic core by measuring each coil's inductance. The SSP1492 can interface directly with each of the LVDT's coils as shown in the schematic below. An inductance versus position profile shown in the figure below can be decoded algebraically and then linearize using tools that are part of the SSP1492s internal math engine.

Since there are active signals from all three coils throughout the measurement range of the LVDT, a temperature stable output value that calculates the ratio of the difference of each secondary's inductance using following equation.

This output is not perfectly linear. Passing the result of this equation though a polynomial linearizing equation can produce a result with very high accuracies.

A small program to orchestrate the process can be stored in an inexpensive serial EEPROM (as small as 1K-byte) and downloaded automatically as a macro program or, alternatively, can be directed by a system's host controller. The enabling math functions are pre-loaded into the SSP1492 ROM memory and are accessible as commands sent via the serial interface from the host controller or are callable from the user's macro program. This implementation shown in the schematic below includes the EEPROM for macro storage, which means the host need only issue a start measurement command and then retrieve the resulting position when the conversion is complete.

In order to provide the highest accuracy sensor measurements possible, the SSP1492 supports the calibration and linearizing of individual LVDT sensors with the use of its internal EEPROM data storage and ROM routines. This additional sensor calibration infrastructure ensures that the LVDT/SSP1492 combination can achieve the highest accuracies possible given any particular LVDT sensors. Click here to learn more or for an Evaluation Kit!