128122-accelerometer & velomitortransduceroperation

8/10/2019 128122-Accelerometer & VelomitorTransducerOperation

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Accelerometer and Velomitor

Transducer System Operation


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Accelerometer and VelomitorSystem Operation Page 1

Rev C 128122

I. INTRODUCTION

An acceleration transducer, much like the velocity transducer, is an

electromechanical device that converts one form of energy to another. In the case

of the accelerometer, this conversion is from mechanical movement into a voltage

output. From this output meaningful casing vibration data can be derived for

frequencies as high as 30,000 Hz (30 KHz).

A. Accelerometer Transducer System

1. The acceleration transducer

(Figure 1) assembly is composed

of a CASINGandBASE

assembly, aPIEZOELECTRIC

CRYSTAL, anINERTIAL

REFERENCE MASS, and an

ELECTRONICS COMPONENT.

To complete the system an

INTERCONNECT CABLEand

INTERFACE MODULEprovide

the transfer medium and signal conditioning necessary to interface with

a readout device or monitor.

2. The CASING and BASE assembly forms the housing for the

transducer mechanism. The base part of the assembly provides the

mounting interface with the machine. It is through this base mounting

MICAINSULATOR

CONDUCTIVE

PLATE

ELECTRICAL

INSULATOR

ELECTRONICS

COMPONENT

PRELOAD BOLT

MASS

PIEZOELECTRIC

CRYSTAL

BASE

ASSEMBLY

Figure 1 - Cut-Away View of Accelerometer


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In addition to the standard and high frequency acceleration transducers

described above, additional acceleration transducer designs are

available for special applications.

5. The part numbering scheme for the acceleration transducer follows

the general form:

MODEL NUMBER-A-B

Where MODEL NUMBER-

A = acceleration transducer model number.

B = transducer mounting adaptor option (when available).

RESONANCE

FREQUENCY

SYSTEM

ACCELEROMETER

FREQUENCY cpm

Typical Amplitude vs Frequency Response

Figure 2 - Typical Accelerometer Response Characteristics

Velomitor

A third type of transducer, the Velomitor, incorporates the

accelerometer design with the addition of an internal

integration circuit for measurement outputs in velocity.Velomitor operation will be described later.


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B. Interconnect Cable

1. TheINTERCONNECT

CABLEfor the transducer

system (Figure 3). Allows

you to connect the

acceleration transducer to

the interface module and

the module to the monitor.

Standard lengths will vary

from 1 foot to 300 feet from transducer to module and up to 1000 feet

from module to monitor. Cables will typically be of either coaxial or

twisted 18 or 22 AWG shielded construction and available with or

without armor. See individual specification sheets and manuals for the

cable composition and length limits of each transducer model.

2. The part numbering for the interconnect cable will follow the general

form:

MODEL NUMBER-A-B

Where:

MODEL NUMBER = cable model number

A = cable length option

B = cable armor option (when available)

INTERCONECTCABLE (coaxial)

INTERFACE MODULE

ACCELEROMETER

Figure 3 - Typical Acceleration Transducer System


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Example 1: 21165-25-01

21165 = Bently cable for Bently high frequency accelerometer

(24147).

25 = 25 foot cable length.

01 = with armor

Example 2: 18622-19-00

18622 = Bently cable for standard frequency accelerometer (23732).

19 = 19 foot cable length.

00 = Teflon coaxial cable without armor.

Example 3: 45358-09

45358 = Bently standard temperature cable for aeroderivative

applications.

09 = 9 meter maximum cable length.

C. Accelerometer Interface

Module

1. The housing for the

standard and high frequency

INTERFACE MODULE

(Figure 4) is very similar in

construction to the 3300

Proximitor. Alternative

-VT (-18 TO -24 VDC)

COM

OUTPUT

INPUT

FROM

TRANSDUCER

TERMINAL

BARRIERCONNECT

Figure 4 - Accelerometer Interface Module Housin


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housing designs exist, however, for other transducer models. It

performs a multitude of functions necessary for the accelerometer

transducer system to operate properly.

