Thread: Reducing the space in my garage.
Thanks Jonathan, I will run the script and see what I am able to do. Irving's spreadsheet was a good place to start. Although my machine doesn't quite fit the profile the sheet expects, I was able to gain a sense of what factors where critical to the design/specifications of the machine.
Should you do decide to write the additional script, it would be interesting to see the distinction between the projected and real world examples. This is v.1, so knowing the maximum top speed at which the machine is able to provide sufficient torque to cut particular materials would help with v.2. The gear ratio on v.1 is just an informed guess and now the holes are drilled, there's no way back!
Yes but, no but, yes but, no but, yes but ......... )
And I did include the phrase 'distinction between projected and real world', covering all my bases! You're just being selective.
This might explain better; http://www.mycncuk.com/threads/2148-...2975#post62975
Last edited by EddyCurrent; 18-11-2014 at 10:20 PM.Spelling mistakes are not intentional, I only seem to see them some time after I've posted
Jazz, I get the point. Using a spreadsheet doesn't distract from the issue of build quality. If the error rate between projected and real world is 20% and I've used all the right parts (as detailed clearly within the forum), then the figures will tell me I can't build a thing worth a damn. My issue, the things I need to be realistic about. I haven't used all the right parts, but then I'm a novic and likely to break, bend and drop things. So I design with this in mind. I have two key issue to resolve within this design. The first is to learn everything I can from it, the second it to buy the electronic responsibly so that don't end up dumped in a foreign country. If I do it right, the worst thing that will happen is the aluminium rail with travel 5 miles to the local smelter.
BTW: I've not quite finished reading all your posts )
Last edited by Rich; 18-11-2014 at 11:17 PM.
Could someone please let me know what the answer is here - I just need to have these thing clear in my mind as all the calculations can be a bit confusing!
The PSU based on the following transformer is quoted within the forum (building a PSU is a risky business) :
When wiring in parallel, the output is +/-70vdc at 12amps, providing 840 watts. This is just less than double the 430watts, measured while a machine was in motion.
An alternative transformer appears to be:
When wired in series, it makes available 700watts (56volts/12.5amps). The figure I have for the motors suggest vmax for the motors at 57 volts (4.2amps,3.2mH).
So why would I build the PSU using the a larger transformer? It affects the number of capacitors required? I'm just running calculation ...
C = 0.1 * A/V for a 5% ripple = 0.1 * 12.5/56 = .022F = 22,000uF with a voltage rating of at least 100V and a ripple rating of 12.5A.
I read somewhere that voltage, in this instance, is like pressure in the system, its re-supplying the motor. I understand there is an issue of impedance, but there are capacitors and back EMF resupplying the circuit. I'm assuming the transformer in continuous use will heat up and so the rate at which is can provide power will slow. Is that the point?
Another point, the BOB (OPPB v.3). Just checking I don't need to worry about its maximum voltage or current unless I am running ancillary items via the BOB. It's not part of the power circuit, its just pulsing the steppers with go/stop signals?
Last edited by Rich; 25-11-2014 at 09:43 PM.
If your drivers can take it go for the higher voltage as this translates directly to upper speed limit. Current requirement is typically 60% of motor rating, so.60% x 3 x 4.2 is 8A approx. This means you can get away with smaller capacitors .1 x 8/70 is approx 12000uF @ 100v, ripple rating can be lower at around 4A.
Re BOB, correct only 5v for signalling and 24v for limit/home switches needed. All i/o should be opto-isolated at the BOB for inputs and at drivers for motion control step/direction or relay outputs for other controls.
Last edited by irving2008; 26-11-2014 at 05:35 AM.
The Gecko site suggestion is working on the safe side but in practice the modern hybrid stepper motors we use and in particular the low inductance 3/4Nm available to us easily handle 70Vdc. Combine this with Modern Digital drives which handle current, motor heating and resonance much more efficiently then performance is greatly increased in several ways. More speed, more torque at higher speeds and smoother running motors.
Regards the transformer then all you really need is to size it so it outputs the voltage required and some extra amps for overhead. The volts are the main thing to concentrate on because if you go to high here you can blow the drives if tolerence peaks higher than there max rating. This is why 80V drives running 68Volts is safe.
You can get away with 500Va and 2x50V wired parallel or 2x25v wired in series with 3 x 4.2a motors without any troubles and I've run these with 4 motors in past without any issues but the 625va is the better option with 4 motors as it gives higher safety margin for say 4th axis.
Reason you can do this because all 4 motors are never pulling full amps at same time and the way the unregulated transformer/drives work they only pull amps on half the PWM cycle so don't draw the full rating of the motor. Combine this together and you can get away with Transformer Amps 60% of total motor amps but you still need the voltage to get the speed/torque from steppers.
Like irving says you can get away with much less caps and using 3 x 4700uf wired in parallel is my prefered way as they are cheaper, easier to find and lower ripple I believe.!
Regards the Bob then not sure what your meaning because voltage is very important to the BOB.? . . Feed it more than it's I/O can handle and you'll fry it every time, infact on the cheaper Bob's like the Oppb v3 you'll drasticly shorten it's life if you run any where near it's limit.
99.9% of BOB's will have voltage requirement or voltage range to operate and run things like Opto's and relays etc. The Motor output side of a Controller is often 5v logic and all the BOB does is pass it thru or in some cases will filter and boost the signal to 5V if lower.
Only the standard I/O side can vary regards voltage/current and even then your BOB will have a limit it can handle. The cheaper BOB's will restrict you to 5V others will be 24 or 30Vdc.
Again in practice your better running 24V thru I/O and the safety system limits Etc to help resist any noise interference.
DONT buy a cheap BOB they are very important and seriously under rated in how they can negatively affect the machine performance and shorten the life expectancy of your Hair.!!
Hope this helps.!
Last edited by JAZZCNC; 26-11-2014 at 11:24 AM. Reason: Thanks Clive
Thanks Irving. I have a parts list together now )
Thanks Jazz, I won't be adding a fourth axis, so the 500va transformer is all I need.
The specs on the BOB haven't changed - OPPB v.3. I have a 12 volt supply to hand, although I may well need to supply 24volts for the alarm circuit, quoted as 30v @ 100mA.
Does anyone have a recommended source for the Leadshine AM882 stepper, someone on Aliexpress perhaps?
Last edited by Rich; 28-11-2014 at 07:00 PM.
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