SI20253A - Method and arrangement for adaptive control of electroerosion based working process - Google Patents

Abstract

The automatic control of the electroerosion based processing according to the invention is performed in such a way that the process adapts to the model for such a process, which during processing concurrently adapts to the current situation, by adaptive learning. The upper and lower limit voltages for the area of the required voltage are progressing during the electroerosion process, depending on the time of the current discharg, are calculated for each individual discharge based on stored data on sampling the time based voltage progress of the active discharge processes. The current discharge is perceived as a non-standard discharge as soon as its voltage leaves the limited area by exceeding one of the two limit voltages. The automatic setting of limit voltages simplifies the control process and allows a higher automation level and at the same time eliminates the operator factor. In order to meet a required progress the adaptive process is reliable and features short reaction times. It allows precise processing providing high quality product surface.

Description

Procedure and preparation for adaptive control of the erosion treatment process

The subject of the invention is a method and device for adaptive control of a process of erosion treatment, by which the process of this process, due to its constant modification, is automatically adjusted to the desired course of the process according to the frequency of abnormal discharges as a criterion for the quality of the process, so that, after each abnormal discharge, a sufficiently long duration is performed disconnection of current pulses.

According to the international patent classification, the invention is classified in class B 23H 1/02.

In view of the disadvantages of the known methods and preparations of the aforementioned type for adaptive control of the erosion treatment process, it is a technical problem of the invention to perform a fully automatic control of the said process by adapting the process to a model for this process, which should be adapted to the current conditions during the treatment by self-learning, and without pre-setting for the process the supposedly appropriate limit values for the operating voltage range.

In the case of electro-erosion treatment, the desired work discharge is uniformly distributed between the electrode and the workpiece, and often unwanted, mostly localized, abnormal discharge occurs. The definition of abnormal discharges is known in the art. In order to improve the effect of the erosion treatment, it is controlled by identifying abnormal discharges, upon detection of which the current pulses are switched off and the pauses between them are extended or the electrode moves away from the workpiece after a certain time.

The patent documents CH 659 966 A5 and DE 33 00 552 C2, both derived from priority patent application YU 51/82, describe the process and preparation for adaptive control of the erosion treatment process. For each current pulse - the current pulse generator is controlled by a clock signal T whose period t consists of a duty period t and a break time t - it is determined whether it has caused a working discharge or an abnormal discharge. A discharge is identified as a discharge in which only after a typical minimum time t _{d has} elapsed after the initiation of the current pulse, the voltage U of the discharge between the electrode and the workpiece drops from the firing value U _p between the prescribed limit values U _H 'and U _L ' and remains here until the end of the service life t of each period of occurrence of current pulses and shows a characteristic oscillation (Fig. 1). Only the Uc curve shows the working discharge, the discharge voltage Ua remained above the upper limit U _H 'until the end of the service life, the discharge voltage Ud and Ue dropped too fast below the limit U _H ', while the discharge Ub does not show significant voltage fluctuations. Labor and abnormal discharges are considered separately. Due to the nature of the discharge process, the flow of the machining process is constantly changing, and with the described procedure it automatically adapts to the desired process flow according to the frequency of abnormal discharges, which is a criterion for the quality of the process, so that after increasing the frequency of abnormal discharges it engages in the processing process with a correspondingly long continuous disconnection of current pulses. The known solution as well as other solutions according to the state of the art require continuous adjustment of the current density, each time adjusting to a pair of electrodes - workpiece and other adjustments, such as changes in capacitance or inductance in the system current pulse generator - connecting cable - electrode - workpiece. Problems are particularly apparent when the operating time of current pulses is less than 10 gs; then abnormal charges are prevented mainly by periodically moving the electrode away from the workpiece, but also by manually adjusting the voltage limit values or a typical minimum time t _d .

Said technical problem is solved by the method according to the invention for adaptive control of the erosion treatment process, according to which the process of this process is automatically adapted to the desired course of the process due to its constant variation according to the frequency of abnormal discharges between the electrode and the workpiece, so that detecting abnormal discharges interferes with the processing process with the frequency of abnormal discharges corresponding to the long-term disconnection of current pulses, the method according to the invention characterized in that the lower limit of the upper region of occurrence of abnormal discharges and the upper limit of the lower region of occurrence of abnormal discharges on the sampling time of the discharge currents in this process, for each liquid discharge separately, the upper boundary voltage or the lower boundary voltage for the field of remarks shall be calculated separately from the duration of the liquid discharge. of the desired course of the process, and that a current discharge is detected as an abnormal discharge as soon as its voltage leaves the said region of the desired course of the process by exceeding one of the two limit voltages.

