Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: Stuart_H on November 30, 2011, 04:51:07 AM
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Thanks to all for this great site.
I am currently setting up to build a 1200mm X 900mm router table and have been trying to get some sense out of Mach 3 running 4 stepper motors ("a" axis slaved to "y" axis).
The problem I am having is with the stepper motor speed. I am lucky if they are turning at about 50 RPM. The following describes the setup:
- The steppers are 380 oz/in being driven by Wantai DQ542MA drivers.
- The power supply is 24 volts which was supplied with the drivers. I have also tried another power supply with no change.
- Breakout board is powered by independant 5V supply and have tried 2 other combinations of supply including 2 separate 5V supplies with the jumpers removed from the BOB
- Have been into the motor tune settings and tried a number of different configurations and calibration settings
- Tried replacing the parallel cable in case there was a pin out there. No change.
- I have another combination stepper driver/ breakout board with 3- 180oz/in steppers and they seem to run without any problems at a pretty normal speed
- Mach 3 driver test is excellent with a very stable trace line at a freq. of 25003 khz.
- The motors all seem to run smoothly but just too slow. Also it does not make any difference whatsoever where the microstep switches are set. It is like it is running in max microstep all the time.
- Is it possibly a faulty breakout board or is there a hidden tick box in mach 3 which I haven't found causing my grief.
I find it hard to believe that all 4 drivers are faulty to this extent, but I have pulled what little hair I have out to try and solve this problem.
Any past experiences or knowledge would be much appreciated.
Thanks,
Stuart
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Stuart,
I take it you have set up each of your steppers for optimum Velocity (reduced by some 20% - 30% for reliability) and Acceleration ??
Tweakie.
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Stuart, can I make a plea to you and everyone else that posts similar problems? Without the motor, driver and power supply data sheet/specs and idealy some info about the gearing and drive type of your machine, all anyone can do is guess. By googling "Wantai DQ542MA" and searching the links for a 380oz motor option (the two clues you've given) I could probably spend some time and home in on which motor, driver and PS you might be using but frankly it would be a lot easier to help you if you did that yourself and posted links.
Thanks
Ian
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I'm not an expert in stepper motors; others here have much more experience than I, but I can try to clarify a few things for you.
24V is generally considered to be low for driving steppers - higher voltage will help "punch through" the inductance of the motors (in layman's terms) to allow for higher speeds and better acceleration without stalling. That being said 24V should normally be sufficient to drive them, just not optimal (you will not even come close to the rated torque value). Our lab product that we manufacture uses 24V (actually, it's 28V now, but used to be 24V) and runs just fine, but our CNC machines use 48V for better torque.
Could you provide the motor inductance? That will help in figuring out the problem. My best guess at this point is the 380oz-in motors may be high inductance, limiting the speed. You said your 180oz-in steppers are being driven fine with the same setup - this points to some difference in the motors - must be inductance, I think. Best to check it first, in any case.
Microstepping is always a good thing - higher setting should help increase max velocity and acceleration as it reduces the affects of resonance which reduces the torque of the motor. More steps = smoother current flow = increased torque = increased speed = better motion.
If anything I've posted here is incorrect or incomplete, other more experienced users will likely jump in. In the meantime, please find out what the inductance of those motors are. That would be the best place to start as your other motors seem to be fine in the same setup. 50rpm would definitely be very slow for a stepper.
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Hi Tweakie,
Yes I was setup almost identical to your screenshot. I had 500 steps per mm but otherwise the same. I changed to reflect what you have here but still no change.
Something I omitted to say in my earlier post is that I am not able to get a driver to work when connected to P2 and P3 on the breakout board. It is now connected to P17 & P16 and operates, but slow like the others. For this reason I am a little suspect of the breakout board. Also the motors are only sitting on the bench top, ie. they don't have any load on them. Is this likely to have any effect?
Stuart
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Ian,
The steppers, drivers and power supply were bought ex China at the recommendation of a friend who has the same setup. As you could probably imagine the supporting documentation was minimal (read Nil), but I will source what I can and reply
Stuart
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Hi Sargon,
I will see what I can find out re the inductance and let you know. As mentioned above, the supporting information is pretty minimal. However, this combination is working fine on a friends machine.
