09-03-2016 #33Figure out what you want your max velocity to be, and gear it so it reaches that at 80% of the servos rated speed.
Boyan, so you are taking the gear ratio the other way then? i.e. screw speed (or ballnut speed) * 1.5 to give motor speed. If I take the same equation from before your 20,000 mm/min with a 10mm/rev screw gives a motor speed of 3000rpm?
What would be the disadvantage to doing a 5mm screw? I know the torque issue with gearing the motors the other way. However I ended up ordering 1.8kw 6Nm 3000rpm motors since they ended up being cheaper than the 1kw motors so I think I can sacrifice some torque to a gear ratio. Are there other issues with 5mm? I havent ordered the long screws so I can go either way just asking the question.
I am thinking about implementing brake resistors, I do this on my VFD's (to run 3 phase woodworking equipment) for quick motor braking. Im just trying to decide if the inertia of the gantry being so high warrants braking resistors or not. Has anyone needed to add them to their setup?
My motor order was as follows (in case anyone is interested)
Qty: 3 - 6Nm 1.8kW 3000 rpm servo motors - these will be for 2 for X axis and 1 for Y axis
Qty:1 - 2.4Nm 0.75kW 3000 rpm servo motor - this one is for the Z axis
i was trying to say / but as i was busy at workshop/ people typically use for woodworking machines xx10 screws on all axis and xx05 on Z, as this more or less gives compromise between speed, precision and step count/pulse train needed to generate movement
So say you have a servo motor that will spin to 3000rpm. From look of the servo it says 2500PPR/ pulses per revolution/ if i am not wrong. And ball screw moves 10mm per revolution.
Then ball screw needs 1/10 rev to move 1mm. If no geared then 3000/max rpm/x10=30 000mm/min max speed , like Gerry says thats your max velocity in Mach3. So from above it seems then you will need 250pulses per mm , so 1mm/250pulses=0.004mm resolution.
There is something important that Gerry told you but you missed his point. The servos should not be greatly OTT as you may think, they must be closed mutch to what effort they will do, otherwise youwill run in other problems. read literature about servo sizing. Its not a good thing to be much bigger than job they are meant to do.
Then read carefully page 14 from my build thread where Johnatan explained me very well how to calculate details and especially about snappiness.
And by the way i was suggesting 20t at the motor and 30t at the rotating ball nut, not vice versa. So you finish with slightly geared up machine, 0.7-1kw servos, and very fast acceleration. But just do your self the calc. My gantry is moved by 2x 400w servos, not 2x1.5kw ha ha
Most industrial servo driven routers use 25-32mm pitch screws, which virtually eliminates whipping. With steppers, you don't want to go that high because you lose resolution, but with 10,000 steps/rev with servos, resolution isn't an issue.
Imo, on a machine that large, you want at least 1000ipm, preferable closer to 1500ipm. With a 10HP spindle, you should have no trouble cutting sheet goods at 1000ipm if you want.
The Following User Says Thank You to Ger21 For This Useful Post:
Alright doing the math for the inertias of the system I did it based on a rotating ballscrew at the moment.
with 2 motors each motor on the X will handle 1/2 of the gantry inertia
2/3 ratio on the pulleys
Ballscrew inertia = 7.4E-4 kgm2
Gantry equivalent inertia (1/2 of total) = 3.44E-4
Pulley inertia Driven = 9.86E-5
Pulley inertia Drive = 1.48E-4
Total Inertia = 1.33E-3
Motor inertia for 1.8kw from BST page = 7.6E-4kgm2
So Inertia ratio with the 1.8kw motor = 1.75
I also ran Jonathans Matlab Script and got
Feedrate in m/min: [x y z]=[20.0 10.0 7.50]
Inertia in g-m^2: [x y z]=[1.56 0.96 0.80]
Torque in Nm: [x y z]=[4.29 2.63 2.27]
Inertia ratios: [x y z]=[2.06 1.27 1.06]
It looks like my X axis needs 4.29 Nm, how much above this number generally do I go?
These numbers show a slightly different inertia ratio for X but pretty close.
I know the general rule of thumb here is the keep the ratio under 10 but is there a lower limit as well? Ive searched to see if there is any information about that particular motor for inertia ratio but I havent found anything. Boyan I see your motors show an allowable of 12 but I dont see that for the motors BST sells (searched the model number online as well) so not sure how high I can go with them.
I guess based on these numbers the motors are probably oversized however by how much? These motors show 6Nm continuous, how much is that number trusted for the chinese motors? If I need 4.29Nm per the torque calculation?
Should I be closer to an intertia ratio of 5,6,7,etc?
Dont know if I know more now than I did or less...
Thanks for the heads up
OK, maybe I am missing something here, but the more I read about proper servo motor design the more I see that the load inertia to motor inertia should be as close to 1:1 as possible. So although the rule of thumb is to be under 10 what I am seeing is for the ratio to be closer to 1. From what I am reading (and it is very possible I am mis-understanding this) the system should be better tuned and more responsive the closer to 1:1 ratio that can be achieved. This can be achieved with gearing or with sizing the motor to the load. I have included a 2:3 ratio in my calculations.
Am I mis understanding this? To me this would tell me that my inertia ratio of 1.75 --> 2.06 that was calculated for the system would be pretty good.
So as long as the inertia ratio doesnt go the other way...i.e. the motor inertia is larger than the equivalent load inertia... it is better to have the load to motor inertia as close to 1:1 as possible.
Of course from there one has to consider continuous torque and peak torque for the motor etc etc...
Scott welcome to the reality's of BIG heavy Machines. Need to Slow down with the Ordering and rushing in. The fact you even considered 5mm pitch screws and Steppers on such large machine shows you need to do much more research.
You enter another world which becomes expensive very quickly if you go rushing in without doing home work.!!
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