To make a BLDC motor act like a servo, use an actual servo and remove the brushed DC motor and install a BLDC in it's place. Then you can use the existing gear reduction and potentiometer feedback system. Of course you'll also need to delete the servo controller IC since you will be implementing your own ESC to replace it.
above is the link to an example of Camera Brushless Gimbal using BLDC like a servo, i think this proj has to use BLDC because a servo doesn't fast enough to react with movement of the camera
This video shows the construction of a servo:http://www.youtube.com/watch?v=-XSXfqd1N58There are some very tiny BLDCs available, less than 20mm in diameter. It should be possible to find one that fits inside a servo.
Is it possible to replace a brushed motor by BLDC? I tried to find some information about these two and i found that we can't use a same technique to control them.
I came to the conclusion, that all three phases must get different positive or negative PWM signals to make them tow against each other. The winding resistance must be very high to keep the current low.With a 8 bit pwm signal on each phase the Motor can do very small and smooth steps. Much better than a servo with their inbuild deadband.
i think this proj has to use BLDC because a servo doesn't fast enough to react with movement of the camera