Servo motor encoder principle
The basic functions of a servo encoder are the same as those of a regular encoder. For example, incremental encoders have signals such as A, A reverse, B, B reverse, Z, Z reverse, etc. However, servo encoders differ from regular encoders in that most servo motors are synchronous motors. When a synchronous motor starts, the position of the rotor's magnetic poles needs to be known in order to start the servo motor with high torque. This requires several additional signals to detect the rotor's current position. For example, incremental encoders have signals such as UVW. Because of these signals for detecting the rotor position, servo encoders appear somewhat complex, making them difficult for the average person to understand. In addition, some manufacturers deliberately conceal certain signals, and related information is incomplete, further adding to the mystique surrounding servo motor encoders.
Since phases A and B are 90 degrees out of phase, the forward and reverse rotation of the encoder can be determined by comparing whether phase A or phase B comes first. The zero-position reference position of the encoder can be obtained through the zero-position pulse.
Encoder disks are made of glass, metal, or plastic. Glass disks have very thin lines deposited on glass, resulting in good thermal stability and high precision. Metal disks have lines directly engraved with both through and non-through surfaces, making them less prone to breakage. However, due to the thickness of the metal, their precision is limited, and their thermal stability is an order of magnitude worse than that of glass. Plastic disks are economical, with low cost, but their precision, thermal stability, and lifespan are all inferior.
Resolution—The number of through or dark lines provided by an encoder per 360 degrees of rotation is called resolution, also known as resolution scale or simply the number of lines. It is generally 5 to 10,000 lines per revolution.
Servo motor encoder classification
1. Incremental encoder
In addition to the ABZ signals of ordinary encoders, incremental servo encoders also have UVW signals. Currently, most domestic and early imported servo encoders use this form, which has more lines.
2. Absolute value type servo motor encoder
Incremental encoders output pulses as they rotate, and their position is determined by a counting device. When the encoder is stationary or there is a power outage, the position is remembered by the internal memory of the counting device. Therefore, the encoder cannot move at all after a power outage, and when power is restored, there must be no interference or loss of pulses during the encoder's pulse output process; otherwise, the zero point remembered by the counting device will shift, and the amount of this shift is unknown until an erroneous production result occurs.
The solution is to add reference points. Each time the encoder passes a reference point, it corrects the reference position into the counting device's memory. Before reaching the reference point, position accuracy cannot be guaranteed. Therefore, industrial control systems employ methods such as finding a reference point before each operation and zeroing upon startup.
For example, printers and scanners use incremental encoders for positioning. Every time you turn them on, you can hear a crackling sound as they search for the reference zero point before they start working.
This method is quite troublesome for some industrial control projects, and it may even prevent zeroing after powering on (the exact position must be known after powering on). Therefore, absolute encoders were developed.
Absolute rotary photoelectric encoders, due to their absolute uniqueness at each position, anti-interference capabilities, and lack of power-off memory, are increasingly widely used in angle and length measurement and positioning control in various industrial systems.
An absolute encoder has many etched lines on its code disk, arranged sequentially with 2, 4, 8, 16 lines, and so on. Thus, at each position of the encoder, by reading the on/off state of each etched line, a unique binary code (Gray) from 2^0 to 2^(n-1) is obtained. This is called an n-bit absolute encoder. Such an encoder is determined by the mechanical position of the code disk and is unaffected by power outages or interference.
An absolute encoder ensures the uniqueness of each position determined by its mechanical position. It requires no memory, no reference points, and doesn't need to constantly count; it only reads the position when needed. This significantly improves the encoder's anti-interference capabilities and data reliability.
Because absolute encoders are significantly superior to incremental encoders in positioning, they are increasingly used in servo motors. Due to their high precision and large number of output bits, absolute encoders require parallel outputs. Each output signal must be securely connected, and isolation is necessary for complex operating conditions. The large number of cores in the connecting cable leads to numerous inconveniences and reduced reliability. Therefore, absolute encoders with multi-bit outputs generally use serial or bus-type outputs. The most common serial output for absolute encoders manufactured in Germany is SSI (Synchronous Serial Output).
From single-turn absolute encoders to multi-turn absolute encoders: Single-turn absolute encoders measure the lines on the optical code disk during rotation to obtain a unique code. When the rotation exceeds 360 degrees, the code returns to the origin, which violates the principle of unique absolute coding. Such encoders can only be used for measurements within a rotation range of 360 degrees, hence the name single-turn absolute encoder. To measure rotations exceeding 360 degrees, a multi-turn absolute encoder is required.
