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Question about velocity and lookahead


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I have a question regarding lookahead on the Power PMAC and how to get arcs more accurate.   We have a linear move at 2000 inches/min going into an semicircle with 0.1" radius at 330 IPM, followed by another linear move at 2000 IPM.  Point A is where the semicircle should start and Point B is where the semicircle should end.  The inner path shown is the actual path the PowerPMAC uses.  


What I was expecting to happen with lookahead enabled (Coord[1].LookAheadSize = 2048) is that the Y axis would slow down from 2000 IPM to 330 IPM at Point A so the desired path is maintained.  The following is the same motion commanded on a Beckhoff control using their lookahead algorithm.  You can see the Y axis Velocity slows down to 330 before the X axis starts to move.  The path is maintained.  


The following plot is the velocity on the PowerPMAC.  The Y axis does slow down but not slow enough when the X axis starts.  The end result is that instead of a rounded slot, we get an egg shape.  


The only way we can get the shape to be close to accurate is to either program exact stops at points A and B (not desirable) or set the Ta and Ts very small (5 mSec) which causes the machine to shake pretty violently.  Is there a way to get the lookahead control to reduce the velocity smoothly so that when it hits the tangent point of the arc, it is at the programmed arc velocity?   I have tried explicitly specifying a feedrate on the arc, letting the acceleration limits of the motor slow the arc down, and using Coord[].MaxCirAccel to slow down the velocity of the arc.  They all seem to yield the same velocity plot above.  

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  • 2 weeks later...

I recommend checking the deviation of the command trajectory first.

The look-ahead function of PMAC will not tamper with the trajectory, but if the blend parameter of PMAC is not set properly, it may lead to deviations in the instruction trajectory.

Usually Ta Ts will affect the accuracy of the trajectory blend, you can try to set Ta Ts to 0 at this position.The premise of setting Ta Ts to 0 is that the Look ahead function is set correctly, otherwise the machine will vibrate.


Edited by Sangmo
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