Servo Motors Flashcards
What are the two approaches to motion control?
Servo Motors - closed loop.
Stepper Motors - open loop.
Describe a Servo Motor.
Any motor used in closed loop control of torque, speed or position.
They require a high ratio of torque to inertia and need to be able to handle high currents as torque is proportional to currents.
What are the characteristics of a servo motor?
Provided the magnetic field is constant, the torque is proportional to the current and the supply voltage is proportional to the no load speed.
The back EMF is proportional to the angular velocity.
Back EMF is the internally generated voltage that opposes current.
What are the different types of Servo Motor?
Conventional (Iron Core).
Ironless.
DC Brushless.
AC Brushless.
What are the characteristics of a Conventional Servo Motor?
The permanent magnets on the stator and armature rotate in a magnetic field.
The armature is made of iron and copper so the moment of inertia is large.
The attraction of the armature poles to the magnets makes for torque ripples and cogging.
What are the characteristics of an Ironless Servo Motor?
The armature windings are made into a self supporting structure - usually copper windings with some form of resin holding them together.
The magnetically conducting core is made stationary and the armature disc runs in the the narrow gap in the circuit so the inertia is kept low.
Ironless servo motors have no cogging and much reduced torque ripple.
What are the characteristics of a Brushless Servo Motor?
The armature windings are placed on the stator so there is no need for brushes.
The rotor is a permanent magnet.
Commutation is carried out using solid state switches rather than a mechanical commutator and requires knowledge of the rotor position.
What are the differences between a DC and AC Brushless Servo Motor?
DC:
Fed with voltage proportional to desired speed.
Switched to each winding in turn using solid state commutation.
Back EMF constant while windings connected.
AC:
Fed with sinusoidal varying supply to each winding via suitable circuitry.
Constructed do back EMF is sinusoidal.
