Method and an arrangement for controlling position and/or force of an elongated rolling device

A method for controlling position or force in an apparatus which has a roll nip between a first elongated rolling device and a second elongated rolling device in a paper or a board machine, the method comprising the steps of:

• measuring a variable related to a position of the first elongated rolling device relative to the second elongated rolling device or the force exerted by the first elongated rolling device on the second elongated rolling device;
• comparing the value of the measured variable to a preset value of said variable in order to obtain a difference value of the variable;
• adjusting on the basis of the difference value of the variable, the position of the first elongated rolling device with respect to the second elongated rolling device or the force the first elongated rolling device exerts on the second elongated rolling device;
• and
• changing a fluid pressure of a hydraulic device or changing a rate of flow of a fluid to the hydraulic device in order to alter the difference value of the variable, by repeatedly driving with a digital signal a plurality of digital valves arranged in parallel to form a flow of fluid which is the sum of the flow from each of said plurality of digital valves, wherein each valve of the plurality has a finite number of discrete states, and wherein driving the plurality of digital valves comprises changing the plurality of digital valves from a first condition where all of the plurality of digital valves are in first particular discrete states directly to a second condition, different from the first condition, wherein at least one of the plurality of digital valves assumes a different discrete state, and wherein the hydraulic device is connected to the second elongated rolling device and changes the value of the measured variable.