The Human Response to a Digital ControlSystemBrianNetz, SeniorElectrical Engineer.Abstract – A brief description of howhuman interaction is a necessity for thecontinual operation of digital controlsystems is presented. This documentcontains a detailed description of someof the components involved with thedigital control system. A strongemphasis is placed on the integration ofall of these components.I. INTRODUCTIONGenesis 1:28a “God blessed them and said to them,‘Be fruitful and increase in number; fill the earthand subdue it’.” This passage can easily be taken out of contextwithout giving serious thought to the implicationsthat this means. The responsibility that this passagebears is mind-numbing! Engineers are moredirectly affected by this command than many otherprofessions and as Christians we are called to learn,understand, and improve on the things that God hasgiven us. This document provides a brief summaryof how digital controls systems are incorporatedinto human control systems to improve responsetimes and to subdue the desired process.II. THE DIGITAL CONTROL SYSTEM Digital control systems feature a number ofdifferent electrical components that are integratedinto one or multiple systems. Digital controlsystems are used in any number of differentapplications. They are used to control robotic arms,airplane flaps, robotic cranes, and conveyorsystems to name a few. Digital control systems areincreasing in complexity and size. They are veryversatile ranging from watches to gigantic robots.The main design of a digital design system is tohave several different components feeding digital oranalog signals to a processor, the processor in turntakes the signals, uses some sort of logic on theinput signals, and then the processor turns on or offcertain outputs determined by the logic in theprocessor. This method of always turning on or offdifferent components without feed back andwithout gain control is known as the “bang bang”approach to control systems. Since digital control systems are so varied innature and design, a specific design will be used forthis paper. Allen-Bradley is one of the leadingmanufactures of control system components andthey produce several families of PLC’s that will bediscussed later in this paper.A. PLC The PLC is known as the programmable logiccontroller and is the “brain” of the entire controlsystem. PLC’s are relatively new to the market butthey have already made a substantial impact. Theywere first made in the late 60’s to eliminate themassive amounts of relays used to make entiresystems operate. They were also more reliable as itwas a software based thing, not a mechanicaldevice which would wear out over time. Naturallythe first ones were not all that fast, and certainly didnot have many features. They could also not storevery many lines of code. The past 40 years hasbrought many changes to the PLC. They are nowfaster, smaller, have more memory, and can now belinked together. The latter has only recently beenan achievement as early on, they did not have anystandards and hardly anything had the samenetwork protocol [1]. PLC’s work by programming code, ladder logic,visual blocks, or many numerous other types oflanguages into the PLC so that it only triggers theright output under the conditions you set for it. Theladder logic is very comparable to IF…THENstatements. See Fig. 5 for an example of the PLCcode. This eliminates the need for a tremendousamount of relays, which also cuts down on cost,labor for wiring, and the chance that it gets wiredincorrectly. It can easily be seen how much thePLC has added to the manufacturing market.The logic that is in most controllers today uses allof the common ones: AND, NOT, OR, NOR,NAND, etc. They also have many advanced logicstatements that vary the voltage of the output basedproportionally on a separate input that the controlleris getting fed. The “digital relays” ,pictured in Fig.3, that are used in the controllers are called“examine if closed” and “examine if open.” Thesetwo pieces are commonly used for the logic andeliminate the mess of having hundreds to thousandsof relays and the mess of wiring them all uptogether. As seen in Fig. 6, the output is highlighted. Whenusing this specific code, if the relay is allowingcurrent through it, the piece of code is highlighted.On the other hand, if no current is being allowedthe relay, there is no color around that specific pieceof code. This makes it incredibly easy fortroubleshooting for engineers. Often digital controlsystems have PC’s hooked up to the PLC thatenables them to “watch” what the system is doing.This enables shorter down times on manufacturinglines, as well as an easier, user-friendly interface. Allen Bradley created many different families oftheir manufacturing processors. Their first largesales processor was the PLC-5. The following isthe brief history of the Allen Bradley’s PLC-5. In the early 1970’s Allen Bradley revolutionizedthe automotive industry. The PLC-5 made its debutin 1985 which lead to the conformation of controlengineering around the world. In 1990 the PLC-5’sinput/output cards were improved and becamestandardized. The communication modes for thePLC-5 were greatly improved in 1993 by addingEthernet capabilities and then broadening it furtherin 1995. By 2000, Allen Bradley had sold over400,000 PLC-5’s and 9million input/output cards.The latest change to the PLC-5 was made in 2001by adding internet capabilities [2]. The PLC-5 wasreplaced by several of the newer families such asthe PLC-500 series and the PLC-5000 series talkedabout below. ControlLogix processor modules are available in arange of memory. Logix5555 processors (1756-L55Mxx) have 750K, 1.5M, 3.5M, or 7.5M bytesof user memory. You can replace the memory sub-module on a Logix5555 processor to change thememory size. The ControlLogix556x processor (1756-L61,L62, -L63) has 2, 4, or 8M bytes of fixed
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