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Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Category: Science

If you measure something often enough, it becomes science

MPCNC: Makerbot-style Endstop Switch Spring Constant

Using a lever-arm switch as a tool length probe works surprisingly well:

MPCNC Tool Length Probe – Plotter Pen

However, probing a pen mounted in a compliant holder means the actual trip point depends on the relative spring constants. Having measured the pen holder’s 100 g/mm spring constant by poking a scale with the pen, I did much the same thing with the endstop Z-axis Autolevel probe:

Which produced a similar graph:

MB Endstop Switch – spring constant

The force increases linearly at 30 g/mm up to the trip point, drops by maybe 16 grams, then increases linearly again.

Obviously, the “constant” applies only to switches on MBI-style endstops in the lot I happen to have, but given the ubiquity of parts from the usual eBay sellers, any identical lever switches may have the same “constant”:

Endstop lever switch – detail

Your mileage will vary, fer shure.

Poking a pen into a similar switch used as a tool setter means the Z-axis coordinate of the trip point will depend on the opposing springs. That’s unlike the situation with a cutter mounted in the DW660 spindle, which (by definition) shouldn’t move in response to the pressure from a little bitty switch.

Eyeballing the graph, the switch travels 2.2 mm to the trip point, where it exerts 64 g of force. The pen holder opposes that force and therefore deflects (64 g) / (100 g/mm) = 0.64 mm just before the switch trips: the trip point will be the same as with a rigid tool, but the tool’s Z axis coordinate will be 0.64 mm lower.

I’d been touching off pens in the springy holder, with enough pressure to draw a decent line. Setting Z=0 with the holder deflected upward by 0.3 mm means the pen first touches the height probe at Z=+0.3 and the switch trips at Z=-0.3 mm (-ish), making the force on the paper 60 g, rather than the 30 g I expected.

I think the pen plots worked out pretty well, despite not getting the numbers and, thus, pen positions, quite right.

2018-03-27
BLDC Fan RPM vs. PWM Duty Cycle

A simpleminded MOSFET circuit provides PWM drive for the BLDC blower:

BLDC Fan PWM Test Fixture – schematic

The Tek P6302 current probe looms much larger in real life than in the schematic:

BLDC fan PWM Test Fixture

A quick dataset shows the RPM variation against PWM duty cycle:

BLDC Blower – RPM vs PWM – doodles

Unsurprisingly, the RPM curve resembles the earlier results against a variable DC supply voltage:

BLDC Blower – RPM I P vs V

Capturing the current waveform is stalled behind another project, but it has exactly the voltage spikes you’d expect from forcibly switching an inductive load.

2018-03-19
Water Hardness

A water hardness test strip recently arrived from Morton Salt:

Water harness test

I call it between 7 and 15 gpg. Based on the feel of the water just before regeneration, I’d been guesstimating 15 gpg, so it’s within reason.

I’ll back the softener off to 10 gpg and see what happens.

2018-03-04
Monthly Science: BLDC Fan Characteristics
I have often asserted, in public, in writing, that you can’t change the speed of a fan’s BLDC motor by varying its voltage, because the fan controller generates the waveforms responsible for the motor speed based on its internal timing.

A pair of BLDC blowers recently arrived and a quick test showed I’m pretty much completely wrong:

BLDC Blower – RPM I P vs V

The data points come from this blower:

Blower label – 24V 0.2A

The blower specs from the eBay listing:

75MM 24V Brushless DC Blower Cooling Fan Exhaust Fan
- Dimension:75(L)x75(W)x30(H)mm
- Connector:2Pin-PH2.0
- Rated Voltage: DC24V
- Rated Current: 0.2±10% Amp
- Rated Speed: 3800±10%rpm
- Air flow:1.8CFM
- Noise: 23±10%dBA
- Bearing Type: Sleeve
- Life: 35000 hours
- Cable Lenght: 32cm(12.5in)
- Weight: 75g/pcs
The case is about 75 mm × 75 mm × 30 mm, so the generic part number seems to be 7530, with many variations. However, they all seem to resolve to the same blower with different models drawing different current at specific voltages (clicky for more dots, JPG blurriness in original):

GDT7530S12B BLDC blower parameter table

The blower in hand roughly corresponds to the bottom line of the 24 V section:
- 0.21 A
- 4000 RPM
- 16.3 CFM
- 1.1 inch H₂O pressure
- 43 dBA
There’s a gross discrepancy between the eBay 1.8 CFM and the chart 16.3 CFM, but the other parameters seem within handwaving distance and, yo, it’s from eBay. ‘Nuff said.

The graph up top shows the results with an unrestricted output opening.

For more realistic results with some resistance to air flow, I taped a small anemometer to the blower output:

Blower air flow test

Which produced:

BLDC Blower – RPM Flow vs V – anemometer

In very round numbers, the anemometer aperture is 400 mm², so the 9 m/s air flow at 24 V works out to 3.6×10^-3 m³/s = 0.13 CFS = 7.6 CFM. Which is maybe half the 16.3 CFM spec, but they’re surely using a fancier anemometer with much lower back pressure. Close enough, anyway. Fer shure, 1.8 CFM is wrong.

