EP0492080B1 - Process and apparatus for controlling the operation of a roller-mill - Google Patents

Description

The invention relates to a method and a device for regulating the operation of a material bed roller mill, in particular for regulating the fill level of the feed hopper and the power consumption of the roller drive motors.

Operating experience shows that maintaining a certain level in the feed hopper above the grinding rollers is advantageous when operating material bed roller mills. In the previously known fill level controls, attempts are made to keep a predetermined set value of the fill level constant, for example by means of a PID controller. For this purpose, the controller changes the mass flow of at least one feed component of the material bed roller mill (e.g. fresh material, semolina and / or Schülpen). If the fill level setpoint is exceeded, the corresponding feed quantity is reduced, if the fill level setpoint is undershot it is increased.

However, this customary control method does not take into account the fact that ground materials with different mechanical properties have different optimal fill levels. If these properties of the material to be ground change during operation, the known regulation becomes unstable. The resulting fluctuations in the manipulated variable, ie the mass flow of those used for the control feed component, have a detrimental effect on the material throughput and the shredding result.

Similar problems are shown in the control of the hydraulic pressure of a material bed roller mill that has been customary up to now. The power consumption of the roller drive motors is directly proportional to the hydraulic pressure, which in turn is determined by the working gap and the spring characteristics. A large working gap corresponds to a high hydraulic pressure. A high power consumption generally gives a good shredding result.

It is known to regulate the power consumption of the roller drive motors by changing the hydraulic pressure. Usually, a specific setpoint is specified for the power consumption, which is kept as constant as possible by changing the hydraulic pressure.

However, this known method has significant disadvantages. If the grindability of the good changes during operation, the easier to grind good is "over-grinding", i.e. an unnecessarily high grinding energy is expended. With frequent control interventions, the hydraulic oil is also heated and must be cooled, which requires an increased outlay on equipment. At the same time, the hydraulic unit is subjected to high mechanical loads during frequent switching on and off, which reduces the service life.

DE-A-34 07 535 is concerned with a device for the automatic regulation of the throughput of a two-stage shredding, in which the first shredding stage by a kind of material bed roller mill and the second crushing stage is formed by a grinding plant with a tube mill and sifter. Here, the material bed roller mill is preceded by a dosing belt scale with a controller, which in turn is connected to a controller of the grinding system (second comminution stage), from which the grinding flow setpoint for the controller of the dosing belt scale is obtained. In this known device, the throughput of the first comminution stage (material bed roller mill) is to be adapted automatically (through appropriate control interventions) to the throughput performance of the downstream second comminution stage (grinding system) which changes as a result of operational fluctuations or product changes. According to one of the exemplary embodiments there, a feed hopper with a minimum and a maximum fill level indicator can additionally be arranged between the dosing belt scale and the first size reduction stage, which is in operative connection with the controller of the dosing belt weight scale, but which in turn has the mass flow setpoint obtained from the controller of the second size reduction stage switched on becomes.

FR-A-25 12 927 is concerned with controlling the coal outflow from a bunker in a coal production line to a coal mill, which is obviously not a material bed roller mill, as in the case of the present invention. The main aim of this known device is to monitor the flow of coal through a line section and to indicate an imminent dangerous empty state. Here, a radiation detector generates a pulse train via the amount of coal present immediately below the bunker in the line section, in order to use it to flow through the line section Determine the amount of coal per unit of time. In this way, e.g. B. via a kind of target and actual value comparison or empty display by appropriate signals for increased coal supply from the bunker or - if there is a risk of empty signal - a closing of the coal line can be initiated.

Also known from EP-A-0 163 890 is a feeding and testing system for the chocolate in chocolate roller grinders, the structure and mode of operation of which are not comparable to a material bed roller mill, the operation of which is to be regulated according to the present invention. In this known embodiment there is a chocolate hopper, which cooperates with a load cell with an electrical load cell, above a chocolate receiving chamber, which is located above the lower pair of rollers of a plurality of cooperating roller pairs arranged one behind the other or one above the other Determine chocolate level in the funnel and display the actual performance and fineness of the finished chocolate film, for example via a data processing and display device. For example, it should be possible to achieve a uniform filling of the chocolate funnel by adapting it to corresponding lengths of this funnel or to corresponding roller lengths, and in addition the speed of the refining rollers there and other structural parameters of the machine can also be used in order, for example, to use a suitable computer to determine and display the respective fineness value.