Stuart
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Thanks for the prompt replies guys,
This is my first attempt at anything CNC, so I apologize in advance if this turns out to be anything basic.
.xml file attached of current setup.
Stuart
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Sorry,
These motors are actually 425oz as per the details below. The power supply is 24 volt 400 watt
Technical Specifications
Part No.: WT57STH115-3004B
Frame Size: NEMA23
Step Angle: 1.8 degree
Voltage: 6.3VDC
Current: 3 A/phase
Resistance: 2.1Ohm/phase
Inductance: 9mH/phase
Holding torque: 30Kg-cm 425oz-in
Rotor inertia: 810 g-cm2
Detent torque: 0.89 kg-cm
Number of wire leads: 4
Weight: 1.55KG
Length: 115mm
Shaft Diameter: 8mm
Front Shaft Length: 21mm
Stuart
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Hi Stuart,
The fact that you cannot use P2,P3 is an indication that something is wrong there.
One difficulty that has cropped up a few times recently is that most newer PC parallel ports have changed from the 5 volt TTL standard to the 3.3 volt TTL standard and some breakout boards will still only reliably work with the 5 volt signals.
Tweakie.
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Stuart - from the info you've given and what I've been able to find...
With inductance of 9mH your "ideal" power supply voltage for your motors to achieve max torque/speed is 96Vdc. Your drivers can only handle a max of 50Vdc and your power supply is only 24Vdc. I'm afraid this is NOT a well matched system if high speed is what you want.
Ian
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Ian,
Even if his system is not that well matched don't you think he should still be able to achieve better than 50 rpm ??
Tweakie.
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Tweakie - well I suppose, but ultimately we'd be guessing wouldn't we?
We know that typically a doubling of voltage will double the stall speed so with the 24V as opposed to 96V we can generally expect a quarter of potential. Is 200RPM still slower than we'd expect then I suppose maybe yes but it's a hard call at this distance. Stuart has stated that they run smooth so I'm thinking the drives and Mach are OK. I'd stick the microstepping as close to 10 as possible for starters but after that - like I say from this distance it's hard to tell what other issues (if any) may be at play.
Ian
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bought ex China ...............the supporting documentation was minimal (read Nil)
Not able to assist without posted info on your drives and bob.
Till then,
RICH
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Hi Ian & Tweakie.
Thanks for your help so far with this problem.
Today I disconnected all drivers from the BOB and connected each one individually one at a time. As I reconnected them, each one still ran slow and the microstep switches still have no effect on speed or torque levels. The motors don't even make a different sound with each selection. Also I went into Mach 3 and changed the kernel speed through the range of 100 hz to 75000 hz with absolutely no effect.
The computer is about 8 years old and is a Pentium 4, 3.2 Ghz processor wit a 2.75 Gb of Ram. I had a fairly new hard drive completely formatted and windows reloaded to be used for this purpose when I first started to set up for this project. the steppers are running off the parallel port on the motherboard but it does have another parallel port PCI card installed. Do you think it is worthwhile trying to use this other parallel port, or maybe removing the card in case it is having any effect on the output pulse rate?
I do have some doubt about the BOB as I still can't get 2 of the output pins to work on any of the stepper drivers but they will work if connected to the other available pins. My next thought is to replace the breakout board with a different type. Do you have any suggestions?
I appreciate this might not be the best matched system, but the attached You tube URL has the identical components on a friends machine and it can be seen how fast it operates. There is no way these steppers are running this fast.
http://www.youtube.com/user/vanhaydn#p/u/121/2zhwnxttPbE
Stuart
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Hi Stuart,
Can I suggest you carry out a couple of very basic tests.
Initially just one controller set microsteps (SW 5-8) at 1600 steps per rev and the motor current (SW1-3) at 3 Amps.
Go into Mach, Motor tuning and set "steps per" to suit your machine screw pitch (theoretical) then adjust motor velocity.
What speed does the motor stall at ??
Connect a voltmeter to your power supply output.
Does it maintain the 24 Volts during the above test ??
Tweakie.
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Today I disconnected all drivers from the BOB and connected each one individually one at a time. As I reconnected them, each one still ran slow and the microstep switches still have no effect on speed or torque levels. The motors don't even make a different sound with each selection.
