Monthly Archives: July 2013

Project and Task Management

After watching a short 2 min video, it dawned on me that I can make better use of my time if I pre-planned in detail most of my hobby projects.
A lot of what is mentioned in that video I felt applies to me, so I’m going to make an effort to streamline how I approach my hobbies.
Often when I have some energy to tackle a task in the evening, I am am grasping at straws for what to do mainly because I don’t know where to start or which project to work on… and for the most part this turns into procrastination. A little bit of youtube, a little bit of facebook… and the night is wasted.

I am going to pre-plan all the tasks I need to accomplish for all/most my hobby projects going on at the moment. The hope is that when I have 15 or 30min available, I can just pick up a short task, and complete it without having to look at the big picture (in the moment). Just get the task done and move on.

3D Printing resolution

With FDM (Fused Deposition Method-printing with a filament) 3D printers there’s always a question about the accuracy or precision of the print.

I read a great description of the misconceptions of how accurate 3D prints using this method can be:
“Clarification: the precision of the printhead in the horizontal plane (X-Y direction) is about .011mm (about 2300dpi). However, this number is a little superflous because we are extruding ABS plastic through a relatively larger .35mm nozzle, and all ABS plastic oozes a bit. So a more realistic & practical estimate of resolution in the horizontal plane is about .1mm. And to be crystal clear – this creates great prints. Trying to define the resolution more accurately than this is similar to trying to define the position of a garden hose nozzle to within millimeters – it’s essentially meaningless since the water is going to expand anyway.”

SOURCE

Timing belt sizing for quadrotor power transmission

Looking for info on designing timing belt power transmissions. Some sites I’ve found of interest:

http://www.roymech.co.uk/Useful_Tables/Drive/Timing_belts.html

http://us.misumi-ec.com/pdf/tech/mech/p2823.pdf

http://www.sdp-si.com/D265/PDF/D265T003.pdf

The tooth profile of the FT2 and HTD belts have a much higher resistance to skipping combined with a higher power transfer.

http://www.hpcgears.com/technical.htm
Technical data on belts, gearing, chains.

http://www.contitech.de/pages/produkte/antriebsriemen/antrieb-industrie/download/TD_Synchrobelt_HTD_en.pdf
HTD belt data sheet

http://www.gates.com/europe/file_display_common.cfm?thispath=Europe%2Fdocuments_module&file=Gates_PU_Catalogue_2012_E2%2Epdf
M
ore HTD belt design data sheet

http://www.skf.com/files/896756.pdf
SKF “Power Transmission Belts” manual ; from page 128 onward for timing belt sizes and calculations

http://www.gates.com/login/register.cfm?requesting=powergripManual
Gates manual for “Powergrip GT2 Belt Drives” (may have to register; it’s free)

http://www.gates.com/login/register.cfm?requesting=llpdesign
Gates design manual for “Light Power and Precision” (may have to register; its free)

SUPPLY

Looking for places to get cheaper timing belts:

http://www.robotdigg.com/category/9/Timing-Pulley&Belt
Chinese outfit. Prices reasonable, minimum order is $25 shipping

http://reprap.org/wiki/Choosing_Belts_and_Pulleys
Supplier list of timing belts and pulleys (bottom of page).

DESIGN

What I learned from going over the SKF and Gates design manuals for timing belts and pulleys: Belts transmit a certain amount of power. That power is based on the rpm of the pulleys. Taking the RPM out of the equation, the force on the belt is relatively constant for (for a certain RPM). The torque varies but only because the gear diameter changes. Between the different timing pulley sizes (at a constant RPM) the torque varies based on the pulley diameter. The force on the belt is relatively constant.

The force on the belt however seems to vary with RPM for some reason. Have to understand why that is. Also have to understand why the belt length affects the force transmitted by the belt.

The torque and power numbers listed in the tables is based on 6 tooth engagement. Less then 6 teeth requires a correction factor.

