Sine

Relation between Acceleration, Velocity, and Displacement

The formulas here describe the relation between acceleration, velocity, and displacement. The formulas are:

The Most Common Formulas of Sine and Random — Relations between Acceleration, Velocity and Displacement

where:

  • A – acceleration in m/s2;
  • V – velocity in m/s;
  • D – displacement in m. Please note that displacement D is listed in a zero-to-peak value;
  • F – sine frequency in Hz.

We then use the above relations to derive the formulas for crossover points in Sine tests:

The Most Common Formulas of Sine and Random — for smoothing crossover point or sweep frequency sine test

where:

  • A – acceleration in m/s2
  • V – velocity in m/s;
  • D – displacement in m. Please, note that displacement D is listed in zero-to-peak value;
  • Dmm – displacement D in mm.
  • F(A-V) — crossover point between acceleration and velocity;
  • F(V-D) — crossover point between velocity and displacement;
  • F(A-D) — crossover point between acceleration and displacement.

Sweep rate calculations

There are two common sweep rate types in Sine: linear and logarithmic. The linear sweep rate is usually nominated in Hz/sec, the logarithmic — in oct/min.

We can calculate the sweep time of a linear sweep using the following formula:

The Most Common Formulas of Sine and Random —  Sweep rate calculations

where: 

  • Tlin – sweep time in seconds;
  • Fe – end sweep frequency;
  • Fs – start sweep frequency;
  • S – sweep rate.

Vice versa, the required sweep rate can be calculated using the desired sweep time:

The Most Common Formulas of Sine and Random — Sweep rate calculations

If we are speaking of the logarithmic sweep rate, the base term here is an octave – the interval between a frequency and its double value.

Therefore, if we want to know the number of octaves between sweep frequencies, we have to use the formula below:

The Most Common Formulas of Sine and Random — octaves and sweep frequencies

Using the formula below, we can calculate the sweep time of a logarithmic sweep:

The Most Common Formulas of Sine and Random — sweep time of a logarithmic sweep

where Tlog is the logarithmic sweep time in seconds.

The required sweep rate also can be calculated using the following formula:

The Most Common Formulas of Sine and Random — sweep rate

Random vibration

Calculation of Acceleration RMS value

Generally, a random profile is defined as a table of PSD and frequency, as shown below:

Frequency (Hz) Amplitude, g2/Hz
20 0.00040
100 0.01
1000 0.01
2000 0.0025

The Most Common Formulas of Sine and Random — Random vibration - Spectrum density vs. Frequency

To calculate the RMS value of acceleration we should calculate the area under the task line. This requires two actions: 1) calculating the area under each line of the spectrum and 2) summing the areas.

The best way to calculate the area depends on the slope of the line. The slope can be calculated using the formula: 

where:

  • Ae – end amplitude;
  • As – start amplitude;
  • Sl – slope.

The area P is calculated using the formula:

The Most Common Formulas of Sine and Random — The area P - RMS value of the acceleration

The acceleration RMS value is calculated using:

The Most Common Formulas of Sine and Random — RMS value

Calculation of Velocity and Displacement Values

Calculating velocity is based on the same principle, as calculating the acceleration, however. the values of amplitude are divided by (2π)2:

The Most Common Formulas of Sine and Random — Calculation of Velocity and Displacement Values

Two peculiarities have to be mentioned here:

  • As and Ae should be in (m/s2)2/Hz units;
  • not RMS, but peak values are usually taken into account. Therefore, we have to multiply the RMS value by the sigma factor:

The Most Common Formulas of Sine and Random — RMS value on sigma factor

where ς– sigma factor.

The same principle is used for displacement calculations:

The Most Common Formulas of Sine and Random — displacement calculations - RMS value

However, the third peculiarity appears here: Dpeak is a zero-to-peak value, so if we want to calculate the peak-to-peak value, we have to multiply Dpeak by two:

The Most Common Formulas of Sine and Random — Dpeak is a zero-peak value

If you want to learn more about the formulas we usecontact our 24/7 technical support via email or call +371 6610 2166.