【正文】 r) is proportional relationship. Automatic control system to overe the errors in the adjustment process may occur in the oscillations and even instability. The reason is due to greater inertia ponents (links) or there is a lag (delay) ponent, can inhibit the role of error, the change always lags behind changes in the error. The solution is to change the role of inhibition of error advanced, that is, the error close to zero, the role of inhibition of error should be zero. This means that only the introduction of the controller proportional term is often not enough, the role of the proportional term is only amplified the magnitude of the error, and the present need to increase the differential item that can change the trend of the prediction error, Thus, the controller has a proportional + derivative, we can advance the role of inhibition of the control error is zero, even negative, thus avoiding the serious charges the amount of overshoot. Therefore, greater inertia of the controlled object or lag, proportional + derivative (PD) controller can improve the system in the regulation of the dynamic characteristics of the process. 5, PID controller parameter tuning PID controller parameter tuning is the core of the control system design. It is based on the characteristics of the controlled process PID controller to determine the proportional coefficient, integral and derivative times the size of the time. PID controller parameter tuning many ways, summed up in two categories: one theoretical Tuning. It is mainly based on the mathematical model, through theoretical calculations to determine the controller parameters. Obtained by this method may not be directly calculated data must also be adjusted by engineering and modifications. Second, the tuning method works, it mainly relies on engineering experience, directly in the control experiments carried out, and the method is simple, easy to master, in engineering practice is widely adopted. PID controller parameter tuning method works, there are critical ratio method, the response curve and decay. Three methods have their own characteristics, their mon ground through testing, engineering experience and then follow the formula of the controller tuning parameters. But no matter what kind of methods used by the controller parameters need to be in the actual operation of the last adjustment and improvement. Now monly used is the critical ratio method. Using this method of tuning PID controller parameters as follows: (1) First, select a presampling period short enough to let the system work。 (2) proportional control only add links to the input until the system appears critical step response oscillations Then write down the ratio of magnification factor and the critical oscillation period。 (3) under a certain degree of control calculated by the formula PID controller parameters. In practice, debugging, you can only experience the first general to set a value, then adjust the effect of changes. The temperature system: P (%) 20 60, I (sub) 3 10, D (sub) 3 For the flow system: P (%) 40 100, I (sub) 1 For pressure systems: P (%) 30 70, I (sub) 3 The level system: P (%) 20 80, I (sub) 1 5 To find the best parameter setting, from small to large order of investigation After the first proportional integral differential plus the last then Oscillation curve is very frequent, the proportion of the dial to zoom Grand Bay floating around curves, the ratio of small to pull the dial Curves deviate from the slow response, to decrease the integration time Curve of a long wave cycle, coupled with a long integration time Fast oscillation frequency curve, first derivative down Great to fluctuations in slow moments. Differential time should be extended Ideal curve of two waves, high to low 4 to 1 More of a look at the second transfer, regulation will not be low quality Digital PID algorithm is simple principle (not entirely plete version) PID is a proportional, integral, differential abbreviation Uo (N) = P * E ??(N) + I * [E (N) + E (N1 )+...+ E (0)] + D * [E (N)E (N1 )] Eerror P P can increase the response speed of change, reducing the static error, but too large overshoot and increased settling time. I and the role of P similar, but to make a static error is 0, you must use the points. D and P, I contrast the role is mainly to reduce the overshoot, reduce the settling time. Three parameters to be considered, the general first I, D is set to 0, and adjusted the P, to meet the basic response speed and error, plus I, the error is 0, then add D, the three parameters to be repeated debugging, and ultimately achieve better results. Different control objects, the difficulty of debugging a big difference, good luck!