stepper motor problems with plasma cutting

newplumber

newplumber

Senior Member
Hi I tried to post a thread earlier but new to posting my problem is when I built my cnc plasma table using 5 stepping motors and 5 step drivers I think the drivers are L293d's with my 20m2 chip it works absolutly great till I run the plasma cutter then everything stops. I tried using many caps to pin 1 an pin 20 like 100un etc I am using a 400 amp power supply from a computer at 5 volts so it don't lose power I soldered a board and put inside a metal box for the 20m2 and drivers and its grounded and I used cat 5 twisted pairs for the wiring I even made another metal box to go over the first metal box with the power supply and grounded that box too, still shuts off my chip. I know plasma is very noisy but sorry about no schematic Im working on one but my question is since I think somehow the l293d are making the 20m2 freak out which would be coming from the stepping motors so thought anyone would have any hints to fix it. I hope to get a scope a soon. My wires from the l293d to the stepping motors are just twisted pair cat 5 same with the wires to the 20m2 to the l293d. thanks anyone that has a thought
 
JimPerry

JimPerry

Senior Member
Almost certainly a spike/surge when the plasma fires - have you tried powering the Picaxe from a separate set of 3 AA cells - and possibly using opto-isolators :confused:
 
R

Reloadron

Senior Member
How are you starting your arc? While this is not my forte I have been involved in it a little. Typically two methods are used to start the arc, either RF (Radio Frequency) or a touch method (Blow Back). With an RF start a burst of RF is imposed on the DC at the torch head to initiate the arc. The torch head uses AVC (Arc Voltage Control) to maintain the arc gap so you start with the torch head off the work piece. With a touch start the torch head comes down and touches the workpiece and backs off the work piece initiating the arc, sometimes called a dead short start or similar. When running are you monitoring your arc voltage and current? If you are monitoring the arc voltage there is a RF Choke needed. I have seen the issues you describe in large tig welding systems if a choke is omitted or the choke windings are shorted. Systems that use an RF start can have the RF wreak havoc on the electronics if the RF is not eliminated and gets a back path to the system.

From your description I am guess you are using a touch start method? I believe to eliminate RF problems that touch or blow back is the preferred method. This method as JimPerry points out can result in a large noise spike. Are you using any banks of large capacitors across the cutting power supply output?

Ron

Ron
 
S

SAborn

Senior Member
One thing i would try is placing a 330R resistor in the lines between picaxe output pins and the L293D input pins.
I use resistors in this type of circuit as a standard practice, simply because steppers can cause lots of EMF problems, or EMF can come from other sources like your plasma cutter, well worth trying i think for very little cost.
 
newplumber

newplumber

Senior Member
Sorry not getting back earlier I work out of town but thanks for your input ... I don't now what kind of arc starts the plasma but I don't even have it close to the table and it freaks out the board i am going to try saborns thought with 330 ohms. reloadron I tried a lot of caps to keep the power up but my power supply never drops since I am using a good power supply I will make a schematic but Im a plumber not a smart electronic person and I love working with the picaxe 20m2 btw the plasma machine is a cheap china made 50 amp but hopefully in the next hour or so i can get u a print so u can see what i have
 
R

Reloadron

Senior Member
newplumber said:
Sorry not getting back earlier I work out of town but thanks for your input ... I don't now what kind of arc starts the plasma but I don't even have it close to the table and it freaks out the board i am going to try saborns thought with 330 ohms. reloadron I tried a lot of caps to keep the power up but my power supply never drops since I am using a good power supply I will make a schematic but Im a plumber not a smart electronic person and I love working with the picaxe 20m2 btw the plasma machine is a cheap china made 50 amp but hopefully in the next hour or so i can get u a print so u can see what i have
Click to expand...
OK and something to keep in mind is that while shielding is a good thing, frequently the noise and problems follow along existing wire paths.

