Physics is constant, but electrical mobility changes the methods and equipment of test-and measurement. In NVH-engineering there are new digital sensors for sound and vibration. Digital output formats are coming closer to the bus-and network structure in and around electric vehicles.
Sound or noise and vibration sources are also generated in the electric vehicle. But the range of signals is changed regarding the levels and also the frequency range. Sensors with wideband frequency range and high dynamics are an ideal solution, but some kind of functions could be also related to special parts of the ranges of frequency and amplitude. Background knowledge of physics of sensors, like microphones and accelerometers and the special digital output signals in the new generation is indispensable also in this new world.
Microtech Gefell from Germany has developed a very new digital test- and measurement microphone, which includes very new functions. The interface is USB, which looks very simple. Of course the handling of everything based on USB is simple for users. Concealed in this supposed simplicity are world firsts in several respects. You can connect the USB-interface of this new kind of sound-sensor with its most innovative part, the digital preamplifier MV 240, connect to each kind of computer and use it like a sound-interface without any additional driver software. That device can work with Microsoft Windows ® as well as Linux®-systems or other operating systems. That looks not new, but where else in the world a high standard capacitor measurement microphone capsule is connected with its associated preamplifier is providing a clean signal with 32bit processing width.
Even newer is the fact, that this USB-microphone is the first microphone in the world, which converts the physical quantity sound pressure into digital output values. These are processed directly in each NVH-software, which allows sound interface inputs as well as common measuring- or researching systems like Labview® or MATLAB®. Two analogue-digital converters with 24bit each are interleaved in the new quality microphone, to which the same microphone signal is applied with different amplifications. Due to the fixed assignment 1µPa = high level of the lowest bit (per definition), it is no longer necessary to calculate with an electric voltage between sound pressure and computer internal signal processing. An own preamplifier signal processing chip allows special calculations with the measured signal like frequency response correction etc. before the signal reaches the computer.
If connected to a single board computer for example, the acoustic signals can be connected to Ethernet® or other network systems. On that way physical values from vehicle bus systems like rpm-signals could be combined with noise-vibration signals in the digital world, what is necessary for common NVH-analyses like order tracking etc.
A similar innovation is a very new kind of accelerometer with USB-interface from DYTRAN® (USA). This model 5384 VIBRASCOUT™ 6DOF USB vibration measurement system is a unique and portable solution for fast, cost-effective 6DoF vibration surveys and data acquisition. One important application is machinery condition monitoring system. The sensor is a 6-dimensional sensor with x-, y,-z-acceleration measurement along with roll, pitch, and yaw. Diverse functions like user defined filters and FFT-analysis or multichannel overlays are available.
Each NHH-engineer from the traditional world learned his trade, which included the levelling of the signals before each measurement. When the measurement systems had a dynamic range from 60dB 40 years ago and 90dB dynamic range 20years ago, it was a lot for these times. But a high standard laboratory microphone capsule has more, about 130dB from 15dBA self-noise level up to 146dB maximum pressure. All the time a high qualified person, a cost-intensive engineer, was necessary to do the levelling and measurement. Otherwise the signal could be lost in the noise of the measurement equipment or was unusable because of clipping effects.
In the new world with 32bit-signal of the MV 240 there is available an INTEGER number range for the sound signal between -2exp31 and +2exp31 (respective from -2147483648 to + 2147483647). This means, 1Pa sound pressure corresponds to a number of 1000000 because of the definition for 1µPa. The hearing threshold 20µPa can be mapped well as a measured value. Otherwise the maximum sound pressure can be 2147Pa, which corresponds to a sound pressure level of about 160dB. That processable range is much more than the dynamic range of standard ½-inch measurement microphone capsule.
Related to the old levelling problem was made a big step to the future in this new digital world. Possible errors by the operator are almost excluded now.
