Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 1998Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 1998Pnömatik konum kontrolü havanın sıkıştırılabilirliği ve sürecin temelde termodinamik bir süreç olması nedeniyle karmaşık bir problemdir. Ancak sistemin yüksek mertebeden olması ve darbe yutucu özellikleri, bilgisayarla kontrol durumunda düşük maliyetli donanımla akıllı kontrol algoritmalarının kullanılmasına uygundur. Bu çalışmada bu tür uygulamaların deneysel sınanmasını sağlayacak donanım tanıtılmaktadır. Hız geri beslemeli ikili kontrol, kayan rejimli ikili kontrol, darbe genişliği modülasyonu ve sürekli basınç kontrolü bu donanımla gerçekleştirilmiş, elde edilen sonuçlar karşılaştırmalı biçimde irdelenmiştir. Bu çalışmada, doğrusal hareket yapan bir pnömatik silindirin, çalışma alanı içinde farklı referans konumları için katı ve hassas konumlamasını yapacak bilgisayar kontrollü, düşük maliyetli, basit donanım yapısına sahip, ancak akıllı hareket algoritmalarının da uygulanabileceği çok amaçlı bir deney tesisatı kurulmuştur. Bu amaçla kurulan deney tesisatında 1adet çubuksuz silindir, 1 adet sayısal ölçüm yapabilen bir cetvel, 2 adet basınç ölçer, kumanda valfi olarak ikili kontrol uygulamaları için 2 adet hızlı(20 Hz.), 3/2 aç-kapa valf, sürekli kontrol için ise 2 adet 3 yollu basınç oransal valfleri kullanılmıştır. Sistem bilgisayara Advantech PCL-PG-812 endüstriyel arayüz kartı ile bağlanmış ve gerekli tüm yazılım Borland C++ dilinde yazılmıştır. Tesisat üzerinde konum kontrolünün istenilen şekilde yapılabilmesi için çeşitli kontrol algoritmaları dener ek, karşılaştırmaları yapılmıştır. Araştırmada "konum kontrolunda ±0,1 mm.'den iyi kesinlik" ve "dış kuvvet değişimlerinde en fazla ±5 mm. sapma" hedef alınmıştır. Yapılan deneyler sonucunda, hız geri beslemeli ikili kontrolün en hızlı cevap veren yöntem olmasına rağmen aşma meydana geldiği ve sistem mertebesinin yüksek olması sonucu birkaç salınımdan sonra referansa oturduğu, kayan rejimli kontrolde aşma yapmadan referansa oturduğu, ancak oturma zamanının hız geri beslemeli kontrole göre fazla olduğu, sistem parametrelerinin değişimine duyarsız olduğu görülmüştür. Her iki kontrol teknolojisi de referans etrafında aynı katılığa sahiptir. Darbe genişliği modülasyonunda ise valfleri sürekli aç-kapa yapmasından dolayı referans etrafında aynı derecede katı davranmadığı ve bozan etkenlere duyarlı olduğu, oransal basınç kontrolünün ise en hassas ve yumuşak davranışa yol açtığı, sistem katılığının çeşitli yöntemlerle istenilen mertebeye ayarlanabileceği görülmüştür.The purpose of this study is the realisation of accurate and robust pneumatic position control by experimental testing of new methods like Velocity Feedback On-Off Control, Sliding Mode Control, Pulse Width Modulation Control, Continuous Pressure Control. To realise an accurate and robust position control, new algorithms for pneumatic cylinders are investigated and applied to experimental test bench. The objective of this research is the determination of dimensioning, system design and control principles to realise "accuracy better than ±0,1 mm. for position control " and "max deviation of 5 mm. under external force variations". Pneumatic Position Control is widely used in food industry and packaging systems because it is a "clean" technology and preferred in mechanical positioning operations where large fo..e/torqi.^s are not necessary, because its components are comparatively cheap with respect to hydraulic and electromechanical components. However, pneumatic position control is more complicated than hydraulic and electrical position control. Air compressibility emerges as an important issue when accurate and robust position control is desired against of external force variations. Technological developments in measurement and control components (accuracy of measurement systems, fast switching directional valves) with advances in Intelligent Motion Control methods and increase of the computational speed prepared the necessary scientific and technological basis to develop performance and low cost solutions to this problem. To observe the effects of the developing algorithms on a real time system, an experimental test bench is constructed (Figure S.l). A sufficiently accurate linear scale (600mm,20um/pulse resolution) are coupled to a double acting rodless XI pneumatic cylinder (500mm) with high precision guide to measure the piston displacement. The cylinder is suitable to be used in robotic applications. To drive the pneumatic cylinder, for application of on-off control theory, two 3/2 on-off valves and for application of continuous time control theory, two 3 way pressure proportional valves are added to pneumatic position control system. Two pressure sensors are used to observe the changes in the pressure at the two chambers in the cylinder. The system has been controlled in "real time" by a computer (486 DX-25) using necesssary interface cards. The control program is written in the C language and the control law is implemented with a sampling period of 7,5 ms. The velocity signals are obtained by differentiating and digitally filtering the position measurements. Pb Pb Figure S. 1 General structure of pneumatic position control system Effects of technological parameters (friction, valve delays, digital measurement resolution, sampling period, external force magnitudes, dynamical behavior parameters of the system) on system performance are investigated and solution methods are developed. Results obtained from this work can be applied and generalized to Pneumatic Velocity and Force Control Systems. In the recent years, pneumatic position systems is especially used in robotic applications. This applications have caused the development of pneumatic position control systems. At the beginning although proportional valves are very expensive, Xll manufacturers have used pneumatic proportional valves. Instead