The Mechanical pulp refining process involves the fibrillation of wood chips between grooved metal plates in a succession of refiners to produce developed fibers. Each refiner uses a movable plate and a fixed plate. The movable plate is driven by a high electric motor and is held against the fixed plate by a hydraulic servo actuator as the force of the conveyed chips tries to push the plates apart. The control process is complex because the intensity of refining varies according to the size and grade of the wood chips, as well as wood species.
Earlier days the pulp refiner machines were built using analog controllers which is very difficult to tune and troubleshoot. The refiner disc gap control is very critical application where damage of the discs can occur if discs are allowed to collide. With the analog control cards it is very difficult for the operator to know the system status and the actual disc gap.
To overcome these problems the RMC70 motion controller by Delta computer System was suggested. The RMC70 obtains continuous information on the position of the movable refiner plate from Magnetostrictive Displacement Transducer (MTS Temposonic). The MDT is mounted next to the hydraulic positioning cylinder and provides cylinder position information to motion controller .
The MTS Temposonic MDTs were added during the control system retrofit phase, replacing linear variable displacement transformers (LVDTs), in order to provide more precise position information. MDTs are highly reliable and have the advantage of providing precise position feedback without requiring a homing cycle at the start of the motion. Though not mounted inside the cylinder in this application because it is a retrofit, MDTs also have high pressure and temperature specifications that allow them to be inserted directly into hydraulic cylinders.