Thread: Let the fun begin !!! ???
If it's the inner then the bearings are in the wrong way around. You want them back to back with a shim between outer races.
Regards the Leadshine Closed loop steppers then they are nice but expensive. If your using Mach3 then just attaching encoders to your steppers won't help at all because Mach is not closed loop and all it can do is report the encoder position it cannot correct for error like the leadshine servo stepper drives do.
The closed loop stepper drives actually close the loop within the drive not in the control software so when you see it correcting position Mach doesn't know anything about it. Now this is good and bad at same time.?
It's good that the drive monitors position using the encoder and corrects for any error but at the same time if any error as happened it's still an error which causes a break in continuity between drive and control software. Because the loop is contained within the drive and never reports any small positional errors to mach then In affect Mach becomes in front of the drive for a short period before the position catches backup in that axis. Other axis will carry on in front So Depending on the error amount it can affect the job so the error still shows up in the finished job.!! . . . . . This is also true of servos or any other drive that doesn't have true closed loop system that works in tandem with the control software to sync axis together.
If the positional or what's called following error gets too great then the drive will fault and e-stop the system (which in affect is what Stall detect does but much cruder) This following error can be set in the drive parameters to a small amount so if goes outside parameters then drive faults and system E-stops.
This is how most servos work but like I say the small errors have still happened and at some point will show up in the work.!!
This is why you'll often see people who have experience with both Servos and steppers on slower machines like mills etc say that Servos are no more accurate than steppers if steppers are setup and tuned correctly.! . . . Error is Error and will show in the work with either system.!!
In some ways servo or Semi closed loop is worse than standard stepper system because you'll never know where the Error comes from as each axis have caught back up so when job finished all looks well and in position but the work piece still shows error. With steppers any lost steps can clearly be seen in the axis as it won't return back to where it started.!!
Servo's rule when heavy loads and high feeds are required Or when going point to point very fast like in positioning systems. For Small/Med routers or Small/Med mills steppers can match servos no problem without closed loop.
Last edited by JAZZCNC; 06-02-2015 at 03:04 PM.
ya , that's the way the shims are in o.k bearings back to back and shim in between on the outer races. cheers for the run down on the closed loop . i think i'll stick with the original plan. steppers it is with the AM882 drives.
well, i've been reading most of the weekend on the electrics and control side of things and i'm still in 2 minds about my next step. i started on sourcing a power supply for the motors / drivers ( AM882) i was quoted 346 euros + vat for a 60 V DC SUPPLY so i think i will be building my own one !!!
i still have a few queries about the twin motor set up on the x axis ball screws. i'm a bit confused about the homing of each ballscrew separately. am i correct in saying that when i home the x axis, both motors engage and head for home simultaneously but because 1 motor might have lost steps during the course of machining they wont reach their respective home switch together? will the motor that has lost steps get to its home switch eventually albeit some time after the first motor and if so why do we need to home them independently.
i was reading( http://www.mycncuk.com/threads/6418-...-CNC-Mk2/page8 ) about independently homing each axis because of missed steps and the strain it buts on the gantry. so my next question is about the distance they travel away from their respective switches before they stop and return. or have i got this concept wrong also!! if i did decide to go the 1 motor route and upgrade the motor to a nema 34 and belt drive both ballscrews would the am8882 drive and 60 VDC POWER SUPPLY need to be upgraded also? it a question of cost at the moment and that's why i'm trying to have some upgradability if i do box myself into a corner with dual motors that need regular independent homing. all these choices are really playing havoc with the hamster in the hamster wheel upstairs.
I THINK I FOUND A ANSWER TO THE ISSUE OF HOMINGhttp://www.mycncuk.com/threads/6565-...dy-Eddy/page30 / 32 , I wouldn't mind but i read that before!!
Last edited by PAULRO; 10-02-2015 at 02:11 PM.
The steppers will generally only lose steps if they're tuned too close to their maximum speed and acceleration from my understanding and reading of Jazz's posts. There's plenty of people on here using twin setups and not many posts to say they're losing steps and racking the gantry. I'm in the same situation as you regarding the twin vs single stepper and I'm leaning towards the single Nema 34 with a different driver. The AM882 will not drive the Nema 34 with any sort of speed, 240V drives are best for them but they are more expensive! I'm going to use my spare 3.1Nm Nema 23 stepper and driver for a 4th axis when I get the Nema 34....Neil...