Ron
 
newplumber

newplumber

Senior Member
thanks ron I have double shielded cat 5 wire ordered and was going to try that plus resistors on wires from pic to l293d driver here is a schematic of how I wired it View attachment 17522
 
Last edited:
newplumber

newplumber

Senior Member
Okay I finally had time to rewire the whole table with double wall shielded wire and grounded everything and still kills the chip when the plasma is fired ..I even ran the chip power on different power supply and put 330 ohm resis between each wire from chip to driver and even tried running chip with a 9 volt battery with a 5v regulator and 1uf cap everything I do still just shuts off the chip Jim perry must be right about the surge spike when it fires and how do I kill that interference well thank you all for giving me ideas and would love to hear more, if I am thinking right then I have to stop the surge spike from getting to the chip which I don't have a clue but will keep working on different ways again thanks for the help
mark
 
hippy

hippy

Technical Support
Staff member
Do you have a scope which will let you examine the power rails and signals at various places to see what is being affected and how ?

Otherwise it's just guessing what type of problem there is to solve and simply a case of throwing everything at it until it works, much of which has been done, but ferrite beads on the cables or individual signal wires may help.

Have you added SERTXD messages to show if the PICAXE is resetting or locking-up ? Is it the PICAXE which stops working or the stepper driver chip ?

One thing you could try is another PICAXE alongside the existing but not connected to the cutter; does that also reset or lock-up ?
 
G

Goeytex

Senior Member
Some Thoughts ...

When using sheilded wire, a common mistake is to ground both ends of the shield wire. The correct way is to ground only one end. Tying both end to ground can sometimes be worse than no shielded wire at all.

The 20 Ohm resistor from serin to ground is too low. This could cause excessive current in the AXE027. It should be no lower than about 1K, but most use 10K.

The serin current limit resistor is missing from the schematic. While not absoultely necessary when using the AXE027 cable and a 5V Picaxe supply, a 22K resistor is generally recommended, however anything from 1K to 22K will help prevent noise on this line from affecting the Picaxe. I would use a 22K.

The L293D datasheet says that the input current needs to be a minimum of 100 microamps. So to be safe a 500 microamp current will more than cover it. That means that with a 5V supply you could use 10K resistors from the Picaxe to the L293D inputs. 4K7 resistors will allow up about 1ma. I think 330 ohm is too low in this case. The higher the resistor value the less noise that will be fed back into the Picaxe. I would try 10K or 4K7.

Set all unused pins as outputs. If c.6 is not connected tie it to ground.

If it is HV noise on the power supply disabling (constantly resetting?) the Picaxe, then a 10 to 20 Ohm resistor can be placed in series with the 5v supply and the Picaxe +V pin. It should be placed close to the Picaxe. Along with this resistor tie a bidirectional transorb diode to ground. The transorb should be on the Picaxe side of the resistor.
I might suggest an ST BZW04-5V8B. This is a 5.8V TVS diode. A TVS diode could also be placed on the 5V input of each L293D to absorb HV Spikes.

As a test I would program the chip with something like the following.

Code: 
#picaxe 20M2
#terminal 4800

dirsC = 255  'all pins are outputs low
dirsB = 255  'all pins are outputs low  

sertxd  "Starting"


do
   pause 100
Loop
Then start the plasma cutter and see if the Picaxe continously resets. If it does, you will see "starting" repeated over and over. Then I would make changes adding dodes, caps, filters, shielding, etc until the resetting problem was solved.

I am not too excited about using the ATX supply for the Picaxe. I might consider taping off of the 12V and then use a "local" 5V LDO regulator near the Picaxe. A 15V TVS didode could then be placed on the input side of the regulator and a 5.6V TVS on the Picaxe side.
A 100uF capacitor might be a good idea on the 5V Regulator output and at least a 10uf cap on the input.

Good Luck
 
newplumber

newplumber

Senior Member
what do you mean opto-isolators? for the battery supply it was just running the picaxe 20m2 the drivers were using the pc power supply I tried 3 different supplys
 
newplumber

newplumber

Senior Member
hippy said:
Do you have a scope which will let you examine the power rails and signals at various places to see what is being affected and how ?

