動(dòng)靜態(tài)液壓試驗(yàn)臺(tái)液壓系統(tǒng)設(shè)計(jì),動(dòng)靜態(tài)液壓試驗(yàn)臺(tái)液壓系統(tǒng)設(shè)計(jì),靜態(tài),液壓,試驗(yàn)臺(tái),系統(tǒng),設(shè)計(jì) KSME International Journal, Vol. 11, No. 4, pp. 397-407, 1997 A Study on the Stability Analysis of a PWM Controlled Hydraulic Equipment 397 Jun-Young Huh* and G. Wennmacher* (Received August 16 1996) PWM control, which is inherently nonlinear digital control, has been used for hydraulic equipment control because of the robustness and the availability of the low priced high speed on-off valve which is required for the system. Since this valve can be directly controlled without any D/A converter, it is easily implemented to hydraulic equipments with microcomputers. The objectives of this study is to analyze the limit cycle which ordinarily appears in the position control system using high speed on-off valve, and to give a criterion for the stability of this sy,;tem. The nonlinear characteristics of PWM and cylinder friction are described by harmonic linearization and the effects of parameter variations to the system stability are investigated theoretically. Experimental results demonstrate the feasibility of the proposed method. Key Words: Electro-Hydraulic Servo System, High Speed On-Off Solenoid Valve, PWM Method, Stability Analysis, Limit Cycle Nomenclature Ns : Transfer function of valve-cylinder sys- tem As : Piston area P1, P2 : Forward and return pressure of cylinder ca : Discharge coefficient Ps : Supply pressure D : Duty (:acting time/T) Pr : Tank pressure Fc : Coulombs friction force q : Input signal of PWM FL : External load c : Amplitude of PWM input signal Fe : Friction force of piston Q, Q2 : Forward and return flows G : Viscous damping coefficient qmax : Magnitude of PWM input signal of duty kl : Flow gain of valve : 1 k2 : Flow-pressure coefficient T : Period of PWM carrier wave k: : Proportional gain Tq : Period of PWM input signal wave km : Maximum displacement of valve poppet Ts : Period of sampling time K : System gain (:k: 9 k,) : Amplitude of PWM output pulse Kq : Sizing factor of valve v : Velocity of piston Ls : Static friction 7 : Amplitude of piston velocity wave M : Piston mass Vt : Total volume of pipe, valve and cylinder MRa : Critical speed starting viscous damping chamber friction w : Area gradient of valve port NpwM : Describing function of PWM x : Displacement of piston xv : Displacement of valve poppet (i:a, b, 9 Department of Mechanical Engineering, Korea c, d) University of Technology and Education, Chonan P.O.B. 55, Chungnam, 330-600, Korea Be Effective bulk modulus of fluid and pipe 9 * Liebherr-Aerospace Lindenberg GmbH, Pfaender : Synchronizing degree Str. 50-52, D-88161 Lindenberg, Germany r : Valve delay time 398 Jun-Young Huh and G. Wennrnacher r(q) : Width of PWM output co : Angular frequency of PWM input signal ton.ea : Switch on delay ton,d,so : Switch on displacement time tof.dead : Switch off delay toff,dsp : Switch off displacement time 1. Introduction The control method using Pulse Width Modu- lation(PWM) is one of the nonlinear control schemes. Hydraulic equipments which are operat- ed in PWM control usually use high speed on-off valves as control valves. The usage of this valve gives several advantages; this valve has very sim- ple structure so that it has robustness to oil contamination and its price is low. And it can be easily implemented to the system by using micro- computer since the control of this valve can be carried out digitally without D/A converter (Wennmacher, 1992a, b; Wennmacher, 1994; Muto, et. al., 1988). Since the hydraulic cylinder system controlled by PWM has highly nonlinear characteristics such as the nonlinearity of PWM, the phase delay of valve poppet and the friction in the cylinder, the nonlinear characteristics should be considered in the analysis of the system stabil- ity. Tanaka(1988) considered this system as a linear time invariant discrete system and derived transfer function using z-transform, then, showed the stable limiting