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Noobish Question On Motors & Drivers
« on: April 26, 2008, 08:57:01 PM »
Hi all,

I would like to say my noobishness knows no bounds when it comes to electronics. Electronics are a bit out of my realm.  Anyways , here goes:  I have had this project in the works for quite sometime now, and am finially moving forward.  I have tried to research this for several weeks on the net and ended up confusing myself even farther.  I want to use servo motors instead of steppers, due to the fact of lost steps.  I would be willing to use steppers if a encoder could be mounted to close the loop.  I am planning on using 1/3 HP 90V Permanent Magnet DC Motors.  These should operate nicely with the off the shelf Gecko Drives. I got the idea from this source:

http://www.truetex.com/servomod.htm

I plan to purchase (3) motors from Grainger (X,Y,Z)    http://www.grainger.com/Grainger/items/2M509 

I already have a 1/3 HP 7,000 RPM Spindle Servo Motor with a cat 40 taper in hand. What I am now overly confused about is this:

1.  I was planning to use this   http://www.geckodrive.com/product.cfm?pid=13   Gecko 320 drice.  Will I need a 80V DC power supply to feed the motors somehow?

2. Does Mach3 feed this unit directly?

3. Does Mach3 Feed the Gecko G100 which in turn then drives the 320's?

I guess I am only asking what hardware would I need? I will then try to go from there. 

I already have started construction on the machine, and most of the pieces are already in hand , Linear Rails, Ballscrews, Etc.  I have built a lot of the components where I work using our CNC laser to cut 3/8 plate steel for the machine parts.  Nothing like laser cut holes to line up the linear rails and such.  Also using our CNC lathe and mill to make the mounts, etc.  As far as building the machine, I feel pretty confident.  I just am lost on the electronic parts of the connections and equipment required for the servos.

Basics on the machine are thus : 36" X 36" x 12" machine area.  All running on THK linear bearings and 1 1/2" ballscrews. The gantry shows in my 3D models to weigh in at around 125 pounds, so that is why I want such beefy motors.

I really apprictae any help I can get on this, and hope the flames for my noobishness will be kept to a low level  ;)

Thanks,

Rob
Re: Noobish Question On Motors & Drivers
« Reply #1 on: April 26, 2008, 10:03:03 PM »
You can probably find some decent dc brush type servo motors with encoders already fitted on ebay for a little less. There is a fellow that pretty regularly has three new in the box reliant servos for aprox $500 .
In addition to the gecko drives you will need a dc power supply that can provide the needed amperage and voltage.

You will also need a breakout board of some sort that will go between the computer and the gecko drives. Computer hooks to the breakout board , break out board attaches to geckos , limit switches , solid state relays etc. The power supply will attach to the drives. There are a number of breakout boards avail , I like this one http://www.cnc4pc.com/Store/osc/product_info.php?cPath=33&products_id=161

Offline jimpinder

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Re: Noobish Question On Motors & Drivers
« Reply #2 on: April 27, 2008, 03:05:08 AM »
You might know a lot of this is you have been reading up on it.

Mach 3 puts out signals on the LPT1 - the old computer printer port. There are only 12 output lines and 5 input lines ( It is the input lines you might run short of) - so run through what you might need.

Each axis requires 2 wires (plus a return wire) to drive it, one wire drives the step, one wire drives the direction. If you have four axis you need eight wires etc. Don't just allow for three axis - most people eventually want a rotary table and other goodies.

Outputs must also drive things like the spindle control (normally three wires) coolant (one or two wires) axis enable wires (if required) one for each axis. etc.

Inputs are for limit switches, home switches, spindle speed, and any other switch you might want to put on your lathe like an "emergency stop" button or other safety device.

You can see that it is very easy to run out of wires, when your system gets a bit advanced - and you end up having to add other ports to your computer.

Many firms offer driver boards and breakout boards etc. A breakout board merely provides an easy way ( usually screw terminals) of connecting your bits and pieces to your computer. Many breakout boards have electronics on board to isolate signals and provide a 5 volt power supply for switches etc.

The disadvantage with a lot of breakout boards is that they are pre- wired and fixed in their applications - rather like painting by numbers - fine to get started with, but in the end, very limiting.

The Mach 3 admin boys are going ******** at the moment about a new interface board called a Smooth Stepper. This is a board that provides inputs and outputs (more than you need), it provides on board pulse engines to drive your axis, which means that your computer does not have to generate the pulses and can get on with calculating the job. I understand that it is opto-isolated , etc etc . The beauty about it is that it connects to your computer via a USB lead. I would recommend that you look at one of these ( if you are starting from scratch).

Whichever type of board you start with - you then need to interface this with your machine.

Geckos are good drives and I am pleased with mine - so I cannot crticise your choice. Use seperate drives - Do not get a single board that has breakout board and three drives all in one - if one goes faulty it all goes faulty. If you have seperate units you can change anything at will.

The output from your breakout/smooth stepper board will drive the Ghecko board directly. Mine require a 5 volt connection, a "step" connection and a "dir" connection to each board. The inputs to the Gecko are isolated from the rest of the board to protect the low voltage side of the system.

I have looked at the Grainger website and your choice of axis motor. I am at a bit of a loss. These appear to be a straight DC brushed motor, and there is no mention in the blurb of feedback for positioning, There is mention of a tacho - is this for positioning.

