Jump to content
Ultimaker Community of 3D Printing Experts

Recommended Posts

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 by Guest
  • Like 1

Share this post


Link to post
Share on other sites

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)

Share this post


Link to post
Share on other sites

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 by Guest

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Our picks

    • How to 3D print with reinforced engineering materials
      Ultimaker is hosting a webinar where we explain how you can achieve and maintain a high print success rate using these new reinforced engineering materials. Learn from Ultimaker's Product Manager of Materials and top chemical engineer Bart van As how you can take your 3D printing to that next level.
      • 0 replies
    • "Back To The Future" using Generative Design & Investment Casting
      Designing for light-weight parts is becoming more important, and I’m a firm believer in the need to produce lighter weight, less over-engineered parts for the future. This is for sustainability reasons because we need to be using less raw materials and, in things like transportation, it impacts the energy usage of the product during it’s service life.
        • Like
      • 12 replies
×

Important Information

Welcome to the Ultimaker Community of 3D printing experts. Visit the following links to read more about our Terms of Use or our Privacy Policy. Thank you!