gain against the damping ratio on every sampling time. Noritsugu(1983) sim- plified the nonlinearity of PWM as a saturation function and considered it with the phase lag of valve poppet and the cylinder friction in the velocity control of pneumatic cylinder. But the nonlinearity of PWM signal generation is not considered fully. Prochnio (1986) utilized describ- ing function based on the harmonic linearization in depicting the nonlinearity of PWM and consid- ered the friction in cylinder, did not include the phase lag of the valve poppet. The objective of this study is to present a method to predict the stability of hydraulic servo system. The electro-hydraulic position control system is theoretically modelled, which have high speed on-off valves controlled by PWM. In this process, the highly nonlinear characteristics of the system such as the dynamics of PWM, the phase lag of valve poppet and the cylinder friction are modelled mathematically. Especially the non- linear characteristics of PWM and cylinder fric- tion are expressed with describing function by the harmonic linearization. The validity of the presented method is investigated for the variation of the system parameters theoretically and experi- mentally. 2. Hydraulic Servo System Description and Analysis 2.1 Dynamic modelling The schematic diagram of the hydraulic servo system employed in this study is shown in Fig. 1. This system consists of 4 high speed on-off valves and a hydraulic actuator(double rod double acting cylinder) as the main parts. The piston displacement (x), which is output of the system is feedbacked to the comparator, which is im- plemented by software inside the computer, to be compared with the reference input value. For the PWM operation with the high speed on-off valve, since the switching characteristic of valve poppet makes a significant influence to the performance of this system, the lag of the valve poppet move- ment should be considered in dynamic modelling. The hydraulic system is modelled in Fig. 2. When the ON signal is applied to the valves a and c, since the valves b and d receive the OFF signal simultaneously, the forward oil flow(Q1) is oc- IIII Fig. 1 ? Electro-hydraulic servo system with high speed on-off valves. A Study on the Stability Analysis of a PWM Controlled Hydraulic Equipment 399 X,= X,)%,R P1 P2 . l I 7- / I 7 TM 5 r I ELI Xvd Xvc I Xvb Xvo I I PT I PT PS Fig. 2 Modelling diagram of hydraulic system. ing, because it is generally not critical or of particular interest in system design, it cannot be altered appreciably by design, and it does not affect system stability. To derive the transfer func- tion of the system, the load flow Eq. in (5) which is a nonlinear equation is linealized by the Taylors expansion in the vicinity of an operating point(xv0, PLo) of interest, It can be written as (Merrit, 1967) QL = kx - k2PL (8) curred through the valves a and c, and this makes the positive piston displacement. Under the assumption that the supply pressure(Ps) is con- stant, the leakage flows between a valve poppet and its seat and that of cylinder are negligible, and the valves are operated simultaneously, the flow rates (Q, Qz) through the orifices of the high speed on-off valves are given by Q = caw v 9 xe (1) Q2=cawvf. xaPUA=-P (2) where the load pressure(PL) and the load flow (QL) are defined as pL = p- p2 (3) QL :- Q1 + Qz (4) 2 The load flow equation can be deriven as, QL = Kqx,/-Ps- PL (5) where Kq - CdW v The continuity equation of the valve-cylinder system is given by QL=AhI lit dPL (6) 4fie dt The force balance equation of the piston is d2x M-t- = AhPL - FR (7) where the viscous damping friction of the piston is included in the term of total friction force of cylinder (FR), and the external load (FL) is omit- ted. The external force is neglected in the modell- 2.2 Describing function of PWM When a high speed on-off valve is operated in PWM mode, the input signal which is inflicted on the valve from the PWM module is a type of pulse row which has