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Posts Tagged 3018 CNC

CNC 3018-Pro: Probe Camera Case for Anonymous USB Camera

The anonymous USB camera I used with the stereo zoom microscope not only works with VLC, but also with bCNC, and it has a round PCB with ears:

CNC 3018-Pro - Probe Camera - PCB
CNC 3018-Pro – Probe Camera – PCB

Which suggested putting it in a ball mount for E-Z aiming:

CNC 3018-Pro - Probe Camera - ball mount
CNC 3018-Pro – Probe Camera – ball mount

Black filament snippets serve as alignment pins to hold the ball halves together while they’re getting clamped. They’re epoxied into the upper half of the ball, because who knows when I’ll need to harvest the camera.

The clamp mount descends from the Tour Easy Daytime Running Lights, with more screws and less fancy shaping:

USB Camera - Round PCB Mount - solid model - build
USB Camera – Round PCB Mount – solid model – build

The clamp pieces fit around the ball with four M3 screws providing the clamping force:

USB Camera - Round PCB Mount - solid model sectioned
USB Camera – Round PCB Mount – solid model sectioned

The whole affair sticks onto the Z axis carrier with double-sided foam tape:

CNC 3018-Pro - Probe Camera - alignment
CNC 3018-Pro – Probe Camera – alignment

It barely clears the strut on the -X side of the carriage, although it does stick out over the edge of the chassis.

After the fact, I tucked a closed-cell foam ring between the lens threads and the ball housing to stabilize the lens; the original camera glued the thing in place, but some fiddly alignment & focusing lies ahead:

Alignment mirror - collimation
Alignment mirror – collimation

It’s worth noting that the optical axis of these cheap cameras rarely coincides with the physical central axis of the lens. This one requires a jaunty tilt, although it’s not noticeable in any of the pictures I tried to take.

All in all, this one works just like the probe camera on the MPCNC.

The OpenSCAD source code as a GitHub Gist:

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CNC 3018-Pro: Probe Camera Case for Logitch QuickCam Pro 5000

The ball-shaped Logitch QuickCam Pro 5000 has a rectangular PCB, so conjuring a case wasn’t too challenging:

Probe Camera Case - Logitech QuickCam Pro 5000 - bottom
Probe Camera Case – Logitech QuickCam Pro 5000 – bottom

That’s more-or-less matte black duct tape to cut down reflections.

The top side has a cover made from scuffed acrylic scrap:

Probe Camera Case - Logitech QuickCam Pro 5000 - top
Probe Camera Case – Logitech QuickCam Pro 5000 – top

The corners are slightly rounded to fit under the screw heads holding it in place.

The solid model shows off the internal ledge positioning the PCB so the camera lens housing rests on the floor:

3018 Probe Camera Mount - solid model
3018 Probe Camera Mount – solid model

The notch lets the cable out, while keeping it in one place and providing some strain relief.

I though if a camera was recognized by V4L2 and worked with VLC, it was good to go:

Logitech QuickCam Pro 5000 - short focus
Logitech QuickCam Pro 5000 – short focus

Regrettably, it turns out the camera has a pixel format incompatible with the Python opencv interface used by bCNC. This may have something to do with running the code on a Raspberry Pi, rather than an x86 box.

The camera will surely come in handy for something else, especially with such a cute case.

The OpenSCAD source code as a GitHub Gist:

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CNC 3018-Pro: Platter Fixtures

Up to this point, the Sherline has been drilling 3.5 inch hard drive platters to serve as as reflecting bases for the vacuum tubes:

LinuxCNC - Sherline Mill - Logitech Gamepad
LinuxCNC – Sherline Mill – Logitech Gamepad

The CNC 3018-Pro has a work envelope large enough for CD / DVD platters, so I mashed the Sherline fixture with dimensions from the vacuum tube code, added the 3018’s T-slot spacing, and conjured a pair of fixtures for a pair of machines.

Because I expect to practice on scrap CDs and DVDs for a while:

Platter Fixtures - CD on 3018
Platter Fixtures – CD on 3018

And a 3.5 inch hard drive platter version:

Platter Fixtures - hard drive platter on 3018
Platter Fixtures – hard drive platter on 3018

The holes sit at half the 3018’s T-slot spacing (45 mm / 2), so you can nudge the fixtures to the front or rear, as you prefer.

