Brushless DC means exactly that - a DC motor with brushless commutation. The controller's job is to switch the phases in and out at the correct rotor angles (not frequency) just like the commutator in a brushed motor. The motor spins at whatever speed it wants to, depending on supply voltage and load.
If the motor has sensors then the controller can be very simple, since it just has to read the sensors and turn on the appropriate phases depending on rotor angle. It has no direct control over commutation frequency, but it can 'control' motor speed indirectly by varying the effective supply voltage (using either a regulator or PWM).
Sensorless controllers have a harder job because they must monitor the back-emf waveform for zero crossings. At startup there is no back-emf so the ESC cannot detect the rotor's position. To get the rotor spinning it pulses the phases at low power like a stepper motor, gradually increasing speed until it gets a strong enough back-emf to switch into synchronous operation.
During this startup period only, the sensorless ESC controls motor speed by varying commutation frequency. However since it is basically dragging the rotor up to speed, any sudden change in load can cause it to loose sync. Also the motor may start in reverse, then it has to stop and try again. This may result in the rotor jumping back and forth a few times until the ESC sees a good back-emf.
Can I use a sensorless control of the motor, by sensing the back EMF even if the motor is spinning very low?
Technically, yes you can. However, in practice, it is not possible/difficult. The reason is that because the back emf voltage is so low, you need to amplify it (extra circuits) or work with low resolution data. Since the resolution is lowered, you get cogging because it becomes difficult to identify exact point of zero crossing. Also, back emf can't be lower than the noise in your system, you won't detect it.
How can I energize properly the phases of the BLDC motor, from standstill, if it is sensorless?
You will do an open-loop start-up sequence and hope that the motor catches up. Continue open-loop operation until a critical back emf speed is achieved.
Can I use the IMU for finding out how to spin the BLDC motor properly without counter rotations (meaning that I know when to commutate)?
IMU generally gives information about accelerations. So, you will integrate that to find the rotor positions. This operation will take some time and there will be calculation errors (You'd get cogging in BLDC motors). I'd say, this method would be more difficult than the back-emf method. IMU method is better for stepper motors. (Stepper motors + IMU = nice gimbal system)
How can I hold the motor standstill when reaching the setpoint?
You will switch the mosfets at a constant frequency. The motor will move at a constant speed. Is that what you mean by this question?
Should I implemebt a speed controller or a torque (current) one for such an application (sensorless driving of BLDC motors at low speeds)?
BLDC motors are inefficient and hard to control at low speeds. Why not use a stepper motor? If you really have to use BLDC, though, use both current and back emf method combined. They have their benefits.
Best Answer
Q: Both are bldc sensorless motors?
A: No, only the first one is a sensorless BLDC motor. The motor in image 2 has three Hall sensors (the ones enclosed in your red circle), thus it's not sensorless.
Q: How can I know the difference between one physically?
A: If you were to open up a sensored BLDC motor you will find three Hall sensors inside. A sensored BLDC motor also has 3 "small" cables for the signals of the Hall sensors, whereas a sensorless BLDC motor has only the three "thick" power cables.
Q: If the 3 power cables and 3 smaller ones come out of a motor, are we talking about the configuration of image 2 and if only 3 of the configuration of image 1 come out?
A: Exactly
By the way, if you're designing a sensorless inverter you don't need to use the Hall sensor signals, you can just only ignore them.