IMHO your not going to be able to detect visible or audio changes with different microstep settings. Though technically microstepping does have subtle effects on torque I really doubt you'll be able to see or hear any difference. Maybe between NO microstepping and (say) 10 but other than that I doubt it very much so I'd not get too bogged down in that. As I said - set them as near 10 as you can and leave them there.
Also I went into Mach 3 and changed the kernel speed through the range of 100 hz to 75000 hz with absolutely no effect.
To calculate the CORRECT kernel speed multiply your steps per unit by your max velocity (in motor tuning) and divide by 60. Do this for each axis and choose the LARGEST figure. That gives you the max steps per sec you'll ever use. Then pick the LOWEST kernel speed above that figure. Any kernel speed above that is not only pointless but may cause other problems. Post your figures so we can check.
The computer is about 8 years old and is a Pentium 4, 3.2 Ghz processor wit a 2.75 Gb of Ram. I had a fairly new hard drive completely formatted and windows reloaded to be used for this purpose when I first started to set up for this project. the steppers are running off the parallel port on the motherboard but it does have another parallel port PCI card installed. Do you think it is worthwhile trying to use this other parallel port, or maybe removing the card in case it is having any effect on the output pulse rate?
I very much doubt this will make any difference.
I do have some doubt about the BOB as I still can't get 2 of the output pins to work on any of the stepper drivers but they will work if connected to the other available pins. My next thought is to replace the breakout board with a different type. Do you have any suggestions?
You may or may not have a couple of pins out on either your parallel card or your BOB but if all four of your axis are working on other pins I very much doubt this is the cause of your speed problem.
I appreciate this might not be the best matched system, but the attached You tube URL has the identical components on a friends machine and it can be seen how fast it operates. There is no way these steppers are running this fast.
Well if you say it's identical fair enough - but are you absolutely sure?
Exactly the same power supply, drivers, motors, AND GEARING? EXACTLY? The reason I'm checking is if you look at this link (http://www.wantmotor.com/ProductsListB.asp?id=88&Pid=75 (http://www.wantmotor.com/ProductsListB.asp?id=88&Pid=75)) the systems APPEAR to be similar to yours but if so, they seem to "randomly" include different power supplies. For example the THREE axis kit has a 24V 350W ps but the FOUR axis system has TWO 36V supplies. It's not clear to me if that's TWO 350W or TWO in parallel give 350W.
IF for example you had TWO 24V 350W supplies you could series them and get 48V at 14.5A which would double the stall speed of your system. (incidentally this is why I say random supplies - 14.5A is waaaaaaaay more than those 4 motors need, so with that supply, the voltage is way too low and the amperage capabilites way higher than neccessary)
Ian
EDIT: didn't see Tweakies post when I posted but FWIW with a supply of 14.5A (if that is indeed what you have) I doubt very much you'll get a voltage drop. Just a thought.
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Thanks guys,
I am tied up with work at the moment. Shall do these tests within 24 hours and report.
Stuart
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Hi Tweakie and Ian,
Well finally some good news.
Tweakie, before actually reading your previous post, I decided to strip the whole thing down to do a rewire from the start and run it in a new profile of Mach 3 with all new configuration settings. So, after reading your post I did the rewire and ran the profile config as you suggested, and whoa, some success. Speed was up to a very respectable speed, however the "X" axis did not work when connected to the suspect P2 and P3 on the BOB. I changed this to P1 and P17 and found it worked fine. I am pretty confident this is a BOB fault. I have a multimeter with a frequency counter built in and I was getting a pulse rate of 6.39 khz at the pulse terminal on the BOB for each axis except the P2 & P3 pins.
I calculated a "steps per mm" figure of 320 (5mm ball screw at 1600 steps) and found the motor would stall at 1600 mm/min. Prior to the stall, the power supply was stable at 24.008 Volts. When stalled there was a minimal increase in voltage to 24.012 volts. The highest current draw with all 4 motors running (no load) was about 3.5 amps which is well below the rated value for the power supply. I dropped the speed down by 25% to 1200 mm/min and they all ran very smoothly.
