Dynomotion

Group: DynoMotion Message: 8486 From: Hugh Sontag Date: 10/17/2013
Subject: Effect of channel parameters on GCode execution
Hi Tom,

I'd like to know how the channel parameters interact with GCode execution.

I find that my Y axis is tuned so that a Move works well with a particular set of V, A, and J in the "motion profile", but when I run GCode with high feed rates, the SnapAmp current limit is tripped, and the GCode can't complete.

I thought that each axis had a max V, A, and J, and that choosing these numbers in the "motion profile" would guarantee that the axis wouldn't attempt to execute moves that exceeded them. Is that true?

If so, then what could cause a particular line of GCode to trip the SnapAmp current limit, when that doesn't happen when testing the Step Response?

The rationale for the max values, I thought, was to ensure that the maximum capabilities of the SnapAmp, motor, and PID parameters were not exceeded, allowing cutting to occur without errors.

Can a GCode program cause motions that exceed the V, A, and J in the "motion profile" ?

Thanks,
Hugh
Group: DynoMotion Message: 8487 From: Tom Kerekes Date: 10/17/2013
Subject: Re: Effect of channel parameters on GCode execution
Hi Hugh,

There are two different types of motions with different parameters controlling them.   The two types of motions are called Independent and Coordinated. 

Independent Motions are 3rd order Jerk Limited motions.  This type of motion is performed in cases of Rapids (G0), Jogging, Homing, and motions initiated by KFLOP User Programs.  These motions are performed using the settings in KFLOP and in the Step Response Screen.  Note these settings are in raw counts and seconds.

Coordinated motions are 2nd order motions (infinite Jerk).   These motions are Trajectory Planned in KMotionCNC using the settings in KMotionCNC's Tool Setup Trajectory Planner Screen.  Note that these parameters are in units of Inches and seconds.

It is common to have completely different settings for the two types of motion.  Simply adding a Jerk constraint will make the motion much smoother but also much slower.  Often the Acceleration and Velocity can be increased with Jerk Limits such that faster motion can be achieved and still have less disturbance that without limited Jerk.  You can simulate 2nd order motion on the Step Response Screen by setting the Jerk Limit to a huge value (ie 1000X the acceleration).

HTH
Regards
TK


Group: DynoMotion Message: 8488 From: Hugh Sontag Date: 10/17/2013
Subject: Re: Effect of channel parameters on GCode execution
Hi Tom,

So if I understand you correctly, Coordinated Motion in GCode has infinite jerk. This corresponds to what seems to happen, in that the Y axis makes a "bump" sound as if it's trying to move out of its bearings, and about the same time the SnapAmp faults.

It sounds like Coordinated Motions are also what is commanded by many GCode words.

So one approach to solving the problem would be to limit acceleration in the Trajectory Planner. Are there others?

Thanks,
Hugh


On Thu, Oct 17, 2013 at 4:49 PM, Tom Kerekes <tk@...> wrote:
 

Hi Hugh,

There are two different types of motions with different parameters controlling them.   The two types of motions are called Independent and Coordinated. 

Independent Motions are 3rd order Jerk Limited motions.  This type of motion is performed in cases of Rapids (G0), Jogging, Homing, and motions initiated by KFLOP User Programs.  These motions are performed using the settings in KFLOP and in the Step Response Screen.  Note these settings are in raw counts and seconds.

Coordinated motions are 2nd order motions (infinite Jerk).   These motions are Trajectory Planned in KMotionCNC using the settings in KMotionCNC's Tool Setup Trajectory Planner Screen.  Note that these parameters are in units of Inches and seconds.

It is common to have completely different settings for the two types of motion.  Simply adding a Jerk constraint will make the motion much smoother but also much slower.  Often the Acceleration and Velocity can be increased with Jerk Limits such that faster motion can be achieved and still have less disturbance that without limited Jerk.  You can simulate 2nd order motion on the Step Response Screen by setting the Jerk Limit to a huge value (ie 1000X the acceleration).

HTH
Regards
TK


Group: DynoMotion Message: 8490 From: Tom Kerekes Date: 10/17/2013
Subject: Re: Effect of channel parameters on GCode execution
Hi Hugh,

Yes coordinated motion in GCode are the G1,G2,G3 type of moves.  Yes you must check if you have reasonable acceleration settings set for your machine.

Another common mistake is to have Acceleration set ridiculously high but not realize it because only relatively small Jerk limited moves are tested.  In those cases there isn't enough time in the move to ramp the acceleration up fully.  

Regards
TK