CN102520257B - Alternating current (AC) impedance spectroscopy automatic testing device of positive temperature coefficient (PTC) thermistor - Google Patents

Abstract

The invention discloses an alternating current (AC) impedance spectroscopy automatic testing device of positive temperature coefficient (PTC) thermistor. The device comprises an electric heating oven for holding a sample, a temperature control and protection circuit for maintaining the electric heating oven at a constant temperature and preventing overcurrent, an impedance measuring circuit for measuring the sample impedance, and a master control machine connected with two circuits via a CAN bus respectively and used for receiving a user instruction and controlling the two circuits to accomplish measurement. The impedance measuring circuit in the invention is based on high-speed synchronous data acquisition and digital signal processing technology, satisfies the measurement requirements for high precision, wide measurement range and wide frequency band, and has features of small volume, low cost and convenient operation. Data communication among the temperature control and protection circuit, the impedance measuring circuit and the master control machine is realized via CAN buses, to achieve good real-time property and high reliability. The invention can ensure constant-temperature testing environment, carries out multi-frequency point scanning measurement to the AC impedance of the tested sample, and realizes full automatic testing of impedance-temperature characteristics of the PTC thermistor under different frequencies.

Description

A kind of PTC thermistor ac impedance spectroscopy automatic testing equipment

Technical field

The invention belongs to component parameter field tests, particularly a kind of PTC thermistor ac impedance spectroscopy automatic testing equipment.

Background technology

The BaTiO of semiconducting with doped with trace rare-earth element ₃pottery at normal temperatures resistance value is very low, but along with the rising of temperature, undergos mutation at vicinity of Curie temperatures, and resistance value sharply rises, and produces significant PTC effect.Be called positive temperature coefficient (PTC) (PTC) thermistor with the element that this pottery is made, PTC thermistor is widely used in the various fields such as household electrical appliance, electric power facility, electronic equipment and automobile industry.

Resistance-temperature characteristics is one of fundamental characteristics of PTC thermistor, and it refers under the voltage of regulation, the relation between the zero-power resistance of PTC themistor and resistive element temperature.The measurement of resistance-temperature characteristics need to, with the resistance apparatus of the high precision of intelligent interface, wide-range, be considered the factors such as reliability, stability, ease for use, and way is to adopt the desk-top multimeter of foreign vendor to carry out the system integration conventionally.In practical application, PTC thermistor is worked under the alternating voltage of being everlasting, as motor starter, overcurrent protector, heater etc.Research shows: due to the impact of grain boundary effect, under alternating voltage, PTC thermistor is not pure Ohmage, but capacitive resistance; The resistance value of PTC thermistor, obviously declines with the increase of test frequency, and when frequency is during higher than certain value when (as 1MHz), the resistance of sample loses positive temperature characterisitic.Therefore it is sufficient not, characterizing PTC thermistor with direct current resistance simply.In other words, by AC impedance but not direct current resistance characterizes PTC thermistor is feasible, the impedometer with high precision, wide-range, broadband measurement ability is also essential.Although high performance impedance measuring Instrument can obtain by order, also there is the problems such as bulky, expensive, complicated operation, use inconvenience in them, is not obviously suitable for industrial automation detection field.

Temperature control aspect, traditional method is to adopt A.C. contactor to control electric furnace heating wire power on/off, and power supply is not taked quarantine measures yet, this kind of mode temperature control poor accuracy, disturbs large, poor reliability, temperature rise out-of-control phenomenon happens occasionally, and when serious, will cause testing apparatus to cross cause thermal damage.In addition, electric furnace heating wire power circuit does not have overcurrent protection measure, makes system reliability further worsen yet, and is not suitable for long time continuous working.

Communication interface aspect, traditional testing apparatus is equipped with RS-232 serial line interface and LPT parallel interface.Communication interface disunity causes system to connect complicated, difficult expansion, versatility is poor.In addition, main flow business computer has been cancelled this two kinds of interfaces at present, even if part industrial computer has still retained this two kinds of interfaces, but hardware configuration is on the low side, and expensive, is difficult to meet the actual needs that use.

