Advanced Vibration Meter Principles of Operation

The ALTA® Advanced Vibration Meter uses a silicon micromachined, variable capacitance accelerometer to measure vibration (Acceleration, Velocity, Displacement, or acceleration peak), frequency (Hz/RPM), and crest factor on all three axes, duty cycle (how much of the report interval vibration was present), and Temperature of the system to which it is attached.

The Advanced Vibration Meter uses an accelerometer to capture g-force on all axes and then calculates vibration, frequency, and crest factor from that acceleration data. The meter reports the Duty Cycle as a percentage of how long the vibration was present during the Heartbeat. A single measurement consists of gathering 256 acceleration data points, analyzing those data points to produce vibration data, and then taking a temperature measurement. The meter will measure on a configurable measurement interval or Heartbeat. Only the most recent set of data points is reported on each Heartbeat.

Example Applications

Determine a Vibration Baseline

Before setting final configurations, one of the most critical steps is gathering baseline readings. A baseline is a set of average readings the sensor measures while the machine, equipment, or system is working well. Before determining this baseline, it’s important to determine your system’s typical operating frequency (or RPMs). If this isn’t clear from the system’s technical literature, this can be determined experimentally with the meter.

Find the typical operating frequency experimentally:

1. Set the Vibration Mode to Acceleration RMS, Power Mode to High Performance, and Accelerometer Range to 16 G.
2. Set the Sample Rate and the Frequency Range Min/Max settings on the meter, considering what you think the system’s operating frequency might be (where the assumed operating frequency falls just below the middle of the Frequency Range settings).
3. Observe the readings of the meter while the system is running. It’s best to gather several readings for a more robust picture of the typical operating frequency.
4. Adjust the Sample Rate up and down and repeat Step 3, looking for what frequency produces the greatest Acceleration RMS readings. Note: Setting the Data Mode to Maximum and Measurement Interval to about 10x less than the Heartbeat can help speed up this process.
5. Once you determine what frequency produces the greatest Acceleration RMS readings, you have found the typical or fundamental operating frequency.
6. Now, adjust the Sample Rate so the typical operating frequency falls within the lower third of the Frequency Range.

This is desirable since harmonics (multiples of the typical operating frequency) may appear when system health degrades. The typical frequency in the lower third leaves better bandwidth at the upper end of the Frequency Range to see harmonics as they arise.

With an appropriate frequency setting found, all that is left is determining the preferred Vibration Mode. The Frequency Range of interest typically determines the ideal Mode.

Select a Vibration Mode:

1. Acceleration: Best for higher frequencies where frequencies of interest are above 1 KHz (60,000 RPM).
2. Velocity: Best for medium frequencies where frequencies of interest fall between 10 Hz and 1 KHz (600 RPM to 60000 RPM).
3. Displacement: Best for low frequencies where frequencies of interest fall below 20 Hz (1200 RPM).

We can now determine the baseline with Vibration Mode and Frequency settings applied. Take appropriate measurements while the system runs and use a combination of Average and Maximum Data modes to determine the baseline.

Using the Baseline

With the baseline determined, set the meter with the desired configurations and Vibration Thresholds. Once the meter is positioned with its final configurations, set up Rules in iMonnit to be notified of readings that significantly deviate from the baseline values.

Even without expert analysis, deviation from the baseline can be interpreted as a change in your system that may require further attention. This may be an appropriate time to contact a technician or expert to assess the system more closely for potential issues. The value of using this meter and this approach is that the system can be evaluated, and often problems addressed long before it becomes a much more expensive repair, preventing unscheduled downtime and more extensive system damage.

Measurement Intervals and Sample Rate

Sample Rate

As mentioned above, the meter takes 256 samples at the configured Sample Rate on each Measurement Interval. These samples are used for two purposes:

1. To determine if the measurement is outside the configured Aware State Threshold and if the meter should immediately transmit an Aware State reading to the gateway.
2. To determine the Duty Cycle reading reported when a Heartbeat occurs.

If the Aware State Threshold wasn’t breached, the sample is noted toward the duty cycle reading reported on the Heartbeat and otherwise disregarded. Unless a Measurement Interval sample is determined to have breached the Aware State Threshold by the dominant frequency, the vibrational data is never reported and can’t be reviewed. Only readings that enter/leave the Aware State will be transmitted to the gateway.

This meter performs Fast Fourier Transform (FFT) calculation, and the FFT data is not reviewable. This is performed on each sample taken by the meter on each Measurement Interval, and the dominant frequency is reported. The Sample Rate can be adjusted to different frequencies. This is key for yielding readings that report vibrational data for the desired frequency range.

