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Dec 20, 2014, 04:58 AM
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Death of constant power regulated throttle

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.

A simple PI controller yielded the power regulated throttle once dreamed of. It needed a starting ramp, just like an autopilot, then it quickly stabilized on the desired power. Unfortunately, it became clear that RPM was higher at lower voltages, despite the power always being constant.

At higher voltages, more of the measured power was burned off as heat before reaching the wheels. It was the same reason an adjustable resistor wastes more power than a switching regulator.

Truly making the amount of power independent of the battery voltage would require measuring the amount of power actually reaching the wheels. This would not be possible while driving. Only the old method of making a table of all PWM for all voltage on a dynamo would account for efficiency changes at the different voltages.

After programming the truck to iterate through several PWM's for each voltage, a rough chart of RPM vs voltage for various PWM ranges emerges. The units of RPM & PWM were arbitrary. The RPM range was confined to a narrow limit by the motor's maximum speed, the motor stalling, & the bench supply's current limit.

Theoretically, one could look up a goal RPM at a current voltage, then interpolate vertically between the 2 nearest PWM lines to get the required PWM.
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Dec 20, 2014, 08:53 AM
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Next on Jack Crossfire's Garage: More T-Wrench Adaptions!

Great stuff JC! I always enjoy your posts.


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