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Archive for December, 2014
Posted by Jack Crossfire | Dec 29, 2014 @ 10:49 PM | 6,767 Views
It takes about a day to process & upload a video to the goog, so many miles of driving video have now accumulated, all on the same test trail. It's accumulated over 50 miles, since its arrival 1 month ago. To drive an RC car 50 miles, you have to run 50 miles.

9 mile Tamiya drive (4 min 2 sec)

9 mile Tamiya drive. Side looking cam (4 min 39 sec)
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Posted by Jack Crossfire | Dec 28, 2014 @ 10:36 PM | 7,138 Views

A straight test of RPM vs PWM showed the dreaded stair stepping.

A test of minutes/mile using the most precise, slowest RPM feedback still showed stair stepping above RPM's where it oscillated. So either the Tamiya ESC wasn't precise enough or the mechanics had some voodoo. It's probably a limitation of all ESCs whether brushed or brushless. The next solution was to install the H-bridge from the G-buggy.

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Posted by Jack Crossfire | Dec 27, 2014 @ 04:19 AM | 6,822 Views
Posted by Jack Crossfire | Dec 25, 2014 @ 11:19 PM | 6,856 Views

The 1st tachometer sensed 1 wheel revolution. It barely had enough deflection to flip a logic level & took a very long time to get a reading.

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Posted by Jack Crossfire | Dec 24, 2014 @ 11:27 PM | 6,837 Views

Whacked on the ages old 5V BEC to try to maximize the range, then tried constant RPM set to 500 or 9m18s/mile.

The result was higher speed going uphill than downhill, with the total speed in the 10minute/mile range. It must have been the lousy tracking of rpm.

Then of course, the range was reduced to 4.5 miles. The BEC was actually less efficient than straight PWM. As for measuring power independent of voltage, only the current feeding the BEC was measured, so the power would vary as the ESC efficiency changed at different voltages. The rough figures showed 15W on the downhill & 17W on the uphill.

Then, there were the usual software problems. Throttle always reset during fast turns & sometimes reset during slow turns. Acceleration was too slow. A faster tachometer is needed.

Ran all 9 miles looking at data on the phone. Probably need to capture the bluetooth data. During these long runs, a wish list topped by automatic path following always forms. Automatic path following can be reduced to a simple problem: determining what's 1 material & what's another material in an image. 1 material is usually asphalt. The other material is grass, gravel, or dirt.

The computer always knows asphalt is on the left & other stuff is on the right. It can get the color on the leftmost & rightmost parts of the image. Then it can work inwards until the colors change.
Posted by Jack Crossfire | Dec 21, 2014 @ 06:10 PM | 6,749 Views
So what happens when power is constant & voltage is changed?

Was hoping there would be an identical percentage decrease in RPM for all power. Instead, the RPM for 25W decreased 9% while the RPM for 10W decreased 4% as the voltage increased.

RPM decrease at 12V vs 9V:
25W 9%
22W 8%
15W 6%
10W 4%

It was a table just as complex as scaling PWM based on voltage. The rainy season was over & there was enough time before the next commute to try out constant power at a roughly constant voltage anyways.

The speed variation on hills was wider than any other method. It became clear that humans need a lot less power going uphill & a lot more power going downhill than a motor to achieve the same speed. It's the same as how humans don't hear all frequencies equally.

The next step would be a hall effect sensor tachometer. It's all purely speculation about what a human can keep up with, faking a speed that feels like constant effort instead of trying to predict constant effort from power. The tachometer could be combined with an accelerometer to give it some variability based on incline. The current sensor is useless.
Posted by Jack Crossfire | Dec 20, 2014 @ 04:58 AM | 6,324 Views
After much playing with LTspice, it became clear that it was erratic with small voltages. Sometimes it would actually work. Usually, it would go to full maximum. With large voltages, it always correctly simulated the difference amplifier.

It was time to build up a circuit with an LM324. Amazingly, it worked. It measured 1-5A nearly linearly, using a length of wire & some spare trim pots.

Using a high rail voltage with voltage divider cut the op-amp dropout voltage by half, making the LM324 reasonable. The bog standard difference amplifier did exactly what a proper INA169 sensor did.

With all the effort in the dynamo & the current sensor, you might as well try making the constant power throttle regulator you always dreamed of. Even with a power factor correcting cap, the current was still oscillating wildly from the PWM. Averaging the current & voltage down to 10Hz made a very stable power reading. It was an actual power reading from a bunch of spare parts.

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Posted by Jack Crossfire | Dec 14, 2014 @ 08:25 PM | 7,323 Views

Building a dynamo was long dreaded, but necessary to have any hope of regulating the speed.

Out of sheer luck, the Tamiya came with a tool which perfectly coupled 1 shaft end with a spare motor. A simple jig could hold everything together. The bench supply couldn't provide stable voltage because the current & voltage were out of phase. The old LM317 system with giant capacitor could provide stable voltage.

