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Offline RICH

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Re: Steppers are too slow
« Reply #50 on: December 08, 2011, 05:24:22 PM »
Hope I haven't started a forum war here
Let not your heart be troubled....
Just a friendly discussion looking for understanding......
Re: Steppers are too slow
« Reply #51 on: December 09, 2011, 06:10:17 AM »

Hope I haven't started a forum war here, but I still have a bit of a problem understanding why a good quality Digital multimeter connected in line with the PS output to the drivers fails to read an accurate current flow compared to a hall effect clamp meter. When i look at the chopwave graph Ian posted at post 44, the current trace has fairly minimal peaks and troughs associated with the switch activity. I read this as the current flow through the coil and not necessarily a waveform of the driver input supply. I would have expected the circuitry within the driver would have provided some smoothing to the input current. Additionally with 4 drivers all connected together I would have expected some further smoothing of this wave form as one goes high and the other goes low.

Is my thinking correct here??



Unfortunately, it isn't as simple as that. Measuring PWM power is actually a substantial problem. One major problem is the phase angle, which is not taken into account at all by a DMM, no matter how good it is. The second problem is the harmonics generated by chopping the DC signal. You can have measurable power up to the 100th harmonic in some cases, although at the power levels we're talking about here it won't be that extreme, and obviously the power going up the harmonics will drop exponentially. Nonetheless, unless these factors are taken into account you will not get an accurate reading. The best DMM on the market is therefore pretty much useless to obtain an actual measurement.

Power output from a PWM can be expressed as Power = V * I * cos(e). Without taking this phase angle into account, the only real use of a DMM would be to compare motors to motors, or drivers to drivers, but the actual value you get is quite erroneous.

In fact, an analog meter will give results closer to reality than either an averaging DMM or true-rms DMM, because it will respond to low frequency harmonics in much the same way as the motors. It will not, however, take the phase angle or higher harmonic power that is also being used/lost into account. Higher harmonic power, from what I understand, will not be converted to usable torque, but it is still being drawn nonetheless.

After a little more research into the topic, I'm starting to also question the effectiveness of a DC clamp meter for the same reasons as above. While I'm fairly certain it would be more accurate, I'm not sure it would be close enough to actually be considered accurate. A power meter, however, will take the phase angle into account and measures both voltage and current simultaneously. With this method you will be within 1% of real life power draw. This, however, doesn't really help with real time current calculations as voltage is not constant, but could still be useful with the right methods.

A digital storage oscilloscope will do a pretty good job, but you will have to know what you're doing to get useful measurements. Also USB oscilloscopes with decent software would also do a good job, and many cases better because of higher resolution sampling. Standard oscilloscopes are 8-bit, better ones are 12-bit. This is relevant to measuring the harmonics.

This is a complex topic because of, in short, the phase angle and harmonics, and is made worse by the very fast rise and fall times created by modern PWMs. In addition you may also have to contend with reflected power depending on cabling lengths and such, but not likely significant in this application. Unfortunately I'm not well versed enough to say for sure one way or the other, I just know the potential is there.

There is obviously much more to this topic, but hopefully this will be enough to provide some insight into why standard meters are so inaccurate at measuring PWM power.
« Last Edit: December 09, 2011, 06:17:59 AM by Sargon »
Re: Steppers are too slow
« Reply #52 on: December 09, 2011, 07:13:50 AM »

Thanks for your explanation. I think I need to do a bit more reading on the matter and refresh my knowledge.

Your post above makes multiple references to measuring power. I have not tried to measure the power in the circuit. It was the current I was trying to measure.

Also your formula is used for calculating the power within a circuit, and to obtain this we multiply the voltage by the current and then multiply it by the Cosine of the phase shift angle. This I understand, but if you are saying my current reading is not accurate, and we don't know the power figure, how can you use this formula? You are missing 2 parts of the equation.

I qualified as an "A" grade electrician in Australia 32 years ago (but have been out of the game for about 20 years) and recall situations where the current wave versus voltage wave were out of phase. We used to come across it if we were fitting a lot of fluorescent lights, where the inductive ballasts in the lights would cause the current wave to lag and we would correct this by fitting capacitors to every 5th light. We referred to the cos(e) part of your equation above as "Power Factor" and had a legal requirement to get this figure above 0.8. (Otherwise the power authorities power meters would not read accurately). The major difference with this situation was however, that we were working with AC circuits on the lights and the output from the PS should be a very stable DC. I was not aware that any of this phase shifting could occur in DC circuits.