Going with a GT2 pulley system. It provides a much better power transfer with less skipping. Even a small GT2 2mm pitch belt at 6mm wide will be able to transmit about 70w of power. The 4mm wide belt should transmit about 47W of power.
Equation for Power: P (W) = Torque (N-m) * RPM * (2*pi/60)  (source)

Using the data from the “Gates Light Power/Precision” design manual, a GT2 5mm(pitch) belt can transfer nearly 3 times the power at the same belt width and RPM then a XL belt.

For the GT2 belts, the 3mm pitch belt can carry 4 times the load of the 2mm pitch belt and the 5mm pitch belt can carry 5 times the load of the 3mm belt. So instead of increasing the belt width, you get much more bang for the buck by going to the next size pitch belt.

In contrast with the  Trex450 belt is 2.03mm(MXL) pitch or 0.8 inch pitch, 3mm (1/8″) wide. This MXL belt can transmit about 30W of power.
From THIS POST I was able to get about 700g of thrust = 50ish W with only 5 teeth engaged on the 11T pulley at around 2000RPM. Since only 5 teeth were engaged (6 is minimum) only 80% of the possible power was transmitted, so i could possibly transmit 40ish W with all 6 teeth engaged. I was actually exceeding the design guidelines in terms of power transmitted but only by a little bit.  Going with a larger puley of 19T (see this post) I was able to get 1080g of thrust = 90ish W.
I was pushing the system way beyond its design capabilities. Likely the timing belt design data is conservative.
With a GT2 belt, I should be able to transmit WAY more power with a similar sized belt. With a 14T pulley, at 3600 RPM,

For a 3kg AUW (2kg quad and 1kg payload), I would aim for a 3:1 power to weight ratio, so need to be able to generate 9kg of thrust. Using two 250 blades (250mm long, 22mm chord) I need about 4500RPM at 10deg blade angle to get 9kg of thrust from all 4 rotors. With a 2 bladed rotor I’d be sucking down 911W. Slowing down the rotor to 3500rpm and adding 2 more blades (4 total) 9kg of thrust can be produced at 880W. Goes to show how much drag increases with RPM.
Designing for 3500 RPMs and 1KW power usage (250W per rotor)

Quadcopter frame rebuild

The Turnigy micro quad V2 I bought from hobbyking was great for the first little while. Then with each crash more and more parts of the frame kept breaking despite my constant reinforcing of the frame after each crash.
So a few months ago I got to a point where patching up splintered frame made no sense anymore.
So I designed my own frame and 3d printed it.

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The white frame is the original frame that came with the quad and weighs 44g. The red frame is the new 3d printed frame and it’s a little heavier at 58g. Only 14g heavier.

So far I’ve flown the new frame for maybe 5 hours and crashed it more times I can count. A good number of crashes are hard landings on grass (so not very stressful top the frame) and another good number of crashes have been quite hard. Amazing into the fence from 10ft, of into the side of my house, or the tree in the back yard, or onto patio stones.
This new frame has takes A LOT of beating with no I’ll effect. Hard landings will break props (I have another bunch coming as I’m running out) but otherwise I’m surprised that everything seems to continue to work.

I’m actually thinking of refining the frame and making it lighter side it’s obviously too strong. It could end up lighter then the original frame if I design it right.

Audio offset in Sony Vegas Video

Editing some video in Sony Vegas Video I noticed that some of the footage has out of sync audio with it.
I don’t know where this sync problem initiated (as the original footage is in perfect sync) but it’s there. I usually convert the MOV file to an AVI and then run it through a video stabilization software before using the footage, so anywhere along that path the probem could have arisen.

Anyway, the solution in Vegas is to ungroup the video and audio tracks, and shift the audio track to the right by 3 frames. With most video footage this is not noticeable, but when viewing a musical performance, the slight shift is very noticeable (at least to me). The vocalist’s mouth movement seen slightly off and the drummer looks not in sync.

UPDATE:
When the final movie is rendered, there is no delay (using original footage) problem only seems to occur when previewing within Vegas Video