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What Everybody Ought to Know About the Transducer & CM Panel Systems – Part 1

Introduction:

I tend to split up machinery types into two main groups –

• Critical Machines
• Non-Critical machines

Critical machines are those which, if they fail in service, will have an impact on:

(a) Production

(b) Safety – both plant and personnel

(c) The High Capital Replacement cost

(d) Those machines which have an extended spare part(s) availability timescale, etc.

Machines which I have noted as “Non-Critical”, are used in this context to separate the (generally speaking) larger, higher capital cost, machines from the smaller ones. However, it must be stated that some of these so-called ‘non-critical’ machines may also have some degree criticality associated with them, but do not justify the high cost of installing an ‘on-line protection system’.

This course will discuss those larger, critical machines, which have an on-line protection system, and those which might have been upgraded beyond a ‘protection’ panel system, to a full ‘condition monitoring system’. A condition monitoring system comprises a protection panel system, but has the additional capability to store (and retrieve) your data, produce plots and reports. Pure protection systems, on the other hand, do not store any of your data.

A machinery protection system comprises:

1. A set of transducers – primarily non-contact displacement probe types
2. A “Proximitor” or “Driver” or “Oscillator / Demodulator” units
3. Field Wiring
4. Safety Barriers
5. A vibration panel system
6. In some cases, an attached computer for data storage, data analysis and reporting

The Transducer:

In order to have some type of measurement quantification and repeatability, an interface device must be provided between your operating machinery and the diagnostician. The devices used for this task are electronic sensors or the transducer. These transducers convert numerous types of mechanical behaviour into proportional electronic signals. The transducer outputs are usually converted into voltage sensitive signals that may be recorded and processed with various electronic instruments.

The industrial transducer used for measurement of dynamic characteristics typically falls into three distinct categories:

• Shaft sensing proximity (displacement) probes (or ‘Eddy Current’ Probes)
• Mechanical motion velocity coils
• Solid state piezoelectric devices (for measuring both velocity and acceleration)

There is no universal sensor that can be used for all measurements, on all machines, under all conditions. The electronic signals from the transducer is quantified in terms of the following parameters:

• Amplitude (i.e., magnitude or severity)
• Frequency (i.e., rate of occurrence)
• Timing (i.e., phase relationship)
• Shape (i.e., frequency content)
• Position (from proximity probes only)

Most protection systems comprise primarily of displacement probes; which are often also referred to as: (a) “Proximity Probes

(b) “Eddy Current Probes

(c) “Non Contact Probes”.

The transducer measures “relative” shaft vibration, which is generally expressed in terms of amplitude by a Peak-to-Peak amplitude value. As its name implies, this measurement extends from the lowest portion of the dynamic time signal (i.e., bottom peak) to the highest portion of the signal (i.e., the top peak). Shaft vibration measurements are quantified in units of microns (μm), or 1 x 10-6 metres. In Imperial measurement terms, it is in units of MILS (i.e., 0.001” or 1/1000th of an inch); and where:1 MIL = 25.4 μm; and 1 μm = 0.03937 MIL’s (or 0.03937”).

In the SI (metric) system, the amplitude is normally expressed in units of microns Peak-to-
Peak (μm Pk-Pk), and in the Imperial system, it is in MIL’s Peak-to-Peak (MIL Pk-Pk).

Shaft Displacement Probes:

This type of transducer measures relative displacement of rotating or reciprocating shaft surfaces. The word ‘relative’ refers to the vibration between the probe tip (i.e., the machine casings) and the shaft surface. These displacement probes are supplied in a number of sizes and configurations. They come in a range of tip diameters, mounting thread sizes, and mounting thread lengths, to suit your desired application. Typical tip diameters are: 5 mm, 8 mm, 16 mm and up to 2” diameter. However, the majority of the probe tip diameters are either 5 mm or 8 mm. The figure below shows a typical displacement probe.

Regardless of configuration, eddy current proximity probes consist of the same basic components, a threaded stainless steel body and a plastic protective tip. A flat wound coil, which is located close to the probe tip, is connected by two wires to a coaxial cable that runs between the probe and proximitor (or ‘Oscillator’ / ‘Demodulator’) unit.

This cable is in two parts:

(1) The integral probe cable – which is typically of the order of 0.5 metres long,

(2) An Extension cable. The two cables are connected by a miniature coaxial connector. These two cables must be electrically tuned to a specific length in order to maintain the proper impedance between the probe and the proximitor. If the interconnecting cable length is altered from the correct value, the transducer calibration will be influenced. In addition errors can occur if the shaft material and probe system are not matched. The ‘free end’ of the probe coaxial cable is connected to the proximitor by a miniature coaxial connector.

