Messages

21

General Mach Discussion / Re: Steppers are too slow

« on: December 07, 2011, 06:54:28 AM »

That's not how choppers typically work.  They normally run at a constant switching rate, with the pulse width varying based on commanded output.  Current limiting will prematurely turn off the output current, ONLY if the limit is exceeded.

I certainly didn't mean to say that choppers don't switch the power at all times. Thanks for pointing that out so others don't get the wrong idea.

22

General Mach Discussion / Re: Steppers are too slow

« on: December 06, 2011, 06:00:13 AM »

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  ).

To figure this out lets take a look at how multimeters (which would be the same as a typical in-line ammeter) work, as well as DC clamp meters. The theory of operation will give us a good clue about what will work in this situation. Time to redeem myself from my earlier stupidity!

For a DC ammeter (or multimeter in DC current mode), the idea is to use a shunt to measure the current. Basically, we have two paths for the current to flow through - one main bypass to move most of the current around the metering device, and a low current shunt to measure the current. This method is actually going to measure the voltage drop across a shunt resistance, and using the value of the shunt, calculate what the current should be. It's important to note that here we are actually measuring DC voltage. In addition to this, there are various ways the internal circuitry can be arranged - the shunt configuration differs from meter to meter so results in this situation will not be same with all meters, but in any case it should not be considered accurate. AC ammeters are built different, measuring an alternating magnetic field by using an iron core with input and output coils. This will not respond to a constant magnetic field, and in turn will not respond to constant current (DC). In short, you're not going to get a good reading on a combination AC/DC signal using either of these measuring techniques.

What will work is a DC clamp meter. These devices will detect the current by making use of the Hall effect. Essentially this type of meter will measure the strength of the magnetic field by creating current in a conductor within a chip that is arranged such that the conductor is at right angles to the magnetic field. The voltage generated by this Hall effect is directly proportional to the strength of the magnetic field and thus current can easily be calculated. It is important to note that this device is not dependant on a reversing magnetic field. It is only looking at the strength of the field. In addition, it will still respond to changes in the magnetic field (caused by an AC component or changes in the DC current) instantly, and therefore will also be able to measure any complex AC that is on the line, or in our case a fluctuating DC signal.

In short, if you want to measure the actual current draw your best bet will not be a multimeter or oscilloscope, but a DC clamp meter. This will give you, by far, the most accurate measurement. That being said, there is always more than 1 way to skin a cat, and there are likely other methods that would work, but this would be the easiest and should be very accurate.

23

General Mach Discussion / Re: Nesting troubles

« on: December 05, 2011, 11:28:57 PM »

If there is sufficient demand I can polish up my improved nesting wizard - has some extra options and such for selecting nesting direction, determining how many parts will fit on a given material size, and few other goodies.

Let me know if there's interest.

24

General Mach Discussion / Re: Steppers are too slow

« on: December 05, 2011, 11:25:26 PM »

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

No, I don't believe you're wrong. I was mixing up theory. My mistake for sure. Power consumed by the motor can drop with increased load, not current. Current should be constant as long as there is sufficient voltage to drive it through the inductance of the motor (Back EMF related). Thank you for correcting me. I really shouldn't get into theory at 5am - brain just doesn't quite think things through like it does later in the day. This lesson will surely make the point stick in my mind - embarrassment is a powerful learning tool, after all.

25

General Mach Discussion / Re: Steppers are too slow

« on: December 05, 2011, 07:46:26 AM »

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 after the motor reaches stall speed).

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. Of course, once the stepper has stalled the power coupling between the rotor and stator will drop off to almost nothing - the motor will, at that point, only be able to provide a small fraction of it's normal torque.

On an aside, after re-reading the post I did mis-speak in calling, or at least alluding to the idea that a chopper is a constant voltage source - which isn't true. The chopper is being fed constant voltage and then typically uses PWM to "chop" the power into pulses in an attempt to provide constant current - with limited success when compared to a linear supply.

If there are any errors here please don't hesitate to correct me! I'm always willing to learn, and I'm not an electrical engineer so I will admit that my understanding of the theory may not be complete or necessarily correct.


Tweakie:
Please specify what you think would be acceptable capacitance, and what would be considered too large. If I were to use a capacitor for this purpose I would be thinking somewhere in the range of 100uF - 500uF. Am I correct? Also, thank you for clarifying where the problem is in using too much capacitance - makes sense to me - can't respond to changing conditions if you can't see them.

26

Newfangled Solutions Mach3 Wizards / Re: Machine jumps when in program and looking at last wizard

« on: December 05, 2011, 05:58:48 AM »

Mach3 at $159.00 is the deal of the century for hobbiest and simple comercial ventures as long as you understand the limitations.

