Abstract: The frequency characteristic of a controller is acquired. The gain characteristic and phase characteristic of a controlled system are acquired. The open-loop transfer characteristic of the control system and the variation range of the open-loop transfer characteristic are computed from a variation range of the frequency characteristic of the controlled system and the frequency characteristic of the controller, and the variation range of the frequency characteristic of the controlled system is a region where a variation range of the gain characteristic and a variation range of the phase characteristic overlap in a complex coordinate system. The stability of the control system is estimated from the open-loop transfer characteristic and the variation range of the open-loop transfer characteristic.

Abstract: An image forming apparatus includes a rotatable image bearing member provided with a mark; a driving portion for rotationally driving the image bearing member; a first detecting portion for detecting speed information of the image bearing member by detecting the mark moved in a predetermined first distance; a second detecting portion for detecting speed information of the image bearing member by detecting the mark moved in a second distance shorter than the predetermined first distance; a filter circuit for suppressing a high-frequency part of an output from the detecting portion and a low-frequency part of an output from the second detecting portion; a calculating portion for calculating a speed of the image bearing member detected from an output from the filter circuit; and a controller for controlling the driving portion so that the calculated speed of the image bearing member is a set speed.

Abstract: The frequency characteristic of a controller is acquired. The gain characteristic and phase characteristic of a controlled system are acquired. The open-loop transfer characteristic of the control system and the variation range of the open-loop transfer characteristic are computed from a variation range of the frequency characteristic of the controlled system and the frequency characteristic of the controller, and the variation range of the frequency characteristic of the controlled system is a region where a variation range of the gain characteristic and a variation range of the phase characteristic overlap in a complex coordinate system. The stability of the control system is estimated from the open-loop transfer characteristic and the variation range of the open-loop transfer characteristic.

Abstract: An image forming apparatus includes a rotatable image bearing member provided with a mark; a driving portion for rotationally driving the image bearing member; a first detecting portion for detecting speed information of the image bearing member by detecting the mark moved in a predetermined first distance; a second detecting portion for detecting speed information of the image bearing member by detecting the mark moved in a second distance shorter than the predetermined first distance; a filter circuit for suppressing a high-frequency part of an output from the detecting portion and a low-frequency part of an output from the second detecting portion; a calculating portion for calculating a speed of the image bearing member detected from an output from the filter circuit; and a controller for controlling the driving portion so that the calculated speed of the image bearing member is a set speed.