t2_proximity transducer system operation

3500 Installation and Maintenance - Section 2: Page 1

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Proximity TransducerSystem Operation

Topic Objectives:

You will be able to explain the general construction and operation of the proximity probe system

You will be able to find probe calibration values both mathematically, and through the use of calibration equipment

You will be able to identify the conditions that lead to problems with proximity probes, including probe cable length(s), supply voltages, types of target material, etc

3500 Installation and Maintenance - Section 2

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Proximitor

SIGNAL COMMON

TRANSDUCER POWER

SIGNAL OUTPUTELECTRICAL LENGTH

The Eddy Current Proximitor

Transducers convert one form of energy into another. In the case of proximity transducers, mechanical energy is transformed into electrical energy using the proximity transducer system. The interface device used for this system is called a Proximitor. This electronic device is mounted in a rugged metal case, and has two basic functions:

1. Generates a radio frequency (RF) signal using an oscillator circuit. 2. Conditions the RF signal to extract usable data using a

demodulator circuit. The Proximitor requires a -17.5 to -26.0 Vdc supply voltage connected between its VT and COM terminals. Bently Nevada systems provide -24 Vdc, which is the recommended supply voltage.


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Proximitor Operation


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Eddy Current Flow

CONDUCTIVEMATERIAL

EDDY CURRENTS

RF SIGNAL

Eddy Current Flow When conductive material is present in the RF field, EDDY CURRENTS flow in the surface of that material. The penetration depth of the eddy currents depends on the materials conductivity and permeability. 4140 steel penetration is around 0.003 inches (3 mils).


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Small Gap

RF SIGNAL 0

Probe Close To The Material Once the probe is close enough to cause eddy currents to flow in a conductive material the RF signal is affected in two ways:

1. Amplitude is at a MINIMUM when distance (GAP) between probe and material (TARGET) is at a MINIMUM. Maximum eddy current flow occurs.


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Large Gap

RF SIGNAL 0

Probe Farther Away From The Material Amplitude is at a MAXIMUM when distance (GAP) between probe and material (TARGET) is at a MAXIMUM. Minimum eddy current flow occurs.


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Changing Gap

RF SIGNAL 0

Vibration Signal If the target is moving SLOWLY within the RF field, the signal amplitude INCREASES or DECREASES SLOWLY. If the target is moving RAPIDLY within the RF field, the signal amplitude INCREASES or DECREASES RAPIDLY. Oscillatory movement of the target causes the RF signal to modulate.


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Demodulator Operation

DEMODULATORINPUT

PROXIMITOR OUTPUT

0

0

Resulting Vibration Signal The demodulator circuit deals with a slowly or rapidly changing signal amplitude in the same way. If the target is not oscillating, as might be the case with a thrust probe, the Proximitor output is a constant DC voltage, called the gap. If the target is oscillating (gap changing slowly or rapidly) the Proximitors output is a varying DC voltage (AC) shown above by a sine wave. If the probe is observing a vibration, the Proximitor will provide both a DC (gap) and an AC (vibration) component in the output signal. A typical system frequency response is from 0Hz (DC) to 10kHz. Newer transducer systems, such as the 3300XL proximity system have responses up to 12 kHz.


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Proximity Probe Usage

RADIAL MOVEMENT

AXIAL MOVEMENT

Applications - Vibration And Distance

Proximity transducer systems have many uses in monitoring the behavior of a machine's shaft (target). The two most common being RADIAL VIBRATION (radial movement) and THRUST (axial movement).


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Proximity Probe Usedas a Keyphasor

NOTCH

Applications - Keyphasor Another common use for the proximity transducer system is as a ONCE PER REVOLUTION marker or KEYPHASOR (K) on a machine shaft. This proximity transducer system is mounted so that it observes a "notch" or a "projection" on the shaft and produces a voltage pulse once each revolution. Passage over the notch or projection causes a much more significant voltage change than expected from normal vibration or distance measurements. This significant difference in voltage allows the 3500 monitoring system to discriminate between a legitimate ONCE PER REVOLUTION signal, and background noise or vibration. The Keyphasor is a very useful tool when diagnosing machinery problems. At a minimum, the generated pulse can be used to measure machine speed.


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Verification ofProximity Probes

Probe response is verified by measuring and creating a calibration curve

Problems that can cause proximity probes to be out of tolerance: probe cable length power supply voltage crosstalk and sideview conditions target size and material

Performance Verification - Proximitor Calibration The Proximitor is designed to give known output voltage changes equal to known gap changes. This is called a SCALE FACTOR. For the proximity transducer system the standard scale factor is set at 200 millivolts per mil (200mV/mil). Scale factor information can be found on the nameplate attached to the Proximitor.


