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EDN, May 26, 2011

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E Control Systems MECHATRONICS IN DESIGN esigners of modern robotics based their systems on modularity. Instead of using one six-axis robot for all applications, mechatronics engineers design a robot for each application. This approach places more emphasis on model-based design and system integration. The H-bot, so-named because it resembles the let- ter H, is an example of such a robot. This 2-D robot, a planar mechanism for positioning an object in XY space, such as a plane, finds use in many industrial applica- tions, such as pick-and-place, sorting, gluing, and inspection systems. It is easy to manufacture because it comprises two motors, a timing belt, and two perpendicularly mounted rails (Figure 1). Despite its dynamic simplicity, friction, backlash, and com- pliance throughout the mechanism are impediments to accurate posi- tioning and repre- sent system-design challenges. As in any coor- dinated-motion sys- tem, the computa- tion of the position command to each motor of the H-bot is just as impor- tant as the control scheme you employ to control the robot. The successful com- bination of these two aspects will lead to accurate position- ing, but that con- cept means different things depending on the application. In point-to-point- system applications, SERVO MOTORS (CONNECTED BY A SINGLE TIMING BELT) Digital Control Systems Software FRESH IDEAS ON INTEGRATING MECHANICAL SYSTEMS, ELECTRONICS, CONTROL SYSTEMS, AND SOFTWARE IN DESIGN Control Electronics mechatronics Mechanical CAD Electro- mechanics Mechanical Systems R Electronic Systems So, you want to build an H-bot? D The H-bot is conceptually simple, but the design of the controls makes it amazing. such as a pick-and-place system, accurately moving to the target posi- tion is the main concern, whereas tracking applications, such as a glu- ing system, require a low number of position-following errors. Motion applications typically use a cascade-control system that comprises position, velocity, and cur- rent loops, all typically proportional integral. Additional features, such as velocity feedforward to reduce posi- tion-following error and accelera- tion feedforward to reduce velocity- following error, are also usually part of the control architecture. Many mechatronics engineers GANTRY (MOVES HORIZONTALLY ONLY) TIMING BELT (FIXED AT ONE END) LIFT ARM (MOVES VERTICALLY ONLY) lack a thorough understanding of the position-command computation. Its complexity depends on the shape of the path the robot must follow. Kevin C Craig, PhD, is the Robert C Greenheck chair in engineering design and a professor of mechanical engineering, College of Engineering, Marquette University. For more mechatronic news, visit mechatronics zone.com. Figure 1 The 2-D H-bot robot comprises two motors, a timing belt, and two per- pendicularly mounted rails. Paths with sharp corners, such as a square, are challenging to accurately reproduce with a machine. The challenge resides in accurately following sharp corners. Poor implementation of the calculation of the position command causes an overshoot on the corner, yielding imperfections in the product. One approach to mitigating this effect produces perfect corners for a square shape with an H-bot. In this approach, each side of the square becomes a segment on the motion profile, which is defined by the geometry of a square projected on X and Y axes. Thus, you obtain the profile X and Y axes in the Cartesian space. You then employ the inverse kinemat- ics of the robot to obtain the position profile at the motor shafts. Use a master axis to obtain synchronization between axes. The motion profile of this master axis plays a key role in creating perfect corners. Four segments that start and end at each corner of the square shape define this profile. To reduce machine vibration, wear, and noise, use a smooth profile, such as a fifth-order polynomial profile, to define the motion of the master axis from corner to corner. You can find details on the design and construction of an H-bot, including modeling, analysis, control design, and experimental validation, at www.multimechatronics.com.EDN 28 EDN | MAY 26, 2011 R D S G X E E N E F R P R I N O U L S C T A E E C C S S A D I Y S U F E S N I N M E T G A M M S A P C T T S O O C O N M U R S O D E U T O A V I M E P R C U S Y M H A P T A E R R G A I O L E A

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