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PLC在冷冻干燥机中的应用
论文介绍了可编程控制器在冷冻干燥机的应用。传统的冷冻干燥机由程序步进器进行机械控制，其触点繁多，故障频繁，维修不便， 严重影响冷冻干燥机的正常工作，对产品的质量也产生了很大影响。
1 根据冷冻干燥机自动控制方式及特点，常用的控制方式有手动控制、继电器控制、单片机控制及PLC程序控制。
1.1 继电器控制
继电器问世以来，一直是真空工程控制的主流，目前国内仍有不少厂家应用继电器控制。其中输人单元为按钮开关等主令电器、逻辑单元是支配控制系统的“程序”。它是由各种继电器的触点通过导线连接，并具有一定的逻辑功能的控制线路。输出单元包括各种阀门的控制线圈，控制各类接触器线圈及信号指示等执行元件。这种控制系统的特点是：价格便宜、性能价格比低。因此，存在继电器抖动、打弧、吸合不良等现象，使控制系统寿命短，可靠性差。因为它的自动控制功能是靠开关继电器的辅助触点动作来实现。而触点动作一次需要几十毫秒，故控制速度橙，由于需要改变控制逻辑就要改变各开关、触点间的连线，故倍改控制逻辑复杂、困难，同时体积大，耗能高。
1.2 单片机控制  
用单片机可以构成各种工业控制系统。有不少利用通用PC单片机或通用计算机系统。大部分都可以用单片机系统或单片机加通用机系统来代替计算机系统，特别是较大型的工业测控系统。如大型自动磁控镀膜机、真空冶炼等。除通用外设(如打印机、硬盘、磁盘等)外，还有许多外部通信、采集、多路分配管理、驱动控制等接口，而这些外设与接口完全由主机进行管理，必然造成主机负担过重，因而出现了新的控制手段，即可编程控制器。
1.3 可编程控制器控制
1968年，美国最大的汽车制造商一通电汽车公司，从国产角度提出了新一代控制器。我国紧跟其后，发展也很快。不少厂家生产了不同类型的可编控制器已在冷冻干燥机获得应用。为了适应工业环境中使用，具有以下特点： 
（1）靠性高，抗干扰能力强。平均无故障时间可达30万小时。  
（2）控制程序可变，具有很好的柔性。冷冻干燥不同品种的食品或生产设备更新的情况下，不必改变硬件设备，只需改变冻干程序或冻干曲线就可以满足工艺要求。   
（3）编程简单、使用方便。这是PC机优于微机的另一特点。目前大多数PC机采用继电器控制形式的“梯形图编程方式”，即继承了传统控制线路的清晰直观，又考虑大多数现场企业电气技术人员的读图习损与微机水平，易于校操作人员接受。   
（4）功能完善。现代PC具有数字和模拟输入、输出逻辑及算术运算完成时，计数、顺序控制、功率驱动、通信、人机对话、自捡、记录和显示等功能，使冻干设备控制水平大大提高。扩充方便，组合灵活。PC具有各种扩展单元，可以方便地适应不同冻干工艺控制的需求，不同输入输出点数及输入、输出方式的系统：少了控制系统设计及施工的工作量。由于PC采用软件编程来达到控制功能，而不于继电器控制所采用硬接线来达到控制功能，因此，减少了设计及施工工作量，PC又事先模拟调试，减少现场工作量。   
（5）体积小、质量轻。一个收录机大小的PC具有相当于三个1．8M高的继电柜的功能一般节电50％以上。PC专为工业而设计，结构紧密、坚固、体积小巧，具有很强的抗干扰能力；而装入机械设备内部，PC还可以与监控计算机联网。
2 冷冻干燥机PLC控制优势  
（1）使用寿命长：从目前反馈情况看，目前冷冻干燥机控制电路的使用寿命大部分与其电路设计、元器件选型、工作环境及控制方式等因素有关，是其本身无法克服的固有缺点。PLC作为工业控制单元，应用于各种控制环境，内部电路、机械结构设计极为精良，所用器件均选用标准工业级产品，其使用寿命一般可保证在十年以上。 
（2）性能稳定可靠，抗干扰性好：PLC应用于各种工业控制现场，其硬件及软件设计均考虑到各种生产环境，其电压适用范围很宽，具有极强的抗电磁干扰、抗震动、抗高温、高湿等特性，性能极为稳定、可靠。
（3）功能强大，实现灵活，可扩展性好：PLC控制型冷冻干燥机作为其老型号的升级产品，其功能得到极大增强，目前可实现的功能有：自动除霜、自动在线清洗、自动灭菌、各种情况的自动报警、设备保护及故障识别等。以上功能可实现各种组合，并可根据实际需要改变上述功能的控制过程及方式，并可根据使用者要求在不增加或少增加硬件的基础上开发新的控制功能。
（4）良好的性价比：虽然PLC型冷冻干燥机性能及功能较现有冷冻干燥机有极大提高，但其成本的增加与其性能的提高并非成线性关系，所以无论将其作为整机用于新品开发，还是作为老设备改进均有其良好的性价比。
采用三菱的小型PLC对整个冷冻干燥机运行系统进行控制，可实现较高的自动化程度，并完成智能控制、状态显和故障报警等任务，同时还提供良好的HMI界面，使得修改各种参数简单、易行，对于机械设备的运行能及时、快速、准确的反映。其控制系统结构清楚、线路简单、工作可靠，减轻了工人的劳动强度，提高了系统的稳定性和可靠性。

APPICATION OF PLC ON THE SYSTEM OF FROZEN DRYERS
The thesis introduced application of programmable logic controller (PLC) on the system of frozen dryers . The traditional frozen dryer is carried on machinery to control by the procedure walking into device, its contact is various, the trouble is frequent, maintain inconvenient. The traditional frozen dryer not only influence the normal work of the frozen dryer seriously, but also have severe effect on the quality of the products .