The Sample Rate has a direct correlation with what frequencies can be detected. The higher the Sample Rate, the more frequencies can be observed. Therefore, you should configure your sensor’s Sample Rate to include the frequency of the vibrational data you monitor. Configure the Sample Rate too low, and it will not detect the higher frequencies you want to observe. Configure the Sample Rate too high, including noise from frequencies higher than the desired frequency, and you will see inconclusive readings.

Measurement Intervals

The Advanced Vibration Meter is not always listening like some Monnit trigger-type sensors (such as an Open-Closed Sensor). This sensor takes samples according to the configured Sample Rate on the configured Measurement Interval. This operational detail is why the meter is excellent for applications in which vibrational events occur in periodic intervals and not ideal for applications in which a specific short-term event occurs (such as impact detection). See the application example below:

Configuration Example

You’re monitoring the health of a Class I motor, which operates at 24,000 RPM (400 Hz) with an intensity of 0.7 mm/s during normal operation. The motor is always running, so you can monitor the vibration signature to see if the motor may need servicing (when the frequency or intensity increases beyond the expected values) or if the motor is no longer running (accidental shut-off).

Sensor configuration

Heartbeat: 20 minutes

Aware State Heartbeat: 10 minutes

Vibration Mode: Velocity

Vibration Aware Threshold: 2 mm/s

Vibration Hysteresis: 0

Minimum Sensitivity (g): 0.1 g

Window Function:: Rect

Accelerometer Range: 8 g

Measurement Interval (seconds): 60

Sample Rate: 800 Hz

Frequency Range Minimum (Hz): 188 Hz (so configured since the motor will never report less than 188 Hz during operation)

Frequency Range Maximum (Hz): 800 Hz (so configured since the motor would be damaged and not operational if it reports more than 700 Hz)

Synchronize: Off (default)

Failed transmissions before link mode: 3 (default)

With this configuration, the Advanced Vibration Meter will always check in with its standard Heartbeat Interval of 20 minutes. During normal operation, the meter will report vibration (frequency and amplitude on X, Y, and Z axes), Duty Cycle (what percentage of time during the Heartbeat Interval the meter detected vibration above the Minimum Sensitivity), crest factor, and temperature readings every 20 minutes. While the motor runs, we expect a frequency reading of approximately 400 Hz (24,000 RPM) and an amplitude of roughly 0.7 mm/s. The meter will check every 60 seconds between the 20-minute Heartbeats to determine:

1. The Aware State Threshold of .2 mm/s (as configured in the Vibration Aware Threshold).
2. The Duty Cycle that the samples calculate (any Measurement Interval that detects the intensity of .1 g, as configured in the Minimum Sensitivity).

If the Measurement Interval does not breach the .2 mm/s threshold, the sensor will discard the reading and go back to sleep until the next Measurement Interval and repeat.

Since the standard Heartbeat is 20 minutes, there will be 20 Measurement Intervals that occur during a standard Heartbeat. Each of these Measurement Intervals will take a sample of 256 samples at the configured Sample Rate (in this case, 800 Hz); since the meter takes 256 samples at 800 times per second, the actual time it takes the meter to complete this Measurement Interval sample is .32 seconds (256 samples divided by 800 Hz).

Entering and Leaving the Aware State

If the meter detects that the vibration intensity goes over .2 mm/s during one of the Measurement Intervals occurring every 60 seconds, the meter will wake its radio and transmit that reading to the gateway. So if the motor started to fail and vibrate excessively, you could configure an Action to trigger in this scenario to notify you that the motor is outside its expected operational threshold.

Once the meter enters its Aware State, it checks in every 10 minutes (Aware State Heartbeat configuration). It continues to take samples on the Measurement Intervals. If the intensity continues over the configured Vibration Aware Threshold, it checks in every 10 minutes. If the meter detects the intensity below .2 mm/s, it wakes its radio, reports the reading to the gateway, and checks in every 20 minutes with its standard Heartbeat Interval.

Mode Considerations

The Mode you select will determine how the data is analyzed and reported. While this configuration would depend on your specific application, and an expert in that application should decide which Mode to use, the following guidelines may be helpful.

1. Acceleration: Use for higher frequency vibration 200 Hz and above

   a. mm/s^2

   b. Harmonics in motors (which can be indicative of a failing motor)

2. Acceleration Peak: Same units as Acceleration but indicates the peak acceleration only.

   a. Helps determine the strongest force that acts on the system per measurement cycle.

