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Topic: weird explanation about motor torque and about motor speed being capped (Read 981 times) previous topic - next topic

Ajaj

In the bike course the student is guided to think the motor input is more or less equivalent to torque when it's a pwm ratio on voltage. Voltage is not that simply linked to torque, i think it's a bad shortcut.
PWM ratio will modulate tension which is somehow related to speed via back EMF and very loosely correlated to torque which would be more related to current.
Previous example are better made in that sense they give the student the feel that pwm and speed are somehow correlated, without ever stating it obviously, leaving room for more detailed course later. Also the assumption pwm/speed is much more true than pwm/torque which work only at very low speed.

Also the course explain that the wheel is not accelerating constantly because of friction when it's mainly from back EMF effect.

I think it's fair not to want to explain everything in detail to student discovering engineering but it would be better to avoid telling false things. If you think it would be too long or unproductive find a better way by hiding some of the concepts, it's better than to be wrong.

Also as teacher we need a way to manipulate the material of the course. For many reasons I won't use it as is so i won't be able to use this pretty e learning platform. i'll have to copy paste and rewrite most of the course and consequently find another way to distribute the course.

pauldanielcox

There's a lot of nice content out there showing the two fundamental equations of a DC motor that relate torque, speed, voltage and current. Measuring current gives a direct view of the torque, and you will see we designed in the capability to measure current on the motor shield that was introduced for this kit. PID can be used to control the torque by thus measuring the current. The motor shield actually includes a microcontroller capable of doing this and the source code is actually installed as part of the motor shield support in the Arduino IDE! Another PID loop could then be used to regulate speed by sending torque commands to the lower-level PID loop and getting speed feedback from the tachometer, and you'd then have the classical cascaded PID technique. And another loop above that could regulate position (adding encoder feedback of course). That's what's done for position control for two of the projects where position is important. For the bike, only torque needs to be controlled. A lot of these fascinating details are not fully addressed in the provided content, but these are things that professors have been teaching well for decades and will no doubt integrate very well over time.

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