Hydraulic Linear Motion Precision Controll

Start with an ordinary analog servo config, Then where the pid's output would be connected to the analog output command. You would connect that instead to your stepgen's position command. If you are using a Mesa hardware stepgen I wouldn't bother with using the usual PID to stepgen velocity command setup. I think i'd just connect directly to the stepgen's position command input.

You might find that the existing screws and other intermediate hardware between the stepper and the control valve are nessisary to have the required resolution to adequately control the valve with the stepper motor. (This is just a guess not having seen how it is really set up.) If that isn't the case simpler is almost always better.

Hi Guys,
It's been a long time since the last update. At least 5 versions of linuxcnc
I have done some interesting modifications to the machine.
I have attached the valve shaft to a lead screw with backlash compensated nut. 
Later, I am going to replace that with the Ball Screw.
Then, the valve is moved via a timing belt connected to the stepper motor.
With a 2mm screw pitch It is possible to locate valve shaft with decent acuracy.
Please see the attached picture.
For the past couple of weeks, I've been trying to tune the PID for that setup. But it is hard.
It oscillates a lot, or it is not compensating the steady state error.
I would love to hear your advice on a couple of points:

1) What is the typical following error for an axis with a linear scale of 0.001mm resolution(1 micron)? In steady state it is able to position the axis within +-0.005 accuracy. Do you think it is possible to get more from that scale resolution?
During acceleration and deceleration, an error jumps up to 0.1mm at 1500mm/min speed. What is a typical result for well well-tuned PID loop?
2) Do you think I should set up a two-stage PID control loop? One stage is to control the position of the valve shaft based on the velocity feedback from the scale. And that another loop to control the position and give a commanded velocity as an output?

Thank you!

PS. I do have some videos that I can share in case you are interested.


 

Hello, yes your editing is very interesting, don't hesitate to share the videos and share, I didn't know this operating principle, thank you

1) What is the typical following error for an axis with a linear scale of 0.001mm resolution(1 micron)? In steady state it is able to position the axis within +-0.005 accuracy. Do you think it is possible to get more from that scale resolution?

Typically, some I term is added to reduce the static error.

During acceleration and deceleration, an error jumps up to 0.1mm at 1500mm/min speed. What is a typical result for well well-tuned PID loop?

That mostly depends on the characteristics of the controlled device (delays, linearity etc)

Can you post a Halscope plot of following error and velocity?