Explain the Secrets of Jerk

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SteveHGraham
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Explain the Secrets of Jerk

Postby SteveHGraham » Thu Jul 07, 2016 5:01 pm

Help me out with velocity, acceleration, and jerk.

I have been trying to set my z motor up so it functions correctly. I have several issues.

1. When I start the motor at zero on a dial indicator and tell it to move 0.500", it goes 0.504" or so. Changing the steps/inch doesn't seem to fix it.

2. As the motor starts, it seems to pop forward about 0.004" before beginning to turn for real. Obviously, this screws things up.

3. I read that I should have V set at 35000 steps/second for a NEMA 23 motor. This is 50000 - 30%, for a board that divides every step into 16 microsteps. I also read that A should be about ten times as high, so I set it at 350,000. I read that J should be about 20A, so I tried 7e6. The motor jerked to life so fast, I thought it was making the shaft slip in the collar, so I changed J to 5e2 (500). Now it's much smoother.

I do not understand why anyone would want the jerk to be so high it makes things snap, so I am wondering what the explanation is. Is high jerk just for industrial users who have incredibly stout, accurate tools and who need to have parts made as quickly as possible? Personally, I don't mind if a part takes five whole minutes. That's faster than I could do it manually.

The backlash correction is off. I am using Mach3. The board is a KFlop. The control software for the board is KMotion. Mach3 tells the board how many steps are in an inch.
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Magicniner
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Re: Explain the Secrets of Jerk

Postby Magicniner » Thu Jul 14, 2016 3:36 pm

I'd never heard of Jerk so I looked it up, this is linked to from the KFlop document which covers V, A & J settings -

https://groups.yahoo.com/neo/groups/Dyn ... sages/9252

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ctwo
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Re: Explain the Secrets of Jerk

Postby ctwo » Fri Jul 15, 2016 1:12 am

so what are your mach3 motor tuning settings, and how fast (ipm) will your motors actually drive before stalling?

did you see this?

http://www.dynomotion.com/wiki/index.ph ... C_and_Jerk

simply, you have:

V=in/s
A=in/s/s
J=in/s/s/s
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Magicniner
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Re: Explain the Secrets of Jerk

Postby Magicniner » Fri Jul 15, 2016 4:50 am

Acceleration is "Rate of Change of Velocity", "Jerk" appears to be "Rate of Change of Acceleration", probably named "Jerk" after the guy involved in the design of the system who then wrote the documentation, seeing no need to explain something which was obvious to him!

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SteveHGraham
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Re: Explain the Secrets of Jerk

Postby SteveHGraham » Fri Jul 15, 2016 9:19 am

I completely understand velocity, acceleration, and jerk. What I do not understand is why a HIGH level of jerk is helpful.

I get the impression that this stuff is geared toward fast production, not hobby work. It seems like they want everything to fly. I don't mind if the lathe takes five minutes to cut a thread; I'm not paying machinists by the hour.
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Magicniner
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Re: Explain the Secrets of Jerk

Postby Magicniner » Fri Jul 15, 2016 10:39 am

After all the recommendations they do proceed to suggest experimenting to get the system to run smooth.
Just ignore the recommendations and use settings that make it run smooth ;-)

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Rick
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Re: Explain the Secrets of Jerk

Postby Rick » Fri Jul 15, 2016 2:00 pm

Steve

Does this only happens in the Z axis? If so it may have to do with the weight of the Z axis. If the axis isn't counter balanced then the stepper system is seeing two different loads depending on if z is going up or down. And as you know you only have one set of parameters. The axis may actually be dropping a bit as the system goes from hold/idle state to run state. I am assuming your system is open loop. Does you system have a idle current ( or hold current) setting, they usually set this to about 50% of full power, if it does set it at 100% for the z and try it out. Another way is to counterbalance the z axis, this is done on some industrial machines also, I have a stepper open loop gantry mill and was having issues in the Z axis (missing steps) I counterbalanced it with a air cylinder and regulator and this took care of my problem. Just a couple of ideas.
Rick

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SteveHGraham
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Re: Explain the Secrets of Jerk

Postby SteveHGraham » Fri Jul 15, 2016 3:07 pm

I found out the motor was jumping because the controller was turning it off between jobs. When it turns back on, it jumps a few thousandths. The answer was to change the initialization code so the motors stay on all the time.
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