GB-A-21 59 306 also has no relation to a material bed roller mill. This publication is concerned with the setting of a comminution gap in a crusher or the like, which can be one with a swash plate or with a crusher cone. An essential idea of this known method is seen in the fact that the comminution gap is monitored and set as a function of the amplitude of the changes in the power consumption of the crusher motor, the respective current comminution gap being compared with a predetermined value and then adjusted in accordance with the deviation from this predetermined value becomes. In a way, a setpoint / actual value comparison is used here.

The invention is therefore based on the object of providing a method and a device for regulating the operation of a material bed roller mill which are characterized by a high stability of the control even when the grindability of the goods changes.

This object is achieved by the characterizing features of claims 1 and 2 or 8 and 9. Appropriate embodiments of the invention are the subject of the dependent claims.

The control method according to the invention thus works without a fixed setpoint for the controlled variable, which can level off freely according to the properties of the regrind to the respectively optimal value. In this way, extremely stable control is achieved even when the grindability changes during operation. The control concept according to the invention can be implemented with any freely programmable control.

These and further details of the invention will become apparent from the following description of two exemplary embodiments.

Example 1: Control of the filling level of the feed hopper

A number of intervention points are defined within the fill level range from 0 to 100%, for example at intervals of 0.5%. Every time the control variable representing the actual level reaches one of these points of intervention, the manipulated variable (i.e. the mass flow of at least one feed component of the material bed roller mill) is changed by a fixed, also adjustable value, and this mass flow is reduced with increasing fill level and with falling Level increased.

In a practical exemplary embodiment, a good bed roller mill of the type POLYCOM was regulated with a freely programmable control S5 (Siemens AG) for a throughput of 600 t / h and a fresh material quantity of 190 t / h in the fill level. When regulating the fill level with a fixed setpoint (according to the conventional method), the quantity of fresh product fluctuated by +/- 50%, whereas when regulating according to the method according to the invention only about +/- 5%. The fresh material throughput averaged over a longer period of time increased from 170 t / h to 190 t / h in the process according to the invention.

If changes in the grindability occurred during operation, the fill level automatically settled to a new value, at which the control again remained very stable and which can therefore be described as the optimum fill level (for the changed grindability of the material). If, on the other hand, external control interventions were attempted to keep the fill level at the original value when the grindability changed, there would be significantly greater fluctuations in the manipulated variable.

In the control of the fill level according to the invention, it has proven to be expedient if the change in the mass flow associated with an intervention point is only carried out after a predetermined, adjustable delay time (for example of 2 minutes), provided that the fill level does not again reverse this engagement point within the delay time reached. Whenever the level reaches the same point of intervention again within the delay time, the delay time that has already expired becomes reset to zero. In this way, the number of control interventions can be reduced to what is absolutely necessary.

In the method according to the invention, when the fill level reaches an engagement point, the mass flow is expediently changed by a fixed, adjustable value. You can change the mass flow at each point of intervention by the same amount (for example 2%). However, it is also possible within the scope of the invention to assign different change values of the manipulated variable to the individual intervention points. In order to prevent the pre-bunker from overflowing, the change values of the manipulated variable were set to 4% with increasing fill level in the example above for fill levels above 80%.

Example 2: Control of hydraulic pressure

With this control method, four points of intervention were defined in the control range of the controlled variable (i.e. the hydraulic pressure generating the pressure in the nip). In the above example, 1850 kW, 1800 kW, 1730 kW and 1700 kW were defined as intervention points.