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Proximitor Calibration Graph

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PROBE GAPmils 0

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Performance Verification - Scale Factors The scale factor is the response of the transducer to target distance in mils or micrometers compared to the voltage resulting from the measurement. The overall average scale factor is an important tool for evaluating the performance of a proximity probe system. In addition, the incremental variances are important as well. When a probe response is evaluated, it should show little deviation from the linear response curve shown. The API 670 conventions have specifications for both the overall scale factors, and the incremental variances.


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Performance Verification - Calibration Process

The graph above verifies the performance of a proximity transducer system. It is created by clamping the probe and a spindle micrometer, with a target attached, in a bracket. With the spindle micrometer set at ZERO, clamp the probe tip flush to the target surface. Gap is increased by rotating the spindle micrometer away from the probe in 5 mil increments and noting the Proximitor output D.C. voltage at each step.

Proximitor Calibration Graph

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PROXIMITOR CALIBRATION EQUIPMENT SETUP


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Calibration of Scale Factor

Average Scale Factor (ASF) equals:

change in gap voltagechange in gap

Performance Verification - Scale Factor Calculations change in gap voltage -18 Vdc - (-2 Vdc)

Average Sale Factor (ASF) = ---------------------------- = ------------------------ change in gap 0.090 in - 0.010 in

A tolerance of 11 mV is allowed for Average SF (189mV/mil - 211mV/mil). Note: Change in gap is within the 80mils LINEAR RANGE which is between 10mils and 90mils. Proximity System Verification Tip: For detailed information regarding Average Scale Factor and verifying transducer system performance, refer to the 3300 5mm and 8mm Proximity System Manual, or the 3300XL Proximity Transducer Manual. A system performance verification record can also be found in the System Maintenance Topic.


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Calibration of Scale Factor

(-18.0Vdc) - (-2.0Vdc)0.08in or 2.0 mm

= 200 mV/milor

= 7.87 mV/um

Performance Verification - Potential Problems If the performance graph does not fall within specified limits, i.e., LINEAR RANGE less than 80mils, or the ASF is outside 11mV tolerance specification, the reason may be one of a variety of problems. The first to consider is potential mismatch of the Proximitor with the extension cable, and/or the proximity probe.


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OUTERSCREEN

CENTERCONDUCTOR

INNERSCREEN

INSULATION

Triaxial Cable

Performance Verification - Proximity Transducer System The transducer system has three individual components. None of these alone are a transducer. The three components are the PROBE, the EXTENSION CABLE and the PROXIMITOR. The PROBE has a tip assembly, made of a generic version of polyphenylene sulfide (PPS), that threads into a stainless steel case. The tip assembly is 8mm in diameter and contains a coil that terminates to a 75 ohm miniature triaxial cable that exits the stainless steel casing. The triaxial cable has one center conductor and two screens. The inner screen is a coil connection and the outer screen is not connected. This prevents unwanted grounding of one side of the coil if the cable is damaged. The cable is very flexible, and is tested to a one inch radius curve without impacting the proximity systems performance.


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Probe NumberingCABLE

FIXEDLOCK NUT

TIPASSEMBLY

P/N 330106-05-30-10-02-00

S/N APRS 416567

Performance Verification - Proximity Transducer System The Probes cable terminates to a 75 ohm miniature coaxial male connector. The probe part number and serial number are attached to the cable. Probe options are denoted by the part number.

Example: 330106-05-30-10-02-00

where: 05 = 5 mm unthreaded length,

30 = 30 mm case length,

10 = 1.0 meter total length,

02 = with connector,

00 = hazardous area approval not required.

Transducer System Numbering For a detailed system part number breakdown refer to the Transducer System catalog. For serial number breakdown refer to the System Maintenance topic


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Extension Cable NumberingP/N 330130-085-00-00

P/N 330101-00-08-05-02-00

Performance Verification - Proximity Transducer System

The extension cable is the part that connects to the probe and allows you to reach a convenient junction box. One end of the cable terminates to a 75 ohm miniature coaxial female connector for connection to the probe. The other end terminates to a 75 ohm miniature coaxial male connector for connection to the Proximitor. Heat shrink, special tape, or rubber boots are usually placed on the cable to be slid over the probe to extension cable connection. This prevents unwanted grounding of one side of the coil. As a note, standard electrical tape must never be used. The extension cable part number is attached to the cable. Cable options are denoted by the part number.

