Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
For the last year or so, the oven temperature control on our Kenmore gas stove has been decreasingly stable, sometimes varying by 100 °F from the setpoint before settling down somewhere close to what it should be. Spotting a replacement control board for a bit over $100, I decided the board used an absolute rotary encoder of the open-frame variety, so I took the thing apart:
Kenmore oven control – PCB overview
The encoder was, indeed, an open frame:
Kenmore oven control – rotary encoder
The red droplet is DeoxIT, the rest of which went inside, just ahead of the contact fingers, and got vigorously massaged across the switch contacts on the wafer by spinning the shaft.
Some time ago, the membrane over the TIMER ON/OFF switch cracked and I applied a small square of Kapton tape. Having the entire controller in hand, I replaced the square with a strip of 2 inch Kapton, carefully aligned with the bezel marks embossed on the membrane, and now it’s smooth all over:
Kenmore oven control – Kapton tape cover
The MIN(ute) ^ switch required a much firmer than usual push, so I tucked a shim cut from a polypropylene clamshell between the membrane and the pin actuating the switch.
I set up an orthotic shoe insert on the MPCNC and unleashed the Z-Axis height probe on it:
Orthotic – bottom probing
In principle, the grid keeps the object aligned with the machine axes and the blocks put the upper surface more-or-less parallel with the platform. The XY origin, at the G28 location I’ve been using for tool changes, is on the midline of the sole, with Z touched off by probing the platform beside the sole.
The only interesting part of the orthotic is the rigid white plastic plate, which extends about 20 mm into a pocket in the black foam, so the probe area excludes the bendy part.
I’m abusing the bCNC Auto-level probe routine to get the height map, because it produces a tidy file of XYZ coordinates with three header lines describing the overall probe area:
The first two lines give the X and Y coordinate ranges and number of samples. The third line is the Z axis range and probe speed (?). After that, it’s just probed XYZ coordinates, all the way down.
Meshlab can import ASC files consisting of XYZ coordinates, with the ability to skip a specific number of header lines:
Meshlab ASC file import – header lines
If you don’t skip those three lines, then you get three additional points, far off in XYZ space, that will confuse the next step.
Checking the Grid Triangulation box (the default) produces a nicely lofted sheet:
Orthotic – R bottom triangulated
It is, however, a single-sided sheet, not a manifold 3D object. After a few days of screwing around, I’m unable to find any (automatic, reliable, non-manual) way to solidify the thing in Meshlab, so just save it as a PLY file in ASCII format:
Import it into Meshmixer, Ctrl-A to select the whole thing, click (Select →) Edit → Extrude, pick Y-Axis and Flat EndType, then extrude a convenient base in the negative direction:
Meshmixer – Y-Axis extrusion
For whatever reason, some 3D programs show machine-tool coordinates with Z pointing upward and others aim the Z axis at your face. Both must have made sense at the time, because Meshmixer defaults to swapping the Y and Z coordinates on import / export.
The Density slider controls the number of generated faces in the extruded section, so tune for best results.
I have no idea what Harden does.
Accept the result and you have a solid object suitable for further modeling.
For reasons not relevant here, we recently decontaminated a second lift chair, this one in bariatric size (so it doesn’t suffer from fuzz-shaving struts) with a six-switch control pod:
Pride lift chair control – dimmed LEDs
The green LED-lit buttons were so bright I took it apart to see what could be done; the picture shows the considerably dimmed result.
Start by prying outward on the tab at the USB charging port:
Pride lift chair control – USB port latch
Done right, you can then release the latches along the sides:
Pride lift chair control – side opened
It’s impossible to photograph the PCB with the LEDs active, but here’s what it looks like without power:
Pride lift chair control – PCB overview
The eight (!) SMD LEDs align with light pipes around the switch openings:
Pride lift chair control – button keys
The black dots come from Sharpie ink daubed in the shallow recesses intended to nestle around the LEDs. Note that the four switch caps have unique keying, so you can’t put them back incorrectly without some effort.
