【正文】 2。當(dāng)然，采樣 操作或離散化有時(shí)完全是假設(shè)，其引進(jìn)目的僅僅是為了簡(jiǎn)化實(shí)際上只包含連續(xù)性元件的控制系統(tǒng)分析。這些瞬時(shí)值可以用 KT 或 Tk表示，指完成測(cè)量的時(shí)間或者數(shù)字計(jì)算機(jī)內(nèi)存的讀取時(shí)間。另一方面，滯后補(bǔ)償明顯提高了穩(wěn)態(tài)精度，代價(jià)是增加了暫態(tài)響應(yīng)時(shí)間。對(duì)于高頻段，幅值曲線應(yīng)該盡可能快的衰減，以減小噪聲的影響。為了獲得較大的速度誤差常數(shù)以及滿意的相對(duì)穩(wěn)定性，我們發(fā)現(xiàn)需要重新構(gòu)造開(kāi)環(huán)頻率響應(yīng)曲線。 用伯德圖設(shè)計(jì)的常用方法是，我們首先調(diào)節(jié)開(kāi)環(huán)增益，以使穩(wěn)態(tài)精度的要求得到滿足。一種是極坐標(biāo)圖法，另一鐘是伯德圖法。當(dāng)開(kāi)環(huán)系統(tǒng)由頻率設(shè)計(jì)完成后，閉環(huán)的零點(diǎn)和極點(diǎn)就可以確定了。 需要注意，在控制系統(tǒng)的設(shè)計(jì)中，暫態(tài)響應(yīng)性能通常是最重要的。 讓我們考慮一個(gè)設(shè)計(jì)的問(wèn)題。在用跟軌跡法設(shè)計(jì)的過(guò)程中，通過(guò)使用補(bǔ)償器改變系統(tǒng)跟軌跡的形狀，以使閉環(huán)系統(tǒng)一對(duì)主導(dǎo)極點(diǎn)位于理想的位置。實(shí)際上，系統(tǒng)的跟軌跡圖表明，僅僅通過(guò)調(diào)節(jié)增益并不能獲得理想的性能。在滯后超前網(wǎng)絡(luò)中，相位滯后和相位超前兩種情況都會(huì)出現(xiàn)，但是出現(xiàn)在不同的頻率范圍內(nèi)；相位滯后出現(xiàn)在低頻段，相位超前出現(xiàn)在高頻段。另外一種補(bǔ)償是反饋補(bǔ)償。在很多情況下，增加增益值將改善系統(tǒng)的穩(wěn)態(tài)性能，但是也將使系統(tǒng)穩(wěn)定性變差，甚至變得不穩(wěn)定。我們應(yīng)該牢記，控制系統(tǒng)設(shè)計(jì)過(guò)程中最重要的一個(gè)環(huán)節(jié)就是把性能要求精確的表達(dá)出來(lái)，這樣才會(huì)設(shè)計(jì)出對(duì)于給定的任務(wù)而言最優(yōu)的控制系統(tǒng)。它們 通常和系統(tǒng)精確度、相對(duì)穩(wěn)定性及響應(yīng)速度有關(guān)。 Setpoint(SP) s relative stability and to slow down the settling of the addition of a zero has the effect of pulling the root locus to the left,tending to make the system more stable and to speed up the settling of the response. The rootlocus approach to design is very powerful when the specifications are given in term 無(wú)錫職業(yè)技術(shù)學(xué)院 外文翻譯 3 of time domain quantities,such as the damping ratio and undamped natural frequency,maximum overshoot,rise time and setting time. Let us consider a design original system either is unstable for all values of gain or is stable but has undesirable transient response this case,the reshaping of the root locus is necessary in order that the dominant closedloop poles be at desired locations in the plex an appropriate lead pensator in cascade with the feedforward transfer function may solve this problem. It is important to note that in a control system design,transientreponse performance is usually most the frequencyresponse approach,we specify the transientresponse in term of the phase and gain margin,resonant peak magnitude,the gain crossover frequency,resonant frequency response is indirect,the frequency domain specification can be met conveniently by means of Bode diagram. Design in the frequency domain is simple and the open loop has been designed by frequency response method,the closed loop poles and zeros can be transient response characters must be checked to see whether the designed system meets the requirements in the time it does not,the pensator has to be modified and the analysis must be repeated until a satisfactory result is obtained. Basically,there are two approaches in the frequencydomain is the polar plot approach and the other is the Bode diagram is more convenient to work with Bode Bode diagram of the pensator can be simply added to the original Bode diagram,and thus plotting the plete Bode diagram is a simple ,if the open loop gain is varied,the magnitude curve is shifted up or down without changing the slope of the curve,and the phase curve remains the same. A mon approach to the Bode diagram is that we first adjust the open loop gain so that the requirement on the steady state accuracy is we plot the magnitude and phase curves of the unpensated open the specification on the phase margin and gain margin are not satisfied,then a suitable pensator that will reshape the open loop transfer function is determined. In many practical cases,pensation is essentially a promise between steadystate accuracy and relative order to have a high value of the velocity error constant and yet satisfactory relative stability,we find it necessary to reshape the open loop frequency response gain in the lowfrequency region should be large enough to satisfy the steadystate accuracy the mediumfrequency region (near the gain crossover frequency wc from both directions),the slope of the logmagnitude curve in the Bode diagram should be 20dB per decade. This slope should extend over a sufficient wide frequency band to assure a proper phase the highfrequency region,the gain should be attenuated as rapidly as possible to minimize the effects of noise. The basic characteristics of lead,lag,and laglead pensation are as pensation essentially yields an appreciable improvement in transient response and a small change in steadystate may accentuate highfrequency noise the other hand ,lag pensation yields an appreciable improvement in steadystate accuracy at the expense of increasing the transientresponse pensation will suppress the effects of highfrequency noise pensation bines the characteristics of both lead pensation and lag pensation. Discretetime Systems and the zTransform Method Discretetime systems,or sampleddata system,are dynamic systems in which one or more variables can change only at discrete instants of intstants,which we shall denote by kt or tk(k=0,1,2,........),may specify the time at which some physical measurement is performed or the time at which the memory of a digital puter is read out, time interval between these discrete instants can be approximated by simple interpolation. 無(wú)錫職業(yè)技術(shù)學(xué)院 外文翻譯 4 Discretetime systems differ from continuoustime ones in that the signals for a discretetime system are in sampledata form. Discretetime systems arise in practice whenever the measurements neccessary for control are obtained in an intermittent fashion,or a large scale controller or puter is timeshared by several plants so that a control signal is sent out to each plant only periodically or whenever a digital puter is used to perform putations