Stepper Configuration

1. Introduction

The preferred way to set up a standard stepper machine is with the Step Configuration Wizard. See the Stepper Configuration Wizard Chapter.

This chapter describes some of the more common settings for manually setting up a stepper based system. These systems are using stepper motors with drives that accept step & direction signals.

It is one of the simpler setups, because the motors run open-loop (no feedback comes back from the motors), yet the system needs to be configured properly so the motors don’t stall or lose steps.

Most of this chapter is based on a sample config released along with LinuxCNC. The config is called stepper_inch, and can be found by running the Configuration Picker.

2. Maximum step rate

With software step generation, the maximum step rate is one step per two BASE_PERIODs for step-and-direction output. The maximum requested step rate is the product of an axis' MAX_VELOCITY and its INPUT_SCALE. If the requested step rate is not attainable, following errors will occur, particularly during fast jogs and G0 moves.

If your stepper driver can accept quadrature input, use this mode. With a quadrature signal, one step is possible for each BASE_PERIOD, doubling the maximum step rate.

The other remedies are to decrease one or more of: the BASE_PERIOD (setting this too low will cause the machine to become unresponsive or even lock up), the INPUT_SCALE (if you can select different step sizes on your stepper driver, change pulley ratios, or leadscrew pitch), or the MAX_VELOCITY and STEPGEN_MAXVEL.

If no valid combination of BASE_PERIOD, INPUT_SCALE, and MAX_VELOCITY is acceptable, then consider using hardware step generation (such as with the LinuxCNC-supported Universal Stepper Controller, Mesa cards, and others.)

3. Pinout

One of the major flaws in LinuxCNC was that you couldn’t specify the pinout without recompiling the source code. LinuxCNC is far more flexible, and now (thanks to the Hardware Abstraction Layer) you can easily specify which signal goes where. See the Basic HAL Tutorial for more information on HAL.

As it is described in the HAL Introduction and tutorial, we have signals, pins and parameters inside the HAL.

Note	We are presenting one axis to keep it short, all others are similar.

The ones relevant for our pinout are:

signals: Xstep, Xdir & Xen
pins: parport.0.pin-XX-out & parport.0.pin-XX-in

Depending on what you have chosen in your .ini file you are using either standard_pinout.hal or xylotex_pinout.hal. These are two files that instruct the HAL how to link the various signals & pins. Further on we’ll investigate the standard_pinout.hal.

3.1. Standard Pinout HAL

This file contains several HAL commands, and usually looks like this:

# standard pinout config file for 3-axis steppers
# using a parport for I/O
#
# first load the parport driver
loadrt hal_parport cfg="0x0378"
#
# next connect the parport functions to threads
# read inputs first
addf parport.0.read base-thread 1
# write outputs last
addf parport.0.write base-thread -1
#
# finally connect physical pins to the signals
net Xstep => parport.0.pin-03-out
net Xdir  => parport.0.pin-02-out
net Ystep => parport.0.pin-05-out
net Ydir  => parport.0.pin-04-out
net Zstep => parport.0.pin-07-out
net Zdir  => parport.0.pin-06-out

# create a signal for the estop loopback
net estop-loop iocontrol.0.user-enable-out iocontrol.0.emc-enable-in

# create signals for tool loading loopback
net tool-prep-loop iocontrol.0.tool-prepare iocontrol.0.tool-prepared
net tool-change-loop iocontrol.0.tool-change iocontrol.0.tool-changed

# connect "spindle on" motion controller pin to a physical pin
net spindle-on motion.spindle-on => parport.0.pin-09-out

###
### You might use something like this to enable chopper drives when machine ON
### the Xen signal is defined in core_stepper.hal
###

# net Xen => parport.0.pin-01-out

###
### If you want active low for this pin, invert it like this:
###

# setp parport.0.pin-01-out-invert 1

###
### A sample home switch on the X axis (axis 0).  make a signal,
### link the incoming parport pin to the signal, then link the signal
### to LinuxCNC's axis 0 home switch input pin
###

