Does the brushed DC motor need position feedback in the control system? why?
If you want a DC brushed
commutated motor to control a position variable then yes it does need a position feedback sensor and signal to a closed lo0p control system. The reason is that the stator and armature magnetic fields
are permanently at right angles to one another when axis through the commutator brush pairs is parallel to the stator field:
In this arrangement to Torque is the vector product of the Stator Field (A) and Armature Field (B) = T = A x B and of magnitude AB sinQ, where Q is the angle between the fields.
See for more on vector product:
Vector Product of Vectors - http://hyperphysics.phy-astr.gsu.edu/hbase/vvec.html
So once the Stator and Armature fields are crosses by some angle greater or less than zero there will be a torque on the rotor one way round or the counter-way. Using a commutator there is no null position when sinQ = zero - there would be if slip rings were used or the axis of the brushes was changed to be at right angles the the stator field. Think about this or make a simple motor:
Does the permanent magnet dc motor need position feedback in the control system? why?
No it does not normally need a controlled loop control system with position sensing. Why? Because the armature now is a permanent magnet mechanically fixed to the rotor
so as the rotor turns so does the direction of the armature field. The stator field depends on the positions of the coils - let us assume for now they are at 12, 3, 6 and 9 o'clock positions. For the simplest method of stator drive, the coils would be energised individually to produce stator fields with magnetic field axis up, right, down or left - depends on the direction of the current through the stator coils. If they were all reversed then it would be down, left, up and right - you get the idea.
The torque on the rotor through the interaction of the stator and armature field is as before S x A and of magnitude SAsinQ - the difference is that the armature field direction is free to rotate and will do so until sin Q is zero. Thus, the position of the rotor is determined by the stator coil distribution and how they are energised by current. In this was the rotor can be made to step by an increment (in my case 90 degrees) from one position to another just by providing appropriate stator currents to the coils. Thus in some applications a stepper motor can be used for open loop position control without a position sensor.
Merry Christmas
Marconi
