Messages

881

General Mach Discussion / Re: Would like opinions on servo motor voltage and PS secondary in series

« on: January 23, 2010, 11:00:45 AM »

The motor will basically respond to RMS voltage.

This is how I though it must work, based on what *seems* logical and also what I was observing

How much voltage loss there is depends heavily on the design of the controller, so there's no way to answer that question without knowing the circuitry of the specific controller in use.

I measured with a DVM and also an analog VM and got fairly similar results. Actually it was the low tech analog VM reading that made me think that a simple DC motor winding probably 'sees' the voltage in the same way.

I'm not concerned about how much loss there is, only with how to measure it for the purpose of oversizing a PS an appropriate amount to compensate. However, if you are familiar with the design of PWM power devices, would you know if there is a way to smooth the PWM output in order to obtain a smooth running motor? I have observed a very wide range of smoothness from different PWM devices, which leads me to conclude that there is no intrinsic and unavoidable roughness inherent in the PWM process, but rather that the designers have different approaches that yield different results.

882

General Mach Discussion / Re: Would like opinions on servo motor voltage and PS secondary in series

« on: January 23, 2010, 06:38:43 AM »

Both drives are specifically current limiting. The Minarik motor control has no voltage limiting, but as mentioned by JH, the servo has a 'max PWM' setting which I would speculate effects the average voltage output, if in fact the motor *sees* it that way ( I have not gotten an answer to that question yet).

I am collecting info on this issue from a couple of sources, and an interesting tidbit that I learned is that the PWM scheme does impart a voltage drop of perhaps 10%. I got a detailed explanation for this that is over my head, but the 10% loss is what I was interested in knowing. The remaining 15% delta in measured voltage may be attributable to the inaccurate measurement via voltmeter (which has been confirmed).

Learning and understanding this stuff is essential for a venture I have coming up. I suppose that over my career I have gotten lazy by being surrounded by other engineers of different disciplines to hand things to.  Now I wear all of the hats and some fit better than others. The one that says 'EE' is still in the shape of a dunce cap . . .LOL!!

Perhaps worse yet, I no longer have the resource of the shop where I could walk out and talk to the mechanics and machinists to get the 'real' story of what works and what doesn't. That's where forums are so valuable.

Rambling again . .  sorry.
  

883

General Mach Discussion / Re: Would like opinions on servo motor voltage and PS secondary in series

« on: January 22, 2010, 05:18:10 PM »

Servo motors will run at max RPM when the rated voltage is applied.  If your servo driver is PWM (dugon 160V 35A), your PS output voltage does not have to match your motors max voltage rating, the PS out can be more, then you set the max PWM parameter on the driver so average RMS voltage to the servo motor is not more than the motors rating.

The motor does run at the predicted RPM based on the voltage I am reading on the voltmeter. i.e. the motor spec is  13.36V per K RPM and that pretty close to where it runs if you go by the measured voltage.

The quandry here is that the voltage coming off both the servo drive (PWM) and the motor controller (also PWM) does not *measure* what it should. I'm interested in knowing the reason, which I can only imagine could be;

1) ?? Full PWM is not really *full* as in full time *on*,

2) ?? There is some sort of loss in between the input and output of the PWM scheme,

3) ?? The full voltage is actually there, but a normal voltmeter cannot read it correctly becuase is is pulsing,

4) ?? A DC brush motor 'sees' the voltage in the same way as a standard voltmeter, in which case it would seem logical to size a PS higher than the motor's rating if you are using a PWM scheme, but I have not been able to find anything difinitive on that, although I have not looked very hard yet.

Interestingly, The Minarik drive is rated at 130VDC output, yet the instructions for setting the various pots uses 90V motors as examples. Is there a message there?

I'll have another piece of the puzzle when I do as Ray suggested and run the motor straight off the 74V PS.

884

General Mach Discussion / Re: Would like opinions on servo motor voltage and PS secondary in series

« on: January 22, 2010, 05:03:26 PM »

That would suggest your drives are current limiting.  What happens if you connect the motors directly to the power supply?  They should then go full speed.

Yes, they both are current limiting, but it was my impression that this would not effect the voltage. In any case, I don't know why it did not occur to me to connect the 74V PS directly to the motor for testing. I'll do that next.

885

General Mach Discussion / Re: Would like opinions on servo motor voltage and PS secondary in series

« on: January 22, 2010, 01:54:59 PM »

Interesting new info. I have the feeling I am asking these question in the wrong place as this forum seems to be 99% steppers, but I'll give it another go.

I checked the PS at the output and both sides are cranking out 74V. Yet the spindle servo motor wires only show 50V at max speed.

The Minarik drive arrived and it's 130VDC output kicked the spindle speed up from 1,300 to 2,700 . .  nice. Yet a voltmeter reads only 105V on the motor wires.

Both the Minarik and the servo drive are PWM, and I recall  reading somewhere that a voltmeter reads PWM inaccurately. Yet it seems that at full speed the PWM should be at 100% and therefor show as the full voltage on a voltmeter.

