Control system for controlling the outer diameter of a strand of plastic material, in particular of a cable

Claim 1

A control system for controlling the cold outer diameter of a strand of plastic material formed by an extruder and thereafter drawn through a cooling path, the strand having a speed at which it is withdrawn from the extruder, the control system comprising a first diameter measuring device which is located immediately behind the extruder and which generates a warm diameter signal representative of the actual outer diameter of the stand at said first measuring device, a second diameter measuring device which is located at a distance from the extruder after the cooling path for the strand and which generates a cold diameter signal representative of the actual outer diameter of the strand at said second measuring device, first and second comparators, a delay unit for delaying a signal fed to the second comparator through the delay unit for an amount of time corresponding to the time which the strand needs to move between the fist and the second diameter measuring devices and a control unit, the warm diameter signal of the first diameter measuring device being fed to the first comparator and to the second comparator through the delay unit, the second comparator receiving the cold diameter signal of the second diameter measuring device and the delayed warm diameter signal and generating a difference signal, the difference signal of the second comparator being also fed to the first comparator along with a signal representative of a cold reference value, and the first comparator generating a comparison signal which is fed to the control unit, and the control unit responding to said comparison signal for the control of either the rotational speed of the extruder, or the speed by which the strand is withdrawn from the extruder, or both.