[cncvirgin]

hey, thx so much for the detailed explanations .. and with all this talk about probing i finally understood the deeper meaning of your forum alias

so .. sorry to be so thick about this ... my knowledge in electronics tends towards total ignorance, and before attempting any type of electric connections i'm always afraid to electrocute myself so i prefer to ask again to make sure i completely understood ... btw, can i electrocute myself with the current coming out of the motor cables ? can i break the stepper motor or driver PCB if i hook sth up wrong ?

so to take things from the top, this is my pinout:

Attachment 246908

i decided to hook up the new motor in series since it draws the least amount of power .. i really have no idea how much power the machine can supply but in series the motor draws 2A/phase (Unipolar:2.8;Parallel:4.0) and the resistance with this wiring is closest to what it says on the sticker of the old motor: 2Ohms (Unipolar:1;Parallel:0.5) .. but as you know i don't have a clue so this was just a guess . i don't mind losing torque at high speed with this connection cause i'm not running super high feeds and am mostly cutting very soft materials.

if i understood your instructions correctly then this would be the wiring for the headphone probe:

Attachment 246910

is this correct ? this would be connecting the headphones without the motor .. how would i connect it with the motors ?

Oh, the 2K resistor value limits the power to the headphone element to around 25mW. The power dissipated by the resistor, however, could be on the order of 1.5W if it is connected to a drive with no motor loading the output. A 1/4 Watt resistor should be fine with a motor connected but you need higher Wattage and/or resistance it you want to listen to the unloaded driver output.

so 2kOhms is not the right resistor ? again my lack of knowledge in these matters is striking: 1/4 Watt resistor ??? Aren't resistors measured in Ohms ?

[probinson]

It will take me a while to write up a clear explanation and I'm in the middle of a project at the moment so I'll get back to you a bit later. Pete

[mactec54]

I doubt it's the cable since it's really well attached and there is no movement of the cable at all when the machine is running since the stepper transmits through a belt.

Your cable I know is what the manufacture used, it is neither Twisted Pair or Shielded, which still could be the problem, so this does point to either the cable or your drive, try this Z axes with a different drive X or Y Drive, if it does the same thing then it is the cable, get a shielded cable & Ground the Shield at the input end

[cncvirgin]

Your cable I know is what the manufacture used, it is neither Twisted Pair or Shielded, which still could be the problem, so this does point to either the cable or your drive, try this Z axes with a different drive X or Y Drive, if it does the same thing then it is the cable, get a shielded cable & Ground the Shield at the input end

thx for this advice, now that my connections are on clip connectors this will be easy to test.

the way i understand it, since the problem remained after switching the cables to different drivers, it is either a software bug or all 3 drivers have the same problem .. or a problem with the power (or power supply) .. but you're right, it might just be the cable ...

should i get a cable like this:

Attachment 247006

with all 4 cables in one, or do i need a single shielded cable for each of the 4 motor/driver connections ?
when you say 'input side' do you mean the side with the motor or on the driver/pcb side ?

[probinson]

The stepper motors and driver outputs are not affected by electrical noise.The object of the shield is to prevent the very noisy motor signals from radiating and disrupting the more sensitive signal paths. A shielded cable for each motor with a shield around all the wires of that motor is all you need.

One thing to note is that you should only connect the shield at one end. Pick a good place to connect all your shield wires to ground at the driver end and don't connect the shield at the motor end. If you connect the shield at both ends, you form what is known as ground loops and your shield can generate noise, sometimes even more noise than it is blocking.

[mactec54]

thx for this advice, now that my connections are on clip connectors this will be easy to test.

the way i understand it, since the problem remained after switching the cables to different drivers, it is either a software bug or all 3 drivers have the same problem .. or a problem with the power (or power supply) .. but you're right, it might just be the cable ...

should i get a cable like this:

Attachment 247006

with all 4 cables in one, or do i need a single shielded cable for each of the 4 motor/driver connections ?
when you say 'input side' do you mean the side with the motor or on the driver/pcb side ?

