On the design of a triaxial accelerometer

Up to now, mainly uniaxial accelerometers are described in most publications concerning this subject. However, triaxial accelerometers are needed in the biomedical field. Commercially available triaxial accelerometers consisting of three orthogonally positioned uniaxial devices do not meet all specifications of the biomedical application. Therefore, a new highly symmetrical inherently triaxial accelerometer is being developed, the advantages of which are higher sensitivity and reduction of off-axis sensitivity

Polydimethylsiloxane as an elastic material applied in a capacitive accelerometer

Polydimethylsiloxane is a silicone rubber. It has a unique flexibility, resulting in one of the lowest glass-transition temperatures of any polymer. Furthermore, it shows a low elasticity change versus temperature, a high thermal stability, chemical inertness, dielectric stability, shear stability and high compressibility. Because of its high flexibility and the very low drift of its properties with time and temperature, polydimethylsiloxane could be well suited for mechanical sensors, such as accelerometers. A novel capacitive accelerometer with polydimethylsiloxane layers as springs has been realized. The obtained measurement results are promising and show a good correspondence with the theoretical values

Mass flow meter of the Coriolis type

A mass flowmeter of the Coriolis type with a tube that forms a mechanically closed loop through which a medium flows during operation, and with excitation means for causing the loop to rotate in an oscillatory mode about an axis of rotation during operation. The loop has a starting point and an end point. The starting and end points are situated close together and are connected to a flexible inlet tube and a flexible outlet tube, respectively, which extend preferably in parallel and close to gether. The loop is resiliently suspended from the frame of the flowmeter by means of the flexible inlet and outlet tubes, which preferably form one integral piece with the tube of the loop

Mass flowmeter of the Coriolis type

A mass flowmeter of the Coriolis type with a tube that forms a mechanically closed loop through which a medium flows during operation, and with preferably electromagnetic, contacless excitation elements for causing the loop to rotate in an oscillatory mode about an axis of rotation during operation. The loop has a starting point and an end point. The starting and end points are situated close together and are connected to a flexible inlet tube and a flexible outlet tube, respectivily, which extend preferably in parallel and close together. The loop is resiliently suspended from the frame of the flowmeter via the flexible inlet and outlet tubes, which preferably form one integral piece with the tube of the loop

The mechanical properties of the rubberelastic polymer polydimethylsiloxane for sensor applications

Polydimethylsiloxane (PDMS) is a commercially available physically and chemically stable silicone rubber. It has a unique flexibility with a shear elastic modulus due to one of the lowest glass transition temperatures of any polymer . Further properties of PDMS are a low change in the shear elastic modulus versus temperature , virtually no change in G versus frequency and a high compressibility. Because of its clean room processability, its low curing temperature, its high flexibility, the possibility to change its functional groups and the very low drift of its properties with time and temperature, PDMS is very well suited for micromachined mechanical and chemical sensors, such as accelerometers (as the spring material) and ISFETs (as the ion selective membrane). It can also be used as an adhesive in wafer bonding, as a cover material in tactile sensors and as the mechanical decoupling zone in sensor packagings

Characterisation of a highly symmetrical miniature capacitive triaxial accelerometer

A highly symmetrical cubic capacitive triaxial accelerometer for biomedical applications has been designed, realised and tested. The sensors are available in two outer dimensions, namely 2×2×2 and 5×5×5 mm3. The devices are mounted on a standard IC package for easy testing. Features of the sensor are a highly symmetrical cubic structure, capacitive coupling of the high frequency input voltage to the seismic mass and the use of the polymers polydimethylsiloxane (PDMS) as spring material between the capacitor plates and the mass and polyimide (PI) as flexible interconnection layer between the capacitor plates. The sensor structure and its basic operating principle, its mathematical mass-spring-damper model and the necessary cleanroom technology are described. The measurement results show a sensitivity of ~0.15 V/(m/s2) with a corresponding resolution of 0.01 m/s2, an off-axis sensitivity of <5 %, a good linearity in the output voltage for accelerations up to at least 50 m/s2 and a bandwidth of DC 500 H

Design, realization and characterization of a symmetrical triaxial capacitive accelerometer for medical applications

Small triaxial accelerometers are needed in the medical field for the monitoring of mobility. For this purpose, a new highly symmetrical inherently triaxial capacitive accelerometer has been designed. The basic structure of the device consists of six capacitors surrounding a central mass which is suspended by springs made of the rubber elastic polymer polydimethylsiloxane. The advantages of the design are a low off-axis sensitivity, an equal sensitivity in all axes and a reduction of the sensor's dimensions. In order to show the practical feasibility of the design, a number of manually assembled prototypes of the triaxial accelerometers have been realized with dimensions down to 2 mm × 2 mm × 2 mm. The prototypes are capable of detecting accelerations in three directions with unfortunately unequal sensitivities per axis (e.g., from 0.8 to 1.1 V (m s2) 1) and a maximum off-axis sensitivity of 3% in the well-assembled devices. Clinical measurements have been carried out with the prototypes. The measurement results indicate that the triaxial accelerometer is sensitive enough to register the kind of movements that occur in healthy persons during normal standing

A sensitive differential capacitance to voltage converter for sensor applications

There is a need for capacitance to voltage converters (CVC's) for differential capacitive sensors like pressure sensors and accelerometers which can measure both statically and dynamically. A suitable CVC is described in this paper. The CVC proposed is based on a symmetrical structure containing two half ac bridges, is intrinsically immune to parasitic capacitances and resistances, is capable of detecting capacitance changes from dc up to at least 10 kHz, is able to handle both single and differential capacitances, and can easily be realized with discrete components. Its sensitivity is very high: detectable capacitance changes of the order of 2 ppm of the nominal value (24 aF with respect to a nominal capacitance of 12 pF) result in a measured output voltage of 1.5 mV. However, due to drift the absolute accuracy and resolution of the CVC is limited to 3.5 ppm. A differential accelerometer for biomedical purposes was connected to the CVC and showed a sensitivity of 4 V/g. The measured rms output voltage noise in the frequency range of 2-50 Hz is 750 μV, resulting in a signal to noise ratio of 75 dB at an acceleration of 1 g in the frequency range of 2-50 Hz

Coriolis mass flow meter using contactless excitation and detection

A mass flowmeter of the Coriolis type with a tube through which a medium flows during operation and with excitation means for causing the entire tube or part thereof to perform a rotational vibration about a primary axis of rotation during operation. The excitation means are electromagnetic and designed such that they do not make contact with the tube during operation and have no components that are fastened to the tube. More in particular, the tube is made of an electrically well conducting material, and the excitation means comprise first means for causing an electric current to flow through the tube during operation and second means for generating a magnetic field at the area of a portion of the tube, which magnetic field is perpendicular to the direction of the current and in operation either the current or the magnetic field changes its sign periodically, with the result that the (Lorentz) force generated by the product of current and magnetic field causes a rotational oscillation of the entire tube or a portion thereof about the primary axis of rotation