Included in its functions are the following:

a. Provides a constant current source for the accelerometer

transducer.

b. Amplifies the accelerometer output signal to 100 mV/g. To

achieve this, the standard frequency module will amplify the signal by

4X (from 25 mV/g ) and the high frequency module by 10X (from 10

mV/g).

c. Provides the proper bias level of -8.5 Vdc for the output signal for

OK detection.

d. Provides a comparator circuit for OK and NOT OK detection.

Circuit will differentiate between a short or open circuit and provide

to the monitor an output within -2.5 V of common for a short circuit

or within -3 V of the input supply voltage for an open circuit

condition.

e. Provides the necessary electronics for driving large capacitive

loads (long cables) (Figure 5).


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2. The part numbering for the interface module will follow the general

form:

Model Number-A

Where:

MODEL# = module model number

A = series number

Note: Interface modules designed for special applications may have

additional available options.

Example1: 23733-02

23733 = Interface module for 23732 standard temperature

accelerometer.

02 = model with circuitry power supply or -24 Vdc only (see manual

TW8029278).

10 (dB)

0 (-0.9dB)

8 (-1.9dB)

7 (-3.1dB)

6 (-4.4dB)

5 (-6dB)

4 (-8dB)

3 (-10.5dB)

100K10K10010 1K

Zero through 0.01f

0.022f

0.05f

0.1f

0.5f1.0f

PEAK

TO

PEAK

OUTPUTVOLTAGE(-VDC)

FREQUENCY (Hz)

Typical Accelerometer Interface Module

Frequency Response vs Capacitive Loading Figure 5


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Example 2: 89130-01

89130-01 = interface module for 89129 accelerometer. No other

standard options.

Example 3: 86497-01-02-01-00

86497 = interface module for aeroderivative applications

01 = 40 Hz high pass option

02 = 350 Hz low pass option

01 = AC power supply

00 = no approvals

required

II. INTRODUCTION - Principles of Operation

The accelerometer is best suited for measuring very high frequency vibration

signals; for example, in blade passage applications, and on gearboxes and high

speed machines with roller element bearings. Displacement levels may be very

low at higher frequencies and velocity measurements may be rendered unreadable

at higher levels by the electronic limitations of the Seismoprobe. Acceleration

measurements, on the other hand, can provide critical information about the

machine operating condition.

A. Operating Principles

Scale Factor

Accelerometer scale factors and optionsmay vary depending upon the manufacturer


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1. When the transducer experiences a vibration source above its

minimum operating frequency the inertial reference mass contained in

the transducer will remain motionless. Motion at the transducer base

will put cyclic (increasing and decreasing) compression on the

piezoelectric crystal.

2. This cycle compression of the piezoelectric crystal induces an

electric charge (measured in picocoulombs (Pc)) across opposing

faces of the crystal. This charge is proportional to the acceleration of

the crystals movement.

3. In order for this electric charge to be employed in the measurement

of meaningful data, it must be amplified to a useful level. A charge

amplifier is, therefore, contained within the transducer which will

convert the signal from picocoulombs/g (Pc/g) to millivolts/g (mV/g).

The output will then have a scale factor of 25 mV/gpfor standard

transducers or 10 mV/gpfor high frequency transducers over the rated

frequency range. The signal is then sent to the interface module for

further conditioning.

NoiseA certain amount of signal noise will be generated in the charge amplifier. As

a result, a small output will be created even when there is no vibration source.

The reader should also be aware of other external sources of noise for each

transducer application. See the noise section for further details.


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4. The interface module is used to provide power to the transducer

unit and to amplify and provide proper biasing of the signal using the

components outlined in the previous section. The supply voltage is

either -18 and -24 Vdc depending on transducer model, the output

sensitivity from the standard interface module is 100 mV/gpwith a -

8.5 Vdc signal bias level (these values may also vary for different

transducer systems). This sensitivity will extend over the rated

frequency of the individual transducer model.

