Messages

1141

General Mach Discussion / Re: Steppers are too slow

« on: December 06, 2011, 04:58:14 AM »

I have G201's and have an in line fuse upstream of each of the drives. The fuse is a fast blow and are below the max 7A rating of the drive. Have only blown a fuse one time and that was when the controller was turned on. Non regulated power supply here is capable of 30amps. You all may do as you wish but i will keep the fuses in mine. 

RICH

On an older rig I have G201's also AND I have fast blow fuses as recommended by the STEP MOTOR BASICS guide at the time I bulit the system back in the day. I've never had a fuse blow so I can't comment on the result. I never had an issue with this until a post by Ray in this thread http://www.machsupport.com/forum/index.php/topic,17090.80.html raised the controversy. In this present thread I was just trying to make the point that even the best sources of info can do a 180. Was Mariss correct then or now - who knows?

Moving back to Stuart's issues, (Thanks for your reply BTW Sargon). I was just raising a question about his current readings back in post #24. If these readings are correct then something's very wrong. At standstill, the total coil currents should be 4*3.32A*2/3 = 8.85A NOT the 3.615A Stuart records. HOWEVER if Mariss's statement that a chopper drive "draws" current at 20KHz from the PS is correct then I agree with Sargon's comment about using a multimeter to try to read this is not going to give useful results. So are Stuart's motors being current starved - who knows? IF IF IF there was a cap in there we could read the steady DC between the PS and the cap and get a more meaningful reading but I've learned here that I was wrong and that apparantly caps on switched supplies is not advised so..... just glad I use purpose built unregulated power supplies with hunky caps (and after tossing a coin - no dc fuses  ).

Ian

1142

General Mach Discussion / Re: Steppers are too slow

« on: December 05, 2011, 04:47:13 PM »

I'm not sure I understand the question. I don't think I said a chopper would "limit" the current to less than the max setting. That doesn't mean the stepper will always draw maximum current.

This and your earlier comment that load affects current is what I don't get. As I understand it, the chopper by it's very nature is monitoring the current through the motor. ONLY when the current through the motor reaches the set point will the chopper start to chop. (There will of course come the time with motor speed when the voltage simply can't drive the required current because of inductance and ultimately the motor will stall but that's not what we're talking about here). I can see no reason how external mechanical load can affect the current through the motor. Servos yes but steppers no. Maybe I'm wrong.

Ian

1143

General Mach Discussion / Re: Steppers are too slow

« on: December 05, 2011, 12:09:23 PM »

Just a note Guys,

Do not fit a large capacitor across the output of a switched mode power supply it affects the sense response.

Tweakie.

Hi Tweakie - I have a slight feeling of deja vu here because Mariss used to have a section in the now famous STEP MOTOR BASICS guide on how important it was to have fuses between the PS and the drives. From what I understand this practice is now apparantly regarded as a complete no no. So I'm quite prepared to accept that the section in question re: caps on switch mode supplies has probably been withdrawn for the same reason. Mariss wrong twice? - next I'll be finding out there's no freakin' Santa 

It'll be a while before I quote from that particular tome again....

Ian

1144

General Mach Discussion / Re: Steppers are too slow

« on: December 05, 2011, 06:47:36 AM »

This is indeed quite expected. You will probably also find the current drop when you provide a load to the motors - it changes the slip angle in the stepper and results in better power coupling within the motor, reducing current requirements. Creating torque without moving is much more difficult for the motor to maintain.

Hi Sargon - can you explain this a bit more please. I just don't get how a current limiting device (a chopper in this case) would LIMIT current to LESS than it was set to. (Not to be confused with not being ABLE to drive the "set" current as the motor approaches stall speed).

Ian

1145

General Mach Discussion / Re: Mach3 digitizing wizard instructions and set up

« on: December 04, 2011, 01:01:13 PM »

May need to make a probe that can be destroyed. Have one that cost a bunch and just don't have the courage to harm it.

