Applied Mechanics and Materials Online: 2013-12-09 ISSN: 1662-7482, Vols. 475-476, pp 1638-1642 doi:10.4028/www.scientific.net/amm.475-476.1638 2014 Trans Tech Publications, Switzerland Design of a AC current source for inductive loads Kuihong Hao Bin Li Aeronautical Automation college, Civil Aviation University of China, Tianjin 300300, China E-mail: 342180226@qq.com Keywords: inductive load, SCM named STC89C52, digital synthesis chip, VCCS Abstract Inductive loads are common in the real time monitoring system of aircraft engines. The excitation for these loads is better to chose current source. A stored program control interchange current resource based on DDS and current feedback amplifier was designed for these loads. When the output frequency of the resource was chosen 5kHz, the export current can reach 10mA and the change of the amplitude is less than 1%. Introduction In order to monitor the running state of airplanes there are many senors were installed on the airplanes. Traditional monitoring methods are off-line but the accidents can be happen anytime, so that an on-line monitoring method has important meaning for early warning bring forward. Abrasive produced from engines of airplane can endanger the safe running state. To monitor the abrasive in lube oil on-line can warn the danger of airplane engine bring forward. The sensor is made of a couple of coils, so it is important to drive the sensor for producing magnetic field inside it. The strength of magnetic field produced inside the sensor can influence the monitoring precision directly, so we hope the alternating magnetic field influenced by the dimension parameter of sensor and environment as less as possible. Based on the Biot-Savart s law (magnetic field strength of current carrying coil ), the magnetic field strength of current carrying coil is proportional to the current through it, so an steady current resource as excitation can reduce much adverse impact. To design a steady current resource is very important to the whole on-line monitoring system. According to the demand, the conference designed a precise program-controlled ac current source based on digital synthesis chip named AD7008 controlled by 89C52. The voltage control current resource made of current feedback amplifier AD844 and ordinary amplifier OP07, After the testing of the resource, when the internal resistance is low and the requirement of frequency is around 5kHz, the ac current resource can output steady current with high internal resistance, so it can used as an excitation for inductive load. Hardware of system 2.1 SCM and Direct digital synthesis chips STC89C52 is a simple microcontroller, it has many advantages like simple programming language, cheap and easy using. The direct digital synthesis chip has high image resolution, high transition speed, small size and other advantages, so that it is used in many fields. The direct digital synthesis chip used in the system is made by AD company, it is an widely used DDS chip. AD7008 is a numerically controlled oscillator, and can achieve delicate adjustment for frequency, range and phase position. Clock rates up to 20 MHz and 50 MHz are supported. output frequency is (0~25)MHz, resolution ratio is Δf =(50x10 6 )/2 32 = 0.01164Hz. The inside structure as in Fig 1. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (ID: 130.203.136.75, Pennsylvania State University, University Park, USA-21/02/16,11:46:12)
Applied Mechanics and Materials Vols. 475-476 1639 Fig 1 Internal structure of AD7008 2.2 Current feedback amplifier AD844 AD844 is a high speed amplifier which is using complementary bipolar manufacturing technique. It can be regard as a current conveyor connects with a follower, it has well performance in high bandwidth and fast feedback. Its schematic diagram is shown as Fig 2. AD844 is the second generation current transmission device and it has better linearity of current feedback [6]. This amplifier offers a closed- loop broadband decided by feedback resistance and it can be nearly regarded as a independent closed loop increment. AD844 can drive a 50Ω load within -2.5V~+2.5V and has 80mA earth leakage protection. Generate of voltage signal Fig 2 Schematic diagram of AD844 The conversion circuit is made by 89C52 and AD7008 shown as Fig 3. Fig 3 To control of the frequency and amplitude of the signal, the reset signal of AD7008 should be efficacious. After reset of the chip all registers are zero clearing, then we should select the running mode of AD7008. When the signal range control was required, AD7008 offers many methods to control the signal range. (1)Writing control words to the register to control the range; (2) Selecting the download resistance or changing RESET to control the signal range [2]. To calculate the output frequency of the direct digital synthesis chip, we should set some parameters: output frequency is F OUT, reference frequency is F CLK, The control word is FCW, so we can acquire a formula: FCW = (F OUT 2 32 )/F CLK. The basis frequency of AD7008 is 50MHz.
