Category: Machine Shop

Mechanical widgetry

Atom D520: Config Files for Smoother Sherline Stepping

The dual-core-ness of the D520, as set up there, allows a distinct improvement in the EMC2 BASE_PERIOD setting, which is exactly why I undertook this adventure.

The 100 µs period I used on the Dell Dimension 4550 ensured the occasional long-latency burps wouldn’t cause much trouble… and they didn’t. The setup used the HAL step generator’s ability to supply a single pulse within one base period, so the maximum stepping rate was 1/100 µs = 10 k steps / second.

However, that also determines the granularity of speed changes, so the controller can only drive the motors at multiples of the basic 100 µs without interpolating. For example, the four fastest step rates are:

1/100 µs = 10 k step/sec
1/200 µs =5 k step/sec
1/300 µs = 3.3 k step/sec
1/400 µs = 2.5 k step/sec

The motors have 200 major steps / revolution and run in quarter-step mode: 800 microsteps / revolution. The axes have 20 turn-per-inch leadscrews, thus requiring 16 k step pulses per inch of travel.

That means the corresponding traverse speeds are (step/sec) / (step/inch):

10 k step/sec -> 0.625 in/sec = 37 in/min
5 k step/sec -> 0.313 in/sec = 18.75 in/min
3.3 k step/sec -> 0.206 in/sec = 12.37 in/min
2.5 k step/sec -> 0.156 in/sec = 9.37 in/min

Those are fairly large jumps between the speeds, which means the motor acceleration when the step rate changes is fairly high. HAL interpolates by bunching groups of pulses, but higher resolution is better.

The Atom CPU has latencies under 10 µs, with no large burps that I’ve seen so far, so I set the BASE_PERIOD to 50 µs. However, that required changing the HAL step generator to produce a pulse during two successive periods (one high, one low) to keep the pulses wide enough for the motor controller. That means the highest step rate is still 10 k steps/sec and the top speed is still 37 inch/min.

However, HAL can now adjust the period in smaller increments with lower acceleration between the jumps. The four fastest rates are now:

1/100 µs = 10 k step/sec -> 0.625 in/sec = 37 in/min
1/150 µs =6.7 k step/sec -> 0.417 in/sec = 25 in/min
1/200 µs = 5 k step/sec -> 0.313 in/sec = 18.75 in/min
1/250 µs = 4 k step/sec -> 0.250 in/sec = 15 in/min

A stock Sherline CNC milling machine is rated for 22 inch/min (0.37 inch/sec) rapid motion on all three axes. That means the maximum step rate is

(0.37 inch/sec) * (16 k step/in) = 5.9 kHz

Quite some years ago, I rebuilt my Sherline controller box to reduce its electrical and acoustic noise, then did a clean-room reimplementation of the firmware in the PIC microcontrollers. After the dust settled, my firmware could handle 8 k steps / sec, which works out to 0.5 in/sec = 30 in/min.

That turns out to be slightly more aggressive than the whole lashup can tolerate; I can hear the motors take occasional hits as they miss the odd step at 30 inch/min.

So I set the overall MAX_LINEAR_VELOCITY = 0.400 inch/sec = 24 inch/min, which is also the MAX_VELOCITY for both X and Y. The Z axis, as always, is happier with a bit slower top speed: 0.333 inch/sec = 20 inch/min. The maximum step rate is 0.4 x 16 k = 6.4 kHz, comfortably under the controller’s upper limit.

The MAX_ACCELERATION for X and Y = 5.0 in/sec², with Z at 3.0. STEPGEN_MAXACCEL for each axis is twice that; I have each axis set for a few mils of backlash compensation.

With all that in mind, the changed configuration files look like this, with the others remaining as described there.

Sherline.hal, with the new stepgen pulse specs

# Generated by stepconf at Sat Aug 23 12:10:22 2008
# If you make changes to this file, they will be
# overwritten when you run stepconf again
loadrt trivkins
loadrt [EMCMOT]EMCMOT base_period_nsec=[EMCMOT]BASE_PERIOD servo_period_nsec=[EMCMOT]SERVO_PERIOD traj_period_nsec=[EMCMOT]SERVO_PERIOD key=[EMCMOT]SHMEM_KEY num_joints=[TRAJ]AXES
loadrt probe_parport
loadrt hal_parport cfg="0x378 out"
setp parport.0.reset-time 60000
loadrt stepgen step_type=0,0,0,0
loadrt pwmgen output_type=0