The frustrating point now is, what was wrong and what fixed it. Hopefully time will tell.
Anyway, thank you both for your help here. We are a little isolated down here to seek advice. I appreciate your time.
I have attached a couple of pics of the configuration I have put together with the power supply, Drivers and cooling duct. Please share any opinion or advice if you think I should be doing anything different.
Cheers
Stuart
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Hi Stuart,
I am pleased it is all starting to work out for you. ;)
Tweakie.
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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
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Hi Ian,
I am fairly confident that my port outage is related to the BOB as I use the same parallel lead from the computer to another combined BOB/ Driver for 3 axis and I am pretty sure this board uses those pins from the computer and it works fine. I am away again with work this weekend but shall be home in the morning and shall confirm then.
I have to admit, the 3.5 amp current rating was taken from a fairly crude analog ammeter fitted internally to another bench top power supply I was trying at the time whilst somebody else manually jogged the 4 motors. I have never really considered how accurate this meter is. I can easily connect my digital multimeter in line with the 4 drivers tomorrow and post an accurate reading. The drivers are set to 3.32 amps from memory.
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.
I have been trying to work out what may have been the cause of my problems as well, and you have just had me thinking about a change I made during the rewire with one of your comments. You will notice the power supply has 3 sets of 24 Volt connections. The way I had it connected before was like this; one set of terminals to 2 drivers, another set to the other 2 drivers and the 3rd set connected to a small printed circuit board with 3 voltage regulators and a couple of filter caps on it. Two of these regulators provide a very stable dual supply of 5 volts to the BOB (The BOB has a couple of jumpers to remove from the board if running dual 5V supply) and the other is a 12 volt regulator to run the cooling fan located in the end of the duct. When I did the rewire I isolated the circuit board with the regulators and provided the 5V to the BOB with a separate single plug pack totally isolated from the PS, and replaced the jumpers. 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). The other test I will make tomorrow is to see if the three 24 V positive and negative terminals are actually connected internally in the PS. (testing for continuity between terminals with an ohm meter) If they are not, it may be possible to series connect 2 of these to get the preferred 48 volts. Will let you know what I find.
Thanks for your feedback Ian, much appreciated.
Stuart
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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
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needing a cap fitted across the terminals of the PS
The cap provides filtering in an unregulated power supply and additionaly acts as holding tank of energy ( sort of like a water tower which feeds the town with water) thus even with a changing demand it has volume to provide constant voltage. A regulated supply as a unit does all this with circuitry to convert ac to dc, filter,maintain the voltage at some current.
I am going to guess that even though a number of terminals are provided they come from a single end source of the ps's output .
Frankly i am not sure on using a cap on the output of the regulated ps since it's electronics already provide the function of the cap. It may actualy affect that ps. Usualy ps will provide a data sheet and additional instructions / information on their use ( which you have not provided ). So i will withhold comment. Relative to amperage, there is probably a max running amp and also a surge rating ( when it's first turned on) so when measuring you would see the current draw changing. Your ac circuit should address the surge ie; 1.5 to 2x of running
or max surge , but, irrelevant should satisfy local authority / code on it's use
I am sure the electrical guys in here can provide more insight,
FWIW,
RICH
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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.
Tests today result in the following:
Rich, you are quite correct. There is no way that PS is going to deliver much more than 24 volts. All negative terminals are common as are the positive terminals. The extra terminals just allow for extra terminal space to connect to.
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.
Ian, I understand your supply star connection. I will run 4 separate figure 8 cables from the PS terminals, with each one dedicated to each individual driver.
Today, I have tried a few other "step per mm" settings and found it to operate quite successfully throughout the range, so hopefully all should be OK now.
Stuart
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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.
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Just to clarify, the surge i mention was on the upstream side of the ps and not on the downstream side ( ie; feed to the stepper).
Like a motor, when running the circuit may be fine, but on motor start-up the surge will be much greater and thus you need a higher rated circuit.
RICH
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Thanks Rich,
I can measure that, but I think a few of my other workshop toys give the power meter a bit of a tickle in the ribs more so than this at the moment.
But I'll take your comment on board, thanks.
Stuart
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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
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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.
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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.
