Messages

1011

General Mach Discussion / Re: Linear encoders and steppers (newbie ?)

« on: August 13, 2009, 12:28:21 PM »

Steppers can be set up to run in series or parallel. From the size motor and only 4A, sounds like series. That would be a mistake in my opinion. By way of example, here is a quick discussion using round numbers:

If those are 8 wire motors, running them in series reduces the current, but doubles the voltage. leaving you with only 10X possibility with the Geckos 80V limit. Parallel would put you at 8A but give you 20X voltage. Take a look at the chart on this similar motor:

http://www.kelinginc.net/KL34H2160_62_8A.pdf

When I first decided to plunge into a retrofit, my initial research quickly introduced me to the never ending stepper vs servo debate. Some of the 'common knowledge' is true and some is not.  I am of the opinion that one cannot  reasonably compare an animal know as 'servos' to steppers, because the term 'servos' is completely inadequate to describe the species. Within the various servo types there are significant differences in cost and complexity that often are not mentioned. AC servos get VERY expensive in a hurry, whereas DC brush servos (arguably the logical first tier upgrade from steppers) are actually very competitive with steppers on a total installed cost basis.

True: DC brush servos are more complex to select and tune for a given application . . UNLESS . .  you have a reliable resource to copy from. In which case, the complexity moves to nearly zero.
        Gecko 203V drives require tuning, albeit very simple compared to servo drives.
        Small and medium sized DC brush servo installs can be nearly the same installed cost as steppers

False: Blanket statements like: 'Servos' cost 3 or 5 or 10 times what steppers cost.
         Blanket statements like: Steppers are 'just as good' as servos. This is equivalent to saying a Corvette is 'just as good' as a pickup truck. Neither is 'better', it depends on what you are trying to move and how fast.
         Steppers can be compared to servos by their torque ratings.

1012

General Mach Discussion / Re: Really need help

« on: August 13, 2009, 10:36:27 AM »

. . . . I was monitoring the following error in the drives software . . . . .

I have been jealose of this ever since you first mentioned it some months ago   

Soon I will have my hands on two new servo drives that have this capability. The tuning software for the CNCdrives products reportedly can both graph and provide textural data for real-time monitoring of the following error.  Should be fun to play with as well as very useful.

FYI, Rutex tuning software has real time textrual following error, but the tuning software cannot run while Mach is active. Leadshine tuning software graphs following error (as well as a host of other parameters) in real time also. I do not know if their software can run concurrently with Mach.

1013

General Mach Discussion / Re: Really need help

« on: August 13, 2009, 10:20:55 AM »

To my mind the driver test is there to weed out the really bad computers that have no chance of running Mach in their present state rather than saying it will definitely run Mach.

Hood

You may view it as a pass/fail, but it also 'grades the papers' so to speak, and provides what appears to be a benchmark useable as a guide to setting us Mach. The logical assumption, and the one that I myself made, is that a passing grade at XX khz is a green light to set up for that speed. This is where the problem lies. Another area where the 'sin of ommission' is committed by the Mach documentation. I discovered that the driver test is not neccessarily indicative of how fast Mach can run on a given computer only after much wasted time and frustration 

Info worth sharing, methinks.

1014

General Mach Discussion / Re: Really need help

« on: August 13, 2009, 10:05:48 AM »

Hood,

Arguments are emotional and useless. Debates are factual and beneficial. Methinks we are both fact guys. You are far more experienced than I on these topics and I certainly would not question your observations. What it means to me is that both behaviors occur, and that becomes the fact that begs explanation.

You mention following error on a lathe at 45k. Three significant differences in my scenario (and I perhaps that of the OP) steppers, three axis moving simultaneously, high speed. Which actually brings up an interesting point; In the past, I had all steppers and therefor had no point of reference outside that realm. However, with my current odd combination of steppers and a servo on the same machine, only the stepper, which was maxed out speed wise in the motor tuning, lost position. The servo was spot on, although it was nowhere near maxed.

I make the assumption that Mach sends only pulses and no positional information, therefor it must have sent all of the pulses to the servo in order for it to maintain its positioning. One interpretation of this would be that these curious errors are confined to steppers. I have observed the lost steps with three different models of stepper motors on two different computers. All were being driven by Gecko203V, so perhaps that might be a suspect.

An interesting twist on this topic is that the Smooth Stepper in invariably described as having a 'high quality' pulse train. The logical implication being that Mach and the PP do not produce same. Although I do not recall ever reading any hard evidence to that effect, experienced an knowledgeable people use the SS, so I have to assign credibility to the concept. Taken together, all of the above evidence might seem to siggest that the Gecko 203V is particularly sensitive to a 'dirty' pulse stream, yet the OP is using differnet hardware, which defeats that theory. It takes time to learn which sources are reliable and which are not. The lesson in all of this is that in a situation where an unresolved problem is manefest, all possible known causes deserve mention.

