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The MOOG servo valve is mainly composed of permanent magnet torque motor, nozzle baffle valve, valve core valve sleeve and other components [2] [6] [8]. A typical servo valve consists of permanent magnet torque motor, nozzle, baffle, valve core, valve sleeve and control chamber [13-14]. The input current drives the torque motor, causing the baffle to move and change the pressure on both sides of the nozzle, thereby pushing the valve core to move and regulating the flow and pressure of the hydraulic oil [2] [8]. When the input coil is energized, the baffle moves to change the throttling effect of the nozzle, resulting in an imbalance of forces at both ends of the valve core and causing it to move, thereby controlling the flow direction of the oil; the displacement of the valve core is proportional to the input current of the torque motor [13-14].
According to the number of amplification stages, MOOG servo valves can be classified as single-stage servo valves, two-stage servo valves and three-stage servo valves. The electro-hydraulic servo valve converts weak electrical signals into large power hydraulic energy (flow and pressure) output, and is the core of the electro-hydraulic servo system [11] [20].
New servo valves are attempting to overcome the disadvantages of complex structure and high requirements for oil in traditional servo valves, such as using super magnetostrictive materials (GMM) or direct-acting servo valves driven by stepper motors. The structure is more simplified and has advantages such as high frequency response and high precision. Electronicization, digitization and intelligence are also important development directions of servo valve technology [14].

The MOOG servo valve is an electro-hydraulic servo valve designed and manufactured by the American MOOG Company (founded by William C. Moog in 1951). It is the core component of an electro-hydraulic servo control system. This valve can precisely convert weak electrical signals into high-power hydraulic power output. It usually adopts structures such as torque motors, nozzle baffle valves, or jet pipe pilot stages, and has features such as fast dynamic response, high control accuracy, and good reliability. The product series are diverse (such as D660, D661, G761, etc.), and are widely used in aerospace, industrial automation (such as injection molding machines, testing machines, metallurgical equipment), and energy fields.

Semiconductor analog excitation regulator units’ functions
(1) Measurement and comparison unit. Measures the generator voltage signal, converts it proportionally into a DC voltage signal, compares it with the set DC voltage, and sends out the generator voltage deviation signal. To ensure that the various units operating in parallel can reasonably and stably share reactive power, a correction unit should be set up.
(2) Comprehensive amplification unit. Composed of the comprehensive amplification section, the proportional-integral section, and the adapter section. The comprehensive amplification section combines and amplifies various basic measurement outputs, feedback and auxiliary limitations generated, as well as various stable and compensatory responses, and outputs them to the proportional-integral section. The proportional-integral section processes them according to the predetermined regulation law and outputs. The adapter section converts the signal voltage through amplification and processing into a phase-shifting control signal voltage to control the excitation voltage.
(3) Phase-shifting trigger unit. Receives the output signal voltage from the comprehensive amplification unit, changes the size of the thyristor trigger control angle, to control the excitation voltage.
(4) Voltage stabilizing power supply. Converts the input AC and DC power supplies into the required voltage-stable power supply for the excitation regulator. The AC and DC power supplies input should be able to be automatically switched in a timely manner.
(5) Various other auxiliary function units:
1) Maximum excitation current limiter. Limits the total current output by the thyristor rectifier bridge below the given excitation peak current.
2) Excitation overcurrent limiter. When strong excitation or excitation overcurrent reaches the allowed time, reduces the excitation current to the long-term allowable maximum value.
3) Under-excitation limiter. According to the needs of generator and power system stability calculation, limits the minimum current output by the thyristor rectifier bridge to the given value corresponding to the active power.
4) Voltage/frequency limiter. Mainly used during no-load operation of the generator disconnected from the grid, to prevent the unit from operating below the industrial frequency due to governor failure, causing incorrect strong excitation and over-excitation faults.
(6) Manual unit. In the early days, manual excitation regulation used an open-loop method, and later adopted a closed-loop regulation method using excitation current as the signal source.
(7) Tracking unit. At each moment when the regulator is in “automatic mode” and closed-loop regulating according to the generator terminal voltage as the signal, the tracking unit “automatically closely follows the corresponding aforementioned voltage signal” to ensure that there is no significant fluctuation in excitation when switching from “automatic” to “manual” mode. When the regulator “automatic” regulation mode has a main and backup dual-channel, it is necessary to ensure that the backup channel automatically tracks the signal of the main channel.
(8) Imbalance (pulse disappearance) protection. Monitors the main channel. When the pulse of the main channel disappears or becomes abnormal, it automatically switches to the standby channel.
(9) Voltage transformer open-circuit protection. When the voltage of the excitation measurement signal uses the voltage transformer voltage and it disappears due to a fault, the excitation regulator is switched from “automatic” to “manual” operation.
Functions of each unit of the microcomputer digital excitation regulator
(1) Analog input and output channels. The sampled quantities include generator terminal voltage, stator current, active power, reactive power, rotor current, and system voltage, etc. The sampling can be AC or DC. AC sampling can be done every 12 points per week.
(2) Switching quantity input and output channels. To ensure safety and prevent interference, the switching quantity input and output channels all need to be isolated by optoelectronics. It is mainly used for the input of signals such as on-site operation, parameter setting, generator status, protection, etc., as well as the operation instructions of the regulator for other excitation equipment on site and the output of various fault signals of the regulator.
(3) Digital phase-shifting trigger. Its function and structure are similar to the analog phase-shifting trigger, consisting of synchronous shaping, phase-shifting calculation, pulse formation, and pulse amplification sections.
Software functions of the microcomputer digital excitation regulator The key feature of the microcomputer digital excitation regulator is that it replaces the various functions of the analog excitation regulator, such as signal comparison, limitation, and integration, with software.

