The Austrian company, Grünewald Paper, recently modernised its process control to improve profitability. Thanks to a new control system and a grammage-weight control valve, the company’s throughput and paper quality have been further improved. Through the introduction of a highly accurate sector ball valve with an internally mounted angle transducer and stepper motor, a significant stabilisation of the process could be achieved together with improved quality at markedly higher throughputs.
The production of papers with grammage in the range of only 18 to 60 g/m² demands very accurate process control. Grünewald, with an annual capacity of 50 000 tonnes, supplies largely food contact paper in rolls for the production of food bags, waxed paper or flower tissue paper, as well as paper table cloths.
The existing out-dated system was replaced by modern process control in order to stabilise paper weight and allow increased production. This necessitated the increase of control valve size from 100 to 150 mm incorporating a Schubert & Salzer DN 150 ball sector valve.
The former valve fitted with a stepper motor operating at 5800 steps proved to be unable to maintain accurate tolerance of the weight. “With a larger valve every incorrect positioning step caused more trouble than would be the case with a smaller valve,” said Reinhard Christes, head of electrical, measurement and control engineering at Grünewald Paper. “The change of valve size led to a completely inadequate level of accuracy and the consequences were unacceptable fluctuations in the paper grammage. For that reason the stepper motor was replaced with a newly developed precision servo motor drive from Schubert & Salzer.”
Precise volumetric flows require targeted measures
The servomotor can be configured in the factory to customer requirements for a command signal either via an analog positioning signal (4 to 20 mA) or through a stepper/directional control with a signal level of 24 V and 400 Hz maximum input frequency. The activation of the positioning drive is performed either by CAN-Bus or with Profibus. The drive control via stepper/directional control is performed, depending on process and/or accuracy requirements, in such a way that the 90° rotational movement can be achieved within a range of 1000 up to 8192 steps. Elimination of backlash is done by the use of compact planetary drive. In this way, the set number of steps corresponds exactly to the actual number of positioning steps transmitted to the ball sector and not to the number of steps of the motor. The vital feedback for uncompromised control accuracy is supplied through a 15 bit absolute-rotational angle transducer which is attached load-free to the bottom trunion, and records the current position of the ball sending this as a digital signal to the control system.
This control circuit, monitoring valve movement internally, ensures that the very highest level of accuracy is achieved between positioning signal and the actual valve position.
Activated through an analog positioning signal the positioning time for this valve drive over a 90° rotation can be set in a range of 1 up to 300 seconds. The control system is enclosed in a compact cabinet. In addition, this delivers an analog return signal which can be evaluated by the equipment controls.
“With this new precision drive system all our problems were solved at once. This 8000 step resolution certainly supplied the basis for the solution but the valve’s internal positioning signal direct to the drive is the outstanding feature of this valve drive,“ said Christes.
High control accuracy without hysteresis
Hysteresis is often an underestimated problem with control valves. High levels of control accuracy require a backlash free transmission of the drive to the valve functioning unit. Only then does a positioning signal correlate with the parameterised opening angle of the valve and the corresponding volumetric flow. With the combination of a very accurate valve drive and ball sector valve, Schubert & Salzer Control Systems has succeeded in producing a precision valve unit for even the highest volumetric flows.
In addition, the resolution of more than 8000 steps to drive the valve through 90° reduces the hysteresis under 0,02%.
In this way volumetric flows such as in the case of paper manufacturing can be controlled and regulated to extremely precise levels. “We can now correlate exactly the valve position of the paper grammage regulating valve to a specific material volumetric flow and the valve adopts precisely this position setting,“ said Christes. “Thanks to the new servo drive of the grammage control valve from Schubert & Salzer, we can run our grammage weights much more accurately than ever before. We are producing today with significantly smaller longitudinal weight fluctuations. The more accurately we run the better this is for the whole line. This precision valve drive also has an impact on the dosage metering of additives. Since there are no more fluctuations in the system, the dosage metering also works considerably better.”
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