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Author Topic: Mach3 Output For DC Motor Control  (Read 17269 times)

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Mach3 Output For DC Motor Control
« on: July 31, 2013, 11:16:53 AM »
Hello Everyone...I'm new here today.

I have a Bridgeport style mill (48" table) with a Leeson 90volt DC motor attached (no speed control other than pulleys)

I plan to convert this mill to CNC shortly, using Mach3 as control software.

I'm well versed in digital and analog electronics, as well as computer science and programming ( basic, C, C++, visual basic )

Sorry, no bragging here, but I don't want anyone to hold back tech info.

My main question is with spindle speed control.

While reading other posts, I notice that with Pusle Width Modulation control of DC motors, most users on this site

convert the PWM output of Mach3 to a 0 - 10 volt analog dc voltage and use it to control a PWM DC motor driver board.

I don't get it. Why not just drive a hefty Mosfet ( like a irf260n (50 amps @ 200 volts) ) directly from the Mach3 output pin? (through a suitable FET driver chip such as  a 4420CN, and  an opto-isolator of course )

Why go from PWM to Analog and back to PWM again to drive the motor?

Am I missing something?

Thanks!



« Last Edit: July 31, 2013, 11:30:50 AM by PurpleThumb »
Re: Mach3 Output For DC Motor Control
« Reply #1 on: July 31, 2013, 11:30:46 AM »
Probably because many of the users you're referring to (like myself) don't have the electronics knowledge to build their own safe, reliable DC speed control from scratch.  Most will buy an industrial controller new or used off ebay, like KB brand or others.  You just can't beat their availability & value.  I have 4 or 5 of the KBIC-120's and paid less than $35 each incl. shipping.  They're older SCR models and not PWM but the motors are very happy with them. 
Milton from Tennessee ya'll.
Re: Mach3 Output For DC Motor Control
« Reply #2 on: July 31, 2013, 11:38:48 AM »
Hello, and thanks for the quick reply!

I understand.

It just did'nt seem efficient to me.

I wonder why someone doesn't sell a board like the one I described?

Seems less complicated.

( Sorry if I sounded like an arrogant know-it-all, I did'nt mean to )

Best Regards,

B.

Offline Hood

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Re: Mach3 Output For DC Motor Control
« Reply #3 on: July 31, 2013, 02:05:29 PM »
Does your spindle drive accept a PWM signal? Most I would have thought look for a 0-10v signal but then again I have never used any DC spindle drives so......
Hood

Offline stirling

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Re: Mach3 Output For DC Motor Control
« Reply #4 on: August 01, 2013, 05:00:12 AM »
Not speaking for PurpleThumb here but as I understand it he's suggesting this as a way to implement your own drive that DOES accept PWM rather than use an off-the-shelf drive that expects an analogue voltage.

Personally I think it sounds like a nice (and dirt cheap) idea for those that want to roll their own.

(But then I know squat about driving large DC motors so I might be backing the wrong horse here for all I know).

Ian

Offline Hood

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Re: Mach3 Output For DC Motor Control
« Reply #5 on: August 01, 2013, 07:45:43 AM »
Ian, yes I think thats what he was meaning but it was just this bit
Quote
most users on this site convert the PWM output of Mach3 to a 0 - 10 volt analog dc voltage and use it to control a PWM DC motor driver board
Which to me seemed like he was saying that normal DC motor drives accepted PWM so why convert to an analogue voltage in the first place only to convert back.
 I know nothing of electronics so I will keep away from that side of things other than to say if it was that easy to make a drive would it not have been used Industry wide long before now rather than complex drives that cost many hundreds or thousands of £ or $.
 

Hood
Re: Mach3 Output For DC Motor Control
« Reply #6 on: August 01, 2013, 08:28:41 AM »
Hello All,

I've found that what is needed is what is called a "H-Bridge" circuit.

Simply said, it takes the PWM output from Mach3, and simply amplifies it to the voltage and current required

by the motor.

Has three control pins...

1. Forward
2. Reverse
3. PWM Input

That connect directly to Mach3 output pins on the computer.

The cost of the parts involved for a 200 volt 50 amp amplifier are less than $30.00, ( 4 fet transistors, 4 npn transistors, and an isolator chip if I want to solder my own)

I would rather purchase the thing assembled if the price is right.

I'll let you know what I find out.

Cheers!
B.
Re: Mach3 Output For DC Motor Control
« Reply #7 on: August 01, 2013, 11:15:03 AM »
An H-bridge is the basic output circuit used in nearly any DC motor controller that supports bi-directional operation, and many that only support uni-directional operation.  But the H-bridge itself is only PART of a usable motor controller.  There is MUCH more required to make something that would be suitable for driving a machine spindle, including voltage and current feedback, I/R compensation, and protection circuits to prevent the H-bridge from going up in smoke if not driven correctly (and you CAN'T count on Mach3 to always drive the PWM output correctly.  Without all that other other logic, you can expect to have something that works very poorly, up to the point where it goes Pfffffffffffffft!  And that WILL happen in very short order.

Regards,
Ray L.
Regards,
Ray L.
Re: Mach3 Output For DC Motor Control
« Reply #8 on: August 01, 2013, 11:43:37 AM »
Wow!

You sound angry!

WILL go up in smoke?

You assume I know nothing?

You don't know anything about me.

If it goes up in smoke, I lose a few dollars.

But I have some fun, and I learn something.

I'll modify the circuit and try again, and  I'll solve any problems.

When it works, I'll enjoy the great satisfaction of solving those problems.

Opto isolation protects the computer, fuses protect the motor.

I'm not selling anything here, just learning, and sharing.

Don't take it personally, its not your money.




« Last Edit: August 01, 2013, 11:58:20 AM by PurpleThumb »

Offline Tweakie.CNC

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Re: Mach3 Output For DC Motor Control
« Reply #9 on: August 02, 2013, 12:01:48 PM »
Hi B,

Please keep us informed of the progress on your own designed PWM speed controller. Once it is all sorted I am sure others would be most interested in the circuit details etc. if you are happy to share it as a project.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.