Hello,
Can I be sure that if I buy a kit of 4 quadcopter motors, the 4 of them will respond equally in speed given a same voltage?
If this is not true, how could I compensate this offset? FYI I am building a quadcopter using only an arduino uno and ultrassonic sensors.
Thank you very much
No, they will not. Even if they did, simply having the motors move at exactly the same speed will not result in a flyable quad.
You need active stabilization. This is done with a gyro or IMU (inertial measurement unit) to detect when the orientation had drifted and some PID logic to know how much to adjust the speed to the motors.
I understand that. But my objective is not a proper flying drone. Just a platform able to level at some point in the air. Wouldn't this be possible?
Wouldn't this be possible?
Sure, with a 3D good orientation sensor and feedback system controlling the motors.
@vinceherman my objective is only to lift it off from a flat surface and control the height from it without any other feature. Just a levitating platform, not a drone.
I thought that if I could control each motor independently using an utrasonic sensor on the tip of each arm and all 4 of them with the same set point value I would be able to stabilize. Only the yaw would require that a CW/CCW pair of motors stabilize at an exact same rotating speed to be fixed from happening and maybe an encoder would do it.
I am trying to figure out why this would no work. I still don't have the materials to build an experiment, but I wanna learn the wisdom from the experienced 
Thank you for your time
Just to be clear, hovering in a stable orientation uses every but of complexity that any flying task uses.
I suspect that ultrasonic sensors will not be valuable input devices. Maybe they will provide some distance information in the noisy ultrasonic environment in the prop wash zone. Maybe they wont.
But since virtually everyone uses IMUs for stabilization input, I think that is the way to go.
I am not knocking experimentation. If you want to try, just to see if it works, have at it! If nothing else, it will give you concrete results for your report on the project. Even a crash provides data.
Speaking of crashing, order extra props. They will take a lot of abuse during your test flights.
sorg:
Hello,Can I be sure that if I buy a kit of 4 quadcopter motors, the 4 of them will respond equally in speed given a same voltage?
If this is not true, how could I compensate this offset? FYI I am building a quadcopter using only an arduino uno and ultrassonic sensors.Thank you very much
Reality has other ideas, this is utterly unworkable. You need to sense and correct orientation dynamically
rapidly and this means some sort of IMU. Otherwise you have something that will reliably immediately crash.
You also need to control height, otherwise you have something that will drift uncontrollable vertically.
Quadcopters are fundamentally not stable. Its a given that multiple control loops are needed, generally 4,
for 3 rotation axes and thrust (ie vertical climb).
Standard sensors are a 10DoF IMU, so you can lock orientation on short and long timescales, and sense
air pressure to stabilize vertical drift. You can add GPS for extra info (compensate for barometric change,
detect sideways drift).
Spot on MarkT.
Having tried to fly model/real helicopters and Quads - the best similarly I heard was hovering is like balancing on top of a ball !
sorg:
@vinceherman my objective is only to lift it off from a flat surface and control the height from it without any other feature. Just a levitating platform, not a drone.
You really need to explain exactly what you are trying to do, this statement just comes across as total
xyproblem.
sorg:
@vinceherman my objective is only to lift it off from a flat surface and control the height from it without any other feature. Just a levitating platform, not a drone.
The only way you'll get it to do that without the sort of complex management that quads usually have is if you construct the device so that it slides up and down a rod or rods. That should handle pitch for you and if you make it a non-round rod or use several round ones at the corners that could handle the yaw for you. THEN you'll only have to worry about the height. But that's probably not exactly what you were intending to make.
Quads/drones are inherently unstable. It takes a lot of computing power to achieve anything like stability.
Steve