Trouble Shooting Industrial Accelerometer Installations

INTRODUCTION

Accelerometer based monitoring systems can be tested to verify proper installation and operation. Testing ensures data integrity and can identify most problems. The trouble shooting techniques presented in this paper are very simple and can be performed with most monitoring systems and data collectors or simple test equipment. Many installation and sensor problems can be detected by measuring the bias voltage of the sensor. The bias voltage will indicate bad cable routes and failed sensors. Many online systems are capable of trending the sensor bias voltage. Other problems can be detected by analyzing the time waveform and FFT spectrum. The following section will explain sensor operation and how it relates to the bias voltage, time waveform and FFT response. The next section is separated into different fault indications. Finally a trouble shooting chart is given.

ACCELEROMETER OPERATION AND RESPONSE

Most accelerometer faults can be diagnosed by measuring the bias voltage of the sensor amplifier. If the bias voltage is within correct limits the sensor is most likely operating properly. Most cabling faults can also be isolated by measuring the bias. After the bias is checked, the time waveform and FFT spectrum will verify fault diagnosis or proper operation.

AC Coupling and the DC Bias Voltage

The sensor output is an AC signal proportional to the vibration applied. This AC signal is superimposed on a DC bias voltage (also referred to as Bias Output Voltage (BOV) or sometimes rest voltage). The DC component of the signal is blocked by a capacitor thereby leaving the AC output signal. Most vibration data collectors, monitors, and sensor power units contain an internal blocking capacitor for AC coupling. If not included, a blocking capacitor must be field installed.

What is Bias Voltage?

The majority of accelerometers, PiezoVelocity Transducers (PVT¨), and many pressure sensors have a biased output. The bias voltage is also referred to as the bias output voltage (BOV) or sometimes rest voltage. Biased outputs are characteristic of two-wire sensors used to measure dynamic AC signals. Vibration and pressure are dynamic signals that vary with time. The BOV can be explained as follows:

The external power supply provides a dc voltage to the accelerometer. This power supply voltage is normally 18 to 30 Volts dc. The accelerometer amplifier circuit pulls this voltage down (or “Biases” the voltage) to a preset level. This BOV is normally 12 Vdc. Although this may vary depending on sensor manufacturer and design. The specification sheet for the accelerometer will provide information on the BOV for each model of accelerometer.

The BOV is determined by the amplifier design and is not adjustable. The BOV will remain the same regardless of the input power to the accelerometer, as long as the input power is within the specified range. For example, if the BOV is 12 Vdc and the input power is specified as 18 to 30 Vdc, then the BOV will be 12 Vdc if the input power is 18 Vdc. If the input power is increased to 30 Vdc, the BOV will remain at 12 Vdc.

The BOV is set by the interaction of the amplifier circuit in the accelerometer and the constant current power supply in the analyzer or data collector. Figure 1 is a diagram representing the performance of the circuit. Actually, the line represented by “Instrument Power” and BOV is one conductor that has two functions. So even though the power supply is providing a voltage higher than the BOV, the BOV voltage is the measured voltage level on the cable connecting the accelerometer to the data collector or analyzer.

The BOV is the zero reference of the AC signal and carries the dynamic vibration signal to the analyzer. The AC signal swings high and low from the BOV level and is limited by the power supply level and ground. For example, if the power supply level is 24 volts, the swing of the AC signal would be limited to not rising above 24 volts and not dropping below ground (0 volts). These limits are theoretical limits. In reality, the limits to this swing occur at about 1.5 Volts above ground and about 1.5 Volts below the power supply level. This is shown in Figure 2.

Most portable data collectors and on-line systems supply 24 volts to the sensor. The sensor selected should have a nominal BOV of approximately one half of the power supply voltage to maximize the amount of swing in the positive and negative directions. Most two-wire sensors produce an 8 - 14 volt bias. Figure 2 shows the change in peak amplitude range as the supply voltage is decreased.

When the signal amplitude runs into the supply voltage or ground, clipping occurs. Clipping the vibration signal distorts the waveform. In other words, it is no longer a true analog of the vibration it is attempting to measure.

Measuring the Bias Voltage.

One may ask: How can the supply voltage line be lowered to the bias voltage and still provide power to the sensor? This requires an understanding of the current regulator built into the power supply. This device is usually called the constant current diode (CCD). The CCD limits the current supplied to the sensor. It provides a constant current to the sensor regardless of the supply voltage or vibration voltage from the sensor. The use of unlimited power supply current will damage most internally amplified sensors. For this reason, most commercially available data collectors and vibration monitors have power supply circuits that include a CCD to regulate the power supplied to the sensor. Most battery power supplies contain a 2 mA CCD to ensure long battery life. Line powered supplies (where power consumption is not a concern) should contain 6 to 10 mA CCDs to drive long cables. For operation above 100^•C, limit the current to less than 6 mA to reduce self heating. Most data collectors supply 2 mA of current to the sensor; most on-line systems supply 4-6 mA. If the power supply is not current limited, then a CCD should be placed in series with the voltage output of the supply. Ensure that proper diode polarity is observed!

