DE102015013343B4 - Process for temperature control when operating an extrusion press for metals - Google Patents

Abstract

Process for the optimal control of an extrusion press for metals controlled by a PLC and equipped with press speed and discharge temperature controls, characterized in that the setpoint set by the operator for the extrusion temperature in conjunction with the parameters preset in the PLC for the press guidance and the recorded profiles of the extrusion temperature, the pressing force, the axial bar temperature profile and the pressing speed, for the process control, namely both for determining and specifying the optimal courses of the target values for the bar temperature and pressing speed control loops before the start of each pressing as well as for the ongoing correction of the specified pressing speed during the pressing, be used that before the start of each cycle the optimal input functions u (l) namely the pressing speed and the optimal axial bar temperature curve θ (l) of the material using a calculation model that shows the relationship between the process variables a) bar temperature, b) press speed, c) press force and d) strand exit temperature, determined and specified as setpoint curves for the lower-level control loops that the pressing time decreases successively from cycle to cycle until it reaches its minimum and the temperature θ (l) of the profile when it emerges from the die is as constant as possible and equal to a specified target value θ, and at the same time the pressing speed and the billet temperature remain within permissible limits and the quality of the pressed profile fulfills specified requirements, where the permissible limits of the pressing speed and the strand exit temperature as well as the pressing force limit caused by the hydraulics and / or the material properties of the die are included in the calculation model, that during the pressing the press given at the beginning of the pressing cycle speed is corrected and that deviations of the strand exit temperature from the setpoint during the pressing are corrected by using a superimposed on-line control, which performs a local correction of the optimal input functions u (l), whereby for the on-line control a nonlinear one tailored to the application predictive control algorithm is used.

Description

The present invention relates to a method for temperature control operation of an extrusion press for metals. Characteristic of the temperature control is the combination of a control, in which the temperature errors are fed back over cycles, and a predictive on-line control.

State of the art

Extrusion presses are generally used for the production of extruded products made of metal, glass or plastic. When extruding metals, e.g. Of aluminum, to which reference is made in the following, aluminum bars are heated in an oven to 400 to 500 ° C and then loaded into a receiver (recipient) (see Fig. 1). This is closed on one side by a die, the opening of which corresponds to the cross section of the profile strand being formed. From the opposite side, the die is pressed through the die with the exception of a small remainder under the influence of a very large force (> 10 MN) generated by the oil hydraulics. At the end of a cycle, a new block is loaded and the pressing process can be repeated.

In this process, which is assigned to the hot-forming processes in process engineering, in order to maximize productivity and homogenize product quality, the aim is to master the thermal budget while taking the mechanical boundary conditions into account. If x and / set the pressed length of the billet or the path that the punch has covered since the start of the press cycle in one cycle, the temperature ϑ _P (I) of the emerging profiles is determined by the preheating temperature of the billet ϑ _B ( _X ), o <x <l, and the pressing speed v _IS (x) , o <x <l. The pressing speed must therefore be used to achieve a certain predetermined target profile for the outlet temperature ϑ _P (I) v _is (l) and bar temperature ϑ _B (I), 0 </ <= L, L = length of the bar, are suitable.

The press speed v _is (I) in turn, in modern plants by specifying a target course v _should (l) (usually a constant), set by the press guide or by a higher-level program. A speed control ensures that the actual pressing speed i _st (l) equal to the target press speed specified by the higher-level program or the press guide v _should (I) provided the pressing force F _is within its limits. In general, speed control is implemented with a programmable logic control system (PLC). In older systems, the pressing speed v _is only implicitly specified by the operator input for the adjustment of the oil flow rate of the hydraulic system. The operator input is the setpoint for the swivel angle of a swivel angle pump or for the valve position. Depending on which of the quantities is manipulated, u (l) in the following denotes the pressing speed, the swivel angle or the valve position.

As well as the pressing speed of the actual axial temperature profile billet temperature (Ingot temperature profile ') θ _a (l) is by specifying a temperature profile θ _{e to} (l) realized on the temperature control of the billet furnace and of the quench.