The method according to the invention is further characterized in that the time-dependent upper and lower boundary voltages in each of at least four time segments of the service life in each period of current pulses are calculated by means of said time sections by statistical processing of the time course of the voltages of the previous operating discharges of their acquired of radius radii in these time sections and that in the case of dividing the service life into four time sections, the first time section lasts 2 gs, starting from the passage of the discharge voltage below the upper limit voltage at the outset of the current pulse, and the second time section a further 4 gs, the third time a further 54 gs section and then a fourth time section until the end of the service life during that period of current pulses.

The process according to the invention is also characterized in that the upper limit or lower limit voltage is rapidly adapted to a rapid change in the processing process by means of a lower or upper decision signal following the passage of the discharge voltage across the lower limit voltage or over the upper limit voltage, and by means of an adaptive signal that rapidly adjusts one of the limit voltages to the other limit voltage if it changes rapidly.

The aforementioned technical problem is also solved by the device according to the invention for adaptive control of the process of erosion treatment, in which this device consists of a device for electro-erosion treatment and an adaptive circuit, to which the first input is supplied by the discharge voltage and from which output is output to the control input of the generator The current impulses provide a disconnect signal, the device according to the invention characterized in that the discharge voltage is also directed to the input of the circuit for the formation of a model of the erosion process, and in this circuit the upper and lower boundary voltage signals obtained for self-learning are directed to the desired discharges. second and third adaptive circuit inputs.

The device according to the invention is further characterized in that the circuit for forming the model of the erosion process consists of a circuit for sampling the discharge voltages and for calculating the curvature radii of both limit voltages in individual time sections of the service life in each period of current pulses and of the circuits for the formation of the upper limit voltage and / or lower limit voltage in the aforementioned operating time segments, and that a discharge voltage is applied to the first input of these three circuits and that a clock signal is generated at their first and second control inputs to generate a sequence of current pulses or a disconnect signal, and that the output is circuits for sampling the voltages of charges and for calculating the radius of curvature of both limit voltages in each time section of the service life in each period of current pulses connected to the second input of the circuits for the formation of upper limit voltage or lower limit voltage.

The device according to the invention is further characterized in that upper and lower boundary voltage signals are output from the second and third inputs of the adaptive circuit to the first input of the first comparator or to the first input of the second comparator, which are contained in the identification circuit of the adaptive circuit and to which the second input a discharge voltage is supplied from the first input of the adaptive circuit and whose output signals are directed to the inputs of the decision circuit contained in the adaptive circuit and which is controlled by a clock signal to generate a series of current pulses and a disconnect signal and at which outputs The discharge voltage crosses the upper limit voltage or lower limit voltage, the first decision signal or the second decision signal appear, which are directed to the modification circuit contained in the adaptive circuit and in which the disconnect signal is generated, the first decision signal and the second decision signal lead on the third feed the lower input of the circuit for the formation of the lower limit voltage, or the third control input of the circuit for the formation of the upper limit voltage, and the circuits for the formation of the upper limit voltage or the lower limit voltage are interconnected by an adaptive signal that rapidly adjusts one of the limit voltages to the other, if only - changes quickly.

The process and preparation according to the invention for adaptive control of erosion treatment makes it possible to no longer need to adjust the upper and lower limit voltages of the desired discharges and many other process parameters, but are automatically created during the working process for the entire working life at the beginning of the working life. adjusted to the upper and lower boundary voltage curve for liquid discharge. Therefore, the control is simplified and a higher degree of automation of the erosion treatment is enabled, and the influence of the operator is excluded. Adaptation is carried out reliably and with short reaction times of the order of 0.5 Unlike the prior art, the solution according to the invention enables reliable fine processing, ie a working pulse life of less than 10 / xs. The advantage of the solution according to the invention is also shown by the higher processing efficiency, lower electrode wear and the quality surface of the products.

