Robust sensorless pressure control of electric injection molding machine using friction-free force observer

Injection molding is a common method for producing various resin products. The quality of the product significantly depends on the injection force. The accurate detection of the force is therefore very important to achieving in stable high-quality molding. The force control system of a typical injection molding machine obtains information about the force from the environment of the machine via a force sensor. However, because a force sensor often has disadvantages such as signal noise, high cost, and narrow bandwidth, a reaction force observer has been developed for electric injection molding machines. The high-order reaction force observer (HORFO) includes a friction compensation element and uses the dither signal insertion method. Although it neither affects the inserted dither signal nor the nonlinear friction phenomenon, steady-state errors remain in the pressure holding process because the torque transmission losses in the mechanical structures are not considered. In this paper, we propose a new Friction-Free Observer. We experimentally confirm the estimation performance of the proposed Friction-Free Observer using different driving points.