Servo Motor FAQ

What is a servo motor?

A servo motor is a rotary or linear actuator that allows for precise control of angular or linear position, velocity, and acceleration. They are often used as high-performance alternatives to stepper motors and consist of a DC motor, feedback decvice, and a control circuit.

What makes up a servo motor?

A servo motor is comprised of dozens of parts, but the basic components remain the same. A stator creates a magnetic field to efficiently generate torque. The winding produces a rotating magnetic field. The shaft transmits the motor output power. The rotor is a permanent magnet that is positioned externally to the shaft. An encoder observes and calculates the number of rotations being completed and watches the position of the shaft.

What is the difference between a servo motor and a stepper motor?

Servo motors are often used as a high-performance alternative to a stepper motor. Stepper motors, by comparison, have an inherent ability to control positions as their motion is based on output steps from a step & direction signal. This allows the stepper motor to be used in open-loop position control applications without any feedback device. This is due to the fact that the signal from their stepper drive specifies the number of steps & direction to rotate per shaft revolution. However, the lack of position feedback limits their performance since the stepper motor can only drive a load that is well within its torque capacity. Missed steps under load lead to positioning errors or a stall condition when no closed-loop feedback is in place. Additionally, load–to-motor inertia matching is absolutely critical when using a stepper motor, but less so when using a servo motor.

What is the function of a servo motor? What are they used for?

Servo motors allow for precise control of angular or linear position, velocity, and acceleration. They can be used in a variety of different industries including electronics, robotics, aerospace, food processing, and pharmaceutical.

What are the types of servo motors?

There are two main types of DC servo motors: brush and brushless. Brush servo motors are servo motors that have a winding in the rotor and permanent magnets on the stator. They produce smooth motion at low speeds with good speed control. Brushless servo motors have windings in the stator and permanent magnets attached to the rotor. No brushes are used here. They can be used in the most precise, demanding applications.

Are servo motors AC or DC?

Servo motors can be either AC or DC, meaning they can run on either AC or DC power. In an AC motor, speed is controlled by the frequency of the applied voltage and the number of magnetic poles in the motor. With a DC motor, speed is directly proportional to the supply voltage, given a constant motor load. Both AC and DC motors can be used as servo motors. When referring to brushed or brushless motors, the implication is that these are DC motors. When referring to synchronous or asynchronous, one is referring to AC servo motors.

What are DC Motor/Brushed Motors?

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

What are Brushless Servo Motors?

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.

What are Linear Servo Motors?

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.

What are Pancake Servo Motors?

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.

What are Direct Drive Rotary Motors?

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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