Electrical Basics for sensors(Dynamic vs. PC420 series)

Ron Denton

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Electrical Basics for sensors(Dynamic vs. PC420 series)

What is Ohm’s law ?

How does a “constant current/voltage modulated” sensor work ?

How does a “Loop Powered Sensor” work ?

What’s the difference ?

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Ohm’s Law

Mathematical expressions of Ohm’s law:

V=I*R I=V/R R=V/I

where: V is Voltage, I is Current, and R is Resistance

Each of these equations is identical, they are simply re-arranged to solve for one particular variable.

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Ohm’s Law - Units and Symbols

Voltage: Expressed in Volts or V, and possibly modified by

(Alternating Current) as VAC or (Direct Current) as VDC

Current: Expressed in Amperes, Amps or A

Resistance: Expressed in Ohms or

Unit multipliers: k = kilo = 1,000 ; M = Mega = 1,000,000

m = milli = 1/1,000 ; p = pico = 1/1,000,000

Examples: 100 millivolts = 0.1 volts ; 20 milliamps = 0.02 amps 1.5 kVDC = 1,500 Volts DC

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Ohm’s Law - Circuit example

V = I * R

I = V / R

R = V / IExample 1: R = 250 , I = 20 mA ; find V V = 0.020 x 250 = 5.0 volts

Example 2: V = 1.0 volt, R = 250 ; find I I = 1.0 / 250 = 0.004

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Voltage Modulated Sensors

A.k.a.- PiezoFET™ , ICP™, and others

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Sensor Connection Circuit

V = Sensor BOV

Constant Current Diode (CCD) Typically 4 mA

18 - 30 VDC

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Sensor Output “rides” on BOV

The vibration signal is superimposed on the BOV

The average value of the output signal will always be the BOV

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Measurement is AC coupled

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Loop Powered Sensor

• What is a loop powered sensor ?• How does it work anyway ?• Is that different from the “current modulated” power that Wilcoxon uses on sensors now ?• Why would anyone want one ?• Who usually buys this kind of sensor ?

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4-20 mA Loop Powered Sensor (LPS™)

The sensor controls the current flowing in the loop

0.0 inch/sec. = 4 ma

1.0 inch/sec. = 20 ma

Example: Voltage Output @ 1 in/sec = .02 x 250 = 5 volts ; @ 0 in/sec = .004 x 250 = 1.0 volt

V+ = 24 VDC, R = 250

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What’s the difference ?

PiezoFET® style

VOLTAGE output

AC output

DYNAMIC sensors

LPS™ Sensors

CURRENT output

DC output

STATIC sensor output

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Graphical Difference

PiezoFET® style

LPS™ Sensors

Sensor Type Vibration Signal Measured Signal

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LPS™ Sensor Operation

“0.34”

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Time Response of LPS™

The PC420 output follows the signal peak

The PC420 has a 1 to 2 second ‘lag’ in responding to a sudden shift

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Frequency Response of PC420’s

If PC420 series is DC output, why do they have a frequency response specification ?

•All sensors have frequency limitations imposed by electrical and mechanical design parameters

•The PC420 series is no different

•The “DC” output is derived from the basic accelerometer

•That output signal is “converted” to DC by averaging

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Why would anyone want an LPS™ ?

Connecting an LPS™ to a plant’s Distributed Control System (DCS) or a Programmable Logic Controller (PLC), allows the overall vibration of machinery to be monitored more frequently than a plant’s predictive vibration program.

Trending this overall vibration can spot trouble faster than a plant’s predictive vibration program.

This is very valuable for critical machinery.

The data can be made available to machine operators.

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Who usually buys this kind of sensor ?

Initially, the predictive vibration group would have to be involved in the selection of a PC420 sensor.

Usually the sensors are purchased by the Instrument & Electronic (I&E) group.

The money usually comes from the production budget, not the maintenance budget.

There is always more money available from the production budget than from the maintenance budget.

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Summary

“Dynamic” Sensors

• Constant Current Supply

• Voltage Output

• AC Output

• Bought by Maintenance (usually the vibration group)

LPS™ Sensors

• DCS/PLC Voltage Supply

• Current Output

• DC Output

• Bought by I&E (usually under the production department)