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My advice would be not to add any additional capacitor at all. ;)
I hadn't really thought about this until I tried it but as far a bipolar stepper motors are concerned rotating an un-energised motor by hand has some magnetic resistance - drive the stepper beyond it's stall point and it is perfectly free to rotate by hand in either direction without any of the previously felt magnetic resistance.
Tweakie.
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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
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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
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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
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Hi Stuart,
you have the same China maid BoB as i have, and the same problem too. I had to replace both 74hc14 chips and it works perfect after that. Recently i purchased 2 more BoB from same the vendor at Ebay....... both got the same problem as my first BoB.
Regards
Stein
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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.
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Hi Stein,
When you say you had the same problems as I have had, are you referring to the port being U/S or the slow stepper motor issue? (Or both)
Stuart
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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 (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 ;D).
Ian
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Stuart,
yea, the problem with port 2 not working, but it looks like the smd-chips have been through a very hot production process and many strange things can happens.
Stein
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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 ;D).
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.
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Thanks for this Sargon - definitely sounds the way to go for Stuart to really find out what's going on.
Ian
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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.
Never seen a current meter work as you describe. A shunt is nothing but a VERY small resistance, on the order of milli-ohms, typically provided by a simple strap of brass or copper, necked down at one point, and trimmed at the neck to provide the required accurate, small resistance. ALL of the current flows through the shunt, and its resistance creates a small voltage, which is displayed on a moving-coil meter movement with, typically, 50mV full-scale sensitivity. A DVM works in exactly the same way, except measures the voltage with an A/D converter, rather than an analog meter movement.
Regards,
Ray L.
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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
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. At low pulsewidths, and/or low loads, you'll never reach the limit, because the current is not turned on long enough to saturate the coil, due to the coil inductance . The whole idea is to vary RMS voltage, allowing the current to do what it will, provided it does not exceed the set limit.
Regards,
Ray L.
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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. At low pulsewidths, and/or low loads, you'll never reach the limit, because the current is not turned on long enough to saturate the coil, due to the coil inductance . The whole idea is to vary RMS voltage, allowing the current to do what it will, provided it does not exceed the set limit.
Ray - I'm not sure where your reply contradicts anything I said. If you check out the attached pic I think I described it adequately for the purposes of the thread. However if we're being Mr. Picky... ;D
Current limiting will prematurely turn off the output current
Actually current limiting turns off the output VOLTAGE. current decays as a result.
because the current is not turned on long enough to saturate the coil, due to the coil inductance
As above, it's because the VOLTAGE is not turned on long enough to allow the current to rise to saturation because of the effects of inductance. We use a larger than rated VOLTAGE to "combat" inductance by reducing the rise time of the current and hence why we need current LIMITING in the first place.
But enough of this merry banter... now to the bit in your reply which is really pertinent to the point we were discussing... (please read post #33)
What I was asking was... please explain how the LOAD on the stepper motor affects the coil current? and in particular because you've said it here... please explain how a LOW load would result in a lower coil current. You may well be correct - as I said earlier in the thread I've had this stated to me before but no one has ever explained (or been able to explain) why. Personally I don't get it - and that's why I'd LOVE to have someone explain this.
Cheers
Ian
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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.
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Never seen a current meter work as you describe. A shunt is nothing but a VERY small resistance, on the order of milli-ohms, typically provided by a simple strap of brass or copper, necked down at one point, and trimmed at the neck to provide the required accurate, small resistance. ALL of the current flows through the shunt, and its resistance creates a small voltage, which is displayed on a moving-coil meter movement with, typically, 50mV full-scale sensitivity. A DVM works in exactly the same way, except measures the voltage with an A/D converter, rather than an analog meter movement.
Sorry, I was actually thinking about an alternative measuring method when describing the shunt configuration and it wasn't complete by any stretch of the imagination - you are right about a typical ammeter having the shunt in series with ALL of the current. That being said the main point I was focusing on is that a DC ammeter samples DC voltage to infer the current flow, and cannot provide an accurate reading for this type of power signal.
If you want to find out how much current is being delivered to your motors via chopper circuitry (ie most stepper drivers), the DC Clamp Meter is the way to go - and the only reasonable option that I'm aware of - because it makes use of the Hall effect which responds to DC, AC and pulsed DC, as well as complex combinations. That's the point I was trying to make.
