Study On Ohmic Heating Temperature Control System Of Food

Thermal processing is still the predominant food processing and food safety toolavailable. It is not only directly related to the safety and quality of the final product, but alsorelated to the energy consumption and manufacturing cost. Ohmic heating provides a newheating method for food processing. However, the control of the temperature in the heatingprocess has been considered as the industry’s difficulty because the constant voltage control isapplied in current ohmic heating systems. Moreover,the temperature in the heating processhas a great influence on food quality. So, the study of temperature control during the heatingprocess has important theoretical value and practical significance.In this study, an ohmic heating temperature control system of food based on SCM wasdesigned. The system can not only set the heating rate,holding temperature,holding time, butalso give a real-time monitoring to the temperature change during the heating process. Themain content of this study is as follows.(1) The overall design of the system was made. Firstly,the main functions of the systemrequired are analyzed. Then, The main hardware modules required and the structuralrelationships of them were determined according to the main functions.(2) The hardware design of the system was completed. The hardware of the systemincludes an automatic voltage regulator circuit based on MCU, a data acquisition circuit,function buttons, a liquid crystal display, a serial communication circuit and a heating trough withparallel plate electrode. The automatic voltage regulator circuit based on MCU includes maincircuit and control circuit, and the selection of the major components in the circuit wascompleted. Including temperature, voltage, current collection, the acquisition circuit consistsof a K-type thermocouple, a MAX6657, a voltage transducer, a current transducer and aADC0832. Buttons are mainly in order to complete the setting of heating rate, holdingtemperature and the holding time. A LCD1602is selected as the liquid crystal display. Itdisplays the corresponding parameter values when setting the system operating parameters,and then displays the temperature of food materials in the heating process. The serialcommunication circuit is used for the communication between MCU and PC. The heating trough with arectangular parallelepiped shape is made of plexiglass. Parallel plate electrodes are made ofstainless steel. (3) The overall control scheme of the system is determined. It displays the default heatingrate instead of starts heating directly when the system begins to work. Pressing the start buttonto start heating after the setting of heating rate, holding temperature and holding time iscompleted. The system will start holding process automatically when the temperature reachesthe holding temperature, and the system will stop heating when the holding time reaches theset value.(4) The design of the heating rate control scheme was completed. Heating rate controlusing composite control method which is a combination of feedforward control and feedbackcontrol.(5) The design of the temperature holding control scheme was completed. Temperatureholding control using time optimal control method which is a combination of ON-OFFcontrol and PID control.(6) The system was used for heating the milk in an experiment. It showed a good linearrelationship between the temperature of milk and time when the setting value of heating ratewas6℃/min,8℃/min,10℃/min and12℃/min respectively, and the relative error betweenactual heating rate and setting value was0.50%~3.83%. The settling time is less than20s, themaximum overshoot does not exceed1℃and the steady-state error in the range of±0.3℃.