EP0095657B1 - Process for the automatic control of foundry sand plants - Google Patents

Description

The invention relates to a method for the automatic control of foundry sand processing plants, in which the moisture and the compressibility of the foundry sand are measured.

Clay-bound foundry sand is usually made after its use, i.e. after pouring, returned to a processing plant, where sufficient amounts of water, binders (e.g. bentonite), additives (e.g. coal dust, starch) and new sand are added to the sand.

The aim here is to proportion the individual additives so that the foundry sand has a uniform quality. However, the load on the foundry sand (for example the thermal load, sand loss) fluctuates depending on the respective production program, so that foundry sand with fluctuating properties or different sand characteristics is constantly returned to the processing plant. The goal of a well-functioning preparation is therefore always to recognize the fluctuations in the old sand and to add the additives accordingly during the mixing process.

According to a printed prior publication by Trischberger KG on February 21, 1978, it is known to measure the moisture of the foundry sand before mixing or in the mixing device, preferably while determining the temperature at the same time. From the brochure GI 910 from Eirich from February 1979, a combined measuring system is also known, in which the moisture and the density of the sand are measured on a capacitive basis at the beginning of the mixing time in the mixing device. This method is considered the closest to the present invention. It is true that one can achieve an early measurement with these two aforementioned systems, i.e. even before mixing, or at least at the beginning of the mixing time, so that a correction of the moisture content of the sand is still possible in a relatively simple manner during the mixing time.

In the case of the first pure moisture measurement, however, it is disadvantageous that here only the moisture is measured as the only variable, without taking into account the other fluctuations or variables in the foundry sand. In addition to the moisture of the sand, the second-mentioned combined measuring system also takes into account its compressibility or deformability, which in turn is related to the bulk density, but disadvantageously the measuring device has to be readjusted when the sand characteristics change. It goes without saying that time-consuming measures are required for this.

Systems are also known in which the compressibility and / or deformability of the sand during the mixing time are determined by sampling from the mixing device or after mixing. With this measuring method, direct results of one or the other sand property important for the processor are obtained. However, it is disadvantageous that the correction of the water addition during the mixing time must prolong it because the addition takes place in stages and the addition quantity must be mixed in first before each new measurement.

All known systems and methods have the disadvantage that the compressibility of the sand can be determined if necessary, but the causes which lead to a change in the compressibility cannot be determined.

The object of the invention is therefore to create a method of the type mentioned at the beginning with which the causes of the change in the compressibility are determined and a re-calibration is possible taking these causes into account

This object is achieved. solved by specifying a setpoint for given sand characteristics, both for the compressibility (C) and for the moisture (Z ') according to a first calibration line (I), the actual value (Y) of the compressibility (A) in one measured in the first stage and by changing the moisture (from Y 'to Z') to the moisture setpoint (Z '), then the cross-sample is again measured in a second stage, the actual value (X) of the compressibility (B) and the difference ( BC) between the last determined (B) and the target compressibility (C) is used as a correction variable (d) for adjusting the calibration line (I) to a second calibration line (11), which is then used in the first measuring stage as a new calibration line for the Actual value measurement is used. In this way you get information about the causes that lead to a change in the measured compressibility of the sand, and you create an automatic control loop also for changing sand properties. In essence, this is achieved by means of two simple measurement stages, a measurement difference which may have been found being used as a correction factor for the adjustment of the first measurement stage. In this way, the measuring process is automatically adapted to shifts in the sand composition.

It is particularly expedient if the first measuring stage is at the beginning of the mixing process and the second measuring stage is towards the end of the mixing process. In the meantime, the moisture content of the foundry sand that has just been measured and is being treated can be changed.

Practice shows that after this change in the moisture of the sand to the moisture setpoint, the last actual moisture value is at the latest from the second or third measurement with the method according to the invention in the vicinity of the setpoint. As a rule, therefore, the calibration lines known per se are adjusted in the first measuring stage at the start of operation in the first batches. In theory, however, any difference that is determined will actually only become effective for the next batch, because the second measuring stage is towards the end of the mixing process; a measure that has proven itself in practice, since the change in sand composition does not usually occur suddenly, but can be recognized as a tendency in good time. In an advantageous further embodiment of the invention, the displacement of the calibration line in the first measuring stage is permitted within a predetermined dead zone. The measuring line is understood to mean the approximately linear dependence of the compressibility on the water content of the foundry sand with the respective sand characteristics, preferably sludge content. This calibration line can shift depending on the sand characteristics. If this tolerance range is specified, then the difference determined when this tolerance limit is exceeded is used according to the invention to meter individual additional mixture components in front of the mixer differently or to correct their adjusted dosage so that the sand composition or its characteristics in turn leads to the desired setpoint or at least brought back into the tolerance range.