Electric drive machines of modern vehicles are working in a much lower sound pressure range than their siblings from the internal combustion engine sector. The movement of mechanical parts, which are exclusively rotating and not alternating as former connecting rods, pistons, rocker arms and valves, is more continuous. Rotations have harmonics at multiples at rotation speed. Alternating elements have shocks, impacts and other pulse-containing components in the signal, which generates in combination with the structure of the vehicle-construction more vibrations and ultimately more noise.
For noise and vibration investigations on electric vehicles are needed much more low noise sensors and microphones. To make a low noise measurement microphone capsule are necessary some complicated tricks of microphone construction. For instance the elements if internal attenuation of the membrane-vibration have to be switched off. A consequence of this is a very non-linear frequency response of this microphone with a pronounced resonance. This is initially extremely impractical for acoustic measurements. A curve correction is necessary, which could be realised in the internal signal processing chip, the arm. Alternatively there is also a new analogue IEPE-microphone MMS 214 from Microtech Gefell, which contains an analogue filter for such a curve correction.
Here it is evident, that the new world of electric vehicles requests new measurement equipment in border areas, what looks like an invitation to do new inventions for solutions if big advantages of the digital world are used for that.
If such kind of data processing is already done in the measurement microphone’s respective sensor, the direct measuring unit gives out a corrected neutral output-signal. Not to forget that this unit, as already mentioned, can be integrated as a simple sound interface at a measurement system.
The curve correction method is also necessary, if the use of a measurement microphone is at various sites of operation. There are conditions of acoustical free-field without any room effects and reflection from walls, random field in rooms with a very long reverberation time and pressure field in very small cavities our calibration device couplers. In each of these cases a microphone has another type of frequency response. In earlier times a correct selection of special types of measurement microphone capsules was necessary. Today this curve correction can be executed in the internal signal processing chip in the digital microphone.
A very new field in the world of two wheels are so-called E-motorbikes and E-scooter. As in former publications explained already, each kind of vehicle must pass an ISO- (our other standardized) measurement procedure, which is called Pass-By (f.e. ISO 362-1:2017). Since 1st of July 2019 AVAS (Acoustic Vehicle Alerting Systems) are obligatory in electrical, hybrid-electrical and fuel cell cars.
Whereas in the past the pass-by measurement method concentrated on limiting the maximum sound level value, today a minimum value must be achieved for the new electric and hybrid vehicles at slow speeds. This requirement also sensitively affects the measurement method, since the very low sound level readings now available are much more difficult to measure accurately with minimized but unavoidable ambient noise. This is particularly complicated by the fact that the same measuring equipment and possibly the same, slightly modified measuring system are to be used both for conventional pass-by measurement of “loud” vehicles with combustion engines and for quiet vehicles with minimum noise requirements.
MV 240 digita Dytran USB
A new type of digital measurement microphone with a processing width for signals of 32 bits and the resulting unprecedented high measurement dynamics is therefore also virtually predestined to solve this demanding measurement problem. Without changing the measurement settings, both cases can be covered herewith. For extreme cases, the usable measuring range can be shifted further into the corresponding limit range by using particularly low-noise measuring microphone capsules on the one hand or ultra-high level measuring 1/4- or 1/8-inch measuring microphone capsules on the other.
In the age of state-of-the-art electric or hybrid vehicles, it stands to reason that new types of modern measurement sensor technology will also be used in the NVH sector. On the one hand, the digital world in the field of measurement technology is increasingly converging with the digital control systems of such vehicles, which leads to synergy effects in measurement signal networking and evaluation. On the other hand, this new type of digital sensor technology in the form of measurement microphones and vibration sensors results in advantages that make it increasingly possible to get a grip on problems that are difficult to solve with conventional measurement technology. It is therefore worthwhile to further expand the links between new drive technology and digital measurement and sensor technology in the future and to follow the latest developments in this field.
Author
Dipl.-Ing. Udo Wagner, Microtech Gefell GmbH
Director Sales & Marketing, Director Calibration Services