The excitation regulator of the self-excited static rectifier excitation system has evolved from discrete semiconductor components to integrated solid-state components, and from analog to digital. Domestic devices can be classified into three major categories: semiconductor analog excitation regulators, microcomputer (including programmable controllers) digital excitation regulators, and hybrid microcomputer (including programmable controllers) analog excitation regulators. Domestic semiconductor excitation regulators have had export records since the early 1970s. The research and development of microcomputer excitation regulators began in the late 1970s. In 1985, the WLT-1 excitation regulator produced by NARI Electric Company was first put into operation on the 50MW unit of Chitang Hydropower Station.

The excitation regulator of the self-excited static rectifier excitation system has evolved from discrete semiconductor components to integrated solid-state components, and from analog to digital. Domestic devices can be classified into three major categories: semiconductor analog excitation regulators, microcomputer (including programmable controllers) digital excitation regulators, and hybrid microcomputer (including programmable controllers) analog excitation regulators. Domestic semiconductor excitation regulators have had export records since the early 1970s. The research and development of microcomputer excitation regulators began in the late 1970s.

A microcomputer assembled on a printed circuit board with certain functions is called a single-board computer. It is equipped with a microprocessor (MPU), a fixed-program read-only memory (ROM or EPROM), a read-write memory (RAM), a programmable input-output interface adapter (PIA and ACIA), a real-time clock, a timer, a bus buffer, a baud rate generator, and other supporting chips. Some single-board computers also have simple input-output devices, such as a small keyboard, a liquid crystal display, and a micro printer. Single-board computers can also be connected to external devices such as floppy disk drives, cassette tape drives, or dot matrix printers. The components of a single-board computer are concentrated on a single printed circuit board, allowing for the achievement of high performance with the least hardware, maximizing the characteristics of the microprocessor, and ensuring high reliability, flexibility, and convenience. It can be inserted as a component in industrial equipment or instruments. Single-board computers are generally expandable and can be assembled into larger microcomputer systems. Since their successful manufacture in 1976, they have been widely used in various industrial control fields to achieve automation and programming of equipment or instruments. Initially, they were mainly 8-bit single-board computers, and after the 1980s, 16-bit single-board computers accounted for more than half. There are many types of single-board computers with various performances, and they support up to 60 chips, with a storage capacity of 4000 to 64000 bytes, and can access up to 8 input-output devices. They have 300 instructions. 8-bit single-board computers use chips such as 8080, M6800, and Z80. 16-bit single-board computers use chips such as 8086 and Z8000.
The hardware of a single-board computer mainly consists of the following five parts: ① Microprocessor and its peripheral circuits. Usually, a simple structure, easy-to-interface, and single-power supply microprocessor is selected to reduce the number of chips. Since the number of components that a general microprocessor can directly drive is limited, additional driver circuit chips can be added. ② Memory and address decoding circuits. Single-board computers are usually used as plug-ins. Once they are finalized, the software is fixed, and only a small amount of storage units are needed. Therefore, the ratio of ROM to RAM in a single-board computer is generally 8:1 to 4:1. Usually, ROM is 4000 to 16000 bytes, and RAM is 256 to 4000 bytes. EPROM can be used first, and then mask ROM production can be carried out after the software is finalized. The RAM capacity is small, and static devices are usually used. The address decoding circuits of the single-board computer should be simplified as much as possible and have an expandable address space. A partial decoding scheme can be adopted. ③ Input-output interface adapters and their accessory circuits. Single-board computers usually have parallel external interfaces and serial communication interfaces. The parallel interface adapter is programmable, and each line can