/// ACCELEROMETERS\\\


think it’s the first example of a manufacturer completing an RDE test programme, includ- ing at altitude, in the UK.”


Production advantage \\\ Through strategic use of the chamber test-


ing facilities at the RDE Centre, Mahle and its customer have managed to turn a poten- tially delaying and costly challenge into a production advantage. Replicating whole drive cycles in one controlled environment as opposed to shipping vehicles, engineers and associated hardware around the globe – even in a Covid-free world – makes a com- pelling commercial and engineering case. Brooks notes that, based on Mahle Power- train’s own experience, engineering time combined with travel and subsistence costs can very quickly reach significant sums, even for relatively modest test programmes.


Controlled environment \\\ The main advantage of the facility is the abil-


ity to test in a precisely controlled environ- ment that can replicate any necessary test condition. This includes aggressive climbs on a 4WD chassis dyno up to 5,000m with rele- vant barometric pressures, variable road gra- dients at temperatures between -40 and +50°C, and humidity levels of between 10 and 80 per cent. “The vehicle manufacturer carried out eight mountain test runs over a three-day pe- riod which would normally take 9-10 days plus travel and engineer time, with no guarantee of the stable conditions necessary for good test data,” continues Brooks. “This test programme provided a guaran-


tee of safety – not only from Covid but from day-to-day road incidents – and crucially, re- peatability. “There are a number of factors to consider


when extending physical location testing, such as logistics, accommodation and vehicle shipping. It is much easier to simply book an- other day of test chamber time. This could mark the start of a real shift in the way pow- ertrain test programmes are planned.” Mahle Powertrain has worked with most major vehicle manufacturers to carry out testing and sees the use of dynamic test pro- grammes as a key factor in accelerating the time to market of new vehicles and technolo- gies. In a time-contracted automotive indus- try, Mahle Powertrain believes this provides a significant competitive advantage combined with reduced cost. In the face of growing demand, the com-


pany is already building an additional climatic chamber, which will be ready for utilisation by vehicle manufacturers early next year. C&VT


Laser light takes the strain


US Technology body develops new breed of accelerometer to meet the needs of wearables, the IoT and vehicle autonomy


A


ccelerometers are becoming more commonplace in practically every- thing that people are involved in. Mo-


bile phones and wearable devices are owned and used by almost everyone. The role of the accelerometer in measuring acceleration is increasingly in demand for small navigation systems and other consumer devices. Responding to this need as well as more critical applications in vehicle navigation and safety systems, America’s National Institute of Standards and Technology (NIST) has de- veloped an accelerometer measuring just 1mm thick that uses laser light instead of me- chanical strain to produce a signal. Although a few other accelerometers also


rely on light, the design of the NIST instru- ment makes the measuring process more straightforward, providing higher accuracy. It also operates over a greater range of fre- quencies and claims to have been more rig- orously tested than similar devices.


Light reference \\\ According to NIST, its optomechanical ac-


celerometer is much more precise than exist- ing commercial accelerometers and doesn’t need to undergo the time-consuming process of periodic calibrations. And as the instrument uses laser light of a known frequency to mea- sure acceleration, it may ultimately serve as a portable reference standard to calibrate other accelerometers already on the market, making them more accurate. The accelerometer also has the potential to


improve inertial navigation in such critical systems as military aircraft, satellites and submarines, especially when a GPS signal is not available. In a recent paper published by NIST, re-


searchers from the organisation explain the basic principles of accelerometers and how they apply to their device. Like all such instru- ments, they record changes in velocity by tracking the position of a freely moving mass relative to a fixed reference point inside the de- vice. The distance between the mass and the


reference point only changes if the accelerom- eter slows down, speeds up or alters direction.


Lasers and mirrors /// The motion of the mass creates a detectable


signal. The NIST accelerometer relies on in- frared light to measure the change in distance between two highly reflective surfaces that bookend a small region of empty space. The mass, which is suspended by flexible beams one-fifth the width of a human hair so that it can move freely, supports one of the mirrored surfaces. The other reflecting surface, which serves as the accelerometer’s fixed reference point, is an immovable microfabricated con- cave mirror. Together, the two reflecting surfaces and the empty space between them form a cavity in which infrared light of just the right wave- length can resonate, or bounce back and forth, between the mirrors, building in inten- sity. That wavelength is determined by the distance between the two mirrors. If the mass moves in response to acceleration, changing the separation between the mirrors, the res- onant wavelength also changes. On the NIST device, the dynamic relation- ship between the displacement of the mass and the acceleration is simple and easy to model through first principles. The mass and supporting beams are designed so that they behave like a simple spring, or harmonic os- cillator, that vibrates at a single frequency in the operating range of the accelerometer. This simple dynamic response enabled NIST


to achieve low measurement uncertainty over frequencies from 1 to 20kHz without having to calibrate the device. This feature is unique because all commercial accelerometers have to be calibrated. Capable of sensing displacements of less than one hundred-thousandth the diameter of a hydrogen atom, the optomechanical ac- celerometer detects accelerations as tiny as 32 billionths of a g. That’s a higher sensitivity than all accelerometers now on the market with similar size and bandwidth. C&VT


2021 /// Climatic & Vibration Testing \\\ 17


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