Vibration Measurement: A Non-Contact Approach Using Proximity Transducers

In Industrial Instrumentation, various measurement systems are used to monitor and control industrial processes. These include pressure, temperature, flow, level, position alongside systems for speed, force, humidity, pH, and gas analysis. Additionally, electrical parameters like current and voltage, as well as density, viscosity, noise, and radiation, are monitored. These measurement systems are essential for ensuring efficient, safe, and automated operation across industries such as oil and gas, power generation, manufacturing, and chemical processing.

I want to explain the proximity transducer system in this article for vibration monitoring, which works on the interesting principle of eddy currents. This system is mainly used for monitoring vibration in shaft-based rotating equipment, and I learned about it after starting my career in the industry.

Typically, seismic transducer systems are well-known in vibration monitoring. I still remember learning about strain gauges, piezoelectric crystals, and electrodynamic type seismic mass-based transducers during my 3rd year engineering classes at IIT. Let's briefly understand its working principle before delving into proximity type. A seismic transducer includes a coil of wire attached to a permanent magnet that's fixed to a spring, which is connected to the device being measured. When the device vibrates, the magnet moves inside the coil, generating an electric voltage. These seismic probes are physically mounted on the bearing housing of rotating equipment.?

Next, let’s understand what vibration monitoring involves.

Vibration in rotating equipment refers to the oscillatory motion of machine components, typically caused by imbalances, misalignment, or mechanical faults within the equipment. As rotating parts like shafts, rotors, or bearings move, any irregularities in mass distribution or external forces can create vibrations. These vibrations can range from minor fluctuations to severe oscillations, potentially causing wear, damage, or even failure of the equipment if not properly monitored and managed. Vibration analysis is used to detect and diagnose these issues early, ensuring machinery operates efficiently and safely.?

Types of Vibration Measurement Devices:

1. Seismic Transducers: These measure absolute vibration relative to an inertial mass and include the following:

a.) Accelerometer:

• Measures acceleration of vibrating components, typically in units of g-force or m/s2.
• Used for high-frequency vibration analysis, providing data on the rate of change of velocity over time. Both accelerometers and seismic probes operate on the principle of a seismic mass, but their designs differ significantly. Accelerometers are built with a stiffer mass-spring system and incorporate damping mechanisms, allowing them to respond quickly to rapid changes and effectively measure high-frequency vibrations. They are engineered for a broad frequency response, making them suitable for applications like rotating machinery. In contrast, seismic probes are optimized for lower frequencies and larger displacements, focusing on structural monitoring. These design differences enable accelerometers to capture high-frequency vibrations that seismic probes may not detect effectively.

b.) Velometer:

• Measures vibration velocity, typically in units of mm/s or in/s.
• Ideal for mid-frequency vibrations and commonly used in machinery condition monitoring, as velocity relates directly to the energy of vibration.

c.) Seismometer:

• Measures low-frequency vibrations or displacements, often used for monitoring large structures like buildings or in geological applications.
• Uses a mass-spring system to detect ground vibrations or structural movement, like other seismic transducers but typically designed for very low-frequency ranges.

2. Displacement Sensors (Proximity Probes): Measure relative displacement between a vibrating object and a reference point, often using the eddy current principle. Primarily used for low-frequency vibrations and for monitoring shaft displacement in rotating machinery such as turbines and compressors.

3. Laser Doppler Vibrometer: Uses a laser beam to measure surface vibrations by detecting changes in the frequency of reflected light. Non-contact method, ideal for measuring vibrations in delicate or hard-to-reach components, providing highly precise measurements.?

Now, let’s look at how the proximity monitoring system works which is our topic of interest among all the above-mentioned devices. It takes input from both proximity and seismic transducers, processes the signal to measure various vibration and position parameters, and compares the processed signals with user-programmable alarms.

In a proximity transducer system, the electrical signal shows the distance between a conductive surface and the probe tip. The proximitor has two key functions:

1. It generates a radio frequency (RF) signal using an oscillator circuit.

2. It processes the RF signal to extract useful data with a demodulator circuit.?

A proximity vibration system utilizes non-contact proximity probes that measure the vibration and displacement of rotating machinery components without touching the target, employing the eddy current principle. As the probe approaches the target, an alternating current generates an electromagnetic field, inducing eddy currents in the conductive surface of the target. The resultant eddy current loss alters the impedance of the probe circuit, which is then measured to determine the distance between the probe and the target. This method provides precise measurements while minimizing wear and tear on both the probe and the machinery, enhancing accuracy and reliability in monitoring for effective condition assessment and maintenance.