be defined as input or output by the program, which is flexible in configuration and convenient to use, and can be directly connected to external devices without the need to configure logic circuit chips. The communication interface adapter is also programmable, equipped with a baud rate generator and programmable counter/timer chips. ④ Bus and bus buffer. The single-board computer has two types of buses; one is the bus for inter-board communication, called the internal bus; the other is the bus for communication between the single-board computer and external devices or control objects, called the external bus. Currently, the popular internal buses include four standards: S-100 bus (IEEE 696 standard), Multibus bus (IEEE 696.2 standard), EXORciser bus, and Std bus. The external bus of the single-board computer can be directly output or buffered by a universal interface, or it can be made into a standard external bus. The parallel bus standard is IEEE 488, also known as HPIB interface bus, and has been widely used as an instrument standard. This bus consists of 8 bidirectional data lines, 3 byte transmission control lines, and 5 general control lines. In order to directly connect the single-board computer to the IEEE488 bus, special interface devices are required. The interface circuit uses a level conversion chip. When connecting to the telegraph machine, a current loop chip (usually in 60mA and 20mA versions) is used, and it is connected to the RS-232C standard interface through a LOOP-to-EIA conversion circuit chip. Both the internal and external buses of the single-board computer need to consider the driving capacity, and configurable buffer drivers can be used. The single-board computer generally adopts a dual-terminal wiring method; one side is the internal bus, which is compatible with a certain standard bus; the other side is the external bus, which is connected to other plug-ins or control objects. This not only makes the wiring convenient but also facilitates the implementation of isolation measures. ⑤ Monitoring program and peripheral control circuit. The single-board computer must have a monitoring program and corresponding external devices. The monitoring program is generally fixed in ROM and requires the configuration of the necessary external devices and related control circuits according to the monitoring program. The software of the single-board computer includes monitoring programs, debugging programs, diagnostic programs, assembly programs, and compilation programs, etc. In 1979, Intel Corporation launched a direct-insert expansion board, called Multi-Template, and adopted a new bus standard, InterSBX bus. After the single-board computer adopts the Multi-Template structure, various new boards can be used to expand the functions of the single-board computer, such as expanding the number of programmable I/O interfaces and increasing high-speed floating-point computing capabilities, etc. Thus, the single-board computer can be expanded into a multi-processor system.

A single-board microcomputer (abbreviated as single-board computer) is a type of microcomputer that integrates a microprocessor, memory, and simple peripherals onto a single printed circuit board. This concept originated in 1976. Early models used 8-bit microprocessors, and in the 1980s, they transitioned to 16-bit architectures, mainly applied in industrial control, automation equipment, teaching experiments, and smart household appliances. A typical representative is China’s first practical single-board computer TP801A (in 1981), developed by a team from Beijing University of Technology based on the Z80 processor. Its memory was four times that of the American prototype, and it was mass-produced through a cooperation between the university and the enterprise and promoted with teaching materials [1].
The hardware of a single-board computer includes a microprocessor (such as 8080, Z80), ROM, RAM, programmable interface adapters, and bus systems. The storage capacity is 4000-64000 bytes, supporting internal bus standards such as S-100 and Multibus, and IEEE 488 industrial interfaces. The monitoring program is fixed in ROM, and the software includes debugging and compilation tools, enabling program-controlled equipment. It adopts a modular design. Some models are equipped with simple input devices or expansion slots, and can be upgraded to multi-processor systems. Compared with single-chip microcontrollers, single-board computers are in a multi-chip assembly form, with advantages of high reliability, strong anti-interference ability, and low cost.