You should use a separate cable for each motor, of the same wire size, that was used by the manufacture, just add shield

Connect the shield at the PCB side to Ground which can be direct to the control box, the shorter this shield wire connection to ground the better, don't add a wire to the shield, you can use also a small copper saddle as well, just over the shield, just remove the insulation around the wire, clamp the saddle tight over the shield, check to see that the power control Box has a good Ground also

[jeffserv]

Yes you can use a Cable with wires... like that...

Single - Wires are inside of cables... so call wires.... "wires", and call the group of them in a cord... "the cable".

Confusing the way things have been asked and replied to above one post.

[probinson]

Sorry to keep you waiting. Here is the probe wiring.
Attachment 247038

You leave the motors hooked up while you run the program that causes the missed steps. Insert the probes into the same socket that connects the motor to the PCB. You want the speaker to be connected to the same 2 wires that drive the coil. This allows you to listen to the steps that are being sent to the motor. When the motor skips steps you can hear whether or not the driver stopped sending the pulses.

O.K. I'm not sure how much information you wanted but for the rest of the questions it's helpful if you have an understanding of electricity.So let's start with basics: Volts, Amperes, Ohms, Watts, and Ohm's Law.

Voltage is a measure of how hard electrons are being pushed. Units - Volts Symbol - E The symbol used is E rather than V (which makes sense when you know it stands for Electro-motive force).
Current is a measure of how many electrons actually make it through the circuit per second. Units - Amperes Symbol - I (capital i) because it was originally referred to as electrical intensity rather than current.
Resistance is a measure of the how hard the circuit resists the flow of electrons. Units - Ohms Symbol - R.
Power is a measure of energy consumed by the circuit. In most instances you are concerned with how this translates to heat. Units - Watts Symbol - P.

Ohm's Law describes the mathematical relationship between voltage, current, resistance, and power.

The equations for Ohm's Law can be written in many ways, depending on what you are trying to solve for:

Voltage	E = I x R	E = P / I	E = SQR(P x R)
Current	I = E / R	I = P / E	I = SQR(P / R)
Resistance	R = E / I	R = E² / P	R = P / I²
Power	P = E x I	P = E² / R	P = I² x R

OK Now on to your questions.

so 2kOhms is not the right resistor ? again my lack of knowledge in these matters is striking: 1/4 Watt resistor ??? Aren't resistors measured in Ohms ?

Attachment 247042
Both of these resistors are 3K Ohms but the top one can handle 100 Watts of power and the bottom one can only handle 0.125 Watts. The power rating is basically how much heat it can dissipate, at room temperature, without deviating from it's rated accuracy or being damaged.

Here is where those equations come in. The small resistor is 3K Ohm at 0.125W. Plugging those vales into the equation E = SQR(P x R) we find that it can only handle 19.4V while the 100W version can handle 548V. Now remember that this is all based on how much heat it can handle. That little resistor can take short pulses of more than 100V as long as the average power doesn't exceed it's ability to get rid of 0.125W worth of heat.

So now for the tricky part. The power supply for the stepper motor driver is probably somewhere between 28 and 50V. The 3K 1/8W resistor can handle 19.4V. The next higher power ratings commonly available are 1/4W then 1/2W. You might think that 1/2W is 4 times more power so it will handle 77.6V, right? Nope. As the voltage increases the resistor draws more current so power consumption goes up as a squared function. Using P = E² / R you find out that 3K resistor dissipates 0.84W at 50V. If we plug in the 2K Ohms I recommended for the headphone probe, rather than the resistors I had for the picture, we come up with 1.25W for 2K at 50V.

Now, if your stepper motor coils measure around 20 Ohms then your driver probably doesn't do PWM current limiting and you need to keep the coil resistance of any new motor in the same range as the original. Many of today's stepper motors are less than 2 Ohms. If you put 50V on a 2 Ohm coil it will draw 25A and dissipate 1250W. Obviously, if your driver doesn't control the output current, something will not last too long. Newer drivers can get better performance by using those low resistance coils and a high source voltage to force the coil to charge up quickly but then control the current so that the motor never draws more than a certain amount of power.