addf parport.0.read base-thread
addf stepgen.make-pulses base-thread
addf pwmgen.make-pulses base-thread
addf parport.0.write base-thread
addf parport.0.reset base-thread

addf stepgen.capture-position servo-thread
addf motion-command-handler servo-thread
addf motion-controller servo-thread
addf stepgen.update-freq servo-thread
addf pwmgen.update servo-thread

net spindle-cmd <= motion.spindle-speed-out => pwmgen.0.value
net spindle-enable <= motion.spindle-on => pwmgen.0.enable
net spindle-pwm <= pwmgen.0.pwm
setp pwmgen.0.pwm-freq 100.0
setp pwmgen.0.scale 1166.66666667
setp pwmgen.0.offset 0.114285714286
setp pwmgen.0.dither-pwm true
net spindle-cw <= motion.spindle-forward

net estop-out => parport.0.pin-01-out
net xdir => parport.0.pin-02-out
net xstep => parport.0.pin-03-out
setp parport.0.pin-03-out-reset 0
setp parport.0.pin-04-out-invert 1
net ydir => parport.0.pin-04-out
net ystep => parport.0.pin-05-out
setp parport.0.pin-05-out-reset 0
setp parport.0.pin-06-out-invert 1
net zdir => parport.0.pin-06-out
net zstep => parport.0.pin-07-out
setp parport.0.pin-07-out-reset 0
net adir => parport.0.pin-08-out
net astep => parport.0.pin-09-out
setp parport.0.pin-09-out-reset 0
net spindle-cw => parport.0.pin-14-out
net spindle-pwm => parport.0.pin-16-out
net xenable => parport.0.pin-17-out

setp stepgen.0.position-scale [AXIS_0]SCALE
setp stepgen.0.steplen 1
setp stepgen.0.stepspace 1
setp stepgen.0.dirhold 60000
setp stepgen.0.dirsetup 60000
setp stepgen.0.maxaccel [AXIS_0]STEPGEN_MAXACCEL
net xpos-cmd axis.0.motor-pos-cmd => stepgen.0.position-cmd
net xpos-fb stepgen.0.position-fb => axis.0.motor-pos-fb
net xstep <= stepgen.0.step
net xdir <= stepgen.0.dir
net xenable axis.0.amp-enable-out => stepgen.0.enable

setp stepgen.1.position-scale [AXIS_1]SCALE
setp stepgen.1.steplen 1
setp stepgen.1.stepspace 1
setp stepgen.1.dirhold 60000
setp stepgen.1.dirsetup 60000
setp stepgen.1.maxaccel [AXIS_1]STEPGEN_MAXACCEL
net ypos-cmd axis.1.motor-pos-cmd => stepgen.1.position-cmd
net ypos-fb stepgen.1.position-fb => axis.1.motor-pos-fb
net ystep <= stepgen.1.step
net ydir <= stepgen.1.dir
net yenable axis.1.amp-enable-out => stepgen.1.enable

setp stepgen.2.position-scale [AXIS_2]SCALE
setp stepgen.2.steplen 1
setp stepgen.2.stepspace 1
setp stepgen.2.dirhold 60000
setp stepgen.2.dirsetup 60000
setp stepgen.2.maxaccel [AXIS_2]STEPGEN_MAXACCEL
net zpos-cmd axis.2.motor-pos-cmd => stepgen.2.position-cmd
net zpos-fb stepgen.2.position-fb => axis.2.motor-pos-fb
net zstep <= stepgen.2.step
net zdir <= stepgen.2.dir
net zenable axis.2.amp-enable-out => stepgen.2.enable

setp stepgen.3.position-scale [AXIS_3]SCALE
setp stepgen.3.steplen 1
setp stepgen.3.stepspace 1
setp stepgen.3.dirhold 60000
setp stepgen.3.dirsetup 60000
setp stepgen.3.maxaccel [AXIS_3]STEPGEN_MAXACCEL
net apos-cmd axis.3.motor-pos-cmd => stepgen.3.position-cmd
net apos-fb stepgen.3.position-fb => axis.3.motor-pos-fb
net astep <= stepgen.3.step
net adir <= stepgen.3.dir
net aenable axis.3.amp-enable-out => stepgen.3.enable

net estop-out <= iocontrol.0.user-enable-out
net estop-out => iocontrol.0.emc-enable-in

loadusr -W hal_manualtoolchange
net tool-change iocontrol.0.tool-change => hal_manualtoolchange.change
net tool-changed iocontrol.0.tool-changed <= hal_manualtoolchange.changed
net tool-number iocontrol.0.tool-prep-number => hal_manualtoolchange.number
net tool-prepare-loopback iocontrol.0.tool-prepare => iocontrol.0.tool-prepared

