Vibration measurement, proximity probes.

Vibration systems use transducers which convert mechanical motions into electrical signals.

What is Vibration?

Rapidly changing too and fro motion either in vertical or horizontal of an object with reference to a centerline is called vibration. Vibration is a mechanical phenomenon. A vibration monitoring system is used to monitor the health of a machine in real-time and helps when the machine would need maintenance.

Types of Transducers:

  • Proximity Transducer System.
  • Velocity Transducers System.
  • Accelerometer transducers System.
  • Velometer Transducers System.

The proximity transducer system consists of three parts

1. Probe – which is basically installed on the machine.

2. Extension cable – which connects the probe and the proximitor.

3. Proximitor – It is the part that contains the electronics.

Proximitor performs two functions: Proximity transducer system converts the mechanical vibration to an electrical signal proportional to the displacement of vibration.

Proximitor consists of two circuits

1. Oscillator

2. Demodulator

The oscillator generates a Radio Frequency (RF) signal. Demodulator conditions the Radio Frequency (RF) signal to extract usable data. To do this, it needs a -17.5 V to -26VDC (with barrier) supply connected between terminals VT and COM. Once the proximitor is powered it will generate an RF signal at a specific frequency.  

The frequency is depending on the inductance (L) value of the probe’s coil and capacitance (C) value of the extension cable plus probe cable. RF frequency will be having a range from 500 kHz to 2 MHz

RF signal creates its field around the probe’s tip. When the conductive material is present in the vicinity of the RF field, eddy currents flow on the surface of that materials.

When the conductive material is away from the RF field, eddy currents will be less inside the conductor, RF signal loss will be less.

When the conductive material is nearer from the RF field, eddy currents will be more inside the conductor, RF signal loss will be more.

Measurement of parameters and concepts:

The proximity transducer system has many uses in monitoring the behavior of the machine shaft. There are three main types of dynamic motion. Radial vibration, Axial movement. One more use is Keyphasor or once per revolution marker.

Axial vibration:

Defined as a change in position of the rotor in an axial direction relative to a fixed point.

Radial vibration:

Defined as the motion of the shaft centerline within the radial bearing.

Key Phasor:

The probe is mounted on a shaft which is having a notch or projection on it. It will see once per each revolution as the shaft rotates and counts the revolutions. Key phasor in turbomachines is necessary to find out the phase angle of unbalance mass at the time of dynamic balancing.

Units of measurement:

Mil

Micrometer (mm).

The relation between mil and mm:

1 mil = 1/1000 inch = 0.0254 millimeter.

1 micrometer = 1/1000 mm

1 mil = 25.4 micrometer.

The figure-A below shows the demodulator output when a conductive material is away from the probe. It means the amplitude is maximum when the distance (gap) between probe and material (Target) is at maximum. Minimum eddy current flow occurs.

The figure-B below shows the demodulator output when a conductive material is nearer to the probe. Amplitude is less when the distance (gap) between probe and material (Target) is at a minimum. A maximum eddy current flow occurs.

Extension Cable:

It is the cable that connects the probe and the proximitor and when combine with the probe length gives the total electrical length.

Proximitor length = Probe length + extension cable length.

How to check proximity cable health status:

Total resistance = R Probe + R Core + R Jacket.  

R Probe – Resistance from center conductor to the outer conductor. Resistance changes as with the increase of cable length. It is 7.45 Ω for 0.5-meter length proximity cable.

R Core – Resistance from center conductor to the center conductor. Resistance increases as with the increase of cable length. 

    For 3-meter extension cable (0.066 Ω).

 R Jacket – Resistance from outer conductor to the outer conductor.

    It is 0.20 Ω for a 3-meter cable.  

As per the above data, the total resistance of the proximity system is = R Probe + R Core + R Jacket     

= 7.45 Ω + 0.066 Ω + 0.20 Ω                                                                                       

=7.73Ω

It varies as per the proximity probe length. Proximity probe lengths are available in 0.5 meter, 1 meter, 1.5 meters, 2 meters, and 5 meters. 

Scale factor:

It is expressed in the ratio of change in gap voltage to the change in probe distance from the target, generally it is a rotor shaft of the machine.  

The scale factor for proximity sensors is set at 200 mV/mill or 7.87 mV/micron (mV is peak to peak).

For example: As per the table below,

The Voltage at 40 mills (1.01 mm) is -7.49 V

                 The voltage at 50 mills (1.27 mm) is -9.48 V 

Gap is 50 mill – 40 mills = 10 mills

          Scale factor = Change in voltage/ Change in gap

                                 = (-9.58) – (-7.46)/10 = 200 mV/mill

The linear range is 2mm (80 mils). The linear range begins at approximately at 10 mils to 90 mils (-1 to -17 VDC).  

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