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Dec 07, 2014, 11:56 PM
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The next vehicle

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.

Having battery voltage & runtime configuration on a phone made the configuration a lot faster & allowed changing settings in the field for the 1st time. Controlling headlights from the phone looked neat, but probably wasn't worth it.

It's old news for commercial RC products, but hopefully there was some advantage in the home made solution. It was the 1st vehicle with 2 radios. Driving used a 433Mhz hand controller & configuration used bluetooth. There could be support for driving on a phone, but it would have utterly no point.

Besides full autopilots, it's still hard to find a baseline RC controller which talks to a phone. The top end Spektrum DX9 still just uses the stick controller.

Non functional 1st revision electronics laid out.

A dual LM317 system would once again try to provide some constant power for all battery voltage. The dual LM317 regulator used a mighty heatsink from an age when dual cores required 2 physical pieces of iron & 2 physical fans. It's not the same to merely read it in a software report.

The mane problem was the regulated voltage of 2 LM317's not providing enough current. The brushed tamiya motor used 4A, at which point the LM317's dropped to 2V. Throttle had to be limited to 30% in the forward direction. Complicating matters, these ground ESC's have different power ranges for fwd & bwd directions, with a breaking mode that toggles. It took forever to figure out why the motor worked in reverse but browned out in forward. The motor worked at 100% in rev, but only 30% in fwd.

The bodged, reworked board with MOSFET for the headlights & regulator moved offboard because it got too hot. Bringing out pads for as many unused pins as possible has proven a vital technique no-one else does.

ESC mounting where no-one else puts it. Sadly, the motor mount would not fit any of the brushless motors in the apartment.

Battery mounting was the next issue. There was just enough room in the underside sling for an unpuffed 3S 3.3Ah, using duct tape instead of the plastic to keep it in. This arrangement left enough room inside for some gopro batteries.

Radios & ancient gyro.

LM317 for headlights & computer.

The 1st drive had massive glitches of the low speed steering to full right. The latest theory is it was still not the ADC but a protocol error. Full right was 65535, a standard event from a communication glitch. The glitch only happened when it was moving.

The range on 3S 3.3Ah was 3.6 miles in the downhill section. In the uphill section, the new battery went 4.5 miles without dying, but it was pretty close. Downhill speed with throttle 25 was 9m per mile. Uphill speed was 9m40s per mile. It had no problem with the ruts.

The 2 LM317s will probably be replaced by a direct battery connection using PWM scaling. What system works better is a matter of side by side comparison. There's no evidence the Tamiya ESC would explode on 12V. The next step after that is a brushless motor.

A constant power regulated throttle was a desperate wish. The key was designing a current sensor, which proved very challenging in lieu of buying a standard RC current sensor for 5A. The mane problem was the tried & true difference amplifier had a hard time if the low side of the shunt resistor was above ground. It might have been an op-amp artifact at high gains or a lack of a negative supply voltage that it didn't really act like a difference amplifier. It didn't work at all if in+ went to the high side. The best circuit amplified to a very high gain, then used a voltage divider to reduce op-amp dropout voltage.
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Dec 08, 2014, 06:30 AM
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rdstarwalt's Avatar
That rear end looks like the old Radio Shack truck I've been messing with forever. Nearly same brushed motor, nearly same suspension.

I've chopped and hacked it looking for way to stick an APM in the thing.
After the next fixed wing project, I should knock the dust off of the box with the parts.

Thanks for the post!

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