In addition to the pressing speed and bar temperature control, the temperature ϑ _P (l) of the emerging aluminum profile is kept at a constant predetermined value in some cases by using non-contact temperature measuring devices and a control in order to accomplish the so-called isothermal extrusion process [1]. Depending on the desired constant outlet temperature ϑ _P and the measured axial temperature profile ϑ _B (l) of the billet to be pressed, the pressing speed profile becomes v _should (l) determined and given.

If you want to minimize the pressing time per cycle while achieving predetermined profile and material properties without violating the force limits on the press and die, you would have to set the values for the profile _{exit temperature} ϑ _POpt and the _{pressing speed} v _Opt as well as the corresponding curve of _{oll Bset} . which correspond to the required product properties, determine exactly in advance and this by specifying the associated target courses v _should (l) = v _opt and ϑ _PSoll = ϑ _POpt and ϑ _{B should} achieve (l) and use of suitable regulations during pressing. However, such a determination is not realistic in practice, since it requires the consideration of influencing factors that are difficult to ascertain, such as material composition, profile geometry, die layout, etc.

In practice, the process engineer mostly relies on his experience and specifies the values for the bar temperature and the pressing speed in advance and transmits these to the operator. The The operator accordingly sets the setpoints - usually as a constant over the entire cycle - for the bar temperature and press speed control loops. The profile exit temperature is rarely taken into account.

A more recent development in the field of extrusion is the profile temperature control, which is used in a few modern plants in the press operation. Three methods are known in the literature for such a control, which is usually implemented with the pressing speed as a manipulated variable: a) simulated isothermal extrusion, b) controlled isothermal extrusion and c) cyclical control of extrusion presses [2]. The method c), which has been successfully tested and used in modern extrusion presses with programmable logic control systems (PLC) and speed control, is specified in [2].

1. a) bei ihr Störungen der Temperaturmessung mit berührungslosen Pyrometern durch nicht kausale Filterung weitgehend eliminiert werden können,
2. b) die große zeitliche Verzögerung zwischen Eingriff (Änderung von Stempelgeschwindigkeit) und Reaktion (Profilaustrittstemperatur) in Regelalgorithmus weggerechnet werden kann, sofern die Prozessbedingungen nur langsam über die Zyklen ändern und
3. c) keine genaue Spezifikation des mathematischen Modells des Strangpressvorgangs erforderlich ist.

Das Verfahren ist insbesondere in den Fällen sehr effektiv, in denen die der Temperaturmessung mit berührungslosen Pyrometern gestört istThe cyclic control, which is also referred to as iterative learning control, has the advantages that

1. a) disturbances in temperature measurement with non-contact pyrometers can be largely eliminated by non-causal filtering,
2. b) the large time delay between intervention (change in die speed) and reaction (profile exit temperature) can be eliminated in the control algorithm, provided the process conditions change only slowly over the cycles and
3. c) no exact specification of the mathematical model of the extrusion process is required.

The method is particularly effective in cases where the temperature measurement is disturbed with non-contact pyrometers

If you consider a direct on-line temperature control, for example with a PI or PID controller, you can see that this also has an advantage: You react quickly and directly to control errors within a cycle by directly changing the pressing speed depending on temperature errors , However, the quick response only provides limited and robust control of the extrusion process to a limited extent, and only with low-fault temperature measurement and with small control errors. Advances in the field of non-contact temperature measurement technology now enable low-interference temperature measurement. The condition that control errors remain small can be achieved by basing the direct control on an iterative learning control. This means that a suitable combination of direct control with an iterative learner control can be used to achieve rapid and stable temperature control in extrusion presses.

task

The object of the present invention is to provide processes for extrusion presses which are equipped with press speed, profile exit temperature and bar temperature controls, with which the temperature control, ie the achievement and compliance with the state "profile exit temperature deviates as little as possible from the target profile", takes place quickly and stably.
It should be taken into account that the possibility of intervention in the process during the pressing process is only given via the pressing speed, and between two pressing processes there is given both the course of the ingot temperature and the course of the pressing speed.

In the patent, the applicant specifies methods according to which the profile exit temperature is controlled in an extrusion press by a combination of an iterative learning control (IL control), in which the temperature error is returned via press cycles, and a predictive online control , The IL control only intervenes in the pauses between two press cycles and provides the 'base trajectory' for the stamp speed curve at the beginning of each press cycle, so that the online control can directly control the profile exit temperature around the 'base trajectory' during the press cycle , Such a hierarchical approach is necessary because the IL control alone does not react to faults within a cycle, and the online control alone can lead to instability for large control errors.

