EP0305574B1 - Method and circuitry for controlling the consistency of fresh concrete in a fixed concrete mixing device - Google Patents

Abstract

According to the method of the invention, the final power value which occurs at the end of the mixing process is forecast during the mixing process as early as the water inflow phase with a discrete Kalman filter, using a relationship between the mean power value of the mixing motor, which correlates with the consistency, and the quantity of water currently admixed. This forecast final power value is compared to a desired final power value, which is fixed as a device and formulation parameter, and the difference is used to determine the residual quantity of water required and hence for controlling the water valve. The circuitry consists essentially of an active-power measuring device (13), which is connected via a time-controlled electronic change-over switch (14) to a first-order Kalman filter (15) or via measuring-transducer units (16, 17) to a third-order Kalman filter (20) and a circuit (23) for determining the final value. The signal present at the output of the circuit (23) for determining the final value, which signal characterises the consistency, is compared to a device and formulation parameter, fixed in a read-only memory, (25) by means of a differential amplifier (24). An evaluation circuit (27) uses the difference to derive the water-valve control times of the mixing installation (5).

Description

The invention relates to a method and a circuit arrangement for controlling the consistency of fresh concrete in stationary concrete mixing plants, the amount of water required to ensure the target consistency being adaptively determined and supplied during the mixing process.

In order to achieve reproducible quality properties of the concrete in the usual batch-wise production of fresh concrete, it is necessary to exactly adhere to the quantity relationships and properties of cement, water, additives and other additives required by the concrete recipe. A precise analysis of all components would therefore be necessary before each mixture, but is not feasible in practice for time and economic reasons.

The water-cement ratio and the consistency of the fresh concrete have a particular influence on the concrete quality. Both are decisively influenced by the intrinsic moisture of the aggregates, which, however, can vary greatly in practice due to storage options, storage conditions, weather conditions, etc. So that the quality of the hardened concrete is always ensured, the risk of fresh concrete production resulting from the unknown moisture content of the aggregates has been reduced by adding cement. An obvious procedure for determining the intrinsic moisture of the additives is to use moisture measuring probes, e.g. B. microwave or neutron probes. With these Probes measure the moisture of the fine-grained aggregates directly before each mixture and these values are used to calculate a corrected amount of water. Disadvantages of these probes are the high cost and the susceptibility to failure due to the complicated structure. The inhomogeneity of the material to be measured, which may additionally contain foreign bodies, is the reason why the sole use of moisture measuring probes has not hitherto led to systems which are effective in practice. Methods have therefore been developed in which the measurement of the inherent moisture and the measurement of the consistency are combined. It is known, for example, to calculate the necessary amount of water with the measured initial moisture, to add a major part of the amount of water and to add the rest at a time during the mixing phase at which a predetermined consistency value occurs (DD-WP 141 129).

The consistency measurement is usually carried out indirectly by measuring the electrical resistance of the material to be mixed (DE-OS 1 784 920) or by determining the active power of the drive motor of the mixing drum. The effective power of the motor is proportional to the frictional resistance of the mix (this value corresponds to the consistency) (DE-OS 1 683 778).

• 1. konstanter Verlauf während der Trockenmischphase,
• 2. Anstieg, Maximum und Abfall während der Zeit der Wasser­zufuhr und -untermischung,
• 3. konstanter Endwert, wenn das gesamte zugeführte Wasser untergemischt ist.

The measurement of the active power as a function of time during the mixing phase results in a characteristic curve which has three essential sections in accordance with the current degree of consistency of the mixture

• 1. constant course during the dry mixing phase,
• 2. increase, maximum and decrease during the time of water supply and mixing,
• 3. Constant final value when all the water supplied is mixed in.

DE-OS 2 855 324 describes a method which uses this curve profile. For each recipe, consistency setpoints are determined for an ideal mixture at time-equidistant intervals, these are converted into active power values and saved in a table as setpoints. The required amount of residual water is determined by comparison with corresponding measured values.

Disadvantages of this method are that a large number of measured values must be determined and stored for each recipe. In addition, the quantitative curve shape and the temporal position of the curve maximum depend on the initial moisture, sieve line and grain geometry of the aggregates, and secondly, the measured values are falsified by harmonic and stochastic disturbances of the mixing plant. This makes it difficult to calculate the amount of residual water required. An exact determination of the amount of residual water is absolutely necessary, because too much water is used to make unusable concrete.