a constant pulse magnitude but a different pulse width and sign according to the magnitude of the input signal of PWM module. And the pulse row has the same interval between the pulse starting points. To investigate the stabil- ity of this system analytically, the characteristics of the PWM signal generation should be depicted analytically. Under the assumption that the on- off operations of the valves are symmetric and the linear part of this hydraulic system has the enough low pass filter function (Follinger, 1991), we derive the describing function of PWM signal generation which has the highly nonlinear charac- teristic. The PWM module catches the input sig- nal(q) at every time interval and knows the end of the pulse width previously. The input signal (q (t) of the PWM module can be represented as the following in the harmonic vibration balance state. q (kT) = c7 9 sin (w 9 kT + q5) (9) where the synchronizing degree (if) represents the phase lag between the first output pulse of PWM module and the input signal of PWM module in the harmonic vibration balance state. For exam- ple, the case of if=90 is shown in Fig. 3. The input signal q(t) of PWM module which is depicted as Eq. (9) is a periodic signal which have a period (Tq). If the ratio between the period (Tq) and the period of PWM carrier wave(T) comes to 2n (n=l, 2, 3, .), Fourier series method can be utilized in calculation of the 400 Jun-Young Huh and G. Wennmacher describing function of the PWM signal genera- tion. The output pulse equation of PWM module u (t) = 0 sgn(q (kT) ) is given by kT Nt kT +rq(kT) kT+rq(kT)t (k+l) T (lO) where rq(kT)= .D DI D Do u (t) is the function of the output signal of PWM module, q(kT) is the Kth value of the input signal of PWM module, and D is a duty, which is determined as I q(kT)/qmaxl. Do is the duty value corresponds to the delay time in the valve operation. A vibration which continues steadily with a constant amplitude and frequency is called a limit cycle. Especially, when the ratio of (Tq/ T) is 2 as shown in Fig. 3, we call it 1 pulse IT I (k -G. Fig. 3 ? i l)T (k+2)T Output pulse trains of 1 pulse limit cycle. fi -0 (k+n)r (k+n+1)T (k+2n-1)T kT (k+l)T (k+n-1)T t Fig. 4 Output pulse trains of n pulse limit cycle. limit cycle. The Fourier coefficient al and bl of basic vibration is given by 2s sin( r (c7) (11) al- 7/ 2s / r 1 b, =-LCOST. r () - (12) then the describing function of PWM signal gen- eration is given as the following for the case of 1 pulse limit cycle. r 2s . ;r , NwM(#, l, TJ-Tgksn,r ) ) Meanwhile, the n pulse limit cycle has succes- sively n positive pulses and n negative pulses for one period time(Tq) of limit cycle as shown in Fig. 4. The describing function of this case is derived as the following, 2s . n-t . NpwM (c7, n, b)= _ e-k_oe- lrq = - e-Jk+ nT r(q(kT) (14) As shown in Eq. (14), NPwM(ff, n, b) is a function of the amplitude () of the input signal of PWM module, the pulse number(n) of limit cycle and synchronizing degree(b), but it is in- dependent to angular frequency (w). 2.3 Describing function of friction charac- teristic The friction characteristic of hydraulic cylinder which is modelled as shown in Fig. 5 consists of static friction, coulombs friction and viscous damping friction which is dependent upon piston velocity. Supposing the hysterisis in friction char- acteristic is negligible, the friction characteristic shown in Fig. 5 can be formulated as the follow- ing: (Back6, 1992) I ( Fe()=Gk. + F+L. l- sgn(,7) (15) A Study on the Stability Analysis of a PWM Controlled Hydraulic Equipment 401 In order to derive the describing function, the above equation could be approximated as (Proch- nio, 1986) FR():Gg+Fc4 L 5 , sgn(v) (16) Where c is a constant required to approximate Eq. (15) to Eq. (16). In harmonic balance, the piston velocity v(t) is given by v(t) = 7-sin(wt) (17) the describin function of friction characteristic is NR() = a(g) (18) qvhere a(7) is the Fourier coefficient of basic vibration as follows: 1 ,-2r _ +F