MONSTER - A learning experience... pyVCP machine
There are two primary parts to fail... the opto-coupler...
Digikey: 160-1783-ND $0.81 USD
The MOSFETS (4 each)
Digikey: IRF5305PBF-ND $1.52 each. ($6.08 USD)
So $6.89 for me... or $35.00 for them... I'll fix it myself. I appreciate the offer... but I've got this... just need to get the time.
Then I found a video of a guy with the same Wells Index 847... only his was a later version with stepper motors . And in his video, he was cutting parts at 100 IPM. (2540 mm per minute) Wait... my machine has a top-end RAPID movement of 60 IPM (1524 mm per minute). Really? That fast? Yes ...
So I thought... what would it take to speed up my machine?
To go faster, I needed to replace the 500-line encoders with something smaller... at $100 each... at least $300. And i wasn't really sure how much speed and accuracy that would get me. My wife's Laguna CNC never misses steps... and has had no issues at all in the 4 years she has had it. So... I thought perhaps I should update to stepper motors.
So this week... I ordered a kit with 4 1850 OZ IN (13 N-M), 4 power supplies, and 4 controllers. With shipping, about $650 USD. They should get here by next weekend... and unless I get pulled to some other project in another state... I'll start the conversion.
Wish me luck... I'm hoping this will be much more stable... and long lasting!!!
Askjerry wrote: So way back in January of this year I ordered the parts to repair my mill. I thought that i would get to it rather quickly. As they say... "Life is what happens when you are busy making plans." So... no... 8 months later and it is still down. I started really thinking about the mill... and the needs of the DC motors... a Geckodrive will support up to 80 volts and up to 7 amps... but the TOTAL power dissipation is only 100W. So at 45 volts... that's 100W / 45V = 2.22A. Two amps? I think I can pull more than that under certain circumstances.
Gecko DC servo drives can push 20 A, so with appropriate dc motors they can move a lot of weight pretty fast.
You have 500 cpr encoders, but you did not say what the ratio is and the pitch, however it is, at 2000 pulses per rotation, if you do not have mesa controlling that, you will hit the limit quite fast, even if you use the included 1/10 ratio divider in GECKO drives.
Personally, i had the same setup with 500cpr and GECKO, and i always remove the dividers in Gecko drives, so i got new 100cpr encoders and things got much much faster. I had a 17:1 gearboxes attached to servos, so i had plenty of resolution.
You can test to see if you are limited bu encoders, disconnect one of the motors power wires and connect them directly to power supply, if you notice a lot of change in speed, that is the problem (encoders), but if you get only 10 to 15% increase in speed, you have a voltage problem, so you will have to change the power supply if the drives can handle more voltage or change the motors to lover rated voltage.
Also the power dissipation equation is not correct, power dissipation you have to calculate the voltage that is dropped in MOSFET's multiplied by current passing through them. MOSFET's are very good at conducting due to very low ON resistance, so however you look at it, there is very little dissipation ( confirmed by the drives staying cool under heavy load. Gecko uses IRF740 mosfets in their drives and they have an RdsON of 0.55Ohm, so at full load that amounts to 11W per mosfet ( 4 each on servo drives and 8 on stepper drives ), and since at any given time there are only 2(servo) or 4(stepper) conducting, you get no more than 25W or 50W per drive when all the rest of the electronic is included.
May be to late since you already ordered, but still worth a try.
Gecko servo drives will instantly die if you push the estop on a machine with no back EMF suppression ( dump resistors ) if it is moving fast, there is a nice easy to do schematics on Gecko website.
Then again, you said earlier that mosfets AND opto couplers died in the same drive, that is for sure a wiring problem.
Again to late, but it might help.
I have these on my servo mill, they're fantastic. Impressive noise immunity too.
As your post stated tommylight, my calculations were probably off... the the current draw is very close to the max... plus the issue of instant MOSFET death if the motor coasts due to an ESTOP or similar. Changing the encoders is a $300 adventure... and if I'm going to drop that much... and still potentially have some wire / current / spike issue... then it's probably better to drop $700, replace all the 35 year old motors, redo the wiring, and replace the single mongo (40 pound) transformer with independent power supplies.
Plus... the new controllers have built-in optocouplers on every input... that's going to isolate the parallel ports. Then... I'll add optocouplers to the inputs for every magnetic switch... because they all have a 9v input... and if any leaked... that could have been the cause of the parallel port zap. The outputs to the 120vac have always had industrial opto units on them... so this way every input and output will be protected.
As for the encoders I was using cncnoel... these are what I was using...
They have an integrated shaft that I connected to the existing upper shaft. It originally had a 50-year old design of resistors and cam followers. (I regret not photographing or video recording the old units... all brass workings.) At the time... this was a HUGE upgrade.
I know that with steppers there is the potential to miss steps... but if it is adjusted properly... with modern microstepping... the chances are low. I said the MONSTER was a learning experience... not just for you... but for me too! I'm hopeful that this really improves the machine. I have not played with steppers this big before... I hope they are enough... I'm committed now.