Xiujun Li - Academia.edu (original) (raw)

Papers by Xiujun Li

Proceedings of the 20th IEEE Instrumentation Technology Conference (Cat. No.03CH37412)

This paper presenfs fhe analysis and experimenlul results on the jitter of a cupacifive-sensor in... more This paper presenfs fhe analysis and experimenlul results on the jitter of a cupacifive-sensor inferfuce. The inlerface is implemented wifh (I simple reluxufion oscillufor, a fusf counfer und U microconfroller. The goal is lafind the criteria lo implemenf U lownoise system, so fhul, even with U short measuring lime, U high-resolution fun be abluined. Experimenfal resulfs (Ire performed in order to prove the vnlidi@ of the fheorelicd predictions. The quanfizalion noise of the 50 MHz counler predominufes over fhe inlernul noise of 1he oscillutor for measuring fimes shorter than 2 ms. The achieved resolufion for measuring fimes of 2 ms and 20 ms is beller than 16 bifs und 18 bifs, respecfively.

In this paper a balanced design approach for implementing a universal sensor interface has been p... more In this paper a balanced design approach for implementing a universal sensor interface has been proposed. With such an approach, the consideration between the measurement accuracy and the flexibility/generality of the system has been balanced so that the resulting system can have a much wider application range and therefore more usefulness compared with the other similar solutions existing today. The system architect is explained in detail and some important implementation circuit blocks are also given. Such a circuit would lower the barrier of using sensors in electronics system so that more innovative systems can be invented.

This paper discusses the problems of impedance sensors in their application. Four case studies of... more This paper discusses the problems of impedance sensors in their application. Four case studies of industrial sensors are presented and show how a systematic approach can reduce the effects of undesired physical influences. It is shown that in a first case of ideal environmental conditions, with capacitive sensors even with simple interface electronics a very high precision can be obtained. A second case study shows that when the sensing elements have resistive leakage, the related problems can easily be solved by modifying the interface circuits. The third case study concerns capacitive detection of water content in soil. It is shown that the presence of salt in water creates measurement problems related to physical conditions and phenomena. With a special probe construction and dedicated electronics, the water content in soil with high salinity can be detected. However, because of physical effects such as skin and proximity effects, the water content is only measured for the local environment of the probe. Finally, the physical problems of a blood-impedance sensor are discussed. Also in this case, application of dedicated measurement techniques and a special probe construction enables to solve the problems. A number of experimental results are given.

This paper describes the design and characterization of a liquid-level measurement system based o... more This paper describes the design and characterization of a liquid-level measurement system based on a grounded capacitive sensor. The sensor electrodes are built with affordable materials: a rod of stainless steel and a PTFE-insulated wire. The interface circuit relies on a common relaxation oscillator (which performs a capacitance-to-period conversion) and a microcontroller (which carries out a period-to-digital conversion). Furthermore, a cable with active shielding interconnects the sensor with the interface circuit. Over a level range of 70 cm, the system has a non-linearity error smaller than 0.35 mm and a resolution better than 0.10 mm for a measuring time of 20 ms.

Measurement Science and Technology, 2006

Active shielding is commonly used to measure remote grounded capacitive sensors because it reduce... more Active shielding is commonly used to measure remote grounded capacitive sensors because it reduces the effects of both external noise/interference and parasitic capacitances of the shielded cable. However, due to active shielding, the measurement circuit can become unstable and inaccurate. This paper analyses these limitations theoretically and experimentally, and then provides guidelines for improving the performance of active shielding. One of the key points is the selection of the bandwidth of the amplifier that drives the shield of the coaxial cable. A wide bandwidth improves accuracy, but a narrow bandwidth improves stability. Therefore, there is a trade-off between stability and accuracy with respect to the bandwidth of the amplifier.

