Consider control subsystems

There’s more to micropositioning, of course, than micron-level movements. Parameters of importance include specifications for move and settle time (step response), maximum operating speed, and throughput- related specs. The design and interaction of both the control and mechanical system ultimately define the operating envelope of the complete system.

A good first order approximation is that the wider the system bandwidth, the better system performance will be in terms of settling times and peak position errors. That’s because mechanical systems exhibit resonant modes or frequencies about which they tend to oscillate. These resonant modes must be outside the system bandwidth and sufficiently attenuated so they’re not excited by stage motion.

In keeping with mechatronic design principles, required system bandwidth is estimated early in the design process based on performance criteria. Bandwidth estimates are used not only to guide the mechanical system design by defining a limit on the lowest allowable system resonance, but also to define the control structure. A finite element analysis (FEA) of the mechanical structure can provide estimates on the values of the system’s primary resonant modes.

This information is then used to define the control requirements in terms of servo update rate, control structure (PID/state space), and the number and order of filters required. Based on the servo update rate, servo algorithm, and number of filters specified, it’s possible to estimate the number of operations per second required to implement the control algorithm, which helps define the processor type (microcontroller, DSP, or microprocessor) as well as operating speed.

System simulation

Beyond FEA analysis, which provides information on the mechanical subsystem’s frequency response, a mechatronics design approach can derive great benefit from full system simulation. Items in the control loop such as processor word width, sensor resolution, power stage, control algorithm, and the mechanical system can be modeled and simulated to verify expected performance. Operational parameters can be varied to attain desired performance levels or identify unexpected problems well in advance of completion of a functional prototype.

Meaningful mechatronics

As devices become increasingly complex and development timelines are reduced to shorten ROI and bring products to market sooner, the advantages of a mechatronics-based system design approach become ever more apparent. Micropositioning applications, in general, are well suited to the mechatronics design approach, as all the core technologies are present with significant interaction between them.

The processes applied in mechatronics are not new. Taken together, they represent a “best practices” approach that has been applied by groups of individuals with specializations in different engineering disciplines for many years. What’s new is the recognition of the value of this capability at the university level. Many universities now have mechatronics-based programs that provide engineers with exposure to mechanical, electrical, software, and control systems concepts with sufficient depth of knowledge in each discipline to understand the synergistic relationship between them.

Ron Rekowski is director of product management for Aerotech’s laser and medical group. To learn more, visit www.aerotech.com.