Essentials of Motion Control Design — in Five Simple Steps

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Here, we give you five simple steps to avoid common errors when evaluating motion-control requirements. This list doesn’t cover every possible scenario or error, but it’s a starting point. (If you’re not an expert, we highly suggest you employ a knowledgeable vendor, especially for expensive and complicated systems.)

Rather than doing a blind search for a solution that may or may not apply to your process, let’s start with the basics. There are three elements to a basic motion-control system. These are:

• The things that actually move, including stages, actuators, platforms, and slides

• Electronics, including drives or amplifiers that accept a user command and convert it to a motion — for example, to convert a command to move one millimeter into a signal for the machine

• The “brains” of the machine, or the actual motion controller.

You are now on your way to a basic understanding of motion control. But novices may still harbor fears of wasting thousands of dollars on a system that’s either unsatisfactory or fails completely — and then endure humiliation or worse. To prevent such potential catastrophes, follow these five steps.

STEP ONE:
Make a list of process parameters that limit your design.

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The default approach is to list all the wonderful things your machine will do. Why would you possibly want to limit the design? But many projects spiral out of control without well-defined parameters. Limitations also help define what the machine really needs to do. Some things to consider are how fast your system must move to deliver sufficient throughput; the precision requirements of the process — laser cutting can only go so fast before it degrades part quality, for example; and what you can afford to put into a system versus what you can risk if you try to cut costs.

STEP TWO:
Create an error budget.

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The process of defining limitations leads nicely into creating an error budget. Here, you can greatly narrow the types of systems to consider for a machine by looking at the accuracy defined in your process parameters. Generally speaking, motion accuracy should be 10 times better than the smallest feature size you are measuring, machining, or processing. Also remember the cost of failing to get exactly what you need — and the cost of overspecifying. Costs rise exponentially with motion accuracy, and there’s no need for nanometer accuracy if micrometer accuracy meets the needs of your application for the foreseeable future. On the other hand, high precision is worth the high cost if anything less puts people in physical danger.

Fortunately, most processes aren’t so critical. In most machines, lower precision might simply mean the inability to move parts precisely enough to get tolerances that ensure fit with other parts. Either way, consider the cost of the alternative and the probability of it happening.

STEP THREE:
Consider how things change. Will the machine need to perform more tasks someday?

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Consider how you might use the machine in the future. Do you plan on changing the weight or size of your parts? Do you think your industry will require better tolerances and precision to satisfy future market demands? There’s serious business risk if you don’t pad a machine’s capabilities upfront with enough performance to let processes evolve at least somewhat.

STEP FOUR:
Consider technical support.

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Headaches abound when you’re putting a system together, so ease the pain by picking motion-control vendors that offer technical support both before and after installation. Without this support, you may find yourself on Google for hours, searching for the answer to some frustrating problem. That’s a waste of precious time.

STEP FIVE:
Be very careful about building machines from scratch.

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Of utmost priority for most design engineers is to keep costs down and hit a budget, even though unforeseen design details inevitably drive up cost. But all too often, we see engineers trying to save money by purchasing low-quality machines, only to have them underperform or fail to complete key tasks. These engineers then find themselves in the unenviable position of having lost considerable time and money. Plus, they now have the added expense of procuring a new machine that actually works for the application.