Rotas Mobile: Vehicle noise measurements + FFT Ordertracks Optimizing test stand to vehicle correlation 9/12/2003 1
Correlating vehicle and test stand data In the vehicle certain speeds and torques are more susceptible to gear noise than others. These speed bands and torque conditions are of primary interest on the end-ofline tester. Die operational conditions of the axles or transmissions can be described with the following parameters: Temperature Gear Drive/Coast Torque Speed Often, the test stand administers performs a speed ramp at a certain torque level for both drive and coast condition. It is essential to test the speeds and the torques that are most critical in the car. 9/12/2003 2
Cabin noise components Noise sources There are many different transfer paths for the sound and the vibration from the gearset to the drivers ear. Each path has its own frequency dependent transfer function Transfer paths Noise from the motor, wind and tires may mask certain frequency bands from the gears. These effects induce the speed, torque and vehicle dependent noise perception of the gear sets. 9/12/2003 3
Acoustic cabin resonance s Standing acoustical waves within the cabin give maxima and minima of the sound pressure level depending on the microphone position From statistical sound field analysis, the number of resonance's is given by Standing acoustical waves Frequency f gr cabin volume V, cabin surface A, total cabin edges L, speed of sound c = 330 m/s Cabin with 1,5 * 1,2 * 2 m. V = 4 m 3, A = 14,4 m 2, L = 4,7 m exhibits the following resonance's: Up to frequency 100 Hz 500 Hz 1 khz 2 khz Number of resonance's 1,5 36 657 4.468 9/12/2003 4
Averaging over microphone postions To compensate for the cabin resonance's, four microphones are positioned within the cabin: Front left, middle and right position, one in the rear. The energy of the microphone signals is averaged in the spectral order domain. Speed M1 M2 M3 M4 Accel Order Resamp One sensor is mounted on the transmission to allow for the simultaneous analysis of the gearbox vibration data. FFT Average + Spectrogramm Ordertracks 9/12/2003 5
Order spectrograms Order spectrograms are recorded for the gearbox vibration signal and for the cabin sound field. 2) Order spectrum at 2241 RPM From these order tracks for arbitrary orders and order spectra for arbitrary speeds can be generated: Order track for the 65. order 1) Ordnungs-Spektrogramm 4. Gang Zug 9/12/2003 6
1 st Gear: Masking of gear mesh by motor orders Transmission vibration 1.Gang: 13 teeth on input shaft gear give 13 th order. Differential is very slow and totally masked. The critical frequency Range from 500 800 Hz corresponds to motor speeds from 2400 to 4000 RPM. Here the motor is load enough to mask the gear mesh as well. Cabin sound 9/12/2003 7
2 nd Gear: Gear mesh noise detectable Gear mesh orders can be heard from 1200 up to 2000 UPM in 2 nd gear. Mostly in coast. Transmission vibration Noise source: gear mesh of the input section. 25 th order passes through 500 to 800 Hz from 1200 Rpm to 2000 RPM. Cabin sound 9/12/2003 8
2 nd Gear: Transmission vibration and cabin sound Transmission Vibration Cabin sound field 9/12/2003 9
3 rd Gear: No critical speed bands 3 rd Gear: Input section is out of car resonances (53 rd Order) Differential section is masked by motor orders (16.65 th order). 9/12/2003 10
4 th Gear: Differential section detectable Transmission Vibration Cabin sound field In 4 th gear, the differential section with order 23.06 enters the car resonance area from 500 to 800 Hz. The critical speed range is 1500 to 2200 RPM. The 5 th and 6 th gear did not show any critical speed bands. 9/12/2003 11
Optimizing gear geometry for car susceptibility Test stand vibration order track of differential section in 4 th gear. Shown are 100 production units (black), one unit before geometry optimization (pink) and after the geometry of the differential section has been changed (blue and green). Order track of differential gear mesh in 4 th gear, coast, low load (10 Nm). Production units (black). Geometry optimization of one unit: Before: pink, After: blue and green The critical speed band from 1500 to 2200 Rpm in the car was diminished by 10 db. At higher speeds, the differential section is out of the car s critical resonance s and the gear mesh level is not so important. 9/12/2003 12