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Robust design of direct drive motor
 
General motor use equipment like gear box, to operate the object system. The gear box do lot of things like speed control, etc. However, as using gear box, problems like energy efficiency decrease caused by friction between gears and backlash problem of gears. The motor, on the other hand, that is connected to the object system directly, is called direct drive motor (DDM). DDM, which do not use gear box, has no problems that mentioned before. Also, it has many strong points that DDM can do position control accurately and it has fast response. However, DDM has one big problem that it is weak at disturbance.

Fig 1. Direct drive motor
 
If disturbance applied, air gap, which is difference between rotor and stator, is changed. Air gap change can cause not only difficulty of motor control but also even serious problem like breakdown of the motor. There are two ways to prevent mentioned problems that occurred from air gap change. First way is design controller which is robust to disturbance, and the other way is design robust DDM itself. In this research, we take second way to minimize air gap difference when the disturbance is applied to DDM.

Fig 2. Air gap of direct drive motor
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