The invention will be further explained in detail on the basis of a description of an embodiment of the method and preparation according to the invention for adaptive control of electro-erosion treatment and on the basis of the corresponding plan, which shows in a schematic manner in FIG. 2a and b show the discharge current of the electrocution in the process of electro-erosion treatment and the flow according to the proposed procedure with self-learning of the obtained upper and lower boundary voltages for the area of desired discharges in this process, or the course of the voltage at two abnormal discharge, Fig. 3a and b show the voltage of the first discharge at the beginning of the erosion treatment and the course of the upper and lower boundary stresses when the self-learning began, or the characteristics of the curves for said boundary stresses, and FIG. 4 is a device according to the invention for carrying out the process of the invention for the adaptive control of the erosion treatment process.

In FIG. 4 shows, among other things, a schematic representation of an edmd device for electro-erosion treatment of a workpiece w. The terminals of the cpg pulse generator are connected to the electrode e and the workpiece w, between which is the liquid dielectric fd. The cpg impulse generator of the current is controlled by a clock signal T whose period t consists of a duty period t _Q and a period of _no pause (Fig. 1 above), with a disconnect signal CO from the output of the adaptive circuit ac, which causes the disconnection of the current pulses from of the cpg generator, and also with the basic control circuit bcc, which also controls the servomotor sm to move electrode e, but does not relate to the present invention and is therefore not explained in more detail. A characteristic parameter in the erosion treatment process is the discharge voltage U between the electrode e and the workpiece w, and to control the machining process, only part of this voltage is removed from the voltage divider vd, which will subsequently act as the discharge voltage U.

The process according to the invention for adaptive control of the process of erosion treatment is based on the realization that in the time dependence diagram the discharge is the area of desired, that is, working, the discharge lies between the area of high-voltage abnormal discharge and the field of low-voltage discharge, and the idea use the result of the self-learning of the device according to the invention on the characteristics of the previous discharges, and on this basis, for the purpose of identifying the liquid discharge, the time course of the upper limit voltage U _H and the time course of the lower limit voltage U _{L are} calculated as the boundary curves of the desired course of the process.

From the beginning of the erosion treatment, the sampling time data of the U discharge currents in this process are stored. Based on the statistical evaluation of these data, the upper boundary voltage U _H (t _g ) and the lower boundary voltage U _L (t) for the area of the desired process runs are calculated for each liquid discharge individually immediately at the beginning of the corresponding service life t _o from time t of the duration of the liquid discharge (Fig. 2a).

In the process according to the invention, the moment when the discharge voltage U at point B at the beginning of the service life t _o falls below the upper voltage limit U _H (t _g ) is taken as t _e = 0 to describe the discharge currents in the model. From now until the end of the service period, the U (t _e ) curve of the discharge current is drawn between the U _H (t _e ) and U _L (t _e ) curves for the upper or lower voltage boundary of the discharge range, respectively. Further details in FIG. 2a will be explained later.

In FIG. 2b, the voltage paths U '(t _e ) and U ”(t) show discharges, which turn out to be abnormal, at the instant t' and t ', respectively, when their voltage path intersects the upper limit voltage curve U _H (t) or lower boundary stresses U _L (t _e ); then the disconnect signal CO in the current pulse generator cpg interrupts the electrical current. The interruption only lasts until the end of his working life in the current period or even for a few subsequent periods in the event of an increased likelihood of abnormal discharge.

The following approximate procedure is used to calculate the U _H (t _e ) and U _L (t _e ) runs for the upper or lower voltage limits of the discharge range, respectively. The working time is divided from the crossing of the voltage U (t _e ) of the current discharge below the upper limit voltage U _H (t _e ) at point B onwards, calculated on the basis of data from the previous discharges, to preferably at least four time sections with the following durations (Fig. 3b ):

- the first time section Δ ₃ lasts up to 2 jus,

- the second time section Δ ₂ takes a further 4 / ac,

- the third time segment Δ ₃ lasts a further at least 54 μ-s and thereafter

- fourth time segment Δ. until the end of his working life t.

o

The specified duration of the time segments was chosen empirically. Function approximations for U _H (t _e ) and U _L (t _e ) for liquid discharge are calculated using the aforementioned time sections Δ _Γ ..., Δ ₄ with the statistical evaluation of the voltage path U (t _e ) for the previous working discharge of the obtained radius curves R _p R ₂ , R ₃ , R ₄ . In fact, each of these radius symbols represents two slightly different radii, namely the radius for the upper and the radius for the lower voltage limit.

