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 Golden Goose Deluxe Brand 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.

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

Fig 1. Bently 7200 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 Golden Goose Superstar Outlet 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”.

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