As for electrical safety, anything less than 100VDC or 60VAC is safe. (We are talking line frequency AC not radio frequencies.) You don't even start to feel voltages with your hands until around 40VDC, 20VAC and most people need to have wet hands to feel that. As the voltage gets up in 100V range it starts being able to force enough current to cause muscles to contract and that's when it starts to be dangerous. Muscle contraction can cause you to grab on and not be able to let go. Touching a 120VAC circuit can scare the crap of of you but it's not lethal. Getting stuck on a 120VAC circuit and not being able to let go, however, will eventually kill you.

Now it's not a simple as saying "I have a 12VDC supply so there is no risk of getting electrocuted." That 12VDC came from somewhere. You need to test all your supplies measured to ground to make sure there is no line voltage present. Before you touch anything you need to test for line voltage both with the machine turned on and when the machine is turned off.

Pete

[cncvirgin]

Both of these resistors are 3K Ohms but the top one can handle 100 Watts of power and the bottom one can only handle 0.125 Watts. The power rating is basically how much heat it can dissipate, at room temperature, without deviating from it's rated accuracy or being damaged.

Here is where those equations come in. The small resistor is 3K Ohm at 0.125W. Plugging those vales into the equation E = SQR(P x R) we find that it can only handle 19.4V while the 100W version can handle 548V. Now remember that this is all based on how much heat it can handle. That little resistor can take short pulses of more than 100V as long as the average power doesn't exceed it's ability to get rid of 0.125W worth of heat.

So now for the tricky part. The power supply for the stepper motor driver is probably somewhere between 28 and 50V. The 3K 1/8W resistor can handle 19.4V. The next higher power ratings commonly available are 1/4W then 1/2W. You might think that 1/2W is 4 times more power so it will handle 77.6V, right? Nope. As the voltage increases the resistor draws more current so power consumption goes up as a squared function. Using P = E² / R you find out that 3K resistor dissipates 0.84W at 50V. If we plug in the 2K Ohms I recommended for the headphone probe, rather than the resistors I had for the picture, we come up with 1.25W for 2K at 50V.

Now, if your stepper motor coils measure around 20 Ohms then your driver probably doesn't do PWM current limiting and you need to keep the coil resistance of any new motor in the same range as the original. Many of today's stepper motors are less than 2 Ohms. If you put 50V on a 2 Ohm coil it will draw 25A and dissipate 1250W. Obviously, if your driver doesn't control the output current, something will not last too long. Newer drivers can get better performance by using those low resistance coils and a high source voltage to force the coil to charge up quickly but then control the current so that the motor never draws more than a certain amount of power.

hey Pete, thank you so much for taking the time to educate me !!!
all this theory got me a little confused now ... i'm still struggling to understand what kind of resistor i should use.

if i understood correctly, with no motors connected i should use a 2kOhms/1.5W resistor and with the motors connected a 2kOhms/0.25W ?

and im connecting the plugs to only one coil of the motor .. ie in my case the Green/Yellow pair or the Red/Blue pair. i suppose it doesn't matter which one of these coils i connect to ?

[probinson]

The Wattage rating of the resistor is maximum power it can handle reliably. The circuit determines the minimum requirement. You can use a resistor with a higher rating than needed but you can't go the other way and use a lower rating than the circuit requires.

I don't know what voltage your drivers run at. I am just guessing that they are under 50V but 75V and 90V are also commonly used voltages. I had also assumed the driver would use controlled current to drive the motor but you said the motor coils are about 20 Ohms so that may not be the case. Anyway, Controlled current drivers cut back the output when needed so the motor draws no more than the preset current. This would mean that the driver output would be at full supply voltage without the motor and would be much lower with a motor connected. That's the reason I estimated different power requirements with and without the motor connected. Again, if your motors are 20 Ohms then attaching the motor will probably not make the voltage go down as I had expected.

So, it boils down to:
1) A 2K Ohm resistor should be enough resistance to protect the headphone element from a high voltage driver and still provide enough volume if it's a low voltage driver.
2) You should use a resistor with a power rating at least as high as the worst case.
3) Now that you know how to calculate the power requirements, you can measure the voltages and use real values instead of me trying to guess what the voltages might be.