DE-AS 2 432 609 proposes to keep the inflow rate of the water very low. However, this leads to uneconomical mixing times, and there is also the risk of segregation of the concrete. Try other methods, a defined initial moisture z. B. by saturating the additives with water and then controlled partial removal of the water (DE-OS 2 756 039) or by supplying the water in vapor form (DE-OS 1 950 910). These processes require additional equipment and increased time per mixture.

Furthermore, it is proposed in DE-A-2124697 and in US-A-3593966 that in a material processing system the current taken up by the drive motor of the mixer is reciprocally proportional to the water content of the processed material, ie the plasticity of the material, so that for a regulation of the water content is merely an actual value.

The aim of the invention is to adaptively determine the expected consistency value of the mixture at the earliest possible time by means of a method and a circuit arrangement for controlling the consistency of fresh concrete, so that the amount of water required to achieve the desired consistency is supplied to the concrete mixture in a controlled manner already during the mixing phase can, whereby an overdose of cement can be avoided. The purpose of the invention is furthermore to avoid water metering errors which can result from the unknown initial moisture and from quantitative changes in a determined active power-time curve.

The invention has for its object to provide a method and a circuit arrangement for controlling the consistency of fresh concrete, which during the wet mixing phase, ie. H. From the beginning of the water supply, the expected consistency value of the mix currently in the drum is determined at discrete time intervals. The target-actual value comparison for controlling the water supply is to be reduced to the comparison of only a known consistency value, namely the stationary end value according to the recipe, with the adaptively determined predicted end value.

The circuit arrangement is also intended to enable the expansion of conventional conventional concrete mixing plants, so that automatic control of the consistency of the fresh concrete to the consistency setpoint is possible in a time-optimized manner by means of controlled water metering.

Based on the knowledge that the consistency of the material to be mixed is determined only by the amount of water currently mixed in and the directly measurable state quantity - mixed quantity of water - is supplied by the measurable state quantity Amount of water - can be described, it is possible to carry out the consistency control process described below. Furthermore, a relationship between the course of the active power as a function of the amount of water mixed in was found and used to implement the control method.

The stated object is achieved according to the invention by the method according to claim 1 and by the circuit arrangement according to claim 3.

An advantageous development of the method is characterized in claim 2.

According to the invention, therefore, before the beginning of the actual mixing phase, a final power value, i. H. a value representative of a corresponding degree of filling of the mixing device or of a recipe specification is determined. The expected final output values are determined once according to the respective filling level and can be fixed as a system parameter. During the loading of the mixing plant, which takes place in a known manner, a dry mixing power value is also determined, for example by determining the active power of the drive motor of the mixing plant.

The dry mix output value and the final output value corresponding to the respective degree of filling or the recipe specification are stored for the further implementation of the consistency control process. Accordingly, at the beginning of the dry mixing process there is a final power value and a dry mixed value in the memory of the mixing plant. From the beginning of the dry mixing process to the end of the mixing, for example, also the determination of the active power of the drive motor of the mixing plant, an average power value is recorded at equidistant time intervals. The actual dry mixing phase takes place in a known manner. During the dry mixing phase, according to the invention, within the consistency control process, the power averages (measurements) are processed using a suitable discrete Kalman filter of the 1st order in such a way that a pseudo actual value, ie a value approximating the actual power value at the end of the dry mixing phase is derivable.

The determination of the pseudo actual value and the comparison with the respective power mean (measurement) value leave incorrect doses, i. H. Detect loading errors and gross deviations from the permissible nominal moisture content of the additives immediately.

At the start of the water supply, the derived pseudo actual value is used as the initial value of a first component of a state vector containing three components. The other components can be freely selected within the system under consideration.

According to the above-mentioned knowledge, the measured power means (measured) values are used to transfer the measurable state variable - amount of water supplied - to the non-measurable state variable - mixed quantity of water - which ultimately determines the consistency of the mix, within the time period from the start of the water supply until the end of the water supply. These values now available represent the dependency between the performance of the motor of the mixing plant and the amount of water mixed in. The control system can be completely described on the basis of the further knowledge mentioned at the outset, ie the depictable relationship between the performance and the amount of water mixed in. Using a suitable discrete 3rd order Kalman filter, control components are provided as estimated values, ie it is determined which values are to be expected at the time of the next power measurements. These control components correlate with the expected final power value. The control variable for stopping or continuing the water supply can be derived with high accuracy from the comparison of the expected end value with the stored power end value.

