So You Want to Build an H-Bot?

Kevin Craig

April 29, 2011

3 Min Read
So You Want to Build an H-Bot?

Modern robotics is based on modularity. Instead of using one six-axis robotfor all applications, the mechatronics engineer is designing a robot for eachparticular application. This approach places more emphasis on model-baseddesign and system integration.

The H-Bot is an example ofsuch a robot. This two-dimensional robot is used extensively in many industrialapplications, e.g., pick-and-place, sorting, gluing and inspection. It is easyto manufacture as it consists of two motors, a timing belt and two rails mounted perpendicular to each other.Despite its dynamic simplicity,friction, backlash and compliance throughout the mechanism are impediments toaccurate positioning and represent system design challenges.

As in any coordinated-motionsystem, the computation of the position command to each motor of the H-Bot isjust as important as the control scheme employed to control the robot. Thesuccessful combination of these two aspects will lead to accurate positioning,but that means different things depending on the application. In point-to-pointapplications, such as a pick-and-place system, moving to the target positionaccurately is the main concern, while in a tracking application, such as agluing system, a low position-following error is required.

The control system for motionapplications is typically a cascade control system that consists of position,velocity and current loops, all typically proportional-integral. Additionalfeatures such as velocity feedforward to reduce position-following
error and acceleration feedforward to reduce velocity-following error are also usually part of the controlarchitecture.

The position command computationis usually not well understood. Its complexity depends on the shape of the paththe robot needs to follow. Paths with sharp corners, such as a square shape,are in general one of the most challenging paths to accurately reproduce withthe actual machine. The challenge resides in accurately following sharpcorners. Poor implementation of the calculation of the position command causes anovershoot on the corner which yields imperfections in the actual product. 

Here's one approach tomitigate this effect and produce perfect corners for a square shape with anH-Bot: Each side of the square becomes a segment on the motion profile definedby the geometry of a square projected on X and Y axes. Thus, the profile X-axis and ?Y-axis in the Cartesian space is obtained. The inverse kinematics of the robot is thenemployed to obtain the position profile at the motor shafts. Thesynchronization between axes is obtained by a master axis. ?The motion profile of this master axis plays a keyrole to create perfect corners. This profile is defined in four segments aswell that start and end at each corner of the square shape. To reduce machine vibration, wear and noise, a smooth profile - such as afifth-order polynomial profile - needs to define the motion of the master axisfrom corner to corner.

Complete details on thedesign and construction of an H-Bot, including modeling, analysis, control designand experimental validation, can be foundhere .

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