High Accuracy GPS with RTK in Orlando

Global Navigation Satellite Systems (GNSS)  receivers have made incredible improvements over the last twenty years. I remember having to stand outside for 10 minutes waiting for a US only Global Positioning System (GPS) receiver to lock onto 4 satellites so that I could get a fix with less than 100m accuracy (due to selective availability).  Now, you can buy a $220 GNSS receiver that can track 60 satellite channels simultaneously, start from cold in 25 seconds, lock into signals from satellites launched by four different countries (the USA, Russia, European Union, and China) and gets 2.5 to 5 meter accuracy all on it’s own without correction signals.

Here is a plot of the calculated location for a stationary antenna over time without correction signals (3D fix mode):

GPS location wondering around a 1m accuracy circle
As you can see, all of the readings are within a 1 meter circle of accuracy, which is quite good for finding your location on earth, but not (quite) accurate enough to drive a robotic lawnmower around and miss the petunias. [And from day to day you may be off by a few more meters…]
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Build an 8×8 play structure with dimensional lumber

How to build an 8×8 play structure out of dimensional lumber.

8 foot by 8 foot play structure around a tree

360 view of the play structure. – Spherical Image – RICOH THETA

List of materials and tools for the basic structure (part 1)

Lumber:

  • 4x Pressure Treated (Ground Contact) 4×4 posts, 12′ long (8′ or 10′ possible for shorter platforms)
  • 2x Pressure Treated 2×8 beams, 8′ long
  • 9x Pressure Treated 2×6 Joists, 8′ long (possibly 10 or 11 needed if building box around a tree)
  • 6-8x Pressure Treated 2x4s (6 required for fancy corner braces and assembly bracing, but 8 suggested to make things easier. A few small pieces of scrap 2×4 are very useful for temporarily shelves to hold up boards if you are working alone.)

Hardware:

  • 12x 5/8″ galvanized lag bolts, 4″ long – mounting beams to 4×4 posts
  • 8x 5/16″ galvanized lag bolts, 4″ long – mounting end joists to 4×4 posts
  • 16x galvanized joist hangers (may need more if boxing the tree)
  • 100x 1.5″ galvanized structural screws – joist hangers to beams (can be replaced with 9 gauge galvanized nails if disassembly is not anticipated)
  • 100x 2.5″ galvanized structural screws – joists to beams (can be replaced with galvanized nails)
  • 1lb box of 3″ deck screws (used for attaching bracing together and temporarily mounting beams/joists, not actually part of the finished play structure)
  • Water sealing wood stain (to color/preserve the wood)

Tools Required:

  • Digging tools: Shovel & Post Hole Digger (+ clippers to cut small roots)
  • Wheelbarrow or other way to transport dirt and mix concrete
  • Hoe or concrete mixer, razor knife to open concrete bags
  • Cordless Drill, drill bits, screw drivers
  • Framing Level (4′ or longer suggested), large framing square
  • Tape Measure & pencil,
  • Metal rod (probe for roots), spraypaint or pegs to mark digging locations
  • Adjustable Wrench and/or ratchet driver and sockets for lag bolts
  • Hammer for setting the joist hangers and “persuading” joists into position.
  • Step ladder if you are building a tall platform, or are short.
  • Brush or sprayer for applying wood stain.

List of materials and tools for the flooring and corner braces (part 2)

  • 2x 4×8 exterior grade plywood (for floor)
    or
  • 17x 5.5″ composite deck boards (for floor)
  • 2 lbs of 2″ deck screws
  • Circular saw to clean up the edges of the deck boards (if used)
  • 4x-6x Pressure Treated 2x4s (8′ long) for the corner braces
  • Miter Saw (chop saw), circular saw, or hand saw (for simple corner braces)
  • Compound miter saw (if doing fancy corners)
  • 8x 5/16″ galvanized lag bolts (3″ long) for corner brace to beam/joist connections
  • 8x 5/16″ galvanized lag bolts (4″ long) for corner brace to 4×4 post connections
  • Premium waterproof wood glue for corner brace assembly
  • 2.5″ galvanized nails (or extra 2.5″ structural screws) for corner brace assembly

 

If you have a VR headset, or just want to wave your phone around in the air, you can watch this 360 video of my son running through the play structure:

Salvage 2013 Nissan Leaf modules – 7 year old range update

Back in January of 2016 I put a set of battery modules harvested from a salvage 2013 Nissan Leaf into my S-10 conversion electric pickup. In march of 2016 I drove the truck for a while to see what its range was. [More than 46 miles, as I got tried of driving. The pack had a capacity of at least 15 kWh at that point in time.]