a(g)=-J 1Gv sin(x) L Ls + 1 +( g sin(X)c .)2- sgn(sin X) sin(x) y Fig. 5 Model of cylinder friction. Eq. (18) the describing function of friction char- acteristic becomes as follows: NR () =Gk+ 4Fc 4 2Lsc 2 . In xv x2/ 2 + c2 t7 + (20) 2.4 Analysis of the control loop In order to investigate the stability of the system which have highly nonlinear characteristics such as PWM signal generation and friction in cylin- der, the describing functions of these nonlinear characteristics are derived. Hence, the well known linear analysis method can be utilized. The above derived Eqs. (6), (7), (8), (14) and (20) are combined to construct the block diagram of the overall system as shown in Fig. 6. In this figure the nonlinear characteristics which is depicted by the describing functions are shown with double square frame. The transfer function Ns of the valve-cylinder system is derived as Eq. (21), the input of which is the valve spool displacement X and the output is cylinder displacement x, Ns(W, g)=- c (21) s ( s2 + c2s + c3 where, C1- k c2= FRM c _ 4J - VM The unstable phenomenon of a vibration sys- tem which has an element with a highly nonlinear characteristic tlsually starts with the limit cycle, Nsr ), F Fig. 6 Block diagram of PWM hydraulic cylinder system. 402 Jun-Young Huh and G. Wennmacher that is, the output of the system oscillate continu- ously with a constant amplitude and frequency. Whether the limit cycle will occur or not in a certain condition can be determined through the investigation of the solution of the closed loop characteristic equation. That is, in the harmonic vibration balance state, the limit cycle will occur if the solution of the closed loop characteristic equation exists (Prochnio, 1986; Follinger, 1991). The objective of this study is to present a method to predict the stability of the system so that it can be used to guarantee the stable operation in a wide range of operating conditions. The charac- teristic equation of the position control system shown in Fig. 6 is given as 1 + Ke - 9 NewM Ns :0 (22) where K is obtained by the product of the propor- tional gain(k1) and the maximum displacement of valve poppet(kin). The characteristic Eq. (22) is so complicated that it is very difficult to solve it analytically. Therefore, a graphical method is utilized to get the solution of the characteristic equation. wave(T) comes to 2n (n=l, 2, 3, .-.), hence, the relation between angular frequency(co) and PWM carrier wave period(T) is derived as the following: z n=l, 2, 3, . (24) co= nZ In the above equation, when the pulse number (n) of limit cycle is fixed, angular frequency(co) is uniquely determined. Therefore the unfixed parameters of Eq. (22) are the gain(K), the amplitude () of the input signal of PWM mod- ule, synchronizing degree(e) and pulse number (n) of limit cycle. At first to investigate whether the 1 pulse limit cycle occur or not, the pulse number of limit cycle is set as n=l. Then, we changed K value variously in the range of interest for the fixed r and value, otherwise we chan- ged value variously for the fixed r and K value, and investigated the solutions which satisfy the Eq. (22). And for the cases of n=2, 3, . it is investigated in the same way. The values of each parameters which is used in computer simu- lation are shown in Table 1. 3. Computer Simulation and Experiment In order to solve the characteristic Eq. (22) by graphical method which is utilized in this study, the first step is to investigate the Eq. (22) and look for the unfixed parameters. In the second term of the left hand side of the Eq. (22) NewM (c7, n, r is the function of the amplitude(p) of the input signal of PWM module, the pulse num- ber(n) of limit cycle and synchronizing degree (r but it is independent to angular frequency (co). Ns(co, g) is the function of angular fre- quency(w) and piston velocity amplitude(f). The relation between the amplitude(p) of the input signal of PWM module and piston velocity amplitude is given by t = k/ g (23) co When this system is operated in harmonic vibra- tion balance state, the ratio between the input signal period (Tq) and the period of PWM carrier Table 1 System parameters used in computer simulation. Parameters Value Dimension A 7.65 cm 2 c 1.5 cm/s Fc 0 kgf L 21 kgf G, 5.46 kgf s/cm kl 4666.7 cm2/s k2 0. 