Integrated Drive System of Robotic Arm Joint Used in a Mobile Robot

Abstract

The objective of the article is the design of compact cycloidal gear with built-in brushless motor and integrated driver. The assumption of the project is lightweight joint with high ratio and durability to use in a mobile robot. The main advantage of a cycloidal gear is low backlash and self-locking. One of the most significant design problems is off-axial work of some gear parts. The internal part which directly transfers torque is being put into off-axis motion by the eccentrical shaft. The solution to restoring axial movement is the application of the second stage which is orientated backwards to the first stage. The created unit is a differential pair of gears with input and output shafts move in one axis. The specificity of this system requires a difference of ratio by one between both stages to work correctly. A brushless DC motor has been applied to maintain compact size of joint. This kind of electric motor allows for precise control of angle and speed. Moreover, it can halt in any desired position while withholding the torque. The internal driver based on STM32 microcontroller unit with ARM-Cortex core is used to control the drive. Thanks to DC/DC converter power supply unit of the driver is adapted to use the same power source as the motor. A measure of the motor current allows the driver to calculate load and prevents winding overcurrent even when the motor is not rotating. The absolute magnetic encoder is fitted to precisely determine the inclination of joint. The most significant advantage of the integrated joint is standalone construction with own control unit. Control algorithm adjusts inclination and load. This independent module communicates with other units using CAN-bus. Robot's central computer sends only set values of angle, speed and acceleration.