Understanding Dynamic Range in Acceleration Measurement Systems. February 2013 By: Bruce Lent

Transcription

1 in Acceleration Measurement Systems February 2013 By: Bruce Lent

2 Topics to discuss Definition of dynamic range The effective range Making full use of the high level Using filters to improve dynamic range Determining the lowest useable level How is dynamic range calculated Effect of signal conditioning and DAQ 2

3 Definition of dynamic range Ratio between the largest and smallest possible values of a changeable quantity such as acceleration levels. Minimum detectable signal Largest signal Expressed in db 3

4 The measurement window Head Room Maximum (5 V) Noise Floor Zero Volts 4

5 The effective dynamic range Head room Maximum Noise floor Zero volts detectable signal Noise floor (threshold) Dynamic range in db 5

6 Dynamic range per specifications Head Room Maximum (5 V) Dynamic Range = 60dB Noise Floor.005 V rms Zero Volts 6

7 Dynamic range per specifications Head Room Maximum (5 V) Dynamic Range = 60dB Noise Floor.005 V rms Zero Volts 7

8 Convert noise floor to peak units Head Room Maximum (5 V) Dynamic Range = 50.5 db Noise Floor Volts peak Zero Volts 8

9 System noise sources Accelerometer DAQ Signal Conditioning 9

10 Noise contribution of the signal conditioner Added circuits (amplifiers, A/D converters) will always add noise. It is important that the input signal be as clean as possible since (in most cases) the most the highest noise originates at the input. Take into account all post sensor noise sources when calculating the dynamic range. 10

11 Resonance excitation digital limitations Assume a 12- bit digitizer Resonance is at full scale, resolution is 1 part in Assume the signal is at 4 bits with a resolution of 1 part in 16 Resonance Signal 11

12 Resonance excitation analog limitations Assume resonance amplitude at 5 volts. Assume signal amplitude at 1 volt and noise floor of Volts peak. 20 log 1/0.015 Dynamic Range = 37 db Resonance Signal 12

13 Resonance mitigation Accelerometer Low Pass Filter DAQ Signal Conditioning 13

14 Limitations from resonance Resonance amplitude can exceed signal levels Signal attenuation reduces useful dynamic range Filtering required at the input states Pitfalls of input electronic filtering include masking of resonance A mechanical filter will mitigate the accelerometer s resonance 14

15 Resonance limitations on dynamic range +/- 5 Volts is a common IEPE transducer output Avoid the Headroom area Possible Distortion The Highest Amplitude Signal should be as close to 5 Volts as possible 15

16 Charge mode piezoelectric noise 16

17 Use low-pass filtering included in most signal conditioners Low pass Filter On 17

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