Dynomotion

Group: DynoMotion Message: 10716 From: cnc_machines Date: 12/23/2014
Subject: Step Response Screen Units
Greetings,

I have spent quite a bit of time tuning my stepper motors using the step response screen. Does anyone know how to convert the Velocity, acceleration, and jerk units?

  1. I am guessing that the velocity is the number of microsteps pulsed per second? Convert this to velocity by the lead on the screw?
  2. Acceleration - the number of change in micro steps per second. Not sure how I would convert this to a G force or a unit a motor manufacture could understand.
  3. Jerk - How many microsteps acceleration changes in a given second?
I am hoping that with this and mass information I can design a new system and select a motor that will closely approximate the performance I am currently getting.

Thanks,

Scott
Group: DynoMotion Message: 10724 From: Tom Kerekes Date: 12/23/2014
Subject: Re: Step Response Screen Units
Hi Scott,

Velocity, Acceleration, and Jerk all have units of microsteps and seconds.  To convert to inches and seconds divide by your system's microsteps/inch.

The lead on the screw is only one of several factors which determine how many microsteps/inch your system has.

After the division your Acceleration will be in standard units of Inches/sec2.   1G = 384 Inches/sec2

Also after the division Jerk is in standard units of Inches/sec3.  Many have a difficult time understanding what Jerk is.  It is the rate the Acceleration Force gets applied.  Given a specified max Acceleration and Jerk the time required for the Acceleration to be applied is T=A/J.  It usually makes sense for the Acceleration to be applied over an interval of 0.01 to 0.1 seconds.  Faster than that will have little benefit (les shock) over being applied instantly.  Slower than that is usually excessively gradual and slows down machine operation excessively.  So a rule-of-thumb is to set the numeric value of Jerk to be 10X to 100X greater than the Acceleration Value.

HTH
Regards
TK

Group: DynoMotion Message: 10795 From: cnc_machines Date: 1/8/2015
Subject: Re: Step Response Screen Units
Tom,

Finally getting back to this after the new year. I think I still may have an issue with my calculations.

Lead Screw 10mm (.394")
Motor step angle - 1.8
Steps/Rotation - 3200
Direct drive
Calculated Steps/IN - 8128

I have the following data in the step response screen:


From what I understand from your explanation we would have the following values:
  • Velocity - 85,000 / 8,128 = 10.45 IN/Sec
  • Acceleration  - 8,000,000,000 / 8,125 = 984,252 IN/Sec^2
  • Jerk - 30,000,000 / 8,125 = 3690

The velocity seems reasonable to me. That comes out to 627 in/min, however the acceleration is greater than 2,500 G's. Doesnt that seem very extreme? I was expecting a value in the 3-5 G range?


Based off of your comments on the ratio of acceleration to jerk. Maybe because Jerk is lower than acceleration, the jerk parameter is controlling motion rather than velocity? Or maybe my math is wrong?


I set all of these parameters through trial and error - watching position error on the encoders. Is there a best practice on how to set these parameters?


Thanks!


Scott
Group: DynoMotion Message: 10796 From: Russ Larson Date: 1/8/2015
Subject: Re: Step Response Screen Units

Scott,

 

Use this simple spreadsheet from CNCzone and it will provide your answer.

 

Russ

 

 

 

 

 

From: DynoMotion@yahoogroups.com [mailto:DynoMotion@yahoogroups.com]
Sent: Thursday, January 08, 2015 4:16 PM
To: DynoMotion@yahoogroups.com
Subject: [DynoMotion] Re: Step Response Screen Units

 

 

Tom,

Finally getting back to this after the new year. I think I still may have an issue with my calculations.

Lead Screw 10mm (.394")
Motor step angle - 1.8
Steps/Rotation - 3200
Direct drive
Calculated Steps/IN - 8128

I have the following data in the step response screen:


From what I understand from your explanation we would have the following values:

  • Velocity - 85,000 / 8,128 = 10.45 IN/Sec
  • Acceleration  - 8,000,000,000 / 8,125 = 984,252 IN/Sec^2
  • Jerk - 30,000,000 / 8,125 = 3690

The velocity seems reasonable to me. That comes out to 627 in/min, however the acceleration is greater than 2,500 G's. Doesnt that seem very extreme? I was expecting a value in the 3-5 G range?

 

Based off of your comments on the ratio of acceleration to jerk. Maybe because Jerk is lower than acceleration, the jerk parameter is controlling motion rather than velocity? Or maybe my math is wrong?

 

I set all of these parameters through trial and error - watching position error on the encoders. Is there a best practice on how to set these parameters?

 

Thanks!

 

Scott

 

 

   @@attachment@@
Group: DynoMotion Message: 10797 From: cnc_machines Date: 1/8/2015
Subject: Re: Step Response Screen Units
Russ,

Thanks for the spreadsheet. Quite helpful, I cross checked it with the one I have and the results are the same.

Really my questions is regarding how to set velocity, acceleration and jerk. Is there a method to do this? I am going to try something along these lines, but not sure if it is a good method:

  1. Set Velocity by doing a move long enough to finish acceleration. Bump up velocity until steps are lost.
  2. Test combinations of jerk and acceleration until you start losing steps?
Thanks,

Scott
Group: DynoMotion Message: 10799 From: Tom Kerekes Date: 1/9/2015
Subject: Re: Step Response Screen Units
Hi Scott,

Its all based on the simple concept of rate-of-change.  If you divide how much something needs to change by they rate-of-change it will tell you how much time is needed.

For example Velocity is the rate-of-change in position.  To go 1000 mi in your car at a velocity of 100mi/hr will require 10hrs.

Velocity is the rate-of-change of Position
Acceleration is the rate-of-change of Velocity
Jerk is the rate-of-change of Acceleration.

In your case to achieve your specified acceleration of 8e9 counts/sec2 with Jerk of only 3e6 counts/sec3 would take 266 seconds!  Normally we would like to apply the acceleration in ~ 0.1 seconds. We want it as quickly as possible but without generating a lot of shock or disturbance.  Much like applying the gas or brakes in a car.

Most systems only accelerate at a fraction of a G.  So reduce your acceleration by ~ 10000X.

HTH
Regards
TK