A New CNC Build

[epineh]

Hi Rod, I just checked the G251 manual and it says the idle current is set to 70% of the motor current, which is set by the current set resistor... I tried to copy and paste the info but it was all messy.

I guess it makes sense, the motor current has to be set somehow, as different motors will have different inductance and the supply voltage will vary with each setup.

See this wouldn't happen with servo's hehe ... sorry Geoff, but Rod and I have a little steppers vs servos debate going every now and then, all in good fun of course

Cheers.

Russell.

[seafurymike]

Geoff,

Try wiring on one half of the coils. This may help understand if it is a set of outputs or not.
swap them even if the unit gets hot and verify the other side.

i think your right in saying it seems the unit has an issue.
Have a look at the board, just to make sure no pins or contacts are touching

[rodm]

Hi Russell,
Yep you are right. When I read their manual it says:

Connect the current set resistor between these two terminals. Select a 1/4W resistor value to match the motors rated phase current. The resistor value in Ohms is 1,000 times the motors rated phase current in Amps. Example: A 1.8A per phase motor requires a 1.8K resistor (1,800 Ohms). Always use a current set resistor even if 3.5A is selected (drive maximum current). If no current set resistor is used for 3.5A then automatic standby current will be 100% instead of the expected 71%.

Me being the electroinics genius (not) I interpreted the last sentence as being the condition set by the resistor. ie no resistor then 100% standby current. Of course when you think about it (which you did) there is no other means of setting the max motor current so the resistor must set the motor current. Just as well I fitted them on mine as I can't risk anything when I play with electronics - obvious hey

Nah servos would be hovering 20mm either side of the exact point of where they shouldn't be. Bit like a blind man looking for a black cat in a dark room, that isn't really there.
I really like your new machine by the way - done anymore?

[epineh]

Nah servos would be hovering 20mm either side of the exact point of where they shouldn't be. Bit like a blind man looking for a black cat in a dark room, that isn't really there.
I really like your new machine by the way - done anymore?

LOL, well I guess I can't argue with that logic

No more on the new machine, I am waiting on some more parts to be machined, my mate isn't charging me anything so I can't really give him the hurry up, or can I...?

I am currently looking at how I am going to do the electronics, (controls and pendant), not the nitty gritty but the physical location, I need it usable but I don't want to lose the advantages of the cantilever, maybe I can swing it from the shed roof

Cheers.

Russell.

[rodm]

Of course you can hurry up your mate Even more so if it is Chich

I like your lateral thinking - swinging from the roof though might lead to monkey business.

[Geoff Sims]

Hi All

Thanks for your suggestions.

I have current set resistors on each of the G251s. Since the steppers are 3.5A, I fitted 3.3k resistors being the closest standard value. I checked the one on the Z axis driver and it was 3.52k which should be about perfect.

Rod, I've checked the wiring from the driver to the plug and there is definitely no short there. I actually disconnected the wires at the gecko end and checked them with the multimeter as well as visually. Reconnected and no change.

Russell, I don't know much about steppers, so I'd be even more confused with servos......I think. On that basis, I have to side with Rod and stick with the steppers. Not sure about the blind man analogy though. Just how do servos compare in accuracy?

Michael, I'm not quite sure what you're suggesting. Are you saying to just connect one motor winding at a time and see if it still gets hot? If it does, on just one side and not the other, what does that tell me? I know the stepper is OK as it runs well on both of the other drivers. With my meagre knowledge, I'm not sure what I'm looking for.

Cheers

[rodm]

Geoff
Logically the problem lies between where you connect your motors and the computer so I would not worry about motor checks at this stage.

The only thing left that I can think of is to check the parallel cable and computer. It is quite likely these could be your problem so don't overlook it.

Disconnect both ends of the parallel cable - at the bob which includes the flat ribbon cable and at the computer.
Leave the computer parallel cable connected to the ribbon cable and check for continuity on all pins and with each other. ie check Pin 1 to pins 1 to 26 on the other end, then Pin 2 to 2 to 26, 3 to 3 to 26, etc. Obviously you should get continuity between each corresponding pin but not with any other pin.

Next is to unhook the signal wires from the bob to X axis and swap them over with Z axis signal wires. Don't change any Mach3 setting but see if the problem is still with the Z axis motor. If the problem has shifted to the X axis motor this will elimante the driver but it could still be the bob or the computer so you are not out of the woods yet and this assumes the parallel cable has been checked for continuity. If the problem is still with the Z axis motor then it is pointing at the Z axis driver being faulty. It will be a little wierd in this test because you will have to jog X axis in Mach3 to make Z axis move and Z for X.