3. Velocity: Use 10 to 200 Hz

   a. mm/s

   b for mid-range frequency vibration. Motor health monitoring

4. Displacement: Use for low-frequency vibration 10 Hz and below

   a. mm

Fundamental Frequency

The frequency that is represented in the data is the Fundamental Frequency. In the case of our meter, the fundamental is the frequency that a majority of the vibration energy is present. The vibration metrics (Acceleration, Velocity, or Displacement) represent the energy in the frequency range defined in the iMonnit UI. For example, you may only have 1000 mm/s^2 of Acceleration contributed by the fundamental frequency. Still, there may be 1500 mm/s^2 total in the spectrum, and that 1500 value will be reported as data.

Crest Factor

The Crest Factor is peak acceleration / RMS acceleration (a type of average). This value starts at 1.41 and goes as high as 3.96. This unitless metric indicates how “noisy” a system is. Lower values are better and indicate only the fundamental frequency/vibration is present in the signal. Larger values indicate more noise and the presence of non-fundamental frequencies in the signal. These non-fundamental frequencies are typically called harmonics and indicate a motor is unhealthy.

Duty Cycle

The Duty Cycle is a percent of how many measurements in the Heartbeat detected enough vibration to be analyzed.

Temperature

The Temperature is a simple temperature measurement from a thermistor. The thermistor may be embedded in the meter cube if it’s a leaded meter or the enclosure for the non-leaded option.

Available configurations

Vibration Mode

Options: Acceleration Peak, Acceleration RMS, Velocity RMS, Displacement

The meter is user-configurable to produce one of four different vibration data types.

Vibration Aware Threshold (VAT)

Configurable Range: 0 to 65530 mm/s^2 for Acceleration, 0 to 655.35 mm/s for Velocity, 0 to 655.35 mm for Displacement

The meter reports immediately when it crosses this threshold (in either direction). This threshold works with the measurement interval since the sensor can only detect changes after measurement. When the sensor goes above this threshold, it will report Aware. When the sensor goes below, it will report Not Aware.

Vibration Hysteresis (VH)

Configurable Range: The lesser of 0 to 25% of VAT or 0 to 1000

This feature is used in conjunction with the VAT and is only used when the sensor is already in an Aware state. This feature prevents rapid triggering when the vibration is hovering near the VAT. For the sensor to go not aware when already aware, it must go below VAT - VH.

Window Filter Function

Options: No Filter, Hanning

The Hanning Filter reduces the start and end of the sampled waveform to 0 to minimize spectral leakage that can occur when the sampling of the vibration waveform doesn’t start and end at the same point in the waveform.

Accelerometer Range

Options: 2, 4, 8, 16 g

The Accelerometer Range can be decreased to increase resolution at lower amplitudes or increased to capture higher amplitude vibrations. If the range is set lower than the input signal, the peaks of the vibration waveform may be clipped, distorting the data and usually resulting in a Crest Factor below 1.41.

Measurement Interval

Configurable Range: Lesser of 1 to 43200 s or the Aware Heartbeat

This is how often the meter takes a measurement.

Sample Rate

Options: 25, 50, 100, 200, 400, 800, 1600, 3200, 6400, or 12800 Hz

This determines how fast the accelerometer samples acceleration data during a measurement. The sensor takes 256 samples for every measurement to reproduce the vibration waveform. This configuration determines the rate at which those 256 samples are gathered. Noise levels increase as the sample rate is increased. See Noise in the Electronic Specifications table for more details.

Frequency Range (Min/Max)

Options: 25, 50, 100, 200, 400, 800, 1600, 3200, 6400, 12800 Hz

The Min/Max frequency configurations define the bandwidth of the sensor. The sensor can only assess frequencies within this bandwidth for Vibration RMS, Velocity RMS, and Displacement mode. Acceleration Peak mode vibration amplitude measurements are conducted in the time domain, so it is possible to get frequency measurements that don’t match the vibration signal in this case.

Power Mode

Options: Low, Medium, High Performance

This feature controls the number of samples the accelerometer averages when the Sample Rate is below 800 Hz. At 800 Hz and above, these power modes perform the same and consume the same power. Below 800 Hz, lower power modes consume less current but will have higher noise levels than the higher power modes. See Noise in the Electronic Specifications table for more details.

Data Mode

Options: Most Recent, Average, Maximum

This feature controls how the data the sensor reports is computed. With the Most Recent Mode, the sensor only reports the results of the last measurement before the Heartbeat. Using Average Mode, the sensor reports the mean of the frequency, vibration, and crest factor readings that occurred since the last Heartbeat. The temperature reading is just the most recent reading when in Average Mode. Using Maximum Mode, the sensor reports the frequency, crest factor, and Temperature associated with the largest vibration measurement in the Heartbeat.

Resources

User Guide - Advanced Vibration Meter

Data Sheet - Advanced Vibration Meter