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Frequency Measurements, Part 1 of 3


Since our newest data acquisition starter kit (DI-149) supports a frequency measurement function I thought I’d burn a few posts to talk about that feature in general terms. In this first part I’ll talk about where frequency measurements are typically encountered with examples of the specific information that can be gleaned from them. In Part 2 I’ll describe the theory behind frequency measurements, how they are typically made and the limitations of typical approaches.Part 3 will describe the DI-149 method of  frequency measurement, and why it’s better than more traditional approaches.

So, who needs to make a frequency measurements, anyway? You might be surprised to learn that it’s many more than the techs who measure motor or engine RPM. Of  them all, that’s certainly the most common and the one that most people think of when the question is first raised. But let’s explore the measurement a bit deeper.

To state the obvious, you need to measure frequency whenever the distance between a consecutive series of waveform transitions carries information. The distance is measured in time, and the reciprocal of time over one period is frequency. In the quintessential application that measures RPM, if you have one detector attached to the flywheel, motor shaft, or whatever, one such pulse occurs per revolution. Two detectors yield two pulses, and so on (for a discussion about how the number of pulses per revolution affects RPM measurement resolution see this link.) There are other applications where a frequency measurement is necessary:

  • Fluid flow measurement where the frequency output of the sensor is directly proportional to flow rate: gallons per minute, liters per second, etc.
  • Anemometers where output frequency is directly proportional to wind speed.
  • Air mass flow sensor to determine the mass flow rate of air entering a fuel-injected internal combustion engine.
  • Geiger counter applications where detected pulses are displayed as frequency.

So, the need for frequency measurements can encroach on many different application environments. The fact that such a measurement can be folded into and synchronously included with analog voltage measurements extends the usefulness of a product like the DI-149 and any other data acquisition system with similar capability.

In the next installment I’ll discuss how frequency measurements are typically made, and some common pitfalls associated with those approaches.


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