RU2504447C2 - Automatic control over dc motors of rolling mill master drives - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to dc motor ACS. Proposed method comprises measurement of actual motor shaft rpm in rolling. Motor rpm is adjusted. Note here that measured motor shaft rpm is compared with preset one. In case inconsistency occurs, square voltage pulse opposed to deviation of motor shaft rpm it is generated and fed to motor armature. Magnitude and length of voltage are selected subject to control over minimum change in motor armature current at preset motor shaft rpm of reaching the preset rpm.

EFFECT: decreased amplitude and time of transients at variation of rpm and/or torque at DC motor shaft.

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Description

The invention relates to automatic control systems for DC motors in the main drives of the stands of the rolling mill, the task of which is to cyclically change the rolling speed and maintain a given rolling speed when the rolling moment changes.

A well-known proportional-integral-differential (PID) control algorithm with feedback (described in VV Denisenko “Computer dressing by technological process, experiment, equipment”), which dominates the control of the engines of the main drives of rolling mills and is described by the expression:

where t is time;

K, T _i , T _d - proportional dimensionless coefficients,

y - adjustable

u is the output value of the controller.

In the particular case, the proportional, integral, or differential components may be absent, and such simplified regulators are called P, I, or PI controllers.

The input of the control object is the controller output, i.e. quantity u, which has the same dimension as the mismatch e, the output quantity y, and the control action r. That is, if the object is controlled by the current or the shaft speed, in all these cases, the control quantity is u, and in the model of the control object P, a value converter must also be introduced into the current or the shaft speed, respectively. In all cases, such an effect should be the value of u (the output value of the controller). However, the rolling mill includes a line of rolling stands containing roll systems and drive systems consisting of gear stands, gearboxes, spindles, shafts and couplings, which is a complex multi-mass system that cannot be fully described mathematically, and there is a mutual influence of the rolling stands each other through the rolled billet. The preform itself has heating uneven in length and may have various structural defects. This leads to unpredictable and uneven changes in the rolling forces, which are transmitted through the drive system to the motor shaft. Despite the results obtained in the field of creating a modern theory of optimal self-propelled guns, the practical application of this theory for the synthesis of real systems is still relatively limited. This is explained by the fact that the construction on the basis of this theory of systems for complex objects in which the system is described by high-order differential equations or has a complex character of limitations is practically not feasible in the current state of control and computer technology. Therefore, the existing self-propelled guns use approximate control algorithms (quasi-optimal), insignificant in the sense of the chosen functional, differing from strictly optimal ones, but simply implemented in practice. In this case, a certain deviation of the process from strictly optimal is obtained, however, the control device of the system is significantly simplified. To determine the optimal coefficients, there are well-known analytical methods, such as the Ziegler-Nichols methods, the CHR (Chien, Hrones and Reswick) method. Calculation of parameters by formulas cannot give an optimal regulator setting, since analytically obtained results are based on highly simplified object models. In particular, they do not take into account the present non-linearity. In addition, the models use parameters identified with some error. Therefore, after calculating the parameters of the regulator, it is adjusted. Adjustment is performed based on the rules obtained from experience, theoretical analysis and numerical experiments.

They come down to the following:

- an increase in the proportional coefficient increases speed and reduces the margin of stability;

- with a decrease in the integral component, the control error decreases faster with time;

- a decrease in the integration constant decreases the stability margin;

- an increase in the differential component increases the stability margin and speed.

Despite the effectiveness and widespread use of PID controllers, there are a number of disadvantages. PID control loops are difficult to tune, their behavior is not always predictable, the required performance is not always possible, and troubleshooting is difficult. In addition, since the error value in the feedback circuit is used in the formation of the control signal, the shape of the control signal is not ideal, but is “blurred” in time (lines of the control (2) and output (4) signals in figure 2). Closest to the claimed invention is a method for automatic control of DC motors of the main drives of rolling mills described in SU No. 1026870 A1, class. ВВВ 37/46, 07/07/1983, a device for controlling the main electric drive of the stand of a continuous rolling mill.

