1. Basic Introduction to Electric Motors
1.1 Servo Motor
Motors are classified in different ways and have different names. Below, we will introduce the classification of motors through several classification methods and flowcharts.
First, based on the type of power supply, motors can be divided into DC motors and AC motors. DC motors, according to their structure and working principle, can be further divided into brushless DC motors and brushed DC motors; AC motors, according to their voltage, can be divided into single-phase motors and three-phase motors. Second, based on their structure and working principle, motors can be divided into DC motors, synchronous motors, and asynchronous motors. Synchronous motors can be further divided into permanent magnet synchronous motors, reluctance synchronous motors, and hysteresis synchronous motors; asynchronous motors can be further divided into induction motors and AC commutator motors. Finally, based on their application, motors can be divided into drive motors and control motors. Drive motors can be further divided into motors for power tools, motors for household appliances, and motors for other general-purpose small mechanical equipment; control motors can be further divided into stepper motors and servo motors.
A common type of servo motor is the permanent magnet synchronous AC servo motor, whose internal rotor is a permanent magnet. The driver generates an electromagnetic field by controlling the U, V, and W phases of the power supply. The rotor rotates under the influence of this electromagnetic field. Simultaneously, an encoder connected to the motor generates feedback signals to the driver based on the motor's operation. The driver then compares the feedback value with the target value and adjusts the rotor's rotation angle accordingly. Therefore, it can be seen that the control accuracy of the motor depends on the accuracy (or line count) of the encoder.
What is an encoder?
An encoder is a type of sensor used to detect the speed, position, angle, distance, etc., of mechanical motion. Besides its application in machinery, many motor controls, such as servo motors, require encoders for commutation, speed, and position detection.
Encoder Classification:
According to their shape: shaft encoders, through-hole encoders, and blind-hole encoders
Based on their working principle, they can be divided into photoelectric type and magnetoelectric type.
Compared to photoelectric: Magnetoelectric type:
It has dustproof, oil-proof, and vibration-resistant properties;
Easy to debug and simple to install;
A smaller size can be achieved while maintaining the same level of precision;
Suitable for harsher environments.
Based on the output signal, signals are classified into analog signals and digital signals. Analog signals are further divided into rotary transformers and sine/cosine encoders.
Rotary transformers have relatively low precision but strong anti-interference capabilities, making them suitable for applications with high levels of interference.
Sine and cosine encoders have relatively high accuracy. However, their anti-interference ability is average, and they are mostly used in high-speed applications.
Digital signals are divided into incremental and absolute signals; incremental ABZ encoders and absolute encoders are divided into single-turn and multi-turn encoders.
Incremental encoders: Position information is lost when there is a power outage or power supply failure. An inverted output is added to each channel signal; the inverted signal is mainly used to eliminate interference and compensate for losses for long-distance output. (Illustration by ABZ)
Absolute position information is always available, even in the event of a power outage.
Single-turn absolute value: Transmits a unique signal at every angular position from 0 to 360°, used for angle measurement and reciprocating motion measurement.
Multi-lap absolute value: Compared to single-lap absolute value, it adds a lap counting function, and each position on each lap is unique.
Encoder communication methods: There are two types, serial and parallel. Serial is suitable for incremental and absolute encoders. Absolute encoders require a battery on the wiring harness.
Parallelism is applicable to both full-line and line-saving modes.
Encoder interaction with the system: 1. Interaction with the driver
Step 1: The driver sends a command to read encoder data;
Step 2: The encoder receives commands and sends position data to the driver;
2. Interacting with the system
Common Fault Analysis: Encoder Faults
Connection cable failure
Encoder +5V power supply drops
Absolute value battery voltage insufficient
The encoder shielding wire is not connected or has come loose.
Grating contamination (photoelectric type)
Servo motor encoder classification
A servo motor encoder consists of a set of interconnected sensors. Each sensor comprises multiple indexing heads of different precision and a certain number of encoders. By analyzing the encoder signals at different positions of the indexing heads, the rotational speed and displacement can be obtained.
Servo motor encoders are also called position sensors or mechanical accelerometers.
It can directly acquire information such as the speed and angular position of rotating machinery, and is one of the main signal sources in modern mechanical devices.
Servo motor encoders come in many types, including direct speed encoders, position resolution encoders, high-precision pose encoding output encoders, and high-sensitivity displacement measurement encoders. Below, we'll explore how servo motor encoders are classified:
I. Direct Speed Encoder
This type of encoder performs well under high loads at speeds above 1000 rpm.
This type of encoder has good vibration resistance and maintains high performance under high loads.
This type of encoder can be controlled using AC drives that are matched with AC servo motors, such as PID controllers and integrated drives.
Direct speed encoders offer excellent stability, making them suitable for most applications, including those requiring high reliability, high accuracy, and long lifespan.
Common types: 1) Direct speed encoder (i.e., servo motor encoder);
2) Direct rotation speed type and position resolution type encoders;
3) Multi-purpose high-speed precision servo drive.