The alignment dots & slots should help touch off the XY coordinate system on the Sherline, although it can’t reach all of a CD. Using bCNC’s video alignment on the hub hole will be much easier on the 3018.

After fiddling around with the 3018 for a while, however, the CD fixture doesn’t have many advantages over simply taping the disc to a flat platen. Obviously, you’d want a sacrificial layer for drilling, but it’s not clear the OEM motor / ER11 chuck would be up to that task.

The OpenSCAD source code as a GitHub Gist:

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CNC 3018-Pro: DRV8825 Hack for 1:8 Microstep Mode

The CAMTool V3.3 board on the CNC 3018-Pro hardwires the three DRV8825 stepper driver chips in 1:32 microstep mode by pulling all three Mode pins high. Unlike most CNC boards, it does not include jumpers to let you select different microstep modes; the designers know you want as many microsteps as you can possibly get.

As it turns out, 1:32 microstep mode requires 1600 steps for each millimeter of travel and, because GRBL tops out around 30 k step/s, the maximum speed is about 18.75 mm/s = 1125 mm/min. Which isn’t at bad, but, because I intend to use the thing for engraving, rather than the light-duty machining it’s (allegedly) capable of performing, running at somewhat higher speeds will be desirable.

For sure, a 3018-Pro does not have a physical resolution of 625 nm.

If you’re willing to settle for a mere 400 step/mm = 2.6 µm, then you can just ground the Mode 2 pin to get 1:8 microstep mode:

DRV8825 - Stepper Motor Controller - Microstep Modes
DRV8825 – Stepper Motor Controller – Microstep Modes

Rewiring the CAMTool board isn’t feasible, but hacking the DRV8825 carrier PCB doesn’t require much effort.

So, we begin.

Clamp the PCB in a vise, grab the Mode 2 pin with a needle-nose pliers, apply enough heat to melt the solder completely through the board, and yank that pin right out:

CAMTool V3.3 - DRV8825 M2 pin removed
CAMTool V3.3 – DRV8825 M2 pin removed

I do wonder how the layout folks managed to reverse the “N” for the Enable pin. Perhaps it’s a Cyrillic И in a dead-simple font?

With that done, add a snippet of wire from M2 to the GND pin in the opposite corner to complete the job:

CAMTool V3.3 - DRV8825 wired for 8 ustep mode
CAMTool V3.3 – DRV8825 wired for 8 ustep mode

Despite that picture, remember to plug the DRV8825 boards into the CAMTool V3.3 board with the heatsink downward and the twiddlepot on the top, as shown in the little instruction book you got with the hardware:

SainSmart Genmitsu CNC Router 3018PRO-User Manual - DRV8825 orientation
SainSmart Genmitsu CNC Router 3018PRO-User Manual – DRV8825 orientation

Recompute the step/mm value in 1:8 microstep mode:

400 step/mm = (200 full step/rev) × (8 microstep/full step) / (4 mm/rev)

Then set the corresponding GRBL parameters:

$100=400
$101=400
$102=400

The 3018-Pro should work exactly like it did before, maybe a little noisier if your ears are up to the task.

Moah Speed comes later …

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CNC 3018-Pro: CAMTool V3.3 USB Power Diode

The CAMTool V3.3 board dispenses with fancy USB power switching circuitry:

CAMTOOL CNC-V3.3 schematic - USB Power Entry
CAMTOOL CNC-V3.3 schematic – USB Power Entry

The NUP2201 is an ESD clamp diode / suppressor IC, which is a nice touch, but FU1, a simple 300 mA polyfuse, is the only thing standing between the USB cable and the on-board +5 V regulator. In real life, it looks like this:

CAMTool V3.3 - USB power fuse
CAMTool V3.3 – USB power fuse

It’s the little black rectangle between the USB jack and the CH340 USB-to-serial chip. The

The far end of the USB cable plugs into a Raspberry Pi, a device known for unseemly fussiness about USB power, so I unsoldered the fuse and installed a diode:

CAMTool V3.3 - USB power diode
CAMTool V3.3 – USB power diode

It’s a BAT54 Schottky diode, pointed toward the right to prevent current from the board getting to the Pi. Pin 2 (toward the bottom) isn’t connected to anything inside the package, either, so it’s all good.

