Regarding the Bentley 3300 voltage issue

What this sentence means is that the Bentley 3300 series eddy current sensors use the distance between the probe and the surface of the conductor being monitored to measure vibration and relative position; the smaller this distance, the greater the voltage signal generated by the sensor. And this voltage signal is a negative voltage signal; this is because when the probe gets close to the surface of the conductor being observed, changes in the magnetic field generate an induced current, and the direction of this induced current is opposite to that of the voltage signal, hence the output voltage signal is negative. It should also be noted that this negative voltage signal is inversely proportional to the square of the distance from the probe, rather than being directly proportional to the distance. This is because the working principle of the eddy current sensor is based on the eddy current effect, which creates an eddy current circulation between the conductor and the probe; this circulation intensifies as the distance decreases, resulting in an increase in the induced current and the output voltage signal. However, the eddy current effect is damped as the distance decreases, causing the output voltage signal to be inversely proportional to the square of the distance. .

. Positive and negative voltages are themselves established by artificially defining a reference voltage point； The concepts of positive and negative voltage arise from people’s habitual tendency to associate the reference voltage (neutral point, virtual ground) with the earth voltage ； Choosing a positive voltage or a negative voltage is for the convenience of circuit design, application, and analysis ； The preamplifier of eddy current sensors uses a negative voltage as the signal transmission method, which is a convention established by early measuring instruments (a de facto standard resulting from technological leadership). As for why this choice was made, it is no longer easy to determine. However, both early transistors and modern high-frequency transistors often use a PNP structure (early electron triodes were also controlled by applying a negative voltage to the gate). It is relatively convenient to supply a negative voltage to a PNP amplifier circuit. As a high-frequency device, the eddy current sensor benefits from the use of negative voltage and a secondary meter for connection. I personally guess this might be the reason why a negative voltage transmission signal was chosen in the first place. .

I’ve learned it now; I used to think as well that the voltage level was proportional to the distance. Learned