Summary of the invention

The object of the present invention is to provide the impedance-temperature characterisitic automatic testing equipment of a kind of PTC thermistor under different frequency, solved that the characterizing method that existing testing apparatus exists is not objective, the problem such as impedance measuring Instrument poor availability, temperature control instrument reliability are low, each discrete instrument communication interface disunity, can be widely used in research, test, sign and the evaluation etc. of PTC thermistor, semiconductor ceramics, various sensitive elements.

A kind of PTC thermistor ac impedance spectroscopy automatic testing equipment; comprise the electrothermal furnace for placing sample; for maintaining electrothermal furnace constant temperature and preventing temperature control and the holding circuit of overcurrent; for measuring the impedance measuring circuit of sample impedance; and join with two circuit respectively by CAN bus; be used for receiving user instruction, control two circuit and complete corresponding main control computer of working.

Further, described impedance measuring circuit comprises ac-excited source, self-balancing bridge circuit and vector ratio detecting unit, ac-excited source provides synchronous sinusoidal test signal and sampling clock to self-balancing bridge circuit and vector ratio detecting unit respectively, self-balancing bridge circuit produces voltage and current vector signal under sinusoidal test signal excitation, and vector ratio detecting unit completes the demodulation to described voltage and current vector under sampling clock control;

Further, described vector ratio detecting unit comprises first signal modulate circuit, secondary signal modulate circuit and a SoC microcontroller, first signal modulate circuit and secondary signal modulate circuit respectively to voltage and current vector signal amplify, filtering and level conversion, voltage and current vector input the one SoC microcontroller after conditioning, the one SoC microcontroller carries out A/D conversion to the signal of input, and by its built-in digital phase-sensitive detector, the voltage and current burst after A/D conversion is made to related operation, obtain modulus of impedance and the phase place of sample.

Further, described digital phase-sensitive detector is made related operation to the voltage and current burst after A/D conversion, obtains the modulus of impedance of sample and the specific implementation of phase place is: the modulus of impedance of sample $\left|Z\right|=R_r\&CenterDot;\sqrt{\frac{{I_u}^2+{Q_u}^2}{{I_{\mathrm{<figure-callout\; id="2"\; label="i"\; filenames="HDA0000116882500000011.png,HDA0000116882500000021.png"\; state="\{\{state\}\}">i</figure-callout>}}}^2+{Q_i}^2}},$ Phasing degree $\mathrm{\&theta;}={\tan }^{-1}\left(\frac{Q_i}{I_i}\right)-{\tan }^{-1}\left(\frac{Q_u}{I_u}\right),$

Wherein, R _rfor the range resistance of self-balancing bridge circuit,

$I_r=\frac{1}{N}\underset{n=0}{\overset{N-1}{\mathrm{\&Sigma;}}}\&CenterDot;A\cos (2\mathrm{\&pi;n}/N+\mathrm{\&theta;})\&CenterDot;\cos (2\mathrm{\&pi;n}/N),r=i,u$

$Q_r=\frac{1}{N}\underset{n=0}{\overset{N-1}{\mathrm{\&Sigma;}}}\&CenterDot;A\cos (2\mathrm{\&pi;n}/N+\mathrm{\&theta;})\&CenterDot;\sin (2\mathrm{\&pi;n}/N),r=i,u$

A _iand θ _ibe respectively amplitude and the phasing degree of current phasor signal, A _uand θ _ube respectively amplitude and the phasing degree of current phasor signal, N is a hits in test period, and n is sampled point, and cos (2 π n/N) and sin (2 π n/N) obtain by cosine look-up table and sinusoidal look-up table respectively.

