高分辨同步数据采集处理系统设计

设计了一种Delta-Sigma技术的24位高分辨率同步数据采集系统.该系统具有2个差分模拟输入通道,并具备扩展到8个差分模拟输入通道的能力,具有高速、高分辨率、低功耗三种工作模式.板载DSP完成数据的缓冲,并通过RS232/USB2.0接口与上位机通信,接收上位机的控制命令,向上位机发送采样转换数据.VB6.0开发的Windows数据采集处理程序具备了诸如采样参数设定、波形和数据显示、滤波、信号参数测量和频谱分析等功能,构成了一个功能完备的24位分辨率的同步数据采集处理系统.     

Analysis of mismatch effects among A/D converters in atime-interleaved waveform digitizer

High-speed A/D conversion can be achieved by employing a parallel array of M A/D converters interleaved in time, each working at 1/Mth of the sampling rate. Theoretically, the resolution of the structure is given by the resolution of the A/D converters in the array (subconverters). In practice, however, mismatches among the subconverters lead to a decrease in the resolution. The effect of such mismatches is analyzed in terms of a signal-to-noise ratio defined as the ratio between the energy of the input analog signal and the energy of the error signal due exclusively to these mismatches. The analysis shows that the distortion is comparable to that generated by nonuniform sample timing in the analog demultiplexer when converting a single high-speed signal into several low-speed sampled-and-held signals. The results of the analysis can be used to specify the degree of precision to be achieved in an actual monolithic implementation     

High-resolution of rotary encoder analog quadrature signals

The paper describes a software technique to provide high-resolution absolute angular measurements from the analog quadrature signals of a rotary encoder. The method uses digitized samples of the sinusoidal quadrature signals and the output of a divide-by-four counter circuit. Dynamic measurements on an external trigger signal are possible allowing instantaneous up-to-date angular readings even at high speed. The resolution and hysteresis errors are only dependent on the encoder itself and the bandwidth and resolution of the sampling circuitry. The scheme allows up to 135 degrees of counter hysteresis and delay without loss of precision, thus also affording excellent noise immunity. The theoretical resolution for a 12-bit digital conversion of the analog signals is 1/3360 of a pitch. Experimental results on an encoder built into a laser-tracking measurement system and using 81000 pitches show a unidirectional precision of 0.3 arcsec (rms), a mean bidirectional hysteresis of about 1 arcsec and a worst case variation for a stationary encoder shaft of 0.06 arcsec     

Improving the signal data acquisition in condition monitoring of electrical machines

The occurrence of a broken bar in induction machines causes an internal magnetic imbalance, which is reflected in the stator current. This effect may be detected by estimating the spectral signature of the stator current, particularly the sidebands around the fundamental frequency. However, the fundamental 60-Hz amplitude is considerably greater than the sideband amplitude, which usually requires an analog 60-Hz notch filter in the data acquisition system. Analog filters are sensitive to temperature variations, which, in this case, may shift the filter resonance frequency and degrade the desired response. It is proposed in this paper that the analog notch filter may be replaced by a digital/analog cancelling technique based on the recursive discrete-time Fourier transform. The technique is shown to attenuate the fundamental 60-Hz amplitude considerably before A/D conversion, leaving only the sidebands in the current signal.     

New Learning Algorithm for High-Quality Velocity Measurement and Control When Using Low-Cost Optical Encoders

A new compensation method that can greatly reduce the slit errors (i.e., transition location errors) due to nonidealities in optical incremental encoders caused by manufacturing limitations in the code wheel, optical components, and analog circuitry is presented. An M/T-type constant sample-time digital tachometer (CSDT), which involves pulse-count and high-frequency timer measurement, effectively time-stamps the encoder transitions. Using CSDT-based data, encoder compensation techniques that improve velocity measurement accuracy are presented. These do not require precise knowledge of the shaft velocity, thereby eliminating the need for high-accuracy reference equipment. During the initial learning stage (possibly performed in situ ), slit errors are calculated through pseudoinverse-based solutions of simple approximate linear equations or an iterative method that requires very little memory storage. Subsequent operation of the motion system utilizes adjusted slit positions for more accurate velocity calculation. The performance improvement in velocity measurement is experimentally demonstrated using motor drive systems, each of which includes a field-programmable gate array (FPGA) and a digital signal processor (DSP). Results from open-loop velocity measurement and closed-loop servo control applications are given, with the latter highlighting the resultant reduction in high-frequency motor torque. Slit error reductions in the range of 60%–86% are obtained (typically approximately 80%) with a similar improvement in velocity measurement error.      

