Sweep Range and Sweep Rate
March 29, 2018
The sine sweep test is used in vibration testing to determine a product’s resonances. Below, we outline several parameters that define the test.
The sweep range defines the minimum and maximum frequencies of the sweep. With a sweep range of 5Hz to 1000Hz, the controller will start the test by shaking the product at the rate of 5cycles per second. Then, it will increase the frequency over time until the controller reaches 1,000Hz. The test could also be defined so that the sweep begins at a high frequency and ends at a low frequency.
The sweep rate defines the speed at which the controller increases or decreases the frequency. It is measured in terms of the width of frequency covered per unit of time. The sweep rate can be either linear or logarithmic.
Linear sweep rates
When the sweep rate is linear, the controller spends the same amount of time at each frequency. The frequency width is measured in Hertz for a linear sweep rate. For example, we could set the controller to sweep over 1,000Hz in one minute (1000Hz/min sweep rate). Additionally, we could define the amount of time to complete the entire sweep.
Logarithmic sweep rates
When the sweep rate is logarithmic, the controller accomplishes the same amount of cycles at each frequency. With a logarithmic sweep rate, less time is spent at higher frequencies than at lower frequencies, as more cycles are happening every second at higher frequencies.
For a logarithmic rate, frequency width can be measured in decades, or a factor of ten. There is one decade between 10Hz and 100Hz, two between 10Hz and 1000Hz, and so on. The frequency width can also be measured in octaves with a logarithmic rate. Two frequencies are one octave apart when the one frequency is double the frequency of the other. 10Hz and 20Hz are one octave apart, 10Hz and 40Hz are two octaves apart, and so on.
Suppose our sweep range or frequency range was from 10Hz to 1,000Hz. If we set our sweep rate at 2dec/min (two decades per minute), then it would take one minute to complete the sweep, as there are two decades between 10Hz and 1,000Hz (10Hz to 100Hz, 100Hz to 1000Hz). Again, suppose our sweep range or frequency range was from 10Hz to 640Hz. If we set our sweep rate at 2oct/min (two octaves per minute), then it would take three minutes to complete the sweep, as there are six octaves between 10Hz and 640Hz (10Hz to 20Hz, 20Hz to 40Hz, 40Hz to 80Hz, 80Hz to 160Hz, 160Hz to 320Hz, 320Hz to 640Hz).