Real-time Control of a Mitsubishi SCARA Robot

  • InterHome
  • Industrial Control
  • Realtime
  • Robotics
  • Wireless
  • Zigbee

This project presents an exclusive real-time, wireless control approach for a Mitsubishi industrial robot (SCARA robot) through the use of an embedded Microcontroller. The novel approach to real-time control has several advantages over a conventional teaching pendant (TP) system, such as ease of operation, lower memory requirements, cost-effectiveness and better real-time properties.

Industrial robots have a large potential for the future [1]. In many situations they relieve humans from dangerous, difficult or monotonous task. Sometimes, it is necessary to control robots remotely in real-time so as to enable them carry out their tasks more efficiently. The real world is simply too complex and unpredictable to be managed by robots autonomously without some form of human intervention either constantly or periodically. In the real world, it is apparent how higher animals with adaptive abilities have been very successful in their environments. Man in particular has been very successful in his attempt to control and influence the environment; however, the currently accessible technology limits robots to passive objects that are incapable of learning very much and thus, they require human intervention at some point of their operation. This necessity in human intervention might not always manifest itself in a physical presence as is often the case in dangerous situations; therefore, tele-presence and real-time operation offers one solution to the problem.

There are several problems that an autonomous robot devoid of human intervention has to cope with such as [2]:

  1. Its model of the world is often inaccurate and out of date
  2. The sensors of the robot will be noisy and its actuators imprecise
  3. The environment is likely to be dynamic and constantly changing

The processes involved in operating a robot can be greatly simplified if the designer does not have to explicitly add every detail of the environment (as is most often done in non-real-time operation) and be within close proximity to the site of operation.

Over the years, a wide variety of techniques aimed at limiting the proximity of human operators within a robot’s working environment have been used [3]. Some of these methods are based on teleoperation combined with real-time control. A large class of telemanipulator systems rely on the knowledge of the human operator and sometimes augmented by external methods, such as data gathered from feedback.

Currently, industrial robots are controlled through command instructions that are written by a programmer and then transferred to the robot control unit (RC). Once these commands are written, the program would have to be downloaded into the robot control unit for execution. Although this method is good, it requires a high level of skill on the path of the programmer and human operator; also, any changes related to the program structure cannot be implemented once the program has been downloaded into the RC. In order to change the program structure, a new version of the program has to be written and downloaded into the robot control unit.

In real-time control, the aforementioned issues do not exist since the robot motion path can be completely altered during operation without the need to rewrite and download a completely new program into the robot control unit each time a change is desired. During real-time operation, the robot controller (RC) receives a series of command data from an external unit (usually a PC or an embedded digital device), within a specified time period and executes these commands immediately.

The combination of real-time programming with a wireless communication technique has many important advantages, some of which are: It enables the efficient use of real-time control technique and at the same time it offers a degree of freedom that is unparalleled in wired communication.

Figure 1: Overall system design
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The system works well when applied on a real robot with real-time constraints, receiving noisy signals within an environment that has other transmitters located nearby. The real-time algorithm shows robust performance even if the robot is operated from a distance (15 meters) away.

The robust behaviour of the robot is partly attributed to the built-in rudimentary encoding capabilities of the real-time control system. The robustness is also partly attributed to the pragmatic approach that was taken in writing the real-time application. The method of approach guarantees that even if a command data was successfully decoded, it could only be executed by the robot if it is valid.

Distributed control system (DCS) method has been proven to do well and robustly in multidimensional control systems. This could be one reason for the robust performance in keeping up with the strict time-constraints of the real-time system. The concept of distributing the total load of a system among different devices with varying capabilities could help to avoid system overload within real-time systems, in regards to meeting deadlines. Simpler approaches such as putting all the functionalities into one device might impede the system from achieving full real-time functionality.

The entire real-time application has also been compiled and run on the mbed Microcontroller thereby eliminating the need for a Personal Computer. This demonstrates the applicability of Compiling real-time programs to control robots on low-end architectures. This technique could potentially be applied to many ubiquitous devices such as mobile phones etc. Also, since a general purpose programing language was used, it is fairly easy to port the program from one platform to another.

[1] International Federation of Robotics: “The Continuing Success Story of Industrial Robots” [viewed 18 October 2013]. Available from: http://www.ifr.org/news/ifr-press-release/the-continuing-success-story-of-industrial-robots-414/

[2] Nordin, P. and Banzhaf, W. (1997), “An On-Line Method to Evolve Behavior and to Control a Miniature Robot in Real Time with Genetic Programming”. Adaptive Behavior pp. 2

[3] Corley Ann-Marie “The Reality of Robot Surrogates” [viewed 24 March 2014]. Available from: http://spectrum.ieee.org/robotics/humanoids/the-reality-of-robot-surrogates/0