If an identical recipe is processed in the next mixing process, it is expedient to control the determined control components, i. H. the final estimated value, to be recorded at the time of the total mixing of the water, to transfer this value to the initial value and as a starting component, d. H. to be used as an initial estimate in the tax process. Such a procedure ensures a short settling time of the overall control process and has a positive influence on the mixing process in terms of time and quality.

• Fig. 1 ein Diagramm zur Veranschaulichung der Verfahrensschritte; und
• Fig. 2 ein Schaltschema der Schaltungsanordnung.

The invention is explained in more detail with reference to the drawing; show in it:

• 1 shows a diagram for illustrating the method steps; and
• Fig. 2 is a circuit diagram of the circuit arrangement.

The time sequence of the method steps according to the invention will now be explained in more detail using an exemplary embodiment and FIG. 1.

1a shows the time course of the active power of the motor of the mixing plant with superimposed interference signals during the mixing of a conventional recipe.

The average active power of the drive motor is advantageously determined by measuring current and voltage at time-equidistant intervals and calculating the product sums.

On the abscissa of Figure 1, the time marks t₀ to t₆ are given, which correspond to the essential steps of the inventive method.

Here mean:

t₀

- start of loading

t₁

- Start of the dry mixing phase

t _1.1

- Start of the comparison of the initial value of the filter with the system parameters

t₂

- End of the dry mixing phase and start of the water supply and mixing phase

t₃

- Time of correspondence between forecast and target end value

t₄

- End of the mixing phase

t₅

- Start of the learning phase

t₆

- End of the learning phase

To use the method, it is necessary to produce a calibration mixture with known quantity relations and properties of all components used once for each concrete formulation and to fix the performance value N₂ at the time stamp t₂ and the performance value N₆ as system and formulation parameters at the time stamp t₆.

In a known manner, the mixing drum is loaded in the time phase from t₀ to t₁ with the dry components of the concrete and a small amount of water, which determines the initial moisture content W₀ with the unknown moisture content of the additives.

In the dry mixing phase t₁ to t₂, the dry components are mixed. The active power N correlating with the consistency is constant for this. The numerical size of these constants varies depending on the initial moisture W₀ and on the weighing inaccuracies.

This constant, which is directly difficult to measure due to the superimposed interference, is predicted with the use of a discrete Kalman filter according to the invention.

With the help of the simple model N _{k + 1} = N _k and the recorded measured values, which are superimposed with disturbances of the variance R, the estimated value N̂ _{k + 1 of} the active power and the error covariance matrix P _{k + 1 are} determined. The values N̂ _k are shown in Figure 1b.

From time t _1.1 onwards, the estimated value N̂ _{k + 1 of} the filter is compared with the determined system and recipe parameters N₂.

In the case of a gross deviation, either a loading error has occurred or there is an unusually high intrinsic moisture content of the starting materials. Appropriate reactions take place in both cases. The power value N̂₀ predicted at the time t₂ correlates with the initial moisture W₀ and is used as the initial value for the estimation of the other power values N̂ during the following phases.

indirekt bestimmt, wobei α die Zeitkonstante der Mischanlage darstellt.With the start of the feed and mixing phase from t₂ to t₄ water flows at a constant flow rate A _w into the mixing drum (Fig. 1c). According to the knowledge that only the currently mixed amount of water W₂ determines the consistency, the amount of water mixed with the relationship
$${\text{W₂ (t) = A}}_{\text{w}}{\text{t - A}}_{\text{w / α}}{\text{· (1-e}}^{\text{-αt}}\text{) + W₀}$$

indirectly determined, where α represents the time constant of the mixing plant.

The dependency of the active power N on the mixed amount of water W₂ is described by a 2nd degree polynomial as follows

Since the constant A of the polynomial is also unknown, the relationship found is shown using an extended state vector with 3 components in discrete form, whereby it must be noted that the power values N now depend on W₂ (N = N (W₂)):

Nichtäquidistanz in W2 auftritt.D is the non-equidistant step size between two performance values, as a result of the transformation of the performance values

Non-equidistance occurs in W2.