The Following User Says Thank You to njhussey For This Useful Post:
that's good to know about the AM882 drive. originally i bought 4 nema 23 4Nm with the intention of using 1 each for the x,y,z and a spare for the fourth axis. but soon realized that i would probably need 2 for the x axis. ( 1 on each ball screw). maybe down the road i might just follow you down the road and replace the 2 23s for a 34, now that i know i will just keep going with the 23s and 60 VDC power supply. cheers for that.
Paulro The homing with the am882 drives is a doddle all you need is two inductive switches like this : http://www.ebay.co.uk/itm/DC-3-Wire-...-/390880485798 mounted one each side and adjusted so that they both trip at the same time when the gantry is square.
Then set homing slaved axis in Mach3 X with A (ask how to do this nearer the time) what happens then is when the home button is pressed the gantry will move to the end with the switches on and trip the switches. Then the gantry will back off the switch until it trips again. You can adjust the gantry square by adjusting one of the switches a little.
The machine will then make sure the gantry is squared every time you home the machine.
When using the AM882 drivers you wire up the stall detect (fault) terminals so that if one motor stalls it estops the router and saves any damage by racking the gantry. Hope this helps. ..Clive
The Following User Says Thank You to Clive S For This Useful Post:
Clives answered the slaving question correctly but I'll give a little more detail.!
When the motors are slaved both move together towards each home switch. The first switch to hit a switch stops just it's own motor not both and reverse's off the switch. The other motor carry's on it's merry way until it hits the home switch and does the same.
Now in practice this happens so fast it appears as one motion. This is because if the gantry is setup correctly the switches should trip exactly at same time.
If your gantry is not quite square you can tweak it square by moving the switches slightly but all the same it still appears like one motion as the difference will and should be tiny.
Regards the Motors and single or twin motors then it's personal thing and close call in some respects. If using quality Digital drives with stall detect like AM882 then I wouldn't worry about it and it's far easier to setup and less messy on wide machines.
One thing I would suggest to get the best from the motors is that you use more than 60Vdc with 80v drives and if using toridal psu you'll easily get away with running 70vdc. The extra 10v makes all the difference and worth sizing psu to maximise the motors.
Regards the electronics then don't get carried away like Eddy current did because for home use it's really not required. YES safety is important and a decent emergency stop circuit is required but you don't have to go to lengths of using expensive safety relays like Pilz. Normal Relays will work just as well if sized correctly.
More important is that e-stop or safety circuit can not be reset unless any fault is cleared and that it removes power to any devise that might do you harm.
Limits I don't really class as emergency stop condition and more positional error so can be dealt with in a differant manner. Instead of killing Power to devices they can just inform the control software limit as been tripped so can halt G-code and at same time remove siganls to the drives by using the drive enable signal.
This should be done in such away that it halts the machine and won't allow reset until limit is cleared or over ridden but at same time leaves the drives powered and holding the motors under torque.
E-stop system should done in such away that it responds to outside events like Fault signals from drives, IE Stall detect, VFD/spindle fault or emergency button press. In which case power should be removed and again system not allowed to restart unless fault cleared.
Building a control box isn't difficult and doesn't need to be complex but it does need to be safe which is easily done using relays.
Last edited by JAZZCNC; 10-02-2015 at 08:54 PM.
well i'm glad i held off with the purchase of the 60 VDC PSU. i must get a sketch of a psu organised and post it here. i remember reading a thread posted here about the grounding issues and potential for error if the power supply is grounded incorrectly . i think it was in relation to the main screw of the coil touching or not touching the base plate . i'll have to dig it out and give it a read again. the homing issue can be put to bed for the time being at least , i 'm clear in my mind following the info from you guys here. with regards to the safety relay i think i will leave it for now and get the baby to take its first step with a decent level of safety using the relays that i have . down the road i will tackle the safety relay when i get more familiar with the whole experience. as always guys ... the advice is first class, cheers
By DarkWalkJRB in forum New Member IntroductionsReplies: 2Last Post: 07-02-2012, 07:31 PM