It is expedient if, according to the invention, the tolerance range for the displacement of the calibration line is determined by the sludge content of the sand. This is a physically known fact, but in connection with the invention means an evaluation for the corrective measures. In other words, it has been found that the sludge content of the sand is a particularly important parameter, and if you can correct it automatically, you have already largely achieved the desired goals.

As already briefly mentioned above, it can be particularly advantageous according to the invention if, when the tolerance range is exceeded, a measured variable is delivered to the dosing computer for adding new sand, additive, etc. Although the specified tolerance range is usually set and specified as a empirical value with a reasonable size in a system in operation, so that this range is generally not exceeded, in exceptional cases and when the foundry sand is subjected to exceptional loads, a corresponding dosage correction may be necessary. which is automatically controllable by the measures according to the invention in the manner described above.

• Figur 1 ein schematisches Diagramm des an sich bekannten Zusammenhanges zwischen der Verdichtbarkeit von Gießereisand und seinem Feuchtigkeitsgehalt, bei verschiedenen Schlämmstoffgehalten,
• Figur 2 schematisch eine Eichgerade mit einem Istwert und einem Sollwert,
• Figur 3 die gleiche Darstellung wie bei Figur 2, wobei jedoch der Toleranzbereich und die Berücksichtigung der Verschiebung der Eichgeraden dargestellt sind, und
• Figur 4 schematisch eine Anlage zur Durchführung des Verfahrens nach der Erfindung.

Further advantages, features and possible uses of the present invention result from the following description in conjunction with the drawings. Show it:

• 1 shows a schematic diagram of the known relationship between the compressibility of foundry sand and its moisture content, at different sludge contents,
• FIG. 2 schematically shows a calibration line with an actual value and a setpoint,
• Figure 3 shows the same representation as in Figure 2, but showing the tolerance range and the consideration of the displacement of the calibration line, and
• Figure 4 schematically shows a plant for performing the method according to the invention.

The deformability of the sand is directly related to the bulk density (in kg / l) or the compressibility (in%) and is a very important factor for the foundry specialist. In Figure 1, the dependence of the compressibility in% on the moisture content in% water is shown as a schematic example. For example, there is a calibration line I for a sludge content of 8%, a calibration line 11 for a sludge content of 10% and a calibration line 111 for a sludge content of 12%. You can see how the straight line shifts as the sludge content changes. In other words, for example, increasing the sludge content and increasing water contents are necessary. Further findings from the theory that these curves are steeper the lower the sludge content of the sand need not be taken into account here because they are not necessary to understand the invention.

FIG. 2 shows one of the calibration curves in a qualitatively identical diagram to explain the measures according to the invention. The point Z shows the desired setpoint on the calibration line, which corresponds to the compressibility C as the setpoint and the moisture content Z 'in% water. If the actual value Y is determined in the first measuring stage I at the start of the measurement on the calibration line, then the actual value A of the compressibility and the actual value Y 'of the moisture content belong to this.

According to FIG. 4, a measurement signal is now sent from the measuring level of the compressibility via line 2 to the dosing computer, which contains the required amount of correction water determined in the moisture correction on the basis of the diagram according to FIG. 2 or 3 as a difference between the size Z'-Y '. If this amount of correction water is added, the actual value Z on the calibration line has theoretically been reached as the setpoint.

Due to the different levels of sand, the actual conditions are not so simplified and cheap. The most important change in the sand characteristics is the sludge content, which causes a displacement of the calibration line I according to FIG. 3, for example, to the calibration line 11.

