Servo Motor System Overview
Servo motors come in so many types and flavors that it is difficult to define them in a way that is accurate and universally acceptable. However, it is possible to describe some of the things commonly found in servo drives.
as typical configurations.
Structure of Servo Motors
The servo motor can perform high resolution and high response positioning operation by detecting the rotor position and speed with the rotation detector (encoder) on the back shaft side of the motor
The encoder is a sensor for detecting the speed and position of the motor.
Light from the light-emitting diode (LED) passes through a position detection pattern on the slit disk and is read by the light-receiving component. Dozens of phototransistors are integrated in the light receiving component. Every pattern for absolute positioning detection varies depending on the encoder rotation angle.
The CPU is installed in the encoder to analyze the pattern for absolute positioning detection. The current position data is transmitted to the servo driver via serial transmission
Servo drives are designed to convert electrical energy into precision controlled motion, for example, controlled torque (torque servo), controlled speed (speed servo) or controlled position (positioning servo). This typically requires at least three elements: the motor, feedback of some kind and an amplifier.
DC brush motors
DC motors can be either rotary or linear. Rotary DC motors typically are long and thin, which allows for quick
acceleration due to the lower inertia, as well higher speed due to the lower centrifugal forces of a smaller diameter
armature . The armature is skewed to help reduce low-speed “velocity ripple.
Linear DC motors have the commutator and windings along the path of travel, and power may be supplied to the brushes by a bus bar or an umbilical cord . The “moving short” with the brushes has a permanent magnet, which is attracted to the energized stationary coil. A linear bearing is used to create an air gap and low friction
DC brushless motors
Brushless DC motors may be either rotary or linear and come in many varieties. They are probably the most prevalent kinds of servo motors due to their quick response time, low inertia-, weight- and size-to-torque ratios, and
reasonable cost. Rotary brushless DC motors have either ceramic or rare earth magnets banded onto the rotor .
AC induction servo motors
AC induction servo motors can be linear or rotary. The rotary design typically is long and thin, making it suitable for
higher speeds and quicker acceleration and deceleration profiles due to the lower inertia. A separate constant-velocity blower motor is often attached to the back of the servo motor for cooling during low-speed operation. The stator has a standard, low-inductance, three-phase AC motor winding, which sometimes has special volts/hertz ratings and/or wye-delta switching.
Servo systems receive feedback from the motor and sometimes from the product and/or process. The focus of this article is servo drives, so elements of process feedback are not included. Brush-style rotary DC servo motors may use tachometer feedback (typically 7 volts/1000 rpm), encoder feedback, and/or resolver feedback. Brush-style linear DC servo motors use encoder or laser feedback. Brushless rotary DC servo motors may use hall feedback and/or encoder feedback, or resolver feedback. Brushless linear DC motors may use linear encoder feedback with halls or sinusoidal commutated linear encoder feedback. Induction rotary motors use a rotary encoder, whereas induction linear motors use a linear encoder.
The encoder is a sensor that notifies the driver of the motor speed and position. The encoders (position detector) used for the servo motor are structurally classified as “incremental encoders” and “absolute encoders.” Oriental Motor uses a 20-bit absolute encoder for our servo motors NX series for low vibration at low speed range.
The encoder, which can detect the absolute position within a single rotation of servo motor, outputs the absolute position of rotation angle. Normally, it transmits the multiple rotation information to the servo amplifier when the power is turned on. After that, the multiple rotation information is output to the current position control
Tachometers use a field of permanent magnet. The armor, which is coupled to the motor shaft, induces a DC voltage
proportional to the speed as the shaft rotates. This voltage It is carried out through a switch and brushes to the amplifier for the speed and address information
The amplifier converts the output power from the distribution panel to controlled output that will cause the motor
to move at the correct velocity. Most servo amplifiers are PWM (pulse width modulated) style. The converter section of a single-axis amplifier uses a diode bridge to change AC input power to DC power, which is then “smoothed out” by the addition of a capacitor. This DC section is called the bus
The output is then “chopped up” by transistors (typically FETs or IGBTs). These devices rapidly turn the DC power to the motor on and off. If the transistor is closed for long periods (motor receiving power) with short open periods (motor not receiving power), the average terminal voltage goes up. If the transistor is open for long periods (motor not receiving power) and closed for shorter periods (motor receiving power), the average voltage goes down. The amplifier can close an “upper transistor” in one phase and a “lower transistor” in another phase, creating a path through the winding