ALTA Advanced Vibration Sensor—Principles of Operation

The ALTA Advanced Vibration Meter uses a silicon micromachined accelerometer 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, then taking a temperature measurement. The meter will measure on a configurable Measurement Interval. Only the most recent set of data points is reported on each Heartbeat.

Example Applications
  • Vibration monitoring
  • Smart machines, structures, and materials
  • Bridge and building seismic activity monitoring
  • Assembly line monitoring

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 transmit an Aware State reading immediately to the gateway.
  2. To determine the duty cycle reading reported when a Heartbeat occurs.

If the Aware State Threshold was not 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 are looking to observe. Configure the Sample Rate too high, and it will include noise from frequencies higher than the desired frequency, and you will see inconclusive readings.

Note: The sensor will automatically account for mismatched Heartbeat/Measurement Interval/Sample Rate configurations.

Measurement Intervals

The Advanced Vibration Meter is not “always listening” as 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): .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 physically be damaged and not operational if it reports more than 700 Hz, and so 800 Hz is not possible with the motor)
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. So 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 is running, we expect a frequency reading of approximately 400 Hz (24,000 RPM) and an amplitude of approximately 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).

Note: The Sample Rate is an important consideration as it will potentially limit your most frequent Measurement Interval. For example, if you set your Sample Rate to 25 Hz, it will take 10.24 seconds to complete the 256 sample Measurement Intervals (256 samples divided by 25 Hz = 10.24 seconds). Therefore your Measurement Interval cannot be more frequent than 10.24 seconds. The meter should automatically adjust, but it is an important consideration.

Entering and Leaving the Aware State

Entering and Leaving the Aware State
If the meter detects that the intensity of the vibration 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 to be over the configured Vibration Aware Threshold, it continues to check in every 10 minutes. If the meter detects that the intensity went 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 be dependent on your specific application, and an expert in that application should decide which Mode to use, the following guidelines may be useful.

  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 for mid-range frequency vibration 10 to 200 Hz
    a. mm/s
    b. 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 all of the energy in the entire 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 whole spectrum, and that 1500 value is what 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 usually 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 in the case of the non-leaded option.

Applications

  1. Just about anything that vibrates
    a. Motors of almost any kind
    i. Vehicle engines
    ii. Industrial machinery
    iii. Drills
    iv. AC Units
  2. Fan
  3. Condenser
    b. Conveyor systems
  4. Sometimes even things you don’t want to vibrate
    a. Static systems: If vibrations occur in these systems, it could be a big problem
    i. Buildings (skyscrapers)
    ii. Bridges (Think Oneida Narrows Bridge)


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