Servo Motors

Servomotors are generally used as a high performance alternative to the stepper motor. Stepper motors have some inherent ability to control position, as they have built-in output steps. This often allows them to be used as an open-loop position control, without any feedback encoder, as their drive signal specifies the number of steps of movement to rotate. This lack of feedback though limits their performance, as the stepper motor can only drive a load that is well within its capacity, otherwise missed steps under load may lead to positioning errors. The encoder and controller of a servomotor are an additional cost, but they optimize the performance of the overall system (for all of speed, power and accuracy) relative to the capacity of the basic motor. With larger systems, where a powerful motor represents an increasing proportion of the system cost, servomotors have the advantage.

Servomotors are used in applications such as robotics, CNC machinery or automated manufacturing.

DC Motor/Brushed MotorsView More

DC Brushed Motors that have winding in the rotor and permanent magnets on the stator. Carbon brushes and a mechanical commutator provide a current path through the windings to achieve motor torque. A DC motor will continuously rotate if a DC power source is applied across its terminals. DC motors require simpler drives but require higher maintenance, and are larger in size for the same output power.

There are two types of brushed permanent magnet DC motors: iron core and moving coil rotor.

Moving Coil Rotor Motors feature:

  • High acceleration due to a low mass inertia
  • Low electromagnetic interference
  • Low inductance
  • High efficiency
  • Linearity between voltage/load & speed, and load & current
  • Small torque ripple

Iron Core Rotor Motors feature:

  • High torque-to-inertia ratio
  • High starting torque
  • Low thermal resistance
  • Low current consumption
  • High inertia for improved load-to-motor inertia matching
  • Low cost

Brushless Servo MotorsView More

Brushless Servo Motors that have windings in the stator and permanent magnets attached to the rotor. No brushes are used. Motor rotation is achieved by means of electrical commutation performed by the drive. Brushless servo motors provide high acceleration, high torque, and no maintenance. Brushless Servo Motors offer the highest torque-to-weight ratio and are commonly used in the highest throughput, precision and demanding applications.

There are two types of brushless servo motors:

Slotted Motors feature:

  • Better accel/decel capabilities
  • Better load to rotor inertia ratio
  • Lower rotor inertia
  • Lower cost vs. slotless motors

Drawbacks of slotted motors include: cogging, high speed operation and lower efficiency vs. slotless design.

Slotless (coreless) Motors feature:

  • Zero cogging
  • Smooth operation
  • Increased heat dissipation
  • Ability to withstand high peak torque
  • High power density
  • Lowest electrical time constant
  • Low inductance

Drawbacks of slotless motors include: low inductance, low moment of inertia, cost and lower accel/decel capabilities vs. slotted design.

Linear Servo MotorsView More

A linear motor provides direct linear motion (rather than rotary). Electromagnetic force is utilized to produce thrust directly, eliminating the need for rotary to linear conversion. Advantages include: high speeds, high precision, fast response, stiffness, zero backlash and maintenance free operation. Disadvantages include: higher cost, required higher bandwidth, larger footprint and heat. Types: Iron core, air core, and slotless.

Pancake Servo MotorsView More

Available in both Brush and Brushless varieties, these flat disc armature motors feature:

  • Low inertia
  • Low axial profile
  • High-pulse torque capability
  • No cogging even at low operating speeds

Drawbacks of pancake servo motors include: cost, delivery, customization, low inductance and low inertia.

Direct Drive Rotary MotorsView More

Direct Drive Rotary Motors are brushless motors with high resolution encoders or resolvers, plus optional radial bearings that turn the table top or integral coupling which is directly attached to the load. Key benefits include high accuracy and torque in a package that does not have a gear reducer.

The key disadvantages include high system cost, larger size, and the requirement of using a specific motor control system, one designed for that specific rotary motor.

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