Quarter circle: Without limiting

Quarter circle: With limiting An X-Y path of a 90° arc move is approximated by 18 short linear moves blended together. Without look-ahead, all initial acceleration occurs in the first half of the first programmed move, covering only 2.5° — and deceleration occurs in the final 2.5°. Velocity transitions are much smoother after look-ahead acceleration limiting.

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Other approaches

Attempts to manually control speed (with a feedrate-override control- panel potentiometer, for example) are almost always unsuccessful. Traditional trajectory generation algorithms don’t fare much better; they can only filter trajectories, so features and curves requiring a high block rate are rounded significantly to the inside, compromising accuracy.

In the machine-tool world, sometimes path programs are modified by a specialized CAM post-processor, which assigns lower feedrates as needed to specific blocks in the program for the machine’s specific limitations. However, this requires an extra processing step, and makes path programs machine-specific instead of universal. In addition, the post-processor must make assumptions as to how the machine blends between different speeds.

In contrast, a look-ahead algorithm within a controller has automatic access to that machine’s limits and blending algorithms. It takes the path program as input (without having to create a modified program) to slow the trajectory generated from the path program as needed.

When executing small move blocks at high speed, look-ahead algorithms calculate acceleration profiles out of a stop, and by scouting ahead, deceleration into a stop, over multiple move blocks. Similarly, the algorithms can calculate a deceleration profile into a tight corner, and the acceleration out of it over many blocks.

Suitable velocity profiles on acceleration- sensitive axes greatly extend acceleration (and deceleration) periods to cover substantially more path — often with several programmed moves each. The path is not changed, even though the speed at which the path is traversed is altered in some locations.

A second case in which look-ahead algorithms are useful is on long moves intermixed with tight, sharp corners between. Traditional trajectory generation algorithms generate constant path speeds when not cornering, so they only accelerate and decelerate during the cornering itself. In these schemes, low cornering accelerations require large corners or slow program speeds. If the required corner is so sharp that it cannot be taken at the programmed speed and still stay within machine capabilities, the entire path must be slowed to navigate the corner properly.

Say we have a two-axis machine in which X and Y exhibit high rates of acceleration at the beginning and end of a sequence. Here, the Y axis might need to accelerate rapidly in the negative direction at the corner as it changes directions.

A look-ahead algorithm can predict excessive acceleration to negotiate the corner at the programmed speed, reduce speed enough to corner within limits, and then work back along the path to generate a better deceleration profile. Coming out of the corner, it computes an acceleration profile along the path, back up to the programmed speed.

An algorithm such as this permits designers to separate corner-size specifications from maximum-acceleration specifications, which is useful for optimizing machine performance.