Gernot Pötzelberger, Matthias Meinhart, Austria

Introduction

It is quite common in Austria, that students of a final class of a school for higher technical education, must prove their knowledge by working on a final project. Thus, two students of the LiTec in Linz, Austria have decided to design a robotic arm. Due to the cooperation with RLS and their generous sponsoring, it was possible to equip that robot with magnetic encoders.

Background

Robotic technology has matured to a point where it has many applications in the industrial section or research. Because of their astounding ability to accomplish repetitive work with enormous precision robotic arms have been used to replace a lot of conventional jobs in production lines. Although there is an enormous number of robotic arms on the free market for sale, nearly none of them meets the price range requirements which a small company can afford.

Therefore, we have decided to design an inexpensive and free programmable SCARA (abbreviation for: Selective Compliance Assembly Robot Arm). In contrast to many other hobby robot arms it should be suitable for industrial applications.

 

Challenge

If you are looking for an engine, what is perfectly fitted for positioning applications, stepper motors are very good choice. Due to that reason, we used them to power the linear axis and the brachial joints of our SCARA.

Although these engines offer many benefits, they come with one big disadvantage; If they became exposed to a massive workload it is possible that they lose some of their precision. Such huge forces often occur when the robot arm should transport objects to another place. Especially during the acceleration and breaking phases, the stepper motors must overcome massive torques.

Solution

To achieve the required positioning accuracy, required for industrial applications, it was decided to implement a closed-loop control for both joint motors. To accomplish that two rotary sensors were needed.

After some research it was found that the AksIM™ rotary absolute encoder module by RLS was the best choice. This magnetic rotary encoder module offers many advantages, such as simple communication with microcontrollers due to the big variety of bus systems, the high resolution or its ability to perform high speed operations.

The two encoders communicate via a SPI bus with a microcontroller, which periodically checks the joints positions. The controller’s goal is to adjust the position of both stepper motors to their corresponding calculated angles.

Results
By implementing this sensor to the construction not only the accuracy of the arm improved but it was also possible to implement a so-called “Teach-In” operation mode, in which the user can move the robot by hand to specific point, which will be recognized by the SCARA’s control unit and can be approached later.

The high accuracy of the chosen encoders has ensured very precise movements which are crucial for Pick-and-Place applications. Because of the good adaptability of the AksIM™ modules, the SCARA can be developed further, so that its field of applications can increase.