17-08-2012, 12:25 PM
PC BASED CONTROL SYSTEMS FOR COMPLIANCE CONTROL AND INTUITIVE PROGRAMMING
1PC BASED CONTROL.pdf (Size: 796.86 KB / Downloads: 57)
Abstract
Nowadays, high costs of automation technologies, sustainable only in manufacturing markets with large volume
production and intensive capitals, do not allow a widespread application of Robotics to Small Medium Enterprises (SME).
To face such a problem the need of low-cost and not-proprietary solutions at all level of the robot work cell is mandatory.
Focusing on the robot control system, two aspects can be addressed to reduce its life cycle costs. Firstly, during the
Research & Development activities, more openness allows to design, develop and integrate functionalities quickly,
reacting to market changes with few engineering efforts both in term of cost and time; secondly, during the common use of
the control system, providing intuitive interfaces for robot programming let the robot end user not to rely on high skilled
operators who require specific knowledge for each specific system with expensive efforts in personnel training. In this
paper, the above mentioned aspects are addressed proposing a PC and open source software based control architecture for
robot controllers. Such an architecture has been used to test compliance strategies; specifically, this work shows the
implementation of teaching by demonstration based on manual guidance using a force/torque (F/T) sensor and a wireless
joy-pad. The same algorithms used on the PC-based controller have been applied to an industrial robot controller in order
to accomplish a manual guidance based on the motor currents feedback.
Introduction
During the last decade, the openness of control systems has been addressed in several ways by worldwide research projects
both in the field of machine tools (i.e. OSEC/JOP, OMAC, OSACA) [1] and in the field of Robotics (i.e. OROCOS) [2].
Although a universal agreement on definition of Open Control Architecture (OAC) has been not yet achieved, a basic set
of common requirements has been drawn and they can be summed up as follows:
· an OAC allows the portability of software on different hardware platforms and operating systems;
· an OAC is modular having the capability of easy replacement of modules with new ones, allowing scalability
both in performances and functionalities
· an OAC allows easy integration of new functionalities. This means, firstly, that it is based on standards to let third
parties develop hardware and software that meet new requirements; secondly, it means that an OAC provides
plug-and-play mechanisms for fast integration of such new functionalities
· an OAC is reconfigurable providing mechanisms of easy adaptation of its parameters in order to customize the
system to different application scenarios
· an OAC is economic since it is based on off-the-shelf modules both with reference to hardware and software
Compliance strategies for Manual Guidance
Manual guidance is a programming method that was widely used in the past especially for painting robots. Such systems
were mechanically balanced; this means that the method used for their manual guidance did not use any external sensor:
the robot was completely passive while the operator had to compensate its inertia.
Nowadays, standard industrial robots are not mechanically compensated, so their manual guidance needs the use of proper
feed-back systems to get forces/torques applied by the operator to the robot. Furthermore, the success of PbD by means of
Manual Guidance [10] and, generally speaking, the success of human-robot or environment-robot interaction, relies mainly
on the planning accuracy and the control performance. The implementation of manual guidance needs the use of
compliance control strategies. Such control algorithms transform the robot from a passive and hard mechanism only
capable of repeating a programmed operation, to an active mechanism capable of adapting to environment modifications.
To understand the property of the Compliance strategies it's easily to think how is essential the tactile information for a
human, in the such way the force information managed from the compliance strategies gives to the robot a real intelligent
way to have an entirely success in the task approach. Compliance control strategies are basically divided in two groups:
active and passive compliance.
Experimental test-beds for manual guidance
The control system described in section 3 and 4 has been used as test-bed of compliance control strategies addressed in
section 2. Such algorithms are needed to have a natural robot programming by means of manual guidance as underlined in
section 1. It is important to note that the adopted compliance strategy is constructed over an inner standard joint position
controller. Then, as far as an industrial robot controller provides an openness at that level, the proposed control
architecture can be easily integrated.
Manual guidance based on F/T sensor feedback
Mitsubishi PA-10 is a commercial robot with an open architecture that makes the manipulator suitable for research and
development. The robot is composed by seven links that allow seven d.o.f. (three rotation and four pivot axes); its structure
is shown in Figure 7. The geometrical characteristics of the PA-10 manipulator are the follows: the length of the
manipulator is 1.37 m and the weight is about 35 kg. The robot joints are actuated through three-phase AC servomotors,
while the power is given through the domestic 230V AC.
The setup that is normally sold consists of a robot arm, a servo controller, a motion control card and an upper control
computer linked to the servo controller by means of a dedicated optic fibre network based on ARCNET. Several levels of
control are allowed by the standard Mitsubishi axis control card, from an higher level controlling the movement in
Cartesian space by using an inner interpolator up to a lower level controlling directly the position of the joints.
Manual guidance with motor currents feedback
F/T sensors are quite expensive; their cost rates from a minimum of 4 K€ to more than 15 K€ and this is the main
drawback of their use in industrial applications. Although the continuous improvements of electronic devices will give a
reduction of the cost of these sensors in the next future, the use of sensor-less methods such as ones based on motorcurrent
feedbacks could be a solution to develop cost-effective intelligent compliance strategies for robotic applications in
SMEs.
Conclusion and future work
In this paper, a reference architecture for the development of PC and Open Source based robot controllers has been
illustrated. Our major goals includes the definition of a modular architecture which can be easily reconfigured with
reference to the kinematics of the controlled robot and which can be easily applied to different RTOSs. Specifically, this
architecture has been implemented with reference to a 7-d.o.f. robot arm manufactured by Mitsubishi. With reference to
such a robot, the reconfigurability and modularity of the control system have been exploited in order to implement
Programming by Demonstration based on manual guidance and in order to use a low-cost programming device. The
proposed architecture allows both the manual guidance based on the feedback provided by an F/T sensor (as shown in the
PA-10 application) and the feedback of motor currents (as shown in COMAU NS-16 application).