I suppose if one were a stickler for detail, one could gimmick the diode in series with the fuse, but I figured that’s a solution for a problem well down on the probability list …

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DRV8825 Stepper Driver: Fast vs. Mixed Decay Current Waveforms

Herewith, a look at CNC 3018-Pro stepper motor current waveforms as a function of supply voltage, PWM decay mode, and motor speed.

The scope displays X and Y axis motor current at 1 A/div, with sensing through a pair of Tektronix Hall effect current probes:

CNC 3018-Pro - XY axes - Tek current probes
CNC 3018-Pro – XY axes – Tek current probes

The X axis driver is an unmodified DRV8825 PCB operating in default mixed-decay mode. The Y axis DRV8825 has its DECAY pin pulled high, thereby putting it in fast decay mode.

The scope timebase varies to match the programmed feed rate. Because the X and Y axes move simultaneously, each axis moves at 1/√2 the programmed speed:

G1 X10 Y10 F100 → 71 mm/min on X and Y

The motor generates minimal back EMF at slow speeds, so the winding sees nearly the full supply voltage. As described in the previous post, the basic problem arises when the current rises too fast during each PWM cycle:

V = L di/dt
di/dt = 24 V / 3 mH = 8 kA/s

The first 1:32 microstep away from 0 calls for 5% of max current = 50 mA at a 1 A peak. The DRV8825 datasheet says the PWM typically runs at 30 kHz = 33 µs/cycle, during which the current will change by 270 mA:

267 mA = 8 kA/s × 33.3 µs

Notice how the current slams to a nearly constant, much-too-high value just after the first microstep. The incorrect current level decreases with lower supply voltage, because the rate-of-change decreases and the commanded current level reaches the actual (incorrect) current sooner.

Varying the motor voltage at a constant 10 mm/min:

3018 XY - Mixed Fast - 24V - 10mm-min 1A-div
3018 XY – Mixed Fast – 24V – 10mm-min 1A-div
3018 XY - Mixed Fast - 20V - 10mm-min 1A-div
3018 XY – Mixed Fast – 20V – 10mm-min 1A-div
3018 XY - Mixed Fast - 15V - 10mm-min 1A-div
3018 XY – Mixed Fast – 15V – 10mm-min 1A-div
3018 XY - Mixed Fast - 12V - 10mm-min 1A-div
3018 XY – Mixed Fast – 12V – 10mm-min 1A-div
3018 XY - Mixed Fast - 10V - 10mm-min 1A-div
3018 XY – Mixed Fast – 10V – 10mm-min 1A-div

Note that reducing the supply voltage doesn’t change the motor winding current, because the DRV8825 controls the current during each microstep, at least to the best of its ability.

Also note that the current overshoots the target for those microsteps, even when the motor is stopped, because there’s no back EMF, so the power dissipation is too high even at rest.

Enough back EMF appears at 100 mm/min to begin tamping down the current overshoot at 24 V:

3018 XY - Mixed Fast - 24V - 100mm-min 1A-div
3018 XY – Mixed Fast – 24V – 100mm-min 1A-div

The current waveform looks good at 12 V:

3018 XY - Mixed Fast - 12V - 100mm-min 1A-div
3018 XY – Mixed Fast – 12V – 100mm-min 1A-div

The back EMF at 1000 mm/min nearly eliminates the overshoot at 24 V, with fast decay in the Y axis causing some PWM ripple:

3018 XY - Mixed Fast - 24V - 1000mm-min 1A-div
3018 XY – Mixed Fast – 24V – 1000mm-min 1A-div

Both decay modes look good at 12 V:

3018 XY - Mixed Fast - 12V - 1000mm-min 1A-div
3018 XY – Mixed Fast – 12V – 1000mm-min 1A-div

At 1500 mm/min, the highest reasonable speed for the thing, and a 24 V supply, both waveforms still look good:

3018 XY - Mixed Fast - 24V - 1500mm-min 1A-div
3018 XY – Mixed Fast – 24V – 1500mm-min 1A-div

However, the back EMF is now high enough to buck the 12 V supply, preventing the current from decreasing fast enough in mixed decay mode (top trace):

3018 XY - Mixed Fast - 12V - 1500mm-min 1A-div
3018 XY – Mixed Fast – 12V – 1500mm-min 1A-div

Tweaking the GRBL config to allow 2000 mm/min feeds shows the waveforms starting to become triangular, even at 24 V:

3018 XY - Mixed Fast - 24V - 2000mm-min 1A-div
3018 XY – Mixed Fast – 24V – 2000mm-min 1A-div

And a 12 V supply opposed by the back EMF simply can’t change the current fast enough to keep up with the DRV8825 microstep current levels:

3018 XY - Mixed Fast - 12V - 2000mm-min 1A-div
3018 XY – Mixed Fast – 12V – 2000mm-min 1A-div

Bottom line: a +12 V motor supply and DRV8825 drivers modified to run in fast decay mode look like the best setup for the 3018-Pro: good current control at low speeds with enough moxie to handle higher speeds.

