Page 1 Time_vs_Freq.ppt 9/21/99 Time vs. Frequency September 21, 1999 Ron Denton
Page 2 Time_vs_Freq.ppt 9/21/99 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 ? Time Domain vs. Frequency Domain
Page 3 Time_vs_Freq.ppt 9/21/99 The Time Domain (for our use) is... A graphical representation of the change of some value with respect to time
Page 4 Time_vs_Freq.ppt 9/21/99 How the Time Domain is Measured
Page 5 Time_vs_Freq.ppt 9/21/99 The Frequency Domain (for our use) is.... A graphical representation of the amount of energy present in a signal at each of many frequencies
Page 6 Time_vs_Freq.ppt 9/21/99 The Frequency Domain amplitude is usually measured in RMS or Peak 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.
Page 7 Time_vs_Freq.ppt 9/21/99 What does this have to do with Vibration ? 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
Page 8 Time_vs_Freq.ppt 9/21/99 The Time and Frequency Domains are related by.... Time Domain Frequency Domain Amplitude
Page 9 Time_vs_Freq.ppt 9/21/99 Sinusoidal Signal Examples
Page 10 Time_vs_Freq.ppt 9/21/99 Sinusoidal Signal Examples
Page 11 Time_vs_Freq.ppt 9/21/99 Sinusoidal Signal Examples
Page 12 Time_vs_Freq.ppt 9/21/99 Sinusoidal Signal Examples
Page 13 Time_vs_Freq.ppt 9/21/99 Sinusoidal Signal Examples
Page 14 Time_vs_Freq.ppt 9/21/99 Sinusoidal Signal Examples
Page 15 Time_vs_Freq.ppt 9/21/99 These examples illustrate that... 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
Page 16 Time_vs_Freq.ppt 9/21/99 A practical example to wrap up 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 ?
Page 17 Time_vs_Freq.ppt 9/21/99 A practical example to wrap up Tell them to check the gain range (amplifier setting) on their equipment ! Their time domain signal probably looks like this...
Page 18 Time_vs_Freq.ppt 9/21/99 A practical example to wrap up 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.
Page 19 Time_vs_Freq.ppt 9/21/99 Clipping Time signals will cause a penalty and cost you data 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
Page 20 Time_vs_Freq.ppt 9/21/99 The moral of the example ? Don’t be a square !
Page 21 Time_vs_Freq.ppt 9/21/99 Questions ?