I would like to set up Mach3 to use a third-order (or higher) motion profile. Basically, this ramps up acceleration slowly instead of instantly. This is important to minimize jerk (a technical term) and vibration.

Some background info that might explain it better: http://en.wikipedia.org/wiki/Jerk_(physics)

Are there any plugins written that will change the default motion profile, or can I write one? Without accounting for jerk (and even higher-order derivatives), you cannot have smooth, vibration-free motion. Instantly changing acceleration means instantly changing the forces affecting the CNC, which is not good.

I think it is not possible with the current version of match, but have read that they are working on a new trajectory planner for next version

You can try Quantum, or the new development planner, Tempest. http://www.machsupport.com/forum/index.php/board,83.0.html

That's fantastic! Unfortunately I can't figure out where Tempest can be downloaded. EDIT: No, wait, I got it. Post #55 in the linked-to thread seems to have the latest version.

Not fired up to tackle the bleeding edge just yet with Tempest, but...

For a machine like my Industrial Hobbies, it would be sweet to do as the OP suggests and start out accelerating slowly.  This would be true even if it was "hacked" to allow 2 acceleration values with a threshold speed after which the second acceleration cuts in.

The reason is sticktion.  My machine has dovetail ways and sticktion is a notorious issue.  The hard part is getting them moving, but once moving it's not so bad.  A slower startup acceleration would facilitate that and I bet it would allow much higher acceleration to immediately follow.

Cheers,

BW

I built some welding manipulators and actually got better motion out of them when using trapezoidal motion. The acceleration is higher when using sinusoidal motion rather than trapezoidal. The higher accel in the middle of the move actually caused a problem with rough motion. We were not moving real fast - welding moves at 10"/min, and rapids at 100"/min.