I know what you mean RICH - it can get a tad scary, but to be fair that's probing. When I was developing the software I trashed plenty but they were all home made cheapies so no matter. I then came up with the "brilliant" idea of adding in a limit switch for overtravel which worked a treat but then discovered it wasn't my idea at all and had been around for ages.  It therefore always seemed strange to me that expensive probes don't come with that protection.

Actually one of these days I must update the software with one or two improvements I've come up with. Time.....

Ian

1146

General Mach Discussion / Re: Mach3 digitizing wizard instructions and set up

« on: December 04, 2011, 12:05:21 PM »

I would suggest you check out Stirlings Crawler type 3d probing routine. I think it is just what you need.

It will crawl around the circumference of the part to define the area then probe the surface of the defined area.

Probably the best routine I have seen for MACH3 .

Cheque's in the post Terry 

Ian

1147

General Mach Discussion / Re: Steppers are too slow

« on: December 04, 2011, 06:21:20 AM »

I have not been made aware of needing a cap fitted across the terminals of the PS. I would be interested to know the theory why. Was the intent to further smooth the output of the DC supply or for some form of protection of the PS from damage.

More savvy folks than me can probably explain it better but as I understand it a chopper drive "draws" PS current at a frequency in the order of around 20KHz. The large cap provides a "reservoir" or "flywheel" for this current "draw". I tend to go on whatever Gecko says and it's allways served me well. Here's the quote from their "original" stepper document.

The drive works best with unregulated power supplies though regulated linear and switching power supplies may also be used. What matters is the power supply must have a large output capacitor and an unregulated supply intrinsically has one.

(I allways use unregulated supplies)

The other change I made was to connect all four drivers to the same PS terminals. (and you are correct, this is done by having 2 sets in series connection joined in parallel at the PS. I can change that to parallel for all 4 without any problem).

Don't confuse series/parallel with star/daisychain wiring, they're different things. At the moment (from your picture) you have the drivers (in parallel as they should be) BUT divided into two pairs where each PAIR is STAR wired BUT the two WITHIN each pair are daisychained. You need to STAR wire from your PS to each driver separately.

Ian

1148

General Mach Discussion / Re: Steppers are too slow

« on: December 03, 2011, 09:41:47 AM »

Hi Stuart

Glad things are improving - of course without knowing exactly what's changed it's difficult to comment. It seems that you do indeed have a port/BOB problem on a couple of pins but let's leave that aside for the moment and concentrate on the fact that you have 4 motors running so obviously the other pins are good.

I have a few suggestions. Some I know to be correct, some I think are correct. I'd be grateful for comments where folks disagree.

You say you measured 3.5A. This doesn't sound right. Where exactly did you measure this? If you have each of your drivers set to limit current at 3A (which for your motors you should have) then I would expect the total current supplied from your PS to be around 8A at 24Vdc for the 4 motors.

Am I correct in thinking you appear to have a switched mode power supply? Mariss at Gecko used to recommend adding a capacitor at the output of a switched supply (like you automatically would have in an unregulated supply) but this advice seems to have been removed from their updated stepper guide so I'm not sure if this still holds but it makes sense to me at least.

From your pictures it looks as if you have a mixture of star wiring and daisychain wiring from your PS to your drivers. I'd suggest you make it all star wiring. Again see the excellent blurb on driving steppers on the gecko site.

I see your supply is actually 17A at 24V - wow that is some overkill - shame it wasn't 8.5A at 48V  .

Just for clarification, don't worry about measuring current with or without mechanical load on your steppers - it makes no difference to a stepper.

Anyway - just a few thoughts.

Ian

1149

Tangent Corner / Re: Theroretical CNC Mill Design

« on: December 02, 2011, 01:12:29 PM »

I don't know what all you guys are on but can I have some? I recognise all the words in this thread but I've never seen them in this order before... 

1150

General Mach Discussion / Re: Scripter Compile Errors

« on: December 01, 2011, 04:22:27 PM »

Hiya Ray there is a reason that a lot of us stay at the 3.042.020 version or very close to it.

Ain't that the truth. I just can't understand why subsequent releases have been made without thorough beta testing. Personally I wouldn't go near them with a barge pole.

Ian