Understaning PWM

I have changed the standard spindle motor on my Boxford lathe as the original no longer functioned correctly.
In place is a 240v servo and drive rated at a maximum of 3000 rpm
The drive uses the 0-10v analogue input for speed control.
I am using LinuxCNC to deliver a PWM signal into a small DAC pcb which is then delivered to the drive as the 0-10v Analogue signal.
Rotation is controlled via a relay PCB to swap the signal from + to -
I can control on/off, direction and rpm
The system works fine, but I need to understand how to control the spindle speed more closely to the actual speed requested by LinuxCNC.
I have in my HAL a scale set to 3000, but I don't understand what frequency value I require or indeed what other parameters I must set to achieve what is required. Currently as I request RPM values via LinuxCNC if I look at the displayed RPM at the actual servo drive there is a mismatch which can either be higher or lower than requested and the differance can vary wildly. It appear the differance is non linear.
I like the sound of Andy's linear component, but I need help in understanding the basics first.
Once I have this set pretty close to what is required, I can then introduce my closed loop encoder to assist a more precise control.

Hi Andy,
I am currently fighting with my spindle setup created by the pncconf wizard and was following your instructions above to adjust the speed.
It will only run at one speed and that only the maxaccel and/or maxvel have any effect and it does not matter what my "s" command value is...
I did look over the docs but it only has an example for a parallel port and no example for my 7i96 card... Also, all the examples assume an encode for feedback..

Your help is greatly appreciated!

[SPINDLE_0]
MAX_VELOCITY = 8.33333333333
MAX_ACCELERATION = 500.0
# The values below should be 25% larger than MAX_VELOCITY and MAX_ACCELERATION
# If using BACKLASH compensation STEPGEN_MAXACCEL should be 100% larger.
STEPGEN_MAXVEL = 10.42
STEPGEN_MAXACCEL = 625.00
P = 0.0
I = 0.0
D = 0.0
FF0 = 1.0
FF1 = 0.0
FF2 = 0.0
BIAS = 0.0
DEADBAND = 0.0
MAX_OUTPUT = 24000.0
# these are in nanoseconds
DIRSETUP   = 2000
DIRHOLD    = 2000
STEPLEN    = 2000
STEPSPACE  = 2000
SCALE = 3125.0

#*******************
# SPINDLE
#*******************
setp pid.s.Pgain [SPINDLE_0]P
setp pid.s.Igain [SPINDLE_0]I
setp pid.s.Dgain [SPINDLE_0]D
setp pid.s.bias [SPINDLE_0]BIAS
setp pid.s.FF0 [SPINDLE_0]FF0
setp pid.s.FF1 [SPINDLE_0]FF1
setp pid.s.FF2 [SPINDLE_0]FF2
setp pid.s.deadband [SPINDLE_0]DEADBAND
setp pid.s.maxoutput [SPINDLE_0]MAX_OUTPUT
setp pid.s.error-previous-target true
# This setting is to limit bogus stepgen
# velocity corrections caused by position
# feedback sample time jitter.
setp pid.s.maxerror 0.012700

net spindle-index-enable <=> pid.s.index-enable
net spindle-enable => pid.s.enable
net spindle-vel-cmd-rpm => pid.s.command
net spindle-vel-fb-rpm => pid.s.feedback
net spindle-output <= pid.s.output

# Step Gen signals/setup

setp hm2_7i96.0.stepgen.03.dirsetup [SPINDLE_0]DIRSETUP
setp hm2_7i96.0.stepgen.03.dirhold [SPINDLE_0]DIRHOLD
setp hm2_7i96.0.stepgen.03.steplen [SPINDLE_0]STEPLEN
setp hm2_7i96.0.stepgen.03.stepspace [SPINDLE_0]STEPSPACE
setp hm2_7i96.0.stepgen.03.position-scale [SPINDLE_0]SCALE
setp hm2_7i96.0.stepgen.03.step_type 0
setp hm2_7i96.0.stepgen.03.control-type 1
setp hm2_7i96.0.stepgen.03.maxaccel [SPINDLE_0]MAX_ACCELERATION
setp hm2_7i96.0.stepgen.03.maxvel [SPINDLE_0]MAX_VELOCITY

net spindle-enable => hm2_7i96.0.stepgen.03.enable
net spindle-vel-cmd-rps => hm2_7i96.0.stepgen.03.velocity-cmd
net spindle-vel-fb-rps <= hm2_7i96.0.stepgen.03.velocity-fb
# ---setup spindle control signals---

net spindle-vel-cmd-rps <= spindle.0.speed-out-rps
net spindle-vel-cmd-rps-abs <= spindle.0.speed-out-rps-abs
net spindle-vel-cmd-rpm <= spindle.0.speed-out
net spindle-vel-cmd-rpm-abs <= spindle.0.speed-out-abs
net spindle-enable <= spindle.0.on
net spindle-cw <= spindle.0.forward
net spindle-ccw <= spindle.0.reverse
net spindle-brake <= spindle.0.brake
net spindle-revs => spindle.0.revs
net spindle-at-speed => spindle.0.at-speed
net spindle-vel-fb-rps => spindle.0.speed-in
net spindle-index-enable <=> spindle.0.index-enable

# ---Setup spindle at speed signals---

sets spindle-at-speed true

If your analog circuit expects PWM value, a stepgen is not going to work very well.

If you want PWM, you need to:

1. Use firmware that has a PWM output available
2. Initialize and connect this PWM hardware
in the hal file

There is firmware for the 7I96 that replaces stepgen-04
(the last channel) with a PWM generator. (7i96d_1pwm.bit)

Thanks PCW,
When I find the listed bit file, is it just a matter of flashing the 7i96?
And, would this overwrite my current setups for the other steppers & switches?

hairyMug wrote:

Thanks PCW,
When I find the listed bit file, is it just a matter of flashing the 7i96?
And, would this overwrite my current setups for the other steppers & switches?
 

No, but you will lose one Stepgen and gain one PWMGen so if your hal file has a num_stepgens=5
statement, it will need to be changed or you will get a LinuxcNC error

++ on using Johns configuration utility. it knows how to setup PWM. Even if you don't want to
start a new configuration from scratch, you can use the utility to make an example PWM setup
hal file section

Yes, I used John's tool and copied the spindle info to the HAL created by the pncconfg tool...
Now, the spindle runs at full speed only... no matter what setting I use...

BTW: I did try using John's tool alone, but I always had to edit the HAL file to fix code that threw errors... >> The "config" line was incorrect. and the "E-stop" code also threw an error...

Can you post your hal/ini files, and also show how the PWM is connected?

Thanks!