Measurement Science and Technology, 2008

This paper presents a high-performance interface for grounded conductivity sensors. The interface... more This paper presents a high-performance interface for grounded conductivity sensors. The interface mainly consists of a sensor driver, an analog front-end, a multiplexer and a voltage-to-time converter. The sensor driver and analog front-end provide a controlled excitation voltage for the sensor and convert the sensor signal (conductance) into a voltage signal. The voltage-to-time converter acts as an asynchronous converter that employs a relaxation oscillator to convert the sensor signals (voltages) into a period-modulated output voltage. The analysis and experiments are performed to optimize the interface circuit with respect to the range of measurable conductance. With a prototype, over a wide conductance range, from 0.01 μS to 1 mS, the experimental results show random errors with a standard deviation of less than 5.6 nS for a measurement time of 160 ms, and a systematic error of less than 22 nS.

IEEE Transactions on Instrumentation and Measurement, 1998

A novel low-cost smart interface for self-generating sensors, e.g., thermopiles and thermocouples... more A novel low-cost smart interface for self-generating sensors, e.g., thermopiles and thermocouples, is presented. The proposed interface acts as an asynchronous converter for dc voltages employing a relaxation oscillator which output a periodmodulated signal. A microcontroller is used to measure the output signal from the interface, to process data, and to output the measured sensor signal. The system provides an intrinsic A/D converter. Experimental results show that the interface is able to measure voltages in the range of 0 V to 5 V, with an accuracy and resolution of 6 6 6642 2 2 10 06 and 20 2 2 2 10 06 , respectively. The measurement time is less than 100 ms.

IEEE Transactions on Instrumentation and Measurement, 1998

A novel low-cost sensor system for the measurement of low angular or linear speed is presented. T... more A novel low-cost sensor system for the measurement of low angular or linear speed is presented. The sensing element consists of a noncontact resistive potentiometer. The applied sliding electrode does not make mechanical contact with the resistive layer. The processing circuit consists of a very linear oscillator which converts the position quantity to a period-modulated signal. This signal can be directly read out by a microcontroller. A novel algorithm is presented which can eliminate or strongly reduce the influence of the many nonidealities, and which results in a short measurement time. The resolution of the low-cost angular speed sensor system is about 8 2 10 02 r/min in a measurement time of only 80 ms. The angular range is limited by the potentiometer geometry and the finite size of the sliding electrode, and typically amounts to about 270. The sensor system also indicates the moving direction.

IEEE Transactions on Instrumentation and Measurement, 1995

A novel smart resistive-capacitive angular position sensor is presented. The main advantages of t... more A novel smart resistive-capacitive angular position sensor is presented. The main advantages of this low-cost system are its simplicity, high stability and high reliability. A very hear oscillator is used in the processing circuit to convert the position quantity to a period-modulated signal which can directly be read out by a microcontroller. The system does not need an A/D converter. The nonlinearity of the smart angular position sensor system is less than f03% (f0.9") over the range of 270".

IEEE Transactions on Instrumentation and Measurement, 2000

Three-layered electrode structures are often employed in multiple-electrode capacitive position s... more Three-layered electrode structures are often employed in multiple-electrode capacitive position sensors. Even when advanced algorithms and well-designed guarding electrodes are used, the electric-field-bending effect is still one of the major contributors to the nonlinearity of capacitive position sensors. In this paper, the effects of electric-field bending on linearities of five capacitive linear-position sensors have been studied based on a physical model of the capacitive sensor. It is shown that the effect of electric-field bending on linearities strongly depends on the sensor structures, and that it is significantly reduced when advanced sensor structures and algorithms are used. The results are very useful for optimizing the sensor structure according to its application.

IEEE Transactions on Instrumentation and Measurement, 2005

This paper presents a low-cost and accurate interface for four-electrode conductivity sensors. Th... more This paper presents a low-cost and accurate interface for four-electrode conductivity sensors. The interface mainly consists of an analog front-end, a multiplexer, and a voltage-to-time converter. The analog front-end is used to provide a controlled excitation voltage for the sensor and to convert the sensor signal (conductance) into a voltage signal. The voltage-to-time period converter acts as an asynchronous converter for the sensor signals (voltage), which employs a relaxation oscillator and outputs a period-modulated signal. Experimental results over a conductance range of 0.1 S to 20 mS show a random error of 1.6 10 5 and a systematic error of 6.6 10 5 for a measurement time of 110 ms.