3536 cmS/kgf s k, 0.015 cm M 5-80 kg T 5 ms Ts 1 ms Vt 306 cm 3 Va 50 cm 3 r 1 ms fie 12000 kgf/cm 2 A Study on the Stability Analysis of a PWM Controlled Hydraulic Equipment 403 I I I I I I I- ! J I r-1 I I I ! I I I I I I I I I I I Ace.2 I I I I I I I I L. -3 r I I I I I I .3 Fig. 7 Hydraulic circuit of experimental equipment. The experimental apparatus is constructed as shown in Fig. 7, which has 4 high speed on-off valves, a double rod hydraulic cylinder, and a microcomputer as the major components. Piston displacement is measured by the digital optical Table 2 Specification of ex 9erimental apparatus Equipment Specification Hydraulic System Electric Equipment Electric Motor 5 PS Hydraulic Pump Qmx:8.9 l/min Actuator Ak = 7.65 cm 2 St.= 10 cm High speed On-off valve Relief valve Filter Accmulator 1 Accumulator 2 Displacement Transducer Microcomputer Controller Qmax:4.2 1/min Xvmax:0.015 cm ton,dead :0.46 ms /on,dlp :0.18 ms toll,dead = 0.30 ms tou,dlsp :0.16 ms Pmax :230 kgf/cm 2 10 m 4 liter 0.075 1, 70 bar 12 bit (0.315ram) CPU386 sensor with + l#m accuracy. Pressure transducer is represented by P/T in Fig. 7, of which accuracy is - bar. It is an analog sensor so that the analog to digital converter is required to transfer measured values in the computer. Tile controller in control loop gets the sampled data on every millisecond, calculates the control law and gives it to 4 channel PWM modulator. PWM carrier frequency is 200 Hz and the period is 5ms. For the various inertia loads, we looked for the propor- tional gain that makes the system on the threshold from stability to instability with limit cycle. The major specifications of the components which is used in this experiment are shown in Table 2. 4. Performance of the Control System In the cases of inertia load 6kg and 50kg, the experimental results for step input are showed with the solid line and the dotted line respectively in Fig. 8 (ks= 1 5). The responses shows instabil- ity with almost regular oscillations which have almost constant amplitudes. For the inertia load 6kg, we can find that the response shows 1 pulse limit cycle because it vibrated with 100Hz fre- quency which have 10 peaks per 0.1 second, and 404 Jun-Young Huh and G. Wennmacher ? i. ,. . I . . . i . i i ; i i . 0.5 ; i i ! 0 0 0.05 0.1 0.15 0.2 .me (sac) Fig. 8 Experimental results of n pulse limit cycle (n= 1, 2). this is the half of the PWM carrier frequency 200Hz. For the inertia load 50kg, the response shows almost 2 pulse limit cycle to have about 5 peaks per 0.1 second. Here the reason that the response does not show the exact 5 peaks per 0.1 second is guessed due to the followirtg vibration of high speed on-off valve after its opening and closing operation. Figure 9 is one of the result figures which is utilized to investigate analytically the existence of 1 pulse limit cycle which is the solution of Eq. (22). One solid line of Fig. 9 (a) is obtained for the case with the inertia load of 11 kg, synchronizing degree (b) of 88 and propor- tional gain(ks) of 0.7, by plotting the value of Ke -s 9 NpWM Ns in s-plane for the variation of the velocity amplitude(7) from 0.01 to 25, and the other solid line curves are obtained by repeat- ing this process for the increased proportional gain(ks) from 0.7 to 1.5 by step of 0.2.Similarly when the proportional gain (kl) varies from 0.1 to 4.5 for the increased amplitude(O) from 0.2 to 3.2 by the step of 0.6, the values ofKe - 9 NpwM 9 Ns are plotted with dotted lines. Herein, when the curve crosses the point (-1, 0) of s-plan, there exist the solution of Eq. (22) which is given by the values of the proportional gain (kj) and veloc- ity amplitude (7). One solid line of Fig. 9 (b) is obt