Glass Scale with UC400ETH

[marndra]

Hello,

could anybody say if it is possible to use a glass scale with my UC400ETH?

Software I want to use UCCNC.
I have a triplebeast as motordriver and a Benezan Breakoutboard for several in/out switchings.
The glass scale I hope I can use such:
https://www.aliexpress.com/item/Ip55-Nine-pin-TTL-Output-Signal-linear-Glass-scale-for-measuring-length/32866997667.html?spm=a2g0x.10010108.1000001.8.592c36b5YCpNDC&pvid=f1cc530e-e2e4-4ba0-afb4-6e6da537dc89&gps-id=pcDetailBottomMoreThisSeller&scm=1007.13339.90158.0&scm-url=1007.13339.90158.0&scm_id=1007.13339.90158.0

Motor I have the https://mecheltron.com/sites/default/files/webresources/MechanicalElectroMech/StepperMotors/pdf/datasheets-steppermotors/86BYGH450BD.pdf

[Robertspark]

what do you want to use the glass scale for??

UCCNC cannot offer closed loop control, that is done by the servo / hybrid stepper driver.

[spumco]

So here's a question - not a complaint or even a feature request...

What would it take to close the loop with external linear encoders? Whatever flavor quadrature encoder - glass, magnetic, inductive...

I'm sure there are some hard limitations with the encoder pulse count vs. axis speed vs. resolution and the maximum 400khz kernel speed. But I'm curious what it would take in general terms in the software (or UC300/400 hardware).

While there are probably more, I only know of a couple non-OEM control systems capable of this. Galil - yes. Kflop and LinuxCNC - maybe?

Just curious.

-Ralph

[cncdrive]

Closing the loop with step/dir interface and stepper motors would be very unoptimal and about useless.
You will probably ask why?
Because steppermotors have inverse speed/torque graph and if the motor looses step is usually because it has an extra unwanted torque on it's shaft and what a PID loop would do in case the position error is growing? It would higher the control signal commanding the motor to run faster. What would then happen? Ofcourse the motor could not run faster, because of the inverse speed/torque graph. A stepper motor should be slowed down in this scenario.
And even if you somehow trick this issue out then still you would have to synronise all the axes loops otherwise this slowdown of one axis would create a large temporarily error between the axes. So, again not good.

How closed loop would be useful is with servos with speed control loops, because servos speed/torque graph is not inverse.
And it is simpler to even syncronise the speed loops together. And so ofcourse this is how it is done in industrial controllers.
In newer ones it is done not with analog command signals, but via etherCAT or CAN etc. bus packets, but the analogy is the same, speed loops inside the position loop, but for that you need servos...
And syncronising the position loop with the speed loops would be impossible with our system, because Windows is not realtime and the motion commands are in a communication buffer and so it is not possible to instantly intervene into the position loop on the computer side since there is a buffer in between the motion controller and the PC.

So, closing the loop in the step/dir line with open loop stepper drives would be about useless in my opinion and how it should be done with a system like that is impossible with our current controllers.

[Robertspark]

Interesting...

Does that mean what is called a hybrid stepper drive (stepper motor + drive + encoder) is pointless / a marketing gimic?

[cncdrive]

Those closed loop steppers are totally different thing, they control the stepper motors directly and so the loop is closed not in the step/dir line, but in the current loop of the stepper drive.
The position of the rotor is measured and the controller has access to the current control and it has 2*PI current controller loops and a lookup table for the electrical commutation and so the PI controllers can change the commanded current to the motor coils based on the rotor position. So, the inverse speed/torque issue in this case is not a problem, it is resolved...
Ofcourse doing this is not possible with a motion controller because it has no access to the current controller of the stepper motor and not the motion controller is controlling the current in the motor coils and so the inverse speed/torque graph of the stepper motor problem can't be resolved.

[laki]

Closed loop can be achieved with the help of such a driver.
https://www.machdrives.com/bra.aspx.

A complete loop (with the use of glass scale) is performed on the driver side.

[cncdrive]

But that is still not what the OP asked about. He asked about closing the loop in step/dir lines and ofcourse this drive you linked does not do that, the step/dir line is still open loop.

[spumco]

Thanks a ton for the very clear explanation.

Steppers - bad idea as you can't make them 'catch up' if they've already lost steps.
Control software/hardware - can't really use if there is any meaningful delay between the linear scale feedback and the moton controller (Windows).

The linear scales to the drive (MachDrives) looked interesting, but reading closely they're only for DC servos. If they offered something similar for AC servos they might be more interesting.

[cncdrive]

Yes, exactly, they can't catch up with speed if they stall because of the step/dir interface connection and the inverse speed/torque graph of the stepper motors.

For some time we working on a 6 axis closed loop motion controller with analog outputs instead of step/dir (earlier I've posted some pics of the prototype), unfortunately we had to put the project apart for some time now, but we will get back to it to finish it.
That controller has analog +-10Volts output for servo drives and you connect the encoder back to the controller.
There is no step/dir interface at all. The servo drive can be DC or AC servo with analog +-10V input interface and the drive should be configured to torque or speed control.
Then the position is closed inside the motion controller, you setup the position PID loops inside the motion controller (in the UCCNC).
So, we working on a closed loop solution already, but the development goes slow, because we have lots of other ongoing projects and developments too.