The corrective measure according to the invention is illustrated in FIG. 3 using the same schematic diagram. The target value Z on the calibration line I is specified. If the actual value Y is measured in the first measuring stage on the calibration line I entered in the computer, this corresponds to a compressibility A and a moisture content Y '. The computer assumes the calibration line I as the correct one and determines the amount between the quantities Z 'and Y' as the correction water quantity. After this amount of water has been added, the mixed material is processed further, so that the water is finally incorporated into the foundry sand. At the end of the mixing process or after leaving the mixing device, a cross-test is carried out in the mixed sand in a second measuring stage. If this counter-sample, ie in the second measuring stage 11, also has the determined value at Z, then it is actually the correct calibration line, because there is no need to make a correction. However, if the actually measured value X deviates from the value Z, it can be assumed that the sludge content of the sand has changed, for example, from calibration line I to line 11 and the line has thus shifted by the amount d. The displacement d determined in this way is transmitted via line 1 to the moisture correction and can be used to shift the calibration line in the measuring device which carries out the first measuring stage I in order to shift this amount d.

With this method it is advantageously possible to carry out an independent regulation and adaptation to changes in the sand composition.

In the example in FIG. 3 it can be seen that after adding the correction water quantity Z'-Y ', the cross-test has produced a value X which does not have the desired and desired compressibility C, that is to say the setpoint, but instead has the size B. In fact, the computer at measurement stage I calculated an incorrect amount of correction water and evaluated it incorrectly via line 2 with the moisture correction FK via the metering computer.

Only the determination of the displacement d and the readjustment of the device at the measuring stage I allow the correct determination of a correction water quantity, so that the value X ultimately measured then coincides with the value. Only then is the compressibility setpoint equal to its actual value.

The lower parallel calibration line is designated T _min and the upper one T _max . These are the specified tolerance limits for the sludge content of the foundry sand. If one of these limits is exceeded when carrying out the method according to the invention, the calibration straight line is readjusted up to this limit, but at the same time a corresponding measured value is given to the dosing system, for example according to line 1 for the dosing computer for adding filter dust or new sand or via Line 3 for the metering computer for bentonite and addition of coal dust, possibly based on the measurement of the compressive strength, as will be explained. There the quantities of new sand, binders or additives can then be automatically increased or reduced accordingly.

The counter-test described can be carried out particularly advantageously with a measuring device which, in addition to the compressibility, also measures the pressure or shear strength of the sand. Such a measuring device for the compressive strength is indicated schematically above the conveyor 6 in FIG. The value obtained in this measurement in the second stage 11 can be transmitted, for example, via line 3 to the metering system, so that a corresponding correction of bentonite and carbon or additives is carried out. When using dosing computers, it is simply possible to automatically correct the addition of bentonite and additives according to the measured strength values of the sand and, moreover, to carry out an additional correction, which is necessary due to the change in the addition of new sand.

Claims (5)

1. A process for automatically regulating foundry sand preparation installations, wherein the moisture content and the compactability of the foundry sand are measured, characterised in that for a predetermined sand characteristic, a reference value (Z) is predetermined both in respect to compactability (C) and also in respect of moisture content (Z'), in accordance with a first calibration straight line (I), the actual value (Y) of compactability (A) is measured in a first stage and set by changing the moisture content (from Y' to Z') to the moisture content reference value (Z'), then as a check sample the actual value (X) in respect of compactability (B) is measured again in a second stage and the difference (B-C) between the last-determined compactability (B) and the reference compactability (C) is used as a correction parameter (d) for adjusting the calibration straight line (I) to a second calibration straight line (II) which is then used in the first measuring stage as a new calibration straight line for the actual value measurement operation.

2. A process according to claim 1 characterised in that the first measurement stage is at the beginning of the mixing operation and the second measurement stage is towards the end of the mixing operation or after the mixing step.

3. A process according to claim 1 or claim 2 characterised in that shift in respect of the calibration straight lines in the first measurement stage is admitted within a predetermined tolerance range (T min to T_max)_'

4. A process according to one of claims 1 to 3 characterised in that the tolerance range in respect of the shift of the calibration straight lines is determined by the slurry material content of the sand.

5. A process according to one of claims 1 to 4 characterised in that when the tolerance range (T_min―T_max) is exceeded, a measurement value is outputted to metering computing means for the supply of fresh sand, additive etc.

Images