Motion System Design

A new spin on motion

Media researchers are constantly analyzing how people read magazines. What they know – although it takes them hundreds of pages to say it – is that some people start in front, while others start in back. I suppose PT DESIGN lends itself to either approach, but for this issue, I suggest starting in the middle with the special pullout section on planetary gearing.

Before you head off to planetary gear land, however, I'd like to point something out. In today's machines, mechanical, electromechanical, and electronic functions are often so highly intertwined that it's difficult, if not pointless, to separate them. This is particularly true in the case of planetary gearing.

Think about backlash for a second. Whether a planetary introduces X amount of play may be a moot point if the associated motor controller compensates for lost motion or avoids move sequences that cause it. A sloppy coupling can also make backlash specifications meaningless – for any component.

This interaction of functions necessitates a corresponding change in the way we think and talk about motion. Instead of imagining torque, for instance, as residing exclusively in one component or another, we need to start thinking of it as a constituent of everything occurring throughout a motion system. In other words, torque – or speed, or precision, or vibration – is not merely what comes from the output side of a gearbox, rather it's the combined result of the workings of software, electronics, electromagnetics, and mechanical and thermal dynamics. If you really want to be precise, you could say that torque (like everything produced in a motion system) is a derivative of the move command, control loop dynamics, amplifier response, winding current, magnetic field interaction, electrical and magnetic losses, rotor and bearing dynamics, coupling dynamics, and any mechanical advantage in the path to the load. It's a function of the system, not a particular component.

Lucky for us there's some linguistic shorthand to describe this interdisciplinary nature of things. The word is "mechatronics," a hybridization of mechanical and electronic.

I imagine that among our readers are some recent engineering graduates with formal training in mechatronics. The rest of us, to one degree or other, have gained this interdisciplinary perspective and knowledge on our own. There's no question in my mind, it is the future.

We are at a point today on the technological time scale where the improvement versus cost curves have saturated for many machine components based on one-dimensional (singlediscipline) thinking. The good news is that there appears to be quite a bit of untapped potential to achieve some fairly significant gains economically by leveraging one technology against another. I like to think of these opportunities as loopholes left behind by a century's worth of conventional engineering.

Let's say you're designing a machine that requires 20% more precision at its current selling price to remain competitive. It's unlikely you'll be able to pull it off through mechanical or electronic means alone applied intensively to a single component. But you may be able to coax the additional precision out of the fabric of the system with a tweak here and a tweak there. Maybe you recode some software, swap couplings, use a different sensor, and add a surface treatment to your bearings or gears. That's what the mechatronic approach is all about; and it works.

Even component manufacturers are getting in on the act. Five years ago, if you wanted to automate a servo axis, you'd most likely go to one manufacturer for a motor, another for a gearhead, and possibly a third for a controller. Back in your test lab, you'd fiddle with the components until they worked together. Today, however, you can often get what you need in an integrated package – motor, speed reducer, and controller in one. If you want to learn more about it, then, like I said, start in the middle of the magazine.

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