Rack & Pinion with Retina Controller

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Rack & Pinion with Retina Controller

Postby PSG » Mon Dec 13, 2010 2:00 pm

I am in the planning stages of building my first laser. I have experience with routers, so the basic construction is not new. In my reading, I see that the Retina Controller must have 1000 steps per inch. This confuses me some because on my router I was shooting for a much higher resolution (for clean cuts) and I am under the assumption that it is even more critical in the laser systems.

Is 1000 steps for speed or quality?

I am trying to find a balance of gear and reduction to come up with 1000 steps. With a rack driven system, I believe I can get all sorts of speed, as indicated by my calculator. I am doing everything possible to avoid screws due to backlash and I have zero experience with belts - although belts are completely not ruled out yet.
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Re: Rack & Pinion with Retina Controller

Postby bdring » Mon Dec 13, 2010 6:45 pm

1000 Steps/in is a realistic output resolution and makes it easier to go faster. Engraving takes forever if at least one axis can't go fast. Do the math on your router for a 4" square engraving at your fastest speed and 0.002 stepover. (4" / 0.002 = 2000 scans).

You might get a little smoother with higher step rates, but you are unlikely to get better real world resolution. If you are really interested in smoothness, go with servos instead of steppers.
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Re: Rack & Pinion with Retina Controller

Postby PSG » Mon Dec 13, 2010 9:10 pm

Agreed - servos are the better way to go. Considering the needed size, they ought to be cheap enough. The biggest problem I have with servos is me.... I know absolutely nothing about setting them up. Steppers I know but I will not discount servos.

After a discussion with another friend, that is also considering a build of his own, we have decided that we really should look at belts vs. racks. I am theoritically ok with belts, as long as I do not deal with screws (did that once & prefer other methods of motion)....... No offense to those that use screws, just a personal preference.

Will the Retina work with servos? I am stuck on that thing - very impressive. Btw...are they still possibly looking for beta testers?? :)
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Re: Rack & Pinion with Retina Controller

Postby bdring » Mon Dec 13, 2010 9:47 pm

You want servo drivers that accept step and direction just like steppers. The encoder you choose determines the "steps" per rev. There is no difference from a control standpoint.

You do need to tune the servos. Be sure to look into the tuning requirements. Some require an o'scope to tune. I use a servo on my big router's Y axis. It is super smooth and quiet, but I think the steppers on the other axes work just as well.
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Re: Rack & Pinion with Retina Controller

Postby PSG » Mon Dec 27, 2010 1:05 pm

Bart,

From what I have read so far, it seems that the Retina is a superior controller. I did read that the Retina requires a calibration spec of 1000steps/in. I am assuming that this is required on all axis.I still want a 4'x8' table with a gantry mounted tube. I plan on using V-wheels and I have gone back and forth between belts and rack (screws still out of the question), at least for the "X". I am not that concerned about the "Y" because the "Z" plate has virtually no weight. The gantry will most likely be constructed of aluminum, to keep the weight down. I have already figured that I will be doing all of the raster (engraving) work with the "Y". As a side note, for the stepper drives I plan on using the Geckos.

Here is my dilemma....

1) My belt calculations indicate that I would need a 10 tooth pulley for an XL belt. This seems rather small for a belt that would most likely be 3/8" in width and 16' in length (8'x2). The concern is belt strength/stretch and ample tooth contact.

2) My rack calculations fail to come up with a "even" reduction ratio. In other words, instead of a reduction of 1.5, I end up with a calculated reduction of 1.572. This yields steps per inch at 1000.766

Is the Retina controller the only way to achieve the results we have seen, for a machine that cuts and engraves?

Is the 1000 step/in. requirement to mean an exact 1000 (I am assuming that it is)?

How does the LightObject controller compare to the Retina controller?
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Re: Rack & Pinion with Retina Controller

Postby twehr » Mon Dec 27, 2010 1:30 pm

I use the LightObject DSP and am very happy with it. During the setup, you can specify exactly how far your axis moves for each step, allowing you to dial it in very accurately.

As for the 1000.772 vs 1000. That requirement simply means that their output is based on 1000 steps for each inch. If your system is at 1000.772 steps per inch, it means your final output will be slightly smaller than intended. (.772 / 1000 = .000772 or 0.0772%) If you can live with an output of 0.9993 inches instead of 1.0000 inches, you will have no problem. Looks to me like a good match if you want the Retina Engrave. If you want it to be more exact than that, you will have to go with the DSP.
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Re: Rack & Pinion with Retina Controller

Postby gavztheouch » Mon Dec 27, 2010 5:15 pm

A good source of cheap servos is Ebay, there is a consistant supply of full servo packs - drivers, motors and cables from brand names like Yaskawa and Panasonic can be bought for relatively small sums of money ie.. $150 per axis secound hand from machine salvage. The plus point is these servos which normally cost 1000's of dollars new come with some nice features like automatic servo tuning and circuit protection.
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Re: Rack & Pinion with Retina Controller

Postby lasersafe1 » Tue Dec 28, 2010 12:27 am

I think servos would be unwise for this application. The retina engraver and other controllers are created to act in real time and fire the laser based on a known position. With steppers, if you give it a step, it is there "now", without having to read an encoder to verify the position. With servos, there can be a small time lag depending on the tuning. Expensive doesn't mean better. The simple NEMA 17 steppers are just fine for lasers with a lightweight mirror assembly.
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Re: Rack & Pinion with Retina Controller

Postby macona » Tue Dec 28, 2010 6:32 pm

Servos will be fine. Even with my mitsubishis on my lathe with a load they are never off more than a couple encoder counts and that a 15000 count encoder on the motor.

You do need to do your research on the servos before buying them. Many companies offer them in different flavors. Many use modbus, sscnet, or other industrial networks to communicate to the drives. These are useless here. You need drives that handle step and direction pulse inputs. Mitsubishi drives that do this have a model number that ends with a "A" Like MR-J2S-40A. Yaskawa drives have a "P" in their model number to indicate positioning drive (Step/dir). Many of Omron drives are actually rebranded Yaskawas.

With Japanese manufactured drives it is rather difficult to mix and match motors. The drives are intended to be used with a specific motor, especially newer ones.Older drives tended to use standard quadrature encoders but the new drives all use serial interface encoders. American and European drives are more versatile. Drives like the ones made by Allen Bradley are fully configurable and almost all the motors have standard quadrature outputs. So as long as you know the specs of the motor you can configure it to work. I am using Elmo drives on my laser.

Another thing to watch out for is input voltage. The smaller drives up to 400 watt are available in 100v (100v to 120v) or 200v (200v to 240v) classes. Mitsubishi drives that are 100v have a 1 a the end of the model number, so a J2S series, 400 watt drive (~1/2HP), 100v input, is a MR-J2S-40A1. Yaskawa drives have a similar designation method. In the last two letters of the part number there is an A or B. A signifies 200v series and B is 100v. One major thing to watch out for with Yaskawa motors are drives versus Mitsubishi. Mitsubishi uses one motor for both the 100v and 200v series drives, Yaskawa does not. There are specific 100v motors and 200v motors.

I recommend using the Mitsubishi or Yaskawa drives. They both have great customer service.
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Re: Rack & Pinion with Retina Controller

Postby macona » Tue Dec 28, 2010 6:34 pm

Oh, supposedly you can modify the steps per inch setting in the parameter XML for the RetinaEngrave.
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