When you are working with components you have on hand, you don't always have the option to go bigger. If all you have a 1/4W resistor (that's the most common) and it turns out to be insufficient then it will get hot and burn out. No big deal. You use a bigger one next time or you use a bunch of resistors in series and/or parallel combinations to build you own higher power resistor.

Pete

[jeffserv]

i suppose it doesn't matter which one of these coils i connect to ?

Of course you would test the one side first, then the other. All in all, this is about trying to discover, if pulses are funny, or not making it to the other end of the cable. I use an ocilliscope for this... but what is being offered here... is a way to listen to the signal.

Also a simple ohm test of each wire, in the cable, can reveal problems. You use a multimeter for that.

If I suspect a cable having wire problems, that is the first test of that cable I would do. Most the time, 5 minutes into the test, I discover; (look wire ?) has super high resistance, and I move the wire a little, in a suspect area, like a bend in the wire... or place that shows some past damage... and there it is... a stranded wire, with only one of its 30 some strands, connected. Or completely broken, but touching. Just enough to get a signal, but no power... very weak signal due to a "broken wire". Or nearly broken wire. Replace, and go back to work.

It will be interesting what you discover...

[cncvirgin]

ok cool .. got it ... the motors BTW have 2 Ohms resistance not 20

so i went shopping in the local electronics store and lucky as i am they didn't really have what i needed. They only had 5 wire cables without shield but the guy sold me a braided metal shield tube to put around the whole cable and said this would have the same effect. the cable they had also has all black wires with numbers printed on it .. i would have preferred colors since my eyesight is not too good and it's hard to read. and of course they did not have a connector that fits the PCB so i had cobble something together with crimps and heatshrink tube.

Here is the contraption i made out of the cable they sold me:

Attachment 247146

the problem i'm now facing is where to connect the ground .. since the PCB with power and drivers sits on this drawer/tray i don't have a ground connection nearby (i think).

Should i connect the braided metal shield to one of the connectors ground wire somehow:



Or would clamping it to the metal of the PCB-tray be enough. This is how the machine looks from the underside, it looks like it is grounded:

Attachment 247144

thx for your continued patience with me

[probinson]

You do have the ground labeled correctly. You want to tie the shields from all 3 cables to one ground screw, preferably at the same point where any other shielded cables leave the chassis.

You can leave the green wire disconnected for now or connect it to the same ground as the shield. I would connect it to the shield ground an use it later to ground the motor's case through a 47,000 pF 250VAC capacitor. This helps reduce electrical noise generated by the motor without producing ground loops.

Pete

[cncvirgin]

You want to tie the shields from all 3 cables to one ground screw, preferably at the same point where any other shielded cables leave the chassis.
Pete

to be honest i wanted only to connect the Z-Axis this way since it's extremely tedious to get to the other motors and i only have missed steps on the Z-Axis. For all these tests i had disabled the XY axes in Mach3.

Also, since it's difficult to get to the bottom of the machine, can i just clamp the shield to the metal of the tray since the bottom ground seems to be connected to the tray holder ?

[probinson]

You do have the ground labeled correctly. You want to tie the shields from all 3 cables to one ground screw, preferably at the same point where any other shielded cables leave the chassis.

You can leave the green wire disconnected for now or connect it to the same ground as the shield. I would connect it to the shield ground an use it later to ground the motor's case through a 47,000 pF 250VAC capacitor. This helps reduce electrical noise generated by the motor without producing ground loops.

Pete

[probinson]

I would run this as a separate cable bypassing the one on the machine and see if it fixes the problem. I wouldn't rewire the machine until you find out if the is a wiring problem.

P.S.

Let me point out that the shield is not to protect the Z motor lines from interference. They are high power signals and are not affected by electrical noise. The shield to to prevent the high power motor signal from interfering with other electronics. If you only shield one motor line you will still have 2 others radiating electrical noise.

Do one for the test but if noise is the problem you will need to do them all.

