Signal Conditioning

 

Signal conditioning associated with differential transformers includes filtering and amplification.

 

Filtering is needed to improve the signal-to-noise ratio of the output signal. Amplification is necessary to increase the signal strength for data acquisition and processing.

 

 Since the reference frequency (carrier frequency) is induced into (and embedded in) the output signal, it is also necessary to interpret the output signal properly, particularly for transient motions.

 

The secondary (output) signal of an LVDT is an amplitude-modulated signal where the signal component at the carrier frequency is modulated by the lower-frequency transient signal produced as a result of the core motion (x).

 

Two methods are commonly used to interpret the crude output signal from a differential transformer: rectification and demodulation.

 

In the first method (rectification) the ac output from the differential transformer is rectified to obtain a dc signal. This signal is amplified and then low-pass filtered to eliminate any high-frequency noise components.

 

 The amplitude of the resulting signal provides the transducer reading. In this method, phase shift in the LVDT output has to be checked separately to determine the direction of motion. In the second method (demodulation) the carrier frequency component is rejected from the output signal by comparing it with a phase-shifted and amplitude-adjusted version of the primary (reference) signal.

 

Note that phase shifting is necessary because, as discussed before, the output signal is not in phase with the reference signal.

 

The result is the modulating signal (proportional to x), which is subsequently amplified and filtered.

 

As a result of advances in miniature integrated circuit technology, differential Tran’s orders with built-in microelectronics for signal conditioning are commonly available today.

 

A dc differential transformer uses a dc power supply (typically, ±15 V) to activate it. A built-in oscillator circuit generates the carrier signal. The rest of the device is identical to an ac differential transformer.