Capacitive Proximity against Inductive Where the Target Material Changes
Metallic target variation demands Factor 1 inductive sensors to eliminate reduction factors, while polymer and liquid swaps require capacitive heads with active guarding.
High-Frequency Carrier
Signal Modulation
Alternating current signals used in instrumentation networks act as vehicles to transport low-frequency measurement data across transmission lines or air gaps. A high-frequency carrier provides the base waveform that is modulated in amplitude, phase, or frequency by the sensor output. This technique shifts the signal spectrum away from low-frequency noise sources such as power line hum or drift.
The frequency of this signal is selected to be at least ten times higher than the highest frequency component of the measured variable.
Noise Suppression
Electromagnetic interference in industrial environments is often concentrated at lower frequencies. By utilizing a high-frequency carrier, the receiver can employ bandpass filters to isolate the modulated signal while rejecting unwanted industrial noise. This filtering action improves the signal to noise ratio of the measurement system in harsh environments.
Sensor Coupling
Inductive and capacitive sensors require a changing field to transfer energy and signals between their stationary and moving parts. The high-frequency carrier allows for smaller coils or plates because the impedance of these reactive elements scales with frequency. This reduction in component size makes the sensor more compact and reduces the overall manufacturing cost.
Demodulation Process
Receiving electronics must extract the original measurement data from the incoming modulated signal. When the high-frequency carrier is removed, the remaining baseband signal is smoothed to represent the physical parameter. This step must be performed with minimal phase delay to avoid introducing lag into the control system.