H型鋼制作矯直機(jī)設(shè)計(jì)含6張CAD圖,型鋼,制作,矯直機(jī),設(shè)計(jì),cad
外文資料
AUTOMATING THE CONTROL OF MODERN EQUIPMENT FOR
STRAIGHTENING FLAT-ROLLED PRODUCTS
The company Severstal’ completed the successful introduction of new in-line plate-straightening machines (PSMs) on its 2800 and 5000 mills in August 2003 [1, 2, 3]. The main design featuresof the machines are as follows:
each machine is equipped with hydraulic hold-down mechanisms (to improve the dynamics and accuracy of the machine adjustments and more reliably
maintain a constant gap);
the machines have mechanisms to individually adjust each work roller with theaid of hydraulicylinders (this broadens the range of straightening regimes that can be realized by providing a measure of control over the change in the curvature of the plate);
each work roller is provided with its own adjustable drive (to eliminate rigid
kinematic constraints between the spindles);
the system of rollers of the PSM is enclosed in cassettes (to facilitate repairs and
reduce roller replacement costs);
the PSM has a system that can be used to adjust the machine from a nine-roller
straightening scheme to a five-
roller scheme in which the distance between the rollers is doubled (this is done to widen the range of plate thick-nesses that the machine can accomodate).
Thus, the new straightening machine is a sophisticated multi-function system of mechanisms th-at includes a wide range of hydraulically and electrically driven components controlled by digital and analog signals. The entire complex of PSM mechanisms can be divided into two functional groups: the main group, which includes the mechanisms that partici-pate directly in the straightening operation (the hold-down mechanisms, the mechanisms that individually adjust the rollers,the mechanisms that adjust the components for different straightening regimes, the mechanism that moves the top roller of the feeder, and the main drive); the auxiliary group (which includes the cassette replacement mechanism, the spindle-lock-ing mechanism, and the equipment that cools the system of rollers). Although the PSM has a large number of mechanisms,the use of modern hydraulic and electric drives has made it possible to almost completely automate the main and auxiliary operations performed on the PSM and the units that operate with it.
Described below are the features and the automatic control systems for the most important mechanisms of the plate-straightening machine.The operating regimes of those mechanisms are also discussed.The hydraulic hold-down mechanisms (HHMs) of the sheet-straightening machine function in two main regimes:the adjustment regime;the regime in which the specified positions are maintained.There are certain requirements for the control system and certain efficiency criteria for each regime.
In the adjustment regime, the control system for the hydraulic hold-down mechanisms must do the following:
synchronize the movements of the hydraulic cylinders and keep the angular deeflection within prescribed limits;
maximize speed in adjusting the machine for a new plate size;
maintain a high degree of accuracy in positioning the mechanisms;
The control system has the following requirements when operating in the maintenance regime:
stabilize the coordinates of the top cassette and the top roller of the feeder with a high degree of accuracy;
minimize the time needed to return the equipment to the prescribed coordinates when deviations occur (such as due to the force exerted by a plate being straightened).
Need for synchronization. Experience in operating the plate-straightening machine in plate shop No. 3 at Severstal’ has shown that the most problematic factor in adjusting the machine
is the nonuniformity of the forces applied to the hydraulic cylinders. This nonuniformity is due to the asymmetric distribution of the masses of the moving parts of the PSM (in particular, the effect of the weight of the spindle assembly). Displacement of the “hydraulic zero point” relative to the “electrical zero point” in the servo valves is also a contributing factor.The latter reason is more significant, the smaller the volume of the hydraulic cylinder.Thus, the HHM of the top roller of the feeder is the most sensitive to drift of the zero point.
There are also other factors that affect the dynamism,simultaneousness,and synchronism of the operation of the hold-down mechanisms:
differentiation of the frictional forces on parts of the hydraulic cylinders due to different combinations of deviations in the dimensions of the mated parts, despite the narrow tolerances;
differences in the “springing” characteristics and the indices characterizing the inertia of the hydraulic supply channels (due to differences in the lengths of the pipes leading from the servo valves to the hydraulic cylinders).
Thus, since the PSM is not equipped with devices to mechanically synchronize the operation ofthe cylinders, the ransmission of signals of the same amplitude to the inputs of the servo valves inevitably results in a speed difference that can seriously damage the mechanisms.
To minimize and eliminate the effects of the above-mentioned factors, we developed an algorithm for electrical synchronization of the hold-down mechanisms.
The HHM of the top cassette, composed of four hold-down cylinders and four balancing cylinders, is designed to ensure mobile adjustment of the machine to set the required size of straightening gap (in accordance with the thickness of the plate) and maintain that gap with a specified accuracy in the presence and absence of a load on the housings from the straightening force.
The hydraulic system of the hold-down mechanism is designed in such a way that only one chamber of the hydraulic cylinders is used as the working chamber.The second chamber is always connected to the discharge channel.The top cassette is lowered when the balancing forces are overcome by the hold-down cylinders.The cassette is raised only by the action of the balancing cylinders.This arrangement has made it possible to eliminate gaps in the positioning of the equipment.
The HHM of the top roller of the feeder consists of two hydraulic cylinders. Hydraulic fluid is fed into the plunger chamber when the roller is to be lowered and is fed into the rod
chamber when it is to be raised.
Control Principles. Individual circuits have been provided (Fig.1) to control the hydraulic cylinders of the hold-down mechanisms.The control signal (Xctl) sent to the input of the servo valve is formed by a proportional-integral (PI) controller (to improve the sensitivity of the system, we chose to use valves with “zero” overlap).The signal sent to the input of the controller (the error signal Xerr) is formed as the difference between the control-point signal for position (Xcpt) and the feedback signal (Xf.b).The latter signal is received from the linear displacement gage (G) of the given hydraulic cylinder.
The gages of the HHM for the top cassette are built into the balancing hydraulic cylinders (HCs).The cylinders are installed in such a way that their movements can be considered to be equalto the displacements of the corresponding cylinder rods, with allowance for certain coefficients.The gages in the HHM for the top roller of the feeder are incorporated directly into the hold-down cylinders.
The integral part of the controller is activated only during the final adjustment stage and duringstabilization of the prescribed coordinate.When the displacements exceed a certain threshold value, the functions of the PI controller are taken over by a proportional (P) controller with the transfer function W(s) = k.Thus, Xctl(t) = kXerr(t).
When there are significant differences between the displacements of the working rollers,the difference (error)between the control point and the feedback signal from the linear displacement gage reaches values great enough so that the output signal which controls the operation of the servo valve reaches the saturation zone.In this case, further regulation of the displacement rate and,thus synchronization of the movements of the cylinders becomes impossible as long as the error exceeds the value at which Xctl is greater than the boundary value for the saturation zone (Xsat).The limiting error–the largest error for which Xctldoes not reach saturation–is inversely proportional to the gain of the controller k: Xerr< Xsat/ k.
Solving the given problem by decreasing k leads to a loss of speed in the adjustment of the PSM and a decrease in control accuracy during the straightening operation.Thus, to keep the control signal from reaching the saturation zone when there are substantial displacements, the system was designed so that the input of the controller is fed not the actual required value (Xrq) but an increment (X) of a magnitude such that the condition kX < Xsat is satisfied.The control point is increased by the amount X after the position of the cylinder has been changed by the amount corresponding to the increment having the largest lag relative to the cylinder’s direction of motion.
The adjustment of the control point is continued until the difference between the required value and the actual position of the mechanism becomes less than the increment:Xrq-xf.b
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