Example: 330130-080-00-00 where: 080 = 8.0 meters total length

00 = without armor 00 = hazardous area approval not required.


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Proximitor

SIGNAL COMMON

TRANSDUCER POWER



As mentioned earlier, the Proximitor is the part that contains the electronics and is usually mounted in a junction box. It has a die cast aluminum case with a blue coat that resists oils, solvents and chemicals. A 75 ohm miniature coaxial female connector is chassis mounted through the casing for connection to the extension cable. A terminal strip is also case mounted for supplying voltage to and taking signals from the Proximitor. The base has an isolation plate mounted on it that will prevent unwanted grounding of one side of the probe coil. The circuit board mounted electronics are fully encapsulated within the casing.


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Proximitor

SIGNAL COMMON

TRANSDUCER POWER



The part number and serial number are attached to the Proximitor. Proximitor options are denoted by the part number. Example: 330100-90-00 where: 90 = 9.0 meters total length (probe with integral cable and extension cable), 00 = hazardous area approval not required. Note: It is an option to have a proximity transducer system with the probe's integral cable and extension cable length as one piece.


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Proximitor

SIGNAL COMMON

TRANSDUCER POWER



All proximity Transducer Systems must have a Proximitor. The Proximitor dictates which probe and extension cable length MATCHES the system. Example: Proximitor Part No. 330100-90-00 Remember: 90 = 9.0 meters total system length of probe and cable (integral or

extension).


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Proximitor

SIGNAL COMMON

TRANSDUCER POWER



Probe and extension cables are manufactured in standard lengths. Proximitors are manufactured that require ONLY TWO standard system lengths. The following examples show some possible combinations of a system. Standard 5.0 meter System. A 5.0 meter Proximitor (330100-50-XX) needs a:

4.0 meter (330130-040-XX-XX) or 4.5 meter (330130-045-XX-XX) extension cable

with a:

1.0 meter (330106-XX-XX-10-XX-XX) or 0.5 meter (330106-XX-XX-05-XX-XX) probe.


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Proximitor

SIGNAL COMMON

TRANSDUCER POWER



Standard 9.0 meter system. A 9.0 meter Proximitor (330100-90-XX) needs a:

8.0 meter (330130-080-XX-XX) or 8.5 meter (330130-085-XX-XX) extension cable

with a:

1.0 meter (330106-XX-XX-10-XX-XX) or 0.5 meter (330106-XX-XX-05-XX-XX) probe.

Note: "XX" in the previous examples denote any other options. 9.0 or 5.0 meter system lengths are quoted as ELECTRICAL lengths and not physical lengths (although they will be close). As mentioned earlier, this is because probes and extension cables are trimmed in length to ELECTRICALLY match Proximitors. To allow maximum usable length extension cables are never physically shorter than their stated lengths but may be up to 30% longer.


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Component Mismatch Effects

PROBE GAP

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SHORT

CORRECT

LONG

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Performance Verification - Transducer Calibration Problems If the performance graph does not fall within specified limits, i.e., LINEAR RANGE less than 80mils, a scale factor outside 11mV, the first possible reason may be that one of the system components is mismatched. The PROBE, EXTENSION CABLE or PROXIMITOR is mismatched in electrical length making overall length too long or too short. The graph above shows effects of having a mismatched system. Where the graph shows a curve that is too LONG a 5 meter (50) Proximitor is used with a 9 meter cable (extension plus probe). Where the graph shows a curve that is too SHORT a 9 meter Proximitor is used with a 5 meter cable (extension plus probe).


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Power Supply Voltage Effects

-24V SUPPLY

PROBE GAP

-16V SUPPLY

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Performance Verification - Transducer Calibration Problems Transducer performance may also be out of tolerance if the provided -24 Vdc power source is out of tolerance. Voltages between -17.5 to -26.0 Vdc may be used by the transducer system; however, a loss of higher range response may occur as shown above. The graph above shows effects of supplying the Proximitor with a lower voltage of -16Vdc. Although the scale factor is within limits, the LINEAR RANGE has been severely reduced. Note that there is approximately a four volt DC offset from the power supplied to the maximum potential output of the transducer system. This means that the maximum output signal will be about four volts less than the power supply voltage.