Performing the process of the invention with four time sections Δ _ρ ..., Δ ₄ gives good results, and a larger number of time sections would require more extensive hardware. The first time section Δ ₁ is important for mastering fine machining; the initial ionization and its discharge channel are still present, and the voltage U (t _e ) drops sharply. In the second time section Δ ₂ , the ionization channel has already evolved and the voltage U (t _e ) drops less steeply, but still changes rapidly at high current density. In the time section Δ ₃ , the plasmin channel develops definitively and the discharge current reaches a nominal value, and the voltage course U (t _g ) stabilizes. In the time interval Δ _4, however, at t> 100 / us the discharge voltage U (t _g ) of the discharge still drops slightly at higher electric currents, due to the good electrical conductivity of the developed plasma channel.

The operating time t of the current pulse starts when the signal T occupies the state of the logic unit. At this point, the curves U _H (t _e ) and U _L (t _e ) begin at points Bj and C _{t, respectively,} and run in horizontal sections to points B 'and C respectively, and end at points B ₂ and C ₂ at the end of their working lives. (Fig. 3b). Point C is placed at the moment when the voltage U (t _g ) passes through the reference voltage U (at least about 2% below the ignition voltage U) at point A, and point B 'lies at least 2 gs behind point B.

When electro-erosion treatment begins, it becomes known and self-taught. In FIG. 2a, two characteristics are indicated which, during self-learning, are determined at the very beginning of the processing process in order to check whether it is a normal discharge, ie a work discharge; from the beginning of the current pulse to the drop of the initial voltage U _{p of the} electrical pulse below the reference voltage U. a typical minimum time t _d must elapse and, in the case of discharge, the voltage fluctuation shows. After the initial self-study is completed, the weekly characteristics are no longer identified.

At the very beginning of the self-study, the baseline approximations are taken for the above and lower boundary stresses U _H (t _e ) and U _L (t _e ), respectively, which are shown in Figs. 3a. The upper limit voltage is represented by the line U _H (t _g ) = U _Hmax , where U _{Hmax is the} high voltage value below the ignition voltage U, for example 60 V, while in the last two time sections A ₃ and Δ _{4 the} lower limit voltage is represented with a constant value of U _L (t _g ) = U _{r min} , which then rises to the point C with the pre-selected values of ^R 2, min ^{and R} _imin , where U _{Lmjn is the} lowest voltage value, for example 18 V. Treatment at the given initial the upper voltage limit is rougher than the very rough treatment with a current of up to 500 A and a working life of up to 2,000 gs, while the treatment at the indicated initial lower voltage limit is finer than the treatment with a current below i A with a minimum active surface of less than 1 mm ² .

At the very beginning of the machining process, self-learning begins by raising U _L (t _g ) by adapting to the most common working discharges U (t _g ); this process takes up to three seconds. Self-teaching continues to lower U _H (t _g ) by continuing to adapt to the most common occupational discharges U (t _g ); this process takes up to two seconds, of course, depending on the frequency of discharges. Between the two curves U _H (t _g ) and U _L (t), the area or window of the desired discharge is formed. After intensive initial self-learning and the formation of boundary stresses U _H (t _e ) and U _L (t _g ), self-learning and adaptation to the most common and mutually similar occurrences of U (t _g ) occurring discharges continue throughout the entire process of electro-erosion treatment.

This window never adapts to the first modified run of the U (t _e ) work discharge, since its representation is negligible. Adaptation of the window to such a course U (t _e ) is made only after several similar discharges have occurred. The value of their voltage is evaluated according to the mentioned time segments Δ _ρ ..., Δ _{4 of} working life with the determined empirical probability for individual discharges. The evaluation of the U (t _e ) discharge runs is made after enough data have been accumulated in about one second to move the boundary curves U _H (t) and U _L (t _e ) of the window.

The limit voltage U _H (t _e ) or U _L (t _e ), however, is rapidly adapted to a rapid change in the machining process by means of the decision signal L and H, respectively, which follows the passage of the discharge voltage U (t _g ) through the lower boundary voltage U _L ( t _g ) or across the upper limit voltage U _H (t _e ). Accelerated adaptation of one boundary voltage to another is achieved by means of an adaptation signal HL directly between circuits b _H , b _L to calculate the boundary stresses U _H (t _g ) and U _L (t _e ) in the mdc circuit to form a model of the erosion process.

