# How to calculate frequency with the voltage received from a vibration sensor?

I have been working on a project recently and I am using Sensor SPM SLD 723 to read variations in vibrations.

The output I received from the sensor is only in volts, how can I convert this voltage into Frequency (Hz)?

Edit : Actual data received from Device • It might help some people with their answers if you can provide some more information on the frequency you're expecting (and the magnitude) as well as your environment for processing the data. Feb 15 '17 at 8:10
• Sure, i will provide actual device data asap. Feb 15 '17 at 10:25
• From spminstrument.com, it looks like you have to be measuring in a selected frequency range. Is it possible that frequency is the input, and voltage would be the output, measuring not so much the frequency of vibrations as the amplitude of those vibrations? I'm not sure, it's the first I've heard of the device: I'm just wondering if this is a possibility. Feb 15 '17 at 13:57
• I can't see how this is an IoT question. You would probably get better help at electronics.stackexchange.com/Althoguh, personally, I would approach the manufacturer and/or their forum first. Feb 16 '17 at 8:37
• Ok, i will checkout for time variation.. but how can we get the frequency if we have the time variation? Feb 17 '17 at 11:39

Key here is the datasheet as linked in Jimmy Westberg's answer. The sensor will output:

The 4-20 mA vibration transmitters are piezo-electric accelerometers of compression type and provide a 4-20 mA output signal proportional to the true RMS value of vibration velocity.

So the output of this sensor is a current signal between 4 mA and 20 mA (not a voltage) that is proportional to the RMS value of vibration velocity. To read this sensors output the current will have to be converted to a voltage using a transimpedance amplifier (current-to-voltage converter) or measuring the voltage drop acros a well defined series resistor.

However as the sensor output is the true RMS value of vibrations in the specified frequency range (2..10,000 Hz) it is not possible to obtain the frequency (or to be more precise the wide frequency band) of the vibration with this sensor. To detect the frequency spectrum a measurement of the time waveform of the vibration amplitudes would be necessary.

This booklet about vibration measurement gives some more insight.

The RMS value is typically used in quantifying the vibration level:

The RMS value is the most relevant measure of amplitude because it both takes the time history of the wave into account and gives an amplitude value which is directly related to the energy content, and therefore the destructive abilities of the vibration.

The purpose of this sensor seems to be for monitoring of machinery where the actual time waveform of the vibration is of little interest. A single value (the RMS value) is sufficient to monitor the operation of the machine against a threshold value. It significantly simplifies measurement.

Experience has shown that the overall RMS value of vibration velocity measured over the range 10 to 1000 Hz gives the best indication of a vibration's severity. A probable explanation is that a given velocity level corresponds to a given energy level so that vibration at low and high frequencies are equally weighted from a vibration energy point of view. In practice many machines have a reasonably flat velocity spectrum.

This is not really an answer to your question but I do find it sometimes hard to understand why you want the real data presented. Take my light and sound sensors that I use. They are both analog and thus give me an signal between (in my case) 1V (low) and 3.3V (high). For the light 1V = to a dark room and 3.3V = daylight or direct lamp light. I could go all the way to try to convert these figures to Lux but it doesn't really make any difference since all I want is to know the relative level of light (or sound). So in my case I instead use % for these analog signals. 0% at 1V and 100% at 3.3V.

You might want to consider trying that in your case as well?

But, if you do want to try to go all the way you might want to take a look in the data sheet. The output signal is 4-20mA and they give you that this correspond to 2-1000Hz.