Apparatus for controlling a plastic extruder

DETAILED DESCRIPTION The present invention is concerned with providing an electronic control system for a plastic extruder which automatically controls the melt temperature and the melt pressure in accordance with preselected melt temperature and melt pressure set points.
The extruder barrel is divided into a plurality of heating zones and the temperature of each zone is controlled in accordance with the respective zone temperature set point.
A melt temperature control loop adjusts a selected number of the zone temperature set points to maintain the melt temperature at its desired value.
The melt temperature control loop and each zone temperature control loop are provided with a separate feedforward controller and a separate feedback controller.
As will be discussed in the detailed description, the feedback controllers are designed to give the desired system response for disturbance signals and the feedforward controllers are designed to give the desired system response for changes in the set point signals.
Thus, the two controllers provide independent specification of set point and disturbance responses and consequently eliminate the saturation problem of prior art controllers when responding to set point changes.
A melt pressure control means controls the speed of the extruder screw such that the actual melt pressure is maintained at its desired value.
In accordance with the present invention, the melt pressure control means is provided with an interaction controller which generates a control signal to adjust a selected number of zone temperature set points such that fluctuations in the melt temperature as a result of changes in screw speed are minimized.
Similarly, the melt temperature control means is provided with an interaction controller which generates a control signal to adjust the extruder screw speed such that the fluctuations in the melt pressure as a result of changes in the selected zone temperature set points are minimized.
Although conventional analog controllers can be utilized to implement the control system of the present invention, preferably a digital computer is used as the control means