B. Operating Limitations

1. The accelerometers lower limit of operation is defined by the

motion of the inertial mass with respect to the rest of the transducer

assembly. At very low frequencies the case and inertial mass will move

together. Since, in this case, there is no relative movement between the

two, there will be no cyclic compression of the piezoelectric crystal, and

therefore, little or no output from the transducer. As the measured

frequency increases to such a degree that the inertial mass first remains

motionless (similar to the motionless of the bobbin on the

Seismoprobe) the piezoelectric crystal will be placed in cyclic

compression since the base assembly is still moving with the machine.

Other ApplicationsSpecial transducers, such as those used in aeroderivative applications, have

been designed with alternative input voltages and output sensitivities to suit

particular application requirements.


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2. Above the low end of the transducers output curve, the output stays

linear for a linear increase in acceleration. As the transducer approaches

the upper limit of operation, however, this will not be the case.

The upper limit of the acceleration is defined by the natural resonant

frequency of the piezoelectric crystal. As the measured frequency

approaches the natural resonant frequency, the output amplitude of the

transducer will begin to increase in non-linear fashion accompanied by a

corresponding phase shift when compared to the amplitude and phase

input to the transducer (Figure 6). Since both of these conditions will

quickly introduce significant errors in both amplitude and phase to the

measurement, operation outside the transducer linear range is

Mounting considerations

Mounting methods and location can often have an enormous effect ontransducer response throughout its operating range. Refer to the

transducer installation section for further details.

Typical Amplitude vs Frequency ResponseRESONANCEFREQUENCY

FREQUENCYSYSTEM

ACCELEROMETER

Figure 6


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undesirable. Accelerometers are chosen for application where the

operating range is well below the transducers resonant frequency but

above its lower limit of operation.

Both the lower and upper limits of the accelerometer operating range are

commonly defined as the frequencies where the amplitude/frequency

performance curves exceeds a certain deviation from linear response.

The limits for the standard (23732) accelerometer are 3 dB (30%) for

a range of operation from 10 Hz to 20 kHz or .5 dB (5%) for a range

of operation from 30 Hz to 10 kHz.

The upper end of the transducers operating range will, as with other

transducers, be affected by the capacitance of the cable, i.e. its total

length from transducer to monitor.

C. Advantages and Disadvantages

There are both advantages and disadvantages when the accelerometer is

compared to a velocity Seismoprobe or displacement transducer.

1. Advantages

a. Extended Range - the accelerometer allows for a much broader

frequency measuring range than any of the other transducer types. It


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has an operating range from 10 Hz up to 30 KHz compared to only

270 Hz to 1000 Hz for the Seismoprobe velocity transducer.

b. Available for extended temperature ranges.

c. Simple external installation. (See transducer section for more

information)

d. Good for high frequency casing measurements.

e. Increased ruggedness from that of the Seismoprobe transducer.

Because of the solid-state nature of its design (no springs or bobbin),

the accelerometer can be expected to have a more extended operating

life than the Seismoprobe.

2. Disadvantages

a. Wide frequency range makes the accelerometer more susceptible

to noise and spurious vibration sources.

b. Difficult calibration check; requires special equipment.

c. Often requires filtering in monitor.


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d. Vibration information is not direct shaft information. This is

because the transducer is externally mounted and measures vibration

from the casing.

e. Poor response at low frequencies.

III. The Velomitor

A. The Velomitor

Piezovelocity sensor (Figure

7) has incorporated the

accelerometer transducer

system design with an

integrator circuit in order to

provide velocity information

for bearing housing

applications. Although similar in size and appearance to an accelerometer

transducer, the design of the Velomitorincorporates nearly all of the

necessary electronics within the housing of the transducer. In other words,

no interface module is necessary to properly link with the monitor. Only a

constant current source (level=.002 mA) is required to provide the

Velomitor with a constant input current to the sensor.

