Archive for category Science

Magnetic Field Visualization

Thinking about springs to apply downforce on plotting pen holders suggested magnets, so I extricated some neodymium bars from my collection of power toothbrush heads:

Magnets - single

Magnets – single

A snippet of magnetic field visualization film shows a dipole pattern:

Magnets - single - field visualization

Magnets – single – field visualization

Snapping two of them together in line:

Magnets - in line

Magnets – in line

… produces a quadrupole:

Magnets - in line - field visualization

Magnets – in line – field visualization

Now, if only I had some magnetic monopoles, this whole thing would be easier!


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Monthly Science: Hawk and Squirrel, with Turkeys

All of the local turkeys come together during snow storms, often lingering in the circle of pine trees in our back yard to get some protection from the wind. Mary spotted a Cooper’s Hawk in the midst of the turkey flock, with its wings spread around a recently captured meal:

Hawk with squirrel - wings spread

Hawk with squirrel – wings spread

When she first saw it, the hawk had its back to us and looked like a cluster of dead pine branches; the recent back-to-back storms have cleared out quite a bit of deadwood.

When I quietly opened the back door for a better view, the hawk noticed and gave me the stinkeye from 100 feet away:

Hawk with squirrel - 2

Hawk with squirrel – 2

The flock had moved out of the pine circle to surround the hawk and examine the situation, although they weren’t harassing it:

Hawk with squirrel - 3

Hawk with squirrel – 3

We’ve counted 27 turkeys, more or less, on some days, well and truly outnumbering the hawk:

Hawk with squirrel - overview

Hawk with squirrel – overview

Fortunately, turkeys feed mainly on insects and seeds, rather than tearing into carrion, so they’re not competing for the prize:

Hawk with squirrel - detail

Hawk with squirrel – detail

Shortly after I gave up and went back inside, the hawk sank her (?) talons into the squirrel, lifted heavily into the air, circled around the pines, and flew off toward the Mighty Wappinger Creek out back.

A casual search suggests both the hawk and the squirrel weigh about 1 lb = 500 g: I’ll never complain about heavy grocery bags again!

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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

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:

IMG_20180305_161831 - MPCNC - Z Autolevel probe force.jpg

Which produced a similar graph:

MB Endstop Switch - spring constant

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

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.

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BLDC Fan RPM vs. PWM Duty Cycle

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

BLDC Fan PWM Test Fixture - schematic

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

BLDC fan PWM Test Fixture

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

BLDC Blower - RPM vs PWM - doodles

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

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.

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Water Hardness

A water hardness test strip recently arrived from Morton Salt:

Water harness test

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.


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

BLDC Blower – RPM I P vs V

The data points come from this blower:

Blower label - 24V 0.2A

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

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

Blower air flow test

Which produced:

BLDC Blower - RPM Flow vs V - anemometer

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

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

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

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.


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

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

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