Further; described temperature control and protection circuit includes supply module, zero cross fired alternating-current switch and current acquisition module; they are electrically connected successively and form loop with the electric furnace heating wire of electrothermal furnace; zero cross fired alternating-current switch also connects the output terminal of the 2nd SoC microcontroller; the first input end of the 2nd SoC microcontroller connects temperature collect module, and the second input end of the 2nd SoC microcontroller connects current acquisition module.

Further, between described sample and impedance measuring circuit, join by five terminal shield terminal.

Technique effect of the present invention is embodied in: a kind of PTC thermistor ac impedance spectroscopy automatic testing equipment of the present invention, test process full automation; Temperature control process adopts expert system intelligence from Tuning PID Controller technology and active current protection technology, accurate, stable, reliable; Between each discrete circuit, data communication is based on CAN bussing technique, fast, reliable, easy care; Impedometer is based on high-speed synchronous data acquiring and Digital Signal Processing, both high precision, wide-range, wide band measurement requirement had been met, there is again the features such as volume is little, cost is low, easy to operate, and the AC impedance providing but not the metering system of direct current resistance more can reflect the real work state of PTC thermistor.Specifically; electrothermal furnace provides constant temperature test environment; temperature control and holding circuit gather the internal temperature of burner hearth by PT100 platinum resistance, and use expert system intelligence self-setting PID arithmetic control electric furnace heating wire power on/off, thereby make burner hearth internal temperature keep constant.In addition, in the time there is overcurrent, zero cross fired alternating-current switch will disconnect automatically, and send warning message to main control computer.Impedance measuring circuit carries out multi-frequency spot scan measurement to the AC impedance of sample, and the impedance data recording is uploaded to main control computer, obtains ac impedance spectroscopy data after data processing.Between each discrete circuit, carry out data communication by CAN bus, realized the unification of data-interface.Whole system, under the control of main control computer, completes the full-automatic testing to PTC thermistor impedance-temperature characterisitic.

Accompanying drawing explanation

Fig. 1 is the present invention's entirety composition frame chart;

Fig. 2 is temperature control and holding circuit theory diagram;

Fig. 3 is impedance measuring circuit theory diagram;

Fig. 4 is digital phase-sensitive detector theory diagram.

Embodiment

Below in conjunction with accompanying drawing, preferred embodiment of the present invention is described.

As shown in Figure 1, the present invention includes electrothermal furnace, temperature control and holding circuit, impedance measuring circuit and main control computer.

Electrothermal furnace is made up of metal shell 1, asbestos filling material 2, electric furnace heating wire 3, insulation cover 4, castor 5 and grounding connection end 6.

Temperature control and holding circuit 9 are used PT100 platinum resistance 8 to gather burner hearth internal temperature, and Real-Time Monitoring is by the electric current of electric furnace heating wire 3, and automatically protect according to pre-set program.

Between sample 7 and impedance measuring circuit 10, be connected by five terminal shield terminal 11, to eliminate the impact of lead resistance, lead-in inductance and Wire Distributed Capacitance.

Main frame 12 accesses CAN bus 14 by CAN interface card 13, and carries out data communication with temperature control and holding circuit 9, impedance measuring circuit 10.

SoC microcontroller is selected the C8051F060 of Silabs company of the U.S., and it has two independently 16 1Msps high speed SAR ADC, and two independently 12 DAC, voltage reference, SPI, I ²many resources such as C, WDT, UART, CAN2.0B, it provides complete data acquisition or control system solution in an integrated circuit.

