Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
As you saw earlier the low-speed waveform looked reasonably good, although the HB-415M driver produces only 71% of its rated current (so it’s actually 1 A peak, not the 1.5 A in the caption):
HB-415M 8-step 1.5A 20V
The driver runs in 1/8 microstep mode, which means 1 revolution = 8 × 200 step = 1600 steps. Each cycle of that stepped sine wave has 32 microsteps = 4 full steps/cycle × 8 microsteps. One cycle is about 27 ms, so 1 step = 840 µs → 1200 step/s → 0.74 rev/s → 44 rpm. The Thing-O-Matic runs at 47 step/mm → 34 mm/rev, so this speed corresponds to travel at 25 mm/s, roughly the usual printing pace.
Admittedly, that hairball on the bench isn’t a realistic arrangement, because the motor runs with no load. On the other paw, assuming you’ve done a good job eliminating mechanical binding, then it’s probably pretty close to what you’d see during constant-speed travel.
Cranking the pulse generator to 6400 step/s = 133 mm/s produces this waveform:
HB-415M 1A 8step 24V
The power supply was 24 V, but there was no visible difference at 20 V. The driver evidently can’t control the winding current on the downward side of the waveform. Adding some frictional torque by grabbing the yellow interrupter wheel improved the situation, but not by much.
A box of 2M542 drivers just arrived from a nominally reputable supplier, although they were actually labeled M542ES. Under the same conditions, they produce this waveform:
M542ES 1A 8step 24V
So there’s something to be said for larger drivers; the HB-415M drivers were operating at their upper limit and the M542ES at their lower limit, both producing close to 1 A peak.
While pondering whether I should use the carcass of an old Dell PC to house the stepper drivers and control logic for the LinuxCNC M2 project, I bandsawed a scrap of aluminum sheet to about the right size. It had some truly nasty gouges and bonded-on crud, so I chucked up a wire brush cup in the drill press and had at it:
Machine jeweled baseplate
It’s obvious I haven’t done jeweling in a long time, isn’t it? Even a crude engine jeweling job spiffs things right up, though, even if a cough showcase job like this deserves straighter lines and more precise spacing. The aluminum sheet is far too large for the Sherline, which put CNC right out of consideration, and I’m not up for sufficient crank spinning on the big manual mill.
I match-marked mounting holes directly from the harvested motherboard and drilled them, whereupon I discovered that the aluminum is a dead-soft gummy alloy that doesn’t machine cleanly: it won’t become the final baseplate.
Memo to Self: Use the shop vacuum with the nozzle spinward of the brush, fool.
Just to see what’s inside, I took those HB-415M drivers apart. They’re not all identical inside:
HB-415M Driver – interior top
The other side shouldn’t come as much of a surprise:
HB-415M Driver heatsinking
Now, admittedly, I’ve applied a heatsink to the top of an epoxy package, but that DIP package has thermal tabs that should connect to the heatsink through a low-thermal-resistance path. A dab (!) of heatsink grease and what might be a thermally conductive plastic sheet atop the package seem, well, insufficient.
The driver chip sports an Allegro A3992 marking that might be genuine. The datasheet goes into some detail as to how you should lay out the PCB; none of its recommendations made it into the finished product. In particular, the hulking current sense resistors surely have more inductance than you’d like.
The resistor color code seems odd: black red red silver brown.
HB-415M current sense resisors
Using black as the first band is unexpected, but it’s probably the only way to indicate a low-value resistance without printing the numbers: 0.22 Ω ±1%.
As mentioned there, the usual eBay vendor shipped HB-415M drivers instead of the advertised 2M415 drivers. Based on the Chinese datasheet and some poking around, I got a test setup working with a bench supply, a signal generator, and a NEMA 17 stepper motor with 2 Ω windings.
First observation: the ENA input is active high. Pulling it low to turn on the optocoupler disables the drive output, which is exactly the opposite of what’s shown in the datasheet, which means that the driver will run quite happily with nothing connected to the ENA pin. The optoisolator current runs about 11 mA from a 5 V supply, close enough to the 10 mA typical spec, but the signal generator thinks it’s providing a TTL pulse output.
Second observation: the driver’s actual winding current doesn’t match the DIP switch setting.
Here’s the 1/8 microstep winding current for the 1.50 A peak setting, with a 0.5 A/div vertical calibration:
HB-415M 8-step 1.5A 20V
Sure looks like 1 A peak, doesn’t it?
The ratio seems close to 0.707 and remains consistent across all current settings, so I’d lay long money that the designer confused “peak” and “RMS” values, then figured the current sense resistor or chose the internal coefficients to produce the corresponding RMS current for the peak value.