Transducer_Shaft Displacement Probe1

Transducer_Shaft Displacement Probe2

Proximitor (or ‘Oscillator’ / ‘Demodulator’):

Bently 7200 Series Proximitor

Fig 1. Bently 7200 Series Proximitor

Fig 2: Bently 3300 Series Proximitor

Fig 2: Bently 3300 Series Proximitor

The word “proximitor” is a copyright word and property of Bently Nevada, and refers to the unit into which all external wiring is terminated, and is normally mounted in a local junction box in the field, and close to the machine and associated probes. Each displacement probe has its own individual proximitor, or ‘driver’ or ‘oscillator / demodulator’ as it is sometimes also referred to as.
The proximitor contains an internal oscillator that converts some of the input energy into a radio frequency signal in the Megahertz range. This high frequency signal is directed to the probe coil at the tip of the probe, via the interconnecting coaxial cabling.

The flat pancake coil at the tip of the probe broadcasts this radio frequency signal into the surrounding area of the probe tip, as a magnetic field. If a conductive material (e.g., the shaft surface) intercepts the magnetic field, eddy currents are generated on the surface of that material, and power is therefore drained from the radio frequency signal.

As the conductive material approaches the probe tip, additional power is consumed by the eddy currents on the surface of the conductor. When the probe tip is in direct contact with the surface of the conductive material (e.g., the shaft surface), the majority of the power radiated by the probe tip is absorbed by the material. As the power loss varies, the output signal from the proximitor also exhibits a change in output voltage. In all cases, a small gap produces a small output signal voltage, and a large gap results in a large output voltage from the proximitor to the panel monitor. Non-conductive materials such as air, gas, fluid between the probe tip and the shaft surface have no effect on the signal.

On the other hand, surface scratches on the probe ‘target area’ does have a significant effect on the signal output, as does magnetic anomalies and other surface irregularities. These
Imperfections and irregularities give rise to output signal errors, known collectively as a
“glitch”.

 

 

Sensonics Proximitor with Coaxial Cable and Probe Assembly

The figure above shows a Sensonics Proximitor with Coaxial Cable and Probe Assembly.

Displacement Probe Calibration:

The relationship between distance (i.e., probe tip to shaft gap) and output voltage to the monitor is achieved by a combination of electronic circuits within the proximitor. For instance, a ‘demodulator’ removed the high frequency carrier signal, and a linearization circuitry provides a reasonably flat curve over a typical range of 80 to 100 MILs (0.080 to 0.100 inches = 2032 to 2540 μm). The probe sensitivity approximates a straight line in the form of :

y = mx + c – which is the linear part of the slope of the calibration curve, which can also checked from the calibration curve- i.e.,

                    Differential Static Gap Voltage (V dc)
Sensitivity = —————————————————- [mV / MIL]
                    Differential Static Probe Tip Gap (MILs)

However, to convert this sensitivity value in mV / MIL to mV / μm, simply divide the sensitivity in mV / Mil by 25.4 = 7.87 mV / μm [for a normal 200 mV / MIL system].

Note: Different shaft materials have a different sensitivity, therefore, the sensitivity value
must be considered in relation to the shaft material used.

The figure below shows the calibration curve for ANSI 4140 Steel, and the “sensitivity” is equal to the slope of the (relatively) straight line graph, and is equal to the value of “m” in the y = mx + c equation above (i.e., 196 mV/MIL). This graph however, is not a totally straight line, and ‘bends’ at the top right and lower left ends. (∴Assuming “C” {the y-axis intercept}= 0: then y = 196 • x + 0; e.g., y = 196 • 50 + 0 ==> so y = 9800 mV = 9.8 V)
Calibration Curve for ANSI 4140 Steel

Proximitor Field Wiring:

This “field wiring” refers to the wiring to the proximitor from the protection panel, which typically, is a fully shielded three conductor 18 AWG (American Wire Gauge – which is 0.0403” in diameter (1.02 mm) and has a cross-sectional area of 0.832 mm2) cable. The cable “shield” is normally grounded at the monitor rack, and the field end of the shield is allowed to ‘float’. This prevents ‘ground loops’ from developing in the transducer wiring.

