Offset Stability Research Articles

Three-Phase Busbar Current Transducer

A true three-phase 1000 A busbar current transducer is based on six micro-fluxgate sensors. Instead of using three independent single-phase current transducers, we use the full information from each sensor. Two TI DRV425 micro-fluxgate sensors are inserted into a hole drilled in each busbar. Our method of data processing is optimized to compensate the crosstalk between the three phases and external fields and gradients up to second order. The crosstalk error between the phases was 0.23% or smaller. The suppression of field from an external current at a distance of 10 cm is improved by a factor of 25 to 150. Our transducer has compact size, high temperature offset stability of 8.5 mA/°C, high current range up to 1000 A, low power consumption, and linearity of 0.1%. The results from a three-dimensional finite-element model and analytical computations confirm the measurements.

Testing and application of an integrated fluxgate sensor DRV425

Abstract The article presents testing of a miniature fluxgate sensor developed and manufactured by Texas Instruments as well as an application of the sensor in a compact magnetic field probe with USB interface and LabView based software. Several basic properties of the sensor were evaluated and compared with the datasheet specifications. Offset stability measurements have indicated some possible problems for applications that require very high DC and low-frequency precision. Jumps in offset were present for some of the devices, frequency and duty of the jumps varying between individual pieces and with respect to the supply voltage. Magnetic probe for fast and handy measurements of DC and AC (up to 30 kHz) magnetic fields (up to ±1 mT) is also presented. The probe contains 24-bit analog to digital converter, microcontroller and high-speed USB interface which allows a live view of the signal in time and spectral charts. The probe proved to be very useful for the DRV425 debugging, application development, EMC measurements and magnetic field demonstrations activities.

Differential AGC with offset stability and wide range frequency synthesis

In modern electronic systems, signal stability is a crucial issue. Many methods have been developed to mitigate the deviation from an ideal signal. One of them is called Automatic Gain Control, which tracks variation in an input signal to settle output amplitude. This paper proposes a Differential Automatic Gain Controller with offset stability and frequency synthesis. It is a Proportional Integral Derivative controller that stabilizes the gain, offset and frequency simultaneously. The proposed model is simulated using spice, and the simulation shows that accurate results are obtained across wide frequency ranges for all three parameters. An arrangement with equivalent circuit models for passive components was also simulated, which provides a novel envelope detector design that operates at frequencies ranging from 1 MHz to 1 GHz.

Fundamental Mode Orthogonal Fluxgate Magnetometer Applicable for Measurements of DC and Low-Frequency Magnetic Fields

Compatibility of low noise characteristic and offset stability is essential for a magnetometer to be applicable for measurements of dc and low-frequency magnetic fields. Fundamental mode orthogonal fluxgate (FM-OFG) magnetometer has advantages in its compact design and low noise characteristic, whereas its disadvantage is the offset drift which is unacceptably large for such measurement. An effective technique called bias switching has been proposed to suppress and stabilize the offset of open-loop FM-OFG magnetometers, yet when it is adopted with a feedback configuration to obtain better linearity, it increases noises attributed to the convergence process. In this paper, we present a design of sensor driving electronics of FM-OFG magnetometers to solve this problem and achieve compatibility of low noise and stable offset characteristics with a feedback configuration. The efficacy of the design was verified through various performance evaluations.

Simultaneous Angular Rate Estimates Extracted From a Single Axisymmetric Resonator

An axisymmetric planar microelectromechanical (MEM) resonator resonator is configured, such that the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n = 2$ </tex-math></inline-formula> elliptical pair of modes near 13.5kHz, and the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n = 3$ </tex-math></inline-formula> pair of modes near 23.8kHz, are both degenerate, i.e., the frequency difference within a given pair of modes is close to the resonance bandwidth. This configuration enables not only the exploitation of the standard elliptical pair of modes for angular rate sensing but also permits the operation of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n = 3$ </tex-math></inline-formula> pair of modes as a high-sensitivity Coriolis vibratory gyro. The performance for each pair of modes separately acting as a vibratory gyro is quantified, however, the control architecture also facilitates the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">simultaneous</i> operation of both pairs of modes. In this scenario, two measurements of angular rate are extracted from a single resonator and although the short-term rate noise associated with the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n = 3$ </tex-math></inline-formula> pair is an order of magnitude larger than the rate extracted from the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n = 2$ </tex-math></inline-formula> pair, the long-term drift in the rate offsets are correlated. Thus, a filter architecture for fusing the rate measurements is proposed and it is shown how the derived rate estimate possesses superior offset stability but also retains the low short-term noise associated with the rate measurement from the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n = 2$ </tex-math></inline-formula> pair.

