What does a mechanical engineer need to know about control theory? More than he did ten years ago. The same is true for electrical engineers and their understanding of mechanical dynamics. More and more, engineers are having to cross disciplinary boundaries, finding themselves in a place uncharted by traditional college engineering curricula. This middle ground formed by confluence of mechanical, electrical, and computer engineering is commonly called “mechatronics” and is changing the complexion of motion system design.

One person quite familiar with the concept, Robert Bigler, President of Animatics, Santa Clara, Calif., gives his take on mechatronics: “It is when you physically meld electronics with mechanics. And, I would say most anyone would agree that’s the future.” The continual advancement of this interdisciplinary approach to machine design has mechanical, electrical, and industrial engineers treading new waters.

Mechatronics involves a deeper and broader melding of both the intelligence and energy coursing through a machine than an “electromechanical” route system, which is based on converting electrical signals into mechanical displacement, or vice versa. Fueled by advances in semiconductor technology, electronic signals and artificial intelligence can now regulate, often strictly, output position and power, while at the same time control any number of machine function parameters along the drive line.

In traditional machines, the intelligence (logic signals) and power (motor current) meet in front of the drive, usually at the controller. In a mechatronic machine, intelligence and power meet in the drive. If the intelligence is in the drive, then the drive controller is a window into the machine. There you can see how the mechanical structure is responding not only to the drive, but also to the control signal, explains Mahito Ando, Venture- Com, Cambridge, Mass.

Little by little, electronic intelligence is taking over functions in interactive toys, semi-active automobile suspensions, point-and-shoot cameras, and other adaptive machines. In other words, electronics are being absorbed into everyday machines and everyday life.

“For three or four decades we have been entering the computer’s world,” says Dr. Victor Zue, associate director of the MIT Laboratory for Computer Science for MIT. In systems now under development, the computer adapts to our world. Cars, houses, and rooms will be instrumented rather than us having to carry data back to external computers, explains Zue.

In five to 10 years, smart technology “will percolate into our homes,” predicts Chris Luebkeman, director of research development at Ove Arup Engineering and a founder of MIT’s Intelligent Homes of the Future project. He envisions buildings that are “aware and adapt to us.”

Engineers involved in the design of such “smart” systems will be required to learn about and apply mechatronic technology. To help them, universities across the U.S. are beginning to offer cross-disciplinary courses and programs in mechatronics.

Mechanical bypass

Before the age of microprocessors, PLCs, and DSPs, state-of-the-art motion systems were heavily reliant on mechanical positioning devices. Complex (and often extremely clever) systems of cams, pulleys, indexers, and gears ran many machines with seemingly clocklike precision.

As applications demanded higher accuracy and electronic instrumentation began to take deeper root, the once-subtle shortcomings of many motion conveyance mechanisms became more imposing. Greater degrees of electronic control and monitoring made it clear that problems with accuracy often resided in the mechanical linkages. No matter how high the resolution on a controller or regulator, a servo could only position as accurately as the sloppiest portion of the system.

So it often was (and is) the case that, where high-grade servo positioning is required, meticulously processed gears, couplings, cams, and the like were used to supply high torsional stiffness and geometric tightness. These “servo class” components remain a huge factor in the scheme of many precision motion equipment designs.

But, even the most finely crafted mechanical part adds complications. As the driving component is linked to the load more and more directly, unwanted dynamics are eliminated and the load moves truer to plan. In the minds of many engineers, for certain designs direct-drive is the hands-down only option.

A direct-driven system may be one of several forms. A simple case is a motor directly fastened to the output shaft. The need for accuracy may preclude gearing or belting, even though the motor is liable to waste much of its energy running at relatively low speed and torque. A speed reducer harnesses the motor’s high-speed power to produce high torque at lower speeds. Thus, for simple rotary actuators, a direct-driven system is generally used only in rare cases where just the slightest backlash and loss of torsional rigidity are intolerable.

Nevertheless, these situations do come up, and they tend to be low-speed and high inertia. Therefore, large-diameter servomotors are often found on direct drives to supply the low-end torque needed. Specialized construction can make a motor better suited for direct drive. One design is an outer-rotor motor, in which the outer portion of the motor turns. The larger rotor radius converts a greater proportion of the electrical energy into torque; less of it goes into speed.

These direct-driven systems obviously rely on sophisticated controls – you’re not going to go to create a seamless motor-toload connection and then manually turn a knob or throw a switch to hit the targeted position. Closed-loop digital control is used. Often, a motion function is input into a microprocessor-based controller, keeping the motor output in strict compliance with the required load movement.

This programmability enables use of such mechatronic functions as electronic cams.

A mechanical cam, to begin with, is a device with eccentric contours traced by a follower. The follower translates to describe a nonlinear path of motion, transferring simple rotary or linear motion into a more complex movement pattern. A mechanical cam is often designed through numerous iterations leading to the finalized shape. Once machined, it’s pretty unlikely you’ll want to change it. These cams are made to provide a specific – and usually permanent – motion profile.

Electronic cams are a different story. With a motor driving an axis, a controller can be programmed in a matter off minutes to produce any number of motion curves. Reprogramming for a different profile takes little time. By driving multiple axes, each with their own independedent motor, you can provide complex synchronized motion that can be changed as required.

Furthermore, even if the motion profile is never expected to change, it’s not unheard of to use an electronic cam; sometimes the cost of machining a mechanical cam justifies the electronic alternative.

Setting up “electronic gearing” is another method of simplifying the mechanical composition of a drive system. Actually, electronic gearing is merely a multiple-axis synchronized motion setup in which a controller is programmed to run separate motors (and axes) at various ratios as required – the master-follower relationship. The mechanical version is a driving axis branching off into several drive paths geared to different ratios, all of which work together. With electronic gearing, there are multiple drives. This can get expensive, but eliminating mechanical linkages will increase accuracy, and the ratios are reprogrammable. They can even be programmed to vary during different phases of motion, becoming position- dependent gear ratios.

And electronic linkages facilitate inprocess adjustability. Jacob Tal, President and co-founder of Galil Motion Control, Rocklin, Calif., uses the example of packaging. “Sometimes in packaging, if you’re pulling paper, plastic wrap, and so on, they can stretch a little,” he says. “Let’s say you’re cutting labels to put on bottles, and they’re made out of a plastic material that keeps stretching. If you always cut at 12 inches and they stretch one mil each time, after a few hundred labels you’re cutting not at the edge of the label but in the middle. This can happen with fixed ratios. With electronic gearing, you can read the mark indicating the edge, electronically compare it to where it should be, and superimpose corrections on top of the gearing.”

Nevertheless, for simpler and less-strict positioning schemes, mechanical linkages are a reliable economical solution, allowing fewer motors in multi-axis systems. For very high torque and power, mechanical linkages are practically indispensable. Most applications require speeds quite a bit less than what a motor is capable of, and much of the motor power goes unused. Designs such as the outer-rotor configuration address this issue to a degree. For very high torque, however, the common (and economical) solution is to couple a smaller motor to a gear, belt, or chain drive.

In linear systems, a linear motor is the direct-drive solution. Although very expensive compared to a rotary motor driving a power screw, belt, or rack, linear motors, when linked with the proper controls, can produce uncompromising accuracy down to the sub-micron level.

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