Kinematics and Delta Robot

Hi Everyone,
This is my first foray into CNC anything, and can already tell I've bit off quite a challenge.
I work in servo drive development, specifically in controls, so I have that part down, but the rest (including Linux) is foreign to me.

I acquired a Delta Robot with high resolution servo motors and 25:1 gearboxes:


And i have servo drives i have configured for step/direction inputs:


And I plan to mount a laser to the delta and use it for a laser etching machine.

So far, using all the tutorials I could find so far, I've been able to:

• Install Linux and locate a machine with ~15000us jitter
• Using Debian 9.8 running x64 with PREEMPT_RT
• Installed LinuxCNC 2.7.14
• Configure Parrallel port for three axis step/dir
• Servo drives are receiving step/dir signals and they move the correct distance LinuxCNC moves

Now, for the thing I have not been able to figure out:
How to setup the inverse kinematics??

I've been through IK work before with Scara robots and seen some sample Delta IK code, but I'm confused on how to utilize the kinematics layer in LinuxCNC.

Step by Step guides seem sparse and I seem to have run out of things to try.

Any assistance would be greatly appreciated!

Best Regards,
Paul

Hi
Welcome
First are you driving the servos with step/DIR signals
loss of Speed
you shoudt go for a MESA board 7i77 for real servo with encoders
or a 7i96 board for Steppers only
the kinetics is online there are also configs that show you in MASTER 2.8 alredy configured just hook up your pins as you need them
forum.linuxcnc.org/10-advanced-configura...lta-robot-kinematics
good point to start

Thanks for the welcome!

"First are you driving the servos with step/DIR signals"
Yes I am, I know this is not ideal, I am sacrificing a lot of the high performance drive loops as well as the 24bit feedbacks on the motors. But since this is my first attempt at anything cnc or linux, I thought this would be a good baby step.
I had pre-emptively bought a 7i92M board as i had planned to run this on a laptop without a parallel port - but i couldn't get the RT latency test to get under 300us (despite it being a pretty high performance laptop it had intermittent spikes i couldn't nail down).

I did stumble on some posts about someone having a branch of LinuxCNC supporting EtherCat - which would be the logical next step since I can configure these servo drives for nearly any industry fieldbus (ECat, CAN, Profinet, EthernetIP, and SynqNet).
But like I said, baby steps...

As for the Kinematics; "good place to start"
I stumbled upon the thread you mentioned already along with the nice deltakins.c code which looks perfect. However I couldn't figure out what to do with it.
I also stumbled upon this thread which seemed eerily similar to my troubles:
forum.linuxcnc.org/24-hal-components/327...matics-again?start=0

He also was encouraged to pull the master 2.8 version instead of the released stable version of 2.7.

Is this what you are alluding to with your reference of MASTER 2.8?

While being a seasoned Windows slave/developer and embedded developer for the better part of 2 decades, I've never even dipped a toe in Linux development so I don't even know where to start (it took me a few frustrating nights to figure out how to get linux switched to a RT kernel and run the latency test to pass well with boot modifications).

I have't been able to find a step-by-step guide to compiling the deltakins and installing it/using it.

Is this setup/structure significantly different/easier in the master build 2.8?

300 usec is probably OK as long as you can reliably run a Ethernet
servo thread at 1 KHz or so, even 500 Hz may be OK if you do not have
very high joint acceleration.

Ok Thanks, I'll keep that in mind - tried 6 machines and just stuck with the machine that gave me the lowest jitter. I put windows back on that machine in the meantime, but I can always go back.

Right now my challenge is learning how to compile and install this deltakins module (the laptop would also need this).

I'm not sure what the frequency limitation of the parallel port is, but the high speed digital inputs on my servo drives is 3MHz, so I should be able to get something working.

I'm closing the Position loop on the drive at 4kHz and the Velocity loop at 16kHz and electronically gearing to a Step/Dir input. I have quite a bit of tuning/filtering available in those loops so I can handle a heavily quantized input coming in on the step/dir channel.

If you install the 2.8 (development) version of LinuxCNC than "rotarydeltakins" is built-in.

You just need In the INI file. If you create a generic config then that will use trivkins. You just need to edit one entry to say rotarydeltakins.

It seems that the kinematics creates a set of HAL parameters which you need to "setp" to the correct numbers for your robot in the HAL file.

github.com/LinuxCNC/linuxcnc/blob/master...otarydeltakins.c#L68

Suggests that you should see pins called:
rotarydeltakins.platformradius
rotarydeltakins.thighlength
rotarydeltakins.shinlength
rotarydeltakins.footradius

I don't know if that is a standard naming scheme, but it seems pretty self-explanatory.