With the measurement equation yk = (1 0 0) N _k + V _k , the Kalman relationship can be established for a 3rd order filter. This leads to the optimal forecast of a state vector

bestimmt.In a further method step, the predicted state vector N̂ _K and the total amount of water W _G given by the recipe become the final power value according to the relationship

certainly.

The course of the predicted final value N̂ end during the wet mixing phase is shown in FIG. 1d.

It has been found that at the latest at time t 3, which is still within the water supply phase, the predicted end value coincides with the actual end value occurring later in time with sufficient accuracy. Therefore, a comparison between the target end value N₆ and the predicted end value N̂ _{end is} carried out at this time. The difference between the two serves as a criterion for controlling the water inlet valve.

If the difference between the target end value and the predicted end value at the time of the comparison is greater than zero, the water supply valve is closed before reaching the specified amount of recipe water, in the opposite case the valve must be closed later.

The duration of the changed valve timing is determined from the absolute value of the difference.

After closing the water supply valve, the remaining water is mixed in in the period t Zeitraum to t₅, whereby the following relationship now applies according to the relationship found:

ermittelt. Zum Zeitpunkt t₅ ist das gesamte Wasser untergemischt, und es kann nochmals ein Vergleich von N̂ end mit N₆ zur Qualitätskon­trolle durchgeführt werden.To control the mixing process, the state vector continues to be used with the method step described

determined. At time t₅, all of the water is mixed in, and a comparison of N̂end with N₆ can be carried out again for quality control.

zur Bestimmung der Kom­ponenten des Zustandsvektors

verwendet werden. Erfindungs­gemäß wird die Komponente N̂₀ während der Trockenmischphase ermittelt. Die Komponente  ist eine Konstante und kann direkt übernommen werden. Die Komponente N̂ ₀ wird auf Grund des ge­fundenen Zusammenhanges mit der Beziehung

bestimmtIf several batches of the same recipe are mixed in series, the state vector can be used

to determine the components of the state vector

be used. According to the invention, component N̂₀ is determined during the dry mixing phase. The component  is a constant and can be adopted directly. The component N̂ ₀ is based on the relationship found with the relationship

certainly

At the same time, the final values of the components of the error covariance matrix of the Kalman filter are converted in a suitable form and used as initial values at time t 2.

Fig. 1e shows the progression of the predicted end values N̂ _end during a further mixture if the above learning process took place in the period between t₅ and t₆.

For the circuit arrangement according to the invention for controlling the consistency, a system metering device known per se is expanded by an output and two inputs, the expansion inlet with the output of the timing control, the first extension input with the input of the emergency switch and the second extension input with the control input of the water valve of the known one Plant are connected. Current and voltage transformers are arranged in the feed lines of the drive motor of the mixing plant. The output of the converter unit formed here is connected to the input of an active power measuring device. The output of the active power measuring device leads to the switching input of an electronic switch with three outputs. The control input of the electronic switch is connected to the extension output of the known system metering device.

The first output of the electronic switch leads to a first order Kalman filter, the second output to a first transducer unit and the third output to a second transducer unit. The output of the first order Kalman filter is connected both to a first input of a first differential amplifier and to a first initial value setting input of a third order Kalman filter. The second input of the first differential amplifier is connected to the output of a first read-only memory. The output of the first differential amplifier leads to the input of a first evaluation circuit, the output of which is linked to the first extension input of the system metering device.

The outputs of the first and second transducer units are connected to one another and lead to the information input of the third order Kalman filter, its second initial value set input leads to the output of a computing circuit and its third initial value setting input is connected to a first output of a final value determination circuit, the second output of which leads to the input of the computing circuit. A first, second and third input of the final value determination circuit are connected to the first, second and third output of the third order Kalman filter. The third output of the final value determination circuit is connected to the first input of a second differential amplifier, the second input of which leads to the output of a second read-only memory. The output of the second differential amplifier leads to a second evaluation circuit, the output of which is connected to a display unit and to the second extension input of the system metering device.

The operation of the circuit arrangement according to the invention is described on the basis of the time course of a typical mixing process according to the consistency control method already described.