Proximity sensor systems are generally preferred over seismic transducers for vibration monitoring in certain applications due to their sensitivity, faster response times, and ability to detect high-frequency vibrations. They are particularly effective in:

1. Rotating Equipment: Proximity sensors excel in monitoring shaft displacement and alignment in rotating machinery, providing real-time feedback on vibration levels.
2. Close-Range Monitoring: Proximity sensors are suitable for applications where equipment is closely monitored, such as turbines, pumps, and compressors, allowing for early detection of issues.
3. Reduced Sensitivity to Noise: They are less affected by external vibrations and environmental noise, making them ideal for high-precision monitoring in industrial environments.

In contrast, seismic transducers may be better suited for broader, low-frequency vibration assessments over larger areas but are often less effective for high-resolution monitoring in specific equipment.

Proximity Probe Eddy Current Flow Example

When a conductive material enters the RF field, eddy currents are generated on its surface. The depth of these eddy currents depends on the material's conductivity and permeability. For example, in 4140 steel, the penetration depth is around 0.003 inches (3 mils).

When the probe is close enough to the target material, the eddy currents affect the RF signal in two main ways:

1. The signal amplitude is at its lowest when the gap between the probe and the target is smallest, causing the maximum flow of eddy currents.

2. The signal amplitude is at its highest when the gap is largest, resulting in minimal eddy current flow.

If the target moves slowly within the RF field, the signal amplitude changes slowly. If the target moves quickly, the amplitude changes quickly. Oscillating movement of the target causes the RF signal to fluctuate (or modulate).

The demodulator circuit handles both slow and fast changes in signal amplitude the same way. If the target is not moving (like with a thrust probe), the proximitor outputs a steady DC voltage, known as the gap. If the target is oscillating (the gap changes slowly or quickly), the proximitor outputs a varying DC voltage (AC), shown as a sine wave. When monitoring vibration, the proximitor provides both a DC (gap) and AC (vibration) signal. A typical system frequency response ranges from 0 Hz (DC) to 10 kHz for the BN3300 proximity transducer system. Newer systems, like the Bently Nevada 3300XL proximity system, have a response range of up to 12 kHz.

For a Bently Nevada (BN) proximity monitoring system,

BN 3300XL proximity probe has the following:

Linear Gap Range : 0-2 mm (0-78.74 mils where 1 mil = 0.001 inch)

Linear Gap voltage Range : -2 Vdc to –18 Vdc (probe zero position or 0 mils is set at -10 Vdc)

Average Scale factor= (change in gap voltage) / (change in gap) = 200 mV/mil whereas power supply to the proximitor is 18 Vdc to 30 Vdc

The 3300 XL series proximity transducer system gives an output voltage that is directly proportional to the distance between the probe tip and the conductive surface being observed. The output voltage is set to negative because the system operates with a negative bias to avoid false readings at zero or near-zero distances, ensuring the sensor is active when the shaft is within measurable range.

The typical zero position is set at -10 VDC, with a range from -2 VDC (closer) to -18 VDC (farther) to ensure the measurement reflects the actual gap distance accurately. Using negative voltage helps provide a clear distinction for small gap distances while preventing saturation of the output, which could happen if the voltage were positive.

It can measure both static (position) and dynamic (vibration) data, making it ideal for monitoring vibration and position in fluid-film bearing machines. It is also used in Keyphasor and speed measurement applications. The BN3500 monitoring system is used to connect with and collect data from the 3300 XL series transducer, offering a solution for more complex and integrated monitoring and protection needs.

Other manufacturers of proximity transducer systems similar to Bently Nevada include GE Measurement & Control, SKF, Honeywell, PCB Piezotronics, and Kaman Precision Products.

Proximity transducer systems have various applications for monitoring the behavior of a machine’s shaft (target). The two most common applications are radial vibration (radial movement) and thrust (axial movement).

Another frequent use of the proximity transducer system is as a once-per-revolution marker or keyphasor on a machine shaft. This system is mounted to detect a “notch” or a “projection” on the shaft, producing a voltage pulse with each revolution. When the shaft passes over the notch or projection, it creates a much larger voltage change than what would be expected from normal vibration or distance measurements. This significant voltage change enables the 3500 monitoring system to differentiate between a genuine once-per-revolution signal and background noise or vibration. The keyphasor timing signal is a valuable tool for diagnosing machinery issues. At a minimum, the generated pulse can be used to measure machine speed.

