Control unit capable of smoothly carrying out a switching operation between position and pressure feedback control systems

A control unit for use in controlling an injection molding machine which comprises a servo motor controlled by a control signal, a screw driven by said servo motor, a position detector for detecting a position of said screw to produce a position signal representative of said position, and a pressure detector for detecting a pressure imposed on said screw to produce a pressure signal representative of said pressure, said control unit comprising:

• a position pattern generator for successively producing a position pattern signal representative of a reference position of said screw concerned with time;
• position control means connected to said position detector and said position pattern generator for comparing said position signal value and said position pattern signal to produce a first manipulated variable related to said position;
• pressure control means for comparing said pressure signal and a predetermined pressure signal representative of a predetermined pressure to produce a second manipulated variable concerned with said pressure:
• a selector connected to both of said position and said pressure control means for selecting a smaller one of said first and said second manipulated variables as a selected manipulated variable;
• and
• supply means for supplying said selected manipulated variable to said servo motor as said control signal;
• said pressure control means comprising:
• subtracting means for subtracting said predetermined pressure signal from said pressure signal to produce a difference signal representative of a difference between said pressure and said predetermined pressure;
• and
• gain adjusting means, having a plurality of gains different from one another, for adjusting said second manipulated variable in response to said difference signal by switching the gain of said gain adjusting means from one to another.