Completely blocking the inlet with a plastic sheet to simulate the blower pulling air from, e.g., a vacuum table:

BLDC Blower – RPM vs V – blocked inlet

The RPM varies more linearly with voltage when the blower isn’t accelerating any air.

Some current waveform show why you really shouldn’t run fans in series to “split the power supply”, as seems common in 3D printers with 24 VDC power supplies.

From a 24 V supply, the current drops to 50 mA every 75 ms (200 mA/div):

BLDC 24V Blower – 24 V – 200mA-div

From a 12 V supply, even weirder things happen (50 mA/div):

BLDC 24V Blower – 12 V – 50mA-div

Note that you can’t reduce the fan’s supply voltage by applying PWM to the current, as happens in essentially all 3D printers for “speed control”. Basically, PWM turns the fan off several hundred times every second, which does not modulate the voltage.

I have no way to measure pressure, but if the 1.1 inch H₂O number comes close to reality, the blower can produce 1.5 lb of clamping force per square foot. Which isn’t a lot, granted, but it might suffice for paper and vinyl cutting.

The DRV10866 BLDC fan controller doc from TI is completely unrelated to the blower in question, but gives a reasonable introduction to the subject.
2018-03-01
Red Oaks Mill: Rt 376 Infrastructure Decay

NYS DOT’s recent Rt 376 repaving projects improved the road surface, but the infractructure seems to be crumbling apace, as we spotted on a recent walk across the bridge over Wappinger Creek:

Red Oaks Mill bridge – dangling concrete

The ragged edge of the deck shows other slivers have fallen into the creek.

My arms aren’t long enough to get a closer view:

Red Oaks Mill bridge – dangling concrete – detail

The concrete roadway is developing potholes in the right hand southbound lane, so the upper surface has begun crumbling, too.

I think the bridge dates to the mid-1990s, based on the aerial photo history from Dutchess GIS, so it’s a bit over twenty years old. Nothing lasts.

Repairing stuff is hard …

2018-02-24
Monthly Science: Wearable LED vs. Dead CR2032 Lithium Cell

Eight months later, the dead CR2032 cell driving the “ruby” wearable LED has dropped to 2.15 V:

Wearable LED – on window

It’s not a true red LED with a 1.5-ish V forward drop, but a white / blue LED with red phosphor or a red filter, with a forward drop well over 3 V.

Against the sunlit backdrop from our kitchen window, the LED looks dark:

Wearable LED – daylight

Seen in a dim room, it’s still glowing:

Wearable LED – dim light

The current is now far below the 1 mA/div of my Tek A6302 Hall effect probe, so I have no way to measure the few microamps lighting the junction.

The coarse grid outside the window is a swatch of deer netting we put up during feeder season to keep the birds from killing themselves on the glass.

2018-02-01
MPCNC: Stepper Motor Back EMF

A plot of the back EMF for an Automation Technology KL17H248-15-4A stepper motor looks like I’m making stuff up again:

KL17H248-15-4A stepper motor – Back EMF vs RPM – data

Maybe the only questions I ask are ones with linear solutions?

Anyhow, the data comes from the Z-axis motor in the lathe:

Stepper back EMF test setup

Scary-looking, but reasonably safe. The chuck holds the motor shaft so it’s not going anywhere, the boring bar prevents any rotation, and the motor bearings do exactly what they’re supposed to. Shorting the motor leads would definitely put a hurt on the PLA frame, so I didn’t do that.

The scope sat on the floor beside the lathe, capturing waveforms and doing calculations:

Motor Back EMF – 500 RPM

Some waveforms look bent:

Motor Back EMF – 300 RPM

I asked the scope to measure the RMS voltage, rather than the peak, because it’s less sensitive to distortions.

Each winding produces one electrical cycle across four mechanical full steps, with the windings in quadrature. One shaft revolution thus produces 200 / 4 = 50 electrical cycles, so converting from shaft RPM into electrical cycles/s goes a little something like this:

Electrical cycles/s = (shaft rev/min) * (50 cycles/rev) / 60 (s/min)

Which works out to a tidy 0.833 Hz/RPM, basically spot on the last data point’s 839 Hz at 1000 RPM.

The motivation for this comes from the third column in the scribbles: back EMF = 22.7 mVrms/RPM = 32 mVpk/RPM.

A rapid move at 12 k mm/min = 200 mm/s shows the motor current collapsing to the ragged edge of not working:

G0 X 200 mm-s – 24V 200mA-div

Converting motor speed to shaft RPM:

RPM = (axis mm/s) / (32 mm/rev) * (60 s/min) RPM = (axis mm/min) / (32 mm/rev)

So the shaft turns at 375 RPM when the X axis moves at 12 k mm/min, with each motor generating 8.5 Vrms = 12 Vpk of back EMF.

The MPCNC wires the two motors on each axis in series, so the 24 V power supply faces 24 V of back EMF (!) from both motors, leaving exactly nothing to push the winding current around. Because the highest EMF occurs at the zero crossing points of the (normal) winding current, I think the current peaks now occur there, with the driver completely unable to properly shape the current waveform.

What you see in the scope shot is what actually happens: the current stabilizes at a ragged square-ish wave at maybe 300 mA (plus those nasty spikes). More study is needed.

2018-01-26