After loading the mixing drum with the quantities of additives and binders corresponding to the recipe, the measured value of the current active power is available at time-equidistant intervals at the output of the active power measuring device. The electronic changeover switch is switched during the dry mixing phase so that the input is connected to the first output and thus the current active power measurement value is present at the input of the Kalman filter of the first order and the filtered active power measurement value is available at its output. This value is compared in the first differential amplifier with the first one-time determined and fixed system and recipe parameter, which is stored in the first read-only memory. At the end of the dry mix phase, the electronic changeover switch is switched via its control input by the system dosing device in such a way that the switch input is connected to the second output of the changeover switch. At the same time, it is checked whether the output signal of the first differential amplifier exceeds a system-specific threshold. The accident is also triggered, otherwise the filtered active power measurement value is taken over at the end of the dry mixing phase via the first initial value input into the third-order Kalman filter. With a first mixture, any initial values or those selected depending on the result of the previous mixture are present at the second and third initial value setting inputs, the output signal of the arithmetic circuit being present at the second setting input and the output signal of the first output of the final value determining circuit being present at the third setting input, thereby improving the accuracy of the Output signals of the third order Kalman filter is reached.

At the input of the third-order Kalman filter, there is the output variable of the first transducer unit, in which the active power measured value is transformed with the aid of a defined relationship between the amount of water added and the amount of water mixed in during the time of water supply. The filtered values describing the consistency are available at the outputs of the third order Kalman filter. These values are led to the inputs of the final value determination circuit, with which the values which characterize the consistency of the material to be mixed at the end of the mixing phase are already determined during the mixing phase. The signal representing the final active power is present at the third output of the final value determination circuit and is used in a second differential amplifier with the second fixed system and recipe. parameter is compared. The output signal of the second differential amplifier is fed to a second evaluation circuit. The output of the second evaluation circuit is connected to a display unit which is used for the visual control of the control variable for the water metering. Furthermore, the output of the second evaluation circuit is connected to the second extension input of the system metering device, the opening time of the water valve being controlled as a function of the sign and size of the output signal. After the water valve is closed, the electronic switch is brought into a third state. The active power measured values now reach the information input of the third order Kalman filter via the second measured value converter unit, in which the measured values are transformed according to a defined relationship that applies when the water is only mixed in. The predicted final values are provided in an analogous manner and can be passed from the first output of the final value determination circuit and from the second output via the arithmetic circuit to the third-order initial value setting inputs of the Kalman filter at the end of the overall mixing phase and can be used on the display unit to control the final consistency value reached.

The circuit arrangement described above is to be explained in more detail using an example and FIG. 2.

The mixing drum 5, which is mechanically connected to the drive unit 6, is fed via the silos 1, 2 and 3 and the water valve 4. The metering of the additives and the binder according to the present recipe is carried out by the known system metering device 28. For this purpose, the outputs A₂, A₃ and A₄ of the system metering device 28 with the elements required for this Silos 1, 2 and 3 connected. The control of the water valve 4 takes place via the connection of the output A₁ of the system metering device 28 with the water valve 4. In the feeder lines of the drive unit 6, the current transformers 7, 9 and 11 and the voltage transformers 8, 10 and 12 are switched on. The outputs of the converter lead to the inputs of the active power measuring device 13. The output of the active power measuring device 13 is connected to the input E₃ of the electronic switch 14. The input E₄ of the electronic switch 14 is connected to the extension output A₅ of the system metering device 28. Furthermore, the output A₆ of the switch 14 are connected to the input of the first order Kalman filter 15, the output A₇ to the input of the first transducer unit 16 and the output A₈ to the input of the second transducer unit 17. The output of the first order Kalman filter 15 leads to the input E₅ of the first differential amplifier 19 and to the input E₇ of the third order Kalman filter 20. The input E₆ of the first differential amplifier 19 is connected to the output of the read-only memory 21. The output of the first differential amplifier 19 is connected to the input of a first evaluation circuit 18 and the output of this evaluation circuit to the first extension input E 1 of the system metering device 28. The outputs of the first and second transducer units 16, 17 lead to the input E _{10 of} the third order Kalman filter 20. The input E₈ of the Kalman filter 20 is connected to the output of a computing circuit 22 and the input E₉ is connected to the output A₉ of a final value determination circuit 23 . The outputs of the Kalman filter 20 are connected to the inputs of the final value determination circuit 23. The output A _{10 of} the final value determination circuit 23 leads to the input of the computing circuit 22, the output A _{11 of} the final value determination circuit 23 connected to the input E _{12 of} the second differential amplifier 24, and the input E _{11 of} this differential amplifier 24 is connected to the output of the second read-only memory 25. Furthermore, the output of the second differential amplifier 24 is connected to the input of the second evaluation circuit 27. The output of the evaluation circuit 27 is connected to the second expansion input E₂ of the system metering device 28 and to the input of the display unit 26.

