A typical electric molding machine employed in the art senses pressure using a pressure sensor in a control target (such as mold open/close, extrusion, and nozzle touch) and, based on a signal from the pressure sensor, configures a closed-loop control circuit to control the propelling power.
Specifically, for control of injection pressure, a load cell is arranged at the root of a screw, for example, to sense a force that pushes the screw (a forward force) in the form of pressure by the load cell. Then, based on the sensed pressure, feedback control is applied such that the pressure to be sensed at the load cell reaches a desired pressure value, thereby controlling the propelling power of the screw.
A measured signal output from a general pressure sensor such as the load cell is a weak analog signal. An electric injection molding machine includes a large number of motorized instruments that also act as noise sources. Accordingly, noises caused from the motorized instruments may superimpose on the weak analog signal output from the load cell. In this case, the propelling power may not be controlled well as a phenomenon. Therefore, devices such as multistage noise filters are located on an analog signal line from the load cell to prevent a noise-caused control failure. Nevertheless, it is extremely difficult to completely eliminate such the control failure.
For adjustment of the load cell, manual works such as zero-point adjustment of an amp and span adjustment are required. Therefore, depending on adjusters, adjusted conditions may differ subtly and result in individually different controlled conditions as a problem.
For that reason, based on an angular velocity or rotational angle and a drive current or torque of an electric motor, a current melt pressure value for use in melt pressure control is estimated using state equations. Such a sensorless melt pressure estimating method has been disclosed (in Patent Document 1: U.S. Pat. No. 6,695,994). In the melt pressure estimating method, state equations indicative of the force exerted on the resin (melt) from forward movement of the ram may be given as shown in Expression 1 (see FIGS. 9-13).E1: PMELT=Finj/ABARREL E2: Finj=(2πeSeBNSP/lNMP)[(T2−JTOTα)−TU]−FLOSS E3: α=ω′  [Expression 1]
In this expression, E1 is a melt pressure equation, E2 is an injection force equation, and E3 is a motor acceleration equation, with PMELT: Melt pressure value, Finj: Injection force, ABARREL: Barrel area, eS: Ball screw efficiency, eB: Belt efficiency, NSP/NMP: Diameter ratio of Transmission pulleys at Ball screw and Motor, l: Ball screw lead, T2: Measured torque value, JTOT: Inertia moment, α: Motor angular acceleration, TU: Support bearing frictional torque, FLOSS: Loss force, and ω: Angular velocity.
In the melt pressure estimating method disclosed in the above-described Patent Document 1, for estimation of the melt pressure, the obtained torque command value and angular velocity associated with the motor are employed to directly solve the state equations shown in Expression 1 to obtain the melt pressure PMELT. Accordingly, the Expression includes a differential term denoted with E3, which lowers the resistance against the noises. As a result, a precise melt pressure control is made difficult as a problem.
The present invention has been made in consideration of such the problem and has an object to provide a method and apparatus for controlling pressure in an electric injection molding machine, which is capable of achieving precise propelling power control without the use of a pressure sensor such as a load cell.