Adding to the puzzle, the actual motor speed calculates correctly using the actual voltmeter reading and not the theoretical voltage, so it would seem that the motor also sees the voltage at the same level as the voltmeter . . i.e. significantly below the rated voltage of either the servo drive or the Minarik speed controller.

What am I missing here?

886

General Mach Discussion / Re: What is a good probe to use with Mach?

« on: January 21, 2010, 10:28:16 AM »

Ray has my wheels turning now and I don't know about a few tenths, but less than .001 is probably doable and would satisfy my needs.

I can do a proof of concept really easily and then make a working prototype if the machining is successful. I don't have the high speed pullleys made for my new mill yet, so this project will have to wait. 


The time eater is probably going to be learning the probe macros and how to use them.

887

General Mach Discussion / Re: Anyone Here Have Experience Milling 6061-T651 Wrought Plate?

« on: January 21, 2010, 07:18:16 AM »

I don't want to get into a science discussion on a hobby forum, but if you are curious about the coatings, you need to look into how cutting works on a microscopic level. The reason a lot of inserts are not sharp is because they don't need to be. The material actually 'tears' off and the very tip does not even contact the material.

The problems occur just aft of the tip where the metal chip is scraping past the tool surface after it is torn from the parent material. If you run inserts as fast as they are designed to run, the wear point is a crater behind the tip. This erosion failure of the cutter surface and is why coatings to improve abrasion resistance are effective.

Look at the part of your end mills where the chip is curled around after the actual cut. The sliding motion here creates intense heat which is the devil in aluminum and other gummy low melting point materials. In this case, abrasion is not so much the problem as heat and 'pick-up' and a coating that reduces friction or imparts a 'lubricity' in these surfaces of the cutting tool are effective in sort of a similar way to flood cooling.

That's as far into it as I want to get, but it is a fascinating subject with lots of technical information available for those with an interest.

As to grinding paste, there are no doubt lots of different ones. I was doing some consulting recently that netted me a sample of Boeing's 'Bio-Lube' product in paste form.  It works well enough that I bought their gel version (have not tried it yet). Normally I use Eastwood's stuff. They call theirs 'grease' for some reason, but both these are more accurately described as 'sticks', methinks.
http://www.eastwood.com/ew-grinders-grease-8-oz.html

888

Third party software and hardware support forums. / Re: DMM Technology Corp.

« on: January 21, 2010, 06:48:58 AM »

The DMM products were good performers. My issue with their drives is that they are not resettable from a fault without powering down and restarting. This is not acceptable for a machine tool where doing so requires re homing the machine, which may or may not be possible depending on what kind of fixtures and workpiece are in place.

DMM is primarily interested in OEM produsts like printers and other single purpose machines that are designed, tested and intended to do a specific predictable operation over and over and their policy is that their drives do not need to be resettable if the machine they used on 'is properly designed'. The same is true for their AC drives, so I did not go forward with evaluating those products.

Frankly, I found this attitude to be not only condescending on its face, but indicates a complete lack of understanding of the needs and function of CNC machine tools.

So my opinion is that DMM has well engineered products for *some* applications . .  but CNC machine tools is not one of them. This comment is based upon their attitude and statements at the time of the review. They may have changed this policy in the interim, so a visit to their site and perhaps some question to their tech support would be in order for anyone contemplating using these products on a machine tools, robotics, or other 'general purpose' CNC application.

889

General Mach Discussion / Re: Opto switches behaving badly

« on: January 21, 2010, 06:32:37 AM »

I don't have limit switches I have never reached end of travel for the work I do.  After thinking about it I don't see why a high debounce would effect the homing switches. 

If you read carefully, you may note that my comment was that if you share homing and limits in Mach. You may *think* that you don't have 'limit' switches, but that is simply a semantics argument that indicates you do not understand how Mach can handle 'home' switches.

Your assumption is theoretical and overly simplistic. You do not take deceleration into account in your calculations. In your case, you have the axis literally crawling up to the switch, but in any case, an axis is not going to stop instantly when the switch is triggered. The table weight (plus the vice, fixture and workpiece), the acceleration setting, the homing speed, 'cushion' between the switch and the hard stop, and finally the debounce setting all are part of the equation that predicts 'bang' or 'no bang'.

I would be interested to see a report that states an opto is 'more accurate' with higher voltage. Do you have a link to that?

890

General Mach Discussion / Re: What is a good probe to use with Mach?

« on: January 21, 2010, 05:46:37 AM »

OK, but if you come up with something that works, you gotta share!  :-)

My idea involves an operation that carries some serious potential hazards if it is done incorrectly, so I am reluctant to post it on a public forum, but no problemo on the sharing it it works.

BTW, everything I come up with works . . .it's only a question of how many redesigns are needed to get there . . LOL!!

However, my special talent is taking something that works fine and 'improving' it to the point where it no longer works at all . . .