一种提高单片机内嵌式A/D分辨力的方法

单片机内嵌式Ａ／Ｄ的使用要比外置式Ａ／Ｄ简单、方便，且容易获得更高的信噪比，但其分辨力相对比较低，难以满足精密测量的使用要求。本文介绍了一种提高单片机内嵌式Ａ／Ｄ转换器分辨力有效实用法，简单外转电路和单片机内部软件计算可以将原分辨力将提高１￣３倍。     

Theoretical principles for establishing a hierarchy of dynamicaccuracy with the integral-square-error as an example

This paper presents the possibility of applying maximum dynamic errors to establish a hierarchy of measuring systems intended for dynamic measurements. To determine maximum errors, it is necessary to apply strictly defined calibrating signals. The shape analysis and necessary and sufficient conditions for such signals to exist are presented with the integral-square-error criterion as an example. Moreover, limitations imposed on calibrating signals depending on the low-pass, high-pass, or bandpass types of calibrated systems are also discussed. The values of the maximum errors determined by means of the signals mentioned above make it possible to create accuracy classes and the resulting hierarchy of dynamic accuracy, valid regardless of the measured signal shape. The solutions presented in the paper refer to both measuring analog systems and systems with analog-to-digital (A/D) conversion     

Digital signal processing techniques for high accuracy ultrasonicrange measurements

Several digital signal processing (DSP) methods are analyzed and compared with respect to the expected errors for an ultrasonic range measurement arrangement. These include L1, L2 norms and correlation with different approaches for envelope extraction. The influence of different factors such as signal-to-noise ratio (SNR), sampling frequency, and digitizing resolution on measurement errors is analyzed using a synthetic approach through nearly 40000 simulations. Results show different performance levels involving accuracy, computing time, and cost for the studied methods, although all of them allow reduction of errors by several orders of magnitude     

基于DSP技术的在线颗粒计数器

颗粒计数器已逐渐应用于水质检测中。本文介绍了基于光阻法颗粒计数器的基本原理，并设计了基于DSP芯片技术的主从机系统，从而提高了颗粒检测的分辨率。文中还给出了相应的控制框图以及信号处理的方法。     

A PC-based coherent sonar for experimental underwater acoustics

An important tool for experimental underwater acoustics research is a versatile sonar system for transmitting, receiving, anti recording acoustic waveforms. In this paper the design, implementation and performance of a variable-frequency (25 kHz to 500 kHz) coherent sonar are presented. The significance of the design lies in its simplicity and versatility. These attributes are a result of a sampling technique that provides quadrature samples of the complex envelope of a bandpass signal without analog demodulation using a single A/D converter. Implementation of the design is accomplished using an ISA bus personal computer (PC), a commercially available digital signal processing (DSP) board, and a small amount of custom electronics. Performance of the sonar, in terms of magnitude and phase errors, is determined from wideband noise characterization, a linear-system model, and transmit/receive measurements     