The outlet pressure was selected so that a power consumption of 1800 kW was achieved for goods that were difficult to grind. If the grindability became better, more ground material was fed into the mill, the grinding gap increasing and the hydraulic pressure and the power consumption increasing. If the power consumption reached the highest point of engagement of 1850 kW, the hydraulic pressure was reduced until the power consumption had dropped to 1800 kW. This process may have been carried out several times in succession, because the better grindability of the material means that the power consumption has repeatedly increased to the highest point of intervention of 1850 kW despite the pressure being reduced.

If the controlled variable (power consumption) reached the uppermost intervention point several times in succession, the control interventions triggered thereby and the control variable (hydraulic pressure) were counted.

If the power consumption then dropped to the lowest intervention point, which was set at 1700 kW, the hydraulic pressure was increased until the second lowest intervention point of 1730 kW was reached. This could also happen more than once. If the controlled variable reached the lowest intervention point several times in succession, the control interventions triggered thereby and increasing the manipulated variable were also counted. Their number was limited according to the number of previous control interventions reducing the manipulated variable (the number of pressure increases was therefore limited to the number of previous pressure reductions, which is a safety measure against an excessive pressure increase).

Even with this regulation of the power consumption of the roller drive motors, it was found that the regulation automatically settled to an optimal value of the hydraulic pressure depending on the grindability of the material.

Claims (9)

1. Method of controlling the operation of a material bed roll mill, characterised in that within the control range of a control variable at least three intervention points are fixed so that when they are reached

   - in the case of a rising tendency of the control variable the setting variable is decreased by a fixed adjustable value

   - and in the case of a falling tendency of the control variable the setting variable is increased by a fixed adjustable value.
2. Method as claimed in Claim 1, characterised in that at least four intervention points are fixed in the control range of a control variable, wherein

   - when the control value with a rising tendency reaches the uppermost intervention point the setting variable is reduced until the control variable reaches the second-highest intervention point,

   - and when the control variable with a falling tendency reaches the lowest intervention point the setting variable is increased until the control variable reaches the second-lowest intervention point.
3. Method as claimed in Claim 1, characterised in that the control variable is formed by the filling level of the feed bin and the setting variable by the mass flow of at least one feed component of the material bed roll mill.
4. Method as claimed in Claim 3, characterised in that the alteration in the mass flow co-ordinated with an intervention point is only undertaken after a predetermined adjustable time lag, unless the filling level again reaches this intervention point with reverse tendency within the time lag, and that whenever the filing level again reaches the same intervention point within the time lag, the time lag already elapsed is restored to zero.
5. Method as claimed in Claim 3, characterised in that when the filling level reaches an intervention point the mass flow is altered by a fixed adjustable value.
6. Method as claimed in Claim 2, characterised in that the control variable is formed by the power consumption of the roll drive motors and the setting variable by the hydraulic pressure producing the pressure in the roll gap.
7. Method as claimed in Claim 6, characterised in that

   - when the control variable reaches the uppermost intervention point a number of times in succession, the control interventions triggered thereby which alter the setting variable are counted,

   - and that when the control variable then reaches the lowest intervention point a number of times in succession, the control interventions triggered thereby which increase the setting variable are likewise counted and the number thereof is limited according to the number of preceding control interventions which reduced the setting variable.
8. Apparatus for controlling the filling level of the feed bin of a material bed roll mill by altering the mass flow of at least one feed component of the material bed roll mill, characterised by a freely programmable control means in which within the control range of the filling level at least three intervention points are fixed in such a way that when they are reached

   - in the case of a rising tendency of the filling level the mass flow is decreased by a fixed adjustable value

   - and in the case of a falling tendency of the filling level the mass flow is increased by a fixed adjustable value.
9. Apparatus for controlling the power consumption of the roll drive motors of a material bed roll mill by altering the hydraulic pressure which produces the pressure in the roll gap, characterised by a freely programmable control means in which in the control range of the power consumption at least four intervention points are fixed in such a way that

   - when the power consumption with a rising tendency reaches the uppermost intervention point the hydraulic pressure is reduced until the power consumption reaches the second-highest intervention point,

   - and that when the power consumption with a falling tendency reaches the lowest intervention point, the hydraulic pressure is increased until the power consumption reaches the second-lowest intervention point.