Sherline.ini, with new periods, speeds, and accelerations

# Ed Nisley - KE4ZNU
# Just do not run stepconf ever again...

[EMC]
MACHINE = Sherline-XYZA
DEBUG = 0
RS274NGC_STARTUP_CODE = G21 G40 G49 G54 G80 G90 G92.1 G94 G97 G98

[DISPLAY]
DISPLAY = axis
EDITOR = gedit
GEOMETRY = AXYZ
POSITION_OFFSET = RELATIVE
POSITION_FEEDBACK = ACTUAL
MAX_FEED_OVERRIDE = 3.0
INTRO_GRAPHIC = /home/ed/emc2/configs/Sherline-XYZA/Sherline.gif
INTRO_TIME = 3
#PROGRAM_PREFIX = /mnt/bulkdata/
PROGRAM_PREFIX = ~/emc2/nc_files
#INCREMENTS = .1in .05in .01in .005in .001in .0005in .0001in
INCREMENTS = 10 mm, 1 mm, 0.1 mm, 90 deg, 45 deg, 10 deg

[FILTER]
PROGRAM_EXTENSION = .py Python Script
py = python

[TASK]
TASK = milltask
CYCLE_TIME = 0.010

[RS274NGC]
PARAMETER_FILE = emc.var

[EMCMOT]
EMCMOT = motmod
SHMEM_KEY = 111
COMM_TIMEOUT = 1.0
COMM_WAIT = 0.010
BASE_PERIOD = 50000
SERVO_PERIOD = 1000000

[HAL]
HALUI=halui
HALFILE = Sherline.hal
HALFILE = custom.hal
HALFILE = Logitech_Gamepad.hal
POSTGUI_HALFILE = custom_postgui.hal

[TRAJ]
AXES = 4
COORDINATES = X Y Z A
MAX_ANGULAR_VELOCITY = 45.00
DEFAULT_ANGULAR_VELOCITY = 36.0
LINEAR_UNITS = inch
ANGULAR_UNITS = degree
CYCLE_TIME = 0.010
DEFAULT_VELOCITY = 0.400
MAX_LINEAR_VELOCITY = 0.400
POSITION_FILE =	lastposition.txt
NO_FORCE_HOMING = 1

[EMCIO]
EMCIO = io
CYCLE_TIME = 0.100
TOOL_TABLE = Sherline.tbl
TOOL_CHANGE_AT_G30 = 1

[AXIS_0]
TYPE = LINEAR
MAX_VELOCITY = 0.400
MAX_ACCELERATION = 5.0
STEPGEN_MAXACCEL = 10.0
SCALE = 16000.0
FERROR = 0.05
MIN_FERROR = 0.01
MIN_LIMIT = -1.0
MAX_LIMIT = 9.5
BACKLASH = 0.003
HOME_IS_SHARED = 1
HOME_SEQUENCE = 2
HOME_SEARCH_VEL = 0.3
HOME_LATCH_VEL = 0.016
HOME_FINAL_VEL = 0.4
HOME_OFFSET = 9.1
HOME = 4.5

[AXIS_1]
TYPE = LINEAR
MAX_VELOCITY = 0.400
MAX_ACCELERATION = 5.0
STEPGEN_MAXACCEL = 10.0
SCALE = 16000.0
FERROR = 0.05
MIN_FERROR = 0.01
MIN_LIMIT = -0.5
MAX_LIMIT = 4.90
BACKLASH = 0.003
HOME_IS_SHARED = 1
HOME_SEQUENCE = 1
HOME_SEARCH_VEL = -0.3
HOME_LATCH_VEL = -0.016
HOME_FINAL_VEL = 0.4
HOME_OFFSET = 0.0
HOME = 4.0

[AXIS_2]
TYPE = LINEAR
MAX_VELOCITY = 0.333
MAX_ACCELERATION = 3.0
STEPGEN_MAXACCEL = 6.0
SCALE = 16000.0
FERROR = 0.05
MIN_FERROR = 0.01
MIN_LIMIT = 0.0
MAX_LIMIT = 6.930
BACKLASH = 0.005
HOME_IS_SHARED = 1
HOME_SEQUENCE = 0
HOME_SEARCH_VEL = 0.200
HOME_LATCH_VEL = 0.016
HOME_FINAL_VEL = 0.3
HOME_OFFSET = 6.93
HOME = 6.5

