Electrical Basics for sensors (Dynamic vs. PC420 series) Ron Denton
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 ?
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.
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
Ohm’s Law - Circuit example V = I * R I = V / R R = V / I Example 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
Voltage Modulated Sensors A.k.a.- Piezo FET ™ , ICP™, and others
Sensor Connection Circuit V = Sensor BOV Constant Current Diode (CCD) Typically 4 mA 18 - 30 VDC
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
Loop Powered Sensor <ul><li>What is a loop powered sensor ? </li></ul><ul><li>How does it work anyway ? </li></ul><ul><li>Is that different from the “current modulated” power that Wilcoxon uses on sensors now ? </li></ul><ul><li>Why would anyone want one ? </li></ul><ul><li>Who usually buys this kind of sensor ? </li></ul>
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
What’s the difference ? Piezo FET ® style VOLTAGE output AC output DYNAMIC sensors LPS™ Sensors CURRENT output DC output STATIC sensor output
Graphical Difference Piezo FET ® style LPS™ Sensors Sensor Type Vibration Signal Measured Signal
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
Frequency Response of PC420’s If PC420 series is DC output, why do they have a frequency response specification ? <ul><li>All sensors have frequency limitations imposed by electrical and mechanical design parameters </li></ul><ul><li>The PC420 series is no different </li></ul><ul><li>The “DC” output is derived from the basic accelerometer </li></ul><ul><li>That output signal is “converted” to DC by averaging </li></ul>
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 .
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.
Summary <ul><li>“ Dynamic” Sensors </li></ul><ul><li>Constant Current Supply </li></ul><ul><li>Voltage Output </li></ul><ul><li>AC Output </li></ul><ul><li>Bought by Maintenance (usually the vibration group) </li></ul><ul><li>LPS™ Sensors </li></ul><ul><li>DCS/PLC Voltage Supply </li></ul><ul><li>Current Output </li></ul><ul><li>DC Output </li></ul><ul><li>Bought by I&E (usually under the production department) </li></ul>