During work, for example, there is a sudden increase in the discharge current. The discharge voltage U (t _g ) rises and exceeds U _H (t _g ). The decision circuit dc detects frequent abnormal discharges and emits a signal H, which in the circuit b _L causes accelerated lifting U _L (t _e ) and consequently, via the adaptive signal HL, also accelerated lifting U _H (t _g ); the adaptation of both the limit stresses U _H (t _g ) and U _L (t _g ) is performed faster than in one second. However, when slower and smaller changes of U (t _g ) occur during processing, the adaptation is performed less intensively over a period of several seconds.

The proposed process successfully detects abnormal discharges over a wide range of the machining process, with operating pulses of between 0.5 pts and 2,000 gs and pulse currents between 0.5 A and 500 A, both independently of the materials involved and the size of the cultivated area.

The apparatus of the invention for carrying out the inventive method of adaptive control of the erosion treatment process is shown in FIG. 4. It consists of an edmd preparation for electro-erosion treatment, an adaptive ac circuit and an mdc circuit to form a model of the erosion treatment process taking place between the electrode e and the workpiece w. From the preparation of edmd, only the voltage of the U discharge signal is guided to the input of the mdc circuit to form the model of the erosion process, as well as to the first input of the adaptive circuit ac.

The adaptive circuit ac consists of the identification circuit ic, the decision circuit dc and the modification circuit mc. The first input of the adaptive circuit ac - its inputs coincide with the inputs of the identification circuit ic - the voltage U of discharge is supplied, and the second and third inputs of the upper and lower boundary voltage signals U _H (t) are fed from the outputs of the mdc circuit to form a model of the erosion process. respectively U _L (t). The mdc circuit produces two self-learning signals based on the input signal U (t) as the discharge voltage.

The mdc circuit for model formation of the erosion process consists of a circuit a, which samples the voltage U (t _g ) of discharge and obtains the curved radii of the upper and lower boundary voltage curve in the individual time segments of the service life for each period of current pulses, and of circuits b _H , b _L to form the upper boundary stress U _H (t _e ) or the lower boundary stress U _L (t) in the aforementioned time sections using the curves obtained previously.

The first input of circuits a, b _R and b _L is led by the voltage U (t _g ) of the discharge, and their first and second control inputs are controlled by a clock signal T to generate a series of current pulses or a disconnect signal CO. The output of circuit a is connected to the second input of circuits ^b H ^inb L ·

In the first comparator c _H and the second comparator c _L contained in the identification circuit ic, the detection of abnormal discharges is made by comparing the voltage U (t _g ) of the discharge with the upper or lower boundary voltage U _H (t _e ), respectively. , U _L (t _e ) for the area of desired discharges.

The output signals of said comparators c _H , c _L , hence the identification circuit ic, are guided to the inputs of the decision circuit dc. It is also controlled by a T clock signal to generate a current pulse sequence and a CO disconnect signal. At its two outputs, whenever a voltage U (t _g ) exits the range of operating discharges by crossing the upper limit voltage U _H (t _g ) or the lower boundary voltage 11 _L U (t _e ), the decision signal H or L. The decision signals H and L are guided to the modification circuit mc in which the frequency of abnormal discharges results in a correspondingly long disconnect signal CO.

On the other hand, the decision signals H and L are directed to the third control input of the circuit b _L for the formation of the lower limit voltage U _L (t _g ) or to the third control input of the circuit b _H for the formation of the upper limit voltage U _H (t _e ). The circuit b _H , b _L for the formation of the upper limit voltage U _H (t _e ) or the lower limit voltage U _L (t) are interconnected with the line for the adaptive signal HL, which rapidly adjusts one of the limit voltages U _H (t _e ), U _L (t _e ) the second if it changes rapidly.