As shown in Figure 2, described temperature control and protection circuit comprises supply module, zero cross fired alternating-current switch, current acquisition module, temperature collect module and SoC microcontroller.Temperature collect module is based on four line Method of constant flow sources, and the precision constant current source of 1mA forms the d. c. voltage signal between 100～200mV by PT100, then amplifies and low-pass filtering through signal, is transformed into the voltage that is applicable to the built-in ADC input range of SoC microcontroller.Adopt zero cross fired be mainly open for fear of power controllable silicon during the saltus step of output voltage and electric current to load with the harmful effect of the miscellaneous equipment of electrical network, be power controllable silicon always in the conducting that is triggered of the zero crossing of alternating voltage, make load current and voltage occur each time all starting from scratch.By the make-and-break time of the pwm signal output port control Over zero bidirectional SCR output photoelectric coupling mechanism MOC3081 of microcontroller, realize the break-make control in load voltage loop.By changing the make-and-break time of Duty ratio control bidirectional triode thyristor of pwm signal, thereby realize the electrodeless adjustable of load voltage loop voltage and power.Temperature control and holding circuit adopt expert system intelligence self-setting PID arithmetic to realize the accurate control of resistance-heated furnace; traditional PID control technology for time become; nonlinear, comprise many unknown parameters or be difficult to obtain good control effect with random disturbance.Expert system intelligence self-setting PID arithmetic has overcome above shortcoming, traditional PID is controlled to situation and segment, can carry out more effectively and accurately controlling to complex situations, has overcome the drawbacks such as the easy overshoot of traditional PI D, poor stability.In addition, when PID imbalance, fire box temperature exceedes ceiling temperature, or electric furnace heating wire loop current is while exceeding upper limit current, and microcontroller will be carried out the processing routine of reporting to the police, and alternating-current switch is disconnected, and to main control computer transmission warning message.

As shown in Figure 3, described impedance measuring circuit comprises ac-excited source, self-balancing bridge circuit and vector ratio detecting unit.Ac-excited source is made up of the DDS frequency synthesizer AD9851 of two synchronous workings, and wherein a road is output as amplitude and the adjustable sine voltage signal of frequency, and another road is output as the low-jitter clock of controlling follow-up dual channel high speed synchronized sampling ADC.Self-balancing bridge circuit is responsible for realization by a slice JFET wideband precision amplifier ADA4817-1, and it has the advantages such as wide bandwidth, low input biasing circuit and low current noise.Self-poise is according to operational amplifier " empty short ", " empty disconnected " theoretical realization, and the impedance of establishing sample DUT is

the voltage at two ends is

the electric current passing through is

in range resistance R _rthe voltage at two ends

with sample DUT impedance

between pass be:

${\stackrel{\&CenterDot;}{Z}}_x=R_r\&CenterDot;\frac{{\stackrel{\&CenterDot;}{V}}_x}{{\stackrel{\&CenterDot;}{V}}_r}---\left(1\right)$

For meeting the actual needs of wide-range, having used an analog multiplexer ADG1609 and four precision is 0.01%, 5ppm, and resistance is respectively 100, the precision metallic paper tinsel resistance of 1k, 10k, 100k is realized automatic range switching.Vector ratio detecting unit mainly comprises 2 signal condition modules and SoC microcontroller.Signal condition module is that 1,10,100,1000 digital programmable gain instrument amplifier AD8253 is constructed by gain margin.The characteristics such as high input impedance, wide bandwidth, low distortion, short Time Created and low noise can be in the front end conditioned signal of various 16 ADC.SoC microcontroller is built-in with two independently 16 1Msps high speed SAR ADC, respectively vector voltage and vector current signal are carried out to synchronous acquisition, when after the signal of a complete cycle, combine digital phase sensitive detection algorithm, realize electric current and voltage resolution of vectors, and calculate in real time required modulus of impedance and phase place, and and these parameters are uploaded to main control computer, carry out data processing and finally obtain the ac impedance spectroscopy data of PTC thermistor by it.

As shown in Figure 4, described digital phase-sensitive detector is the core of impedance measuring circuit, different from implementations such as traditional employing analog multiplier, electronic switch, Type Multiplicative DAC, digital phase-sensitive detector is without the reference signal of two-way strict orthogonal, and " the sine and cosine table " of Flash storage carries out related operation in use sheet, and adopt finite impulse response (FIR) (FIR) filtering algorithm to replace analog low-pass filter circuit.The most important thing is, in digital phase sensitivity detection, the side-play amount being caused by the phase place of non-linear gain and analog element can be eliminated, and is not also existed by the deviation that temperature is floated or component ageing causes.Therefore, no matter digital phase-sensitive detector is in computing velocity, stability, accuracy, or has significant advantage in all many-sides such as realizing cost, dirigibility, extendability.

