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Angular motions are at least as prevalent in the automobile and its surroundings as linear motions but their accurate measurement has been difficult.  The requirement to measure angular motion has become more critical during the last 10 years due to rapid changes in automotive design and testing technologies.  The greatest changes have been brought about by the integration of computers into automotive designs and automated testing which creates the need to capture accurate motion and vibration data for use in modeling, simulation and test fixtures.  The safety regulation of the automotive industry has also resulted in large efforts to standardized crash testing with additional needs to accurately measure angular motions in both vehicles and in crash test mannequins.  Out of these necessities, MHD (Magnetohydrodynamic) angular rate sensors were developed to provide a wide range of solutions for the automotive and aerospace industries.

The prior technology for angular motion analysis used techniques requiring use of linear accelerometers in arrays such as the 3-2-2-2 array described by Padhaonkar, et al.1(1975) or the 3-3-3 array of Nusholtz2. (1993).  However, these techniques require positioning multiple linear accelerometers per axis measured with multiple data channels per axis measured.  They have more complex calculations based on the arrays used, and may include compounded effects of measurement error in the accelerometers and the multiple data channels per axis measured. MHD sensors allow single data channels per axis and direct reading of angular rates which may be easily converted to acceleration using optional in-line-converter modules, or the conversion may be done in the measurement and analysis equipment.

The evolution of new MHD sensor designs for vibration and angular motion analysis have resulted in a range of products that can be used to measure angular rates as high as ±11,000 degrees/second or vibration motions as small as 50 nano-radians over a basic frequency range of less than 1 to greater than 1,000 Hz.  Digital filtering techniques allow the basic frequency response to be extended to below 0.2 Hz and above 1,500 Hz.  The typical modified frequency and phase response curves are shown in Figure 1. Typical results of standard MHD sensors with the digital filtering program yield noise equivalent rates and noise equivalent angles as shown below over the full range from 0.1Hz to >1000 Hz.

The low cross-axis sensitivity and insensitivity to linear shocks make the MHD sensors particularly suited to motion and vibration analysis in automotive applications, and units are hermetically sealed so the use environment may cover the full range for which automobiles are designed.  The sensors have excellent linearity over the frequency range as shown in Figure 2.  MHD sensors are suitable for suspension and chassis motion and vibration analysis, engine and drive train torsional and vibration analysis, as well as for the uses in crash testing previously reported³.  MHD angular rate sensors may be mounted anywhere on the housing of rotating equipment with the angular axis of sensitivity in the same plane as the rotation and still provide a complete spectrum of the vibrations.  This flexibility allows the use of MHD angular sensors and 6 degree-of-freedom arrays in the gathering of data on vehicles under all test conditions. Typical sensors use dual (+/-) voltages inputs between 5 and 15 Vdc, but dynamic automotive applications have resulted in new models which can be operated off of a single +10 to +12 Vdc.

New uses of the MHD sensors include providing typical driving condition shock, vibration, and motion data for use in programming simulators and motion test by attaching analytical instrumentation with angular sensors to each of the suspension or wheel assemblies, supplementing these with 6 degree-of-freedom arrays at the center of-gravity of the vehicle chassis, and utilizing angular motion sensors at the steering components, complete measurements of vehicle motion is achieved.  The resulting analysis of angular and linear motions and vibrations can then be used to program simulators and test stands to emulate exact driving conditions, or to emulate the vibrations, shocks and motions acting on specific subassemblies and components.  MHD sensors are uniquely suited to these applications because of their small size and ease of mounting on moving vehicles, their ability to withstand the broad environmental limits, the simple data analysis equipment required, and their rugged design.

Angular rate sensors also measure the movements of automobile and truck chassis and suspension components in braking tests and in evasive handling maneuvers.  They can be used as roll-over sensors, and are finding use in race cars for analyzing and setting the chassis adjustments for different race tracks for repeatable performance.

In measuring the torsional velocity and vibration of rotating equipment such as power trains, power generation units and engines, MHD angular sensors replace laser systems and accelerometers for direct measurement in multiple axis.  Used by engine and generator manufacturers such as Cummins Power Systems’ Onan Division for torsional vibration measurements, MHD angular rate sensors has been verified to yield accurate results over the usable response curve of the sensor.  Test results compare favorably with laser systems as shown in Figure 3.  Amplification and phase response have been found to be very repeatable and accurate, with phase response for specific applications brought to within +/- 3 degrees over the large frequency range by component selection in ARS-01 on-board signal conditioning electronics.

In automotive crash and roll-over testing, the versatile MHD sensors again play a variety of rolls including use in anthropomorphic test mannequins for measurement of head and pelvis rotation. Extensive studies by the U.S. Department of Transportation, the U.S. Navy Biodynamic Laboratory^4,5 and others with the ARS-01 type MHD sensors proved the MHD devices to be as accurate and responsive to measuring angular motions any of the other methods available while requiring less data channels and calculations.  The ARS-01 products have been made into triaxial arrays and 6 degrees-of-freedom arrays for use in the Hybrid III crash test dummy heads and for use in aircraft ejection seat testing.  Further work by the University of Virginia and Ford Motor Company⁶ with the ARS-04 and Dynacube™ sensors validated measurement techniques and extended the use of MHD angular rate sensors to use in joint kinematics for neck, spine, subtler joints, and ankles.

In other areas of automotive crash analysis, the MHD sensors help automotive designers understand the dynamic forces affecting various components such as passenger seats, sheet metal structural components and collapsible or deformable structures.  During crash testing, the sensors measure the angular motions of the structures to allow understanding of the motions and forces involved for failure analysis and design of better components.  With the ability to operate to linear shock levels well above 1000g and survive shocks over 3,000g, MHD angular rate sensors are ideal for these crash testing applications.

The next generation of MHD angular rate sensors is now under development.  New products like the ARS-12 have extremely high sensitivities for measuring vibrations and very low noise floors.  These products incorporate internal transformers and unique component designs to achieve new levels of performance for automotive and aerospace angular motion and vibration analysis and testing.  Combined with solid state pumps for the internal fluids, new MHD effect gyroscope designs based on the ARS-12 are also emerging which offer the promise of supplementing GPS navigation systems when the GPS signals are blocked by “urban canyons” of high buildings and natural geographic features.  Whenever there are new needs for measurement products, MHD sensors will have all the angles covered.

             
        Figure 1a.  Overlay of the Normalized Magnitude Response of the ARS-01 MHD sensor, H(s), the
        compensation filter C(s), and the extended (compensated) ARS-01 Response, H_c(s)=H(s)C(s).

             
        Figure 1b.  Overlay of the Phase Responses of the ARS-01, H(s), the compensation filter C(s),
        and the extended (compensated) ARS-01 Response, H_c(s)=H(s)C(s).
                 
        Figure 2.  Linearity of MHD angular rate sensors calibrated against linear accelerometers.
              
        Figure 3.  Correlation of various measurement methods shows that MHD sensors have responses
        equal to laser based systems and linear accelerometer based methods while offering multiple axes
        and single data channel per axes simplicity. (Data courtesy of Onan Division, Cummins Power
        Systems).

                 

All data is believed accurate at time of publication.  Specifications are subject to change without notice.
Copyright © 2002-2008  Applied Technology Associates. All rights reserved.  Revised:  02/13/2008