View Full Version : clarification of accel settings

01-25-2007, 09:29 PM
Is there someone out there that can explain in greater detail the settings in the acceleration dialog box in deskcnc. I understand the start velocity and max velocity setting. But the others to me seem rather abstract. "Keep increasing the acceleration value" ? Ok I can do that, but Id rather know what I was doing.
I understand that every motor and drive combo and every machine set up is going to be different so there is no right or wrong setting. I would like to know what that value means, I know the units are steps per second, but is it saying that the speed will increase by that many steps per second every second till it hits max? I dont think. Now we come to the truly bamboozling one, acceleration scale. Good grief.
"This is a scale factor that will reduce the acceleration most notably in high SPS ranges. Entering a Acceleration Scale greater than 1 will decrease the acceleration."

Its enough to make me,
well... write this post.
If anyone is still with me, im also curious about the CMC value, 0 is off, 1 is on, and anything higher is on with that value as a threshold. That makes sense just fine, but the following description?
"An this mode, if any axis has a change in Steps per Second that is greater than this value, will decel to the Start Velocity. If all axis' have a change in velocity that is less than this value, then no decel takes place. "

Can someone out there break it down for me? Feel free to be as condescending as you would like, I wont mind. My goal is to make my very slow very sedate foam cutter run without the odd pulsing that I assume is coming from improper accel settings.
PS is there an up to date manual somewhere?

Adobe Machine
01-25-2007, 11:21 PM
I need to know your encoder value (cpr)or just look at your encoder and give me the P.N. and manufacturer..most are a 200 count that yeilds 800 cpr, but let us not guess.

I need to know the reduction and lead ..I.E.

Reduction = small pully (usually at the servo)
large pully ( usually at the ball or acme shaft)
Just count the teeth on each one
Lead = number of turns the ball shaft or acme shaft turns / inch.
I will need to know this info for all axis.

Then I will plug that into one of my machines and test it, then return the values for you to use.

The top number in the accel tab is just a number that "sets" the maximum accel ( sps). The instructions say to enter the max velocity (sps) ( that is the 3rd line down,) then start velocity, then an accel scale, then pick a number for the top line and start adding zero's untill your accel scale starts from the bottom left and goes to the top right in a smooth line. But before any of this can be done, you must set your axis tab 1st..The maxium sps is a math calculation of the theoritical max rpm of the servo, times the encoder CPR ( I term it "clicks per rev ")divided by 60 = the very maximum sps, but should be 5 to 10 % less to keep the servo amps from faulting.As an example:
encoder cpr= 800 times (*)2700 ( servo rpm)=2,200,000 divided by (/) 60 =
36,666. which represents the maximum sps. I would enter 35000 as the sps.

Look at "Filling Machine Velocities Tabs" by KOLOKITHAS, as he just asked the same questions..I set his values in one of my machines and gave him a conservative set up..He stated it all worked well.

Another way to visulize this is to go to the machine set up ( again) and at the bottom click " Modify Accel ramp", open that and you will understand how the accel ramp is made..do not change that yet untill after you get the machine set up and running, but it is fun to go back and customise your accel ramp once you know what your machine dynamics are.

Once you get past the accel values and the rest of the set up, you will really like Desk CNC..it is smooth, has look ahead, the screen although not jazzy, has everything you need to run a 3 or 4 axis machine safley. Depending on the type of servo amps used, you can run some real high IPM in GOO.

Anyway, get the info, I will set it up and give you the info.

Adobe (old as dirt)

Adobe Machine
01-25-2007, 11:29 PM
Up date manuals: Go to imserv@imsrv.com

Fred Smith has collected and written everything you kneed to know about Desk CNC, you can down load all the manuals, trouble shooting and some tips. I keep them in a seperate note book, and still have to refer to them occasionally.

