Recently, many plastic products have been produced and used. These plastic products are mostly manufactured using injection molding machines. The quality of plastic products depends on the injection force. Therefore, it is important to develop a fine force-control system. Generally, in force-control systems, the force information from the environment is detected by a force sensor. However, control systems using force sensors present problems related to signal noise, sensor cost, narrow bandwidth, and other factors. To overcome these problems, this paper presents a proposal for a new sensorless force-control method using a reaction torque observer. The injection molding machine, which uses a ball screw, has a resonance frequency that is affected by the torsion phenomenon. This new reaction torque observer considers the torsion phenomenon and friction phenomena.
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