How the combined control procedure works

The proposed method is explained using Figure 2. Only the process variables relevant to the new control process are considered.

During a pressing - e.g. in the k-th pressing cycle, k = 2, 3 ... - the N (e.g. N = 100) equidistant intervals of the bar length, the punch speed v _is , the profile temperature ϑ _Pist and the pressing force p _is continuously measured and transferred from PLC to industrial PC. The sizes are scanned and transmitted when the pressed ingot length is n · (L / N), with L = total ingot length and n = 0, 1, 2, (N-1). The sizes depending on the scanning are with v _is (n) , ϑ _Pist (n) or p _is (n) designated. The values are in memory S1 collected in the PC, so that at the end of the kth press cycle in the PC the consequences {ϑ _{PIst, k} } and { v _{is, k} } are available. Also in the PC is the sequence {ϑ _{B, (k + 1)} } of the N measured measured values of the temperature of the ingot which is to be pressed in the next, i.e. (k + 1) th cycle, which is transmitted by the PLC.

dass der prädizierte Regelfehler $$\left\{{\widehat{\text{e}}}_{\text{k}+\text{1}}\right\}=\left\{{\mathrm{\vartheta }}_{\text{Psoll}\text{,k}+1}\right\}-\left\{{\mathrm{\vartheta }}_{\text{Ppräd}\text{,k}+1}\right\}$$

für das (k+1) -ten Zyklus gemäß dem Modell möglichst klein wird.For the (k + 1) th cycle, a calculation model is used to calculate the outlet temperature {ϑ _{Pist, k} } and the results of the measured values of the bar temperatures {ϑ _{B, k} } and {ϑ _{B, (k + 1)} } the press speed { v _{is, k} } the history of the input size { u _{should, (k + 1)} so determined $$\left\{{\text{u}}_{\text{should}\text{.}\left(\text{k}+1\right)}\right\}=\mathrm{\Gamma }\left[\left\{{\mathrm{\theta }}_{\text{pw}\text{, k}}\right\}.\left\{{\mathrm{\theta }}_{\text{PActual}\text{, k}}\right\}.\left\{{\text{v}}_{\text{is}\text{, k}}\right\}.\left\{{\mathrm{\theta }}_{\text{pw}\text{+ 1}\text{, k}}\right\}.\left\{{\mathrm{\theta }}_{\text{bars}\text{, k}}\right\}.{\mathrm{\theta }}_{\text{bars}\text{.}\left(\text{k}+1\right)}\}\right].$$

that the predicted control error $$\left\{{\widehat{\text{e}}}_{\text{k}+\text{1}}\right\}=\left\{{\mathrm{\theta }}_{\text{psetpoint}\text{, k}+1}\right\}-\left\{{\mathrm{\theta }}_{\text{Ppräd}\text{, k}+1}\right\}$$

for the (k + 1) th cycle according to the model becomes as small as possible.

How this can be done in detail is described, for example, in [1,2].

According to the ILR method according to patent 1, the sequence of the input variable { u _{should, (k + 1)} } before the start of the (k + 1) th cycle to the PLC, where it is stored in memory 2. This sequence becomes the sequential setpoint in the PLC u _should , n = 0, 1, 2, ... generated and specified for the speed control loop.

$$\text{x}=\text{Kp }\text{.e}\left(\text{n}\right)+\text{Steig}\left[\text{e}\left(\text{n}\right),\text{e}\left(\text{n}-1\right)\mathrm{,.,}\text{ e}\left(\text{n}-\text{m}\right))\right]$$

$$\mathrm{\Delta }{\text{u}}_{\text{soll}}\left(\text{n}\right)=\text{Fpk}\left(\text{x}\right)$$

berechnet.In contrast to the method according to patent 1, the new method uses size u _should a share Δu _should (n) , which is obtained with the help of a direct, on-line controller tailored for the application. In the on-line controller that is implemented in the PLC, in the nth step, ie when the pressed bar length = n · (L / N), Δu _should (n) according to $$\text{e}\left(\text{n}\right)=\left({\mathrm{\theta }}_{\text{psetpoint}}-{\mathrm{\theta }}_{\text{P}}\left(\text{n}\right)\right)$$