# net Xhome parport.0.pin-10-in => joint.0.home-sw-in

###
### Shared home switches all on one parallel port pin?
### that's ok, hook the same signal to all the axes, but be sure to
### set HOME_IS_SHARED and HOME_SEQUENCE in the ini file.
###

# net homeswitches <= parport.0.pin-10-in
# net homeswitches => joint.0.home-sw-in
# net homeswitches => joint.1.home-sw-in
# net homeswitches => joint.2.home-sw-in

###
### Sample separate limit switches on the X axis (axis 0)
###

# net X-neg-limit parport.0.pin-11-in => joint.0.neg-lim-sw-in
# net X-pos-limit parport.0.pin-12-in => joint.0.pos-lim-sw-in

###
### Just like the shared home switches example, you can wire together
### limit switches.  Beware if you hit one, LinuxCNC will stop but can't tell
### you which switch/axis has faulted.  Use caution when recovering from this.
###

# net Xlimits parport.0.pin-13-in => joint.0.neg-lim-sw-in joint.0.pos-lim-sw-in

The lines starting with # are comments, and their only purpose is to guide the reader through the file.

3.2. Overview

There are a couple of operations that get executed when the standard_pinout.hal gets executed/interpreted:

The Parport driver gets loaded (see the Parport Chapter for details)
The read & write functions of the parport driver get assigned to the base thread
[the fastest thread in the LinuxCNC setup, usually the code gets executed every few tens of microseconds]
The step & direction signals for axes X,Y,Z get linked to pins on the parport
Further I/O signals get connected (estop loopback, toolchanger loopback)
A spindle-on signal gets defined and linked to a parport pin

3.3. Changing the standard_pinout.hal

If you want to change the standard_pinout.hal file, all you need is a text editor. Open the file and locate the parts you want to change.

If you want for example to change the pin for the X-axis Step & Directions signals, all you need to do is to change the number in the parport.0.pin-XX-out name:

net Xstep parport.0.pin-03-out
net Xdir  parport.0.pin-02-out

can be changed to:

net Xstep parport.0.pin-02-out
net Xdir  parport.0.pin-03-out

or basically any other out pin you like.

Hint: make sure you don’t have more than one signal connected to the same pin.

3.4. Changing polarity of a signal

If external hardware expects an “active low” signal, set the corresponding -invert parameter. For instance, to invert the spindle control signal:

setp parport.0.pin-09-invert TRUE

3.5. Adding PWM Spindle Speed Control

If your spindle can be controlled by a PWM signal, use the pwmgen component to create the signal:

loadrt pwmgen output_type=0
addf pwmgen.update servo-thread
addf pwmgen.make-pulses base-thread
net spindle-speed-cmd motion.spindle-speed-out => pwmgen.0.value
net spindle-on motion.spindle-on => pwmgen.0.enable
net spindle-pwm pwmgen.0.pwm => parport.0.pin-09-out
setp pwmgen.0.scale 1800 # Change to your spindle’s top speed in RPM

This assumes that the spindle controller’s response to PWM is simple: 0% PWM gives 0 RPM, 10% PWM gives 180 RPM, etc. If there is a minimum PWM required to get the spindle to turn, follow the example in the nist-lathe sample configuration to use a scale component.

3.6. Adding an enable signal

Some amplifiers (drives) require an enable signal before they accept and command movement of the motors. For this reason there are already defined signals called Xen, Yen, Zen.

To connect them use the following example:

net Xen parport.0.pin-08-out

You can either have one single pin that enables all drives; or several, depending on the setup you have. Note, however, that usually when one axis faults, all the other drives will be disabled as well, so having only one enable signal / pin for all drives is a common practice.

3.7. External ESTOP button

The standard_pinout.hal file assumes no external ESTOP button. For more information on an external E-Stop see the estop_latch man page.