Four Voltmeter Vector Impedance Meter Based on Virtual Instrumentation

In this paper, a digital vector impedance half-bridge meter based on virtual instruments is designed, implemented and tested. Here, not only the accuracy of the magnitude of the impedance is considered but, more importantly, its phase measurement accuracy. The meter utilizes a four-voltmeter method which is a basic modification of the well-known three-voltmeter method. The half-bridge is constructed with commercially available data acquisition (DAQ) board in the form of peripheral control interconnect cards incorporated in personal computers. The DAQ board is used only to acquire the voltages instead of using four separate voltmeters, while the excitation signal is produced by an integrated circuit signal generator. The main error in this method arises from the error in measuring the voltage values. Since the resolution of the DAQ board used here is 16 bits; expect that absolute errors due to A/D conversion will be around 0.305 mV for ±10 V range. Detailed error analysis of the method is included in the context of the paper. It is found that the errors in the impedance magnitude is fairly small and relatively less sensitive on the resolution of the voltmeters because of the relative measurements in the half bridge with a precise reference resistance. The original three voltmeter vector impedance meter has relatively large error in the phase especially in the small phase angles. To decrease the phase error to an acceptable range, one has to increase the resolution of the voltmeter appreciably, which makes them expensive. The other solution to reduce the error in the phase angle with less cost is to add a fourth voltmeter which acquires directly the small phase angles. In this case, it is found that, a much lower resolution voltmeter can be utilized while achieving an acceptable measurement accuracy of the impedance.     

A high-performance EIT system

This paper presents the development of a new electrical impedance tomography system for online measurement of two-phase flows with axial velocities up to 10 ms/sup -1/. The system is designed in a modular fashion and can consist of several data acquisition modules and computing modules. The data acquisition module includes a voltage controlled current source with a direct-current-restoration circuit, an equal-width pulse synthesizer unit and a synchronized digital demodulation unit. A new concept of current switching scheme is developed to enhance the ac coupling speed. The computing module includes a digital signal processor (TMS320C6202/6713) with memory, multichannel buffered serial ports and an IEEE1394 communication interface. Several DSP modules can be pipelined for a series of tasks ranging from measurement control to image reconstruction to flow velocity implementation. The performances have been tested and some trial results are reported. A data acquisition speed of 1164 dual-frames (2.383 million data points) per second has been achieved with a root mean square error less than 0.6% at 80 kHz in static test application. An application in the measurement of vertical oil-in-water pipe flow is reported.     

Self-calibration/compensation technique for microcontroller-basedsensor arrays

Described is a generalized technique for real-time gain and thermal compensation of embedded microcontroller-based sensor arrays. By incorporating a single or multiple low-cost, uncalibrated thermal sensor(s) into a software feedback loop, a self-normalized calibration/compensation table for each sensor can be generated and stored in EEPROM for later use in real-time signal acquisition. The compensation is accomplished by executing a one-time initialization software routine as the sensor array is cycled through the expected temperature range. To achieve compensation to within the system resolution, the required correction loop bit width will be different from that of the system bit width. In addition to temperature compensation, the technique also includes correction for gain and voltage offset errors introduced by the analog signal conditioning as well as A/D conversion errors. An example of the technique is presented using a 68HC711E9 microcontroller for real-time acquisition and compensation of a four-element strain gage array     

A Digitally Programmable A/D Converter for Smart Sensors Applications

Nonlinear analog-to-digital conversion in smart sensor applications is an important topic since signal digitization and linearization can be performed in a single step near the transducer. In this paper a double pulsewidth modulated (PWM) scheme for nonlinear analog-to-digital conversion is presented. Calibration or auto-calibration data stored in the smart sensor's memory define the nonlinear profile characteristic of the transducer and provide the required data to obtain the inverse function of the analog-to-digital converter (ADC) transfer curve. Basically, as a function of the transducer's nonlinearity degree, the input voltage range of the ADC is segmented in a continuous set of subintervals and, for each of these subintervals, a second-order correction term based on a PWM A/D conversion is used to obtain a linear characteristic for the smart sensor. Additional advantages of this method result from its easy implementation in low-cost microcontrollers that include generally comparator inputs and PWM outputs. A flexible and programmable A/D conversion solution can be dynamically adapted to variations of the transducer's nonlinearity profile, and an increased resolution can be achieved at the expense of a lower conversion rate. Some MATLAB simulations and experimental results obtained with a square-root airflow transducer will be presented in the final part of the paper     