The load could be adjusted by shorting out different motor leads with different lengths of wire. An op-amp measured the current between 2 motor leads to give RPM. It was surprising that the op-amp was so sensitive, it could detect the current in a short piece of wire.

An input had to be grounded for this to work. Letting the motor leads float above ground only showed 0V. There was a lot of ringing when the motor leads weren't shorted, too.

Relying on the phone app to show voltage & the oscilloscope to show RPM. It had 1 more problem: the speed fluctuated. If it started slow & ramped up, the speed would be higher than if it started fast. Slight variations in wheel pressure also affected speed.

A software program would have to step though the entire PWM range for each voltage to build a table of RPMs. Then, given a target PWM value & target voltage, it could look up a modified PWM value for the real voltage that would give the same power. There are still dreams of measuring the current accurately enough to make constant power feedback.
Posted by Jack Crossfire | Dec 13, 2014 @ 10:33 PM | 10,724 Views

Some video of the 7.4V drive:

8 mile lunchbox drive (3 min 49 sec)

Tamiya Lunchbox run highlights (3 min 58 sec)
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Posted by Jack Crossfire | Dec 13, 2014 @ 02:48 AM | 7,169 Views
So oil prices started plummeting at the same time quantitative easing ended. The media promoted it as a victory for Obama's energy policies & supply catching up to demand, but if that was true, prices would have fallen back in 2008.

More likely, the decline in housing prices that was averted in 2008 by printing money has resumed in a new form. Too few people are making too little money to sustain $100 oil. The government guarantees all mortgages against all losses, so $100 million bungalows can thrive even in a world with no jobs, but it doesn't guarantee oil. Even in Calif*, gas fell below $3 for the 1st time in 10 years despite a doubling in taxes.

So assuming it is the lack of money & not the reincarnation of Einstein you elected, & the government isn't going to subsidize oil prices with another $5 trillion, lots of people in N Dakota, Canadia, & Alaska, are going to lose their jobs. What's going to happen to Tesla when the allure of avoiding those scorching $3 gallons of gas is gone or are people buying Teslas because of something besides the fact that they're electric?

The promise of gas disappearing caused huge investments in lithium production for electric cars & a plummeting in lipo prices. To be clear, Teslas never used lipos. They used traditional, higher capacity ion batteries in metal cans. Still, they did create a lot of speculation in lithium production that benefited everyone. The world reinvented itself for a future without gas, heavily dependent on electricity storage.

The millions of people extracting oil & producing electric cars in a world with plummeting gas prices now look a lot like the millions of people working at webvan in a world with plummeting demand for mail order groceries, 15 years ago, but there wasn't socialism back then. Socialism is a new beast. The government could subsidize Tesla & the gigafactory with $4 or $5 billion. Google could easily buy them out for $50 billion. Technically, $100,000 electric cars can thrive in a world with no jobs & no demand for electric cars.
Posted by Jack Crossfire | Dec 07, 2014 @ 11:56 PM | 6,339 Views

The lunch box finally started moving, after 2 weeks of baby steps between commutes. An extra 2 days were burned in a last ditch effort to get it going on a dspic33. The pickit ended up completely bricked when it tried to update its firmware in a virtual machine. Virtualbox can't emulate USB ports accurately, which probably causes any USB firmware update to fail. Fortunately, the ARM solution took a lot less time to implement.

The servo steering was bang on, with no D term required. Didn't have any stripped gears, despite using the cheapest plastic Tower Hobbies servo. Making it talk to a phone app took an immense amount of code, but proved vital.

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Posted by Jack Crossfire | Dec 04, 2014 @ 11:46 PM | 5,935 Views
Gantry rolled back from Orion (5 min 26 sec)

It's a really important launch, because it's the 1st time a prototype of a vehicle which could someday carry humans is being launched since 2011.

The prototype can't carry humans, has no seats, no instrument panel, & has never been photographed on the inside, but we're told it simulates the requirements of a crewed vehicle, if it's ever funded.

It took 10 years to build this one. There isn't enough money to launch another one for 4 more years. Humans are going to ride Dragon long before Orion ever takes them anywhere.

It's being launched on your favorite rocket, the one you saw in person twice, carrying She Jehovah's payloads. The fact that it takes the largest rocket in the world to launch a tiny capsule shows how much more energy it takes to put a spaceship around the moon than the nominal mission to low Earth orbit. It's been 40 years since such a large rocket for such a small capsule was part of the human experience.

The manned version will be partly made in Europe, by trading investments in the space station from the 1980's for services today for which there was no money. The space station has proven an invaluable bank account for services NASA couldn't possibly afford today.

The rounded cover looked like something from the old west, when it was 1st unveiled 10 years ago. It turned out to not be a stylized artist rendition, but the actual shape.