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Probe Crosstalk

Performance Verification - Transducer Response Problems While some installed conditions may cause the transducer system to be out of tolerance, others may cause an incorrect or unacceptable response. For example, CROSSTALK occurs when two probes are mounted too close together so that their RF fields interact with each other. Probe RF frequencies are unlikely to be the same therefore when mixed together a DIFFERENCE frequency is generated. This difference is usually within the normal band of frequencies expected for vibration. Therefore, a target may appear 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 probe tip widths.


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Sideview Effect

Performance Verification - Transducer Response Problems Another installation problem called SIDEVIEW occurs when the probe is mounted in an area that has insufficient side clearance around its tip. Eddy currents will be generated in any conductive material within that area. This results in losses in the system that are not due to the real target. This problem may occur when a probe is being installed in a bearing, and it is not known whether the probe has cleared the mounting hole and approached the shaft correctly. If the installation decision is made solely on voltage readings, the installer may not realize that the probe is measuring the wrong surface.


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Effects of Target Size

YES

NO

Performance Verification - Transducer Response Problems The next installation issue to consider is TARGET SIZE. The surface area being observed by the transducer system must be large enough to make contact with all of the radiated RF field in FRONT of the probe. The minimum observed shaft diameter is 20mm (.80 in) for the 8mm probe. The effect on LINEAR RANGE and scale factor, with an under-sized target, will vary depending on the amount of eddy currents created.


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Target Material Effects

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4140 STEELTUNGSTENALUMINUMCOPPER

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Performance Verification - Transducer Response Problems The final transducer calibration issue is related to the surface material being observed by the transducer system. If the Proximitor nameplate does not give target material information, the target material must be AISI 4140 Steel. The following graph gives examples of the effect of different materials when observed by a Proximitor calibrated to a AISI 4140 Steel target. The Proximitor can be re-calibrated for some different target materials. This process must be implemented at the Bently Nevada manufacturing facility, or an authorized repair center.


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Proximitor Calibration Equipment

Performance Verification - Transducer Verification Tools The two components shown above can be used to thoroughly test the respons15 e of a transducer system. The Digital Voltmeter, or DVM, is used to measure the DC component of the transducer output, called the gap. The Bently Nevada TK-3 (Test Kit 3) can be used to mechanically input a signal to a transducer between 0 mil and 10 mils (thousandths of an inch) vibration. By rotating a cambered plate under the probe, a vibration signal can be generated; since the motor speed can be varied, different frequency inputs can be generated as well. In addition, the same rotating plate has a notch on one side, so it can be used to generate a Keyphasor signal in a transducer mounted there. Finally, for purposes of finding the calibration curve mentioned earlier, a spindle micrometer is mounted at the top of the unit. This is used to measure the distance to a probe while the attached DVM captures the DC voltage data.


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Proximitor Calibration Equipment Setup

Performance Verification - Transducer Verification Tools The DVM and TK-3 are shown above in the process of measuring the response of a transducer system.


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Q1. The three parts of a proximity transducer system are: a. __________________________ b. __________________________ c. __________________________ Q2. The ruggedised Proximitor cable used is comprised of what three components?

____________, ____________, and ________________. Q3. What is the electrical length of the following probe? 330106-05-30-05-02-00

___________. Q4. What is the electrical length of the following extension cable?

330130-045-00-00 ___________. Q5. What is the total electrical length required by the following Proximitor?

330100-50-00 ___________. Q6. The Proximitor must be supplied with a dc voltage between ______________, and ______________. Q7. An _______ field is created around the probe. It extends away from the face of the probe for linear range of at least ____________. Q8. When a conductive material is within range of the probe, what kind of

electrical flow is induced in the surface of that material? _________________. Q9. A Proximitor must be calibrated to suit the _____________ it has as a target.


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Q10. The dc output from the Proximitor will become less ______________ as the

target moves closer to the probe. Q11. A proximity system frequency response is from _____________

to ____________ and its output may contain an _________

and a _________ component.

Q12. Name three applications for a proximity transducer system. a. ___________________ b. ___________________ c. ___________________ Q13. Calculate the scale factor from the following: 90mils = -18.5Vdc, 10mils = -2.25Vdc. _____ Is it within tolerance? ___________ Q14. Name six reasons why a proximity system could be out of tolerance. a. ___ _______________ d. ____________________ b. __________________ e. ____________________ c. __________________ f. ____________________


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Proximity TransducerSystem Operation

Topic Objectives Revisited:

You will be able to explain the general construction and operation of the proximity probe system

You will be able to find probe calibration values both mathematically, and through the use of calibration equipment

You will be able to identify the conditions that lead to problems with proximity probes, including probe cable length(s), supply voltages, types of target material, etc

t2_proximity transducer system operation

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