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Old Aug 08, 2004, 02:54 AM
Rajesh L G is offline
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Bangalore, India
Joined Nov 2003
151 Posts
Rave
brushless power to the micros

It started with Prashanth giving me a small brushless motor pulled out of a laptop HDD. There was a controller IC on the PCB attached to the stator. Got the datasheet of the BA6840 controller and found that it had torque control input, that gave me an idea that the speed might be controllable. I set up the reference voltage and a pot to control the torque and there it was working as I hoped. Ok, Prashanth started it, so I borrowed his JMP receiver too and set about interfacing the controller to JMP output. And here are the results.

Power source: 8 nicd cells
current: 750mA
Thrust: 38gm
Prop: GWS 5x3

Pictures and video here. Please excuse the poor quality of the video, I have only a cheap webcam.
http://rcgroups.com/gallery/showgall...0&ppuser=23146

The interface circuit is very simple. I didn't want to make any modifications to JMP rx (I am sure Prashanth would kill me if I did, so that option was ruled out ). So I derived the control voltage by low pass filtering the ESC output. The reference voltage was provided by a led in series with a current limiting resistor. I used a pot to set the control range, this can be replaced by a fixed resistor. Thus the whole interface circuit is 1 led, 1 capacitor and 3 resistors.

The whole motor + controller weigh 16gm. The back plate is huge metal piece as you can see in the pictures, so I guess the weight can be brought down to about 10-12gms easily by replacing it. The can alone weighs 6gms.

And there is scope for further improvements. The controller is working at its power dissipation limit at this current. The saturation voltage of the builtin bipolar output transistors is pretty high. Dual mosfets could be added to outputs to improve this and increase the current handling. This might add a gram of weight, but my guesstimate is thrust can be improved to atleast 60gms. More if the motor is rewound with less turns.

-- Rajesh
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