Design and application of the on-line monitoring s

Design and application of double led ion concentration monitoring system

1 introduction

the optical path system is shown in Figure 1-1, which is mainly composed of light source, filter, absorption cell and monitor. Optical path I is the reference optical path, and the target concentration solution of the measured copper ion is put into the reference cell; Optical path II is the measurement optical path, and the measured solution flows slowly from the bottom of the measurement cell and returns to the system from the top. The pH value and ionic environment of the two solutions are roughly the same

the light source is composed of two in series Φ 5.0 the red super bright LED (light emitting diode) is composed of a filter, instead of using the traditional sodium light source and the light splitting mirror structure, which reduces the cost and volume. Moreover, compared with the common sodium light source, the LED has the advantages of good linearity, low power consumption, long service life, concentrated luminous wavelength and so on. Because the copper ion solution is blue and has a large absorbance to red light, the red LED is used, which is composed of a precision reference voltage source MC1403. Each dustpan is powered by a constant current source (2 ~ 5mA) with a span of 1 mile (about 1.6 km). The absorption cell adopts two 1cm glass cuvette absorption cells, which are placed in parallel and perpendicular to the optical path. The sensor adopts SPD (silicon photodiode, Φ 5．0，TOSHIBA）。 SPD has good frequency response, wide linear range, small dark current, not easy to fatigue, small volume, and easy to obtain working conditions. It is very suitable for long-term uninterrupted work. At present, the widely used vacuum photocell or photomultiplier often requires a very high working voltage, which is very large in volume and power consumption, and it is very easy to fatigue, so it cannot work continuously for a long time

2 hardware system

the hardware system is composed of amplification circuit, a/D, control circuit and computer interface. Figure 2-1 shows the amplification circuit. Under weak light, the carrier formed by the excitation of the PN junction of SPD forms a reverse saturated drift current (10-7 a order of magnitude) under the action of an applied electric field. This current is selected by the analog switch ad7501, and then voltage current conversion is carried out by the operational amplifier lf353, and then voltage amplification is carried out by AD620. When the ad7501 is turned off, the signal current passes through a 1m Ω resistance grounding, which is a necessary instrument for enterprise production. It can not only make the SPD in a normal working state, but also make the input and output terminals of the ad7501 maintain a very simple and small differential voltage. Since the SPD output is a current signal with high impedance, the on resistance of ad7501 has little effect on the signal. Lf353 input stage is JFET, which basically has no absorption of signal current. AD620 is a precision integrated instrument amplifier produced by ad company. It has high common mode rejection ratio, low noise system and temperature drift coefficient, and simple peripheral circuit. The transfer function of the whole amplification circuit is 10m Ω

after the current is amplified, a voltage of 0 ~ 3V is obtained and sent to ICL7109 for a/D conversion. ICL7109 is a double integrating chip with 12 bit accuracy, and has polarity bit and overflow bit output. Since 50Hz power frequency interference inevitably occurs in the modulation of power supply and light source, the integration time is taken as a power frequency cycle, and the corresponding crystal frequency is 6mhz. The sampled data is read into the computer through ISA bus for processing. In order to enhance the anti-interference performance of the circuit, tlp521-4 optoelectronic coupling chip is used to send the conversion result to the computer, and the computer also transmits the control signal in reverse through the optocoupler to realize optoelectronic isolation between the two. The application circuit of TLP 521-4 is shown in Figure 2-2

3 measurement principle

since the luminous characteristic of LED is linear when the current is less than 10mA, assuming that the current flowing through the LED at a certain time is I0, the corresponding luminous intensity:

g1 = l1i0, G2 = L2 i0

and the reverse saturation drift current of SPD is also proportional to the intensity of incident light, assuming that the attenuation factor of the optical path itself is β， The ion concentration is C, the beer coefficient is k, the attenuation caused by the beer absorption of the solution is e-kc, the transfer function of the amplifier is a, and the SPD photoelectric conversion coefficient is m, then the final output voltage of the amplifier can be expressed as:

u1 = AM1 β 1L1 e－k1 C1U2＝AM2 β 2l2e has a large buffer effect on strong impact - K2 C2

the above treatment method eliminates the LED current term, and the temperature drift of device parameters m and l offset each other in the process of division, so B can be regarded as a constant. Let C1 = C2, ln (U1/U2) at this time is B, so the essence of B is the zero point of system software. Using it to correct the zero point of measurement results saves the cumbersome zero adjustment process of common instruments. Since C1 is known, the constant k at this concentration can be obtained experimentally, and then the concentration C2 of the measured solution can be obtained from the measured value of Ln (U1/U2) and B

4 software flow

the software is written in Visual C + + and is mainly composed of system flow control, data acquisition, software filtering and numerical analysis, as shown in Figure 4-1

the system first measures the background voltage (generated by SPD dark current and ambient background light) when the light source is turned off, then puts the reference liquid into the measuring cell under the control of the computer, turns on the light source, measures the software zero point, and then turns to the measurement state. Considering the slow change of concentration signal, samples are taken every 1s or longer, depending on the specific situation. Because Windows 98 does not protect ports, you can directly use the - inp() and - out() instructions to read and write ports. In order to improve the sampling accuracy, the method of combining median filter and average filter is adopted, which can not only eliminate accidental errors (such as pulse electromagnetic interference or a/D error often in industrial occasions), but also eliminate the errors caused by periodic pulses. In addition, the sampling results are also analyzed