Measuring THD to Evaluate Shaker Performance

March 29, 2018

An ideal sine tone is smooth and without distortion. However, the potential energy in harmonic content will result in a distorted sine wave signal.

Total harmonic distortion (THD) is a measurement of the total amount of harmonic content in a signal. More specifically, it is the ratio of the root-mean-square (RMS) amplitude of a set of higher harmonic frequencies to the RMS amplitude of the first harmonic or fundamental frequency.

Equation for Measuring THD

(1) $\begin{equation*} THD_{f}=\frac{\sqrt{V_{2}^2+V_{3}^2+V_{4}^2+V_{n}^2}}{V_{1}} \end{equation*}$

V_n = RMS voltage of nth harmonic
V₁ = RMS of fundamental frequency

THD_f is commonly used in audio distortion specifications and is usually expressed in % or dB (%THD).

Measuring THD to Evaluate Shaker Performance

In VibrationVIEW, the THD compares the RMS acceleration of the first ten harmonic frequencies (starting at the second harmonic) to the RMS acceleration of the fundamental frequency. The THD graphing option is enabled with the Sine Analyzer functions.

THD not only measures the quality of a sine wave but can also determine how well a system is replicating the signal from the controller. Small THD values indicate the system is performing correctly. In the physical test environment, a high THD suggests a weakness or malfunction in the shaker system.

Therefore, THD can measure the accuracy of a vibration system and help to track system changes over time.

Swept THD Option

A test engineer can evaluate shaker performance by running a sine sweep and analyzing the data from the perspective of the THD. They can establish the shaker’s THD when the shaker first arrives at the laboratory testing facility. The value should be less than 10% across the frequency spectrum.

After a baseline THD is established, any future THD responses can be compared to the original value. If the THD response varies from the baseline value, then there is likely an issue with the shaker.

While data from a sine sweep test can be analyzed in many ways, we recommend using the THD plot to evaluate shaker performance. The THD plot provides clear visual clues if something is awry with the shaker system.

The THD method covers an entire range of frequencies and not just an isolated, single frequency such as in System Check. Additionally, engineers use sine sweeps to produce shaker performance standards, so it is logical to evaluate shaker performance with the same type of test.

Watch the video below for a guide on measuring THD in VibrationVIEW.