If the current limited power supply is probed with a voltmeter the supply voltage will be measured between the power supply and the CCD. The bias voltage will be measured on the side of the constant current diode connected to the sensor. Figure 3 shows a schematic of a sensor power supply containing a 2-10 mA CCD.

The BOV should be measured periodically to check sensor operation. The best measurement device is a voltmeter. However, most portable data collectors can measure the BOV if the sensor is powered from a different source (other than the data collector). When using the data collector as a voltmeter the dc voltage input is used. Oscilloscopes can also measure the BOV by selecting the dc coupled input.

The BOV can be trended with many on-line systems. Trending the BOV provides a record of sensor operation. If the sensor is disconnected or slowly develops a fault, the BOV data will show when the event occurred.

The BOV will also indicate the condition of the cabling and connectors. If the BOV level measurement is equal to the supply voltage, the sensor maybe disconnected or reverse powered. A measurement of zero volts indicates a short in the system. An unstable bias voltage can indicate poor connections, but can also be caused by a clipped signal or severe electro-magnetic interference.

Time Waveform and FFT Spectrum Fault Analysis

Time waveform can be measured with an oscilloscope, most data collectors and on-line systems. The time waveform will give an immediate indication of a clipped signal. Usually the signal looks truncated or flattened on one side and normal on the other. Severely clipped signals will cause the waveform to look jumpy. Poor connections can also cause a similar jumpy reading.

The FFT spectrum will give another quick indication of signal quality. The one times operating speed vibration is usually present and a good indication of proper operation. Presence of a large ski slop can indicate distortion from sensor overload. However a noisy accelerometer that has been integrated to velocity or displacement may also produce ski-slope for various reasons.

Cable routing faults can also be detected by analyzing the FFT. Multiples of line power frequency usually indicate improper shielding or grounding. If the time and frequency measurements read zero, the sensor is disconnected or is not operating.

FAULT INDICATIONS

Open Bias Fault: Supply Voltage (18 - 30 V)

When the measured bias voltage equals the supply voltage, the sensor amplifier is disconnected or reverse powered. In most cases the problem is in the connector or cabling. First check the cable termination at the junction box, data collector or monitoring system. If the cable is connected to a terminal block, make sure the wires are secure and in the proper terminal.

If a connector is used, it can be disassembled or replaced. However, avoid disassembling or removing the connector until all other fault sources have been checked.

Next check the cable connection to the sensor. Many times the sensor must be disconnected for maintenance - sometimes no one reconnected the sensor. If each end appears good, check all other terminations, splices and connectors. Also ensure that the cable is not crushed or cut.

If the cable route and connections appear good, further test the connectors. Cable continuity can be tested by shorting the signal leads to the shield wire and measuring the opposite end with an ohmmeter. Depending on the cable length, several ohms to several hundred ohms should be measured from each wire to the shield. If the cable and all connections are in proper working order the fault is in the sensor. However, open faults inside the sensor are very rare.

Short Bias Fault: 0 Volts

When the bias measures zero volts, power failure or a system short is usually the problem. First ensure that power is turned on and connected. If the power supply is on then there is usually a short in the cabling. Like the open fault, it is very rare to have a shorted inside sensor.

The most common fault location is in junction box terminations. Check to make sure that a frayed shield is not shorting across the signal leads. Many times a crushed cable can produce a short. Use an ohmmeter to check electrical isolation between the leads.

Disconnect the cable from all other devices and measure between all signal leads and shields. When measuring the resistance between the cable conductors, the ohmmeter should measure infinite or at least above 50 megaohms.

Damaged Sensor: Low bias, high bias

Out of specification bias readings other than those listed above usually indicate sensor damage. Common sources of sensor damage are exposure to excessive temperature, excessive shock, mis-powering, and electrostatic discharge. Excessive temperature is the most common cause of sensor failure. Sensors caught in a fire are usually destroyed and can show various bias readings depending on the failure mode within the sensor. Long term temperature failures are marked by a slowly rising or declining bias voltage. In many cases bias returns to normal as the temperature decreases. However, the damage to the amplifier is permanent and the sensor amplifier may continue to deteriorate. Figure 4 shows the bias trend of a sensor failing from long term temperature degradation in a paper machine dryer section. Note how the bias voltage increases when the machine cools during machine shut down. Excessive shock, mis-powering and electrostatic discharge can permanently damage the amplifier of unprotected sensors. Industrial sensors should contain protection devices to prevent failure.

Erratic Bias and Time Waveform

The bias voltage should remain stable and unchanging. Shifting bias indicates a very low frequency signal that is not filtered out by the bias meter. In rare cases this indicates an actual low frequency signal, however in most cases this indicates a fault. Primary causes of erratic bias are thermal transients, poor connections, ground loops, and signal overload. Each of these faults will also be visible in the time waveform as an erratic jumping or spiking of the signal.

Thermal transients cause uneven thermal expansion of the sensor housing materials. This can be detected by the sensor as a low frequency signal. The problem is most evident when using low frequency sensors.