The claimed invention is characterized by the fact that it allows to solve the problem of optimal control of a DC motor, that is, to achieve a predetermined rolling speed in a minimum time with a minimum diagram of the armature current, avoiding overheating of the motor and avoiding the above disadvantages due to the formation of a rectangular voltage pulse at the moment of occurrence of a deviation of the engine shaft rotation frequency from the preset one. A similar deviation occurs at the moment of changing the set rotation speed, due to the inertia of the engine and the entire drive system of the rolling stand as a whole, as well as at the time of changing the rolling force (input and output of the workpiece from the stand, uneven heating of the workpiece along the length, etc. )

To determine the optimal control action, a mathematical model of a DC motor of independent excitation is constructed, which is described by the following system of differential and algebraic equations:

where u is the voltage at the armature winding,

e - EMF of the anchor,

i is the armature current,

F is the flow created by the excitation winding,

M is the electromagnetic moment of the engine,

M _s - moment of resistance to movement,

ω is the angular frequency of rotation of the motor shaft,

R is the active resistance of the anchor chain,

L is the inductance of the anchor circuit,

J is the moment of inertia of the armature and drive,

With _ω is the coupling coefficient between the angular frequency of rotation and EMF,

C _M is the coupling coefficient between the armature current and the electromagnetic moment.

To obtain transfer functions, the Laplace transform is applied to the equations under zero initial conditions. Equation (2) gives a function connecting the armature current and the voltage drop across the armature:

Equation (3) gives a function connecting the dynamic moment and the angular frequency of rotation:

A diagram of the engine model is shown in figure 1.

от величины и длительности импульса (табл.1). Таблица 1 значений может составляться исходя как из результатов испытаний, так и на базе математической модели привода, при условии ее адекватности реальному приводу. Как известно, для получения оптимального по быстродействию переходного процесса необходимо поддерживать максимальный уровень управляющего воздействия. В данном случае необходимо с максимально допустимым ускорением разогнать систему, а по достижении заданной частоты вращения снизить управляющий сигнал (линии управляющего (1) и выходного (3) сигналов на фиг.2). В идеальном случае при данном подходе можно достигнуть сколь угодно малого времени переходного процесса, однако реальный привод накладывает на способ формирования сигнала следующие ограничения:To form the optimal control action, a table of dependence of the expression values is compiled

on the magnitude and duration of the pulse (table 1). Table 1 of values can be compiled on the basis of both test results and on the basis of a mathematical model of the drive, provided that it is adequate for a real drive. As you know, to obtain a transient process that is optimal in speed, it is necessary to maintain the maximum level of control action. In this case, it is necessary to accelerate the system with the maximum allowable acceleration, and upon reaching the specified speed, reduce the control signal (control line (1) and output (3) signal in figure 2). In the ideal case, with this approach, you can achieve an arbitrarily short transient process time, however, a real drive imposes the following restrictions on the signal generation method:

- the maximum allowable voltage supplied to the motor armature;

- the maximum allowable current overload of the motor armature (line 1 in figure 3);

- the inertia of a multi-mass system that limits maximum acceleration;

- signal processing time by the existing process control systems. The proposed engine control method can be implemented on

existing PID controllers. For this, the obtained value of the value of the control pulse is introduced as the gain of the proportional component, and after the necessary regulation time, this coefficient is equal to one, until the next deviation occurs. The integral and differential components are equal to 0. The setpoint value of the rotation frequency is supplied to the input of the controller, and the deviation value is used to determine the pulse start time.

The proposed method allows you to change the regulation parameters in a wide range to achieve various tasks (given the above limitations):

- the fastest output of the drive to a given speed;

- minimization of current loads;

- the exclusion of oscillatory processes of overshoot;

- optimization of control over the aggregate time to reach a given speed with a minimum armature current.

A table of values, graphs of control and output signals are constructed for the MP 7000-115U4 engine with the following nominal parameters - speed w = l2.075 s ^-1 , voltage u = 930 V, armature current I = 7150 A, maximum current overload 2.25 rel. units, field winding flux Ф = 0.4258 W, inductance of the armature circuit L = 0.000156 H, resistance of the armature circuit R = 0.003682 Ohms, the cumulative moment of inertia of the multi-mass drive system J = 172000 kgm ² , coefficients C _ω = 176 and C _M = 172. The parameters of the pulse signal are taken based on the equality of the maximum current in the anchor circuit with PI and the proposed regulation (figure 3).

Claims (1)

A method for automatically controlling DC motors of the main drives of rolling mills, including measuring the actual speed of rotation of the engine shaft during the rolling process, characterized in that the speed of rotation of the motor shaft is controlled to maintain a predetermined frequency of rotation, while the measured actual speed of rotation of the motor shaft is compared with a predetermined and if there is a deviation, a rectangular voltage pulse is formed which is opposite in direction to the deviation of the motor shaft rotation speed, which rye is fed to the engine armature, while the magnitude and duration of the voltage pulse is selected based on the conditions for ensuring a minimum change in the current of the motor armature at the maximum output speed of the motor shaft at a given speed.

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