CNC hardware for PM-940

[banctecbobn]

Hi,

My friend just purchased a manual PM-940 with harden ways and has asked me to help with installing CNC. I'm only a hobbyist and added an open loop stepper system with Desknc to CNC my shoptask gold machine around 15/20 years ago. Since then a lot has changed and I am coming here for guidance and direction to help my friend. Original plan was going with an open loop system using MACH3, Gecko drives, and stepper motors. Quickly I found this plan to be outdated.

The PM-940 is a tank of a machine and I'm impressed with the quality and function, I want one myself. The only thing decided is adding double nut ballscrews and a closed stepper system. I'm liking the idea of using linear encoder/scale for feedback to the driver card to eliminate ballscrew tolerances/variances/backlash. Here is where all the questions start and will list as follows.

1) Is the linear encoder/scale the best for feedback over a closed loop stepper motor?

2) Can the feedback from encoder/scale be routed to driver card or the PC for software control or either by design option? Would like to route feedback from linear encoder/scale to the driver card for control.

3) What is the best software to use for a closed loop system?

4) Can Mach3 be used in a closed loop system if feedback is routed to driver card.

5) Good motor to use around 1700 or 12Mn Nema 34?

Goal is to have a 3 axis CNC closed loop system for a hobbyist at around $3,000.

Thank you and have a great day,
Bob

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[CS900]

welcome back to the game. I think you'll find that things haven't changed as much as you might think. Mach3 is still a very viable piece of software at a good price point. Let me see if I can address most of your questions with a rant...

As of yesterday I just got my mill (PM-45) under motion again. I have been using the closed loop steppers from automation technologies for several years now with great success. They are fast, powerful and they just worked right out of the box without having to tune them. Very user friendly and reliable. I run them in conjunction with an ethernet smoothstepper and mach3. Everything plays nice together and works every time.

I recently decided to do some upgrades on the mill. One of them being a switch to linear encoders. It was just yesterday that I got one axis moving again using the linear encoders, so i can tell you that it does work...IF..you are able to connect to the drive and change the firmware. You'll have to change the encoder count in the firmware otherwise you'll get a positioning error anytime you try and move the motor. I am able to connect to the drives with the leadshine protuner software. I can't say that it will work for all closed loop stepper systems, but it connected to my drives with no issues.

So with all that said, Yes you can use linear encoders and I believe they are superior to the rotary encoders on the motor, but they are a lot more work to install as well. The feedback loop is done at the driver level. Mach3 is completely ignorant to the true position of the machine. All the drives that I have seen, mine included, have a fault output signal. So if I get a positioning fault, my driver sends a signal to mach3 telling it to stop motion. It's not as good as feedback control right at the controller, but it's certainly better than having a dead axis while the others continue to jump around.

If you wanted to do the loop control at the controller level you'll want to take a look at the klop board from dynamotion.. It has encoder inputs and I believe you can use standard stepper drives as the controller does the position monitoring. Dynamotion has their own software but they also have a plug-in for mach3 if that's the interface you are used too. If I were to do this all over, this is likely the approach I'd take but I have no reason to change as my current system works great. People also use the Galil boards, which I guess you can get used on ebay for a decent price, but I don't know a whole lot about them.

As far as sizing the motor, i think 1700 is a bit oversized for your machine. Mine are 1128 and I think they are likely a bit overkill.

Hopefully that answers some of your questions.

[banctecbobn]

Thank you for answering some of my questions.

Without ever using a closed loop system, doesn't the driver card or the controller software adjust steps on the fly to keep the position and the software in sync? Was hoping system would add steps or limit steps to motors to make up for backlash and ballscrew variances using the linear scales for feedback. The cheaper ballscrews list tolerances up to .004 per foot.

Thank you and have a great day,
Bob

[CS900]

yes, the driver will do this within a certain error amount. After that error the drive will fault.

[banctecbobn]

Is the amount of error before drive will fault adjustable????

Is it possible to know how much error will cause drive to fault?

Could excessive chatter cause drive to fault?

[CS900]

on my drives i can adjust the error limit. It's measured in encoder counts, so you can do the math to figure out the linear distance.

I wouldn't think so. Chatter is usually vibration in the tool or workpiece and shouldn't effect the sensor if it's securely mounted. Maybe someone else can chime in on this, as this is my first experience with linear encoders.

[LHejza]

on my drives i can adjust the error limit. It's measured in encoder counts, so you can do the math to figure out the linear distance.

I wouldn't think so. Chatter is usually vibration in the tool or workpiece and shouldn't effect the sensor if it's securely mounted. Maybe someone else can chime in on this, as this is my first experience with linear encoders.

Can you tell us which linear encoder you used an dhow you connected it to the drive?

[CS900]

Can you tell us which linear encoder you used an dhow you connected it to the drive?

I'm using Dirton DMR5000 encoders. They are wired to the drive just like the rotary encoder. The power, ground, and A and B phases are wired directly from the encoder to the stepper drive.