Digital control on a dividing head

@MikeInOr - just too many discussions going on!
I took a good look a the Parker SX6 stuff and, yeah it does look good. Your comment about handling missed steps intrigues me. Can that be done without adding an encoder? In other words, is there some feature in with the SX that will detect it account for it?

Based on my research, It looks like I'd need a single SX6 controller, ~80-100$ on ebay and a stepper with an encoder for another ~100-150$ ish. The SX clearly has a very rich feature set based on my first read of the manual.

I also took a good look at Automation Direct SureStep, and for about the same price I'd get a new integrated motor/controller and a power supply. It would however still require me to use one of my arduino boards to provide step & direction pulses. More work, fewer features.

Do you think I'll want an encoder on the stepper driving the dividing head?

-Dave
 
dbb-the-bruce said:
I also took a good look at Automation Direct SureStep, and for about the same price I'd get a new integrated motor/controller and a power supply. It would however still require me to use one of my arduino boards to provide step & direction pulses. More work, fewer features.
Click to expand...

They are expensive, but no step & direction input needed. They can be commanded by an extensive command library, I have one I run that way over a RS232 link. See the SureStep programming manual. https://cdn.automationdirect.com/static/manuals/surestepmanual/scl_manual.pdf

dbb-the-bruce said:
Do you think I'll want an encoder on the stepper driving the dividing head?
Click to expand...

Probably not needed, but nice to have. I think standard on the integrated motor/controllers. This is a new product and I'm not familiar with it yet.
 
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dbb-the-bruce said:
@MikeInOr - just too many discussions going on!
I took a good look a the Parker SX6 stuff and, yeah it does look good. Your comment about handling missed steps intrigues me. Can that be done without adding an encoder? In other words, is there some feature in with the SX that will detect it account for it?

Based on my research, It looks like I'd need a single SX6 controller, ~80-100$ on ebay and a stepper with an encoder for another ~100-150$ ish. The SX clearly has a very rich feature set based on my first read of the manual.

I also took a good look at Automation Direct SureStep, and for about the same price I'd get a new integrated motor/controller and a power supply. It would however still require me to use one of my arduino boards to provide step & direction pulses. More work, fewer features.

Do you think I'll want an encoder on the stepper driving the dividing head?

-Dave
Click to expand...


Because the stepper controller and stepper driver are integrated into a single package the controller is able to monitor the current going into each step of the stepper motor. By monitoring the current the controller is able to detect events way beyond the ability of seperate controller / driver solutions. ...it has been many years but MY recollection is that the controller can detect some missed step events WITHOUT the need for a seperate encoder... but my recollection could be wrong. I specifically remember the ability of the SX to detect stall conditions without an encoder. I know the SX controllers have encoder inputs and can definitely track missed steps with the use of an encoder.

If it were me I would implement without the encoder... then add one if needed... which I doubt it will. I don't think you will have to worry about missed steps unless you are driving your stepper motors REALLY hard... which I do not think you will be.

My understanding is missed steps generally occur when operating a stepper motor at the upper end of its torque range. There are several parameters that you enter for the specific stepper motor you are using that the SX uses to gauge where in its envelope a stepper motor is operating. Of course there are built in parameter sets for Parker brand (CompuMotor) stepper motors so you don't have to configure each parameter. :) These parameters are used to detect missed steps (I recall) but also vary the wave form put out for the particular operating condition of the stepper.

My applications were not so accuracy critical that I worried about missed steps... so my understanding is from what I remember reading quite a while ago... not any direct experience.

I have a couple SX6's and an SX8. The SX6 is a nice some what compact size, the SX8 is about double the width of the SX6 and quite a bit heavier. From my completly novice familuarity with the SX drives I think the SX6 drive is a great match for your application... I think an SX8 would be unnessisarily big and bulky.

I also recall the ability of the SX drives to operate in different modes. There was a follow mode where the SX would drive its stepper motor to follow another input. When steppers are driven at a slow speed they no longer freely spin instead the rotation is a series of many rapis starts and stops which induce vibrations into the movement. At slow speeds the SX drives change the wave forms sent to the stepper motor to make the rapid starts and stops into a smooth motion.

There are also provisions to chain together multiple SX drives to operate together... I think this would be along the lines of... drive one stepper to go from one limit to the other limit and quick return to the first limit then tell a second SX controller to index its stepper a set amount before the first SX goes through its cycle again. There is another thread on here where someone wants to automate his surface grinder so he doesn't have to spend hours behind the wheels. I think a pair of SX drives would be a good solution for this... but the poster found an application specific surface grinder stepper controll to do exactly what he wants without any additional programming.
 