1 According to freeze drying machine automatic control mode and characteristics of commonly used control method with manual control, relay control, microcomputer control and PLC program control
1.1Relay control
Relay, have been is the mainstream of the vacuum engineering control in China at present there are still many manufacturers use relay control. The input unit command electrical appliances, such as push button switch logic unit is the "program" command control system. It is composed of relay contacts through a wire connection, and has a certain logic control circuit. Output control coil unit includes a variety of valves, control all kinds of contactor coil, indicator actuators, etc. This control system features are: the price is cheap, low price performance ratio. Therefore, there are relay jitter, bowl, and wait for a phenomenon, make the control system of life is short, poor reliability. Because it's automatic control function is accomplished by the auxiliary contact of switch relay action. And contact action need tens of milliseconds at a time, so the control speed is orange, because the need to change the control logic is about to change the attachment between the switch and contact, so The Times change control logic is complex, difficult, at the same time, large volume, high energy consumption.
1.2The single chip microcomputer control
With the MCU can constitute a variety of industrial control system. There are quite a few use ge PC MCU or a computer system. Most can use single chip microcomputer system or single chip microcomputer and general machine system instead of a computer system, especially in the larger industrial measurement and control system. Such as large automatic magnetic control film coating machine, vacuum smelting, etc. In addition to general peripherals (such as printers, hard drives, disks, etc.), there are a lot of external communication, collection, distribution of multiplex management, drive control interface, and the peripherals and interfaces is completely managed by the host, is bound to cause the host burden is overweight, so a new control method, can be programmed controller.
1.3 PLC control
In 1968, America's biggest carmaker, a charge the company, from the domestic point of view put forward a new generation of controller. Soon followed, development in our country. Many manufacturers produced different types of programmable controllers had been used in the freeze drying machine. In order to adapt to industrial environment, has the following features:
(1) on high, anti-jamming is strong. Mean time between failures of up to 300000 hours.
(2) the control variable, has the very good flexibility. Freeze drying or different varieties of food production equipment update, don't need to change the hardware equipment, only need to change the lyophilization process or lyophilization curve can satisfy the technological requirements;
(3) simple programming and easy to use. This is another feature of PC is superior to the microcomputer. Currently most PC machine adopts relay control in the form of a "ladder diagram programming approach", which inherits the traditional control circuits of clear and intuitive, and most of the enterprise to read electrical technical personnel's figure loss and micro level, easy to school operators to accept;
(4) the function is perfect. Modern PC with digital and analog input and output logic and arithmetic operation is complete, counting, sequence control, power, communication, man-machine dialogue, since pick up, record and display, and other functions, the freeze-drying equipment greatly improve the control level. Convenient expansion, flexible combination. PC with various extension units, can be easily adapted to different freeze-drying process control requirements, different input and output points and the form of input and output system, control system design and construction work. Because of using PC software programming to achieve the control function to instead of relay control adopts hard wiring to achieve control function, therefore, reducing the workload of design and construction, PC and simulation debugging in advance and reduce the workload;
(5) small volume, light quality. A recorder with PC is equivalent to the size of three high 1.8 M relay function is generally the saving electricity was above 50%. PC specially designed for industrial, tight structure, strong, small size, strong anti-interference ability; And loading machinery equipment internal, PC can also with monitoring computer connected to the Internet.
2 Freeze drying machine PLC control advantage
(1) long service life: from the current feedback, at present the service life of the freeze dryer control circuit most instead of circuit design, components selection and the way of working environment and control factors, is unable to overcome the inherent shortcomings of its own. PLC as industrial control unit, used in various control environment, internal circuit and mechanical structure design is very good, the device used for standard industrial product, its service life can generally be guaranteed within ten years.
(2) stable and reliable performance, good antijamming: PLC applied in all kinds of industrial control field, the design of hardware and software were given all kinds of production environment, its voltage range is very wide, have extremely strong resistance to electromagnetic interference, shock resistant, resistance to high temperature, high humidity and other characteristics, performance is very stable and reliable.
(3) a powerful, flexible implementation, good scalability, PLC controlling freeze drier as the old type of upgrade products, greatly enhance its function, can realize the function is at present: automatic defrost, automatic online cleaning of automatic alarm, automatic sterilization, situations, equipment protection and fault identification, etc. Above function can realize various combinations, and according to actual needs to change the function of the control process and method, and can request according to the user without any increase or less increase of hardware based on the development of new control function.
(4) the good performance-to-price ratio: although PLC type freeze-drying machine performance and functionality from existing freeze drying machine has greatly improved, but the cost increase and performance is not a linear relationship, so no matter as a whole machine for new product development, or as old equipment improvement have its good cost performance.
Small-scale PLC which adopt Mitsubishi runs the system to control to the whole frozen dryers , can realize higher automatic degree , such tasks as it is apparent to finish the intellectual control , stating and fault alarm ,etc. at the same time and the operator proceeds hand over with each other, make modify every kind of parameter simple, easy line, reflect the movement of the machine equipments can on time, fleetness. Its control system construction is clear, the circuit is simple and work dependable, alleviated the worker's labor strength,