Otherwise it's just guessing what type of problem there is to solve and simply a case of throwing everything at it until it works, much of which has been done, but ferrite beads on the cables or individual signal wires may help.

Have you added SERTXD messages to show if the PICAXE is resetting or locking-up ? Is it the PICAXE which stops working or the stepper driver chip ?

One thing you could try is another PICAXE alongside the existing but not connected to the cutter; does that also reset or lock-up ?
Click to expand...
Hippy your too smart see I do things the hard way I try everything that an't wrong then decide to buy a scope but for reals I need a scope it's kinda like redoing all the plumbing pipes in a house when its just a toilet that's plugged infact everyone here is too smart someday I will get there but thanks
 
newplumber

newplumber

Senior Member
Goeytex said:
Some Thoughts ...

When using sheilded wire, a common mistake is to ground both ends of the shield wire. The correct way is to ground only one end. Tying both end to ground can sometimes be worse than no shielded wire at all.

The 20 Ohm resistor from serin to ground is too low. This could cause excessive current in the AXE027. It should be no lower than about 1K, but most use 10K.

The serin current limit resistor is missing from the schematic. While not absoultely necessary when using the AXE027 cable and a 5V Picaxe supply, a 22K resistor is generally recommended, however anything from 1K to 22K will help prevent noise on this line from affecting the Picaxe. I would use a 22K.

The L293D datasheet says that the input current needs to be a minimum of 100 microamps. So to be safe a 500 microamp current will more than cover it. That means that with a 5V supply you could use 10K resistors from the Picaxe to the L293D inputs. 4K7 resistors will allow up about 1ma. I think 330 ohm is too low in this case. The higher the resistor value the less noise that will be fed back into the Picaxe. I would try 10K or 4K7.

Set all unused pins as outputs. If c.6 is not connected tie it to ground.

If it is HV noise on the power supply disabling (constantly resetting?) the Picaxe, then a 10 to 20 Ohm resistor can be placed in series with the 5v supply and the Picaxe +V pin. It should be placed close to the Picaxe. Along with this resistor tie a bidirectional transorb diode to ground. The transorb should be on the Picaxe side of the resistor.
I might suggest an ST BZW04-5V8B. This is a 5.8V TVS diode. A TVS diode could also be placed on the 5V input of each L293D to absorb HV Spikes.

As a test I would program the chip with something like the following.

Code: 
#picaxe 20M2
#terminal 4800

dirsC = 255  'all pins are outputs low
dirsB = 255  'all pins are outputs low  

sertxd  "Starting"


do
   pause 100
Loop
Then start the plasma cutter and see if the Picaxe continously resets. If it does, you will see "starting" repeated over and over. Then I would make changes adding dodes, caps, filters, shielding, etc until the resetting problem was solved.

I am not too excited about using the ATX supply for the Picaxe. I might consider taping off of the 12V and then use a "local" 5V LDO regulator near the Picaxe. A 15V TVS didode could then be placed on the input side of the regulator and a 5.6V TVS on the Picaxe side.
A 100uF capacitor might be a good idea on the 5V Regulator output and at least a 10uf cap on the input.

Good Luck
Click to expand...
Okay thank you alot but my schematic is wrong because I invented it (naturally) but I do have a 20K ohm on serin it just don't say on the schematic but your awesome for finding out that 10k ohm will work between the pic an driver I will try that ... another thing is it works awesome without the plasma firing up so like hippy says in my personal words .. I should get a scope! and then I can see what is freaking out .... ohh by the way this hobby project thing is just passed its 100 hr of trial an error but im stubborn and if I do get this thing to work its cause of the kewl people on forms an maybe a scope
 