Most definitely I can understand your theory if you were trying to measure the power in the motor coils but It's going to take me a bit of getting my grey haired head around the PS side of things.

Thanks again for your explanation.

Guess I'll try to find a good book on the topic and refresh a little on the theory.


Re: Steppers are too slow
« Reply #53 on: March 28, 2012, 11:50:26 AM »

I have just set up a small china cnc router bought on Ebay from MIB instruments. I was able to get everything running, but I am having two problems. The first is that when I set the configuration for inches I only get mm readings on the DRO. I set the G80 F6.0 G20 in the initialization line, but that did not help. I re-set the system to mm and that did changed the DRO to increment in what seemed to be inches, but I don't know if this is correct. The second problem (challenge) is that when I MDI a movement of 1 or 2 inches, the axis I program says 1 or 2 on the DRO, the table slides move much more (at least twice). What am a setting wrong? The spec sheet from the manufacture (MIB) lists the parameters as: SPI = 460, Vel = 250, Accel = 20. This makes the table creep along. I played with the Vel and Accel as suggested in one of the video tutorials and that made the movement faster but still not accurate. The other thing is when I am in motor tuning the motors run nice and smooth and the tables have a nice rapid movement. When I exit out and use the Jog mode, the motors are very noisy and the tables look like they are jerking. The motors are ICAD 57 2 phase 3A 1.8 Degree step motors. Does anyone have a suggestion on what to do?
Re: Steppers are too slow
« Reply #54 on: February 23, 2016, 10:54:22 AM »
ola tambem estou com problemas com dq542ma perde passo se por mais que 500

Offline SKS

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Re: Steppers are too slow
« Reply #55 on: March 31, 2018, 08:19:21 AM »
I build the CNC engaving machine by using Mach3  demo Version R3.043.066 and paralall port breakout board.I used the Dell 4310 win 7 /32 bit  laptop with docking station,
everything works fine.
Later on same version of demo mach 3 installd to dell 6420 windows 7 32bit also.docking station and All breakout bord steppers and machine still old one
When I do the drive test also excellant.
When I run the machine with Gcode, machine is running in slow and its not moving to the smae reading which is in the Gcode file.
finally I found one thing
When I run the axis in MDI mode with D00 x(some value)- its running exacly
But Run with D01 its become very slow.
Previously I used core 2 duo desktop also I expereance same thing.At that time I thought that deskto pc has some issue.Thats why I use this laptop again.
Many time I unstall install mach3 also format that laptop and desktop.
Other thing is still my Dell 4310 win 7/32bit laptop can use withe same machine with same docking station.
Before I buy the mach3 licen I want to slove issu.

Please help me to slove this issue

Thanks for all
Re: Steppers are too slow
« Reply #56 on: March 31, 2018, 08:43:33 AM »
laptops are not recommended for Mach3, they have powersaving code that causes the CPU to run slow. Having said that they usually do work and I don't think
that's why your machine is running slow.

I don't know where you get D00 from, I rather think you mean G00....it is after all called 'G Code'.

G00 Xnnnn Ynnnnn Znnnnn  causes your machine to drive to the coordinate location at the machines maximum speed as determined by your motor tuning
G01 Xnnnn Ynnnnn Znnnnn  causes your machine to drive to the coordinate location at the prevailing feedrate. If you want to change the feedrate  use"
G01 Xnnnn Ynnnnn Znnnnn Fnnnnn which causes your machine to drive to the coordinates at the feedrate  specified by Fnnnn up to the maximum axis speeds
determined by your motor tuning.

My wife left with my best friend...
     and I miss him!

Offline SKS

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Re: Steppers are too slow
« Reply #57 on: April 05, 2018, 03:08:35 AM »
Hi Craig,
Thank you for your replay.
Re: Steppers are too slow
« Reply #58 on: April 07, 2018, 05:59:07 AM »
I have noticed that sometimes Mach3 gets a bit hung up on feed settings. Starting from cold, it can stick at 6 mm/min despite g-code commands otherwise. However, if I reset the feed to, say, 100 mm/min by typing into the DRO on the screen, then it is fine for the rest of the day and does register any reprogramming.
It's a bug.