How DynaVibe Measures Vibration
We sometimes refer to our offices here at RPX Technologies HQ as "nerdville" because our work involves lots of "techie" tools like 3D printers, laser cutters, compilers, mathematical simulation software, and an array of gadgets and electronic test equipment that bring out the inner nerd in us. So, when we get questions about the science behind how DynaVibe actually works, there's almost a competition to see who can be first to answer the question!
Last week we got a question that we loved, because it lets us talk about the principles behind how DynaVibe prop balancing equipment works. Here's what came in:
Q: Balance limits are usually specified in IPS (inches per second). But an accelerometer measures acceleration (G-forces) which are measured in inches per second per second (in/sec^2 [IPSS]). Which is actually measured? If it is IPS, how is that determined from the acceleration?
Kudos to Tom, the submitter of this question through our email@example.com email address (where anyone is welcome to correspond with us)! Here's the answer:
A: You are correct. Vibration is often measured using accelerometers, which directly measure acceleration (e.g., IPSS). Acceleration must be converted to velocity (e.g., IPS) by also considering the frequency of interest and doing the math. The DynaVibe is doing this conversion continuously using one of several methods depending on the operating mode.
You may be wondering why IPS (velocity) is reported, and not IPSS (acceleration). The reason is that velocity is more indicative of machine damage from vibrations in the "thousand cycles per minute" range (for example, engine or prop RPM). While not aviation specific, various standards (notably ISO 10816) cover vibration analysis for machinery of various types, sizes, and rotating speeds. Charts like the one below (notice that velocity in IPS is on the left vertical axis) are quite common in the machinery world:
At very high frequencies (i.e., ultrasonics), acceleration is commonly used and is often reported in "g" units. (This is what the DynaVibe GX3 with Turbine Kit, with our high speed accelerometer, measures and reports in VibeSurvey and Williams modes).
With many of the DynaVibe GX3 modes that utilize our standard accelerometer, such as Spectrum mode, we default to velocity, in IPS, with the option to switch views to acceleration, in IPSS.
By the way, as frequencies get very low, displacement is used (i.e., low frequency fatigue/bending). This typically uses different instrumentation than for acceleration and velocity.
How does all of this lead us back to velocity and IPS? Propellers just happen to be in that middle range shown in this graph, where velocity is the best indicator of damage and the preferred U.S. unit of measure for propeller vibration is IPS.
We love getting questions like this, so if you have one you want to ask, please use our "Contact Us" form to let us know what's on your mind.