[AXIS_3]
TYPE = ANGULAR
###WRAPPED_ROTARY = 1
MAX_VELOCITY = 40.0
MAX_ACCELERATION = 250.0
STEPGEN_MAXACCEL = 275.0
SCALE = 160.0
FERROR = 1
MIN_FERROR = .25
MIN_LIMIT = -999999999.9
MAX_LIMIT =  999999999.9
HOME_SEARCH_VEL = 0
HOME_LATCH_VEL = 0
HOME = 0.0

2010-12-21

An Atom for the Sherline Milling Machine
Somewhat against the recommendations of the experts on the EMC2 mailing list, I bought a Foxconn R30-D2 with an Intel Atom D520 from Newegg during a sale: add 2 GB of memory from Crucial, a spare SATA drive from my collection, and it’s ready to go. I also bashed a spare parallel printer port card into the box, although it isn’t really needed right now: unlike the Intel system board, Foxconn brings the on-board parallel port directly to the back panel.

The Foxconn support site is a nightmare and was, AFAICT, dead for the first few weeks I had the box. The key fact to remember is that the -D2 part of the number specifies the system board / CPU, so the same downloads / BIOS updates apply to the R10, R20, R30, and R40 models. There is no new BIOS available to fix the “fan runs all the time” problem reported by so many people.

I installed Ubuntu 10.04 LTS from the distro CD, then ran the EMC2 installation script. All that is routine, as described there. You probably want to install the EMC2 Live CD, though, and to get much the same result with less fiddling.

Turn off Hyperthreading in the BIOS, which seems to make the RTAI real-time hypervisor happier. Under those conditions, the default install has an interrupt latency of about 13 µs.

The Atom D520 is a dual-core processor and you can devote one core to EMC2’s real-time functions, thus eliminating much of the usual contention and interrupt latency. That works surprisingly well and is completely automagic after you add the isolcpus=1 kernel option to the appropriate line in /boot/grub/grub.cfg. Thusly:
```
linux	/boot/vmlinuz-2.6.32-122-rtai root=UUID=57fe2b04-ffe4-4de3-a597-89bd4ed01018 ro  vga=758  noquiet nosplash isolcpus=1
```
With that done, the latency drops down under 8 µs, which is entirely satisfactory. I can push it to 10 µs by doing stupid things: scrubbing a glxgears window over a Flash video in Firefox, for example.

The catch is that the wonderful new grub2 bootloader rewrites its boot configuration file on the fly, based on a set of rules that, evidently, cannot apply different kernel configuration parameters to different kernels within the same partition. As a result, you must choose between:
- Running stock Ubuntu on one core
- Manually tweaking grub.cfg after every kernel update
Given that an Atom isn’t exactly a blinding flash and a deafening report in the performance department, I opted for the second method. If Ubuntu was still using Legacy Grub, then I’d just tweak menu.lst and be done with it. This is, I suppose, progress.

Memo to Self: Adjust grub.cfg every mumble time.

[Update: There’s now a fix for that, as described in the EMC2 wiki. Go for it!]
2010-12-20
Opening a USB Stick

After I failed to fix that old USB memory, my friend suggested a brain transplant: swap the Flash chip from the dead stick into a new one. That has a low chance of success because the innards will be different for every manufacturer and, even for USB sticks of the same vintage, nothing remains the same from lot to lot.

Anyhow, I removed the fancy end caps from the donor stick, which looks to be swag from a medical conference:

Case caps removed

The key step is to crack the case open without damaging anything inside. This technique works wonderfully well:

Cracking the case

Grab diagonally opposing corners in a bench vise and slowly increase the clamping force until the case snaps apart. Poke a screwdriver in the gap, remove the case from the vise, and pry the thing open.

As it turned out, the innards were completely different: different Flash controllers and different Flash memory chips. So it goes.

Not surprisingly, you can find the data sheets / manuals / configuration utilities for the Flash controller chips by searching for the obvious keywords. Machine translation is your friend…

2010-12-19
Salvaged Heatsink Reconstruction

Heatsink mounting flanges

I decided to replace the sawed-off flanges on that salvaged heatsink to make all three use the same mounting arrangement, whatever that might turn out to be.

Nothing particularly fancy about it: two random chunks of aluminum sheet and two thinner strips, sanded to roughen their surfaces, and epoxied into place.

The repaired heatsink is marginally taller than its siblings, but not so anybody will ever notice, and it’s no more off-kilter than they are, either.