The reduced current produces not very much torque at all; negotiations are in progress for a partial refund based on eBay’s “item not as described” process…
Just got a quartet of 12 V 7 A·h lead batteries, prompted by a big Belkin UPS that instantly shut down during a power blink. It needs only two batteries, but the shipping was the same for two or four and I’m sure the spares will come in handy.
A stiff 2 A discharge test shows that SLA batteries really don’t like high currents, which is exactly what they must provide in a UPS:
Rhino SLA – 2013-01
The capacity is barely 4 A·h at 2 A, not to mention that I’m using a conservative 11.4 V cutoff.
The two batteries with the highest capacity also were the closest matches, so they’re now in the UPS.
Turns out that the anonymous parallel port breakout board isn’t compatible with an optoisolated stepper driver: each output has a 1 kΩ series resistor that limits the current well below the driver optocoupler’s expectations. The driver has an internal 300 Ω resistor on each input, too, which doesn’t help in this situation.
A detailed look at the resistors lined up in front of the connectors:
Anonymous parallel breakout board – series resistors
The breakout board would work fine with non-isolated drivers, like the Pololu breakout boards, so it’s not really at fault. The fact that there’s no doc anywhere to be found means you (well, I) couldn’t discover this without buying it first, but … I suppose it’ll come in handy for something.
One could short across the resistors, but I intended to use this board for the initial bringup and all that soldering defeats the purpose.
Parallel port breakout boards of this ilk run about $14, complete with cable, on eBay:
5 axis parallel port breakout board
The PCB has no part number and the inferred URL isn’t productive. The “driver CD” accompanying it has doc for every possible board the vendor might sell and, absent a part number, the file names aren’t helpful. An exhaustive search suggests it corresponds to the HY-JK02-M 5-axis interface board manual.doc file.
Despite any implication to the contrary, the board does not have optoisolators between the parallel port pins and the outside world. The stepper driver bricks should, but the input signals from limit switches and suchlike connect directly to the guts of your PC.
This overview (from the manual) shows the physical pin layout (clicky for more dots) and reveals the hidden silkscreen legend:
HY-JK02-M Breakout Board – overview
It looks like the board I got added a spindle relay driver transistor, plus a few resistors over by the manual control connector on the right.
Notice that the fourth terminal on each axis is GND, not the positive supply required for the optoisolators on the 2M415-oid driver bricks, which means you can’t just run a section of ribbon cable from the breakout board to the brick. You’ll need a separate +5 V (or whatever) power supply wire for each brick, with a common return to the system ground for this board. Those terminals are firmly bonded to the top and bottom ground planes on the board, so there’s no practical way to re-route them.
The small switch in the upper left, just to the right of the parallel port connector, selects +5 V power from the USB port (which has no data lines) or the power connector in the lower left. The LED near the switch won’t light up until you have both the parallel port cable and the USB cable plugged in.
The doc includes a timing diagram with no numeric values. I established that it can’t keep up with a 500 kHz pulse train and seems content at 100 kHz, but that’s conjecture. Setting the timing to match whatever the stepper driver bricks prefer will probably work. The diagram suggests the setup and hold times for direction changes are whatever you use for the minimum time between step pulses.
This shows the functional labels:
HY-JK02-M Breakout Board – function labels
The parallel port connector output pins, sorted by function:
Pin
9
1
2
14
16
3
7
8
6
5
4
17
Function
Spindle
motor
Enabled
X step
X dir
Y step
Y dir
Z step
Z dir
A step
A dir
B step
B dir
The parallel port connector input functions, sorted by pin:
X -Limit
Y- Limit
Z- Limit
A- Limit
Emerg Stop
10
11
12
13
15
The table uses Chinese for Pin 15: 急停.
It’s not clear whether the pins on the manual control connector are inputs or outputs, nor what the three separate Enabled lines do:
P1
P2
P3
P4
P5
P6
P7
P8
P9
P10
P11
P12
P13
P14
P15
B step
B dir
A dir
Z step
Y step
X step
X dir
Enabled
5V/VDD
5V/GND
A step
Z dir
Y dir
Enabled
Enabled
The three white connectors in the middle drive an LED readout board that’s probably most useful as a DRO for CNC-converted manual mills using the pendant for positioning.
The small white connectors duplicate the functions of the green screw terminals. They’re probably useful in a small machine that I’m not building.
This isn’t the board I intend to use in the final setup, because I need far more I/O pins, but it’ll serve for the short term.
The Smell of Molten Projects in the Morning 