As a matter of interest can you post your Mach3 steps, velocity and acceleration figures for each axis. If you want send me your xml file and I will have a look at it for you.

Don't worry too much about the servo stepper thing with Russell. This is a long standing joke we have had and one day I might admit he is right but not yet.

[epineh]

Russell, I don't know much about steppers, so I'd be even more confused with servos......I think. On that basis, I have to side with Rod and stick with the steppers. Not sure about the blind man analogy though. Just how do servos compare in accuracy?

Hi Geoff, sorry about the thread hijack...

As Rod mentioned this has been going for a while and isn't all that serious

It really is a big can of worms the stepper vs servo debate (when being serious) but for the accuracy well put it this way, depending on the application, servo's use different count encoders, the higher the count the more accurate the servo. Think of the counts as steps on a stepper (kinda). Lets say you have a 2048 count per revolution encoder, now as it is in quadrature, this means you multiply this by four to get the actual pulses per revolution, so you have 8192 pulses per revolution, so like a stepper, give the drive 8192 steps and it will turn one revolution, as Rod mentioned the servo can move slightly at stop, a good drive should hold it at one pulse either way, but with such a high degree of control, the effect is insignificant....or as Rod says, 20mm of where it shouldn't be

Actually now I think of it, I hooked a printer head up to a servo drive, the printer has a linear encoder and dc motor that make up the servo (instead of a rotary encoder) I could see the strips that made up the encoder and they were not all that small so the resolution was pretty ordinary, but the print head was quite stable, surprisingly so. I made a vid, check out my username (epineh) on YouTube, if you can't find it I can post a link, didn't want to here as it will embed in this post and I have already hijacked this thread enough...

Anyway, enough of my blabbing.

I don't have any more ideas about the G251, maybe it is actually just faulty, Gecko support is awesome, if you can't get it going then they will do all they can to help.

Cheers.

Russell.

[epineh]

Actually Rods post got me thinking, when I made up the parallel port ribbon cable I buzzed it all out and it checked out fine, pin for pin, but as it went via post it is possible it got stood on or run out over by a truck, and they are only crimp on so maybe it got damaged in transit.

I used a spare DB25 male connector plugged in one end to make testing easier, don't know if you have anything like that (I can mail you one) but it makes life easier.

Cheers.

Russell.

[rodm]

A bent paper clip works well for the female end.

[epineh]

Is there any problem that cannot be solved with a paperclip ?

[Geoff Sims]

Hi Rod

I have checked out the connections all the way from the BOB to the computer and they are all OK. As you suggested, I then swapped the signal wires from the BOB between the Y axis and the Z axis so that the Y axis signals went to the Z driver and vice versa. The fault remained with the Z driver, which pretty much rules out the BOB being the problem, and the cable back to the computer.

I have reached the conclusion that the G251 driver is faulty and have removed it to send back to Gecko. I have explained everything I've done to Marcus at Gecko, and he agrees that it should be returned.

My big issue now is that I can't continue with the machine. DOH!

When I find it, I'll send you the XML file. Thanks.

[epineh]

My big issue now is that I can't continue with the machine. DOH!

If you are really in a hurry I can lend you a drive.

Russell.

[Geoff Sims]

Hi Russell

I've checked the ribbon cable and connectors you generously sent me, and it is fine. All connections are intact from pin to pin and there are no shorts between pins. Thanks again for that.

Don't be worrying about "hijacking" this thread. I don't see these comments as hijacking and find them interesting, and very often entertaining. At the same time, maybe I will learn something useful.

Now that I've come to a standstill with my machine, this thread is likely to be very quiet for a while anyway, so "Hijack away".

Cheers
Geoff

[Geoff Sims]

If you are really in a hurry I can lend you a drive.

Russell.

Holy smoke you're quick. While I'm slowly picking at my keyboard to send you a message, you've already got one to me.

That's a very generous offer, but it would mean you might get a second hand drive back. I don't know if Gecko would repair mine or just replace it with a new one. If it's a new one, that would be OK cos I could return that to you. If it's repaired, you'd either get your now used one back, or my repaired one. I wouldn't like to do that to you.

I'm not really in a hurry, just getting excited as I near the end of the build. I just need to be patient.

Cheers
Geoff