Mach 3 gives out step and direction signals. It is preprogrammed with the number of steps per inch of axis movement, so if you want to move one inch - it puts out say 60,000 pulses and says - "there you are - one inch". With steppers you hope the motor got all the pulses, but with servos (as I understand it) there is a measurement system either inside the motor, or external to,  which checks that your motor has gone the relevant distance. I do not see that system mentioned in the Grainger.

If you read the PDF file on the Gecko 320 it tells you how to wire them, and also about the feedback circuits. You can get external feedback circuits, but I thought that servo motors ( marketed as such) came with them ready installed integral to the motor. I would check the motors.

If you need any more - come back again.









Not me driving the engine - I'm better looking.
Re: Noobish Question On Motors & Drivers
« Reply #3 on: April 27, 2008, 10:01:59 AM »
Thanks for the fast responses.  In answer to your concerns on the axis motors, I referenced an articale that demonstrated how to mount a digital encoder to the back of the motor to turn a regular dc motor into a servo. My gantry alone shows a weight of 125# on my 3D model, not counting the Z axis assembly, Rails, Drives, etc. which is why I wanted to go as large as I can on the axis motors.

http://www.truetex.com/servomod.htm

I was going to use that feed back to make the "closed loop" on the motors.  I had planned on using those motors, and have machined a plumbers block to support the ballscrew and attached a Renco digital encoder to the ballscrew, not the motor.  I am using Lovejoy couplings and they will flex and compress a few thousands as the machine runs, so in order to maintain true positioning, I am enconding the drive shaft, not the motor shaft.  After reveiwing this website I am loosely basing my machine on his:

http://www.oneoceankayaks.com/madvac/madvac_index.htm

I would be willing to go with stepper motors if I could find a driver that would accept encoder feedback and drive steppers. I just don't like the thought of lost steps.

I was also looking the usb driver board you mentioned, but I thought it said mach3 didn't support the USB at the moment.

After reading your post, I think I may understand a bit more.  You can use a breakout board / smooth stepper to "run" seperate drives, OR you can use a all in one wonder like the gecko g100. 

Offline Hood

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Re: Noobish Question On Motors & Drivers
« Reply #4 on: April 27, 2008, 12:05:32 PM »
The SmoothStepper works great with Mach, it still has a few things to get worked on such as the soft limits and threading sync for turning but I know Greg is working on that now and with the excellent work he, Brian and Art did getting the plugin issues sorted during the Beta testing I have no doubts whatsoever that they will be finished shortly.
 As for steppers you can get the encoder Interface board from Ron Rogers www.rogersmachine.net  This allows you to use encoders with steppers and if the steppers get out of position by the user set following error it will halt Mach and thus parts wont be ruined. In reality a well matched and setup stepper system will never lose steps but it is always an added safety feature.
 Servos are the way to go and  if you can afford them AC are the best but if you can compromise on speed then steppers will suffice.

Hood

Offline Hood

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Re: Noobish Question On Motors & Drivers
« Reply #5 on: April 27, 2008, 12:07:30 PM »
Oh just another thing regarding mounting the encoders on the ballscrew rather than the motor, if you have play anywhere, and you have said you will in the couplings, then you may get issues with the motors constantly hunting back and forth trying to keep position.
Hood
Re: Noobish Question On Motors & Drivers
« Reply #6 on: April 27, 2008, 01:40:18 PM »
Ahhhhhhh  the Good Old Lovejoy Coupling!  I love 'em but I think you're going to see a lot of backlash.  I guess you could weld the 2 couplings together to form a rigid coupling.  But then, why not just get a rigid coupling?  Better still, check out http://www.sureservo.com/couplingconsiderations.htm there is a list of servo coupling manufacturers there.  One Company, Ruland has a really good pdf explaining the differences between the diff types.

(Quick Hijack) Hey Hood, have you ever tried the Rogers encoder interface?  I was speaking with Ron the other day about how it works and it seems like it would be great for steppies & Servies?

Regards,
Sid

Offline Hood

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Re: Noobish Question On Motors & Drivers
« Reply #7 on: April 27, 2008, 02:03:47 PM »
Sid
 Yes I borrowed one way back in the very early days of the encoder interface. It worked well as far as when I deliberately made the following error so small or deliberately stalled the steppers Mach would get stopped. There were quite a few niggly problems with it back then, cant really remember but I think homing was one of them. Since then however Brian has done wonders with it , think its now a plugin that controls things  rather than the macropump. I dont have one now so I cant test it out but I have only heard good  and all issues are reported to be in the distant past.
Hood
Re: Noobish Question On Motors & Drivers
« Reply #8 on: April 27, 2008, 02:08:23 PM »
Cool!  Thanks for the NON-"Silly Answer"   ;)  BTW- What happened to your "Bad-A$$" Avatar- I kinda liked it!

Regards,
Sid

Offline Hood

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Re: Noobish Question On Motors & Drivers
« Reply #9 on: April 27, 2008, 02:11:27 PM »
it was Benny that gave it too me, I let him have a few days of fun then removed it, well you know what these Ozzies are like, they go in the huff easily if you dont humour them :D

Hood