$$\text{x}=\text{kp}\text{.e}\left(\text{n}\right)+\text{Steig}\left[\text{e}\left(\text{n}\right).\text{e}\left(\text{n}-1\right)\mathrm{,.,}\text{e}\left(\text{n}-\text{m}\right))\right]$$

$$\mathrm{\Delta }{\text{u}}_{\text{should}}\left(\text{n}\right)=\text{fpk}\left(\text{x}\right)$$

calculated.

$$\begin{array}{cc}\text{y}=\text{c}\cdot \text{x}& \text{für a}<\left|\text{x}\right|<\text{b}\end{array}$$

$$\begin{array}{cc}\text{y}=\text{b}\cdot sgn\left(\text{x}\right)& \text{für }\left|\text{x}\right|>\text{b}\end{array}$$

wobei a, b und c positive Zahlen sind, die unter Berücksichtigung der Prozessbedingungen festgelegt werden. Als Faustregel gilt: man sollte sie so wählen, dass der Anteil Δu_soll(n) höchstens 10 - 15 % von u_soll(n) beträgt.The function Rise [e (n), e (n-1),., E (nm)] orders the sequence (e (n), e (n-1),., E (nm)) the slope of the regression line by the points e (n), e (n-1),., e (nm), where m represents the length of the sliding window, eg m = 5.
The function y = Fpk (x) represents a five-point limitation characteristic with a dead zone: $$\begin{array}{cc}\text{y}=0& \text{For}\left|\text{x}\right|<\text{a}\end{array}$$

$$\begin{array}{cc}\text{y}=\text{c}\cdot \text{x}& \text{for a}<\left|\text{x}\right|<\text{b}\end{array}$$

$$\begin{array}{cc}\text{y}=\text{b}\cdot sGn\left(\text{x}\right)& \text{For}\left|\text{x}\right|>\text{b}\end{array}$$

where a, b and c are positive numbers that are determined taking into account the process conditions. As a rule of thumb: you should choose it so that the proportion Δu _should (n) at most 10 - 15% of u _should is.

This is the setpoint for the speed control v _should formed according to: $${\text{v}}_{\text{should}}\left(\text{n}\right)={\text{u}}_{\text{should}}\left(\text{n}\right)+\mathrm{\Delta }{\text{u}}_{\text{should}}\left(\text{n}\right),$$

If necessary v _should limited to values in a permissible range. During the pressing, the control signals for the valve control of the oil hydraulics are dependent on v _should generated and specified continuously after each step in order to regulate the oil flow rate. The resulting actual stamp speed v _is (n) is measured and transmitted to the PLC at intervals of n · (LVN) of the pressed bar length, which in turn forwards the value to the industrial PC.

1. 1. Mess- und Regelungssystem für die Pressgeschwindigkeit (bzw. Öldurchflussmenge / Ventilstellung der Hydraulik).
2. 2. Mess- und Regelungssystem für die Profilaustrittstemperatur.
3. 3. Mess- und Regelungssystem für die Barrentemperatur
4. 4. Übergeordnete Optimierungsrechner.

To implement the processes, the extrusion press is supplemented with the following measurement and control functionalities or components; if they do not already exist:

1. 1. Measuring and control system for the press speed (or oil flow rate / valve position of the hydraulics).
2. 2. Measuring and control system for the profile exit temperature.
3. 3. Measuring and control system for the bar temperature
4. 4. Higher-level optimization calculator.

