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Time vs. Frequency September 21, 1999 Ron Denton

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Time vs. FrequencySeptember 21, 1999Ron Denton

- What is the Time Domain ?
- How is the Time Domain Measured ?
- What is the Frequency Domain ?
- How is the Frequency Domain Measured ?
- What does this have to do with Vibration ?
- How are the two domains related ?

- A graphical representation of the change of some value with respect to time

- A graphical representation of the amount of energy present in a signal at each of many frequencies

- The frequency domain values are really just a series of sinusoidal equivalents, so amplitudes are usually represented the same as in the time domain.
- The frequency domain is usually derived by computing a Fast Fourier Transform (FFT) of a time domain signal.

- Transducers, such as accelerometers, convert mechanical energy into electrical signals
- The electrical signals are measured with various devices
- Most of these devices record the time data
- The time data is converted to the frequency domain within these devices
- Vibration analysts use the time and frequency domain data to interpret machinery condition

Amplitude

Time Domain

Frequency Domain

- The time and frequency domains are related
- A sinusoid in the time domain has a unique value in the frequency domain
- Complex combinations of time domain sinusoids can be separated and displayed in the frequency domain
- Signals other than sinusoids can be represented in the frequency domain accurately

- A customer is using an accelerometer to measure sinusoidal vibration . . . . .
- You receive a phone call . . . . .
- “Your accelerometer is all out of whack! I have a simple signal and your accelerometer is generating all kinds of harmonics.”
- You look around and can’t find an application engineer . . .
- (Panic grips you !) What do you do ?

- Tell them to check the gain range (amplifier setting) on their equipment !
- Their time domain signal probably looks like this . . .

If the amplifier range of the measuring system is not set correctly, it will “clip” the signal and “chop off” the highest and lowest parts of the signal.

This is not good !

It means you are trying to turn a sine wave into a square wave.

Just as clipping is illegal in football -in data acquisition, clipping the time data will cost you by causing invalid data (spurious signals) in the frequency spectrum.

Original Signal

Attenuated Signal

Spurious Signals Added

- Don’t be a square !

Questions ?