PLC in Food Industry

Food industry within the agro-industrial complex is one of the largest sectors of the world’s economy. Development of the canning industry goes through the construction of new plants and modernization of existing enterprises with the assistance of the private sector. One of the most important processes in the canning industry, which depends largely on the quality and storage ability of the finished product, is sterilization of food. In the process of sterilizing, capped jars with product are subjected to high temperatures, making microorganisms in jars die, thereby ensuring safety of food products. The quality of finished products directly depends on the quality of the sterilization process, which is the most complex and important stage of preservation technology. Autoclaves are the most frequently used devices to sterilize canned food. Periodic articles and books concerning the study and development of sterilization processes are being published recently in a certain amount, especially the ones concerning management of this process by means of automatic control and regulation devices.

High quality canned food is provided by a clear and well-coordinated work of all parts of the conversion process: starting with the sorting, washing, blanching, packaging and ending with packaging and sterilization. Sterilization is one of the most critical stages of processing of agricultural products (Gutierrez-López, Barbosa-Ganovas, Welti-Chanes, & Paradas-Arias, 2008). Machines batch, which include autoclaves, are used for the sterilization of canned food. To sterilize canned food, the technology uses autoclaves and sterilizers of continuous action. During autoclave sterilization process, cans are loaded in perforated baskets that are loaded via the hoist to the autoclave. There are single, two, three, and four basket autoclaves. Sterilization may depend on the type of product, on the sterilization temperature, and on the amount of jars (sterilization temperature is 113-120 C⁰) (Gutierrez-López et al., 2008).

Modern industrial autoclaves are units with high productivity. They are used in vertical and horizontal configurations. The most widely used configuration in the canning industry is vertical autoclaves as they sterilize all kinds of canned food in tin and glass containers (Safefood 360, Inc., 2014). While there are different configurations and models of autoclaves, the algorithm of their functioning is the same. This algorithm consists of heating a product to high temperatures under pressure, which is usually packaged in a glass or tin container. Increased pressure in the autoclave as compared to atmospheric pressure compensates thermal expansion of the heated product, preventing destruction of the package. Pressure is calculated according to autoclaving and depends on the temperature of sterilization, type of packaging, the coefficient of thermal expansion of the product, etc.

The sterilization process can be divided into several stages. The first step is loading. The autoclave is filled with cold water, which then is heated. A product is put into it upon reaching the desired temperature in the autoclave and then the lid is sealed. The second step is heat reaction. Food should be heated until the temperature reaches a certain value set, whereby the processing mode is supported for several dozens of minutes. The third step is cooling. Heating is discontinued and autoclave is filled with cold water, providing smooth cooling and pressure equalization. Complete food pasteurization is performed after this process (Safefood 360, Inc., 2014).

Currently, the market of industrial automation presents many program-controlled devices for different technological processes. However, the following devices are the most widely-spread: programmable logic controllers (PLC) and industrial computers, including the panel ones (Schwartz, Mulder, Trent, & Atkins, 2010). The first device mentioned above is the most suitable for the sterilization process. PLC is a software-controlled digital device with a plurality of inputs connected to the object that is to be controlled by using sensors and a plurality of outputs connected to actuators. PLC controls the status of inputs and produces a definite software-defined sequence of actions, which are reflected in the output change.

PLCs have found their wide distribution in the food industry due to the fact that they have high resistance to environmental influences, small size, modularity, and expandability. They also stand out in terms of high speed repairs (low recovery time), use of flash memory, presence of a watchdog timer, which protects the system management of hovering, presence of a large number of industrial interfaces, etc. Such controllers traditionally work in the bottom tier of enterprise’s automated management systems that are directly related to production technology.

The use of PLC as a control device allows creating management system of any complexity. However, some difficulties still may arise in its further implementation. The main problem is to create a program to control temperature and pressure in autoclave. Hence, the main criteria to be met by the control system are as follows: accuracy of the process with the temperature up to ± 0,01 θ_st and time up to ± 1 min, where θ_st is temperature of canned food sterilization, C⁰); and maintenance of the counter-pressure to avoid pressure derivation in the jar with an accuracy of ± 0,01P_st, where P_st is the pressure of sterilization, Pa).

Achieving this accuracy is possible by using modern technical control means. Virtually all manufacturers of controllers and programming systems provide the developer with controls library, which traditionally contains one of the PID controller (Mittal, 1996). Proportional-integral-derivative (PID) controller is the most effective and common tool of control, which provides a high enough precision in the management of various processes. The market of automation tools is rather diverse, offering both expensive and cheap, but functional and reliable PLCs. These PLCs are programmed in accordance with IEC 61131-3 in the CoDeSys programming environment (Hanssen, 2015). Realization of temperature control system based on PID in the CoDeSys programming environment uses the FBD programming language (IFM Electronic, 2007).

An experiment was conducted to test efficiency of the block of PID heating element functioning, which showed that the temperature control system was operational.

However, it should be underlined that the dynamic deviation (overshoot) is present on the set parameters in the temperature of the output process although the regulatory process has no static error (deviation from the set temperature in the steady state). Besides, there appear undamped self-oscillations while transitioning to a lower working temperature. All this could adversely affect the sterilization process, in which deflection temperature requirements are quite stringent.

It should be noted that autoclave processing during sterilization is being done at different temperatures. This is due to the fact that the sterilization process has a stepwise character (steps of heating, sterilization, and cooling). Therefore, the regulatory system should work well equally on all parameters of temperature. The use of PID control blocks in the autoclave control system is advisable. However, there is an additional correction controller setting possible, depending on the stage of the process, which can reduce the overshooting. To do this, the mathematical model must take into account process parameters at different stages in the preparation process. Improving the quality of regulation is possible together with adaptive methods and classical control laws (PID). Confirmation of the above is a graph of temperature and pressure control obtained by experimental studies. The system operates stably even when applying disturbance in the form of cold water as the system returns to a steady state, which confirms presence of the desired astatism therein.

The objective of automation is to control two main parameters of the autoclaving process: temperature and pressure. A system of automated control and management of the water sterilization process of canned food includes an automated workplace and serves all steps of canned sterilization. These are filling the autoclave with water, its heating, product loads, temperature and pressure regulation, proper sterilization, and cooling of cans. As the main control unit, the system uses a programmable logic controller, which allows organizing any degree of the control system’s complexity (Hanssen, 2015). Measurement of the operating pressure within the autoclave is carried out by sensors with an output unified signal. Information from them is sent to the controller’s inputs. Control parameters, i.e. pressure and temperature, are interconnected so that the system has two related control channels. In terms of the output device management, output module is used, which is connected to the controller by RS-485 interface (Hanssen, 2015).

The cold water supply pipe is fitted with a pressure sensor, which monitors water pressure to eliminate emergency in the process of the products cooling operation when the autoclave is being drained of hot water with further filling it with cold water. The system will give a signal to the workstation if the pressure drops below the permissible level. The operator should close the valve connecting the autoclave to the backbone of cold water supply and connect the input to the spare tank autoclave; then, the autoclave cycle of work will continue with the same program. Data transmission on the top level of management is carried out via the Ethernet interface. The process uses CoDeSys programming environment as the system for visualizing, archiving, and managing (Hanssen, 2015).

Thus, to resume its work, autoclave automatic control system provides a centralized data collection on the process and the status of autoclaves on a PC, which displays relevant data, pressure and temperature in autoclave, and current information in text and graphic forms. It also makes calculations, fulfilling management functions for the implementation of automatic control of the sterilization process. It is all possible with the PLC usage since it ensures exact implementation of the established sterilization formula, regulating main parameters and largely decreasing complexity of the service.

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