Within the three wire cable, one wire is used for the output [vibration] signal (White) to the
monitor, the second wire provides a common ground (Black) and the third wire is the power input to the probe coil (red). A power input of typically – 24 V dc is applied to the proximitor from the associated panel monitor (or a regulated DC power supply). Normally, this cable length (for 18 AWG cable) is restricted to about 1000 feet (305 metres), for non- hazardous areas, and less for hazardous areas. This is to avoid a reduction in frequency response and high capacitive loads, particularly where hazardous areas are involved, which involve stricter conditions for capacitive loads.

Displacement Probe Output Signals:

The distance between the displacement probe tip and the shaft surface is (when the shaft is rotating), oscillating in a uniform manner. This type of repetitive motion is referred to as Simple Harmonic Motion (SHM), and it is convenient to describe with a sine wave. This oscillating motion is translated
by the probe calibration curve into an oscillating voltage. More specifically, this is commonly referred to as an alternating, or an AC voltage. This AC voltage is proportional to, and represents, the vibration displacement (time-domain) signal.

The probe also outputs a second signal, which is proportional to the static gap between the probe tip and the shaft surface, and is referred to as the “Gap Voltage” or DC voltage. This is also called the “DC Offset”. The maximum DC voltage is coincident with the peak gap distance. Similarly, the minimum output DC voltage matches the point of the closest gap. The average distance between the probe tip and the ‘target’ (i.e., the shaft surface), is referred to as the Average Shaft Position.
Thus, the AC output voltage is proportional to vibration, and the DC output voltage indicates the average distance of the oscillating target with respect to the stationary probe tip. The figure below shows the average DC output gap voltage (the vertical line from the ‘x-axis’) with the AC (vibration signal) voltage superimposed on the DC Offset voltage.

Probe Calibration Curve

Keyphasor Probes:

Another application of proximity probes is to provide a timing signal. Typically these are synchronous, once-per-revolution pulses that may be used for accurate speed measurements. They are also employed for phase measurements and determination of precession, when combined with other probes on the machinery train. These 1x timing pulses are typically referred to as “Keyphasor” signals, and the transducers which provide these signals are called “Keyphasor Probes”. These probes are often positioned over a shaft notch (i.e., a keyway) or a drilled hole. With this arrangement, the probes produce a negative going pulse as the shaft indentation passes under the timing probe. In some cases, a raised surface such as the top of a shaft key is observed, giving a positive going pulse. The keyphasor signal should have a pulse height of between 5 an 15 Volts. The necessity for a strong and
consistent keyphasor signal cannot be overstated. Thus these probes only provide a dc gap voltage pulse, dependent on whether it is a keyway or shaft projection which triggers the keyphasor pulse. The image shown below, shows the effect of a “notch” (e.g., a keyway) in the shaft surface [Fig 6-5], or a “projection” (e.g., a key) on the shaft surface [Fig 6-6].

Negative & Positive Trigger Pulse Signal & Associated Blank Bright Sequence

The radial, axial and keyphasor probes are combined in many different combinations. In most instances, a pair of mutually perpendicular radial probes are installed at each journal bearing (X-Y Probes). A pair of axial probes are typically mounted at each thrust bearing, and a radial keyphasor probe will usually be installed for each shaft speed.

Barrier Protection:

A large portion of plant in the Oil & Gas industry (and also in other industries) is located in a ‘Hazardous Area’, which requires the use of “IS” equipment and systems. Intrinsically Safe (IS) equipment is defined as:

Equipment and wiring which is incapable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture in its most easily ignited concentration”.

In this context, this is achieved by limiting the amount of power available to electrical equipment in the hazardous area to a level below that which will ignite the gases.
Protection panel systems require power to be sent out of or into a hazardous area. This is achieved by installing Zenner Barriers, which are located in a “Safe Area”.

These barriers are passive devices, which are used to safely divert excess electrical energy to ground, to ensure that the maximum energy possible at the terminals in the hazardous area fall far below the explosive point of any volatile gasses present.

Field wiring is connected to/from these diode devices, thus limiting the power supplied to the proximitors and displacement probes, which are located in a hazardous area, and also the power returning from the probe and proximitor to the protection system panel monitors. These barriers can be installed externally in the cubicle which houses the protection panel rack, alternatively, they can be installed internally within the panel rack system.

Note: There is always a voltage drop across these Zenner Barriers. Therefore the voltage output measured at the probe / proximitor will not be the same as that measured at the
monitor input terminals.

Do you have any questions so far ?

Stay tuned for Part 2.

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