Research On The Intelligent Electronic Analytical Balance Based On DLS

The electronic analytical balance is a high sophisticated precision measuring instrument which merges multiple technologies based on the machinery,materials,electronic circuit,information processing,precision manufacturing,etc.It is also a standard instrument of mass measurement widely used in the field of trade or science and technology,such as the quantity transmission,the food security,the biological medicine,chemical analysis,aerospace equipment,mass control of valuables,etc.With the development of science and technology and the improving of the industrialization,the requirement for mass measurement of goods becomes higher and higher in every walk of life,the demand for the electronic analytical balance significantly increases.As the low level of materials manufacturing,machining,partial assembly and fabrication processing at home,and the lack of theoretical researches on the mechanical model of the key part,i.e.,the electromagnetic force compensated load cell,the displacement sensing device,the structural model of the magnetic steel circuit,and the relevance external control circuit of electronic analytical balance.Moreover,the exiting nonlinear rectification for balacne,temperature drift compensation,time drift compensation,drift compensation of acceleration of gravity,the digital signal processing of weighing and so on,still fall behind,the mass measurement performance of domestic electronic analytical balance is far lag behind import products.Therefore,the domestic balance products have weak competitiveness and poor market penetration,and leading to the high prices of imported products,these seriously constrain the development of the domestic electronic analytical balance industry.Along with the increase of the raw materials price and the labor cost,the domestic balance enterprise is in dire need of a home-grown intelligent electronic analytical balance that reaches advanced international level.Aiming at the gaps of the electronic weighing technology between home and abroad,several researches on the electronic analytical balance based on Double Lever Sensor(DLS)are carried out as follows:(1)Aiming at the poor weighing precision and stability of the domestic electromagnetic force compensated load cell,the optimum design method for DLS is proposed.The working principle of DLS is described,and the formation mechanism of the eccentric load error from the Roberval mechanism in the mechanical elastomer of DLS is analyzed.The force models of the critical components,i.e.,the reed,the coupling elements and the beam,are established,after the alloy material is selected,the dimension parameter of each reed is obtained by effectively giving consideration to both the sensitivity and repeatability design criterions of DLS.A precision displacement detection device is designed,the detection of the minimum displacement caused by the discriminability weighing is guaranteed.A magnetic circuit structure based on double magnetic steel is presented,and then the nonlinear error from DLS is eliminated,the dimension parameter of each element in the magnetic circuit structure is optimized.The force model of the mechanical elastomer and the structural model of the magnetic circuit are both built based on ANSYS Workbench,the rationality of the dimension parameter of each element is verified by simulation.(2)In order to make the DLS closed loop outstanding transition process quality and control precision,and guarantee the mass measurement accuracy and stability of electronic analytical balance,the design and setting method for the dynamic calibration link of closed-loop system is proposed.The kinetic equation of mechanical elastomer of DLS and the transfer function of the displacement detection device are analyzed in detail,based on the P regulator formed by the signal amplifying circuit of the error voltage from the photoelectric conversion circuit,and deducing the transfer function of each link in exterior control circuit according to the operation time sequence of the Pulse Width Modulator(PWM),the closed-loop structure of DLS is established before dynamic calibration.The transient process of the closed-loop system before dynamic calibration is deeply analyzed,the impact caused by the forward path and the backward path on the closed-loop system is concluded,and the requirement of closed-loop system for the type of the dynamic calibration regulator is educed.The PID regulator is selected as the dynamic calibration regulator for DLS closed-loop system,on the premise that the closed-loop system possesses approving transient process,the design and the setting of relevant parameters for the PID regulator is finished.The errors from the closed-loop system are preliminarily analyzed,the handling methods for the drifts and noises are mentioned briefly.(3)Aiming at the complex influence on electronic analytical balance from variations of temperature,time and space,all kinds of drift mechanism and relevant fabrication processing method are respectively cleared and proposed.The temperature drift and time drift mechanism from the mechanical elastomer of DLS is intensely researched,the suitable type of material for the mechanical elastomer is selected,and the fabrication processing method for the material of the mechanical elastomer is summarized.The temperature drift and time drift mechanism from the magnetic circuit structure of DLS is also researched in detail,the suitable type of material for the magnetic circuit structure of critical component is chosen,and the fabrication processing method for the material of the magnetic circuit structure is proposed.The operating principle of constant current source in exterior control circuit which is used for driving coil and producing electromagnetic torque under the action of magnetic field(namely,electromagnetic torque creates constant current source)is expounded in detail,the temperature drift and time drift mechanism from the moment generating constant current source is deeply investigated,the suitable type of the key electronic component is selected,and the fabrication processing method for the electronic component in circuit is concluded.The generating mechanism of gravity acceleration is disscussed,the cause of change of gravity acceleration and its impact on electronic analytical weighing are both summarized.All of the above provide the comprehensive theoretical support and technical reference for the drift compensation and calibration method of electronic analytical balance.(4)Aiming at the impacts on the mass measurement accuracy of electronic analytical balance from drift,an effective drift compensation and calibration method is proposed.The mass measurement model of electronic analytical balance under the impact of drift is deduced,a drive circuit based on PWM mirrored pulse is designed,the influence of the dynamic thermal effect in moving coil is eliminated,and the temperature drift and time drift are both simplified according to the relevant stability parameters of the mechanical elastomer,the magnetic steel,the moving coil and the key electronic components,thus the complexity of the drift compensation algorithm is decreased.A high precision temperature measurement circuit based on PT1000 is designed,the temperature drift compensation algorithm by using piecewise linearity weighting,the drift tracking algorithm based on the rate change threshold of the indicating value,the gravity acceleration calibration method based on threshold evaluation are proposed respectively,and then the impacts on the mass measurement performance of electronic analytical balance from drift are effectively reduced or even eliminated.In the work environment constructed by Chinese National Standard"GB/T 26497-2011 electronic balance",the accuracy and efficiency of the drift compensation and calibration methods are both tested and verified.(5)In order to further guarantee the mass measurement performance and the integrality of test results of electronic analytical balance,the method for anti-noise design and the uncertainty model of test results are proposed respectively.A closed-loop system of DLS based on continuous time(CT)?-? modulation structure is designed,a composite filter composed of two SincN filters is established and optimized,and thus the influence of the thermoelectric magnetic interference noise in the closed-loop system is significantly reduced.According to the characteristics of influence of the external environment vibration on the mass measurement of electronic analytical balance,an ant-vibration filtering algorithm based on Newton interpolation is proposed,which restrain the effect of force disturbance noise in the closed-loop system effectively.Based on this,An intelligent electronic analytical balance with automatic drift compensation,fault self-diagnosis,smart calibration and multiple application functions is successfully developed,which have weighing performances of excellent stability,fast response and high accuracy.Finally,reference to Chinese National Standard "GB/T 26497-2011 electronic balance",the mass measurement performance index of electronic balance including the indication error,the eccentric load error,the repeatability error,the discrimination and the creep and return-to-zero,etc.,are tested.In addition,according to Chinese National Standard"JJG99-2006 weights verification regulation",the uncertainty model of test results is established based upon the sources of test errors.The test results of intelligent electronic analytical balance under specified conditions show that the error of indications in all of the scale is not greater than 0.5mg,the error of eccentricity is 0.2mg,the repeatability is 0.3mg,the discrimination test results change obviously,main performance figures of the balance meet the error requirements for the 1st class balance specified by Chinese National Standard "GB/T 26497-2011 electronic balance" and have intelligent function for international advanced intelligent electronic analytical balance.