Basic characteristics and applications of servo motors
With the increasing level of industrial automation, servo motors are being used more and more widely in industrial settings. This article starts with the basics of servo motors, introducing their definition, structure, and fundamental principles; it also provides a professional overview of the bearing requirements for servo motors.
01. Definition and characteristics of servo motors
A servo motor is an electric motor that drives mechanical components in a servo system. It is a controllable mechanical and electromagnetic device used in precision motion control to convert electromechanical energy and signals.
A servo motor rotor shaft can very accurately reproduce the position, speed, and torque commands required by the host computer under controlled conditions to drive the load. In automatic control systems, servo motors are used as actuators and possess characteristics such as low electromechanical time constant, high overload capacity, and high linearity.
01 Good performance
It adopts high-performance neodymium iron boron excitation, which has a large torque-to-inertia ratio, low stator current and stator resistance losses, and measurable rotor parameters and better control performance; it also has a small electromechanical time constant and high linearity.
02. High efficiency: No reactive excitation current required, high power factor, low copper loss, low heat generation, and high efficiency; maintains high output efficiency over a wide range of speeds and loads; strong overload capacity.
02. Composition of Servo Motors
Stator design:
1. The SM series servo motor adopts a segmented solid iron core structure design with ultra-high slot fill factor, reducing heat generation and increasing output power. 2. The stator uses an aluminum housing and vacuum epoxy potting design, maximizing the motor's heat dissipation capacity. 3. The vacuum epoxy potting design improves the motor's insulation capability and effectively protects the motor windings, enabling the motor to adapt to harsh application environments. Rotor design:
1. High-energy-level magnets effectively increase motor output torque. 2. Segmented, staggered magnet structure effectively reduces cogging torque, resulting in smooth motor operation, easy speed control, and precise positioning. 3. Rotor balance weights ensure smoother high-speed motor operation.
Brake design:
A brake is a fundamental option for robot motors. Nearly 95% of servo motors require brakes. To ensure constant brake engagement, especially reliable operation during emergency stops, the brake needs a sufficient safety factor. The static torque is approximately 1.5 times the motor's rated torque, while for heavy-duty robot motors, the safety factor needs to reach 2.0 or even 2.5 times. It's important to note that a robot motor brake is a safety brake, not a traditional brake. The control system must ensure that the servo driver's braking circuit activates through a braking resistor during emergency stops, and the brake engages when the motor speed approaches zero. To improve brake response speed, permanent magnet brakes are superior to electromagnetic spring brakes.
Encoder design:
An encoder, installed at the tail end of the motor, is a sensor for motor speed and rotor position. It measures the rotor's position for servo control magnetic field positioning and provides the control computer with the rotor's actual position and speed for motion trajectory calculation. Robot motor encoders generally have low accuracy, but require multi-turn absolute position measurement to ensure that the position before power failure can be remembered upon restarting. Currently, three common methods address the issue of robot motor encoders. The first method uses Gray code photoelectric or magnetic code disks for single turns and mechanical gears for multi-turn turns. The advantage of this is high measurement accuracy; after a power failure, the encoder's mechanical position is remembered, and the position can be directly read upon power-up. However, the disadvantage is that the encoder is too thick, making it excessively long in limited installation space. The second method uses photoelectric or magnetic Gray code memory for single turns and battery-powered electronic memory for multi-turn turns. This allows for a very short encoder, making it very suitable for small servo motors less than 60mm in diameter. The disadvantage is a relatively short battery life, ranging from 2-3 years to sometimes requiring battery replacement within one year. The third method is used in situations where high precision is not required to measure the position of a single turn using a rotary transformer, while multi-turn information is obtained through a battery-powered circuit board in the control box.
03. Requirements of servo motor bearings
Bearings:
1. High Precision: Stable and high-precision motor bearing dimensional tolerances ensure a perfect fit between the motor bearing and other components. 2. High Speed and Flexible Rotation: Motors require high-speed motor bearings, which Chichuang bearings can meet. 3. Low Noise: Motors require sufficiently low noise levels in their bearings, and every Chichuang motor bearing undergoes noise testing. 4. Low Friction: Complete manufacturing processes and appropriate lubrication minimize friction and resistance during operation, ensuring smooth operation. 5. Long Lifespan: The lifespan of a servo motor is closely related to its bearings. Due to the reliability and durability requirements of robots, bearings must ensure a lifespan of at least 30,000 hours. Based on an 8-hour workday, this translates to a robot lifespan of at least 10 years, requiring the bearings to operate continuously at 6000 rpm.
04. Servo Motor Failure Modes
Failure Mode 1: Improper installation causing early damage
Problem Review – Improper Installation, Axial Force Impact on Bearings
Note: Do not use an iron hammer to strike the servo motor during installation, as excessive axial force will damage the bearing's inner and outer raceways or the surface of the steel balls.
Note: Since both the rotor shaft and stator have magnetic force, and the magnetic force of medium and large servo motors is relatively large, during the installation process, the installation personnel (new employees) are not familiar with the process and the rotor shaft is directly sucked into the stator, resulting in phenomena such as tilting, large and irregular suction force, which causes bearing damage and abnormal noise from the motor.
Failure Mode 2: Electrolytic Corrosion Failure
Electrolytic erosion refers to the phenomenon where, when an electric current flows through the contact area between the raceway and rolling elements of a rotating bearing, sparks are generated through a thin lubricating oil film, causing localized melting and unevenness on the surface. (Mechanism: A potential difference exists between the inner and outer races.)
Failure Mode 3: Fretting Wear Failure Caused by Transportation
Fretting wear failure characteristics: Theoretical analysis was conducted on the normal deformation, tangential deformation, and sliding on the contact surface between the steel ball and the inner raceway. Reciprocating oscillation tests were performed on a radially loaded ball bearing. The results showed that when the oscillation angle was small, tangential sliding was the main cause of fretting wear; as the oscillation angle increased, fretting wear was mainly caused by repeated sliding. Damage caused by tangential sliding occurred around the contact area; damage caused by sliding occurred at both ends of the contact area. The part with the greatest stress at the center of the contact surface was undamaged, and the damage to the inner ring was much more severe than that to the outer ring.