Low Offset Drift–Low-Noise Orthogonal Fluxgate With Synchronized Polarity Flipping

In this paper, we present an orthogonal fluxgate in fundamental mode with extremely stable offset and low noise. The typical solution to stabilizing the offset in this type of sensor is to flip the dc bias of its excitation current and subtract the output obtained with opposite polarities. In this case, on the contrary, we flip both the ac and dc components of the excitation current and sum the output voltages. This is performed using fast solid-state switches with low on-resistance. In this way, we achieved a large suppression of the dependence of the offset on temperature, obtaining an average offset drift of 0.012 nT/K, making this sensor on the top class for offset stability. We digitally sampled and numerically demodulated the output voltage of the sensor to extract the first harmonic. The value of the first harmonic shows noise right after switching of the current. Therefore, we numerically erased those few noisy values of the first harmonic before integrating it. In this way, we managed to limit the noise growth given by switching of the current. Finally, we strongly improved the offset stability with minor increment of noise.

Spin axis offset calibration on THEMIS using mirror modes

Abstract. A newly developed method for determining spin axis offsets of magnetic field instruments on spacecraft is applied to THEMIS. The formerly used determination method, relying on solar wind Alfvénic fluctuations, was rarely applicable due to the orbital restrictions of the mission. With the new procedure, based on magnetic field observation of mirror modes in the magnetosheath, updated spin axis offsets can be estimated approximately once per year. Retrospective calibration of all THEMIS magnetic field measurements is thereby made possible. Since, up to this point, spin axis offsets could hardly ever be calculated due to the mission's orbits, this update represents a substantial improvement to the data. The approximate offset stability is estimated to be &lt; 0.75 nT year−1 for the complete course of the mission.

On determining fluxgate magnetometer spin axis offsets from mirror mode observations

Abstract. In-flight calibration of fluxgate magnetometers that are mounted on spacecraft involves finding their outputs in vanishing ambient fields, the so-called magnetometer offsets. If the spacecraft is spin-stabilized, then the spin plane components of these offsets can be relatively easily determined, as they modify the spin tone content in the de-spun magnetic field data. The spin axis offset, however, is more difficult to determine. Therefore, usually Alfvénic fluctuations in the solar wind are used. We propose a novel method to determine the spin axis offset: the mirror mode method. The method is based on the assumption that mirror mode fluctuations are nearly compressible such that the maximum variance direction is aligned to the mean magnetic field. Mirror mode fluctuations are typically found in the Earth's magnetosheath region. We introduce the method and provide a first estimate of its accuracy based on magnetosheath observations by the THEMIS-C spacecraft. We find that 20 h of magnetosheath measurements may already be sufficient to obtain high-accuracy spin axis offsets with uncertainties on the order of a few tenths of a nanotesla, if offset stability can be assumed.

Handling the influence of chemical shift in amplitude-modulated heteronuclear dipolar recoupling solid-state NMR.

We present a theoretical analysis of the influence of chemical shifts on amplitude-modulated heteronuclear dipolar recoupling experiments in solid-state NMR spectroscopy. The method is demonstrated using the Rotor Echo Short Pulse IRrAdiaTION mediated Cross-Polarization ((RESPIRATION)CP) experiment as an example. By going into the pulse sequence rf interaction frame and employing a quintuple-mode operator-based Floquet approach, we describe how chemical shift offset and anisotropic chemical shift affect the efficiency of heteronuclear polarization transfer. In this description, it becomes transparent that the main attribute leading to non-ideal performance is a fictitious field along the rf field axis, which is generated from second-order cross terms arising mainly between chemical shift tensors and themselves. This insight is useful for the development of improved recoupling experiments. We discuss the validity of this approach and present quaternion calculations to determine the effective resonance conditions in a combined rf field and chemical shift offset interaction frame transformation. Based on this, we derive a broad-banded version of the (RESPIRATION)CP experiment. The new sequence is experimentally verified using SNNFGAILSS amyloid fibrils where simultaneous (15)N → (13)CO and (15)N → (13)Cα coherence transfer is demonstrated on high-field NMR instrumentation, requiring great offset stability.

Offset stable piezoresistive high-temperature pressure sensors based on silicon

Abstract. The exploitation of new application fields and the drive to size reduction even in highly stable pressure sensing systems makes the extension of the operating temperature range of the microelectromechanical sensors (MEMS) essential. For this reason a silicon-based pressure sensor with an application temperature ranging up to 300 °C and the associated manufacturing technology was developed. With special design and manufacturing approaches mounting stress-insensitive sensors with high linearity, excellent offset stability, low hysteresis and low sensitivity changes over the entire temperature range were developed. At the moment, the sensors are tested till 300 °C at wafer level and between 135 and 210 °C as a first-level package.