Figure 7


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The basic construction and theory of operation of the Velomitor is the same

as that for the accelerometer. The

output signal is developed from a

charge induced on a piezoelectric

crystal, amplified and then

integrated to provide velocity

information to the monitor with an

output sensitivity of 100 mV/in/s.

Because of the integration process

used in the Velomitor, a significant

phase error will be introduced to the signal at the low end of the

transducers operating range (Figure 8). This is due to the integration of the

noise portion of the signal which is present at lower frequencies.

B. Advantages and Disadvantages

There are both advantages and disadvantages to the Velomitor design when

compared to a velocity Seismoprobe.

1. Advantages

a. Its extended range. The Velomitor has an operating range from 10

Hz to 5000 Hz compared to only 270 Hz to 1000 Hz for the

Seismoprobe velocity transducer.

Frequency (Hz)

Typical Phase Shift Between Output

and Vibration

Typical Phase Response

(at Low End of Operating Range)

Figure 8


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b. Increased ruggedness - similar to that of the accelerometer.

c. Ruggedness or stiffness of design also makes the Velomitor

much less susceptible to cross axis vibration.

d. Containing the integral electronics in the transducer means that the

noise created in signal transfer through the extension cable will not be

amplified as is the case with the accelerometers interface module.

2. Disadvantages

a. The integral electronics of the Velomitor is susceptible to

temperature variations or transients. Care must be given to strictly

maintaining the transducer within thermal specifications for

operation.

b. Inherent presence of noise from signal amplification (creates a

signal output even when no input is present).

C. The part numbering scheme for the Velomitor Piezo-velocity transducer

follows the general form:


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MODEL NUMBER-A-B

Where:

MODEL # = Velomitor model number

A = transducer mounting adapter option

B = Agency approval option

Example 1: 330500-03-00

330500 = Velomitor model number

30 = with -28 UNF threaded mounting stud

00 = No agency approval required

Example 2: 330500-02-02

330500 = Velomitor model number

02 = M8 X 1 threaded mounting stud

02 = BASEEFA agency approval

The part numbering scheme for the Velomitorcable follows the general

form:

MODEL NUMBER-A

Where MODEL NUMBER = acceleration transducer model number

A = Cable length option (min. 3 feet max 99 feet)

Example 1: 85661-36

85661 = 22 AWG armored, 2 conductor cable

36 = 36 feet long


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Application Exercise

1. Why is the design of the accelerometer and Velomitor transducer systems

considered to be similar?

______________________________________________________________

______________________________________________________________

______________________________________________________________

2. Why is the interface module a necessary part of the acceleration transducer

system?

______________________________________________________________

______________________________________________________________

______________________________________________________________

3. What are the interface modules (5) principle functions?

1.__________________________________________________________

2.__________________________________________________________

3.__________________________________________________________

4.__________________________________________________________

5.__________________________________________________________


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4. What are some of the differences between the accelerometer and Velomitor?

a) the signal output units of measure:

___________________________________________________________

b) Location of electronics components:

___________________________________________________________

5. What two cable designs are used to connect accelerometer transducers to the

interface module?

________________________________________________________________

________________________________________________________________

6. What component of the displacement transducer system is the standard (and

high frequency) interface module very similar in appearance to? Is this the case

for all other acceleration transducer models?

________________________________________________________________

________________________________________________________________

________________________________________________________________

________________________________________________________________

7. Which transducer system has been specified without an interface module?

________________________________________________________________

________________________________________________________________

________________________________________________________________


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8. What maximum capacitive load from the interface cable is allowable without

signal degradation (reduction of the transducers sensitivity range) for the high

frequency accelerometer?

________________________________________________________________

9. Name 2 advantages that the Velomitor transducer design has over the

Seismoprobe design. Name 2 disadvantages.

ADVANTAGES DISADVANTAGES

1.________________________ 1.______________________

2.________________________ 2.______________________

128122-accelerometer & velomitortransduceroperation

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