These cards typically have 8 to 16 outputs of the same type and can be purchased with different current ratings。 A common choice when purchasing output cards is relays, transistors or triacs。 Relays are the most flexible output devices。 They are capable of switching both AC and DC outputs。 But, they are slower (about 10ms switching is typical), they are bulkier, they cost more, and they will wear out after millions of cycles。 Relay outputs are often called dry contacts。 Transistors are limited to DC outputs, and Triacs are limited to AC outputs。 Transistor and triac outputs are called switched outputs。 Dry contacts - a separate relay is dedicated to each output。 

There is no need for processor module, resources of PC are used。 Some Soft PLC-s are still formed as processor cards for PC 。 Reliable communication networks are essential。 Soft PLC-s are well suited for data acquisition applications because of data storage possibilities of PC-s。 

Fourthly, combo-devices (figure 2) that contain both operator panel and medium size processor module gain popularity (Siemens AG, 2002)。 In this solution possibility of disturbances is low and reuse of some components make whole package cheaper。 The development of low cost computer has brought the most recent revolution,the Programmable Logic Controller (PLC)。 The advent of the PLC began in the1970s, and has become  most common choice for manufacturing controls。

Fifthly, processor software takes over properties from PC software。 Data collected from production can be easily transferred into a office applications (Siemens AG, 2000)。

This allows a mixed voltages (AC or DC and voltage levels up to the maximum), as well as isolated outputs to protect other outputs and the PLC。 Response times are often greater than 10ms。 This method is the least sensitive to voltage variations and spikes。 Switched outputs - a voltage is supplied to the PLC card, and the card switches it to different outputs using solid state circuitry (transistors, triacs, etc。) Triacs are well suited to AC devices requiring less than 1A。 Transistor outputs use NPN or PNP transistors up to 1A typically。 Their response time is well under 1ms。The development of low cost computer has brought the most recent revolution,the Programmable Logic Controller (PLC)。 The advent of the PLC began in the1970s, and has become the most common the choice for the manufacturing the controls。 

4、Conclusion

Control engineering has evolved over thetime。 In the past humans were the main method for controlling a system。 More recently electricity has been used for control and early electrical control was based on relays。 These relays allow power to be switched on and off without a mechanical switch。 It is acommon to use relays to make simple logical control decisions。 The development of low cost computer has brought the most recent revolution,the Programmable Logic Controller (PLC)。 The advent of the PLC began in the1970s, and has become the most common choice for manufacturing controls。 The development of low cost computer has brought the most recent revolution,the Programmable Logic Controller (PLC)。 The advent of the PLC began in the1970s, and has become the most common choice for manufacturing controls。 

PLCs have been gaining popularity on the faactory floor and will probably remain predominant for some time to come。 Most of this is because of the advantages they offer。  • Cost effective for controlling complex systems。 

• Flexible and can be reapplied to control aother systems quickly and easily。 • Computational abilities allow more sophisticated control。 

• Trouble shooting aids make programming easier and reduce downtime。 • Reliable components make these likely to operate for many years before the failure。摘要：控制工程随着时间的演变。过去的人们主要致力于控制方面研究。最近电力已被应用于控制，早期电气控制是基于继电器的。这些继电器使其可以在没有机械开关的情况下被开动和关闭。这是通常使用继电器进行简单的逻辑控制的方法。低成本计算机的发展带来了新的，可编程逻辑控制器（PLC）出现于70年代，它已成为制造控制的最常见选择。PLC的功能受到越来越多的工厂欢迎并可能作为主要控制手段再今后的一段时间内。而这其中绝大部分原因是因为PLC它的优点很多。论文网