radar echo delay, black holes and gravitational waves. It is worth pointing out that even here the author goes beyond a mere introduction, for example in the analysis of the phase diagram of cosmological parameters, the calculation of the first acoustic peak in the spectrum of the cosmic microwave background, the discussion of the precession of gyroscopes, the utilisation of Penrose diagrams to understand the causal structure of spacetime and the study of rotating black holes. Gravitational lensing, despite its important role in modern astronomy, is not covered at all. After just under 600 pages, the reader is already well versed in general relativity and many of its most important applications. Where many other textbook authors would have stopped, Zee goes on and continues with more advanced and modern subjects that will prepare the student to start doing research in the area. Maximally symmetric spaces, the vielbein formalism, conformal algebra, de Sitter and anti de Sitter spacetime, Kaluza– Klein theory, brane worlds, topological field theory and twistors are some of the more exotic topics the author addresses. These chapters make Zee’s book incredibly worthwhile, since treatments at the undergraduate level of these subjects hardly exist. All in all, Zee succeeded to write a unique textbook on general relativity. His narrative is exciting and well structured, but he also does not shy away from computations when they are necessary. The logical steps in attacking a problem are always presented very transparently, making it easier for the novice to become familiar with the material. The book is beautifully typeset and richly illustrated. It contains exercises at the end of each chapter and solutions to selected exercises in the appendix, together with a detailed index and a collection of important formulae. I strongly recommend this book to anyone interested in understanding general relativity.

Optical Magnetometry: Scope: reference. Level: postgraduate

To improve the performance of the surface permanent magnet synchronous motor drives, a sensorless control scheme based on the sliding-mode observer with an orthogonal phase-locked loop (PLL) incorporating two synchronous frequency-extract filters (SFFs) is proposed. The rotor position estimation errors are analyzed. The analysis results show that the harmonic errors of the estimated signal are hard to eliminate completely. Therefore, an improved adaptive notch filter - The SFF is proposed to extract the fundamental wave of the rotor position estimation before applying to the PLL. This method of the PLL combined SFFs can compensate the estimated back electromotive force harmonic error efficiently and adaptively. An experimental driveline system used for testing the electrical performance of the developed magnetically suspended motor is built. The effectiveness and the feasibility of the proposed method are validated with the experimental results.

Adaptive Compensation Method for High-Speed Surface PMSM Sensorless Drives of EMF-Based Position Estimation Error

Chip-scale atomic devices combine elements of precision atomic spectroscopy, silicon micromachining, and advanced diode laser technology to create compact, low-power, and manufacturable instruments with high precision and stability. Microfabricated alkali vapor cells are at the heart of most of these technologies, and the fabrication of these cells is discussed in detail. We review the design, fabrication, and performance of chip-scale atomic clocks, magnetometers, and gyroscopes and discuss many applications in which these novel instruments are being used. Finally, we present prospects for future generations of miniaturized devices, such as photonically integrated systems and manufacturable devices, which may enable embedded absolute measurement of a broad range of physical quantities.

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Chip scale atomic devices

Accurate rotor position detection is essential for the surface permanent magnet synchronous motor (SPMSM) drive system to realize the stable and reliable operation. In this paper, two linear hall-effect sensors are utilized to detect the rotor position. However, the estimated errors in the measured signals, especially the harmonic components, impact on the accuracy of the rotor angle estimation significantly. For this reason, a high-precision rotor position detection method, which is based on the synchronous frequency extractors (SFEs) is presented. Two SFEs are used to filter out the high-order harmonics contained in the hall-effect signals. Further, the estimation of the rotor speed is used for the field-oriented control scheme of the PMSM. The proposed method is advantageous since it is simple, low cost, and high estimation accuracy. Finally, an experimental driveline system used for testing the electrical performance of developed high-speed magnetically suspended motor is built. The experimental results validate the feasibility and effectiveness of the proposed method in whole speed range.

High-Precision Rotor Position Detection for High-Speed Surface PMSM Drive Based on Linear Hall-Effect Sensors

This paper presents a novel self-correction method of commutation point for high-speed sensorless brushless dc motors with low inductance and nonideal back electromotive force (EMF) in order to achieve low steady-state loss of magnetically suspended control moment gyro. The commutation point before correction is obtained by detecting the phase of EMF zero-crossing point and then delaying 30 electrical degrees. Since the speed variation is small between adjacent commutation points, the difference of the nonenergized phase's terminal voltage between the beginning and the end of commutation is mainly related to the commutation error. A novel control method based on model-free adaptive control is proposed, and the delay degree is corrected by the controller in real time. Both the simulation and experimental results show that the proposed correction method can achieve ideal commutation effect within the entire operating speed range.

