【導(dǎo)讀】近年來(lái)隨著科技的飛速發(fā)展，“定時(shí)器”總的來(lái)說(shuō)有兩種類型。另一種是基于數(shù)字技術(shù)的新一代產(chǎn)品，這種產(chǎn)品功能強(qiáng)，是前者的替代之。本設(shè)計(jì)開(kāi)發(fā)了一種基于單片機(jī)的多用途定時(shí)器。它造價(jià)低，功能全，整體功。能性價(jià)比高，可適應(yīng)各種場(chǎng)合的定時(shí)預(yù)警之用。計(jì)功能、單片機(jī)的中斷使用方法及LED顯示等技術(shù)?？梢詫?shí)現(xiàn)任意時(shí)間和日期的。的閃爍的提示功能。本設(shè)計(jì)的硬件電路主要包括：STC89S52單片機(jī)、DS18B20溫度傳感器。顯示程序的設(shè)計(jì)，最終完成本設(shè)計(jì)。根據(jù)美國(guó)儀器學(xué)會(huì)的調(diào)查，是1821年由德國(guó)物理學(xué)家賽貝發(fā)明的，這就是后來(lái)的熱電偶傳感器。后，另一位德國(guó)人西門子發(fā)明了鉑電阻溫度計(jì)。溫度傳感器使用范圍廣數(shù)量多，居各種傳感器之首。這種測(cè)溫方法精度比較高，并可測(cè)量物體內(nèi)部的溫度分布。最新推出TS-18B20的數(shù)字溫度傳感器，采用美國(guó)DALLAS公司生產(chǎn)可組網(wǎng)數(shù)。適用于各種狹小空間設(shè)備數(shù)字測(cè)溫和控制領(lǐng)域。接控制打印監(jiān)測(cè)數(shù)據(jù)。但其中所采用的溫度傳感器直接輸出為模擬電壓信號(hào)，該信號(hào)在傳輸過(guò)程中

　　

【正文】 其使用方便、可靠性高、能適應(yīng)惡劣工作環(huán)境等突出優(yōu)點(diǎn) ,在各個(gè)工業(yè)部門獲得極其廣泛的應(yīng)用 .PLC 與機(jī)器人 CAD/CAM 一起 ,被稱為現(xiàn)代工業(yè)自動(dòng)化的三大支柱 . 從單機(jī)自動(dòng)化、生產(chǎn)自動(dòng)流水線到數(shù)控系統(tǒng) CNC 、計(jì)算機(jī)集成制造系統(tǒng) CIMS 和智能制造系統(tǒng) IMS ,都可看到 PLC的應(yīng)用 .在 PLC控制的工業(yè)自動(dòng)化系統(tǒng)中 ,常常用定時(shí)器控制過(guò)程時(shí)間的長(zhǎng)短 ,以確定受控物理的大小 .文〔 1,2〕闡述終端時(shí)間控制在節(jié)能控制系統(tǒng)中的意義和應(yīng)用 。文〔 3〕則研究利用定 時(shí)器參數(shù)的校正 ,補(bǔ)償電壓波動(dòng)對(duì)電加熱系統(tǒng)控制精度造成的不良影響 .這些 ,都表明物理過(guò)程的時(shí)間是控制系統(tǒng)的一個(gè)極為重要的物理量 .作為 PLC 定時(shí)器的應(yīng)用 ,有兩個(gè)問(wèn)題必須研究 . 1 定時(shí)器參數(shù) ,如何根據(jù)生產(chǎn)現(xiàn)場(chǎng)的實(shí)際情況進(jìn)行設(shè)定 ,在設(shè)定中可能存在什么問(wèn)題 ,文獻(xiàn)〔 4,5〕對(duì)此有深入的討論 . 2 受定時(shí)器控制的物理過(guò)程的實(shí)際時(shí)間長(zhǎng)短和定時(shí)器的設(shè)定值比較 ,有多大誤差 ,這就是本文要討論的內(nèi)容 .由于 PLC 采用循環(huán)掃描的工作方式 ,而 PLC 內(nèi)的定時(shí)器是獨(dú)立于循環(huán)掃描之外的計(jì)時(shí)裝置 ,故在應(yīng)用定時(shí)器時(shí)可能造成誤差 .在討論 PLC 中 定時(shí)器的工作過(guò)程時(shí) ,將指出造成誤差的兩種原因 ,給出誤差的范圍 ,并提出減少誤差的方法 . 1 定時(shí)器動(dòng)作條件及其造成的誤差 圖 1為定時(shí)器動(dòng)作的實(shí)驗(yàn)程序 .定時(shí)器 TIMOO動(dòng)作時(shí) ,其常開(kāi)觸點(diǎn)閉合發(fā)出脈沖 ,計(jì)數(shù)器 CNT46和 CNT47對(duì)這脈沖計(jì)數(shù) T46和 CNT47設(shè)定值相同 ,這 2個(gè)計(jì)數(shù)器中任 1 個(gè)即時(shí)值減至零 ,定時(shí)器的線圈電路即被切斷 .在 PLC 上電后 ,當(dāng)循環(huán)掃描至 TIMOO 線圈電路 ,定時(shí)器開(kāi)始計(jì)時(shí) .如考慮循環(huán)掃描時(shí)間 ,定時(shí)器什么時(shí)候動(dòng)作 ,可根據(jù)文〔 6〕認(rèn)為其即時(shí)值減至零時(shí) ,定時(shí)器便動(dòng)作 .對(duì)于圖 1 的電路 ,設(shè)定時(shí)器TIMOO 即時(shí)值減至零時(shí) ,循環(huán)掃描處在梯形圖的 A 點(diǎn)以前或 D 點(diǎn)以后 .由于 TIMOO動(dòng)作，當(dāng)循環(huán)掃描過(guò) A 點(diǎn)時(shí) ,CNT46 將計(jì)到 TIMOO 的脈沖 .接著 ,當(dāng)循環(huán)掃描到定時(shí)器的線圈電路 ,TIMOO 的常閉觸點(diǎn)斷開(kāi) ,則線圈沒(méi)電 ,定時(shí)器復(fù)位 .這將導(dǎo)致CNT47 沒(méi)有計(jì)數(shù)到 TIMOO 的脈沖〔 6〕 .也就是說(shuō) ,CNT46 和 CNT47 計(jì)數(shù)值將不同 .