IEEE Transactions on Instrumentation and Measurement, 2005

This paper presents the analysis and experimental results on the noise performances of a capaciti... more This paper presents the analysis and experimental results on the noise performances of a capacitive-sensor interface. The interface is able to measure low capacitance values in the order of picofarads and is implemented with a simple relaxation oscillator, a fast counter, and a microcontroller. The goal is to find the criteria to implement a low-noise system, so that, even with a short measuring time, low noise can be obtained. Experimental results are performed in order to prove the validity of the theoretical analysis. The achieved resolution, with a measuring time of 20 ms, was better than 14.2 10 7 for the measurement of a capacitance value of 2.2 pF.

IEEE Transactions on Instrumentation and Measurement, 2000

IMTC/99. Proceedings of the 16th IEEE Instrumentation and Measurement Technology Conference (Cat. No.99CH36309)

Three-layered electrode structures are often employed in multiple-electrode capacitive position s... more Three-layered electrode structures are often employed in multiple-electrode capacitive position scnsors. Even when advanced algorithms and well-designed guarding electrodes are used, the electric-jield-bending effect is still one of the major contributors to the nonlinearity of capacitive position sensors. In this paper, the effects of electric-field bending on linearities of jive capacitive linear-position sensors have been studied based on a physical model of the capacitive sensor. It is shown that the effect of electric-field bending on linearities strongly depends on the sensor structures, and that it is significantly reduced when the advanced sensor structures and algorithms are used. The results are very useful for optimizing the sensor structure according to its application.

2008 9th International Conference on Solid-State and Integrated-Circuit Technology, 2008

ABSTRACT

2008 IEEE Instrumentation and Measurement Technology Conference, 2008

This paper presents an interface circuit, which is optimized for reading the output signal of the... more This paper presents an interface circuit, which is optimized for reading the output signal of thermopile-based sensors. The target sensor has an output voltage less than 10 mV and an output resistance of about 40 kOmega. In such a system, errors come from various sources, e.g. limited settling time, components' offset, 1/f noise, thermal noise, long term drifting as well as the noise from the sensors. To achieve the highest possible signal-to-noise ratio, different levels of error correction techniques have been applied. At the system level, three-signal autocalibration technique and oversampling with digital filtering have been applied. At transistor level, the geometry and biasing conditions have been optimized. The interface has been designed and implemented in 0.35 mum CMOS technology of AMI Semiconductor. The measurement result shows a noise level of less than 15 muV for a measurement time of 0.5 ms. When the measurement time is increased to 150 ms the noise level is less than 1muV.

Measurement Science and Technology, 2008

This paper proposes and analyses a novel interface circuit for capacitive sensors in which one of... more This paper proposes and analyses a novel interface circuit for capacitive sensors in which one of the electrodes is grounded. The novel design makes a charge-balanced relaxation oscillator (applied so far to floating capacitive sensors) suited for the measurement of grounded capacitive sensors and applies advanced measurement techniques, such as auto-calibration and chopping. Furthermore, these techniques are combined with feedforward-based active shielding instead of the usual feedback-based one, thus avoiding instability problems. A prototype of the novel interface circuit has been implemented with discrete components and tested for sensor capacitances between 27 pF and 330 pF and for different lengths of the interconnecting cable. The nonlinearity error amounts to less than 0.1% FSS for a 10 m cable.

IEEE Transactions on Instrumentation and Measurement, 1997

The distributions of the potential and charge density in a capacitive angular-position sensor wit... more The distributions of the potential and charge density in a capacitive angular-position sensor with three electrode disks in an axis-symmetrical configuration have been found by creating a suitable physical model and solving Laplace's equation. The influences of geometrical parameters on the nonlinearity, which originates from the electric-field-bending effect, are discussed in detail for a capacitive angular-position sensor. The approximately analytical results are verified by a numerical analysis and are found to be very useful to predict the influences of geometrical nonidealities. The calculations are experimentally verified by using a novel, absolute angular-position sensor with a nonlinearity of less than 617" and 650" over measurement ranges of 15 and 90 , respectively.

