1、 1 Programmable Logic Controllers Programmable logic controller (plc) is a solid-state device used to control machine motion or process operation by means of a stored program. The PLC sends output control signals and receives input signals through input/output (I/O) devices. A PLC controls outputs i
2、n response to stimuli at the inputs according to the logic prescribed by the stored program. The inputs are made up of limit switches, pushbuttons, thunbwheels. Switches, pulses, analog signals, ASCLL serial data, and binary or BCD data from absolute position encoders. The outputs are voltage or cur
3、rent levels to drive end devices such as solenoids, motor starters, relays, lights, and so on. Other output devices include analog devices, digital BCD displays, ASCII compatible devices, servo variable-speed drives, and even computers. Programmable controllers were developed (circa in 1968) when Ge
4、neral Motors Corp, and other automobile manufacturers were experimenting to see if there might be an alternative to scrapping all their hardwired control panels of machine tools and other production equipment during a model changeover. This annual tradition was necessary because rewiring of the pane
5、ls was more expensive than buying new ones. The automotive companies approached a number of control equipment manufacturers and asked them to develop a control system that would have a longer productive life without major rewiring, but would still be understandable to and repairable by plant personn
6、el. The new product was named a “programmable controller”. The processor part of the PLC contains a central processing unit and memory。The central processing unit (cpu) is the “traffic director” of the processor, the memory stores information. Coming into the processor are the electrical signals fro
7、m the input devices, as conditioned by the input module to voltage levels acceptable to processor logic. The processor scans the state of I/o and updates outputs based on instructions stored in the memory of the plc. For example, the processor may be programmed so that if an input connected to a lim
8、it switch is true (limit switch closed), then a corresponding output wired to an output module is to be energized. This output might be a solenoid, for example on each scan to see if that limit switch is, in fact, closed. If it is closed, the processor energizes the solenoid by turning on the output
9、 module. The output device, such as a solenoid or motor starter, is wired to output modules terminal, and it receives its shift signal from the processor, in effect, the processor is performing a long and complicated series of logic decisions. The PLC 2 performs such decisions sequentially and in ac
10、cordance with the stored program. Similarly, analog I/O allows the processor to make decisions based on the magnitude lf a signal. Rather than just if it is on or off. For example, The processor may be programmed to increase or decrease the steam flow to a boiler (analog output ) based on a comparis
11、on of the actual temperature in the boiler (analog input) to the desired temperature. This is often performed by utilizing the built-in PID (proportional, integral, derivative ) capabilities lf the processor. Because a PLC is “software based”, its control logic functions can e changed by reprogrammi
12、ng its memory. Keyboard programming devices facilitate entry of the revised program, which can be designed to cause an existing machine or process to operate in a different sequence or to respond to different levels of, or combinations of stimuli. Hardware modifications are needed only if additional
13、, changed, or relocated input/output devices are involved. All programmable controllers consist of the basic functional blocks shown in Figure 10.1. We will examine each block to understand the relationship to the control system. First we looked at the center, as it is the heart of the system. It co
14、nsists of a microprocessor, logic memory for the storage of the actual control logic, storage or variable memory for use with data that will ordinarily change as a function of the control program execution, and a power supply to provide electrical power for the processor and memory. Next comes the I
15、/O bloke. This function takes the control level signals for the CPU and converts them to voltage and current levels suitable for connection with factory grade sensors and actuators. The I/O type can range from digital, analog, or a variety of special purpose “smart” I/O which are dedicated to a cert
16、ain application task. The programmer is normally used only to initially configure and program a system and is not required for the system to operate. It is also used in troubleshooting a system, and can prove to be a valuable tool in pinpointing the exact cause of a problem. The field devices shown
17、here represent the various sensors and actuators connected to the I/O. These are the arms, legs, eyes, and ears of the system, including pushbuttons, limit switches, proximity switches, photosensors, thermocouples, position sensing devices, and bar code reader as input； and pilot light, display devi
18、ces, motor starters, DC and AD drivers, solenoids, and printers as outputs . An adaptive control system is one whose parameters are automatically adjusted to compensate for corresponding variations in the properties of the process. The system is, in a word, “adapted” to the needs of the process. Nat
19、urally there must be 3 some criteria on which to base an adaptive program. To specify a value for the controlled variable is not enough-adaption is not required to meet this specification. Some “objective function” of the controlled variable must be specified in addition. It is this function that de
20、termines the particular form of adaption required.The objective function for a given process may be the damping of the controlled variable. In essence, there are then two loops, one operating on the controlled variable, the other on its damping. Because damping identifies the dynamic loop gain, this
21、 system is designated as a dynamic adaptive system.It is also possible to stipulate an objective function of the steady-state gain of the process. A control system designed to this specification is then steady-state adaptive.There is, in practice, so little resemblance between these two systems that
22、 their classification under a single title “adaptive” has led to, much confusion.A second distinction is to be made, this not on the objective function, but rather on the mechanism through which adaption is introduced, if enough is known on a process that parameter adjustments can be related to the variables which cause its properties to change, adoption may be programmed. However, if it is necessary to base parameter means of a feedback loop. This is known as a self adaptive system.
