This creates intervals or commutation zones which allows one to monitor the angular position of the rotor to within a specified resolution. With this configuration, three Hall effects and a magnetic disc with four sectors on it, it allows the designer to know the motor shaft angle to an accuracy of 30 degrees. So this information is fed back to the controller and the job of the controller is, based upon what angle that the rotor is at, to determine which coils need to be turned on and off to give it the maximum torque in the desired direction that one wants the motor to spin. So using Hall effect sensors to sense the angle of the rotor shaft actually works very well. It is a very robust solution and it goes all the way down to zero speed. However, there are some problems associated with using Hall effect sensors and the first one and perhaps most obvious one is the additional cost that is involved with using Hall effect sensors. Not only does one have the cost of the sensors themselves and in many cases the additional magnetic disc on the motor, but one also has the additional connectors and wiring involved to get those signals back to the controller. This can be a very big deal especially if the controller and motor are separated by any appreciable distance. The second problem is reliability. Remember, if the designer is going to be using connectors to get those signals back to the controller, connectors tend to be in many cases vibration sensitive and a lot of times there will be problems with the signals getting through the connectors. It also means that an additional power supply to power the Hall affect sensors and if that power supply goes bad or anything happens to any of the Hall affect sensors, the designer has lost the ability to commutate to the machine. So for these reasons and perhaps other ones, as well, the designer would really like to get rid of the Hall effect sensor. In fact, the designer would like to get rid of sensored feedback, period. This brings up the topic of sensorless control of brushless DC motors. Now, there is a right way and a wrong way to control a motor using sensorless technology and before being introduced to the InstaSPIN technique, the next slide will show the wrong way to do it.