Proceedings of the 20th IEEE Instrumentation Technology Conference (Cat. No.03CH37412)

This paper presenfs fhe analysis and experimenlul results on the jitter of a cupacifive-sensor in... more This paper presenfs fhe analysis and experimenlul results on the jitter of a cupacifive-sensor inferfuce. The inlerface is implemented wifh (I simple reluxufion oscillufor, a fusf counfer und U microconfroller. The goal is lafind the criteria lo implemenf U lownoise system, so fhul, even with U short measuring lime, U high-resolution fun be abluined. Experimenfal resulfs (Ire performed in order to prove the vnlidi@ of the fheorelicd predictions. The quanfizalion noise of the 50 MHz counler predominufes over fhe inlernul noise of 1he oscillutor for measuring fimes shorter than 2 ms. The achieved resolufion for measuring fimes of 2 ms and 20 ms is beller than 16 bifs und 18 bifs, respecfively.

In this paper a balanced design approach for implementing a universal sensor interface has been p... more In this paper a balanced design approach for implementing a universal sensor interface has been proposed. With such an approach, the consideration between the measurement accuracy and the flexibility/generality of the system has been balanced so that the resulting system can have a much wider application range and therefore more usefulness compared with the other similar solutions existing today. The system architect is explained in detail and some important implementation circuit blocks are also given. Such a circuit would lower the barrier of using sensors in electronics system so that more innovative systems can be invented.

This paper discusses the problems of impedance sensors in their application. Four case studies of... more This paper discusses the problems of impedance sensors in their application. Four case studies of industrial sensors are presented and show how a systematic approach can reduce the effects of undesired physical influences. It is shown that in a first case of ideal environmental conditions, with capacitive sensors even with simple interface electronics a very high precision can be obtained. A second case study shows that when the sensing elements have resistive leakage, the related problems can easily be solved by modifying the interface circuits. The third case study concerns capacitive detection of water content in soil. It is shown that the presence of salt in water creates measurement problems related to physical conditions and phenomena. With a special probe construction and dedicated electronics, the water content in soil with high salinity can be detected. However, because of physical effects such as skin and proximity effects, the water content is only measured for the local environment of the probe. Finally, the physical problems of a blood-impedance sensor are discussed. Also in this case, application of dedicated measurement techniques and a special probe construction enables to solve the problems. A number of experimental results are given.

This paper describes the design and characterization of a liquid-level measurement system based o... more This paper describes the design and characterization of a liquid-level measurement system based on a grounded capacitive sensor. The sensor electrodes are built with affordable materials: a rod of stainless steel and a PTFE-insulated wire. The interface circuit relies on a common relaxation oscillator (which performs a capacitance-to-period conversion) and a microcontroller (which carries out a period-to-digital conversion). Furthermore, a cable with active shielding interconnects the sensor with the interface circuit. Over a level range of 70 cm, the system has a non-linearity error smaller than 0.35 mm and a resolution better than 0.10 mm for a measuring time of 20 ms.

Measurement Science and Technology, 2006

Active shielding is commonly used to measure remote grounded capacitive sensors because it reduce... more Active shielding is commonly used to measure remote grounded capacitive sensors because it reduces the effects of both external noise/interference and parasitic capacitances of the shielded cable. However, due to active shielding, the measurement circuit can become unstable and inaccurate. This paper analyses these limitations theoretically and experimentally, and then provides guidelines for improving the performance of active shielding. One of the key points is the selection of the bandwidth of the amplifier that drives the shield of the coaxial cable. A wide bandwidth improves accuracy, but a narrow bandwidth improves stability. Therefore, there is a trade-off between stability and accuracy with respect to the bandwidth of the amplifier.

