Archive for December, 2014
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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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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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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.
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.
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.
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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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.
...Continue Reading
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.
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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