Pete

[probinson]

OK Let's recap:
1) You have a problem with the Z retract in one place in a specific program.
2) You tried a few things with the software, including loading an older copy that used to work to check for accidental code errors, but the problem persists.
3) You try some suggested fixes for common problems and that didn't help.
4) It's now time to pin down whether it is a hardware or software issue so you don't keep poking around blindly.
a) A good place to do that it at the driver outputs. If the drive signals are good the problem is somewhere in the wiring or the mechanics. If the drive signals are corrupt then the problem is somewhere from the driver output toward the actual code.
b) The appropriate piece of test equipment is an oscilloscope but if you don't have an oscilloscope you can use headphones to perform the task.

This was intended as a quick way to perform the test using things you have at hand but has morphed into hijacking the tread and I apologize for that.

So, let's confirm the symptoms and get back to actually finding the problem.

Are these true statements?
The machine runs other programs just fine.
It is only this program and only this one retract that exhibits missed steps.

[cncvirgin]

OK Let's recap:
1) You have a problem with the Z retract in one place in a specific program.
2) You tried a few things with the software, including loading an older copy that used to work to check for accidental code errors, but the problem persists.
3) You try some suggested fixes for common problems and that didn't help.
4) It's now time to pin down whether it is a hardware or software issue so you don't keep poking around blindly.
a) A good place to do that it at the driver outputs. If the drive signals are good the problem is somewhere in the wiring or the mechanics. If the drive signals are corrupt then the problem is somewhere from the driver output toward the actual code.
b) The appropriate piece of test equipment is an oscilloscope but if you don't have an oscilloscope you can use headphones to perform the task.

This was intended as a quick way to perform the test using things you have at hand but has morphed into hijacking the tread and I apologize for that.

So, let's confirm the symptoms and get back to actually finding the problem.

Are these true statements?
The machine runs other programs just fine.
It is only this program and only this one retract that exhibits missed steps.

1) yes correct except that the problem is with the z-retract of 'one type of toolpath', the parallel finishing of several objects, it's on the retract before the spindle is moving to the next object that the missing steps happen
2) yes correct - tried everything in software, including different velocity/acceleration settings, different kernel speeds, sherline mode, and whatever else i could find or that has been suggested

The machine runs 2.5D programms just fine, no missed steps there. The problematic toolpaths are the ones where there is a lot of continuous Z-Axis motion (ie Parallel finishing), but the lost steps occur only on the retract move at the end of one object.

I would run this as a separate cable bypassing the one on the machine and see if it fixes the problem. I wouldn't rewire the machine until you find out if the is a wiring problem.

yes this is what i'm doing, it's a separate cable, the original ones are still in place. It's just about the ground i'm confused, a friend told me i could just clamp the shield to any part of the machines fram as long as the machine is grounded. The ground on the bottom is connected to the metal tray holder .. so can i just clamp the braided metal shieeld of my new cable to the PCB tray to ground it .. or should i run a wire from th ground connector on the bottom of the machine and connect it to the braided metal shield ?

[probinson]

Grounding first so you can keep working:

The shielding is most effective if all signals exit the driver's metal case at one place all of your shielding is grounded at the same point. You don't need the absolute best possible shielding for this test. You just need a significant improvement to see if it makes a difference so, for the shielding test, it's not critical where you get ground but it should be grounded to the metal case of the driver not the frame of the machine.

[probinson]

Your machine misses steps on the retract when it finishes a really big block of code just before moving to a new part.

What this implies is that the computer is accessing either disk or memory in preparation for the next part and that is tying up CPU time. The CPU is not getting control back in time to service the motor control thread and steps are being lost.

If this assumption is correct, then inserting a dwell after the retract should make the program wait for the machine to stop before loading the code for the next part.

Here is what I think is happening:

You run a really big routine that eats up lots of memory.
The routine completes and sends a retract command.
The entire program didn't fit in memory so the interpreter starts loading more code for the next part.
The access to disk or swapping of memory pages doesn't give back control often enough for the motor control thread to keep track.


If you put a dwell before the retract, the program stops for a little while, starts a retract, and then begins loading while the retract is running.

If you put a dwell after the retract, the program performs the retract, stops for a little while, and then begins loading while the machine is stopped so no steps get lost.

Try that and see if it works.

Pete