While we’re inside, here’s a closer look at the cable entry point, just in case I must replace the industrial-strength coily cord:
Pride lift chair control – cable entry
Unfortunately, it has a five-conductor cable, so a cheap phone coily cord (remember when phones had coily cords?) won’t suffice.
The PCB sports a pair of PICs, one of which seems to handle the buttons. I betcha the cable dates back to the days of hard-wired power switches, with the PIC now handling the intricate logic of deciding which motors to actuate for each function, then controlling MOSFETs as fake switch contacts.
The other PIC snuggles against the USB interface, which the manual describes as a charging-only port. It might also serve as a programming interface for the main PIC; admittedly the notion of a firmware upgrade for a lift chair seems far-fetched.
Reassembly is in reverse order with a resounding snap at the conclusion. It works fine and you (well, I) can now look at the control pod without sunglasses.
I picked up a pair of Raspberry Pi V1 cameras, both of which arrived unstuck to their breakout board:
RPi V1 camera adhesive
Requiring the customer to peel off the white layer and stick the camera to the PCB helps keep costs low. They’re $4 if you’re willing to wait two months or $7 from a “USA Seller”.
I got an Alead / Nolan HearLinks (many adjectives) Telecoil receiver to boost my ability to hear music & presentations at Vassar, because they recently slotted telecoil loops into the floors of their public venues. It took a few concerts to get the appropriate volume setting, after which I wondered how sensitive the receiver was:
Alead T-coil receiver – test setup
The small T in the upper right corner marks the receiving coil location, with the coil oriented parallel to the body’s long axis. It’s the secondary winding of an air-core transformer with a single-turn (perhaps using Litz wire) primary embedded in the floor, with the induced voltage obeying the usual transformer equation:
For a given installation and receiver position, pretty much everything is fixed, with the voltage depending only on the H field caused by the primary winding current.
The induced voltage is linearly dependent on the frequency, but the transmitter equalization filters apparently flatten the spectrum to get equal receiver amplitude between about 100 Hz and 5 kHz.
The coil in that picture has nine turns, with four passing through the Tek current probe. Applying 10 mVpp to the winding produces a corresponding current:
JDS6600 10mVpp 1 kHz – 4 turns – 1 mA-div
The scope sees 14 mVpp = 1.4 div at 1 mA/div = 1.4 mA. Dividing by 4 turns means the coil actually carryes 350 µA. The signal generator has a 50 Ω output impedance, so 10 mV should produce about 200 µA, which seems a bit low. On the other paw, the signal generator sees the coil as a dead short at 1 kHz, so I don’t trust the numbers.
Whatever magnetic flux it may be produces a 1 kHz tone at a somewhat higher volume (for the same receiver setting) than the fancy Vassar loops, so the flux is in the right ballpark. With a bit more attention to detail, perhaps I can tinker up a current-mode loop drive amplifier.
The Alead receiver has an internally generated tick audible at the audio volume I need for the Vassar loops, which is 5 to 7 steps down from the maximum volume at 15 steps. It seems related to the internal Bluetooth hardware, although it’s present even when the receiver is not paired with my Pixel phone and, in fact, is unchanged even when 100 feet from the nearest electronic device.
When I reported the problem, they said:
Yes, you can hear very minor tick sound on telecoil mode. It is caused by some electronic and current to make those tick sound. Sorry for this defective on the design.
It had one job that it doesn’t do well, so it’s on the way back for a refund.
Evidently, I must build an audio loop receiver to get what I want …
The display on Mary’s Cateye Astrale “Cyclocomputer” had once again faded to gray, so it’s time for a new CR2032 lithium cell:
Cateye Astrale – battery change 2019-09-22
The old cell read 2.5 V, well below what it should be.
The notes scrawled on the cell become readable under better light:
Cateye Astrale – CR2032 life
Seven years (at 1942 mile/yr) ain’t bad at all!