R

Reloadron

Senior Member
newplumber said:
Okay I finally had time to rewire the whole table with double wall shielded wire and grounded everything and still kills the chip when the plasma is fired ..I even ran the chip power on different power supply and put 330 ohm resis between each wire from chip to driver and even tried running chip with a 9 volt battery with a 5v regulator and 1uf cap everything I do still just shuts off the chip Jim perry must be right about the surge spike when it fires and how do I kill that interference well thank you all for giving me ideas and would love to hear more, if I am thinking right then I have to stop the surge spike from getting to the chip which I don't have a clue but will keep working on different ways again thanks for the help
mark
Click to expand...
Here is something you can think about. Years ago I reluctantly became involved with several types of large welding systems. In this maze was the AVC systems which controlled the torch head location, the torch head was motor driven by servo motors. The actual arc voltage (the voltage drop across the arc) was constantly monitored and recorded as well as other parameters. Many of these machines used an RF Start to initiate the arc as well as Touch Start. There was a voltage pick off right at the torch head and that signal was sent through a choke (inductor) to eliminate the HF RF as well as the arc noise once the arc was initiated. This is not the large inductors used in some machines to maintain a good constant arc current but a small inductor to choke the arc voltage being sampled. These things were common on all automated machines and without one the measurement plane was toast. Unfortunately I can't recall the value of the chokes that were used. Anyway, you may want to give this some thought. Ideally a good welding engineer should be familiar with what I am getting at.

Ron
 
newplumber

newplumber

Senior Member
okay I am going to buy a scope hopefully a good one... but so far tried the 10kohm resistors and it resets the chip and as long as I have the plasma trigger on the chip is off no response like no power to it... I tried another 20m2 chip right next to it and put a led to one of the pins to have flash without anything else connected and it stops the same way the driver chip does with no led flashing as long as I hold the plasma trigger down... so something tells me that the plasma arc through the air is screwing up something in the chip... If anyone knows of a good scope I could buy and not trade my house for it I would appreciate it thanks
 
newplumber

newplumber

Senior Member
Reloadron said:
Here is something you can think about. Years ago I reluctantly became involved with several types of large welding systems. In this maze was the AVC systems which controlled the torch head location, the torch head was motor driven by servo motors. The actual arc voltage (the voltage drop across the arc) was constantly monitored and recorded as well as other parameters. Many of these machines used an RF Start to initiate the arc as well as Touch Start. There was a voltage pick off right at the torch head and that signal was sent through a choke (inductor) to eliminate the HF RF as well as the arc noise once the arc was initiated. This is not the large inductors used in some machines to maintain a good constant arc current but a small inductor to choke the arc voltage being sampled. These things were common on all automated machines and without one the measurement plane was toast. Unfortunately I can't recall the value of the chokes that were used. Anyway, you may want to give this some thought. Ideally a good welding engineer should be familiar with what I am getting at.

Ron
Click to expand...
Thanks Ron I tried different chokes thinking it would help but I am not smart enough to know what size choke would help but again with a scope I would think I could see what changes are in different ones ... I sure am glad of all the reply's to help, just learning one step at a time.
 
newplumber

newplumber

Senior Member
Okay I wanted to quit this project throw in the towel ..buuuutt!!! im to stubborn anyway I still don't have it perfect but I bought a scope and it works and it shows huge spikes like every 5 nano seconds i belive when the plasma fires so I added caps made a DIY high freq filther and the caps brought the spikes down a bit like from 8 volts to 6 volts but I can't remember who commented "ground" ground" so I did a star ground I grounded everything and shielded everything well almost everything i even scrapped out a old pc box and put drivers + chip + power supply in it then ground the box and shielded the plasma cutter hose too .. friends think i lost my mind which i already knew that but anyway after all the grounds I added and shields plus every little hole i used metal tape and then tried it and my scope still shows 5 to 6 high/low volts when connected to the power supply but i think its reading the scope connectors themselfs but my motors did stay on for a bit i mean waaaay better then nothing so i think after I ground alittle more of things ..i might win thanks to this site and my ground rods but hopefully my next reply will be a video of this DIY working so sorry for the long boring reply I get excitied when 2 years later it might work there is hope and star ground!
 