Once the phase reference signal (Keyphasor) and time base signal are combined, we can display information about the current position of the shaft's vibration motion. The time base signal is achieved by combining the vibration signal—typically obtained from an accelerometer or other vibration sensor—with the Keyphasor signal, serving as a consistent timing reference to measure the shaft's vibration over time. The absolute phase angle of a vibrating shaft can be determined using the time base slot. This absolute phase represents the number of degrees in the vibration cycle from when the Keyphasor triggers (once per turn reference pulse) to the first positive peak in the vibration signal. By definition, this is known as a phase 'lag' angle.

The transducer system typically consists of three components: the probe, the extension cable, and the proximitor.

A typical BN probe features a tip assembly made from a generic version of polyphenylene sulfide (PPS), which is threaded into a stainless steel case. The tip assembly can come in various diameters and contains a coil that connects to the center conductor and inner screen of a 75-ohm miniature triaxial cable housed within the stainless steel casing.

The triaxial cable has a center conductor and two screens. The inner screen and center conductor connect the coil in the tip to the miniature connector at the probe cable end. The outer screen is not connected to the coil or the connector, meaning it does not contribute to the electrical properties of the system. Instead, it provides mechanical protection for the inner screen, preventing unwanted grounding of one side of the coil if the cable’s outer Teflon coating becomes damaged.

3300XL Probe Part Numbering:

The probe’s cable terminates to a 75 ohm miniature coaxial male connector. The probe part number and serial number are attached to the cable. The part no denotes the probe options. E.g. 330101-05-30-10-02-00 where

05= 0.5 in unthreaded length

30= 3.0 inch case length

10= 1.0 meter total length

02= miniature coaxial ClickLoc connector, standard cable

00= hazardous area approval not required

For the 3300 XL probe, the available probe cable lengths are 0.5, 1.0, 1.5, 2.0, 5.0 and 9.0 meters.

3300XL Extension Cable Part Numbering:

The extension cable connects to the probe and allows you to reach a convenient junction box. One end of the cable has a 75-ohm miniature coaxial female connector for connecting to the probe, while the other end has a 75-ohm miniature coaxial male connector for connecting to the proximitor. To prevent unwanted grounding of one side of the coil, heat shrink, special tape, or rubber boots are typically placed over the connection between the probe and the extension cable. It's important to note that standard electrical tape should never be used. The part number for the extension cable is attached to the cable itself.

e.g. 330130-080-00-00

where 080 = 8.0 meters total length

00= without armor

00= hazardous area approval not required

For the 3300 XL proximity transducer system, the available extension cable lengths are 3.0, 3.5, 4.0, 4.5, 7.5, 8.0 and 8.5 meters.

3300XL Proximitor Part Numbering:

The proximitor contains the electronic components and is typically mounted in a junction box. It features a die-cast aluminum case coated in blue, which is resistant to oils, solvents, and chemicals. A 75-ohm miniature coaxial female connector is chassis-mounted through the casing for connecting to the extension cable. There is also a terminal strip mounted in the case for supplying voltage to and receiving signals from the proximitor. The mounting base provides electrical isolation, so there is no need for separate isolator plates.

Mounting adapters are available for DIN rail mounting or for fitting the "footprint" of the 3300 proximitor. The electronics are fully encapsulated within the casing. It is important to ensure that the proximitor, probe, and extension cable are matched as a complete system. The total electrical length of the system should be either 5 or 9 meters, and the proximitor label specifies the required total electrical length.

e.g. 330180-50-00

where: 50= 5.0 meter (16.4 feet) system length

00 = hazardous area approval not required?

Probe Installation Common Issues:

Some installation conditions may cause the transducer system to fall out of tolerance, while others may lead to an incorrect or unacceptable response. For instance, crosstalk occurs when two probes are placed too close together, causing their RF fields to interact. Since the RF frequencies of the probes are unlikely to match, mixing them generates a difference frequency, which usually falls within the normal range of frequencies expected for vibration. As a result, a target may seem to be vibrating when it is not. The minimum distance between probe tips should be 0.70 inches (17.8 mm) for the 8 mm probe, or approximately three times the width of the probe tips.

Another installation issue is called Sideview, which happens when the probe is mounted in an area with insufficient side clearance around its tip. In this situation, eddy currents will form in any conductive material nearby, leading to losses in the system that are not related to the actual target. This issue may arise when a probe is installed in a bearing, and it’s uncertain whether the probe has cleared the mounting hole and is positioned correctly next to the shaft. If the installation decision relies only on voltage readings, the installer may not realize that the probe is measuring the wrong surface.

The next installation concern is target size. The surface area observed by the transducer system must be large enough to interact with the entire radiated RF field in front of the probe. The minimum observed shaft diameter is 20 mm (0.8 inches) for the 8 mm probe. The impact on the linear range and scale factor with an undersized target will vary based on the amount of eddy currents produced.

