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Author Topic: changing kernel speed and maximizing travel speeds ?  (Read 5623 times)

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changing kernel speed and maximizing travel speeds ?
« on: May 03, 2008, 07:32:33 PM »
I am finishing up my large (30hp) cnc lahe retrofit and I have a few questions about tunning for the highest possible rapids on the x and z axis with Mach . I am using 2kw, 3 phase brushless AC servo motors and matching drives from Automation Direct. Doing the math with their servo-sizer software my setup should be cabaple of rapids in excess of 1000 IPM  . Using the out put signal from another machine with Yaskawa PLC I was able to easily run the x and z axis at a sustained 800 IPM with no problem , motors and drives never got above room temp

I would like to use mach3 with this machine , I have installed the c11 breakout board from cnc4pc and mach 3 , the x axis requires 90k pulse per inch and the z requires 50k pulses per inch . With the kernel speed set at 45k hz the maximum IPM that can be input x axis is 30 and the max for z that can be input is 52 IPM . The software will not allow me to type in a larger value. The machine seems to run fine at these speeds but is painfully slow. If I change the kernel speed to a higher value I can then change the IPM in motor setup to a higher value but when I try to move the machine at anything over aprox 65 IPM the motors run in pulses , pause and then run , stopping and starting at regular intervals. The motors are not stalling , its as if the drives are receiveing the steps in burst with a pause in between.

What do I need to adjust or experiment with to get rapids of at least  400IPM? Could this be an issue with the computer or memory? its a p4 2.6 ghz
« Last Edit: May 03, 2008, 07:37:38 PM by panaceabeachbum »

Offline Hood

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Re: changing kernel speed and maximizing travel speeds ?
« Reply #1 on: May 03, 2008, 07:58:22 PM »
What you need is a higher kernel speed with a smooth pulse, 45Khz may well be the max you are going to get out of your parallel port, I know it certainly is on the Lathe I have. You should however be able to do one of two things. First is your drives will probably have electronic gearing, this allows you to send a lower pulse rate out of the parallel port and the drive multiplies it for your encoder/motor. The drawback to electronic gearing is you can get cogging at low speeds, this may or may not be a problem but it is not ideal and it is why I have limited my gearing 2:1. The best option is to get a SmoothStepper, there is a waiting list for them at the moment and at present the sync for threading is not done and that is the only reason I dont have mine on the lathe yet. The smoothstepper has external pulse generation and can pulse up to 4MHz which is almost 100 times faster than the parallel port at 45KHz. So if you get 30IPM theorectically you could get 3000IPM with no electronic gearing.

Hood
Re: changing kernel speed and maximizing travel speeds ?
« Reply #2 on: May 04, 2008, 02:02:13 AM »
Is this with no mechanical gear reduction- I.e. the motor is directly coupled to the lead screw (or other transmission method)?

Just Curious?

Thanks,
Sid

Offline Hood

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Re: changing kernel speed and maximizing travel speeds ?
« Reply #3 on: May 04, 2008, 04:54:44 AM »
Sid you meaning what I was talking about?
 If yes then no its electronic gearing in the servo drive that I am talking about. If its the steps per unit that panaceabeachbum is meaning then I would think he is 1:1 coupled on the Z and 1.8:1 on the X. I am sure the encoders on the AD  motors are 2500 count so thats 10,000 as far as Mach is concerned, would imagine 0.200 pitch screws so 5 x 10,000 to move 1 inch on the Z = 50,000 steps per unit. He says  90,000 on the X axis so 90,000/50,000 =1.8

Hood
Re: changing kernel speed and maximizing travel speeds ?
« Reply #4 on: May 04, 2008, 11:37:15 AM »
Yeah Hood- I was.  Thanks!  Doesn't your head hurt being crammed with all that info up there?   ;D

I was a bit confused- I saw a system the other day that had a Brush DC servo w/ 500 ppr, 20:1 gear box coupled to a 3.3" pinion. and I was wondering why they used the reducer.  But, now I think I get it- without the SS, there is no way to get that kind of resolution (somewhere around .0002 pulse per inch I think) unless you use the gearbox-right?

As always!  Thanks!
Sid

Offline Hood

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Re: changing kernel speed and maximizing travel speeds ?
« Reply #5 on: May 04, 2008, 02:53:59 PM »
Sid
 Usually 500ppr encoders is what people go for, thats because its a decent compromise between resolution and rapids. With 500ppr thats 2000pulses from Mach, 5mm pitch screws  means 400 pulses per unit. With a 45,000 kernel speed that means 2,700,000 per min /400 = 6750mm/min. The encoders on my motors are 2000ppr so thats 8000counts for Mach, I could use secondary encoders and set it up in the drives but with the SmoothStepper it gives the best of both worlds ie smooth fast pulses and high count encoders = high resolution and breakneck rapids :)
Hood
Re: changing kernel speed and maximizing travel speeds ?
« Reply #6 on: May 04, 2008, 03:41:32 PM »
yep HOOD you nailed it on the encoder counts and ratio, I am headed out now to try the electronic gearing , I think I am on the smooth stepper list but am probably pretty far down on the list and threading is very important to me as I am in the sound suppressor business. Thanks for all your help