37.4 miles on trip meter.

Today I drove the truck for 35.8 miles before the low cell warning beeper from the BMS started to alert. After I got home [37.4 miles total], the average cell voltage of the pack was 3.75, while my (one) lowest cell was down at 3.3 volts. As it turned out, that cell must have started the trip out at a lower state of charge / voltage from the other cells, as it was still low when charging finished and I had to manually add charge to it individually. [My BMS does a good job of alerting at high/low voltage conditions, but does not do much for balancing the pack.]

According to my JuiceBox, the pack required 14.74 kWh to recharge, which is a good estimate on the battery pack’s current capacity. [This is almost exactly the same amount of power that I used in the trip in 2016, but I didn’t go as far due to different driving conditions. And I also hit the bottom of (at least one cell’s) state of charge.

The 2016 trip averaged 322 watt-hr/mile. This trip consisted of a lot of stop & go city driving as well as a few lengthier stretches of 49 mph arterial streets, and I wasn’t light on the accelerator. My measured watt-hour / mile (from the wall, including charger losses) was: 394 watt-hr/mile

Assuming that the pack has a 15 kWh capacity, this is 63% of the brand new 24 kWh capacity, which means I lost 37 % of the capacity over 7 years. (Some of that was in the original Nissan Leaf, but most of it was in my s-10 conversion.)

I’ll repeat the test after balancing my cells a bit better and see how things go.

Update: I drove the truck until the low cell beeper came on again. I went a total of 38.5 miles, and recharged the pack with 16.69 kWh (16690 watt-hours). The relatively higher  433 watt/hours per mile number is a result of the weather being a lot cooler so I was running the heater in the truck and more 45 mph roads. Balancing the cells got the usable pack capacity (measured from the wall with charging inefficiencies) to 16.69 kWh (which could have theoretically gotten me to 42 miles at 394 watt-hour/mi or 51 miles at 322 watt-hr/mile)

The main take-away is that at 16.5 kWh, I still have access to 68% of the brand new 24 kWh capacity Leaf pack, which isn’t too shabby for a 7 year old battery.

 

 

 

Thermoelectric cooler mark 3.5

I took my version 3 prototype ThermoElectric cooler and removed two of the four TEC modules, bypassing them in the cooling loop, to reduce the power draw.

Running two TEC’s at 12v each (in parallel, a sort of “turbo” mode) the whole system draws 136 watts. When I put the TEC’s in series (6v each, or “eco” mode) the whole systems draws 46 watts. This breaks down at 5 watts for the power supply, 11 watts for the fans & pump, and 30 watts for the two TEC’s.

Later on, I also moved the fans to 6v each and reduced the total power draw to 41 watts (the fans went down 5 watts when I reduced their voltage by half).

I’m using a cheap low-efficiency 12v power supply that draws 5 watts all on it’s own just idle, so we could get a 4 watt savings by running if off of a nicer power supply, or a 5 watt savings by running it from a 12v battery directly.

The cooling power is significantly reduced from the 4 TEC version, but I think that having a “turbo/eco” switch that would allow the unit to go from 12v operation on the TEC’s and fans to 6v operation (jumping from 136 watts down to 41 watts) would give the user flexibility to either cool things down when excess power is available, or just maintain temperature when operating off of battery power.  However, even in “eco” mode it takes almost a kWh per day of operation.  But at least it outperforms the Chefman TEC.

 

 

Insulation & heat loss of my DIY cooler

My DIY TEC Cooler has an interior volume of 480 cubic inches (6x8x10) and an interior surface area of 376 square inches (2.61 sq ft). It has an exterior volume of 1.55 cubic feet (12x14x16) and an exterior surface area of 1168 square inches (8.11 sq ft).  It generally has 3 layers of 3/4″ poly-iso insulation (R5) plus a small amount of one-part urethane expanding foam (say, R2?) in some areas, for an estimated R 17 insulation value (sorry, I’m using imperial units here as my insulation comes with R values….)

To calculate the amount of heat that will escape from inside my cooler to the outside (the amount of heat loss I need to counteract with the TEC system to maintain a set 34°F temperature on a 77°F day), we need to know the thermal delta between the inside and outside of the fridge.  (I’ll use 34°F for a good refrigeration value, and 77°F for the exterior temperature).

