MEASUREMENT SYSTEM NOISE

Transcription

1 MEASUREMENT SYSTEM NOISE Gary C. Foss The Boeing Company Seattle, Washington ABSTRACT Clean structural dynamic measurements are obtained when frequency response function maxima and minima fall within the dynamic range window of the measurement system. During a measurement process, attention is usually paid to maximum amplitudes and the prevention of overloads. Less attention is paid to sources of noise in the system that degrade minimum measurable levels. This is unfortunate because useful information about the structure is lost when the response function anti-resonances are truncated by an elevated noise floor. This paper considers how noise can affect measurements and offers a survey of noise floor measurements of various system components from several vendors. NOMENCLATURE: Signal to noise ratio (S/N): The ratio of signal voltage to noise voltage, usually expressed in decibels. FRF: Frequency response function FS: Full scale voltage level : Spectral density in units of RMS nanovolts per square root Hertz INTRODUCTION Frequency response functions quantify the dynamic behavior of a structure and are usually the basis for further parameter estimation and modal analysis. The response normalized to a unit of input as a function of frequency has a well known shape, characterized by alternating regions of high and low activity. At system poles, the amplified response to unit excitation is only limited by the amount of damping; at the anti-resonances, the response may be vanishingly small. To properly measure the full range of amplitude response over the desired frequency range, a data system is required that can discriminate both large and small signals. The large limit is fixed by the full scale voltage capability of the electronics. The small limit is set by either the A/D converter resolution or measurement system noise. Frequency response functions can be degraded by noise in the force input signal (usually a load cell) or noise in the response output (usually an accelerometer). Electronically, the issues are the same. Numerically, there are advantages and disadvantages in how the FRF is calculated, depending on whether the noise is largest on the input or output. This subject is well covered in the literature [1]. The focus of the present effort is on noise in response signals, and how it limits the full description of dynamic behavior. SIGNALS AND NOISE A fundamental specification of a measurement system is its signal to noise ratio, S/N; the ratio of the largest to the smallest signal directly measurable. [2,3] The theoretical S/N achievable in a digital data system is limited by the lowest voltage it can measure, i.e the quantization level of the analog to digital converter. Most structural dynamic measurement systems today range from 13 bit (HP35655A) to 16 bit (HP35670A and VXI), corresponding to S/N ratios of 78 and 96 db, respectively. A few systems are now on the market with 24 bit resolution. The total system noise floor is the root of the sum of the squares of all contributing sources. Contributing sources include the quantization noise, input electronics (amplifier and anti-alias filter), signal conditioning, internal accelerometer electronics, and ambient environmental noise (thermal, acoustic, electromagnetic, and motion). The most efficient strategy for lowering system noise is to identify the greatest contributors and lower them to the mean level. Further lowering of any single source below the mean level does not offer much bang for the buck. Consider the frequency response functions in figures 1 and 2. Both measurements were made with an HP35670A analyzer performing an impact test on the free-free H frame structure shown in Figure 3 with side by side accelerometers near the driving point. The analyzer internal ICP supply was used and the full scale voltage range was the same. (300 mvolts FS on the hammer, 50 mvolts FS on the accelerometers). 914

2 Freq Resp 2:1 100 g/lb Tap test FRF using 308B Freq Resp 2:1 100 g/lb Tap test FRF using 336M30 Fig. 1: FRF measured with an older accelerometer The accelerometer sensitivities were the same; 100 mv/g. The force spectrum was common to both FRFs, which were measured concurrently. The only difference between these measurements was the noise floor specification of the accelerometers. The accelerometer in Figure 2 had a quieter noise floor than the accelerometer in Figure 1. Clearly, we prefer the data in figure 2. These measurements were made at the driving point out at the tip of one of the extremities in figure 3, so they should represent the best opportunity for clean data. The difference in voltage noise floor between these two sensors is shown in the spectral density plots of Figure 4. The noisier upper trace is 2-8 times the noise level of the quieter lower trace, depending on frequency. These noise measurements were made with the accelerometers mounted to a large seismic mass on springs to limit the environmental input from ambient ground motion. Fig. 2: Same FRF measured with a newer, quieter accelerometer It should be noted that both of these sensors have noise specifications falling within the range of commonly available commercial units. We can better understand how noise limits dynamic measurements by examining the accelerometer response spectra used to calculate the above FRFs. Figures 5 and 6 show the accelerometer response spectra for the FRF measurements above in relation to the maximum signal ceiling and minimum noise floor. Both response spectra represent five tap test averages. The ceiling was established by measuring the spectrum of random noise overdriven into clipping. The noise floor was found by measuring the response in the absence of any excitation. V/rtHz PCB308B noise floor PCB336M30 noise floor 1E Hz Figure 3: H-Frame test arrangement Figure 4 Voltage noise floors for the above accelerometers 915

