[peteeng]

Hi Mactec- There is no "engineering jerk free" system. However engineering jerk minimisation is possible. "lay jerky" motion is rough accelerations not necessarily anything to do with engineering jerk. Just want to clarify lay "jerk ie rough motion" verses physics jerk or jolt.

There are two schools of thought it seems a) since the velocity/accel (hence eng jerk) can be calculated by the CAM system and velocity profile are generated by the CAM then the whole thing can be controlled in the code. This presumes the motors and machine can follow the required path and velocity profile independent of the inertial conditions eg volumill b) Since smooth response of the system depends on the inertial conditions then some sort of feedback loop needs to be used. ie place an encoder on the motor (and or linear scales on the axes) measure its actual position, velocity and accel compared to the programmed vel/accel and then use that feedback to correct the response eg DMM C) do both? Depends which side of the commercial border you are on the software side or the machine side. Peter

other school is like movensys - They take the g code stream and look ahead in the code. They calculate the vel/accel/jerk and correct the velocity/acceleration profiles, then change the gcode stream to suit. This can be vel/accel profiles and path smoothing.... I'd put them in the motor camp....

Mactec - by "Jerk" do you mean rough motion or "engineering jerk"?

[PCW_MESA]

Hi Mactec- There is no "engineering jerk free" system. However engineering jerk minimisation is possible. "lay jerky" motion is rough accelerations not necessarily anything to do with engineering jerk. Just want to clarify lay "jerk ie rough motion" verses physics jerk.

There are two schools of thought it seems a) since the velocity/accel (hence eng jerk) can be calculated by the CAM system and velocity profile are generated by the CAM then the whole thing can be controlled in the code. This presumes the motors and machine can follow the required path and velocity profile independent of the inertial conditions eg volumill b) Since smooth response of the system depends on the inertial conditions then some sort of feedback loop needs to be used. ie place an encoder on the motor measure its actual velocity and accel compared to the programmed vel/accel and then use that feedback to correct the response eg DMM C) do both? Depends which side of the commercial border you are on the software side or the machine side. Peter

I think its valuable to divide jerk control into tangential jerk and centripetal jerk:

Tangential jerk (along the tool path) is full controllable by the machine.

Centripetal jerk is totally controlled by CAM if you are doing constant tool speed machining (which is usually desirable for accuracy).

[deadlykitten]

since the velocity/accel (hence eng jerk) can be calculated by the CAM system and velocity profile are generated by the CAM then the whole thing can be controlled in the code. This presumes the motors and machine can follow the required path and velocity profile independent of the inertial conditions eg volumill

hy peteeng my sugestion is to focus 1st on mcs, and leave cam system where it is

even if a cam takes as input some speed, acc, in order to do whatever with it ( like smoothing and cycle estimation ), most results are bull***t

cam is extensions, is not critical; is not like take this toolpath, it will work; no, is like it should work if you don't push the machine too much

my dear petteng, your thinking is ok, thus like tring to smooth things out as much as possible before creating the g-code, but truth is that actual stage of things is not like that, otherwise there wouldn't be so much complains about shaking/screaming machines

smoothnes is required to push machines, but achieving that top smoothness is not because of cam software

then some sort of feedback loop needs to be used

the feedback loop... this is it

C) do both?

yes, but let's leave this a bit for latter, to smooth out waters at actual discusion, otherwise it won't fit in

please, i sugest this, because it requires a perspective shift with 180 degrees : you see, most cam are extensions, simply doing something and what you see is what you get, but there are things really tailored to an mcs

Hi All - The issue with cnc machine control is that there are three main domains to control. rapids ie no tool load, roughing and finishing.

1) rapids - we want very fast AtoB motion without overshooting or exciting the machine ie minimise settling time
2) Roughing - don't need accuracy but do need to maximise MRR. Heavy MRR will lead to vibration so smoothing jerk minimises delta Accel which minimises inertial forces which means better cutting (generally speaking)
3) finishing - we need position accuracy but at max speed so path accuracy and smooth paths require smooth accels. Constant velocity machining means we will have pathway accels due to inflections in path eg lines to rads

close, but no cigar ... machine has not clue if it sees finish, roughing, inertia or a crash

yes, those are different scenarios, but also different scenarios are these :
... trapez vs S motions
... read ahead of 4 lines or 50 lines
... diff 0 or diff 1
this enumaration above has things in common with your enumaration ( rapid rough finish )

again, here we are, discussing the mcs / kindly

[deadlykitten]

measure its actual position, velocity and accel compared to the programmed vel/accel and then use that feedback to correct the response

me again

as you allready know i supose, terms for comparison is not direct g-code data, but planed toolpath, thus estimation/proposed data based on g-code

it is correcting the response only if there is still available resource to do so

thus, if i may reformulate, it tryes to correct while still cutting, and maybe will succed ... who knows ?

ps : sorry, i did not mean to say "perspective shift with 180 degrees", but only a bit; please excuse me i did not intend that, and it may sound different when you read it

[mactec54]

Hi Mactec- There is no "engineering jerk free" system. However engineering jerk minimisation is possible. "lay jerky" motion is rough accelerations not necessarily anything to do with engineering jerk. Just want to clarify lay "jerk ie rough motion" verses physics jerk or jolt.

There are two schools of thought it seems a) since the velocity/accel (hence eng jerk) can be calculated by the CAM system and velocity profile are generated by the CAM then the whole thing can be controlled in the code. This presumes the motors and machine can follow the required path and velocity profile independent of the inertial conditions eg volumill b) Since smooth response of the system depends on the inertial conditions then some sort of feedback loop needs to be used. ie place an encoder on the motor (and or linear scales on the axes) measure its actual position, velocity and accel compared to the programmed vel/accel and then use that feedback to correct the response eg DMM C) do both? Depends which side of the commercial border you are on the software side or the machine side. Peter

other school is like movensys - They take the g code stream and look ahead in the code. They calculate the vel/accel/jerk and correct the velocity/acceleration profiles, then change the gcode stream to suit. This can be vel/accel profiles and path smoothing.... I'd put them in the motor camp....

Mactec - by "Jerk" do you mean rough motion or "engineering jerk"?

It can be a combination of both, what you can see most of is rough machine motion.

Rough machine motion affects the machined part, trying to cut the part faster than what the machine can handle,(but the machine can go faster it says that in the specs) is most of the problems you see, so it does not matter what you do in the software side, if the machine can't handle the programed cut speed and feed, it goes back to the machine, # 1 machine rigidity #2 Motor / Encoder #3 Drives #4 Control #5 Cam in this order.

At the Hobby level of machine building, you see everything from vibrating aluminum frames to concreate, the most successful you see are the solid built machines that run at moderate Speeds and Feeds, you mostly find that the Hobby machines that are built of a solid construction are also using a quality modern servo system.