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How do you imagine your automated rotary table working? Dial in x number of steps, press a button and have the stepper motor move that number of steps? Inputting X was a bigger project than I expected with the SX controlling my table saw fence... I ended up using a bcd encoded 4 digit thumb dial where the first 2 digits were inches and the second 2 digits were 64ths of an inch. It worked well but was a VERY basic human interface and took a bit of doing to make it work... something you might want to consider when selecting your solution.

Wow... that automation direct stepper controller manual looks a LOT like the Parker SXdrive manual. :)

Like the Automation Direct controller the SX controller can be run via serial interface from a PLC or PC. It definitely gives you the ability to implement a much nicer interface... but also takes away from the simplicity of a single box solution.
 
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dbb-the-bruce said:
As far as direct driving a chuck for a forth axis, sure that would work but I've already got (almost, just need to put it back together) a functional dividing head that I can mount my chucks and face plates on.

I'm looking for a single axis, wire up a stepper to a controller and talk serial to it that's on the cheap side.

A simple encoder readout could also work, but it would be nice to not have to do the cranking by hand.
Click to expand...
It's likely that the divider head will have some ratio (40:1 is typical) from its worm drive, and to really do it right,
you'd want the encoder on its worm gear, not on the motor that drives it (because the motor shaft coupling
isn't perfectly free of backlash), and have a motor with position precision on the order of 1:1000 or better
on its output shaft. Even a fine stepper goes 1.8 degrees per step (so, 1:200 granularity:==step size).
That will let you do as good a job with the readout/motor as a pins/plates setup, and ought to be
achievable. It's likely that you'll want to apply some retarding torque on the divider during motion
(to prevent backlash at the worm), and it would be convenient to activate the brake
or clamp that holds the position (so you could power-down the stepper during a cut). The
usual 'positive locking mechanism' will take a separate actuator, not dependent on the motor shaft.

Stepper acceleration/deceleration control is relatively easy with digital control, I've done it with analog
as well. But, a good readout/encoder on the worm shaft is relatively difficult, if you want to match
the precision of plates/holes indexing.
 
It's ALIVE!!

I ended up going with the smallest Automation Direct integrated driver/motor/encoder I could get. Works great! the 40:1 ratio of the worm means that there is very little torque needed for the motor. I've currently got the acceleration set low with a reasonably fast max V - that's why it takes so long to wind up and down. Most of the time you are making short steps and hitting it accurately is the most important part - however, if you do make long steps you don't want to wait forever to get there - doesn't really matter as it all very easy to change and I haven't scripted the user interface code yet.

The fact that drive, motor, and encoder are all one unit makes getting this hooked up and working EXTREMELY easy - hook up a power supply and serial cable and you are done. This is exactly what I was looking for.

The hardest part was figuring out the RS485 and getting the manufacturer's software working correctly - this was complicated by the fact that I had to download/install/setup a Windows Virtualbox and was using a new unproven RS485 USB dongle etc, etc. Still, all in all, it was only a couple of hours before I had the motor spinning.

Regarding resolution/accuracy - As expected, this is currently limited by mechanics of the dividing head. The worm shaft has about .0005 end-to-end (axial?) play in it. This can be adjusted tighter, and I'm going to give it another shot to see if I can reduce it more. The other adjustment is the worm engagement which seems to be just about perfect right now. The tricky part is it's difficult to change one without changing the other. Pretty sure I'll be able to improve it. Right now there is about .0015 backlash/slop measured tangentially at the rim of the headwheel/plate (about a 2" lever - small diameter parts will have proportionally less error).

With the motor micro-stepping and encoder feedback, I've got way more resolution than I need. The controller has an EG - electronic gearing parameter that can be set to determine any number of counts/rev up to some huge number. I've set it up (for now) so that 1 count == .001 deg at the dividing head. With mechanical backlash at about .02 deg (for now), I'm good.

Cost - As I said earlier in this thread, while not wanting to burn money, the cost was less important than time to get it running.

$296 for integrated driver/motor/encoder
$127 for power supply
$35 for flex coupling (motor to dividing head shaft)
$20 for USB RS485 dongle

I hemmed and hawed about getting the motor with the encoder, which added an additional $50 - I knew that I didn't need it. If the drive and micro-stepping worked as advertised, however, it would really suck to get to the 101st tooth on a 127 tooth gear and lose your place! I also justified it knowing that going forward I could move this motor/encoder around to other projects that come up.

So I could have saved ~$50 on the motor, I'm sure I could have gotten a cheaper linear power supply somewhere else but it would have been more of an unknown. I also could have shopped for used parts - the SX6 stuff that Mike recommended came to around the same ~$300 - $400 but being used would have had a lot more unknowns.

Here is the video I did of the tear down / clean up of the dividing head (it's kind of boreing and 24 min long but there is some interesting stuff in it).

I'm working on another that details all of the improvements I made to it other than adding the stepper motor, I'll post it when it's done.

-Dave
 
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