實(shí)驗(yàn)表明 ,無(wú)論 CNT46 和 CNT47 設(shè)定值為多少 兩個(gè)計(jì)數(shù)器設(shè)定值相同 ,由程序長(zhǎng)短決定的掃描周期如何改變 ,TIMOO 即時(shí)值減至零時(shí)循環(huán)掃描落在程序的哪一點(diǎn) ,都可得兩個(gè)計(jì)數(shù)器的讀數(shù)總是相同的 .它們同時(shí)減到零 ,這說(shuō)明定時(shí)器動(dòng)作的條件 ,除了其即時(shí)值減至零以外 ,還要求循環(huán)掃描到定時(shí)器的線圈電路 .從定時(shí)器即時(shí)值減至零 ,到循環(huán)掃描到定時(shí)器的線圈電路 ,這段時(shí)間就是誤差時(shí)間 .定時(shí)器即時(shí)值減至零 ,如果循環(huán)掃描正到 B點(diǎn) ,這段誤差最小 。而如果循環(huán)掃描正到 C點(diǎn) ,則誤差最大 ,此誤差值等于一個(gè)掃描周期 ,到底循環(huán)掃描正在 哪一點(diǎn) ,決定于定時(shí)器的設(shè)定值和循環(huán)掃描周期 .這很難確定 ,故這種誤差是不可避免的 . 程序越長(zhǎng)誤差最大值越大 .從相對(duì)誤差的角度考慮 ,定時(shí)器的時(shí)間設(shè)定值越小 ,相對(duì)誤差越大 。 當(dāng)應(yīng)用高速 定時(shí)器指令 TIMH FUN15 ,而設(shè)定值又很小 如 時(shí) ,相對(duì)誤差可達(dá) 100%,設(shè)計(jì)時(shí)應(yīng)予注意 . 2 定時(shí)器工作過(guò)程指令次序造成的誤差 在定時(shí)器工作過(guò)程中 ,指令先后和指令順序不當(dāng)各造成定時(shí)器的誤差 ,分別如圖 2,3 所示 . 循環(huán)掃描使程序執(zhí)行有先有后 .定時(shí)器線圈通電條件滿足時(shí) ,并不立即計(jì)時(shí) ,而是等到掃描到線圈電路時(shí)才開(kāi)始計(jì)時(shí) .另外 ,定時(shí)器動(dòng)作后 ,其觸點(diǎn)控制的電路也得等到掃描至該電路時(shí)才工作 .在圖 2 梯形圖中 ,從 0002“ ON ”到 0500 有輸出之間的時(shí)間 ,比定時(shí)器 TIMOO 的設(shè)定值長(zhǎng) ,差值的一部分是 由本文上面提到的定時(shí)器動(dòng)作條件產(chǎn)生的 ,另一部分為循環(huán)掃描從指令 LD0002 到OUT0500 的時(shí)間 .這種誤差和指令順序及位置安排關(guān)系密切 .當(dāng)把圖 2 中三段程序緊靠在一起 ,中間沒(méi)有插入其它程序 ,則誤差最小 .當(dāng)指令順序安排不當(dāng) ,此誤差可達(dá) TS 的 2 倍 . 這種情況發(fā)生在圖 3 所示的梯形圖程序中。 Programmable controller in the timer precision analysis Set puter technology, automatic control technology and modern munications technology integration in the PLC, because its convenience, high reliability, and can adapt to bad work environment in all industrial advantage, the department to get extremely extensive application. PLC and robot, CAD/AMC, called the modern industrial automation of the three pillars. From single automation, producing automatic assembly line to the numerical control system C , puter integrated CN manufacturing system C and intelligent IMS manufacturing system IM , can see SPLCPLC. In the industrial automation control system, often use timer control process to determine the length of time, the physical size. Wen controlled 1 this time in saving energy control terminals, the significance and the application of control system, Wen [3] is studied using the parameters calibration, timer pensation voltage fluctuation on electrical heating system control precision of adverse effects. These are that the physical process of time is a very important control system of quantities as PLC timer. The application, there are two problems must study. 1 the timer parameters, according to the actual situation of production site set in the Settings may exist, what problem, literature [4] It has further discussion. 