 

1015

General Mach Discussion / Re: Really need help

« on: August 13, 2009, 04:52:30 AM »

Something I forgot to mention is to leave the task manager running on top of Mach to observe the CPU load. If the CPU pegs, you loose steps.

I just chased down a new problem where my X axis stepper (640 oz-in) was loosing .020 after every flat out run (135 in/min 10 accel). If I drop it back to 90 and 8.5 (where it is normally set), it stops loosing steps. I am switching to servos and decided to have a foot race between the stepper and servo while I had one one each axis. Needless to say, the servo won. I don't know where in that range the stepper started loosing steps and since the stepper is out of there in about a week, there is no incentive to dig into it. I have to measure each time to find if the stepper has been a bad boy as there is only an audible clue if it skips quite a few steps at a time. Onesy - twosy misses are acoustically invisible.

1016

General Mach Discussion / Re: Really need help

« on: August 13, 2009, 04:17:07 AM »

Hood,

That may be true in theory, but in practice, I can reproduce an error every time if I max out the stepper speed on more than one motor.

With the exact same motor settings, increasing the pulse rate makes the problem go away. This is true for XY with two steppers or also with one stepper and one servo (I am in the process of changing over).

My conclusion is that Mach is unable to provide the full pulse rate to all axis simultaneously and needs some headroom for whatever else it is doing at the same time.

Your results may be different.  

I don't know what the breakpoint is, but if someone wanted to do a bunch of testing, maybe some guideline could be formulated, for example not to exceed 90% of maximum theoretical speed, or something like that.

Another area where theory and practice collide is in the driver test vs actual program execution. My observation is that there is a correlation between CPU load and kernel speed when Mach is running. I can state with confidence that a successful driver test at a given speed is no guarantee that Mach can actually perform reliably at that speed, as theory may suggest.

1017

General Mach Discussion / Re: CAT30 vs BT30 vs NMTB30 vs ????

« on: August 13, 2009, 03:09:52 AM »

Isn't the pressure mentioned used for releasing the drawbar?

Seems to me that the release pressure would need to be adequate for breaking loose the taper and could be many times the actual drawbar tension.

1018

General Mach Discussion / Re: apparent loss steps?

« on: August 13, 2009, 02:56:41 AM »

Starting with the assumption that you have already checked things like the slides binding and any other mechanical causes, you should look into what speed is the pulse stream output set to. It is under config>ports and pins.

With high microstepping, you probably have a huge number of steps per unit. If you can, set the microstepping off (and reset Mach appropriately) and see if the problem goes away.

Mach needs lots of pulse speed to be able to run all three axis simultaneously, so you will need a high pulse stream speed with a lot of microstepping dialed in.

1019

General Mach Discussion / Re: Really need help

« on: August 13, 2009, 02:32:43 AM »

Stepper motors should run on 3 to 25 times their rated voltage. I have had trouble with lost steps below about 10x voltages.  Stepper problems do not manifest at a steady state. That is when they are the most powerful. It is at speed where the power drops off sharply that the problems occur.

You mentioned a parallel port monitor. That sounds like a bad idea. If the program is, or if it uses a TSR type of utility, it may be polling the port occasionally. That would be enough to disrupt the pulse stream. Also, remove any printer drivers, and run the port without interrupts.

If you look back a couple of months in the postings, or do a search, I posted info on an add-on parallel port that works with Mach. I have found it to be faster and a little more robust that the built in port on older machines. It is dirt cheap and would answer the question as to whether your existing port is the problem.

You will see a lot of posts showing calculations for max stepper speed based on the pulse rate Mach is running at. Remember to consider that the pule rate is the total and must service all of the drives. While it is unlikely all of the motors would run at full speed simultaneously, it is unwise to set a speed based on all pulse frequency being available for each motor. This is probably why your motors work fine individually and loose steps when both are running.

Make sure any wire shields are grounded on one end only, preferably at the control box.

Lastly, use the driver test program to see how fast your computer can generate the pulse stream, but do not assume you can run Mach that fast. Back off at least one speed setting slower than what the driver test determines.

1020

General Mach Discussion / Re: Linear encoders and steppers (newbie ?)

« on: August 12, 2009, 07:49:59 PM »

Gecko 204V is a great stepper drive. I just sold the last of mine and they did a great job for me while I had them.

You are handing them a tall order with those big steppers. The Geckos top out at 7A

Why have you decided on steppers over servos?