Measurement Science and Technology, 2008

This paper presents a high-performance interface for grounded conductivity sensors. The interface... more This paper presents a high-performance interface for grounded conductivity sensors. The interface mainly consists of a sensor driver, an analog front-end, a multiplexer and a voltage-to-time converter. The sensor driver and analog front-end provide a controlled excitation voltage for the sensor and convert the sensor signal (conductance) into a voltage signal. The voltage-to-time converter acts as an asynchronous converter that employs a relaxation oscillator to convert the sensor signals (voltages) into a period-modulated output voltage. The analysis and experiments are performed to optimize the interface circuit with respect to the range of measurable conductance. With a prototype, over a wide conductance range, from 0.01 μS to 1 mS, the experimental results show random errors with a standard deviation of less than 5.6 nS for a measurement time of 160 ms, and a systematic error of less than 22 nS.

IEEE Transactions on Instrumentation and Measurement, 1998

A novel low-cost smart interface for self-generating sensors, e.g., thermopiles and thermocouples... more A novel low-cost smart interface for self-generating sensors, e.g., thermopiles and thermocouples, is presented. The proposed interface acts as an asynchronous converter for dc voltages employing a relaxation oscillator which output a periodmodulated signal. A microcontroller is used to measure the output signal from the interface, to process data, and to output the measured sensor signal. The system provides an intrinsic A/D converter. Experimental results show that the interface is able to measure voltages in the range of 0 V to 5 V, with an accuracy and resolution of 6 6 6642 2 2 10 06 and 20 2 2 2 10 06 , respectively. The measurement time is less than 100 ms.

IEEE Transactions on Instrumentation and Measurement, 1998

A novel low-cost sensor system for the measurement of low angular or linear speed is presented. T... more A novel low-cost sensor system for the measurement of low angular or linear speed is presented. The sensing element consists of a noncontact resistive potentiometer. The applied sliding electrode does not make mechanical contact with the resistive layer. The processing circuit consists of a very linear oscillator which converts the position quantity to a period-modulated signal. This signal can be directly read out by a microcontroller. A novel algorithm is presented which can eliminate or strongly reduce the influence of the many nonidealities, and which results in a short measurement time. The resolution of the low-cost angular speed sensor system is about 8 2 10 02 r/min in a measurement time of only 80 ms. The angular range is limited by the potentiometer geometry and the finite size of the sliding electrode, and typically amounts to about 270. The sensor system also indicates the moving direction.

IEEE Transactions on Instrumentation and Measurement, 1995

A novel smart resistive-capacitive angular position sensor is presented. The main advantages of t... more A novel smart resistive-capacitive angular position sensor is presented. The main advantages of this low-cost system are its simplicity, high stability and high reliability. A very hear oscillator is used in the processing circuit to convert the position quantity to a period-modulated signal which can directly be read out by a microcontroller. The system does not need an A/D converter. The nonlinearity of the smart angular position sensor system is less than f03% (f0.9") over the range of 270".

IEEE Transactions on Instrumentation and Measurement, 2000

Three-layered electrode structures are often employed in multiple-electrode capacitive position s... more Three-layered electrode structures are often employed in multiple-electrode capacitive position sensors. Even when advanced algorithms and well-designed guarding electrodes are used, the electric-field-bending effect is still one of the major contributors to the nonlinearity of capacitive position sensors. In this paper, the effects of electric-field bending on linearities of five capacitive linear-position sensors have been studied based on a physical model of the capacitive sensor. It is shown that the effect of electric-field bending on linearities strongly depends on the sensor structures, and that it is significantly reduced when advanced sensor structures and algorithms are used. The results are very useful for optimizing the sensor structure according to its application.

IEEE Transactions on Instrumentation and Measurement, 2005

This paper presents a low-cost and accurate interface for four-electrode conductivity sensors. Th... more This paper presents a low-cost and accurate interface for four-electrode conductivity sensors. The interface mainly consists of an analog front-end, a multiplexer, and a voltage-to-time converter. The analog front-end is used to provide a controlled excitation voltage for the sensor and to convert the sensor signal (conductance) into a voltage signal. The voltage-to-time period converter acts as an asynchronous converter for the sensor signals (voltage), which employs a relaxation oscillator and outputs a period-modulated signal. Experimental results over a conductance range of 0.1 S to 20 mS show a random error of 1.6 10 5 and a systematic error of 6.6 10 5 for a measurement time of 110 ms.

