Modern robotics is based on modularity. Instead of using one six-axis robot
for all applications, the mechatronics engineer is designing a robot for each
particular application. This approach places more emphasis on model-based
design and system integration.
The H-Bot is an example of
such a robot. This two-dimensional robot is used extensively in many industrial
applications, e.g., pick-and-place, sorting, gluing and inspection. It is easy
to 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 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 employed to control the robot. The
successful combination of these two aspects will lead to accurate positioning,
but that means different things depending on the application. In point-to-point
applications, such as a pick-and-place system, moving to the target position
accurately is the main concern, while in a tracking application, such as a
gluing system, a low position-following error is required.
The control system for motion
applications is typically a cascade control system that consists of 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
The position command computation
is usually not well understood. Its complexity depends on the shape of the path
the 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 with
the actual machine. The challenge resides in accurately following sharp
corners. Poor implementation of the calculation of the position command causes an
overshoot on the corner which yields imperfections in the actual product.
Here's one approach to
mitigate this effect and produce perfect corners for a square shape with an
H-Bot: Each side of the square becomes a segment on the motion profile defined
by 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 then
employed to obtain the position profile at the motor shafts. The
synchronization between axes is obtained by a master axis. ?The motion profile of this master axis plays a key
role to create perfect corners. This profile is defined in four segments as
well that start and end at each corner of the square shape. To reduce machine vibration, wear and noise, a smooth profile - such as a
fifth-order polynomial profile - needs to define the motion of the master axis
from corner to corner.
Complete details on the
design and construction of an H-Bot, including modeling, analysis, control design
and experimental validation, can be found
to visit the Mechatronics Zone for the latest mechatronics news, trends,
technologies and applications.