The fixtures are centered at X±70.0 mm / Y=0.0 from the G54 workspace coordinate origin dead-center in the middle of the platform, with G55 centered on the HD fixture to the left and G56 on the CD fixture to the right.
So the engraving workflow amounts to homing the CNC 3018 when I turn it on, taping a platter in a fixture, selecting the corresponding WCS, loading a suitable G-Code file, and firing it off. It seems bCNC returns to G54 after completing the file, so verifying the WCS selection every time is Very Good Practice.
The friable lacquer coating on some CDs fills my world with glitter whenever I engrave a pattern on their label side. I didn’t plan on a dust shoe for this thing!
A strip of double-sided foam tape attaches it to the Pi’s case, which is Velcro-ed to the M2’s frame. The cable may be too long, but avoids sharp bends on the way out of the case.
The whole lashup works fine:
That’s a second set intended for the CNC 3018-Pro, but it didn’t fit quite as well. The B brackets are slightly too long (or their pivots are slightly too close to their base) to allow the C plates to turn 90° to the mount:
Nothing one can’t fix with nibbling & filing, but I long for parametric designs …
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:
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 …