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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. At low pulsewidths, and/or low loads, you'll never reach the limit, because the current is not turned on long enough to saturate the coil, due to the coil inductance . The whole idea is to vary RMS voltage, allowing the current to do what it will, provided it does not exceed the set limit.
Ray - I'm not sure where your reply contradicts anything I said. If you check out the attached pic I think I described it adequately for the purposes of the thread. However if we're being Mr. Picky... ;D
Current limiting will prematurely turn off the output current
Actually current limiting turns off the output VOLTAGE. current decays as a result.
because the current is not turned on long enough to saturate the coil, due to the coil inductance
As above, it's because the VOLTAGE is not turned on long enough to allow the current to rise to saturation because of the effects of inductance. We use a larger than rated VOLTAGE to "combat" inductance by reducing the rise time of the current and hence why we need current LIMITING in the first place.
But enough of this merry banter... now to the bit in your reply which is really pertinent to the point we were discussing... (please read post #33)
What I was asking was... please explain how the LOAD on the stepper motor affects the coil current? and in particular because you've said it here... please explain how a LOW load would result in a lower coil current. You may well be correct - as I said earlier in the thread I've had this stated to me before but no one has ever explained (or been able to explain) why. Personally I don't get it - and that's why I'd LOVE to have someone explain this.
Cheers
Ian
Your original post said: "ONLY when the current through the motor reaches the set point will the chopper start to chop." This is not necessarily correct, depending on exactly how the PWM is implemented. The waveform you provided is correct for some PWM power supplies, but NOT necessarily for a PWM stepper motor drive running at a low duty cycle. As I said, the H-bridge will be switched on and off at a fixed rate, based on the commanded duty cycle that will vary from near zero, to near 100% (again, sinusoidally, if micro-stepping is used). Each pulse can be shortened if, and ONLY if, the corresponding coil reaches the limit current. This will likely NOT happen at low duty cycles, as the pulse is not long enough for the coil to saturate. At some greater duty cycle, limiting will start of occur, but that may not be until the duty cycle approaches 100%, heavily dependant upon the motor inductance, and voltage used. The whole point of the chopper, as I said, is to modulate the RMS voltage across the coils, which implies a corresponding modulation of the RMS current. The actual current is not directly controlled (beyond simple peak limiting). If it operated as you suggest, the ripple in the coil current would be running essentially full-scale all the time, which would do some really serious heating in the motor. The PWM is there to allow the current to be modulated, in part to reduce ripple, by lowering the peak currents.
For a stepper, I don't believe motor load does affect current, unless there is some small effect related to back-EMF/slip angle, but I'm not sure about that. It's been a loooooong time since I studied the details of the operation of motors...
Regards,
Ray L.
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Ray - if only I'd said "ONLY when the current through the motor reaches the set point will the
chopper start to chop. drive start to limit the current" I really must be more careful. But in the context of the thread surely to goodness it's clear what I meant. It was never about how a chopper works. (have you read the thread?)
The discussion had reached a point where we were discussing whether LOAD affects coil current. I stated... well I guess you'll read it if you're interested.
But then Ray YOU actually re-stated the EXACT point of where we'd got to when you said...
At low pulsewidths, and/or low loads, you'll never reach the limit
I thought EUREKA! - Ray knows the answer we've been searching for - we're about to get somewhere. BUT you then said in your last post...
For a stepper, I don't believe motor load does affect current, unless there is some small effect related to back-EMF/slip angle, but I'm not sure about that. It's been a loooooong time since I studied the details of the operation of motors...
So it appears after all this we've made zilch progress - marvelous....
Cheers
Ian
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Gents,
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??
Stuart
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Hope I haven't started a forum war here
Let not your heart be troubled....
Just a friendly discussion looking for understanding......
RICH
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Gents,
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??
Stuart
Stuart,
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.
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Sargon,
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.
Stuart
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Hi,
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?
Thanks
Martyondrums
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ola tambem estou com problemas com dq542ma perde passo se por mais que 500
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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
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Hi,
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.
Craig
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Hi Craig,
Thank you for your replay.
sajeewa
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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.
Cheers
Roger