Closed loop with steppers

Hello everybody.

I'm a little new in LinuxCNC , and I wanna make a simple question.

How could I implement a closed loop for a stepper motor in LinuxCNC?

I'm gonna to do the most clear that I can to explain my problem.

I have a machine that have different backlash in different points. I have a very precise rulers that can measure the position of the tool and I want to connect this rulers to LinuxCNC (I guess that is possible with HAL) and correct the trajectory in real time creating the closed loop.The trajectory correction , I guess in this case, is only add more pulses to the motor if the tool not reach the indicated position by G-Code. I understand that in theory is simple and I read that is possible but I don't know how that is possible.
I read, also , that is possible using a PID but I don't know if it is real or no.

Beyond my previous question , I want to know if this is implemented in LinuxCNC or I must design a HAL component to do it possible.

Thanks!

You would do it using a PID loop, but be careful step motors do not always respond well to this. (it can lead to stalling)

The problem being if the pid is detecting a stall, it will command the stepper to move faster to catch up which will insure that it will stall, the opposite of what is needed to prevent a stepper from stalling.

Ok thanks Todd , but after I researched in many sites I reach at the conclusion that I must write a HAL component because the PID will cause problems, but first I need understand how PID is configured in HAL file I guess. Basically I guess I need a very good example of PID configuration , I write to you because the linuxcnc's forum is large and maybe you know about a very good one configuration example .
My first doubt is how configure the 'feedback' in HAL with PID. If I know that, I guess that I can start with some tests

I will apreciate your answer.

Bye!

Heres a schematic I did (one axis shown only), with a couple of other contributers links. I don't think you need a new component. I use 2000 ppr encoders in quadrature with index. It's useful for keeping track of true position, and homing.