Merlin Jerk Speed

[jhinkle 1]

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?

Thanks

Edited November 6, 2015 by Guest

[nallath 1,117]

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

[jhinkle 1]

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)

[eldrick 64]

Just for the record, it's "Marlin", not Merlin.

[jhinkle 1]

I figured it out -- Thanks.

All is well.

Here I thought this was all magic -- now I know it's just fishy! -- Merlin vs Marlin.

Edited November 6, 2015 by Guest

[tinkergnome 926]

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.

Edit:

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

Edited November 6, 2015 by Guest