Adobe (old as dirt)

01-26-2007, 12:27 AM
ive actually been using it for 3 years, but ive just become annoyed with the lack of support with it and when you really try to find your own answers, they just arent there. ive had a few serious problems with it in the past, trying to buy a replacement board and being hung up on by the woman because i didnt have the original disk that came with my machine 3 years ago which it seems is how they verify you paid for it. i still have 3 posts on their website for the last 2 months that are completely ignored by the support guy, who I have absolutly no problem with. this is turning rantish now i guess.
im trying to smooth the opperation of my machine since it seems to be a tad on the rough side as far as smoothness of motion. you can sort of hear the motors trill(accelerate and decelerate every 0.010 as the software is siet to break the dxf into) as they run in straight lines. no doubt due to accel settings being sub par. im sure i could do it if i understood what i was changing as apposed to just trying different numbers till it looks right.
anyway i appreciate the help, cant look a gifthorse in the mouth. I just would like a better explination of what their settings do, for a very slow light machine I would imagine the settigns would be much different than for a big iron mill. Id like to reserve the right to pick your brain in the future on the constant motion contouring setting if i cant figure out whats helping or not

01-26-2007, 10:00 AM

Try increasing the acceleration scale. It increases the amount of time the machine takes to accelerate, which should smooth it out substantially. It worked for me anyway.


Adobe Machine
01-26-2007, 12:55 PM
Yes, all of my machines run over 0.300 seconds accel time which really smooths it up. If , as you say your up and going and have been for some time, I would go to the modify accel ramp and begin there..You would be suprised how well you can tune your machine for what you want.Believe me , will take more than one time, and there are a lot of numbers to change over a full scale, but the results are worth it.And the big thing is , it will show you how the accel ramp works..Not just experimitting with numbers.

You did state you are hearing the servo's "Trill" ? Have you tried setting the gain, limit and dampen on the servo amps ? Improper setting may cause the poor performance you seeing ..along with the "noise". but you should hear the servos ramp up and down in corners , ect..That is because Desk CNC has look ahead, and will not slam into the corners or direction changes.

Work with it and ask questions, seems like its been a rough go from the start with your project, and that is no fun..supposed to be a hobby, right ?

Adobe (old as dirt)

02-06-2007, 05:13 PM
Hi there!

I have to confirm that when Adobe gave me the starting values to fill the tabs,i went with 2650 (free)rpm(2800 servo rating)and all worked fine.Also i have to confirm that is true to experiment with the acceleration scale.The only way to make your machine work smoothly is to EXPERIMENT.Because every machine is different(masses,slides,screws,etc) you can't work it other way!!Just experiment, trial and error(on the safe side always).

And eventually sometime it will come to you(the near perfect setting).I have to admit that i have a long road to get my machine smooth as it can be, but...
I will die trying!!!:banana:

See you laser!!!

02-06-2007, 06:27 PM
there must be a difference in the way deskcnc sets up foam cutting files as opposed to those for a regular machine. Ive never used it to run a mill or router, so maybe its hard for me to explain. when i create a file for foamcutting, there is a setting that asks how long of segments i want to break the motion into. so if i cut a 1 inch square with the setting at .1 the machine makes 40 tiny moves, 10 on each side. the software is apparently trying to decelerate and accelerate for each and every one of these moves. so when the machine is making a perfectly long straight cut, the motors (steppers adobe not servos here) are constantly pulsing. I imagine with a normal file for a mill, that the machine is only asked to decelerate and accelerate at the end of straight cuts and perhaps many small steps aroud a radius, is this true? A foam cutter can not change speed thru a cut, it must remain a constant (there are exeptions), there is no high speed moves above a z plane, in fact there are no such things as a G0 anywhere in any foam cutter g code. I guess the perfect thing for my machine would be no acceleration or decelleratoin between moves, but only when beginning motion from a standstill, or when there is a direction change of greater than 45 degrees between any two adjacent steps of motion. does this description help anyone understand what my problem is? Not slamming into and around corners, but rather slamming along in a straight line. The pulsing of the motors sets up a vibration in my 9 foot long wire which causes a poor cut, and accessive kerf. I guess I need to figure out what is the fastest speed that i can start my motors from zero without losing steps. then finding the max speed at which i cut and choosing the fastest accel between the two.
Im hoping that the constant motion contouring feature would help with this, but the description of its opperation is not made very clear in the help files, and i cant find the manual on line and would be surprised if it helped much more than the help files.
hope to hear from someone