A quartet of 5/16-inch lathe bits provided the right spacing to hold the heatsink over its new flanges while the epoxy filled in all the gaps and irregularities. I probably should have paid a bit more attention to squaring things up, but it’s good enough for what it’ll need to do.

Heatsink up on blocks

2010-12-18
Tour Easy: New Rear Brake

While I had the bike up on the stand to replace the seat strut screws, I installed a new rear brake. The old brake hadn’t been braking well for a while, which I attributed to different brake pads, but nothing seemed to help.

New rear brake

I had to drive the old brakes off the mounting studs with a drift punch; the studs were pretty well rusted after a decade of continuous use under the hostile conditions that pass for normal around here. Shined them up, applied a generous layer of Never-Seez, and bolted the new brakes in place.

Turns out that the rear brakes on a Tour Easy are backwards from their orientation on an upright bike: the studs point spinward, so the cable exits on the right side of the frame. Doesn’t make any difference, as that’s how the front brake studs work, but if you’re thinking of buying some fancy brake with odd mounting requirements, you probably shouldn’t.

The installation specs require “more than 39 mm” of cable between the clamp bolt and the bracket on the other arm. The Tour Easy frame tubes are closer together than that, allowing a bare 25 mm of cable.

Rear brake cable and boot

I trimmed the boot to fit, but the real problem is that the arms aren’t at quite the right angle with respect to the braking surface on the rim and provide a bit less leverage than you’d like; the pad alignment is also trickier. I tried adding spacers to the brake pads, but the mounting studs aren’t quite long enough for that.

The first road test indicates the new brakes work much better than the old ones…

2010-12-15
Another Fractured Seat Strut Screw

Having replaced both screws back in March, I wasn’t expecting this:

Fractured screw surfaces

Of course, it broke at the first pedal stroke while pushing off across an intersection, which is why I never try to ace out oncoming cars.

This was, mercifully, on the left side of the bike, so I could replace it without removing the rear wheel. Being that sort of bear, I now carry spare screws and we were back on the road in about ten minutes.

A closer look at the head end of the screw shows some interesting details:

Fractured screw – head

The tail end has matching cracks:

Fractured screw – tail

Notice how the cracks are all oriented in the same direction. The screw fractured at the edge of the brazed-on frame fitting, so I suspect the seat stay clamp must be moving just enough to flex the screw across that plane.

I mooched a pair of hardened socket head cap screws from Eks, ground down the head of the right-side screw for better chain clearance around the sprockets, buttered ’em up with Never-Seez, and we’ll see how long Real Steel lasts.

Right-side screw with ground-down head

I really should conjure up a clamp that mounts to the frame tubing, rather than depend on that puny brazed-on fitting, shouldn’t I?

It appears that new Tour Easy ‘bents come with more brazed-on fittings and a more secure seat stay mounting bracket. A photo was there when I looked.

2010-12-14
Aluminum-Housed Resistor Hole Locations and Derating
The battle plan is to mount some resistors on those heatsinks to warm up the disinsector.

These seem to be the right hammer for the job:

Aluminum housed resistors

The big one is rated 50 W @ 25 °C ambient. Use two, derated by 50%, times three air-cooled heatsinks for 150 W of low-temperature heating. The little one is 25 W @ 25 °C.

The derating curve is linear from 100% @ 25 °C down to 10% @ 250 °C, when mounted to a square foot of flat aluminum plate: -0.40% / °C.

Assuming a max heater ambient of 150 °F = 65 °C, you can use 84% of full power. Derating by 50% isn’t all that unreasonable.

The relevant hole locations:
- 50 W: X=1.562 inch / 39.67 mm Y=0.844 inch / 21.44 mm
- 25 W: X=0.719 inch / 18.26 mm Y=0.781 inch / 19.84 mm
- 10 W: X=0.562 inch / 14.27 mm Y=0.625 inch / 15.88 mm
Divide those by 2.0 for from-the-center offsets, which may be more useful for manual CNC operations: zero at the resistor mounting center, then back-and-forth from there.

The mounting hole size for 25 & 50 W resistors: 0.125 inch / 3.18 mm diameter, just exactly what you want for a 4-40 mounting screw. Tap drill #43, clearance drill #32 (close fit) or #30 (loose fit).

The mounting hole size for 10 W resistors: 0.094 inch / 2.39 mm to fit a 2-56 screw. Tap drill #50 (better: #49 for 50% threads), clearance drill #43 (close) or #41 (loose).

The Vishay-Dale data sheet is there
2010-12-13