Software for personal instruments

When a personal computer (PC) is used as the computing system of an intelligent instrument, software devoted to measurement process control and to measurement process outputting can be specially developed to assist the operator throughout the measurement process in a friendly way. When these conditions are met, the intelligent instrument is called a personal instrument (PI). The main features of a PI are discussed, and the requirements for PI software are given. The performance of an original software package for PI is illustrated, showing how all the requirements are satisfied. This software, coupled to a good commercial data acquisition system, features high-precision measurement and a high level of friendliness and can be useful for lab tests and educational purposes. For lab tests, this software is a good basis for an automatic test station with extended help facilities. It ensures better performances than that of a digital scope because it allows dedicated measurement routines to be developed and executed. The software is effective for educational applications, since it allows direct application of all capabilities offered by computers in instrumentation, when associated with suitable A/D (analog/digital) conversion hardware     

Magnetic crosstalk compensation for an optical current transducer

In this paper, we analyze the errors associated with magnetic crosstalk within point type, or unlinked, optical current transducers (OCTs) working in a three-phase electric current transmission systems. For many practical conductor arrangements, the magnetic crosstaIk may introduce errors unacceptable for the accuracy requirements demanded from the OCT. A solution to this problem is devised around a unique compensation method which solves, in real time, a set of linear equations, each representing the instantaneous output signal from one phase current sensor     

A／D量化,非线性,微分非线性误差对非同步,准同步采样算法误差影响 …

本文提出了具有量化误差、非线性误差和微分非线性误差的Ａ／Ｄ转换器的数据学模型,在该数学模型的基础上,仿真分析了非同步采样法、准同步采样递推算法各种情况下的误差,包括功率测量中电压线性、电流线性、相位变化误差及电压测量的误差。由于分析中的条件更近接应用中的实际情况,因此,仿真分析的结论为测量仪器设计中的误差估计提供了重要依据。     

The Absolute Measurement of the Ampere by Means of NMR

This paper describes the principle, apparatus, and uncertainties of the absolute measurement of the ampere by the nuclear magnetic resonance (NMR) method. A conversion factor between the as-maintained ampere of NIM and the absolute ampere is given by k = 1 -(5.81 ± 1.8) × 10-6 with a total uncertainty (1?) of 1.8 ppm. Since two methods have been used to determine the gyromagnetic ratio in proton at the same laboratory, systematic errors of the as-maintained unit can be eliminated.     

A new method for reducing DNL in nuclear ADCs using an interpolation technique

The paper describes a new method for reducing the DNL associated with nuclear ADCs. The method named the “interpolation technique” is utilized to derive the quantisation steps corresponding to the last n bits of the digital code by dividing quantisation steps due to higher significant bits of the DAC, using a chain of resistors. Using comparators, these quantisation steps are compared with the analog voltage to be digitized, which is applied as a voltage shift at both ends of this chain. The output states of the comparators define the n bit code. The errors due to offset voltages and bias currents of the comparators are statistically neutralized by changing the polarity of quantisation steps as well as the polarity of analog voltage corresponding to last n bits) for alternate A/D conversion. The effect of averaging on the channel profile can be minimized. A 12 bit ADC was constructured using this technique which gives DNL of less than ±1% over most of the channels for conversion time of nearly 4.5 μs. Gatti's sliding scale technique can be implemented for further reduction of DNL. The interpolation technique has a promising potential of improving the resolution of existing 12 bit ADCs to 16 bit, without degrading the percentage DNL significantly.     

Microcantilever Array Pressure Measurement System for Biomedical Instrumentation

An analog signal processing integrated circuit for microcantilever array has been designed for pressure measurement in biomedical applications. The chip consists of analog multiplexer, instrumentation amplifier, sample-and-hold circuit, on-chip voltage and current references, successive approximation register analog-to-digital converter (ADC) and digital control unit. Root sum square (RSS) error from the overall pressure measurement system including microcantilever array and the application specific integrated circuit (ASIC) is only ±1.79 KPa within the measurement range of 0-300 KPa. The 8-bit ADC attains 45.4 dB signal-to-noise-and-distortion ratio (SNDR) and 56.4 dB spurious-free dynamic range (SFDR), while operating at 772 KHz. The integrated circuit has been fabricated using 0.35-?m 2-poly 4-metal CMOS process technology. The chip occupies an area of 1.54 mm² and consumes 17.8 mW of power with a single 3.3 V supply.