1. 1. Mess- und Regelungssystem für die Pressgeschwindigkeit (bzw. Öldurchflussmenge / Ventilstellung der Hydraulik) Diese Stellt die Einrichtung für die Operatoreingabe dar und wird in den meisten Fällen vorhanden sein. Es wird die Stempelgeschwindigkeit bzw. die Öldurchflussmenge / Ventilstellung der Hydraulik gemessen und mit dem vom Operateur bzw. von der übergeordneten Regelung vorgegebenen Sollwert u_soll verglichen und geregelt.
2. 2. Mess- und Regelungssystem für die Profilaustrittstemperatur Die Profilaustrittstemperatur wird mit einem berührungslosen Pyrometer gemessen und mit Hilfe eines Regelungsverfahrens, wie z. B. mit des im Europäischen Patent EP 0615 795 B1 [1] angegebenen Verfahrens, geregelt.
3. 3. Mess- und Regelungssystem für die Barrentemperatur Die Temperatur der Barren, die meist in einem Gas oder Induktionsofen geheizt werden und eventuell mit einem Quenchsystem abgekühlt werden, ist unmittelbar vorm Laden in Rezipienten zu messen. U.U. steht ein regelbarer Barrenofen zur Verfügung, mit welchem dem Barren ein vorgegebenes axiales Temperaturprofil, genannt Taper, aufgeprägt werden kann. In solchen Systemen wird durch Aufbringung des optimalen Tapers isothermes Pressen bei konstanter Geschwindigkeit erzielt. Üblicherweise wird der Sollverlauf spezifiziert durch die Kopftemperatur und die ‚Taper‘, d.h. die Differenz zwischen der Kopftemperatur und der Temperatur am hinteren Ende des Barrens. In älteren Anlagen dagegen ist nur möglich einen konstanten Sollverlauf vorzugeben.
4. 4. Übergeordnete Optimierungsrechner Aufgabe des übergeordneten Optimierungsrechners ist es, Sollwertverläufe für die Regelung der Pressgeschwindigkeit (bzw. Öldurchflussmenge / Ventilstellung der Hydraulik) und des Barrenofens zu erzeugen, die unter Berücksichtigung der zulässigen Grenzen der Prozessgrößen sowie der Produktqualität die Presszeit zu einem Minimum machen.
5. 5. Speicherprogrammierbare Steuerung SPS Die SPS ist die Instanz, die für die Steuerung der Anlage unmittelbar zuständig ist. Alle Sicherheitsvorkehrungen sind in ihr Implementiert. Für das neue Reglungsverfahren wird die on-line Regelung in ihr realisiert. Damit ist die notwendige Sicherheit auch bei Eingriffen während der Pressung gewährleistet, sofern die Eingriffe geeignet begrenzt sind. Man denke an die Faustregel, dass Δu_soll(n) höchstens auf 10 - 15 % von u_soll(n) betragen soll.

These are used to implement the following functions

1. 1. Measuring and control system for the press speed (or oil flow rate / valve position of the hydraulics) This represents the device for operator input and will be available in most cases. It is measured the piston speed or the oil flow rate / valve position of the hydraulic and _should u with the predetermined by the operator or by the superordinate control target value are compared and controlled.
2. 2. Measuring and control system for the profile exit temperature The profile exit temperature is measured with a non-contact pyrometer and with the help of a control procedure, such as. B. with the in the European patent EP 0615 795 B1 [1] specified procedure, regulated.
3. 3. Measuring and control system for the bar temperature The temperature of the bars, which are usually heated in a gas or induction furnace and possibly cooled with a quench system, must be measured immediately before loading into recipients. A controllable ingot furnace may be available, with which a given axial temperature profile, called a taper, can be impressed on the ingot. In such systems, isothermal pressing at constant speed is achieved by applying the optimal taper. Usually, the target course is specified by the head temperature and the 'taper', ie the difference between the head temperature and the temperature at the rear end of the ingot. In older systems, on the other hand, it is only possible to specify a constant target course.
4. 4. Higher-level optimization computer The task of the higher-level optimization computer is to generate setpoint curves for the control of the pressing speed (or oil flow rate / valve position of the hydraulics) and the ingot furnace, which, taking into account the permissible limits of the process variables and the product quality, minimize the pressing time.
5. 5. Programmable logic controller PLC The PLC is the entity that is directly responsible for controlling the system. All security measures are implemented in it. For the new regulation process, the on-line regulation is implemented in it. This ensures the necessary safety even during interventions during pressing, provided that the interventions are suitably limited. Think of the rule of thumb that Δu _should (n) at most to 10-15% of u _should should be.

It is taken into account that A controllable ingot furnace / quenching device with several cooling water nozzles is available, with which a predetermined axial temperature profile, called a taper, can be impressed on the ingot. In such systems, isothermal pressing at constant speed is achieved by applying the optimal taper.

