Mechatronics: Trajectory planning with electronic cams

Is the mechanical engineer or the electrical engineer most responsible for understanding motion? Electronic cams can help mechatronic designs.

11/19/2010


Kevin C. Craig, Ph.D., Marquette UniversityWhat 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.

Electronic cams are replacing mechanical ones in integrated mechantronic systems. Source: Control Engineering, Plant Engineering, Design NewsTo 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.

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 pointto- 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.

- Kevin C. Craig, Ph.D., Robert C. Greenheck Chair in Engineering Design & Professor of Mechanical Engineering, College of Engineering, Marquette University.

This appeared in November editions of Plant Engineering and Control Engineering, in cooperation with Design Newswww.designnews.com.

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