Here are the times it takes the first bending mode displacement of various machine base materials to decrease by 50% after a force impulse.

The goal is to produce the perfect part. Machine tool designers are inching closer with new motion technology that squeezes almost every micron of imprecision out of cutting, positioning, shaping, and assembly functions. But even new technology has limits. To chip away at those last few microns, designers need to refocus on machine dynamics, particularly settling time.

Settling time is a measure of how long it takes a motion system to come within a set distance of a desired point after the initial positioning move. It can also be a measure of how long it takes to lock in on a desired velocity. For equipment that makes short, quick, repetitive moves, such as punch presses, pick-and-place machines, and laser cutters, settling time can mean everything in terms of performance, drive sizing, and stage design.

Linear motion stages can take 1.5 to 2 times longer than the programmed move time to settle within the positioning tolerance (static accuracy). For example, if a stage moves to a rough position, 2 to 5 times the tolerance, in 30 msec, it may take an additional 30 msec to pull within tolerance. Over a series of moves, settling times quickly accumulate, effecting the desired throughput.

Tolerances and resolutions

To shorten settling times, designers should look at the whole system. It may even be necessary to conduct an FEA analysis on the interactions between resolution and tolerance.

Digital motion controllers determine position by counting. The number of counts per unit distance determines system resolution. For example, a motion stage using a linear encoder with 25 lines/mm and a 4x multiplier in the controller has a resolution of 100 counts/mm, or 10 μm. For a 40-mm move, the controller sees 4,000 counts and sends the appropriate command to the motion programmer. The programmer sends instructions to the stage, which then moves the commanded distance to its programmed position tolerance. That tolerance depends on the mechanical capability of the stage.

Energy from an oscillating ballscrew can transfer to the stage, increasing settling time. Replacing the ballscrew with a linear motor, as was done on an Ingersoll HVM 600 machine tool, is one way to reduce the excess motion.
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Tolerances are set in controls as a number of counts. They affect the overall commanded static accuracy of the stage, measured at the stage feedback device. This accuracy is different from the total mechanical system accuracy at the part because of the design of the machine, stiction, traveling friction, temperature, encoder location, load location, and other factors. For a tolerance of ± 20 counts, a move of 4,000 counts could be considered complete when the stage reaches 3,980 to 4,020 counts. With a 10 μm resolution, this is a position accuracy of ± 200 μm, which may be insufficient for the application.

So it's important to specify the tolerance when defining position accuracy for one or more moves in a time frame. Customers will usually state this as: "The stage will move 1 mm and settle to within ± 3 μm in less than 40 msec." The machine tool builder then knows to control the motion and stage settling oscillations to within ± 3 μm. If this tolerance is large enough, the machine tool builder may not have to alter the design to gain the desired throughput.

Weighty matters

But many machining functions, such as those called for in contouring moves, require increasingly tighter tolerances. The settling time of a linear stage can have a dramatic affect here. The machine controller does not consider a move complete, and therefore will not execute the next command, until the stage is in position. If settling takes too long, it can ruin the part.

The material used in the stage can hinder or help settling times. While all materials respond to step and vibration inputs, steel and aluminum take longer to settle, making them unsuitable for motions with small tolerances. Cast iron meehanite, the mainstay of most machine tools, and polymer concrete quickly dampen vibrations induced by stage motion.

Designers must examine the individual components of each subsystem to develop a light, stiff system with good vibration dampening features.

Motion stages are made up of three main sections: a moving platform, a stationary base, and the drive system. The motion energy that drives the moving component transfers to the stage base by Newton's third law of motion (every motion has an equal and opposite reaction . . .). From here, the energy moves to the machine base. If the base is not sufficiently massive or securely anchored to the floor, the stage base may oscillate, increasing the settling time of the whole system.

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