of using very expensive proportional valves to reach a high accuracy, on-off valves, cheaper than proportional valves, can be used in pneumatic position control. On the other hand, to achieve precision position control by using on-off valves, it is necessary to investigate in detail the dynamical behaviour of the system. To achieve accurate pneumatic position control, a mathematical model is derived for a symmetrical linear actuator which drives a mass with viscous and coulomb friction. By selecting, xi=y X2= V X3=Pl X4=P2 system equations may be written in state-space form as; xi =x0 ~B A / N FS X2 = x_ + - -(x, -x.) *- x3 yıo + xı -x2.x3 + R.T q A VT.a x4 = y20 ' Xl x2.x4 + RT \_ A Vf a.Y" Control technologies applied to system can be considered in three parts. These are; 1- On-off control a- Velocity feedback on-off control b- Sliding mode control 2- Pulse width modulation control 3- Continuous pressure control xiii The first control algorithm applied to the pneumatic system is on-off control. In this control, valve is completely on or off. Control signal is; u=sign(yref-y) This control technology causes oscillations around reference position. If dead-zone is large, load stops around the reference point. This is not accurate and robust position control. Another control algorithm is basically same with the on-off control but in addition to on-off control, velocity feedback control is added to on-off control algorithm. Control signal is; u=sign[(yref-y)-Kv.y] This type of control is applicable for position control. The behaviour of the system strongly depends on Kv. If Kv is increased, control signal changes very frequently and that will cause increasing of the settling time. Adding dead zone around the reference point is essential for stopping oscillations. The width of the dead zone must be smaller than the d<. red position accuracy. Velocity feedback control shows the minimum time control characteristics. At the high values of Kv, control signal changes frequently and motion of load is not affected from parameter variations. Frequent signal changing will cause pressure oscillations in chambers of the cylinder. The system behaves as an overdamped systems. Settling time will increase, system will be much accurate and robust according to the low values of the Kv and there will not be an overshoot. This type of control is called Sliding Mode Control. Otherwise, smaller Kv values decreases the settling time, but the system behave as underdamped system and can oscillate around the reference point. Pulse width modulation is often useful to control the voltage across a motor in such a way that only two values are used, usually zero and some other value. The usefulness derives from two sources: XIV a- The easiest signal to produce with a computer is a binary signal b- Most efficient way to modulate power flow is with amplifiers that can only operate at full-on or full-off. Control signal is derived from a control parameter, a. According to this parameter, it is decided which part of a period, control signal will be on or off. a= 0,5+ 0,4.u* Figure S. 2 Characteristic of u*- a A pulse width modulated signal is a constant frequency, two-valued signal in which the proportion of the period for which the signal is on and the proportion for which it is off can be varied. For pulse width modulation to be effective, the chosen frequency of the PWM signal must be high enough so that valve will not have time to respond to the rapid on-off changes, but, instead, the speed of the valve will reflect the average power level over many cycles. This average power level is controlled by the duty cycle. Shortly, to reach an accurate and robust position control, control parameters must be adjusted carefully. XV Continuous pressure control technique is applied to the system to compare the control efficiency of on-off and continuous pressure control. The valve used in our system, can control the pressure between 0,5 and 6,5 bar (gauge). For deriving 0-6,5 bar, 0-1 ampere must be sent to valve. Au is a control signal. uı= ü+Au, U2= ü-Au Au=Kp.(yref-y) - Kv.y Experimental investigation on the robustness of the system has shown that, if dead zones are the same, velocity feedback on-off control has a same robustness with sliding mode control at the reference point. But, settling time of sliding mode control is bigger than settling time of velocity feedback on-off control. However, there is not an overshoot at the sliding mode control. Although robustnesses of these control are the same, maximum errors at reference point are different, velocity feedback on-off control 0,5 mm., sliding mode control 0,3 mm. The behavior of PWM control at reference point is not robust enough because of continuously changing position of valves. Maximum error at reference point is around 0,6 mm. Application of continuous pressure control at the position control is completely new approach. Continuous pressure control has a good robustness, maximum error around 0,2 mm (Figure S.3). Algorithm of velocity feedback, SMC and PWM are easy but algorithm of Continuous Pressure Control is complex. In other terms, adjustment of control parameters in SMC and Continuous Pressure Control are difficult and parameter variations affect the quality of control. In conclusion, desired accuracy ± 0,1 mm is not achieved at this stage but it is shown that this problem can be solved by using a cylinder which has a low friction coefficient and difficulty of control parameter adjustment is also shown. If cost price, accuracy and robustness of control technologies are compared, most optimal qualifications will be found at sliding mode control. XVI 0,5 1 1,5 2 time(sec.) Figure S. 3 Velocity feedback on-off control 2,5 0,5 1,5 2 2,5 time(sec) 3,5 E E, c" o tn o o. Figure S. 4 Sliding mode control Figure S. 5 Pulse width modulation o,5 1,5 time(sec) Figure S.6 Continuous pressure controlYüksek LisansM.Sc
