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Merlin Jerk Speed

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Posted · Merlin Jerk Speed

I'm going through the Merlin code to get a complete understanding of its control over the steppers.

I believe JERK is defined as the instantaneous allowed change in velocity (mm/sec) that can ignored at a junction so that the speed does not have to go all the way to zero.

Merlin has the default XY jerk defined as 20mm/sec.

Merlin has the default XY acceleration defined as 4000 mm/sec2.

Kinematics states that v(i) = sqrt(((v(i-1) * (v(i-1)) + (2 * Acc))

If we want to see the delta change in speed (v) we can choose v(i-1) to be 0 therefore

DeltaV = sqrt(2 * Acc);

Placing the Merlin values in .... DeltaV = sqrt(2 * 4000) = 89 mm/sec which is about 4.5 times the default Jerk value.

My question is .... if Jerk is defined as the maximum change in speed that can occur at a junction and be ignored --- then should it not also be the maximum change in speed allowed during acceleration or deceleration?

If at a junction you don't want more than an change in Jerk speed (20mm/sec) then why should acceleration be allowed to produce a change of speed of 89mm/sec and be acceptable?


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    Posted · Merlin Jerk Speed

    Reprint from Merlin code:

    // The speed change that does not require acceleration (i.e. the software might assume it can be done instantaneously)

    #define DEFAULT_XYJERK 20.0 // (mm/sec)

    #define DEFAULT_ZJERK 0.4 // (mm/sec)

    #define DEFAULT_EJERK 5.0 // (mm/sec)

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    Posted · Merlin Jerk Speed

    I always thought that jerk was the acceleration of the acceleration.


    Interesting math either way...

    I found this (fairly old) discussion on the RepRap forum.

    As far as i understand, "jerk" is the derivation of acceleration (mathematically)...

    ...but... ... in Marlin it is not implemented this way (strictly speaking).

    I may be wrong, but i think it's simply the maximum change of speed at which Marlin joins two segments in the planner.


    You should also take a look at this thread - with a detailed explanation from @gr5.

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