I’ll also use the average value of the interior and exterior surface area ((2.61+8.1) / 2) = 5.36 sq ft for this calculation. As a reminder, the equation to calculate heat loss in BTU/h is:

equation for calculating heat loss in btu/hours

In imperial measurements:
[5.36 * (77-34) ] / 17 = 13.55 BTU/h

13.55 BTU/h  divided by  3.41 = 3.97 watts

Using the SI system with things translated appropriately gives similar numbers:
0.49796 (25-1) / 2.99 = 3.99 watts

Of course, the above numbers may be completely incorrect, so I also did an experiment after building the cooler:

At 4:45pm I placed 3 refrigerated 12oz (355ml) cans of generic Dr. Pepper in my homemade DIY cooler with a temperature of 3.8°C.  [The active TEC elements were turned off, as I was just testing the insulation properties.]

At 4:21am the next day (11 hours 36 minutes later, rounded to 11.5 hours hereafter) I opened the cooler and one can, measuring the interior temperature at 14.6°C. [we’ll assume all three cans gained the same amount of heat…I only wanted to drink one can.]

So, 36 fl oz (1065 ml) of (basically) water gained enough energy to raise its temperature 10.8°C in 11.5 hours. The specific heat of water is 4.184 J/g-K. 1065ml = 1065 grams of water, or just about 1 kg. Nice how that works out.

4.184 J/g-C * 1065 g * 10.8 C = 48124.3680 Jules = 48.124 kJ = 0.0134 kWh = 13.4 Watt/hour

13.4 wh / 11.5 h = 1.1652173913 watts of continuous energy transfer from the outside to the inside of my cooler (heat gain, or cold loss).

You might notice that the calculated 3.9 watts is not equal to the observed 1.16 watts.
The main reason for this is that the interior of my cooler was never at 34 °F. It started at 3.8°C (39°F) and then raised up to 14.6°C (58°F) over 11.5 hours. It spends more time a higher temperatures, as the rate of heat transfer decreases as the thermal delta decreases. [Also, the ambient temperature was closer to 71.6°F, so the difference between the interior and the exterior was significantly smaller than in my previous calculations.]

For example:
[5.36 * (71-39) ] / 17 = 10.09 BTU/h / 3.41 = 2.95 watts
[5.36 * (71-58) ] / 17 = 4.099 BTU/h / 3.41 = 1.20 watts

However, the integration of the above numbers over 11.5 hours would still give me more heat loss than I observed. So either my experimental measures had a flaw, or the R value of my cooler is higher than the estimated 17.

However, as the results are of the right order of magnitude (4 watts vs 1.1 watts), I’m happy with my calculations and the experiment, and feel that the 4  watts of cooling power needed to maintain a 34°F interior temperature is a good upper bound on the performance needed by my TEC system to maintain temperature.

DIY Thermo-Electric Cooler prototype

I have been playing around with building a DIY Thermo-Electric cooler. Yes, I know the TEC’s are horribly inefficient when compared to a compressor based refrigerator. And I know you can buy basic TEC micro-fridges for $20-$50 online.  We have a camping van that has a small odd sized hole that doesn’t quite fit any of the commercially available car/van coolers, so I’m investigating building my own. This post will discuss prototype number 3.

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Installing Garage Door Slide Locks

My garage has four doors (two in the front, and two in the back) which gives a lot of cross-ventilation potential, but unfortunately some of the doors had the slide-locks installed incorrectly, such that there was no available slots to lock the doors in a “slightly open” position to let air circulate.  They also only had one lock per door, so I rectified that situation by adding a 2nd slide lock to the other side of each door, and moving a few of the original slide locks so that two of the doors can be locked with a 2″ gap below them.  I spent less than $30 for all four slide locks and a box of self drilling sheet metal screws, so it was a relatively quick and inexpensive improvement.

DIY 4×8 Floating Dock section

My last 8×8 floating dock section was built from mostly salvage materials. I’m slowly adding sections until it reaches shore. Unfortunately, I can’t use the cylindrical foam floats as the base of walkways, as they will rotate/spin in the water. (Also, I have plans for the other 2 cylindrical foam sections….)

Two sections of floating dock on lake

So this 4×8′ section of floating dock uses two commercial roto-molded dock float sections (48x24x16″), which drove the price up to around $680 in materials. (But I have a decent number of composite deck boards and hardware left for the next (3×12′) section I plan on building.  [Yes, every section of my dock will have a different width, deal with it.]