Use these buttons to explore the different proximity probe solutions available for your specific application:

Two blue PRO Line proximity probes mounted using black mounting brackets to secure them above the rotating shaft of a machine with a gold gear.

Selecting the Right Proximity Probe Driver for Your Application

CTC offers a wide variety of proximity probe driver configurations. Regardless of configuration, all PRO Line drivers come equipped with a buffered BNC that allows the user access to unfiltered voltage data.


CTC PRO Line 8 mm proximity probe driver in black Aluminum enclosure on DIN rail mounting feet, with a blue faceplate on top and silver BNC connector coming out of the blue faceplate.

The voltage driver has a dynamic voltage output that is compatible with industry-standard continuous vibration monitoring systems and is the format specified in API Standard 670. The dynamic voltage output is proportional to the DC gap between the target material and the probe tip.

View DD100180

4-20 mA DRIVERS:

4-20 mA Drivers Proportional to DC Gap:

Illustration of a PRO Line stainless steel proximity probe with a black probe tip mounted on the shaft of a journal bearing inside an industrial machine.

Probe showing proportional output for the 4-20 mA signals. With shaft surface at 50 mils, 4-20 output is 12 mA. If the gap increases to 70 mils, 4-20 output will be 16 mA.

Radial Application Drivers:

In radial applications, the probe drivers select the average shaft surface distance and the 4-20 is proportional to the overall peak-to-peak vibration in mils around the average surface of the shaft.

Axial Application Drivers:

Illustration of a stainless steel proximity probe mounted horizontally to the front of the journal bearing shaft with a gap between the probe tip and the shaft surface.

In the thrust position the probes auto-range to the face of the shaft or thrust collar and the 4-20 is proportional to the positive or negative vibration away or toward the probe, as shown.

Speed/Phase Analysis Hardware Solutions

For speed/phase analysis applications, CTC recommends using our PRO Dynamic Voltage Probe Systems. Probes utilized in speed/phase applications should be mounted over the portion of the shaft that has one or more keyways in its rotational path. PRO Proximity Probes used in this manner will produce a once-per-revolution pulse or multiple event-per-revolution pulses from a rotating shaft or gear. The PRO Proximity Probe System provides data that can be used to trigger a device that generates a digital pulse signal. A third-party device, like a relay, is used to get a pulse signal equivalent to a Bently Nevada™ Kephasor™ module. PRO Proximity Probe systems will only provide the displacement data that will show when the keyway passes by the probe tip; it will not generate any kind of signal itself.

PRO Proximity Probe Systems with Voltage Drivers can be used with Bently Nevada™ 3500 Series modules and external diagnostic equipment to measure the shaft rotative speed and vector parameters such as 1x vibration amplitude and phase when used with a Keyphasor™ module. The input signal range of these keyway modules is +0.8 V to -21.0 V, they are designed to take in dynamic voltage data from standard proximity probes. The real-time displacement measurements from these probes give the modules an indication of when the keyway(s) on the shaft travel in front of the proximity probe.

The figure above shows the real-time displacement data from a voltage probe mounted over a keyway in a rotating shaft. In this example, you can see two displacement spikes that occur as the keyway rotates in front of the probe tip. The data shows two full rotations of the shaft since there are two spikes and only one keyway on the shaft. If the proximity probe set was used with a Bently Nevada™ Keyphasor™ module, it would produce a digital pulse signal each time one of these displacement spikes occurs. the time between pulses can be utilized to calculate the speed of the shaft. When this Keyphasor™ data is used simultaneously in conjunction with two radially-mounted probes (on a different part of the shaft not over the keyway) it allows the analyst to perform phase analysis. Phase analysis allows the analyst to detect which direction during a shaft's rotation that vibration and position changes are occurring.

A diagram showing Keyphasor™ signal and vibration signal

For applications not utilizing Bently Nevada™ Keyphasor™ module, dynamic voltage probe systems can still provide speed and phase measurements when mounted over a keyway. Depending on the DAQ system and software being used, an analyst will have to set certain amplitude thresholds to trigger a "keyway pulse" when the voltage probe system detects a keyway. For example, if an end-user has a shaft with a 0.5 mm keyway, they should set their "Keyphasor™ pulse signal" to trigger at 0.5 mm so a speed/phase pulse will only occur when the keyway passes through the measurable field of the probe system.

Diagram showing a proximity probe mounted over a shaft with a keyway, along with a chart depicting two rotations of the shaft and keyphasor trigger levels

Dynamic Voltage vs. 4-20 mA Proximity Probe Systems

A standard dynamic voltage proximity probe 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.

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 (1) vibration and (2) 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 Series Drivers
These 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) measurement. 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:

chart showing what peak-to-peak measurement of a simple 60 Hz sine wave looks like

4-20 mA Axial Probe Driver Series
This driver series 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.

diagram of two proximity probes mounted perpendicular to the thrust collar and shaft of a machine.

When to Use a Signal Conditioner with a Proximity Probe System

CTC's SC300 Series Signal Conditioners can be used with Dynamic Voltage Proximity Probe Systems to filter out the DC portion of the voltage signal in order to convert the AC signal (vibration) into a 4-20 mA signal proportional to the severity of vibration. However, this provides a very similar output as CTC's PRO Line 4-20 mA Radial Drivers. As a result, for most new installations CTC typically recommends purchasing 4-20 mA current drivers instead of a proximity probe system plus signal conditioner, to save costs and eliminate unnecessary equipment.

The use of SC300 Series Signal Conditioners is ideal for cases where there are already voltage probes installed (or in stock) and the end-user would like to get a 4-20 mA output, or where specialty configurations are desired. Utilizing an SC300 Series Signal Conditioner will allow for additional configuration options to modify the 4-20 mA signal including selecting a specific frequency range to monitor, additional scaling options, alternative methods of measuring the severity of vibration like Pk-Pk, Peak, RMS, etc., and customized output smoothing features to avoid false alarms and trips. (Note: Drivers and signal conditioners cannot be daisy-chained to their power supply - one 24 V power supply for each unit, two power supplies for each channel of measurement). 

Click here to view the SC300 + Proximity Probe Driver Manual
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