Dynamic Voltage vs. 4-20 mA Proximity Probes Explained
A standard dynamic voltage system provides a voltage signal which is composed of detailed data for complex analysis and condition monitoring. While this is fantastic for those who have the correct equipment to analyze this data, not everyone has the training or hardware to perform complex analysis.
CTC's PRO Line 4-20 mA current drivers are great, low-cost solutions for applications where it is important to monitor the condition of the machine, but there is no budget or need for a full DAQ rack or MPS (Machine Protection System). 4-20 mA signals are very common process control signals utilized by PLC, DCS, and Control Room Monitors. 4-20 mA signals are very popular because they can travel long distances without degrading, and provide a constant reading that is very easily and cheaply read by a wide variety of commonly utilized equipment. All PRO Line 4-20 mA Drivers also feature a fully-isolated BNC on the front of the case, which provides the analyst access to the unfiltered voltage output of the probe.
The two main functions of a proximity probe system are to identify the vibration and position of a shaft relative to its housing. In order to provide monitoring solutions for these two metrics, CTC offers different 4-20 mA current driver types:
4-20 mA Radial Driver Series
This series of drivers focus only on the severity of the radial vibration occurring in the shaft measured as the distance between the negative and positive peaks of the vibration sine wave that is generated, also known as peak-to-peak (Pk-Pk) measurements. This is accomplished by only focusing on the AC portion of the original dynamic voltage signal and converting it into a 4-20 mA process signal.
As vibration severity increases, the amplitude of the AC sine wave indicating vibration will increase, thus increasing the distance between the positive and negative peaks in the sine wave. This increase in distance between the peak-to-peak value is represented by an increase in the 4-20 mA output of the probe driver. So, as vibration grows more severe, the AC sine wave portion of the voltage signal will grow larger, which increases the peak-to-peak value and is represented by an increasing 4-20 mA signal. The scaling of this 4-20 mA signal is configurable when ordering this driver type. Below is an example of what peak-to-peak measurement of a waveform looks like:
4-20 mA Axial Probe Driver Series
This series of drivers only focuses on the DC portion of the original voltage signal and will provide the user data on the position of the shaft while filtering out the AC portion of the signal which indicates vibration. These are primarily used at the end of the shaft or on a shaft collar if there is one available to measure thermal expansion or axial thrust. Often this information is used to trigger alarms and switches to shut down a machine if the shaft moves beyond a designated distance from its original position.