2 under the timer control of physical process of actual length and timer value, how much is this, the error due to discuss the content of PLC. By the way, work cycle scanning and PLC within the timer is independent of cycle time outside of the scanning device, the application in the timer can lead to errors in PLC. In the working process of the timer, will cause error of two kinds of reasons, the scope of error, and puts forward the method of reduce error. One the timer moves and errors Figure 1 for the experimental procedure. Timer moves timer TIMOO action, it often open contacts closed a pulse, counter CNT46 and 47 of the CNT pulse count. CNT46 and CNT47 set identical, this two counter for a duration reduced to zero, namely the coil timer circuits which was cut off. In the PLC electricity, when circulation scanning to TIMOO coil timer circuits, timing starts. If considering scanning time cycle timer, when action, can according to article [6] think its duration reduced to zero, the timer is then moves to figure 1. The circuit, set the timer TIMOO namely duration reduced to zero, the circulation of A scan in ladderdiagram before or after DTIM. Because, when the cycle OO scan A point, CNT46 will plan to TIMOO pulse. Then, when the cycle of coil timer circuits scanned, TIMOOoften closed contacts disconnected, coil, timer reset. This will cause CNT47. Not counting to TIMOO pulse 6 . In other words, CNT46 and 47 plan CNT numerical experiments show that different whether CNT46 and 47 value of CNT how much the same value two counter, the length of the scanning process decision to change cycle, TIMOO namely duration reduced to zero in the circular scanning procedure, can have two counter readings will always be the same. They also reduced to zero, the timer moves, except for its duration reduced to zero, which is required to coil timer circular scanning circuit from the timer namely duration reduced. Zero to cycle the scanning to the coil timer circuits, this paragraph of time is error. The timer is reduced to zero, if cycle as scanning is to B, the minimum error, But if cycle is to scan C, error, this error is a scan cycle timer namely duration for TS. Circulation, exactly zero point is, decided to scan the value and circulation in the timer. It is very difficult to determine the scanning period, this error is inevitable. The imum error of the program longer. From the relative error of the view of value, the smaller the timer time, the relative error, When the application of highspeed timer instruction T IM H FUN15 , and setting and small such as s , the relative error can reach 100%, design sho