Agreed!

MACH4 and building your on GUI ?? NOT EVEN AN OPTION. IF the controller is not ready OUT OF THE BOX what good is it to an OWNER / machinist. The idea is to MAKE PARTS not spend all your time working on the machine that makes parts.

Just presenting an option - not for the faint of heart for sure. I thought he had mentioned paying someone to make changes, but now I can't find the reference - must of been confusing 2 different posts - it's really early after all!

27

General Mach Discussion / Re: Normal condition ? and a few other questions

« on: December 05, 2011, 05:50:36 AM »

I do understand work g54/9, as I said zero in g54 but still has .0007" and .001" in the abs screen, I bet it's in this bloody DSPMC motion board from vital systems.
Ray.

Unless the motion controller has encoders connected that are providing feedback, this would not be possible. Generally external motion controllers are just slaves that do what Mach3 tells them to do.

Are you absolutely sure that when you home the machine, that the machine coordinates are actually being reset to 0? With Machine coordinates of 0, and work coordinates of 0, then G54 must also be 0. I'm not sure how anything else would be possible - not saying you're not right, I just can't figure out how Mach3 could justify that internally - there is only one correct answer in math! 0 + 0 = 0.

Check under Homing/Limits and verify that G28 is also set to 0,0,0. That's the only other thing I can think of.

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General Mach Discussion / Re: Steppers are too slow

« on: December 05, 2011, 05:38:34 AM »

Hi Ian and Rich,
I understand your reasons for the capacitor across the supply terminals without much problem. I will give that one a bit of thought before fitting one though. The fact that the PS will hold a very stable output voltage leaves me to think that it already does contain capacitors internally as part of the regulation process. Shall try to get some sense from the manufacturer and seek their advice.

You cannot determine the stability of a supply with a meter. Meters, especially digital meters, tend to average out the voltage, and you will be unable to detect spikes unless they are quite drastic. A decent oscilloscope will give you a much more detailed and accurate idea of what is happening. Generally speaking, switched supplies almost always have relatively low capacitance (ability to store the charge and resist voltage changes). Note that the important difference in supplies in this application is not regulated vs unregulated, but switched mode (using chips - ie a chopper circuit) vs linear (transformer, bridge and caps). A regulated linear supply may also have additional circuitry to limit current and possibly other protection features. The problem with using switched mode is that you are driving an inductive load which by it's nature is trying to maintain constant current - therefore it will fight the supply, which is trying to maintain constant voltage (inductors want constant current, capacitors want constant voltage).

I personally haven't ever heard of adding capacitance causing a problem, but that being said I haven't needed to add any to my machines either (48V switched mode supplies). I am, however, planning to eventually replace them all with linear supplies (much, much better for inductive loads, like motors). Switched supplies are much cheaper to produce (cost is in copper and winding the large transformer) and excel at driving non-inductive loads, such as electronics (ie computers).

The current draw was quite a surprise. All readings were taken by placing a digital multimeter in line with the 4 drivers and the power supply. ie. disconnected the +ve supply to the drivers and inserted the meter between the PS +ve terminal and the +ve leads to the drivers. The results were: all four stationary  3.615 amps. 1 motor running  2.911 amps, 2 motors running  2.343 amps, 3 motors running  1.726 amps and with all 4 running it was 1.177 amps. I guess this makes good sense as I would expect the current draw to be highest whilst the motor is not able to rotate. I have the ability to hold the peak reading on my meter during the start up, but didn't take it on this occasion. I can do if you are interested in the result.

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.

29

Newfangled Solutions Mach3 Wizards / Re: Machine jumps when in program and looking at last wizard

« on: December 04, 2011, 06:07:05 AM »

Mach4 will help this issue when it comes out. I'm not sure if Mach3 is capable of disabling this, not that I'm aware of at lease. If the problem hasn't been addressed in Mach4 you will have the option of creating your own application for the user interface, or paying someone to program it for you - based on information that Brian has leaked to us in the yahoo forum.

Come on Mach4!

Hopefully it will be as beautiful as it sounds, and any limitations that exist in the stock user interface will be able to be addressed with custom frontend software. For now, BR549 is absolutely correct in that there are some limitations/caveats that the user MUST be aware of to operate the machine safely and without error. The way I see it, $159 is not enough for what you get! As a business user I would pay considerably more, but that would make it more difficult for the hobbyist to get into the CNC world - and AFAIK that is what Mach3 was originally intended for.

30

General Mach Discussion / Re: x axis not moving when using one computer, does when using another

« on: December 04, 2011, 05:54:26 AM »

IBM systems also work well, in my experience. We use IBM and HP. Acer's work also, but are not reliable enough.