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Message |
    
Gerald_D
Registered
Username: Gerald_d
Post Number: 885
Registered: 11-2005
| | Posted on Wednesday, February 14, 2007 - 08:30 pm: | 
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I keep picking up bits and pieces of info on why bigger steppers are not necessarily better...
Today Mariss said on his forum:
"We have one of those infamous 1,200 in-oz motors. The problem with the
motor is its poor electrical angle to mechanical angle linearity.
1) Advancing 90 electrical degrees results in 1.8 degrees of motor
motion (one full step). A 10-microstep drive advances 9 electrical
degrees per step and ideally the motor should move 0.18 degrees
mechanical for each step. A graph would show a linear relationship
between electrical and mechanical angles (a linear slope thru the
origin).
Motors with poor linearity have a pronounced S-shaped curve (initial
steep slope rounding to a shallow slope, rounding to a steep slope
again. This means the microsteps bunch at the half-step location and
spread at the full-step locations. This results in motor deceleration
where the microsteps bunch and acceleration where they spread. Since
this acel/decel occurs over the span of one full step, the motor is
pumped into resonance when the acel/decel frequency (full-step rate)
matches the motor's mechanical resonant frequency.
A motor with good linearity will have evenly spaced microsteps and
therefore has no acel/decel component to its motion. It will exhibit
no low-speed resonance.
Our 'figure of merit' for motor linearity is the ratio of the biggest
microstep to the smallest microstep. Good motors have a ratio of 1.1:1
or less. This motor has a rather miserable 1.9:1 ratio and it does
exhibit pronounced low-speed resonance.
2) The "no free lunch" thing comes into play here as well. This
particular motor has a corner speed of only 210 RPM at 80VDC. This
means torque rolls-off rapidly above 210 RPM. Quick calculations show
a power output of 186 Watts mechanical.
You can get more power (about 250W) from a lower torque NEMA-34 motor
and have excellent linearity as well."
Art Fenerty said on his forum:
"Don't be too tempted to get Nema42's with 1200oz inch power, as steppers get larger, the detent torque (that clicking you feel as you turn a stepper) increases, and it can be a devil to tune them or get great acceleration. I recommend (just me..) that you don't go higher than 800oz in for a stepper" |
Gerald_D
Registered
Username: Gerald_d
Post Number: 1085
Registered: 11-2005
| | Posted on Saturday, March 03, 2007 - 10:27 am: |
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Another exchange of comments last night:
Art:
Yeah, 1200 Oz in motors can be a problem on any system, very high
detent torque. Try .053, it has a ramped backlash correction now, it
may help alot.. hard to say though, Im still tweaking the fine
details.. Let me know how it does.. ( Personally, if one needs more
than 800 oz in or so, I always recommend servos, steppers over 1000
often give bad performance is many situations, on the up side they
have very great power, down side though is that they have very great
power.. 
Mariss:
Art, I agree. NEMA-34 motors over 800 in-oz holding torque are
designed for high torque at the expense of motor linearity. Motor
linearity is paramount for smooth motion at low speeds.
Secondarily, This high holding torque comes at the expense of speed.
It is part of the "there is no free lunch" principle. Gobs of holding
torque means you have a motor that cannot get out of its own way as
speed increases.
Given the examples I've tested, I think of the over 800 in-oz NEMA-34s
as vanity motors. They give the motor mfgs bragging rights to say "Our
NEMA-34 motors go up to 2,100 in-oz !" Some gullible users buy them
and find what they have is a real dog (or boat anchor) on their hands.
The hardest concepts to pound into people's heads is it's power that
gets things done. Power is proportional to V / SQRT L on a step motor.
Detent torque is always present; high detent torque motors consume
more power to turn themselves than smaller, low detent torque motors
and that power comes at the expense of what's left to apply to the
load. Never mind the non-linearity (low speed vibration) of these
unfortunate monsters.
Use a sub-800 in-oz NEMA-34 motor. A good one will be smooth at all
speeds. Spin it fast enough to develop real power (torque times RPM).
Gear it properly to the load instead of taking the brute-force 1:1
approach. You will out-perform any muscle-bound 2,000 in-oz beast of a
motor.
Art:
Yes, Id rather have a small fast motor on a reduction gearing anyday, than a big honker motor 1:1 connected. The first one IS
impressive, the latter only looks impressive.. Its why I think the faster speed drivers over the next few years may change the way
people design drive systems, small and fast can be pretty impressive when fast is determined to be around 300Khz to the motor..
lots of room for gear reduction there.. as well as much higher resolutions ...
But thats just me. ... truth be known, Ill use anything that works well... |
glenn Crawford
Registered
Username: Shopteacher
Post Number: 1
Registered: 03-2007
| | Posted on Thursday, March 29, 2007 - 01:25 pm: |
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Hi,
New member here trying to gain some knowledge and insight on building a CNC and converting 2 convential machines o CNC. My question is what size motor would you think I would need for a 9" South Bend Lathe and a small Enco knee mill? I'm not well vrsed in electronics so this is pretty much all new to me. Also is there a means of compensating for the backlash in the lead screws without going to ball screws?
Thanks to all you really got an interesting site here and I hope to learn a lot from it. |
Gerald_D
Registered
Username: Gerald_d
Post Number: 1189
Registered: 11-2005
| | Posted on Thursday, March 29, 2007 - 02:18 pm: |
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Hi Glenn, I'm sorry to tell you that we specifically want to cover only "big" CNC routers on this site.
However, having looked at your profile, I feel jealousy (you have a Multicam) and sympathy (you are a schoolteacher(I think)).....
Non-CNC routers are tools that need 2 hands to push around. Small non-CNC lathes and mills can be operated with one hand. Therefore you are needing smaller motors to replace the humans for small lathes/mills than for routers. (Big lathes and mills are a different story). Where we are using NEMA 34's for routers, you should be okay with NEMA 23.
www.cnczone.com would really be the right place for you to get all your answers. Good luck. |
Shopteacher
Registered
Username: Shopteacher
Post Number: 2
Registered: 03-2007
| | Posted on Thursday, March 29, 2007 - 04:33 pm: |
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Hi Gerald,
Sorry to tell ya, you ain't getting rid of me that easy. LOL I have a second Multicam coming in the next school year (yes I am a school teacher and I appreciate your sympathy). In addition to converting the two machined above I intend to build a mechmate for home. I downloaded your plans and haven't had time to fully review them, but what I have seen and read really look great.
My biggest weakness in in the setup and understanding of the motors and the controls, but have been getting a better understanding from sites like yours.
Glenn |
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Motors