【導(dǎo)讀】控制工程隨著時(shí)間的演變。過去的人們主要致力于控制方面研究。最近電力已被應(yīng)用于。這些繼電器使其可以在沒有機(jī)械開關(guān)的情況下被開動(dòng)。革命，可編程邏輯控制器出現(xiàn)于70年代，它已成為制造控制的最常見選擇。功能受到越來越多的工廠歡迎并可能作為主要控制手段再今后的一段時(shí)間內(nèi)。部分原因是因?yàn)镻LC它的優(yōu)點(diǎn)很多。梯形邏輯編程法是主要的PLC編程方法。控制系統(tǒng)仍然包括繼電器，但這些都是很少的邏輯使用。當(dāng)電壓作用于輸入線圈產(chǎn)生的磁場，產(chǎn)生電流領(lǐng)。我們可以想像激活24V直流在PLC. 請注意，在實(shí)際情況下PLC的輸入繼電器，常常又是輸出繼電器。注意，兩個(gè)輸入的推按鈕常開，但里面的PLC梯。第一個(gè)是PLC的編程，一個(gè)基礎(chǔ)技術(shù)的繼電器邏輯接線示意圖。圍的PLC設(shè)備，開啟或關(guān)閉則是如燈光或馬達(dá)之類的。將檢查輸入到PLC，如果將它關(guān)閉記得11（或真），如果它會記住一個(gè)0（或假）。記憶形式使用前由PLC。此示例使用一個(gè)PLC的內(nèi)存位置島該內(nèi)存位置為整數(shù)，也將在后

　　

【正文】 Sinking When active the output allows current to flow to a mon ground. This is best selected when different voltages are supplied. Sourcing When active, current flows from a supply, through the output device and to ground. This method is best used when all devices use a single supply voltage. This is also referred to as NPN (sinking) and PNP (sourcing). PNP is more popular. This will be covered in detail in the chapter on sensors. Inputs In smaller PLCs the inputs are normally built in and are specified when purchasing the PLC. For larger PLCs the inputs are purchased as modules, or cards, with 8 or 16 inputs of the same type on each card. For discussion purposes we will discuss all inputs as if they have been purchased as cards. The list below shows typical ranges for input voltages, and is roughly in order of popularity. PLC input cards rarely supply power, this means that an external power supply is needed to supply power for the inputs and sensors. The example in Figure shows how to connect an AC input card. 沈陽建筑大學(xué)畢業(yè)設(shè)計(jì) 20 Figure An AC Input Card and Ladder Logic In the example there are two inputs, one is a normally open push button, and the second is a temperature switch, or thermal relay. (NOTE: These symbols are standard and will be discussed later in this chapter.) Both of the switches are powered by the positive/ hot output of the 24Vac power supply this is like the positive terminal on a DC supply. Power is supplied to the left side of both of the switches. When the switches are open there is no voltage passed to the input card. If either of the switches are closed power will be supplied to the input card. In this case inputs 1 and 3 are used notice that the inputs start at 0. The input card pares these voltages to the mon. If the input voltage is within a given tolerance range the inputs will switch on. Ladder logic is shown in the figure for the inputs. Here it uses Allen Bradley notation for ControlLogix. At the top is the tag (variable name) for the rack. The input card (’I’) is in slot 3, so the address for the card is bob:, where ’x’ is the input bit number. These addresses can also be given alias tags to make the ladder logic less confusing. Many beginners bee confused about where connections are needed in the circuit above. The key word to remember is circuit, which means that there is a full loop that the voltage must be able to follow. In Figure we can start following the circuit (loop) at the power supply. The path goes through the switches, through the input card, and back to the power supply where it flows back through to the start. In a full PLC implementation there will be many circuits that must each be plete. A second important concept is the mon. Here the neutral on the power supply is the mon, or reference voltage. In effect we have chosen this to be our 0V reference, and all other voltages are measured relative to it. If we had a second power supply, we would also need to connect the neutral so that both neutrals would be connected to the same mon. Often mon and ground will be confused. The mon is a reference, or datum voltage that is used for 0V, but the ground is used to prevent shocks and damage to equipment. The ground is connected under a building to a metal pipe or grid in the ground. This is connected to the electrical system of a building, to the power outlets, where the metal cases of electrical 沈陽建筑大學(xué)畢業(yè)設(shè)計(jì) 21 equipment are connected. When power flows through the ground it is bad. Unfortunately many engineers, and manufacturers mix up ground and mon. It is very mon to find a power supply with the ground and mon mislabeled. One final concept that tends to trap beginners is that each input card is isolated. This means that if you have connected a mon to only one card, then the other cards are not connected. When this happens the other cards will not work properly. You must connect a mon for each of the output cards. Modules As with input modules, output modules rarely supply any power, but instead act as switches. External power supplies are connected to the output card and the card will switch the power on or off for each output. Typical output voltages are listed below, and roughly ordered by popularity. These cards typically have 8 to 16 outputs of the same type and can be purchased with different current ratings. A mon 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. This allows 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.