Initial positioning of a smart actuator using dual absolute encoders

In order to precisely control motor positions, most motor systems use an encoder as the feedback sensor. In the case of a simple system, an incremental encoder and limit switches are used for initial position setting. However, this system loses the current position data when the power goes down and the final position is changed due to an external force. For this reason, absolute encoders are used in full-fledged systems and to facilitate the best performance, a high-resolution absolute encoder should be directly attached at the output axis. However high-resolution absolute encoders are usually large and expensive, and in many cases, it is structurally difficult to directly install the absolute encoder to the output axis. An alternative to this direct sensing of the output angle is a multiturn absolute encoder, which is attached to the motor shaft. However, the multiturn absolute encoder also loses its position data when the backup battery runs out and an external force is applied. In this paper, we therefore suggest a dual absolute encoder system, in which one absolute encoder is attached to the motor shaft and the other low-resolution absolute encoder is attached to the output shaft of the harmonic drive speed reducer (the output shaft of a smart actuator). The required conditions for the errorless operation of this dual encoder system are derived and verified by simulations and experiments.