Device and method for pressure control of electric injection molding machine

Abstract

{Problem}The exact method with small time-lag of detecting injection pressure for controlling pressure in an electric-motor driven injection molding machine without using a pressure detector has been asked for because the pressure detector is very expensive, necessitates troublesome works for mounting, an electric protection against noise and the works for zero-point and span adjustings and causes a complicate mechanical structure. {Solution}The present invention uses a high-gain observer which contains the discrete-time arithmetic expressions derived from a mathematical model of an injection and pressure application mechanism in an electric-motor driven injection molding machine consisting of a state equation and an output equation and outputs an estimate of injection pressure, which is one of the state variables of the above state equation, by using an injection velocity signal and a servomotor current demand signal or actual motor current signal as inputs. The high-gain observer obtains the exact injection pressure estimate with very small time-lag without using a pressure detector. Thus the estimate of injection pressure fed by the high-gain observer can be adopted as a feedback signal of actual injection pressure for controlling injection pressure.

1 . An apparatus for controlling pressure in an electric-motor driven injection molding machine having an injection and pressure application mechanism in which rotation of a servomotor is transferred to rotation of a ball screw through a reduction gear and rotation of said ball screw is converted to a linear motion of a nut of said ball screw and said nut drives a moving part and a linear motion of a screw is realized through said moving part and pressure application to a melted polymer stored at the end of a barrel and fill-up of a cavity with polymer melt are realized by a movement of said screw, comprising:
 an injection controller operative to provide a motor current demand signal for said servomotor to external, which comprises
 a high-gain observer operative to feed a high-precision estimate of injection pressure with very small time-lag which is obtained by using an injection velocity signal detected by a rotary encoder mounted on a servomotor axis and a differentiator and a motor current demand signal applied to said servomotor or an actual motor current signal as inputs and by executing at a contant time interval a discrete-time expression of time integration derived by applying a standard method of trapezoid rule to a continuous-time time integration equation between a screw position and an injection velocity and by executing at a constant time interval discrete-time arithmetic expressions derived by applying a standard method of forward rectangular rule to continuous-time calculation procedures which are derived from a continuous-time mathematical model of an injection mechanism representing motion equations of said injection and pressure application mechanism and consisting of a state equation having two state variables of an injection velocity variable and an injection pressure variable and having two input variables of said motor current demand signal applied to said servomotor or said actual motor current signal and a screw position signal and an output equation having one output variable of an injection velocity signal as a measurable state variable, 
 an injection pressure setting device operative to feed an injection pressure command signal, 
 a subtractor operative to feed a difference signal between said injection pressure command signal from said injection pressure setting device and said estimate of injection pressure fed by said high-gain observer, and 
 a pressure controller operative to derive said motor current demand signal for said servomotor by using said difference signal from said subtractor so that said estimate of injection pressure follows said injection pressure command signal; and 
   a motor controller fed said motor current demand signal from said injection controller.