II. Encoders with Position Resolution
A position resolution encoder is one that converts the angular displacement of each encoded point into an incremental position in a coordinate system.
The principle is that each point is arranged in a certain direction and angle. When the encoder is rotated, the angular displacement of the rotating point changes, and its corresponding position on the shaft also changes accordingly.
In two position differences in the same direction, one is zero and the other is positive (negative) zero.
Because position resolution encoders can measure displacement through rotation, they offer high measurement accuracy and resolution.
Position resolution encoders typically employ a three-axis configuration:
Two direction angle (zero value) signals are input to two sets of output signals respectively:
III. High-precision pose encoding output
This servo motor encoder uses a very high-precision technology, allowing the encoder output to be read directly.
In order to directly read the output signal at the encoder output terminal, such encoders also need to provide high-precision displacement measurement.
This high-specification pose measurement system can be used in conjunction with other types of servo motor encoders.
These encoders are designed to provide a range of up to 1/10 μm, thus enabling them to provide very high-precision pose measurement.
Under normal circumstances, this type of encoder cannot be directly connected to external sensors to obtain displacement data. Instead, it needs to be installed inside the servo motor to read data detected by external displacement sensors (such as angle, position, speed, etc.).
In addition, some specialized instruments are needed to acquire the signal output by the encoder.
IV. High-sensitivity displacement measurement
It adds a signal generator to the displacement measurement. The signal generation circuit consists of three main parts: a digital signal generator, a linear amplifier, and an amplification unit.
A linear amplifier is a device used to amplify the width of an input pulse. It combines two pulses into one pulse to compensate for the nonlinear effect of the system on the pulse width, so that the input voltage reaches the limit of the linear output voltage, which is the maximum value allowed by the input signal.
A high-sensitivity displacement measurement module (SAPS-A) is constructed by combining a linear amplifier and a gain control circuit. It employs a high-sensitivity, low-power operational amplifier to ensure high output sensitivity and a long service life.
This module integrates two channels: one for amplification and feedback; and the other for measurement, which can improve resolution.
V. Servo Motor Angle Sensor
A servo motor angle sensor is a device for measuring angular position (angular velocity) and can be applied to various rotating machinery, such as rolling mills, rolls, winding machines, embossing machines, spinning machines, etc.
Its structure and installation location are shown in Figure 4-2.
The principle is to calculate the size of the rotation angle by measuring the angular displacement of the rotating object, thereby obtaining the angular velocity of the rotating axis or object relative to the dividing head.
The sensor is implemented using two eccentric gears and two eccentric wheels mounted on the indexing head.
When installed, the two eccentric gears mesh with each other, thereby generating radial force on the eccentric wheel. The rotation angle parameter is obtained by measuring the distance between the two eccentric wheels.
When the torque changes, it also produces changes in position and radial force, thus outputting a signal; the torque (input torque) provided by the motor can also be used to achieve angle measurement.
A servo motor encoder is a device that directly measures the angular velocity and position parameters of rotating machinery.
VI. Direct drive system (can be directly driven by a motor)
A direct drive system refers to a servo motor drive system where the encoder generates signals directly. In other words, the motor drives the encoder shaft, increasing or decreasing the number of teeth on the encoder to control its working position.
Direct drive systems can generally be divided into two types: one is where the motor directly drives the indexing head shaft to generate a signal; the other is where the indexing head is connected to a servo driver.
1. Direct drive system: This method can effectively avoid the mutual interference between the mechanical encoder and the servo driver, making the encoder work stably and reliably;
2. Optical isolation is used between the indexing head and the rotating shaft: This method can effectively prevent signal distortion and noise caused by external interference to the encoder and the rotating shaft.
3. The shaft and the indexing head are connected using Hall elements: Since the Hall element is directly connected to the encoder, there is no need to consider the problem of the Hall element generating induced current in the electromagnetic field and affecting its operation.
4. Using magnetic shielding: This method is mainly used in situations where there is interference between the motor and the encoder. However, due to the use of magnetic shielding materials, the electromagnetic interference generated between the motor and the encoder can be effectively eliminated.
VII. Enables automatic position detection and control without mechanical movement.
In detection and control, servo motor encoders are widely used as tools for position measurement.
The main types of servo motor encoders include: photoelectric encoders (such as photoelectric encoders), electromagnetic encoders (such as electromagnetic, electromagnetic type, magnetic field type, etc.), capacitive encoders, and photomultiplier tubes, etc.
In this type of sensor, electromagnetic encoders are used to measure signals. When the object being measured passes by, the mechanical movement of the sensor generates a corresponding electrical signal. Electromagnetic induction encoding works on a similar principle and is used to measure position.
Electromagnetic encoders are the most widely used in practical applications. For example, sensors, amplifiers, transducers, or servo motors can all be used to measure position.
Electromagnetic encoders achieve position detection by forming a circuit between a set of magnetic poles mounted on the shaft and a capacitor mounted on the shaft. These sensors typically consist of three electrodes: one set for input signals; two sets for output signals; and a third set for control switches.
