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achieve sharper corner with minimum velocity reduction


lukewang
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I am using Clipper to control a laser cutting machine. When cutting a square or a sharp corner, I experienced a problem:

 

the corner has a noticable arc inserted automatically. Only way to reduce this arc seems to reduce TS, or use exact mode allowing machine a short dwell at corner.

 

Both solutions are not ideal. The first one caused machine vibration, the second caused corner over-burn.

 

Just wonder if there are any Clipper parameters which can fine tune this corner with minimum arc radius and maximum velocity.

 

Thanks in advance.

 

Luke

 

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The Clipper (and all Turbo PMACs) does provide tools for dealing with this issue, but there is a fundamental physical trade-off that will always be there.

 

When your path changes direction, there is a centripetal acceleration perpendicular to the path of magnitude V^2/R, where V is the vector speed, and R is the instantaneous radius of curvature. With a large acceleration/blending time from TA & TS parameters, your cornering radius stays large, and keeps the acceleration small, so you can do it smoothly. As you reduce your TA/TS time, your cornering radius gets smaller, and your cornering acceleration gets larger, and you have noticed that this rocks your machine. Of course, a perfectly sharp corner (R=0) would require infinite acceleration at any non-zero speed, so you must stop to get this sharp corner.

 

The "lookahead" feature of Turbo PMAC helps you manage the trade-off. As most people use it, the TA and TS times are set small enough to get the desired corner size without excessive rounding. Turbo PMAC initially calculates this path at the programmed speed (usually from the F value), and it often results at this point in excessive acceleration at the corners. This initially computed trajectory is then passed through the lookahead algorithm, which checks the trajectory segment by segment against motor acceleration limits in Ixx17. When a violation is found, motion is slowed enough to keep the acceleration within limits, but the path is not changed. The result is that speed will decrease into relatively sharp corners, the corner is taken just slow enough to keep acceleration within bounds, and then speed increases back up to the programmed speed as the corner is exited (the process ends up being just like how you take a corner in your car).

 

Most users are able to find settings that manage the trade-off well enough, but as I emphasize, the trade-off will always be there. If you want pretty sharp corners with accelerations small enough not to vibrate your machine, the corners will need to be taken more slowly than the straight sections of the path. If this is so slow that your laser over-burns the part at corners, you may need to modulate your laser power based on instantaneous vector speed (many people do this with a 1 or 2-line PLC program).

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The corner is inserted automatically due to blending.

 

Usually, if you want a sharp corner, you do need to either insert a dwell (e.g. dwell 0) or disable blending (Isx92) for this particular transition.

 

If a dwell is not desired, the only other solutions are:

- Optimize TA and TS until satisfied. Tuning can be relevant, as well as mechanic's maximum accel and decel.

- Program your corner to specs. break it down to segments, use segmentation with circle commands.

 

Laser over-burn

The over-burn is usually handled by reducing the laser intensity around corners. This is done by calculating the tool tip velocity on the fly (e.g. in a PLC) and sending a proportional laser output.

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  • 2 weeks later...

Hi Luke,

Great advice above from Curt and Richard. Yes for sure on any laser cutter I've worked on we had to implement an algorithm running either in Open Servo or Kinematic buffer to offset laser power based on actual cutting speeds...

 

I suggest you look at this post with download for a good idea on doing this:

http://forums.deltatau.com/showthread.php?tid=147&highlight=predict+future

 

Search this forum for "Can the PMAC Predict the Future"

 

This shows clearly how to use the kinematic buffer and lookahead to provide very clean way to do your calcs in advance of speed changes and then implement a output such as laser power at exactly the time when it is needed... in other words with the kinematic buffer you can predict the future and have the laser power adjusted at same time as speed and not just follow or react after the speed change.

 

Hope this helps...

 

 

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