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Lange R.,Schubert and Salzer Control Systems GmbH | Grubitz R.,Schubert and Salzer Control Systems GmbH
Professional Papermaking | Year: 2011

The Grünewald company from Sauerland, Germany, has improved its throughput and paper significantly due to a new process control system and a new grammage weight control valve. The introduction of a highly accurate Sector Ball Valve with an internally mounted angle transducer and stepper motor has resulted in significant stabilization of the process with improved quality at higher throughputs. The feedback for better control accuracy is supplied through a highly accurate 15 bit absolute-rotational angle transducer, which is attached load- free to the bottom trunion and records the existing position of the ball sending this as a digital signal to the control system. This control circuit, monitoring valve movement internally, ensures that the highest level of accuracy is achieved between positioning signal and the actual valve position.

The Grünewald company in southern Westphalia has modernized its process control. Throughputs and paper quality are further optimized with a new system and a new surface weight valve. A highly accurate servomotor with step control and a direct position feedback servomechanism across the spherical sector of the valve significantly stabilize the process and therefore maintain quality with higher throughputs. The discussion covers introduction; precise volumetric flows requiring purposeful measures; and freedom from hysteresis at the highest control precision. The manufacture of papers with 18-60 gm/sq cm surface weights requires a highly precise control. The Grünewald company supplies with its capacity of 50,000 tonnes/yr mainly paper that comes in contact with foodstuffs in the form of paper rolls selling the paper finisher that makes service packaging such as bakery bags, wax paper, or tissue paper for flowers and tablecloths. In the modernization of the plant, the 30 year old surface weight control valve DN 100 was replaced with a Schubert & Salzer spherical sector valve DN 150. The step motor for the old valve with a resolution of 10,000 steps at 90° was no longer suitable for the new DN 150 valve size. The step motor drive of 5,800 steps at 90° was shown to be too imprecise, leading to fluctuations in the weight of the paper. The step motor after a short time was exchanged for a newly developed precision drive for the surface weight control valve. For the rotary motions the servomotor of the new drive is configured to command via an analog positioning signal (4 to 20 m amps) or via a step-/direction control with a signal level of 24 volts and 400 Hz maximum input frequency. The control of the positioning drive per CAN-Bus or Profibus is also possible. The control is so configured via step-/direction according to the process and precision demands that a 90° rotation is effected in 1,000 to 8,192 steps. The rotary motion of the motor is transmitted free from play via a compact planetary gearing to the sphere sector. The movement is free of hysteresis. The resolution of a 90° rotating motion or swing in over 8,000 steps reduces the hysteresis to less than 0.02%.

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