So, you want to build an H-bot?
Kevin C Craig, PhD - May 26, 2011
Designers 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 letter 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 applications, 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 compliance throughout the mechanism are impediments to accurate positioning and represent system-design challenges.
As in any coordinated-motion system, the computation of the position command to each motor of the H-bot is just as important as the control scheme you employ to control the robot. The successful combination of these two aspects will lead to accurate positioning, but that concept means different things depending on the application. In point-to-point-system applications, such as a pick-and-place system, accurately moving to the target position is the main concern, whereas tracking applications, such as a gluing system, require a low number of position-following errors.
Motion applications typically use a cascade-control system that comprises position, velocity, and current loops, all typically proportional integral. Additional features, such as velocity feedforward to reduce position-following error and acceleration feedforward to reduce velocity-following error, are also usually part of the control architecture.
Many mechatronics engineers lack a thorough understanding of the position-command computation. Its complexity depends on the shape of the path the robot must follow. 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 kinematics 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.