Hello Guest it is December 02, 2021, 12:10:55 PM

Author Topic: Accuracy of Homing Devices? Pro's and Con's of Different Sensors  (Read 31769 times)

0 Members and 1 Guest are viewing this topic.

Offline Dan13

*
  •  1,208 1,208
    • View Profile
    • DY Engineering
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #60 on: November 11, 2012, 02:00:02 PM »
Here's the link to Bertho's test:

http://www.vinland.com/Opto-Interrupter.html

The machine components expansion would be much more of a factor than the photosensor thermal deviations.

Dan

Offline Hood

*
  •  25,838 25,838
  • Carnoustie, Scotland
    • View Profile
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #61 on: November 11, 2012, 02:51:30 PM »
Thanks Dan, thats the one :)
Hood

Offline mc

*
  •  380 380
    • View Profile
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #62 on: November 11, 2012, 05:51:02 PM »
The leadshine closed loop stepper system can be bought new a lot cheaper than a servo system, plus the steppers have the benefit of having more low speed torque than an equivalently sized servo.
For example the Leadshine HBS57 with the smallest closed loop stepper motor produces 0.9Nm at stall, and still supplies 0.5Nm at 1300RPM
The servo you've fitted to your connect, which is a similar size, is 0.17Nm, so to get the same torque at stall, you need to run a 5:1 ratio, meaning it peaks out at 900RPM, or if you gear at 2:1 you get 0.34Nm and 2250RPM.
Now for something with only 5" travel, that speed is pointless, as even with the 2.5mm pitch screw, it's only 50 turns end to end.

The servo with gearing could theoretically get you more accuracy, however will it be any better than a direct coupled 4000step/rev encoder?
It's all a case of understanding the tradeoffs, which I obviously spend too much time doing!


Interesting stuff about the opto repeatability.

Offline Hood

*
  •  25,838 25,838
  • Carnoustie, Scotland
    • View Profile
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #63 on: November 12, 2012, 02:38:14 PM »
Quote
The leadshine closed loop stepper system can be bought new a lot cheaper than a servo system, plus the steppers have the benefit of having more low speed torque than an equivalently sized servo.

Not sure how much the leadshine stuff is but I would reckon on £300 plus for motor and drive? Regarding the torque, not quite true as I will talk about later.

Quote
For example the Leadshine HBS57 with the smallest closed loop stepper motor produces 0.9Nm at stall, and still supplies 0.5Nm at 1300RPM
The servo you've fitted to your connect, which is a similar size, is 0.17Nm, so to get the same torque at stall, you need to run a 5:1 ratio, meaning it peaks out at 900RPM, or if you gear at 2:1 you get 0.34Nm and 2250RPM.
Now for something with only 5" travel, that speed is pointless, as even with the 2.5mm pitch screw, it's only 50 turns end to end.

That is a common mistake when people compare servos and steppers.
The units quoted for both are the same (Nm usually) but a big difference in what they actually refer to. Steppers it is Holding Torque, ie torque at a standstill. Servos there are two values, Continuous and Peak. Continuous means exactly what it says on the tin but Peak is usually 2 or 3 times greater and can be used for short bursts, exactly what you want on a machine tool  as you need a quick burst to get you moving, once on the move the torque requirement drops away hugely. So Servos can provide that extra for very fast acceleration which is of great benefit on a mill or lathe.
 You said that to get the same torque as your 0.9Nm stepper I would need to gear 5:1, that is not the case, I would get 0.96Nm by gearing 2:1 as the peak of my motor is 0.48Nm. Also note that my servo is quite a bit smaller than the stepper you are talking about, the stepper weighs in at 0.75Kg, the servo 0.5Kg, so ½  as much weight again in the stepper.

Ok so lets  compare another,  heres a link to a servo on eBay ( yes second hand but still a nice price BTW ;) ) http://www.ebay.co.uk/itm/Samsung-servo-drive-CSDJ-01BX2-with-a-motor-of-CSMT-01BB1ABT3-or-CSMT-01BB1ANT3-/180947522668?pt=LH_DefaultDomain_0&hash=item2a2150146c
It weighs in again at 0.5Kg but is only rated 3000rpm. Torque  is over 0.3Nm cont and over 0.9 Nm Peak, see torque curve below. Compare that to your stepper, again pic below, quite a difference which you will see most noticeably in the difference in acceleration between the two. Yes the stepper looks like it has more torque but again if you look at peak then its not the case and again peak is there for the acceleration and the continuous keeps you on the move.
Gear that servo 2:1 and the difference is even more.

Quote
The servo with gearing could theoretically get you more accuracy, however will it be any better than a direct coupled 4000step/rev encoder?
It's all a case of understanding the tradeoffs, which I obviously spend too much time doing!

Encoders are usually 2000 or 2500 count so 8 or 10000 pulses per rev , usually it is easy to achieve a following error of less than 20 counts and 20mS.
One other very nice thing about ultra modern AC servo technology is there is no tuning, its done on the fly and is adjusted in mS to take account of varying loads, your stepper system will likely use similar technology however.
Another thing I like about AC servo drives of this type, they take mains in so no messing about with power supplies as the drive has an appropriately sized one internally.

Now everyone to their own and steppers do have their place with me, on the coil winding machine I built. It doesnt require fast accelerations, in fact it wants the opposite so the thin wires dont snap but for a machine tool I personally would not go back to a stepper.
Obviously these are my own personal feelings on the stepper/servo debate and I am sure you and others will think very differently.