Claims (8)

Patent claims 1. A method for adaptively controlling an erosion treatment process, according to which the process of this process is automatically adjusted to the desired course of the process due to its constant variation, according to the frequency of abnormal discharges between the electrode (s) and the workpiece (w), so that detects abnormal discharges interferes with the processing process with the frequency of abnormal discharges corresponding to the long-term disconnection of current pulses, characterized in that, as a lower limit of the upper region of occurrence of abnormal discharges, and an upper limit of the lower region of occurrence of abnormal discharge based on stored data, of the operating discharge in this process, for each liquid discharge separately, the upper limit voltage U _H or the lower limit voltage U _L, respectively, are calculated from the duration of the current discharge, and that, as an abnormal r azelectricity detects a liquid discharge as soon as its voltage U leaves the said region of the desired course of the process by exceeding one of the two boundary voltages U _H , U _L. Method according to claim 1, characterized in that the time dependent voltage limits U _H , U _L in each of at least four time segments of the service life in each period of the current pulses are calculated by means of said time sections by statistical processing of the time course of the voltage U previous working discharges of their radius curves obtained over these time segments. Method according to claim 2, characterized in that in the case of dividing the service life in each period of the current pulses into four time sections, the first time section lasts up to 2 ps, starting from the passage of the discharge voltage U below the upper limit voltage U _H at the very beginning of the current pulse, and lasting a second time section of a further $ 4 μ, a third time section of at least a further $ 54 μ, and then a fourth time section until the end of the service period in that period of current pulses. Method according to one of the preceding claims, characterized in that the limit voltage U _H or U _{L is} rapidly adapted to a frequent rapid change in the machining process by means of the lower decision signal L or the upper decision signal H following the passage of the discharge voltage U through the lower the limit voltage U _L, or via the upper limit voltage U _R , and with the help of an adaptive signal HL, which rapidly adjusts one of the limit voltages U _H , U _{L to the} other limit voltage, if it changes rapidly. 5. A device for carrying out the process for adaptive control of the erosion treatment process according to one of the preceding claims, existing from the erosion treatment device (edmd) and from the adaptive circuit (ac), to which the first input is supplied by a voltage U of discharge and from which output the control input of the generator (cpg) of the current pulses is guided by the disconnect signal CO, characterized in that the voltage U of the discharge is also directed to the input of the circuit (mdc) to form a model of the erosion process, and that the above signal is obtained in this circuit (mdc) by self-learning. the lower limit voltages U _H and U _{L, respectively,} for the area of the desired discharge lead to the second and third inputs of the adaptive circuit (ac). Device according to claim 5, characterized in that the circuit (mdc) for generating a model of the erosion process consists of a circuit (a) for sampling the discharge voltages and for calculating the curved radii of the limit stresses U _H , U _L in individual time sections of the service life in each period of current pulses and from circuits (b _H , bj to form the upper limit voltage U _H or the lower limit voltage U _L in the mentioned time sections of the service life, so that the voltage U is discharged at the first input of these three circuits and at their first and the second control input control the clock signal T to generate a series of current pulses or a disconnect signal CO, and that the output of the circuit (a) is for sampling the discharge voltages and for calculating the curvature radii of the limit voltage U _H , U _L in individual time sections of service life in each period of current pulses connected to the second input of the circuits (b _H , b _L ) to form the upper limit voltage U _H or the lower limit voltage U _L . Device according to claim 5 or 6, characterized in that the upper and lower boundary voltage signals U _H and U _L, respectively, are fed from the second and third inputs of the adaptive circuit (ac) to the first input of the first comparator (c _H ) or to the first input the second comparator (c _L ) contained in the identification circuit (ic) of the adaptive circuit (ac) and to which the second input is supplied from the first input of the adaptive circuit (ac) by a voltage U of discharge and whose output signals are directed to the inputs of the decision circuit ( dc) contained in an adaptive circuit (ac) and which is controlled by a clock signal T to generate a series of current pulses and a disconnecting signal CO and at which outputs always occur when the discharge voltage U exceeds the upper boundary voltage U _H or the lower boundary voltage voltage U _L , the first decision signal H or the second decision signal L occurs, which are guided to the modification circuit (mc) contained in the adaptive circuit (ac), in which the disconnect signal is generated. ala CO., LTD. Device according to one of Claims 5 to 7, characterized in that the first decision signal H is directed to the third control input of the circuit (b _L ) to form the lower limit voltage U _L and the second decision signal L is directed to the third control input of the circuit (b _H ) for forming the upper limit voltage U _H, and that the circuits (b _H , b _L ) for forming the upper limit voltage U _H or the lower limit voltage U _L are interconnected with the line for the adaptive signal HL, which rapidly adjusts one of the limit voltages U _H , U _L second limit voltage if it changes rapidly.