To replace the cell fast enough to maintain the odometer reading, just unscrew & remove the battery cover, slam the back of the Astrale on the bench, and pop in the new cell.
Maybe I should replace the cell twice a decade, regardless of how feeble it might be?
Having previously concluded running the CNC 3018-Pro steppers from 12 V would let the DRV8825 chips provide better current control in Fast Decay mode at reasonable speeds, I wondered what effect a 24 V supply would have at absurdly high speeds with the driver in 1:8 microstep mode to reduce the IRQ rate.
So, in what follows, the DRV8825 chip runs in 1:8 microstep mode with Fast Decay current control. You must apply some hardware hackage to the CAMTool V 3.3 board on the CNC 3018-Pro to use those modes.
In all the scope pix, horizontal sync comes from the DRV8825 Home pulse in the top trace, with the current in the two windings of the X axis motor in the lower traces at 1 A/div. Because only the X axis is moving, the actual axis speed matches the programmed feed rate.
Homework: figure out the equivalent two-axis-moving speed.
The 12 V motor supply works well at 140 mm/min, with Fast Decay mode producing clean microstep current levels and transitions:
3018 X – Fast – 12V – 140mm-min 1A-div
The sine waves deteriorate into triangles around 1400 mm/min, suggesting this is about as fast as you’d want to go with a 12 V supply:
3018 X – Fast – 12V – 1400mm-min 1A-div
Although the axis can reach 3000 mm/min, it’s obviously running well beyond its limits:
3018 X – Fast – 12V – 3000mm-min 1A-div
The back EMF fights the 12 V supply to a standstill during most of the waveform, leaving only brief 500 mA peaks, so there’s no torque worth mentioning and terrible position control.
Increasing the supply to 24 V, still with 1:8 microstepping and Fast Decay …
At a nose-pickin’ slow 14 mm/min, Fast Decay mode looks rough, albeit with no missteps:
3018 X – Fast – 24V – 14mm-min 1A-div
At 140 mm/min, things look about the same:
3018 X – Fast – 24V – 140mm-min 1A-div
For completeness, a detailed look at the PWM current control waveforms at 140 mm/min:
3018 X – Fast detail – 24V – 140mm-min 1A-div
The dead-flat microstep in the middle trace happens when the current should be zero, which is comforting.
At 1400 mm/min, where the 12 V waveforms look triangular, the 24 V supply has enough mojo to control the current, with increasing roughness and slight undershoots after the zero crossings:
3018 X – Fast – 24V – 1400mm-min 1A-div
At 2000 mm/min, the DRV8825 is obviously starting to have trouble regulating the current against the increasing back EMF:
3018 X – Fast – 24V – 2000mm-min 1A-div
At 2500 mm/min, the back EMF is taking control away from the DRV8825:
3018 X – Fast – 24V – 2500mm-min 1A-div
The waveforms take on a distinct triangularity at 2700 mm/min:
3018 X – Fast – 24V – 2700mm-min 1A-div
They’re fully triangular at 3000 mm/min:
3018 X – Fast – 24V – 3000mm-min 1A-div
In round numbers, you’d expect twice the voltage to give you twice the speed for a given amount of triangularity, because the current rate-of-change varies directly with the net voltage. I love it when stuff works out!
At that pace, the X axis carrier traverses the 300 mm gantry in 6 s, which is downright peppy compared to the default settings.
Bottom lines: the CNC 3018-Pro arrives with a 24 V supply that’s too high for the DRV8825 drivers in Mixed Decay mode and the CAMTool V3.3 board’s hardwired 1:32 microstep mode limits the maximum axis speed. Correcting those gives you 3000 mm/min rapids with good-looking current waveforms.
I’m reasonably sure engraving plastic and metal disks at 3000 mm/min is a Bad Idea™, but having some headroom seems desirable.
The Smell of Molten Projects in the Morning 