3 1 g/rthz System max range 308B Tap test response 308B Noise floor 308B 1 g/rthz System max range 336M30 Tap test response 336M30 Noise floor 336M30 1E-06 1E-06 Figure 5: Response spectrum of older accel showing signal ceiling and noise floor. The range from ceiling to floor in figure 5 is about 67 db at 10 HZ and shrinks as the frequency is reduced. The same range in Figure 6 is about 86 db at 10 HZ and is flatter with frequency. Smooth measurements can generally be made anywhere between the ceiling and the floor, though a large number of averages may be required as the noise floor is approached. One reference recommends a safety margin of 10db above the noise floor. [3] As figure 5 shows, where the accelerometer response spectrum closely approaches the noise floor, the FRF becomes degraded. If the structural response drops below the floor at an anti-resonance, the response spectra will be truncated at the floor, as will the FRF. In this case, the response channels were set to 50 mv. full scale. Had the signal levels been higher (harder taps/bigger hammer), a greater full scale would have been needed. This would have made the noise floor more defined by A/D resolution and less sensitive to accelerometer noise, resulting in less difference between the FRFs in figures 1 and 2. On the other hand, a lower signal level at locations of less response may have produced more pronounced differences. A crossover point where the dominant noise transitions from accelerometer to analyzer is discussed in a subsequent section. These figures illustrate the benefits of having a system with as large a dynamic range as possible. That is, we want to maximize the space between the floor and the ceiling. This not only allows more room for the FRF peaks and valleys, it also makes the gain setting or autoranging process less critical (or even unnecessary). While the Figure 6: Response spectrum of newer, quieter accel showing signal ceiling and noise floor. accelerometer shown in figures 1 and 5 is acceptable for measuring the maximum responses at the modes, it is marginal for measuring the full range of accelerance or mechanical impedance. CHARACTERIZATION OF NOISE Noise is usually not constant with frequency, so a proper characterization should specify spectral density at several frequencies which frame the range of interest. Also, noise frequently shows up at discrete power line related frequencies. The best way to characterize this is to measure a broadband RMS noise figure, which includes the power line frequency and several harmonics. For modal analysis applications on common structures, it was decided to compare the voltage spectral density at 10 and 100 HZ, as well as the broadband RMS noise from HZ. To identify the relative contributions, the noise levels of the transducers, signal conditioning, and analyzers were considered separately. First considered was the analyzer. ANALYZER NOISE Analyzer noise floors were measured by bypassing the internal ICP circuitry, setting the most sensitive full scale voltage range, and shorting the input. A typical noise floor measurement on the HP35670A is shown in Figure 7. Table 1 summarizes the findings. is nanovolts per square root Hertz, the units of voltage spectral density. The VXI figures reflect less available onboard gain (100 mv minimum FS). The HP35670A has more A/D resolution than the HP35655A, but as previously noted, the input amplifiers define the noise at maximum gain. 916

4 V/rtHz 1E-08 Figure 7: Typical analyzer noise floor Analyzers (shorted input) 10HZ 100HZ HP35670A HP35655A VXI Table 1 Analyzer noise at minimum FS HP35670A noise floor 5-800HZ uv 800 Hz SIGNAL CONDITIONER NOISE Another potential noise contribution is the signal conditioning. An ICP signal conditioner can be as simple as a constant current source and a decoupling capacitor. At the other end of functionality is the current source/preamplifier/filter/post-amplifier with Transducer Engineering Data Sheet (TEDS) programming. As might be expected, the simplest circuitry usually generates the least noise. A variety of signal conditioners were evaluated for noise floor. A passive transducer simulator was built using a 5000 ohm resistor in parallel with a 1000 microfarad capacitor. The resistor biases the ICP current source correctly, and the capacitor greatly attenuates the resistor noise. This simulator was verified on the HP35670A analyzer in the internal ICP mode to have a noise floor lower than the evaluated signal conditioners. The test then consisted of the HP analyzer measuring the signal conditioner output with the passive simulator connected in the ICP mode. All measurements were made with a gain of one. Figure 8 shows a typical noise floor spectrum. Table 2 summarizes the results for a number of units. V/rtHz 1E-08 PCB482A Hz Figure 8: Typical noise floor, unbuffered signal conditioner Signal Conditioners 10HZ 100HZ 5-800HZ uv PCB 480D PCB482A PCB 483A PCB Endevco A Dytran M1 Endevco M4 PCB 442A Table 2: Signal conditioner noise (gain=1) The first four models in the list were simple, unbuffered units with no active electronics (the 480D06 is a battery powered unit which bypasses the amplifier at a gain of 1.) The other four models had some active electronics. It is apparent from the table that the addition of active signal conditioning raises the noise floor of the system over that of the analyzer alone. Whether the additional noise of active signal conditioning is acceptable or not will depend on the noise of the accelerometer, and that was the final system component evaluated. ACCELEROMETER NOISE In the absence of any environmental input, the noise floor of an ICP accelerometer is due to the internal amplifier. To measure amplifier noise only, the accelerometers were attached to a seismically isolated platform consisting of a large concrete mass on springs with a suspension frequency less than 1 HZ. The facility was also located on stable bedrock away from heavy traffic and industrial activity. A padded enclosure was put over the mounted accelerometer to attenuate acoustic inputs and minimize air currents. The noise floor measurements for the two accelerometers considered above were previously shown in Figure 4. The tabulated findings for a variety of accelerometers are reported in Table 3. Ordering is based on the 100 HZ column from quiet units at the top to noisier units below. Sensitivity and mass information are also included. The first thing to note is that voltage noise floors of commercially available accelerometers vary over a wide range (20 to 1) and the newer products tend to have improved electronics (i.e. lower noise) compared to the older models. Another observation is that the voltage noise floor is unrelated to the sensitivity and only weakly related to the mass. A third observation is that most accelerometers in the list are significantly noisier than the analyzers and most of the signal conditioning. Therefore the noise in a system is frequently driven by the accelerometer specifications, and this is where the greatest improvement can be made. Choosing an accelerometer should start with an estimate of the maximum motion to be measured. Then the sensitivity should be selected using an allowance for 917