Construction of an AMR magnetometer for car detection experiments

A new construction of magnetometer with commercially available AMR (anisotropic magnetoresistive) sensors intended for vehicle detection experiments is presented. Initial experiments with simple AMR gradiometer indicated viability of the approach in a real- world setup. For further experiments and acquisition of representative data, a new design of precise multi-channel magnetometer was developed. The design supports two models of commercial AMR sensors: the proven and reliable, but obsolete Honeywell HMC1021-series sensors and newly available Sensitec AFF755B sensors. In the comparison the two types are similar in most achieved parameters, except offset stability in flipped operation regime. Unfortunately, the new AFF755B sensors seem to have perhaps inferior coupling of the flipping (set/reset) coil to the ferromagnetic core that causes insufficient saturation of the AMR material. The issue is being solved by Sensitec, current deliverables of the AFF755B have “product sample” status (September 2015).

Enhanced analogue front-end for the measurement of the high state of wide-band voltage pulses with 87 dB common-mode rejection ratio and ±0.65 ppm 1-day offset stability.

An improved analogue front-end for measuring the high state of trapezoidal voltage pulses with transition duration of 3 μs is presented. A new measurement system, composed by a front-end and the state-of-the-art acquisition board NI PXI-5922, has been realized with improved Common Mode Rejection Ratio (CMRR) of more than 87 dB at DC and 3-sigma stability of ±0.65 ppm over 1 day. After highlighting the main design enhancements with respect to state-of-the-art solutions, the CMRR measurement is reported. The output drift due to temperature and humidity is assessed to be negligible. Finally, the worst-case repeatability is measured both with shorted-to-ground inputs and with an applied common-mode voltage of 10 V, which represents the nominal working condition.

Charge offset stability in Si single electron devices with Al gates

We report on the charge offset drift (time stability) in Si single electron devices (SEDs) defined with aluminum (Al) gates. The size of the charge offset drift (0.15 e) is intermediate between that of Al/AlOx/Al tunnel junctions (greater than 1 e) and Si SEDs defined with Si gates (0.01 e). This range of values suggests that defects in the AlOx are the main cause of the charge offset drift instability.

Circuit Design of Digital Closed Loop Control System for FOG

Fiber optic gyroscope (FOG) is a new angular rate sensor based on the Sagnac effect. It has been widely applied in the navigation control system of aircrafts, spacecrafts and ships. But its stability, reliability and miniaturization are always the research focuses and difficulties. This paper presented a circuit design method for the digital closed loop control system of FOG based on FPGA. Based on a large number of experiments, this paper summarized the parameter demands for each module of closed loop control circuit and designed a corresponding circuit using FPGA. The proposed dual closed loop technique improved the zero offset stability of FOG. The using of FPGA brought the digital signal processing by software, the system reliability and agility enhancement as well as the system miniaturization. Particularly, we discussed the problems of component selection and the anti-jamming measures for PCB design to improve the performances of the system. We also developed some samples of FOG using this design method. The experiments and tests show that the proposed method is efficient and valuable. The stabilities of zero-offsets of all samples are less than 0.075deg/h.

The millage rate offset and property tax revenue stability

Abstract One of the alleged virtues of the property tax is that it produces stable revenues regardless of price movements in real estate markets. One explanation for this is that local governments adjust their millage rates to offset changes in their property tax base. Little evidence, however, exists on the strength of this millage rate offset mechanism. We hypothesize that the importance of this mechanism will vary among local governments depending upon the monopoly power that they possess. The results provide strong support for our hypothesis.

A Fully Differential Charge-Balanced Accelerometer for Electronic Stability Control

An accelerometer for electronic stability control utilizes a two-mass mechanical sensor element to implement a fully-differential signal path, achieving robustness against electromagnetic interference (EMI) without the need for external shielding in the package. The EMI rejection is augmented further with a pseudo-random chopping scheme, which spreads the interference over a wide bandwidth, reducing its in-band portion to the level of the noise floor. The chopping function maintains zero-mean voltage waveforms across the sensor electrodes, which is also beneficial for the long-term offset stability of the device. A charge-balanced capacitance-to-voltage converter provides linear transduction for displacements of the proof-mass up to 70% of the gap and minimizes the residual electrostatic forces. A dual-axis design occupies 1.1 mm 2 in 0.18- μm CMOS and consumes 820 μA from an internally regulated 1.9-V supply. The system achieves 380 μg/ √Hz noise floor and 84-dB dynamic range. The offset variation in the automotive temperature range of -40 to +140°C has a 3 σ range of ±11 mg.

