| The paper describes hardware design of the on-line automatic analytical instrument used to detect nine kinds of trace heavy metals(Zn,Cd,Pb,Cu,Mn,As,Fe,Cr,Cd and Hg) of seawater, which adopts multiple measurement techniques such as light-addressable potentiametric sensor, differential pulse stripping voltammetry and chalcogenide ion selective electrode.The instrument can be divided into two parts on the basis of function: measurement and control system. Various hardware designs are applied for different measuring techniques.Moreover, the noise model of the system is set up for the optimization of the whole circuit realization. The core of the Lock-in amplifier used to measure response of LAPS consists of a high precision modulator/demodulator and DDS as signal generator. The Electrochemical measurement system used to perform DPSV measurement is constructed with two parts, a low-current measurement system consists of low input bias current operational amplifier and a high resolution SAR ADC and a potentialstat consists of high resolution DAC and high current buffer amplifier. The instrument used for the measuring electrode is a high precise multi-channel system constructed with a Σ-AADC as the core ,a high input impedance amplifier and a low input current multiplexer. Special functions are accomplished by the embedded firmware , including the simple potential sampling and the complicated process of electrochemical scanning.The core of control system consists of MCU DS80C320 and Programmable System Device PSD854F2, which features high reliability, high integrity ,flexible configuration and wonderful extensibility. According to the requirement of online measurement, the control system is expanded by adding SRAM for storage sample data, the real time clock for counting, and the power logic addressable latch for peripheral such as valve and DC Motor. According to the requirement, theinstruments can limberly configure the parameters such as open/close sequence and delay time of valve and pump, thus guarantee the accuracy of measurement.The prototype has been fully checked, including the curernt resolution of the lock-in amplifier and the electrochemical system, the stabilization of the Potentiostat and bias voltage of lock-in amplifier, the effective resolution of the instrument used for the measuring electrode. The result shows that all thetechnical data of the instrumental hardware reaches the requirement of the design. Up to now, the instrument is tested in the Bohai Sea for on-line sea water measurement. The system, which consists of automatic sample injection and analysis, is quite suitable for measurement of trace heavy metal ions. |
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