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Jerk Limit can be possible?


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Hello Everyone,


I have a path by a series point(not equal distance) in XY plane,usually we are using lookahead to control the max speed and the max accel in liner mode,but can't control the Jerk as my mind.

If i want to control the Max Jerk of the path,include the max speed and acc of course,What can i do? Or is't a simple way to control TS? because it seems the TS can control the Jerk.

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Limiting jerk in the general case is a very difficult problem. When all of the moves are long -- long enough so that they always have a constant-speed section between blends -- it is not so difficult, and properly computed TS times can limit jerk. In Power PMAC for these types of moves, you can specify maximum jerk and let PMAC calculate the TS time automatically.


But when you use a TS time to limit jerk, the blend between moves (and to or from a stop) always starts and ends at zero acceleration. Again, for long moves with a constant velocity section, this is fine. But in this case, you would not need lookahead even just for acceleration control.


When you have short moves, for example when you need to accelerate up to full speed over many moves, jerk limiting with S-curve at the programmed move level is not appropriate, because each little move starts and stops at zero acceleration. If you would plot the velocity versus time profile here, it would look like a series of steps (or escalators). This is almost never desirable, so in these applications, virtually everyone sets TS to 0.


Ideally for these applications, you would have a maximum jerk parameter for lookahead just as there is maximum acceleration and velocity. But it turns out that jerk limiting in lookahead is much, much more difficult than acceleration control in lookahead, just as acceleration control in lookahead is much more difficult than velocity control.


When a lookahead algorithm tries to solve a jerk problem in one axis, it can make the jerk, acceleration, or velocity of other axes worse. Even if a general multi-axis solution is available, it would take at least 10 times the calculation of the present lookahead.


Some users add a low-pass filter to the trajectory after lookahead. With Turbo PMAC, you can use a 1st order filter with Ixx40. With Power PMAC, you can use up to a 4th-order filter. This is not as good as an ideal lookahead limit, but some people find that it is good enough.

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First,Thank you very much for so long and detal answer。

But,the filter solution have problem,because it's change the path,one circle will not be one circle after low pass filter in one plane.

I ask this question because one of my customer do Laser cutting software based PMAC,But the speed can't reach the machine controlled by PA,

I check the PA controller,Jerk control inside.So the Impact will be controlled,so acc can be more big.

How holpful Jerk will be control in lookahead mode by PMAC.

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There are controllers that do explicit jerk limiting for single-axis moves, where the math and logic are much easier, but limit jerk by low-pass filtering for multi-axis path moves. Check the implementation very carefully.


The low-pass filtering needed for jerk limiting once acceleration limiting has been done through lookahead can have a much higher cutoff frequency and so have very little effect on the path.

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