Mechatronics: Optimizing the Design of New Tools


The relatively well-understood gears and cams that once served as the mechanical brains of motion control systems are quickly being replaced by digital technology. Mechanical systems are increasingly controlled by sophisticated electric motor drives that get their digital intelligence from software running on an embedded processor.

To help facilitate a more integrated design process for electromechanical systems, software developers are partnering with control design companies to add motion simulation capabilities to CAD environments to create a more unified mechatronics workflow. This approach simplifies design because the simulation uses information that already exists in the CAD model such as assembly mates, couplings, and material properties.

It also yields more accurate results, because force and torque data are highly sensitive to the shape of the velocity profiles. As a result, virtual prototyping helps designers reduce risk by locating system-level problems, finding interdependencies, and evaluating performance tradeoffs.

Picking the Right Parts

Consider an analysis of the torque load for the bottom lead screw actuator in a four-axis pick-and-place machine. If you violate the limits specified by the manufacturer, the mechanical transmission parts may not last for their rated life cycles.

Using simulation software, you can find the mass of all the components and determine the resulting center of mass by creating a reference coordinate system located at the center of the lead screw table and calculating the mass properties with respect to that coordinate system.

With this information, you can then calculate the static torque on the lead screw due to gravity caused by the load. You can then quickly reproduce this same analysis for widely different conditions, such as having a loaded gripper or increasing the velocity limits on the motion profiles.

Optimum Motor Size

In principle, motor sizing is pretty simple: Just pick the best motor to match the speed, inertia, and torque requirements of your application. However, the Department of Energy estimates that about 80 percent of all motors in use are oversized. Although modest oversizing compensates for rising friction over the life of the machine, going too far decreases performance because of higher inertia, lower motor operating efficiency, and higher energy costs. The DOE estimates that 96 percent of the lifetime cost of a motor comes from the energy it consumes, rather than the cost of buying the motor itself.

You can augment the tools by using features built into your CAD package to make more accurate estimates of your speed, inertia, and torque requirements. Typically, you do this by creating a reference coordinate system at the center of the motor coupling and measuring the rotational inertia with respect to that location.

If the ratio between the load inertia compared to the motor shaft inertia is greater than 6:1, switching to a lead screw with a different pitch to change the transmission ratio, substituting a different motor, or adding a gearbox, should all be factors to consider.

Using multi-axis motion profiles to drive your simulation can provide more accurate torque and velocity requirements, which depend on the acceleration characteristics of your motion profiles and the mass, friction, and gear ratio properties of the transmission. So when evaluating simulation results, it is important to compare statistical values for torque and velocity with the manufacturer's rated performance curve for the motor.

Material Stiffness

Analyzing stiffness and selecting the right material thicknesses for the job might be considered the bread-and-butter stuff of mechanical engineering. However, when the prime mover is an electric motor under the control of a complicated power electronic chopper drive, governed by closed-loop electronic control systems with software-configurable PID control gains and real-time motion trajectory, you can't blame a mechanical engineer for asking a few questions.

Mechanical compliance issues that lead to unintended vibrations are one of the most common causes of problems in the motion control industry, and they are not easy to fix in controls software. Most of the time, a mechanical team considers compliance issues when it designs the assemblies, but incorrect assumptions about operational forces and torques may lead to problems. If accurate motion trajectories aren't considered until after the mechanical design is finished, any reasonable project manager cannot blame the mechanical team.

On-Time Product Launches

The Boston-based Aberdeen Group recently conducted a survey of about 160 companies involved in the design of mechatronics systems to learn what separates the best performers from the rest of the pack. It was found that the best performers implement processes to improve collaboration and communication among the members of the design team, such as using electromechanical simulation and design analysis tools.

Successful mechatronics design teams conduct frequent design reviews with the entire team and actively try to identify design decisions that affect more than one group. Using these strategies, the most successful companies were able to meet product launch dates 21 percent more often and budget targets 25 percent more frequently than the industry average for mechatronics projects.

[Adapted from "Right the First Time," by Brian MacCleery and Nipun Mathur, for Mechanical Engineering, June 2008.]

Successful mechatronics design teams conduct frequent design reviews with the entire team and actively try to identify design decisions that affect more than one group.


August 2011

by Brian MacCleery and Nipun Mathur