I should hack the DRV8825 boards into 1:8 microstep mode to reduce the IRQ rate by a factor of four, then see what happens to the back EMF at absurd speeds.

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DRV8825 Stepper Driver: Forcing Fast Decay Mode in a (Likely) Counterfeit Chip

The DRV8825 stepper driver chip defaults to mixed decay mode, which TI defines thusly:

Mixed decay mode begins as fast decay, but at a fixed period of time (75% of the PWM cycle) switches to slow decay mode for the remainder of the fixed PWM period. This occurs only if the current through the winding is decreasing (per the indexer step table); if the current is increasing, then slow decay is used.

The 24 V supply on the CNC 3018-Pro provides too much voltage for the motors, because slow decay mode can’t handle those rising slopes:

3018 XY - Mixed Fast - 24V - 10mm-min 12V 1A-div
3018 XY – Mixed Fast – 24V – 10mm-min 12V 1A-div

Note that “rising” means the current increases with either polarity from 0 A at the midline. The DRV8825 uses a MOSFET H-bridge to drive winding current in either direction from the +24 V motor supply voltage.

Both traces show motor winding current at 1 A/div, with the XY axes creeping along at 10 mm/min (thus, 7.1 mm/min each). The upper trace is the X axis, with a stock DRV8825 module in mixed decay mode. The lower trace is the Y axis, with its DRV8825 hacked into fast decay mode.

The basic problem, about which more later, comes from the current rising too fast during each PWM cycle:

V = L di/dt
di/dt = 24 V / 3 mH = 8 kA/s

The first 1:32 microstep away from 0 calls for 5% of max current = 50 mA at a 1 A peak. The DRV8825 datasheet says the PWM typically runs at 30 kHz = 33 µs/cycle, during which the current will change by 270 mA:

267 mA = 8 kA/s × 33.3 µs

Some preliminary measurements suggest these (probably counterfeit) DRV8825 chips actually run at 16 kHz = 66 µs/cycle:

3018 X - ripple 1 step - 18V - A0 B-90 500mA-div
3018 X – ripple 1 step – 18V – A0 B-90 500mA-div

During those cycles the current can increase by more than 500 mA. The first scope picture shows an abrupt increase to maybe 700 mA, so, yeah, that’s about right.

Having the wrong current in one winding means the motor isn’t positioned correctly during those microsteps. The 3018-Pro runs at (an absurd) 1600 µstep/mm, so being off by even a full step isn’t big deal in terms of positioning.

The real problem comes from running nearly 1 A through both windings. Those little motors run really hot: they’re dissipating twice what they should be.

Anyhow, the pin layout shows the DRV8825 DECAY mode selection on pin 19:

DRV8825 pinout
DRV8825 pinout

Which sits on an inconveniently skinny little PCB pad, fifth from the left on the bottom:

DRV8825 PCB - open Decay pin
DRV8825 PCB – open Decay pin

Memo to Self: Don’t make that mistake when you lay out a PCB. Always put a little pad or via on a disconnected pin, so as to have a hand-soldering target big enough to work with.

The objective is to pull the pin high:

DRV8825 DECAY pin settings
DRV8825 DECAY pin settings

Pin 15, in the lower left corner, provides the output of a 3.3 V linear regulator, with its PCB trace connected to the left side of the ceramic cap:

DRV8825 PCB - Decay pin wired low
DRV8825 PCB – Decay pin wired low

On the scale of TSSOP packages, even 30 AWG Wire-Wrap wire looks like a bus bar!

Those are two different PCBs. The crappy TI logos, not easily visible in those low-res pix, on both ICs suggest they’re by-now-typical counterfeits, so seeing a factor-of-two difference in PWM frequency isn’t surprising.

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