What do Leonardo da Vinci and the
Nautilus exercise machine have in common? Leonardo da Vinci invented the cam
hammer (see photo) around 1497 and the Nautilus exercise machine, invented
around 1970, uses a cam to modulate resistance. The cam - an irregularly shaped
member on a rotating shaft that transfers motion - has been around for
centuries. Up until recently, the study of cam design and application was
foundational in a mechanical engineering curriculum. But today, it seems its
study is nowhere to be found.
In mechatronic design, integration is
the key as complexity has been transferred from the mechanical domain to the
electronic and computer software domains. Cams are a prime example of that
mechatronic principle as mechanical cams are gradually being replaced by
electronic cams. But transfer implies that we first understand the fundamental
principles in the mechanical domain. Since MEs aren't learning cam fundamentals
anymore and it was never part of an EE's training, motion systems today most
often use crude motion trajectories that stress the machine and motor, produce
unwanted vibrations, and result in poor performance.
To get up-to-date insight into this
issue, I met with Aderiano da Silva, an expert in motion control and automation
machine design for Rockwell Automation in Mequon, WI. His view is that
trajectory planning and its real-time implementation is not well understood and
therefore often neglected. It typically becomes an after-thought add-on - and a
crude one at that.
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Trajectory planning is the computation
of motion profiles for the actuation system of automatic machines, e.g.,
packaging machines, machine tools, assembly machines, industrial robots.
Kinematic (direct and inverse) and dynamic models of the machine and its
actuation system are required. Desired motion is usually specified in the
operational space, while the motion is executed in the actuation space, and
often these are different. The trajectory is usually expressed as a parametric
function of the time, which provides at each instant the corresponding desired
position. Once the trajectory is defined, implementation issues include time
discretization, saturation of the actuation system and vibrations induced on
the load.
In past decades, mechanical cams have
been widely used for transferring, coordinating and changing the type of motion
from a master device to one or more slave systems. Replacing them are
electronic cams, with the goal to obtain more flexible machines, with improved
performances, ease of re-programming and lower costs. With electronic cams, the
motion is directly obtained by means of simpler mechanisms with electric
actuators, properly programmed and controlled to generate the desired motion
profiles, which also allows synchronization of actuators on a position or time
basis.
Once the displacement and its duration
have been defined, the choice of the manner of motion from the initial to the
final point has important implications with respect to the sizing of the
actuators, the efforts generated on the structure and the tracking error. The
engineer must carefully consider the different types of point-to-point
trajectories which could be employed with a specific system. Both time-domain
and frequency-domain analyses must be performed on the complete system, i.e., actuator,
mechanism and load, along with the motion profile, to achieve optimal
performance. Input shaping and feedforward control are two techniques used to
improve tracking performance.
A key reference is "Trajectory Planning for Automatic
Machines and Robots" by Luigi Biagiotti and Claudio Melchiorri. Knowledge from
the past combined with new technologies results in innovation. Engineers must
never forget this fact.
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