You are probably referring to a proximity probe, and specifically a probe manufacturered by Bently Nevada (now GE Power). Actually the negative DC voltage is bias power that the sensor requires in order to return a voltage (also negative) corresponding to the actual gap between the sensor tip and the target. The gap between the sensor and target must be set prior to using the probe at a distance relative to the probe sensitivity, this gap varies depending on what type of probe is being used. The output DC voltage from the signal conditioner used by the probe can be checked to set the gap. At the far end of the sensitivity range, the output voltage may be close to -24 VDC. As this gap decreases the voltage becomes more positive and at a nearly closed gap it is almost 0 VDC.

There is no good reason for this supply voltage or output voltage to be negative, as other types of proximity probes use positive bias voltage. I suspect that the Bently company used a minus bias voltage for product protection. They have been selling their hardware for 50 years, so it\'s impossible to switch now.