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Topic: New robot:battery drainage advice, mechanical design improvement (Read 3 times) previous topic - next topic

supercap

i just finished my first arduino based robot/vehicle. But i have a few problems:
- the vehicle only goes about 3mph (w/ x2 9-18v dc motors, opperating at 18v). I thought that the motors (even with only 2) would be able to go much faster than that...
- the motors have massive battery drain. after 1 minute of use with x2 new 9v batteries the voltage dropped to 16v. I didnt think that the drain would be that bad.

What can i do about the drain? How can i increase the speed?
The vehicle w/ x2 9v batteries weighs 519g, 9v battery weighs 47g, wheel (from hobbyshop, plastic, black, with no traction) weighs 5g, motors weigh 53g.

supercap


RuggedCircuits

First, 9V batteries do not have a lot of capacity. It is easy to drain them quickly when driving motors. I would instead start with a AA-battery pack. Besides, a 9V battery will lower its voltage while it is being drained of current (the bigger the drain, the bigger the voltage drop) so measuring 16V from a 2-battery pack under current load is not a surprise.

It's also not clear what you are using for driving the motors. It looks like you have a homebuilt H-bridge on a breadboard. What kind of transistors are you using? Often times people will use MOSFET's that are not logic-level and need more than 5V to turn them on, so they don't turn fully on and their motors do not perform as expected.

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The Rugged Motor Driver: two H-bridges, more power than an L298, fully protected

supercap

-RuggedCircuits
Thanks for the explanation. Im fairly certain that its not the transistors (i was going to mention that). They're NPN's. Here's the datasheet http://www.google.com/url?sa=t&source=web&cd=1&ved=0CBgQFjAA&url=http%3A%2F%2Fdalincom.ru%2Fdatasheet%2FFJL6920.pdf&ei=OM9-TZ_dAsia0QGxz5yBCQ&usg=AFQjCNHe1lRh5CMgc0MqnxRBijgwsSa67Q. They aren't currently set up in an h bridge (though i will implement this later). The arduino pins drive the transistors without a biasing resistor, so i figured that the current provided would be enough to fully saturate the tranistors. But maybe i am wrong? probably yes...if iam, i guess i will just use smaller signal transistor to provide the current necessary to turn on the power npns. Thanks for the advice.

RuggedCircuits

Those are really killer transistors....1700V! But overkill for this application. And a transistor rated for that voltage won't have as good current gain as a lower-voltage transistor. Indeed the datasheet suggests the current gain is only about 8 at Ic=1A meaning you'd need 125mA out of the Arduino pin to get 1A of motor current.

You'd be better off with a MOSFET or a higher-gain BJT. And driving a BJT base without a resistor is a bad idea as you may damage your Arduino. There really isn't anything to limit the current out of the I/O pin.

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The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons

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