Although electrothermal furnace shell and impedance measuring circuit are altogether, suppress to a certain extent power frequency interference, still have to be positioned at frequency spectrum and to sneak into measured signal compared with the random noise of low side.For discussing conveniently, the frequency of establishing measured signal is f _t, the frequency of sampling clock is f _s, the measured signal that note contains noise e (n) is:

x(n)＝Acos(2πn/N+θ)+e(n) (2)

Wherein, A is amplitude, and θ is initial phase, N=f _s/ f _t, value is 64～1024, the orthogonal reference sequences of two-way is respectively S _i(n)=cos (2 π n/N) and S _q(n)=sin (2 π n/N),, after digital phase-sensitive detector, in-phase component I and quadrature component Q are respectively:

$I=\frac{1}{N}\underset{n=0}{\overset{N-1}{\mathrm{\&Sigma;}}}[A\cos (2\mathrm{\&pi;n}/N+\mathrm{\&theta;})+e\left(n\right)]\&CenterDot;\cos (2\mathrm{\&pi;n}/N)=\frac{A}{2}\cos \mathrm{\&theta;}---\left(3\right)$

$Q=\frac{1}{N}\underset{n=0}{\overset{N-1}{\mathrm{\&Sigma;}}}[A\cos (2\mathrm{\&pi;n}/N+\mathrm{\&theta;})+e\left(n\right)]\&CenterDot;\sin (2\mathrm{\&pi;n}/N)=-\frac{\mathrm{<figure-callout\; id="2"\; label="A"\; filenames="HDA0000116882500000011.png,HDA0000116882500000021.png"\; state="\{\{state\}\}">A</figure-callout>}}{2}\sin \mathrm{\&theta;}---\left(4\right)$

Obviously,, after digital low-pass filtering is processed, in-phase component I is only relevant with amplitude and phase place with quadrature component Q, and irrelevant with the random noise of sneaking into.Therefore, can obtain accurately amplitude and the phase place of measured signal by these two components:

$A=2\sqrt{{\mathrm{<figure-callout\; id="2"\; label="I"\; filenames="HDA0000116882500000011.png,HDA0000116882500000021.png"\; state="\{\{state\}\}">I</figure-callout>}}^2+{\mathrm{<figure-callout\; id="2"\; label="Q"\; filenames="HDA0000116882500000011.png,HDA0000116882500000021.png"\; state="\{\{state\}\}">Q</figure-callout>}}^2},$ $\mathrm{\&theta;}=-{\tan }^{-1}\frac{Q}{I}---\left(5\right)$

Method as discussed above, carries out digital phase sensitivity detection to u (n) and i (n) respectively, demodulates I and Q separately, can try to achieve impedance and the phase place of sample:

$\left|Z\right|=R_r\&CenterDot;\sqrt{\frac{{I_u}^2+{Q_u}^2}{{I_i}^2+{Q_i}^2}},$ $\mathrm{\&theta;}={\tan }^{-1}\left(\frac{Q_i}{I_i}\right)-{\tan }^{-1}\left(\frac{Q_u}{I_u}\right)---\left(6\right)$

Because digital phase-sensitive detector utilization and measured signal have the reference signal benchmark as a comparison of same frequency and phase relation, only measured signal itself and those and reference signal are had to response with the noise component of frequency, homophase, and external interference noise and pumping signal are very low with the probability of frequency homophase, therefore digital phase-sensitive detector has very strong inhibiting effect to harmonic wave and random noise.

