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Author Topic: Limiting motor acceleration  (Read 11484 times)

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Offline jimpinder

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Re: Limiting motor acceleration
« Reply #10 on: July 28, 2008, 12:52:00 PM »
O.K. - I'm sorry -  ??? :-[ :-[ :-* :'(
Not me driving the engine - I'm better looking.

Offline budman68

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Re: Limiting motor acceleration
« Reply #11 on: July 28, 2008, 01:14:45 PM »
Nah, Jim, I don't think our comments were meant that way, I think once you do finally run out of your demo amount, you'll be happy to contribute to this software, no?
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Offline jimpinder

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Re: Limiting motor acceleration
« Reply #12 on: July 28, 2008, 01:27:11 PM »
We are getting a bit off the post here - Yes - I think Mach 3 is the best thing since sliced bread ( to coin a phrase). It let me convert a lathe/mill into a CNC machine for a very small price. It is, as far as I can see, complete in evey way, and opened up a whole new approach to my machining. I was getting a lot of my stuff done professionally - in steel and in plastic - and I am capable of doing this at home now (if my machine is good enough). At least I can understand what the CNC guys are on about.

I haven't run up against a limit with the download version - and being from Yorkshire is like being a Scotsman with no arms, we don't like spending money if it is not necessary.

Now another short coming is threading - I think - and I want to have a go at that sometime - at the moment I have a good range of taps and dies - .

Yes - you are right - when the shortcomings start limiting my work I will gladly pay my money for what I consider to be a superb product.

In a post reply to Ftec -
Yes - Mach 3 can be set to any acceleration - and the ones you are quoting are at the bottom end of the scale.



Not me driving the engine - I'm better looking.

Offline ftec

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Re: Limiting motor acceleration
« Reply #13 on: July 29, 2008, 10:37:58 AM »
...
In a post reply to Ftec -
Yes - Mach 3 can be set to any acceleration - and the ones you are quoting are at the bottom end of the scale.

I see. I have no installed Mach yet as I need to get another computer for it so I havent been able to see what it says. So what do you think, does a low accel value have a negative impact? (The gantry weights actually appr. 95 kg. With the accel time of 0.1s from 0 to top speed 166.7mm/s the 300W servo is running in it's countinuos range. It can deliver a 900W peak if necessary).

Offline ger21

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Re: Limiting motor acceleration
« Reply #14 on: July 29, 2008, 11:06:19 AM »
So what do you think, does a low accel value have a negative impact?

It can. If you cut parts in Constant Velocity mode, corner rounding may occur. The degree of rounding is related to acceleration. Faster accel = less rounding.
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html

Offline ftec

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Re: Limiting motor acceleration
« Reply #15 on: July 29, 2008, 02:59:55 PM »
So what do you think, does a low accel value have a negative impact?

It can. If you cut parts in Constant Velocity mode, corner rounding may occur. The degree of rounding is related to acceleration. Faster accel = less rounding.

Well, that was what I orginally was asking, if Mach can optimize the axis/routing path according to what the motor can deliver acceleration. So you mean it can't?

Offline ger21

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Re: Limiting motor acceleration
« Reply #16 on: July 29, 2008, 09:10:07 PM »
The corner rounding when in CV mode is the only time that Mach will change the toolpath from what it's supposed to be. There are a few different CV settings that will let you customize how Mach behaves during CV mode. http://www.machsupport.com/docs/Mach3_CVSettings_v2.pdf

There is no "optimisation" of the toolpath based on acceleration. You tell Mach where to cut and how fast to cut, and it accelerates using the accel rate that you set. If the accel is too low to reach your intended velocity, then it won't reach it.

Your original question asked if mach would modify the toolpath so that the max accel would not be exceded. Mach ALWAYS accelerates at the rate you set, never more, and never less/ The toolpath has nothing to do with the acceleration.
« Last Edit: July 29, 2008, 09:12:20 PM by ger21 »
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html

Offline ftec

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Re: Limiting motor acceleration
« Reply #17 on: July 30, 2008, 02:40:55 PM »

Your original question asked if mach would modify the toolpath so that the max accel would not be exceded. Mach ALWAYS accelerates at the rate you set, never more, and never less/ The toolpath has nothing to do with the acceleration.

Ok, my question was obviously inaccurately put. However, what I had in mind is that when the tool travels a certain curve, the path, and a change in it's shape occures, the velocities (in a 2 dimensional plane) dx/dt and/or dx/dt will need to change. Thus there will be changes in the time derivates of these velocity components ie. the accelerations.  Now if Mach could make sure that neither of these accelerations exceed the given max value by changing the resultant velocity everything should be ok as far as I can see, even with a weak servo. The pdf file you gave only gives an overall picture, how can I get into detail what I can expect to happen in real life with an acceleration mentioned?

Thanks,
Risto

Offline ger21

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Re: Limiting motor acceleration
« Reply #18 on: July 30, 2008, 09:41:53 PM »
After reading your example, I see that I mis spoke in my last post. Mach doesn't always accelerate at the set rate.  It will not, however, ever exceed it. Which answers your question I think. :)
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html

Offline RICH

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Re: Limiting motor acceleration
« Reply #19 on: July 31, 2008, 07:09:16 AM »
Risto,
"how can I get into detail what I can expect to happen in real life with an acceleration mentioned?"

A practical approach would be to look at similar machines by vendors and  individuals and see what they are using.
Make note of the sizes, weights, axis or drive chains, and speeds provided and also what they doing with the machines.
Use your computer program and compare its output to what others are using that works in real life.

I am not familar with servo's or the program you mentioned but have used other programs for steppers. The programs provide general paramters as input guides. So you can be conservative on those parameters. Now they usualy ask for additional loads you expect and only you can provide those. These values  can certainly have an influence on sizing.
So play around with the computer program and see what it tells you. The sizing software may use some inertia guideline ,like a ratio of 10:1 of calculated to actual motor inertia, and often these are general guidelines / rules of thumb which they have found to be applicanle over time. It's just one parameter which provides for motor recomendation.
Like the story on computer use goes, "garbage in garbage out".

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Think I,m correct in saying, when going around a circle  Mach uses the lesser of the either axis velocity for the movement. I know that in some other programs this dosn't happen. I also know that my faster computer will give me 
quicker moves around a circle especially at the axis transition points.

Hope this helps
RICH




 






A computer program is a great tool for doing the calc's for a system and usualy provide for motor selction motor options.