Area-Efficient Three Axis MEMS Lorentz Force Magnetometer

A micro-electromechanical-systems (MEMS) three-axis Lorentz force magnetometer based on a 0.24 × 0.4 mm2 MEMS resonator that is the smallest Lorentz force sensor reported to date is presented in this paper. A magnetic field can be detected in two axes using a single MEMS structure. Placing two structures perpendicular to each other in a single die makes three-axis sensing possible. Sensing is performed by exciting the MEMS resonator at its in-plane and out-of-plane mechanical resonant frequencies of 40.5 and 107.4 kHz, respectively. A modest die-level vacuum packaging results in in-plane and out-of-plane mechanical quality factors of 110 and 310, respectively. The sensor has a bandwidth of 184 Hz for z-axis and 189 Hz for x/y-axis magnetic field. With an excitation power of 2 mW, the sensor resolution is 285 nT/√Hz for z-axis magnetic field inputs and 344 nT/√Hz for x/y-axis magnetic field inputs. The resolution is limited by Brownian noise in the z-axis whereas the x/y-axis resolution is limited by electronic noise. With an averaging time of 288 s, the sensor shows an offset stability of 23 nT.

The Impact on the Positioning Accuracy of the Frequency Reference of a GPS Receiver

Despite the pervasive use of the global positioning system (GPS) as a positioning technology for its high efficiency and accuracy, several factors reduce its performance. This study examines to which extent the frequency offset and the frequency stability of the internal quartz oscillator or of an externally supplied rubidium oscillator have an influence. Observations were made at the Taiwan Ching Yun University (TCYU) tracking station, where a quartz oscillator and a rubidium oscillator were applied alternatively on a monthly basis throughout a 16-month period. Moreover, the accuracy of the local oscillator used in this study was calibrated by the National Standard Time and Frequency Laboratory, Taiwan. The frequency offset and frequency stability calculated via the remote method at the TCYU station were compared with values (uncertainty is 3.0E−13) measured directly at the National Standard Time and Frequency Laboratory, Taiwan. Analytical results show that the two methods vary by 1.4E−10 in terms of frequency offset and by 6.5E−12 in terms of frequency stability, demonstrating that the remote method can yield computational results almost as accurate as direct measurement. Positioning precision results also show that rubidium oscillator accuracy improved by 5, 11, and 15 % for short-, medium-, and long-baseline positioning, respectively, indicating that clock quality is more influential for long-baseline GPS relative positioning and that the frequency stability of a receiver clock is far more critical than the frequency offset. On the other hand, the positioning performance noted is essentially independent (max. 15 % change) of the reference frequency stability, which indeed differed by 4 orders of magnitude.

Three-Axis Lorentz-Force Magnetic Sensor for Electronic Compass Applications

A low-power microelectromechanical-systems (MEMS) three-axis Lorentz-force magnetic sensor is presented. The sensor detects magnetic field in two axes with a single MEMS structure. Three-axis sensing is performed using two perpendicular structures on the same die. The MEMS device is a micromechanical resonator, and sensing is conducted using excitation currents at the device's in-plane and out-of-plane mechanical resonant frequencies which are 20.55 and 46.96 kHz, respectively. A die-level vacuum seal results in in-plane and out-of-plane mechanical quality factors of 1400 and 10000, current, the sensor's noise is equivalent to 137 nT/√Hz for the respectively. With 0.58 mW used to provide the two-axis excitation z-axis magnetic field inputs and 444 nT/√Hz for the x-and y-axis fields. For the z-axis field measurements, Brownian noise is the dominant noise component, while the xand y-axis field measurements are limited by the electronic noise in the discrete capacitive-sensing electronics. The major source of offset error is residual motion induced by electrostatic force. The offset is reduced to 14 μT using a dc compensation voltage applied to the MEMS structure to null the electrostatic force. After compensation, the offset stability is 400 nT with a 0.7-s averaging time.

Precision Signal Conditioning Electronics for Cryogenic Temperature and Magnetic Field Measurements in SST-1

As a part of refurbishment of Steady-state Superconducting Tokamak-1 (SST-1) at the Institute for Plasma Research, India, all 16 toroidal field (TF) magnets of SST-1 have been individually tested in cold with nominal current in a dedicated experimental cryostat. The magnets were cooled down to 4.5 K using either supercritical or two-phase helium, after which they were charged up to 10 kA of transport current. Precise cryogenic temperature and magnetic field measurements in the experimental configuration were mandatory. Temperatures were required to be accurately measured at several locations in the magnet and hydraulic circuits, whereas the field measurements were carried out at few predefined locations on the magnet case. Highly accurate in-house modular signal conditioning electronics had been developed for low temperature and high magnetic field measurements for such large magnets. This paper describes the scheme of the measurement, precautions taken for enhanced reliability, precautions taken for long-term offset stability in cryogenic environment, and the results of SST-1 TF magnet tests.