premelec

premelec

Senior Member
Glad to hear of your almost success and learning about practical shielding etc. Sometimes copper and mu metal boxes with all lines filtered in and out with optocouplers are required - and you have to do what works when you are in a tough environment. - ferrite beads can help too. Post a picture of what you've done and perhaps someone can suggest further improvements - the power supply voltage should not be varying 5 to 6 volts if your common grounds are in good shape and the supply is solid regulated. Sometimes just knowing what components are available [like feed through capacitors] can solve a situation.... Persist on!
 
newplumber

newplumber

Senior Member
thanks premelec I didn't know about feed through caps ,I will try that and learn more on ferrite beads.. I will take some pictures after I redo my board for the
3rd time but hopefully this time using green clamp connectors everywhere ..for pins etc to switch out or in whatever... some can say use a bread board which i have done but when I jam this in the old pc box I want to make sure no wires disconnect.
 
J

john2051

New Member
Hi, As a former repairer of switch mode supplies including some very iffy pc supplies, I cant help but feel that a 400watt pc supply might not be a good choice. Which type is it?
If its a ps2 type ( with a single large plug + drive connectors ) the main wattage in the modern ones appears to be mainly at 3.3v , although in conjunction with the 5v line.
It might be interesting to put a scope on the 5v line and see it jump when the arc strikes. One of the problems with using these things as bench type supplies is the smoothing, or lack of.
Most of the smoothing was done on the motherboards regulator circuits.
I will be interested to see how things go, as I'm looking at a laser cutter at the moment, and I'm bound to run into all sorts of problems.
Regards john
 
newplumber

newplumber

Senior Member
Thanks John H i didn't know about MB smoothing I will try to check which power supply but I work out of town alot so I really don't get time to work on this. On the fourth of july I was able to have it cut a piece of 1/8 metal from learning another video they placed a ground clamp on the tip of the plasma which I did the same and it worked so all america celebrated for me :) but I took a short vid of it working before the whole thing flies apart (or just in case) but I believe with a little more tweaking this machine might just be my friend. When I get back I will take pictures of the box and power supply and stuff.

https://www.youtube.com/watch?v=dW79nN35Pck&feature=youtu.be
 
E

edmunds

Senior Member
newplumber said:
Thanks John H i didn't know about MB smoothing I will try to check which power supply but I work out of town alot so I really don't get time to work on this. On the fourth of july I was able to have it cut a piece of 1/8 metal from learning another video they placed a ground clamp on the tip of the plasma which I did the same and it worked so all america celebrated for me :) but I took a short vid of it working before the whole thing flies apart (or just in case) but I believe with a little more tweaking this machine might just be my friend. When I get back I will take pictures of the box and power supply and stuff.

https://www.youtube.com/watch?v=dW79nN35Pck&feature=youtu.be
Click to expand...
Awesome machine! Great achievement!

I have built a CNC machine that does a great deal of things around the house, but I did all of it with Chinese components. Even learned to modify up to cutting traces, replacing components etc, but never thought picaxe could be the core. I'm now in the process (slow process) of building a picaxe based controller for everything (heaters and fans for 3D printer, RPM regulator for mill spindle, etc.) except steppers. Would love to see your code and full schematics if you happen to put one together some day. What software are you running on the computer?

Regarding your problem my only suggestion was the download circuit of 22k and 10k resistors or 4k7 from SerIn to ground if you take the chip out for downloads. I have had perfectly working projects until one day they stop working because of no apparent reason. Only to discover (after many hours or desperation) that the download circuit is entirely missing.

Regarding the scope - I went for the PCBScope from PICAXE website in a situation unable to sort out I2C protocol problems. While it is not perfect in many ways (missing a decent case and plugs to mention the most important thing :)) you don't have to break a bank to get one. I don't use it very often, but there is nothing else that will quite do it when I reach for it.


Good luck,

Edmunds
 
newplumber

newplumber

Senior Member
Thanks Edmunds
You will laugh when you see how simple I made this machine for the code its very simple I just used GOSUB routines . I can't cut a circle because my math for those steps fly over my head. I go by up, down ,left, right ,upright,downright,upleft,downleft. I figure a octagon is close enough for my circle.
 
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