IEEE Transactions on Instrumentation and Measurement, 2005

This paper presents the analysis and experimental results on the noise performances of a capaciti... more This paper presents the analysis and experimental results on the noise performances of a capacitive-sensor interface. The interface is able to measure low capacitance values in the order of picofarads and is implemented with a simple relaxation oscillator, a fast counter, and a microcontroller. The goal is to find the criteria to implement a low-noise system, so that, even with a short measuring time, low noise can be obtained. Experimental results are performed in order to prove the validity of the theoretical analysis. The achieved resolution, with a measuring time of 20 ms, was better than 14.2 10 7 for the measurement of a capacitance value of 2.2 pF.

IEEE Transactions on Instrumentation and Measurement, 2000

IMTC/99. Proceedings of the 16th IEEE Instrumentation and Measurement Technology Conference (Cat. No.99CH36309)

Three-layered electrode structures are often employed in multiple-electrode capacitive position s... more Three-layered electrode structures are often employed in multiple-electrode capacitive position scnsors. Even when advanced algorithms and well-designed guarding electrodes are used, the electric-jield-bending effect is still one of the major contributors to the nonlinearity of capacitive position sensors. In this paper, the effects of electric-field bending on linearities of jive capacitive linear-position sensors have been studied based on a physical model of the capacitive sensor. It is shown that the effect of electric-field bending on linearities strongly depends on the sensor structures, and that it is significantly reduced when the advanced sensor structures and algorithms are used. The results are very useful for optimizing the sensor structure according to its application.

2008 9th International Conference on Solid-State and Integrated-Circuit Technology, 2008

ABSTRACT

2008 IEEE Instrumentation and Measurement Technology Conference, 2008

This paper presents an interface circuit, which is optimized for reading the output signal of the... more This paper presents an interface circuit, which is optimized for reading the output signal of thermopile-based sensors. The target sensor has an output voltage less than 10 mV and an output resistance of about 40 kOmega. In such a system, errors come from various sources, e.g. limited settling time, components' offset, 1/f noise, thermal noise, long term drifting as well as the noise from the sensors. To achieve the highest possible signal-to-noise ratio, different levels of error correction techniques have been applied. At the system level, three-signal autocalibration technique and oversampling with digital filtering have been applied. At transistor level, the geometry and biasing conditions have been optimized. The interface has been designed and implemented in 0.35 mum CMOS technology of AMI Semiconductor. The measurement result shows a noise level of less than 15 muV for a measurement time of 0.5 ms. When the measurement time is increased to 150 ms the noise level is less than 1muV.

Measurement Science and Technology, 2008

This paper proposes and analyses a novel interface circuit for capacitive sensors in which one of... more This paper proposes and analyses a novel interface circuit for capacitive sensors in which one of the electrodes is grounded. The novel design makes a charge-balanced relaxation oscillator (applied so far to floating capacitive sensors) suited for the measurement of grounded capacitive sensors and applies advanced measurement techniques, such as auto-calibration and chopping. Furthermore, these techniques are combined with feedforward-based active shielding instead of the usual feedback-based one, thus avoiding instability problems. A prototype of the novel interface circuit has been implemented with discrete components and tested for sensor capacitances between 27 pF and 330 pF and for different lengths of the interconnecting cable. The nonlinearity error amounts to less than 0.1% FSS for a 10 m cable.

IEEE Transactions on Instrumentation and Measurement, 1997

The distributions of the potential and charge density in a capacitive angular-position sensor wit... more The distributions of the potential and charge density in a capacitive angular-position sensor with three electrode disks in an axis-symmetrical configuration have been found by creating a suitable physical model and solving Laplace's equation. The influences of geometrical parameters on the nonlinearity, which originates from the electric-field-bending effect, are discussed in detail for a capacitive angular-position sensor. The approximately analytical results are verified by a numerical analysis and are found to be very useful to predict the influences of geometrical nonidealities. The calculations are experimentally verified by using a novel, absolute angular-position sensor with a nonlinearity of less than 617" and 650" over measurement ranges of 15 and 90 , respectively.