Encoder manufacturers utilize the mechanical principle of clock gears. When the central code disk rotates, it drives another set of code disks (or multiple sets of gears and multiple sets of code disks) through gear transmission. This adds more turns of encoding on top of the single-turn encoding, thereby expanding the encoder's measurement range. Such an absolute encoder is called a multi-turn absolute encoder. It also determines the encoding by mechanical position, and each position encoding is unique and non-repeating, so there is no need to memorize it.
Another advantage of multi-turn encoders is their large measurement range, which often provides ample margin for error in practical applications. This eliminates the need for painstaking zero-point finding during installation; a midpoint can be used as the starting point, greatly simplifying installation and debugging. Multi-turn absolute encoders offer significant advantages in length positioning, and most new servo motors in Europe currently utilize multi-turn absolute encoders.
3. Sine/cosine servo motor encoder
A photoelectric encoder disk with a central axis has circular light and dark markings. Photoelectric transmitters and receivers read these markings and obtain four sets of sine wave signals, which are combined to form A, B, C, and D. Each sine wave is 90 degrees out of phase (360 degrees in one cycle). The C and D signals are inverted and superimposed on the A and B phases to enhance signal stability. Additionally, a Z-phase pulse is output every revolution to represent the zero-position reference.
A typical sine and cosine encoder has a pair of orthogonal sin, cos1Vp-p signals, which are equivalent to the AB quadrature signals of an incremental encoder with square wave signals. These signals repeat many times per revolution, such as 2048. It also has a narrow-amplitude symmetrical triangular wave Index signal, which is equivalent to the Z signal of an incremental encoder. It usually appears once per revolution. This type of sine and cosine encoder is essentially an incremental encoder. Another type of sine/cosine encoder, in addition to the aforementioned orthogonal sin and cosine signals, also possesses a pair of mutually orthogonal 1Vp-p sinusoidal C and D signals that appear only once per revolution. If the C signal is sin, then the D signal is cosine. Through high-ratio subdivision technology of the sin and cosine signals, the sine/cosine encoder can achieve a more refined nominal detection resolution than the original signal cycle. For example, a 2048-line sine/cosine encoder, after being subdivided by 2048, can achieve a nominal detection resolution of over 4 million lines per revolution. Currently, many European and American servo manufacturers offer this type of high-resolution servo system, while it is still rare among domestic manufacturers. Furthermore, the C and D signals of the sine/cosine encoder with C and D signals, after subdivision, can also provide high absolute position information per revolution, such as 2048 absolute positions per revolution. Therefore, the sine/cosine encoder with C and D signals can be regarded as a type of analog single-turn absolute encoder.
The advantage of sine and cosine servo motor encoders is that they allow servo drivers to achieve high-precision subdivision without the need for high-frequency communication, thus reducing hardware requirements. Also, due to the presence of single-turn angle signals, the servo motor can start smoothly with a large starting torque.
Precautions for using servo motor encoders
(1) Installation
Do not apply direct impact to the shaft during installation.
The connection between the servo motor encoder shaft and the machine should use a flexible connector. Do not force the connector onto the shaft. Even with a connector, improper installation can still apply a load greater than the allowable load to the shaft or cause the encoder core to snap. Therefore, special care must be taken.
Bearing life is related to operating conditions and is particularly affected by bearing load. If the bearing load is less than the specified load, the bearing life can be greatly extended.
Do not disassemble the encoder, as this will impair its oil and drip resistance. Drip-resistant products should not be immersed in water or oil for extended periods; wipe clean any water or oil residue from the surface.
(2) Vibration
Vibration applied to the encoder often causes false pulses. Therefore, attention should be paid to the installation location. The more pulses per revolution and the narrower the slot spacing of the rotating disk, the more susceptible it is to vibration. When rotating at low speed or stopped, vibration applied to the shaft or body causes the rotating disk to vibrate, which may result in false pulses.
(3) Regarding wiring and connections
① Wiring should be done with the power off. If the output wire comes into contact with the power supply when the power is on, the output circuit may be damaged.
② Incorrect wiring can sometimes damage internal circuits, so special attention should be paid to the polarity of the power supply when wiring.
3. If the wiring runs parallel to high-voltage lines or power lines, it may sometimes be affected by induction, causing malfunctions and damage. Therefore, it should be wired separately.
④ When extending the power line, it should be less than 10m. Furthermore, due to the distributed capacity of the power line, the rise and fall times of the waveform will be relatively long. If problems arise, a Schmitt trigger circuit or similar device should be used to reshape the waveform.
⑤ To avoid induced noise, use the shortest possible wiring distance. Special attention is needed when inputting to integrated circuits.
6. When the wire is extended, the rise and fall times of the waveform are lengthened due to the influence of conductor resistance and inter-wire capacitance, which can easily cause interference (crosstalk) between signals. Therefore, wires with low resistance and low inter-wire capacitance (twisted pair, shielded wire) should be used.