Figure 1 shows such an arrangement.

LIST OF REFERENCE NUMBERS

e

control error

e _k (l)

Control error depending on the stamp position

L

Barren length

I

Stamp position = pressed bar length

N

Number of equidistant measuring points along the bar length, e.g. N = 100

p _is (n)

the measured pressing pressure

ϑ _B (x)

Course of the measured bar temperature as a function of length x

ϑ _P (l)

Strand exit temperature as a function of length l

ϑ _pist (n)

Strand exit temperature as a running variable depending on the sampling index n

{ϑ _{Pist, k} }

Strand exit temperature as a result of N individual values

(ϑ _{Psoll, k + 1} )

Setpoint of strand exit temperature as a result of N setpoints, _generally all values = ϑ _{Psoll, k + 1} maximum outlet temperature

(ϑ _{B, k} )

Course of the measured bar temperature in the k. Cycle as a result of N setpoints

u _should , _k (n)

the proportion of the speed setpoint calculated by the IL control

Δu _should (n)

the percentage of the speed setpoint calculated by the online control

v _is (n)

the measured stamp speed

v _{should, k} (n)

Course of the setpoint for the speed control loop

{u _{Soii, (k + 1} )}

the sequence of N speed setpoints calculated by the IL control at the beginning of the (k + 1) th cycle

v

Press (also stamp) speed

v _k (l)

Pressing speed in k. Cycle at punch position = extruded bar length I

.DELTA.L

Elementary length when discretizing the stamp position = L / N

n

Ortsindex

k

Zyklus

indices:

n

location Index

k

cycle

literature

1. [1] EP 0615795 Temperature control of an extrusion press
2. [2] DE 10 2005 047 285 Process for guiding an extrusion press for metals controlled by a PLC and equipped with a profile temperature control and press speed control

Claims (4)

Process for the optimal control of an extrusion press for metals controlled by a PLC and equipped with press speed and discharge temperature controls, characterized in that the setpoint set by the operator for the extrusion temperature in conjunction with the parameters preset in the PLC for the press guidance and the recorded profiles of the extrusion temperature, the pressing force, the axial ingot temperature profile and the pressing speed, for the process control, namely both for the determination and specification of the optimal courses of the target values for the ingot temperature and press speed control loops before the start of each press as well as for the ongoing correction of the specified press speed during the press, that the optimal input functions u _{k + 1} (l) namely the press speed and the optimal axial bar temperature profile θ _{Bk +} before the start of each cycle ₁ (l) of the material to be pressed using a calculation model that shows the relationship between the process variables a) bar temperature, b) press speed, c) press force and d) strand exit temperature, determined and specified as setpoint curves for the underlying control loops that the pressing time decreases successively from cycle to cycle until it reaches its minimum and the temperature θ _{ak + 1} (l) of the profile as it emerges from the die is as constant as possible and equal to a predetermined setpoint θ _aw , and at the same time the pressing speed and the billet temperature within permissible limits stay and the quality of the pressed profile meets specified requirements, the calculation model taking into account the permissible limits of the pressing speed and the strand exit temperature as well as the pressing force limit caused by the hydraulics and / or the material properties of the die, so that the pressing speed specified at the beginning of the pressing cycle is corrected during the pressing and that deviations of the strand exit temperature from the setpoint during pressing are compensated for by using a superimposed on-line control, which carries out a local correction of the optimal input functions u _{k + 1} (l), one for the application for the on-line control tailored nonlinear predictive control algorithm is used. Procedure according to Claims 1 characterized in that in the nonlinear predictive control algorithm, a regression model with a sliding horizon is used to predict the control error. Procedure according to Claims 1 characterized in that in the nonlinear predictive control algorithm a five-point characteristic curve is used for the calculation of the correction component of the input functions u _{k + 1} (l). Procedure according to one of the Claims 1 characterized in that the optimal courses of the input function u _{k + 1} (l) and the axial bar temperature course θ _{Bk + 1} (l) are determined with the aid of a simulation model and algorithms of the iterative learner control and are carried out in the higher-level optimization computer and the on-line Regulation is carried out in the PLC.

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