On 2/19/20 4:16 AM, Les Newell wrote:
On 18/02/2020 23:54, Marshland Engineering wrote:
Just a correction, I have setup a few stepper systems with LinuxCNC
and step
and direction is no problem. The problems came in when using 2KW
servo drives
and linear scales on the axis.
The subject of using linear scales for feedback pops up every so often
on this list and always the recommendation is not to do it. I just
checked back through the list history and you were also told this.
It's not a LinuxCNC or competence issue. It's a hardware issue.
Feedback loops like a very stiff connection between the motor and the
encoder. Even a tiny amount of backlash or spring has a major effect
on loop stability. Have you ever tried driving a car with badly worn
steering? It's hard to keep a straight line. Linear scales have have
the same effect on the feedback loop. If you fit encoders directly on
your motors most of your tuning issues will go away. If the machine is
very stiff and has very little backlash you MAY get away with using
velocity mode drives with your scales. Even then it's likely to be
twitchy to set up. In fact this was also suggested to you at the time.
Using linear scales for secondary feedback in addition to encoders on
the motors can in some cases improve accuracy but this is a pretty
advanced configuration and off hand I don't know of anyone who has
done it with LinuxCNC. It is done on some high end commercial machines.
http://wiki.linuxcnc.org/cgi-bin/wiki.pl?Combining_Two_Feedback_Devices_On_One_Axis
I have purchased and tried a 80V 20A Servo Drive upgrade Geckodrive
G320X
DG4S-08020 DCS810 from ebay and it worked well on a small motor.
See above. A motor with a directly connected encoder is easy to set
up. If you tried the G320 on your linear scale setup you would have
had exactly the same tuning issues.
I think having the servo tuning external to the PC is a good option.
This makes the
LinuxCNC servo installation much easier.
I have used position mode drives (step/direction), velocity mode and
torque mode with LCNC, Mach, Eding CNC, my own custom controller and
others. I've tested Gecko, Mesa, Rutex, Granite Devices, Xylotex,
Kinco, Bosch, Yaskawa and others in various configurations so I have a
reasonable sample set. In my opinion the best control scheme overall
is torque mode with LinuxCNC doing the feedback. Velocity mode comes
second and position mode (step/dir) a long way behind.
The drives like Gecko, Rutex R9x series and Xylotex are very basic and
you have no way of knowing what following errors you are getting until
the drives trip. It's difficult to get a really good tune without
knowing what the drive is doing.
Configuring digital drives such as Kinco, Yaskawa etc with a couple of
buttons and a tiny LED display is frustrating to say the least. GD,
the later Rutex drives Kinco, and many other digital drives have their
own tuning software (mostly Windows only) but I haven't yet found one
that is easier to use than LinuxCNC with Halscope. Some drives have
auto tuning but I've not yet seen auto tuning that does a really good
job.
My Hurco mill came with Yaskawa brushless servos in velocity mode. The
velocity tuning in the drives was very touchy and I struggled to get a
good balance between accuracy and oscillation. Some hunting is to be
expected with velocity drives but this was too much for my liking. I
then switched to torque mode and my problems went away. No hunting and
much lower following error. My lathe is torque mode with Mesa 2KW
drives. Again very smooth, no hunting and very low following error.
They are not without their issues though (see below). My router uses
Bosch analog velocity mode drives. Following error is adequate for a
router but being velocity mode they do hunt a bit when the machine is
stationary. I will say velocity mode is generally a little easier to
tune than torque mode so if you aren't fanatical about pushing the
limits of your machine and you can put up with the hunting it's not a
bad way to go.
Here are some of my impressions of various drives:
Gecko G320. Step/dir DC brushed. Rather rudimentary feedback loop.
Fixed following error limit. You need to take them apart and attach an
oscilloscope to tune them properly. I suffered from issues with random
tripping that I never fully solved. Physical pots to adjust which is
nice. Would I buy again? No.
Mesa 7i29. Analog PWM DC brushed. Cheap and powerful. Generate lots of
electrical noise so it's well worth adding some filtering. I had
issues with noise pickup on the encoder inputs, to the point where I
ended up using them for my handwheels and used a separate encoder
board for the servos. The onboard dc-dc converter let the smoke out
once but I managed to fix it by replacing a couple of small surface
mount transistors. I've run my lathe on this drive for many years and
the DC-DC converter circuit looks pretty well designed so I am
chalking this one down to random bad luck. Excellent support from
Mesa. They even sent me schematics. Would I buy again? The price is
tempting but the encoder noise issues put me off. Might be something
specific to my setup but I'm not willing to take the chance. I would
be very interested to hear anyone else's experiences with this drive.
Rutex R9x series. Step/dir DC brushed. Proper PID loop. Physical pots
to adjust. Some control over the following error limit but no way to
monitor the following error while tuning. Not bad drives but they have
little protection and have a reputation for letting the smoke out when
pushed hard. I never killed one even running some pretty huge motors.
Would I buy again? Possibly if I needed step/dir. It's a bit of a moot
point as they are no longer made. I have no experience with the later
models.
Granite Devices. Analog or step/dir, brushless or brushed. I can't
remember the model number and this model is out of production. The
config software requires a special USB dongle and is a bit laggy and
awkward to use. It took a while to get the tuning sorted in step/dir
mode. I had some problems with their power up sequence but apart from
that they have been rock solid reliable in heavy industrial use. Would
I buy again? Maybe but overall I prefer the Kinco drives.
Xylotex. step/dir DC brushed. If you want a small no frills
step/direction DC driver they are a reasonable bet. Physical pots to
adjust and no control over the following error. Would I buy again?
Probably if I needed a small step/dir driver. This one is a bit of a
niche market due to it's size.
Kinco brushless. Analog or step/dir. Can't remember the exact model
number. Configuration through operator panel (painful) or RS232. The
Windows software is a bit quirky but mostly gets the job done, though
documentation could be better. Unfortunately my customer point blank
refused to use Linux so I used Eding CNC with step/dir and I ended
having to do a couple of return visits on site before I got the tuning
dialed in. The machine's been operating in an industrial environment
for a couple of years with no more issues. Direct mains power so easy
to install. Separate 24V DC power for the encoder and logic so you can
track the encoder while in estop. Would I buy again? Yes, though I'd
try to avoid step/dir.
Bosch analog. These are 80's era velocity mode DC brushed drives and
still trucking on. Huge, annoyingly noisy due to their low switching
frequency and so much circuitry compared to modern drives. Designed to
run on 110V three phase but will work on 110V single phase. They work
well but due to their age are crazy expensive to get repaired. I'm
told they can do odd things if the tacho brushes wear but I've had no
problems. If they ever pop I'll replace them with something a few
decades newer.
Yaskawa brushless. Analog or step/dir. I'm too lazy to go into the
cold workshop and check the model number. These are fairly old models
and configuring through the operator panel sucks big time. Velocity
mode is a pain to set up and very sloppy. Didn't try step/dir but if
vel mode is anything to go by, probably a nightmare to set up. Work
well enough in torque mode. Can't monitor the encoders in estop which
is a pain.
All of the above is my personal experience and opinion. Your mileage
may vary.
Les
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