Hood

Offline Hood

*
  •  25,838 25,838
  • Carnoustie, Scotland
    • View Profile
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #64 on: November 12, 2012, 03:49:45 PM »
Ok here is a nice comparison from real life facts and figures.
 My Bridgeport had steppers and I could achieve 2200mm/min rapids at 120mm/s/s accel. I could increase the rapids to 2500mm/min but accel had to drop down to 40mm/s/s so the former is a much better compromise.
Fitted some servos to it, same gearing etc only difference was servos instead of steppers. I could easily get 6,000mm/min at an acceleration of 1200mm/s/s. Could have got a lot more acceleration if I had fitted active shunts buit hat was more than ample for a Bridgeport.
So that’s almost a threefold increase on rapids and almost tenfold on accel yet the specs of the motors if going by torque would suggest that to not be possible hence why comparing a servo and stepper on torque is not a good idea.
Stepper was 6.4Nm
Servo was  3Nm cont

Hood
« Last Edit: November 12, 2012, 03:51:37 PM by Hood »

Offline mc

*
  •  380 380
    • View Profile
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #65 on: November 12, 2012, 05:15:35 PM »
How do you manage to find all the figures for servos?
The reason I used the 0.17nm, was that was the only figure google threw up.
I understand the cont/peak ratings, it's just for alot of servos the figures are not easy to find, more so if you're looking at older motors.

Cost wise, the HBS57 (smaller of the two closed loop drivers) with the bigger stepper option (2Nm) is £180 from Zapp. The larger HBS86 with 8Nm steppers are £250 or £265 if you want the one that can accept AC or DC power.
The big issue with my Conect is the x-axis motor is mounted at the rear, so unless I do some major modifications, I'm severly limited for space.
Using a HBS57 with the smaller 0.9Nm motor, I can fit a proper bearing block and still direct mount the motor, all with minimal control box changes.
I did figure out a way to mount a bigger motor, but it's a lot of extra work for power which I won't need. The hardest job I want to do is part-off, and the original 0.69Nm managed through 3/4" steel with a chipped 3.2mm insert, so the 0.9Nm should work perfectly.


For the Matchmaker, I'm currently swaying towards steppers with high voltage drivers, and closing the loop using a K-flop with Kanalog and encoders, purely for cost reasons. I can get all the parts excluding the K-flop/kanalog (which I'm planning on using anyway) for the cost of one new servo.
I know I could go secondhand for servos, but unless I can pick up suitable servos for a good price, without much hassle, I'd rather go with the limitations of steppers and upgrading later when I've got more money.
Plus, by using the high voltage drivers, upgrading to servos later will involve minimal wiring changes.

Offline RICH

*
  • *
  •  7,419 7,419
    • View Profile
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #66 on: November 12, 2012, 06:43:17 PM »
Quote
limitations of steppers and upgrading later when I've got more money

Maybe waite to get what you really want and in the long run you'll save money. The reason I stay with steppers is because
I have 5 stepper drives and frankly they satisfy my need.

RICH

Offline Hood

*
  •  25,838 25,838
  • Carnoustie, Scotland
    • View Profile
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #67 on: November 13, 2012, 12:10:18 AM »
Easy enough to find the torque values, just find the manual for the motor/drive and its usually all there, in fact most motors have it on their label.
Price is definitely a big drawback to servos, especially in the UK but if going with steppers I personally dont see any real advantage to trying to close the loop. If you lose position with a stepper it is because you are pushing it beyond its limitations and if that is the case then you need to either lower your expectations or get better sized motors/drives.
Reading the info on the closed loop steppers it seems you would get better resolution but their is still the issue that pushing it too hard will stall it. Ok same could be said for servos, push them too hard and you will trip the drive on following error or overcurrent but servos are much more forgiving than steppers due to their high peak values.
 My Conect had the stepper at the front originally but I shifted it to the back to allow more travel and room. Dont know how yours was set up but the bearings in mine were pathetic, the motor bearings were used to take up the play in one direction and a thrust for the other.

Hood

Offline mc

*
  •  380 380
    • View Profile
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #68 on: November 13, 2012, 01:23:27 PM »
Rich, that is an option, however I could probably do with getting it running sooner rather than later. The Matchmaker is going to be one of three bigger projects fighting for money, with the main priority currently being to improve the Conect as it's the one that's actually started making money, but I've got a few jobs where having the Matchmaker running would help alot. Plus if I get it running, it means I can sell my Harrison mill to fund the next project.

Hood, the only thing I found while trying to search for manuals, were sites selling the motors with minimal information.
Anyway, I've just dug out my notes for the Conect, and with the existing set-up, I've got about 135mm of room behind the cross-slide to fit motor, mount and coupler, unless I start cutting big holes in the cabinet.
And yes, mine has the same hopeless bearing set-up, which I think is the reason behind having to continually tweak tool offsets, and why it's top of my list to upgrade.

I wouldn't mind a second hand servo for the spindle if you happen to know of any, that you aren't wanting for yourself...

Offline Hood

*
  •  25,838 25,838
  • Carnoustie, Scotland
    • View Profile
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #69 on: November 13, 2012, 02:43:28 PM »
Look at the manuafacturers sites and you should get plenty info on motors/drives etc, sometimes they take a wee bit of digging to find them though.

Sounds like your conect may have less room than mine, I was thinking mine was more like 200mm but would have to look tomorrow to be sure. As said mine originally had the stepper mounted on the front but I decided to mount to the rear to give me more travel.

Afraid I dont have any motors/drives spare, sold one recently to a guy for a wee Boxford HMC, it was the same motor and drive I have put on the Conects spindle except the one I sold had a brake on the motor.
Hood