Self-Correction of Commutation Point for High-Speed Sensorless BLDC Motor With Low Inductance and Nonideal Back EMF

A sensorless control method for a high-speed brushless DC motor based on the line-to-line back electromotive force (back EMF) is proposed in this paper. In order to obtain the commutation signals, the line-to-line voltages are obtained by the low-pass filters. However, due to the low-pass filters, wide speed range, and other factors, the actual commutation signals are significantly delayed by more than 90 electrical degrees which limits the acceleration of the motor. A novel sensorless commutation algorithm based on the hysteresis transition between “90-α” and “150-α” is introduced to handle the severe commutation retarding and guarantee the motor works in a large speed range. In order to compensate the remaining existing commutation errors, a novel closed-loop compensation algorithm based on the integration of the virtual neutral voltage is proposed. The integration difference between the adjacent 60 electrical degrees interval before and after the commutation point is utilized as the feedback of the PI regulator to compensate the errors automatically. Several experiment results confirm the feasibility and effectiveness of the proposed method.

Sensorless Control for High-Speed Brushless DC Motor Based on the Line-to-Line Back EMF

In this paper, a novel drive method, which is different from the traditional motor drive techniques, for high-speed brushless DC (BLDC) motor is proposed and verified by a series of experiments. It is well known that the BLDC motor can be driven by either pulse-width modulation (PWM) techniques with a constant dc-link voltage or pulse-amplitude modulation (PAM) techniques with an adjustable dc-link voltage. However, to our best knowledge, there is rare study providing a proper drive method for a high-speed BLDC motor with a large power over a wide speed range. Therefore, the detailed theoretical analysis comparison of the PWM control and the PAM control for high-speed BLDC motor is first given. Then, a conclusion that the PAM control is superior to the PWM control at high speed is obtained because of decreasing the commutation delay and high-frequency harmonic wave. Meanwhile, a new high-speed BLDC motor drive method based on the hybrid approach combining PWM and PAM is proposed. Finally, the feasibility and effectiveness of the performance analysis comparison and the new drive method are verified by several experiments.

A Novel Drive Method for High-Speed Brushless DC Motor Operating in a Wide Range

The brushless dc (BLDC) motor can be driven by either pulse-width modulation (PWM) or pulse-amplitude modulation (PAM) techniques. Unfortunately, neither of them can be the most suitable method for the high-speed BLDC motor in the wide speed range, so a hybrid drive method combining PWM and PAM is first introduced. Then, this paper proposes a new sensorless control method based on the virtual neutral voltage for high-speed BLDC motor to suit for this hybrid drive method. In order to obtain the commutation signals from this seriously unclear voltage signal due to the commutation notches and electromagnetic interference, a low-pass filter with super low cutoff frequency is employed at the price of a large phase delay. Meanwhile, a novel sensorless control method based on the transition between “90–α” and “150–α” is introduced to handle the severe phase delay. At last, the feasibility and effectiveness of the proposed drive method and the sensorless control method are verified by several experiment results.

Sensorless Drive for High-Speed Brushless DC Motor Based on the Virtual Neutral Voltage

We demonstrate a single-beam atomic magnetometer (AM) capable of measuring a three-axis magnetic field with high-sensitivity, achieved by applying a small DC offset field and a high frequency modulation field. To satisfy the miniaturization demand of AMs, an elliptically polarized light detuned by 50 GHz from the resonance transition center is employed. The circularly polarized component is used to polarize the alkali-metal atoms, while the linearly polarized light is used to detect the dynamics of the polarized spin under a magnetic field. Based on theoretical analysis, parameters that significantly affect the performance are optimized, and a sensitivity of 20 fT/Hz1/2 in x-axis, 25 fT/Hz1/2 in y-axis, 30 fT/Hz1/2 in z-axis is achieved with a miniature 4 × 4 × 4 mm 87Rb vapor cell. Moreover, we also verify that the operation principle of AMs can be used to null background magnetic fields in-situ with isotropic compensation resolution of 6.7 pT, which provides an effectively precise method for zeroing ambient magnetic field. The high-sensitivity operating of an elliptically-polarized-laser-based magnetometer provides prospective futures for constructing a compact, low-cost AM, which is particularly applicable for non-invasive bio-magnetic imaging such as array-based magnetoencephalography (MEG) and magnetocardiography (MCG).

Gang Liu

Papers

Sensorless Control for High-Speed Brushless DC Motor Based on the Line-to-Line Back EMF

A Novel Drive Method for High-Speed Brushless DC Motor Operating in a Wide Range

Sensorless Drive for High-Speed Brushless DC Motor Based on the Virtual Neutral Voltage

High-sensitivity operation of a single-beam atomic magnetometer for three-axis magnetic field measurement

Unbalance vibration suppression for AMBs system using adaptive notch filter