Claims (2)

1. a PTC thermistor ac impedance spectroscopy automatic testing equipment, comprising:

Electrothermal furnace, for placing sample;

Temperature control and holding circuit, use PT100 platinum resistance to gather burner hearth internal temperature, and Real-Time Monitoring is by the electric current of electric furnace heating wire, to maintain described electrothermal furnace constant temperature and to prevent overcurrent;

Impedance measuring circuit, for measuring the impedance of sample;

Main control computer, is connected with described temperature and current protecting circuit and described impedance measuring circuit respectively by CAN bus, for receiving user instruction, controls described two circuit and completes corresponding work;

Described impedance measuring circuit comprises ac-excited source, self-balancing bridge circuit and vector ratio detecting unit, ac-excited source provides synchronous sinusoidal test signal and sampling clock to self-balancing bridge circuit and vector ratio detecting unit respectively, self-balancing bridge circuit produces voltage and current vector signal under sinusoidal test signal excitation, and vector ratio detecting unit completes the demodulation to described voltage and current vector under sampling clock control; Described ac-excited source is made up of the DDS frequency synthesizer of two synchronous workings, and wherein a road is output as amplitude and the adjustable sine voltage signal of frequency, and another road is output as the low-jitter clock of controlling follow-up dual channel high speed synchronized sampling ADC; Described self-balancing bridge circuit is realized by a slice JFET wideband precision amplifier;

Described vector ratio detecting unit comprises first signal modulate circuit, secondary signal modulate circuit and a SoC microcontroller, first signal modulate circuit and secondary signal modulate circuit respectively to voltage and current vector signal amplify, filtering and level conversion, voltage and current vector input the one SoC microcontroller after conditioning, the one SoC microcontroller carries out A/D conversion to the signal of input, and by its built-in digital phase-sensitive detector, the voltage and current burst after A/D conversion is made to related operation, obtain modulus of impedance and the phase place of sample; Described digital phase-sensitive detector uses " the sine and cosine table " of Flash storage in sheet to carry out related operation, and adopts finite pulse response FIR filtering algorithm to replace analog low-pass filter circuit;

Described temperature control and protection circuit includes supply module, zero cross fired alternating-current switch and current acquisition module, they are electrically connected successively and form loop with the electric furnace heating wire of electrothermal furnace, zero cross fired alternating-current switch also connects the output terminal of the 2nd SoC microcontroller, the first input end of the 2nd SoC microcontroller connects temperature collect module, and the second input end of the 2nd SoC microcontroller connects current acquisition module; Temperature collect module is based on four line Method of constant flow sources, and the precision constant current source of 1mA forms the d. c. voltage signal between 100～200mV by PT100, then amplifies and low-pass filtering through signal, is transformed into the voltage that is applicable to the built-in ADC input range of SoC microcontroller; Adopt zero cross fired be open for fear of power controllable silicon during the saltus step of output voltage and electric current to load with the harmful effect of the miscellaneous equipment of electrical network, be power controllable silicon always in the conducting that is triggered of the zero crossing of alternating voltage, make load current and voltage occur each time all starting from scratch; By the make-and-break time of the pwm signal output port control Over zero bidirectional SCR output photoelectric coupling mechanism of microcontroller, realize the break-make control in load voltage loop; By changing the make-and-break time of Duty ratio control bidirectional triode thyristor of pwm signal, thereby realize the electrodeless adjustable of load voltage loop voltage and power; Temperature control and holding circuit adopt expert system intelligence self-setting PID arithmetic to realize the accurate control of resistance-heated furnace; When PID imbalance, fire box temperature exceedes ceiling temperature, or electric furnace heating wire loop current is while exceeding upper limit current, and microcontroller will be carried out the processing routine of reporting to the police, and alternating-current switch is disconnected, and to main control computer transmission warning message.

2. PTC thermistor ac impedance spectroscopy automatic testing equipment according to claim 1, is characterized in that, between described sample and impedance measuring circuit, joins by five terminal shield terminal.

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