1640 Sensors, Measurement and Intelligent Materials II Conversion circuit Fig 4 Program flow The performance of the current conveyor is based on the performance of the voltage buffer among the two input ends. Based on require of the whole system, the conversion circuit was simulated by proteus software as Fig 5. OP07 is ordinary amplifier, the V/I transform is accomplished by OP07 and AD844. AD844 can bring error because the first precision is low. This problem can be solved by using an amplifier as feeding buffer, this method can make output resistance near 0. To avoid the direct current signal making the stopping capacitor saturation, the system designed a direct current feeding circuit. Circuit parameters selecting Fig 5 Conversion circuit R IN in fig 2 and R 3 in fig 5 are in series, because the current in R 3 and in the 5 th pin of AD844 are mirror-image relation, so the size of R 3 can influence the output current of the circuit. If the R 3 is oversize or undersize the circuit performance will be influenced seriously. When R 3 =380Ω, the output current of circuit can reach 8mA. When R 3 is less than 380Ω, the output wave form will be distortion gradually, it becomes square wave when R 3 approaches 0. The select of R 1 isn t influenced by the input frequency and range of OP07. C and R 1 in fig 5 make up a low-pass filter circuit, if the C was selected bigger, the current through R 1 will larger. We choose a 10μF capacitance. Carrying load ability test Output impedance is the most important parameter of the voltage controlled current source. To make the input and output signals of the sensor in on-line motoring system of lube oil have no phase difference, the frequency range of the voltage controlled current source should be 3kHz~10kHz, and the output impedance should be large. Testing method is simple, a variable resistance R P should be series connection with R 1 as the loading of the voltage controlled current source. If the size of R P is change, we can get the output resistance, the theory is as shown in fig 5 [4].
Applied Mechanics and Materials Vols. 475-476 1641 Fig 5 Circuit of load capability Z S in fig 5 is the output resistance of the voltage controlled current resource, R L and variable resistance R P are as the load of the voltage controlled current resource. We can get different load size through changing R P, so that we can obtain the output resistance. i i I (1) S Z1 1 i i I (2) S Z2 2 I I i i (3) 1 2 Z2 Z1 I ( V V ) Z2 Z1 1 I2 (4) ZS The voltage of R P is equal to it of output resistance Z S. We can get the formula: RP2I 2 RP 1I1 ZS I1 I2 The testing results of different resistance load R P is shown in tab 1 Tab 1 Different internal resistance with different frequency F(kHz) 2 2.5 3 3.5 5 R(MΩ) 2.036 1.977 1.892 1.821 1.699 We can get the result that when the frequency is 3kHz, the internal resistance is about 2MΩ. The inductor is made of coil, adding a 2Ωresistance is in order to limit the current. The exciting frequency of lube oil on-line monitoring system is 3.5kHz, the testing frequency is selected 3.5kHz±10%. The output current keeps about 8mA. The testing result is shown in line graph as fig 6. (5) Fig 6 Test result of inductive load The variation range of inductance value is 1mH~2mH, the inductive reactance changes from 12.56Ω to 62.8Ω, we can get a law from the line graph that the variation range of current in the same frequency is very small within 0.15% and the current keeps about 8.2mA in different frequency. Conclusions From the test result we can get that the ac current resource designed above can output 8mA stable current and the variation range is within 1%, it can adapt to the requirement of small internal resistance of the sensor. The voltage controlled current resource can run smoothly in inductive load system, it can be controlled easily, the frequency and phase can be adjusted fast with low distortion acknowledgements
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Sensors, Measurement and Intelligent Materials II 10.4028/www.scientific.net/AMM.475-476 Design of a AC Current Source for Inductive Loads 10.4028/www.scientific.net/AMM.475-476.1638 DOI References [5] PBertemes-Filho et al. Comparsion of current generators. Physiol. Meas. 21(2000)1-6. 10.1088/0967-3334/21/1/301