Claims (16)

1. A method of controlling the consistency of fresh concrete in stationary concrete-mixing plants, by measuring the active power of the motor driving the mixing plant, characterised in that
- a final power value representing a specified formulation or a corresponding filling ratio of the concrete-mixing plant is determnined once before the beginning of the mixing phase and is set as the plant parameter,
- a dry-mixing power value is obtained while the concrete-mixing plant is being loaded in known manner,
- the dry-mixing power value and the final power value determined in accordance with the respective filling ratio or specified formulation are stored,
- a mean power measurement is made at equidistant time intervals from the beginning of the dry-mixing phase to the end of the mixing operation,
- during the dry-mixing phase, which occurs in known manner, the mean power measurements are processed at equidistant time intervals in a discrete first-order Kalman filter in such a manner that a pseudo-actual value near the actual final power value at the end of the dry-mixing phase is forecast,
- the forecast pseudo-actual value is compared with the respective stored final power value and, if any difference is detected, known steps are taken to correct faulty metering or loading or deviations from the permitted set natural moisture content of the aggregates,
- at the beginning of the water supply operation, the forecast pseudo-actual value is used as the initial value of a first component of a vector of state containing three components and, by means of the measured mean power, the measured variable of state -­the amount of water supplied - is transferred to the non-measurable variable of state - the amount of water already present - within the time between the beginning of the water supply operation and the end of the mixing phase, and
- the expected final power value is derived by means of a discrete third-order Kalman filter, after which the expected value, with no change in the water supply, is compared with the stored final power value and the water supply is influenced in known manner on the basis of the comparison.

2. A method according to claim 1,
characterised in that
when a number of mixtures of identical formulation are made in succession, the forecast final values are transformed into the initial forecast value, constituting a learning phase of the system.

3. A circuit arrangement for controlling the consistency of fresh concrete in stationary concrete-mixing plants, using a plant metering device, characterised in that

- the plant metering device (28) comprises an extension output connected to a time control means (14), a first extension input connected to the damage switch and a second extension input connected to the control input of a water valve (4),

- the supply lines to the motor (6) of the concrete­mixing plant (5) contain a transducer unit made up of a current and voltage transformer (7-12), and the transducer unit is connected to the input of an active-­power meter (13),

- the output of the active-power meter (13) is connected to the switching input of an electronic change-over switch (14),

- the control input of the electronic change-over switch (14) is connected to the extension output of the plant metering device (28),

- the first output of the electronic change-over switch (14) is connected to a first-order Kalman filter (15), the second input is connected to a first measurement transformer unit (16) and the third input of the electronic change-over switch (14) is connected to a second measurement transformer unit (17),

- the output of the first-order Kalman filter (15) is connected to the first input of a first differential amplifier (19) and to the first initial-value setting input of a third-order Kalman filter (20),

- the second input of the first differential amplifier (19) is connected to a first fixed-value store (21),

- the output of the first differential amplifier (19) is connected to the input of a first evaluation circuit (18), and the evaluation circuit (18) is connected to the first extension input of the plant metering device (28),

- the output of the first measurement transformer unit (15) and the output of the second measurement transformer unit (17) are connected to the information input of the third-order Kalman filter (20),

- the second initial-value setting input of the third-­order Kalman filter (20) leads to the output of a computer circuit (22) and the third initial-value setting input of the third-order Kalman filter (20) is connected to the output of a final-value determination circuit (23), whose second output is connected to the input of the computer circuit (22),

- a number of inputs of the final-value determination circuit (23) are connected to the outputs of the third­order Kalman filter (20),

- the third output of the final-value determination circuit (23) is connected to the first input of a second differential amplifier (24) and the second input of the second differential amplifier (24) is connected to the output of a second fixed-value store (25), and

- the output of the second differential amplifier (24) is connected via a second evaluation circuit (27) to the second extension input of the plant metering device (28) and the input of a display unit (26).

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