4. Sensors

4.1 Encoders

Encoders are essential for assessing the robot's current status. Very high accuracy is required, particularly for the legs, to prevent impact when the flight leg lands. With an electric actuator, the encoder, usually an incremental encoder, is fixed on the motor shaft, which, given the reduction ratio, gives very good joint displacement accuracy. Typically, the joint accuracy of a robot like the HRP-2 is of the order of a few thousandths of a degree. The position of the body to which the actuator is attached is calculated from the model of the relationship between encoder and actuator. Errors can be caused by actuator elasticity, flexibilities and mechanical clearances. Few robots, such as ARMAR or iCub, have a direct readout of joint positions, i.e. after reduction, to...