5 Accelerometers (On iso table) 10HZ 100HZ 5-800HZ uv Sensitivity mv/g Mass gm. PCB393B Endevco Endevco PCB393A PCB336M Dytran 3041A PCB356M PCB352C Endevco 2255A Endevco 7255A PCB353B PCB356A Endevco PCB352C Endevco PCB353B Endevco 63C PCB333B PCB333B PCB356A PCB356A PCB333B PCB308B PCB309A Endevco 7251HT Endevco 2251A Endevco 2250AM Table 3: Accelerometer voltage noise floors uncertainty. Frequency range, size, mass and cost considerations will further narrow the field of candidates. From the final list, the sensor with the best noise specifications will measure the cleanest data over the widest range of structural response amplitudes. EFFECT OF ANALYZER FULL SCALE As previously noted, the dynamic range in an analyzer can t get any better than the A/D converter quantization will allow. In the case of the HP35670A shown in Figure 9, the broadband (5-800HZ) dynamic range levels off at 81 db for the larger full scale ranges. At the most amplified ranges however, this drops to about -72 db as the input electronics becomes a significant noise source, and limits the benefit of further gain. During a large structural dynamic test with dozens or hundreds of channels, many response measurements are made out of axis, near fixturing, or other areas of low motion. Signal levels may range from a few millivolts to ten volts or more. While the analyzer dynamic range is relatively constant, the absolute noise floor is a function of analyzer full scale voltage. This relationship is illustrated in Figure 10. The noise floor varies from less than a microvolt to almost a millivolt. Also shown is the noise floor of a PCB336M30 accelerometer, which is constant at 8 microvolts. Where the two curves intersect is a crossover between accelerometer domination and analyzer domination of the total system noise floor (assuming no external signal conditioner). For this example, if we could lower the accelerometer noise floor to 1 microvolt, the analyzer would dominate the system noise at all levels and we would get the full dynamic range shown in Figure 9 for both large and small motions. As more 24 bit data systems become available in the future, the accelerometer will increasingly determine system noise floors at all signal levels. Sensor manufacturers will be pressured to build quieter products that take full advantage of improved analyzer performance. Dynamic Range, db HP35670 Dynamic Range vs Full Scale Range Full Scale, mv Figure 9: Analyzer broadband dynamic range 918

6 Broadband Noise floor, uv Comparative Noise Floors Accelerometer noise predominates Analyzer Full Scale, mv Analyzer noise floor Accel noise floor (8 uv) Analyzer noise predominates Figure 10: Effect of HP35670A analyzer voltage range on system noise floor Figure 11 Figure 11 shows a lineup of the accelerometers used in this evaluation, ordered by voltage noise floor. The quietest sensors are on the left. CONCLUSION Measurement of FRFs at locations of low motion is made more difficult by the system noise floor, particularly near anti-resonances. Contributors to the noise floor may include unwanted environmental inputs, transducer noise, signal conditioner noise, analyzer electronics, and quantization noise. To measure high quality frequency response functions at all degrees of freedom, we need to minimize these noise sources as much as economically possible. At measurement points of low to moderate motion, the primary noise source is usually the accelerometer. Selecting quiet accelerometers is the most cost effective way to extend the dynamic range of a measurement system and improve the quality of measurements. REFERENCES 1. McConnell, Vibration Testing: Theory and Practice, Wiley, 1995, pg Vergers, Handbook of Electrical Noise: Measurement and Technology, TAB Books 1979, pg Formenti, Norsworthy, Accelerometer Dynamic Range, Sound and Vibration, June 1999, pg 1 919