Noise is an unwanted signal component that is always present in any vibration signal. Although it can never be completely eliminated, it should be minimized or kept within acceptable limits. Noise can be introduced at almost any point in the measurement system, especially where the instrumentation connects to the application environment. To reduce noise, use a single-point ground for the entire system and take precautions to avoid ground loops. Employ only shielded cables and ensure that the shields are properly grounded. Keep power and signal cables in separate trays or conduits. To prevent noise, connections between signal common and earth should occur at earth ground points of equal voltage potential. This is achieved by connecting signal common and earth ground at a single point, typically at the monitor rack, but isolated from the earth ground at the proximitor side.

Now, let’s talk about runout: the measurable changes in a vibration signal that are not caused by shaft motion. Runout refers to any alteration in the transducer signal that does not result from the movement of the shaft. There are two types of runout for proximity transducers: mechanical runout and electrical runout.

Mechanical runout is a noise component in the output signal caused by geometric imperfections. It occurs when the distance from the probe tip to the shaft centerline remains constant, but the distance from the probe tip to the shaft surface changes. Sources of mechanical runout include scratches, dents, rust corrosion, other physical surface defects, and an out-of-round shaft.

Electrical runout, on the other hand, arises from non-uniform conductivity or permeability properties of the shaft being observed. Sources include uneven alloying, surface plating, localized stress concentrations in the shaft material, and areas of localized magnetization on the shaft. Unlike mechanical runout, electrical runout is usually not visible.

To eliminate noise, we use filters to remove everything within the frequency reject band of the filter, including both noise and useful information. Compensation is the process of removing noise from a signal by subtracting only the noise components. However, not all noise can be compensated, and accurate compensation can be difficult and costly. For effective compensation, the noise must be measurable, constant, and repeatable.

Slow roll compensation refers to the speed at which a machine operates with little or no dynamic motion. At very low speeds, the dynamic forces are so small that the resulting motion is too minor to measure. As the machine speed increases, dynamic forces also increase. At a certain threshold speed, the dynamic motion becomes large enough to measure. This threshold speed is at the upper end of the slow roll speed range.

Vector compensation is a common method of compensation where the 1X (or nX) slow roll vector component is subtracted from the measured 1X (or nX) vector. To do this, the runout and bow must be measured at the machine’s slow roll speed. Consequently, the 1X vector component of the runout and bow is often referred to as the 1X slow roll vector.

Bow in vibration analysis refers to a specific type of shaft misalignment or bending that occurs when a rotating shaft is not straight. This condition can arise from several factors, including thermal expansion, bearing wear, or improper installation. When a shaft bows, it leads to an uneven distribution of mass and stiffness along its length, resulting in increased vibration amplitudes and frequencies during operation. This situation can cause operational inefficiencies and excessive wear on bearings, seals, and other components, potentially leading to equipment failure if not addressed quickly. Detecting bow is essential for ensuring the reliability and performance of rotating machinery.

Runout compensation involves taking many synchronous samples during a slow roll of the shaft and storing these values. Then, at the machine’s operating speed, the shaft is sampled again in the same way. By subtracting the corresponding slow roll sample from each sample taken at operating speed, you create a runout compensated waveform that eliminates runout noise.

A runout compensated waveform reflects the true dynamic waveform without any runout noise.

Actual vibration pp = Measured vibration pp – slow roll pp

e.g. measured vibration pp = 1.4 mils pp, slow roll pp = 0.6 mils pp

In-phase compensation Actual vibration pp = 1.4 -0.6 = 0.8 mils pp

Out-of-phase compensation Actual vibration pp = 1.4 – (-0.6) = 2.0 mils pp

In conclusion, eddy current-based proximity vibration transducer systems provide a reliable and accurate method for monitoring the health of rotating machinery. By measuring the distance between the probe and the target without physical contact, these systems reduce wear on both the probe and the equipment, leading to longer service life and less maintenance. Interestingly, these systems leverage eddy current loss as a key part of their measurement principle, which is not typically desirable in electric or electronic systems. However, in this case, the induced eddy current loss alters the impedance of the probe circuit, allowing for precise readings of vibration and displacement. This enables early detection of potential issues, enhancing operational efficiency and safety across various industries. Contactless measurement is becoming increasingly popular in instrumentation due to its ability to provide accurate and reliable data without physical interference with the process, which minimizes wear and tear on equipment and enhances safety in critical applications. Overall, this technology is an essential tool for effective condition monitoring.