Method for controlling a piston fed wood grinder

Details Method for controlling a piston fed wood grinder Method for controlling a piston fed wood grinder

1983-12-13

1985-05-07

Rosenbaum, Mark

Solid material comminution or disintegration

Processes

Wood and similar natural-fibrous vegetable material

241 34, 241 35, 241282, B02C 2500

Patent

active

045153183

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The present invention relates to a method for controlling a piston fed wood grinder, wherein a plurality of wood charges are fed by pressure medium driven pistons against stones for grinding.
Previously the work of a piston fed wood grinder has been controlled by maintaining the feed pressure of the pistons or the feed power of the pistons at a constant value or by maintaining the control valve of the feed pressure in a constant position. These earlier methods have the common advantage of being realizable in a simple manner. On the other hand they have the common serious draw-back that due to unevenness in the quality of the wood to be ground the rate of movement of the piston will vary, wherefore the pulp produced will not be of uniform quality. The importance of uniform quality of the pulp, again, has in recent times steadily grown.
Although this problem had been recognized a long time ago, no satisfactory solution has been found by means of which the rate of movement of the piston would be maintained essentially constant; one reason for this being that the feed piston moves very slowly and speed changes which may be absolutely small in toto relatively great have been difficult to observe and to compensate for.
It is the object of the present invention to provide a new control method which overcomes the afore mentioned difficulties.


SUMMARY OF THE INVENTION

The object is achieved by means of the method according to the present invention which is characterized in that the rate of feed movement of each piston is controlled to an essentially constant value on the basis of determining the rate of movement of the piston by direct digital measurement.
Preferably the rate of the feed movement of the piston is determined on the basis of each time interval between two consecutive digital pulses.
The time intervals occurring between the pulses are processed in a computer by division or by means of a calculator performing the corresponding work to a value of the rate of feed movement and in this manner the value is readily available for the control process. In practice the measured value of the rate of feed movement is received in a time less than 100 milliseconds and still at a great accuracy, the error being less than 2%, and thereby it has became possible to control the rate of feed movement for each feed unity in such a way that it is possible to grind at an optimal rate all the time.
The digital rate measurement can be performed e.g. by means of a rack coupled to the feed piston, the rack rotating a wheel the circumference of which moves past a pulse emitter trigging pulses at a rate proportional to the speed of the piston. The pulse emitter may e.g. be a photo-electric device, whereby the circumference of the wheel is provided with alternating zones permeable and impermeable to light.
As mentioned before, the rate of feed movement of the piston is varied to a great extent in control methods based on a constant feed pressure or a constant power. In case one rigidly tries to maintain a certain optimal rate of feed movement there will correspondingly be a risk of overloading the grinder, of dropping out of balance or of dropping out of the network completely.
In order to avoid these situations it is preferable to complement the control signal based on the measured rate of movement of each piston by an additional signal related to the feed pressure, whereby a more even distribution of the feed pressure between different pistons is achieved, and/or by an additional signal related to the power acting at the piston grinding stone, whereby overloading and the grinder dropping out of balance are avoided. In each case the rate of feed movement of the piston is regulated smaller when approaching preset limit values for the pressure or the power, the changes of the rate movement are still small and even whereby the quality of the produced pulp remains uniform.
In addition to these complementary signals the feed control can take into account an additional signal relating to the over-all pow

REFERENCES:
patent: 3314615 (1967-04-01), Hill
patent: 3599154 (1971-08-01), Carol, Jr. et al.
patent: 3690568 (1972-09-01), Alexander
patent: 3693891 (1972-09-01), Remmer
patent: 3776475 (1973-12-01), Perry
Int'l. Mech. Pulping Conf. Helsinki, 1977, Proceedings, vol. 111 Sess. 4 & 5.
Tampella Grinding Meter TGM, Tampella AB, Eng. Works, Tampere, Finland.

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