Development Of Teslameter Controlled By The Chip Microcomputer

Teslameter is to be used to measure the surface intensity of magnetic field of various permanent-magnets and the intensity of magnetic field of air-gap field. It is a permagnag based on Hall Effect. At present, Teslameter is widely used in research and production for its high sensitiveness, strong adaptability, high stability and easy micromation. But Hall probes are different and their measurement processes are nonlinear Moreover they are brittle and their picketageing processes are complicated. Therefore, to change this situation and make the application of Teslameter simple, it is meaningful to design a scheme to realize automatic picketaging and nonlinearity-area-measurement.Since Hall probe has such defects, this paper makes chip microcomputer AT89C52 control Teslameter to realize automatic picketaging of Hall probe and measuring of nonlinear area. The meter can measure steady field, alternating magnetic field and pulsed magnetic field, and frequency of alternating magnetic field.Follow Hall effect we can find the relationship among Hall generated voltage, magnetic induction intensity and current flow: V H = RHIdB. RHI/d can be treated as the proportion factor between VH and magnetic induction intensity B. It can be seen that if only every Hall probe is given different current, the proportion factor can be consistent. When the probe is changed, we can control the current to realize picketaging. The solution of the nonlinearity is: The corresponding relationship form between nonlinear region Hall generated voltage of the probe and magnetic induction intensity value is stored in an E2PROM chip. When it enter nonlinear region, the chip microcomputer fetches the data form, connect with interpolation method to acquire magnetic induction intensity value. Sample-maintaining circuit is added in the Teslameter. It realizes measuring of alternating magnetic field by measuring positive and negative peaks through maintaining the sample of the alternating field peak value. Besides the system uses the AT89C52, which has three timer/ counters. Therefore a module of measure frequency of alternating field is designed to make use of timing/counting function. This paper introduces the chip microcomputer Controlled Teslameter by means of hardware and software structures. Hardware system uses the AT89C52 as the core. It mainly includes Hall probe, main control circuit, picketaging circuit, signal acquisition and process circuit, circuit of photoelectricity isolation, circuit of measure frequency, data storage circuit, display circuit, keyboard and communication with PC etc. The measurement circuit of magnetic field is mainly composed by the Hall probe, constant-current source, the non-idiostatic offset circuit, the signal amplification circuit, the D/A and the A/D transducer circuit. The data form of nonlinear region and the picketaging parameter store in serial memory X24128 for microprocessor fetching to realize picketaging and measuring of nonlinear region. D/A transducer is serial 16-bit DACs MAX541. The voltage controlled constant-current source, which is composed by dual operational amplifiers LM358, is controlled by the voltage-output of the D/A transducer. And it offers working current for picketaging of Hall probe. The non-idiostatic offset circuit is a summing circuit/subtraction circuit. In the process of system initialization, it stores non-idiostatic from every range in E2PROM for digital zeroing. Programmable amplification circuit is composed by option switch, resistance network and an operational amplifier OP07. It can program control different magnification times of measurement, and realize auto-switch of measurement. A/D transducer is realized by±14-bit serial MAX110. It changes the different magnetic field value into a digital signal and communicates chip microcomputer The peak value measurment circuit of alternating magnetic field adopts sample-and-hold amplifier LF398, through I/O port P1.2, P2.0, P2.1 controls sample and hold of positive peak, and P1.3, P2.2, P2.3 controls negative peak. Frequency measurement by timer/ counters T0, T1 and INT0 requires the signal zero adjust, appropriate amplification, shaping by zero passage rotary comparator generate square wave signal, passing a four frequency demultiplier again. If the frequency is greater than 5 kHz counting process measure is used, else periodic time process is used. All low-frequency control lines about analog circuit are isolated with photocoupler. Low cost MAX232 is used in the communication interface of serial circuit, function of which is to convert voltage from TTL to RS232 and communicate with PC computer. The display circuit is realized by the IC 1602, which can sequential show the data of magnetic induction intensity and frequency of alternating magnetic field. The keyboard is directly controlled by chip microcomputer I/O control and realizes six keys: power, pickage, zero, range-switch, auto-measure, pause-show. The depressed sky adopt interrupt mode to response.The Teslameter's software is program by assembly language. The procedure includes the main program, the picketage program, the data acquisition program, the frequency measurement program, the key interrupt program, the display program, the zero adjusting program, the D/A convert program, the A/D convert program, the serial communication program, etc. The instrument'PC software is programmed by VB 6.0.