Servo motors are often used as a high-performance alternative to a stepper motor. Stepper motors, by comparison, have an inherent ability to control position 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 since 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 usually 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.
Servo Motor Types
Servo motors come in two main types: brush and brushless. Both servo motor types operate in a continuous torque range as well as a peak torque range. Peak torque can be much higher than continuous torque, but can only be achieved for short periods of time. By comparison, continuous torque can be reached throughout the regular operation of a servo motor.
The brush type motor is an older technology that can be run with simple motor controls, such as 2 or 4 quadrant servo drives, but have parts (graphic or precision metal brushes) that require maintenance. Todays preferred servo motor is brushless. The brushless servo motor has no component maintenance but does require a servo drive that can electronically commutate the motor. This is normally done with a primary feedback device like Halls Sensors. A brushless servo motor will normally have an encoder or resolver for accurate position feedback that can be used used to control both position and/or velocity.
Brushless servo motors can be rotary, linear or frameless in construction. They can perform very simple motion or be used in highly dynamic requirements. Applications include: robotics, CNC machinery, laser cutting, packaging, printing, material handling and many other automated manufacturing processes.
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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