Poor or contaminated connections can also cause low frequency bias and contact noise. Look for corroded, dirty or loose connections. Repair or replace the connection as necessary. Non-conducting silicone grease should always be applied to connectors to reduce contamination.

Ground Loops are developed when the cable shield is grounded at two points of differing potential. Always ground the shield at one end only! An easy test for ground loops is to disconnect the shield at one end of the cable. If the problem disappears it was probably a ground loop fault. Figures 5 and 6 show a connection susceptible to ground loops and a correct installation where the shield is tied at one end only.

Sometimes spurious spikes from fast thermal shifts, lightning strikes, and shocks can overload the sensor and cause a momentary shift in the bias voltage. The shift in bias can trigger alarms and protection system shutdown devices. To prevent triggering alarms and shutdown a longer delay can usually be programmed or hardwired into the monitoring system. The delay prevents the system from taking action until the sensor has settled.

High frequency, high amplitude vibration signals can also overload the sensor and in severe cases cause bias shift and erratic time waveform. However, overload problems are usually detected by observing truncated waveforms and large ski-slope spectrums.

Truncated Time Waveform: Sensor Overload

Truncated (flattened) time waveforms indicate that the signal is clipping into the supply voltage or ground. The clipping causes the amplifier to saturate and become overloaded. Some common causes of overload are severe pump cavitation, steam release, impacts from loose or reciprocating parts and even gearmesh.

One way to reduce clipping is to use a higher power supply voltage and ensure that the bias voltage is centered between supply voltage and ground voltage. However the bias voltage and power supply are rarely adjustable. For example, if you are using a 15 volt power supply and a 12 volt bias, clipping will occur sooner than if you used a 20 volt power supply.

Long cables in excess of 200 ft. can also reduce the amplitude swing at high frequency and may be a problem in some applications. The easiest solution is use a lower sensitivity sensor. A sensor with 10 mV/g sensitivity will have a hundred times more high amplitude range than a similar 1V/g sensor.

Ski-slope Spectrum

Sensor overload may also produce a ski-slope spectrum. If the amplifier saturates, intermodulation distortion occurs. This causes low frequency noise also referred to as washover distortion. Figures 7 and 8 show distortion due to pump cavitation and gear mesh overload.

Sometimes the ski-slope response can be caused by the circuits used to integrate acceleration signals to velocity or displacement. High amplitude vibration from other close machines can also produce ski slope problems. In these cases the ski-slope levels are much lower. Figure 9 shows integration noise on an accelerometer. Figure 10 shows ski-slope due to ambient machinery noise at very low frequency.

Mounting Resonance Spectrum

Mounting resonance can give false indication of high frequency machinery faults such as gear mesh and bearing problems. The problem is most evident when using probe tips and magnets. However mounting the sensor on thin plates such as machine guards can also lower the mounting resonance. Figure 11 shows the resonance of several common mounting techniques - Mounting Resonance Plots for: (A) Probe Tip; (B) Magnet; (C) QuickLINK; and, (D) Stud Mounted Configurations

Re-measuring with higher resolution will usually discriminate machine vibration from mounting resonance. However amplitudes of machine signals at sensor mounting resonance will be greatly increased. In some cases the mounting resonance can cause sensor overload if it is excited by a machinery.

Line Frequency Harmonics in Spectrum

Harmonics of AC line power frequency usually indicate interference from motors, power lines and other emissive equipment. First ensure that the sensor shield is grounded (at one end only!). If the shielding is good, check the cable routing. Avoid running the cable along side high voltage power lines and only cross power lines at right angles. For example, if a power cable is 440 Volts and the vibration signals from the sensor are at the milli and microvolt levels, any cross talk can severely corrupt the data.

Trouble Shooting Chart

Attached is a trouble shooting chart for sensors with a 12 volt bias. For sensors with other bias voltages, the same concepts apply — only the stable bias range will be different.

BOV	Spectrum	Time Waveform	Fault Condition	Action
0	No signal	No signal	No power or cable/connector short	Test/turn on power Test cable isolation Repair/replace cable
2.5 - 5V	No signal	No signal	Damaged amplifier	Replace sensor
10 - 14V Stable	High low frequency “ski slope”	High amplitude high frequency noise	High frequency overload (steam release, air leak, cavitation, etc.)	Repair steam leak/dump Use less sensitive sensor Place rubber pad under sensor
10 - 14V Stable	Very high low frequency “ski slope” no high frequency signal	Choppy	Damaged amplifier	Replace sensor
10 - 14V Stable	Good signal strong 50/60Hz	50/60 Hz	Inadequate shielding	Connect/ground cable
10 - 14V Stable	High low frequency noise	High frequency spikes	ESD Arcing impacts	Reroute cable Use less sensitive sensor Place rubber pad under sensor
10 - 14V Stable	High low frequency noise	Jumpy/Choppy	Intermittant connection	Repair connection
18 - 30V	No signal	No signal	Reversed powering	Reverse leads
18 - 30V	No signal/weak 50/60 Hz	No signal	Open cable connections	Repair connection