Position proportional gain
1. Set the proportional gain of the position loop regulator;
2. The larger the setting value, the higher the gain, the greater the stiffness, and under the same frequency command pulse conditions, the position lag will be smaller. However, if the value is too large, it may cause oscillation or overshoot;
3. The parameter value is determined by the specific servo system model and load conditions.
Position feedforward gain
1. Set the feedforward gain of the position loop;
2. The larger the setting value, the smaller the position lag under any frequency command pulse;
3. A larger feedforward gain of the position loop improves the high-speed response characteristics of the control system, but it will make the system position unstable and prone to oscillation;
4. When the required response characteristics are not very high, this parameter is usually set to 0 within the range: 0 – 100%.
Speed proportional gain
1. Set the proportional gain of the speed regulator;
2. The larger the setting value, the higher the gain, the greater the stiffness. The parameter value is determined according to the specific servo drive system model and load value conditions. Generally, the larger the load inertia, the larger the setting value;
3. Under the condition that the system does not produce oscillation, try to set a larger value.
Speed integral time constant
1. Set the integral time constant of the speed regulator;
2. The smaller the setting value, the faster the integral speed. The parameter value is determined according to the specific servo drive system model and load conditions. Generally, the larger the load inertia, the larger the setting value;
3. Under the condition that the system does not produce oscillation, try to set a smaller value.
Speed feedback filtering factor
1. Set the low-pass filter characteristics of the speed feedback;
2. The larger the value, the lower the cutoff frequency, and the less noise generated by the motor. If the load inertia is very large, the setting value can be appropriately reduced. If the value is too large, the response will be slower and may cause oscillation;
3. The smaller the value, the higher the cutoff frequency, and the faster the speed feedback response. If a higher speed response is required, the setting value can be appropriately reduced.
Maximum output torque setting
1. Set the internal torque limit value of the servo motor;
2. The setting value is a percentage of the rated torque;
3. At any time, this limit is valid for the positioning range;
4. The pulse range for positioning completion in the position control mode;
5. This parameter provides the basis for the driver to determine whether positioning is completed in the position control mode. When the remaining pulse count in the position deviation counter is less than or equal to the setting value of this parameter, the driver considers positioning complete, the到位 switch signal is ON， otherwise it is OFF；
6. In the position control mode， output the position completion signal， acceleration and deceleration time constant；
7. The setting value represents the acceleration time or deceleration time of the motor from 0 – 2000 r/min or from 2000 – 0 r/min；
8. The acceleration and deceleration characteristics are linear to the speed range；
9. Set the reaching speed；
10. In non-position control mode， if the motor speed exceeds this setting value， the speed reaching switch signal is ON， otherwise it is OFF；
11. In position control mode， this parameter is not used；
12. Independent of the rotation direction.

Requirements for servo feed systems
1. Wide speed range
2. High positioning accuracy
3. Adequate transmission rigidity and high speed stability
4. Fast response, no overshoot
To ensure productivity and processing quality, in addition to requiring high positioning accuracy, it is also necessary to have good rapid response characteristics, that is, the response to follow the command signal should be fast, because the numerical control system requires sufficient acceleration for start-up and braking, shortening the transition time of the feed system and reducing the contour transition error.
5. Low speed with large torque, strong overload capacity
Generally, servo drives have an overload capacity of more than 1.5 times for several minutes or even half an hour. They can overload 4 to 6 times without being damaged in a short period of time.
6. High reliability
It is required that the feed drive system of the numerical control machine has high reliability and good working stability, has strong adaptability to temperature, humidity, vibration, etc., and has strong anti-interference ability.
Requirements for motors
1. The motor can operate smoothly from the lowest speed to the highest speed, with small torque fluctuations, especially at low speeds such as 0.1 r/min or lower speeds, there is still a stable speed without crawling phenomenon.
2. The motor should have a large and long-term overload capacity to meet the requirements of low speed and large torque. Generally, DC servo motors require an overload of 4 to 6 times within several minutes without being damaged.
3. To meet the requirements of rapid response, the motor should have a smaller rotational inertia and a large no-load torque, and have as small a time constant and starting voltage as possible.
4. The motor should be able to withstand frequent start, stop and reversal. Testing platform
Broadcast
The test platforms for servo drives mainly include the following types: the test platform using servo drive – motor mutual feedback drive, the test platform with adjustable simulated load, the test platform with an actuating motor but no load, the test platform with the actuating motor driving the inherent load, and the test platform using online testing methods.
1. Test platform using servo drive – motor mutual feedback drive
This test system consists of four parts: a three-phase PWM rectifier, the tested servo drive – motor system, the load servo drive – motor system, and the upper computer. The two motors are connected through couplings. The tested motor operates in the motor state, and the load motor operates in the generator state. The tested servo drive – motor system operates in the speed closed-loop state to control the speed of the entire test platform, and the load servo drive – motor system operates in the torque closed-loop state to change the torque of the load motor by controlling the current of the load motor, simulating the load change of the tested motor. Thus, the mutual feedback drive test platform can achieve flexible adjustment of speed and torque, and complete various test function tests. The upper computer is used to monitor the operation of the entire system, send control instructions to the two servo drives according to the test requirements, and receive their operation data, and save, analyze and display the data.
For this test system, using a high-performance vector control method to separately control the speed and torque of the tested motor and load equipment can simulate the dynamic and static performance of the servo drive under various load conditions, and complete a comprehensive and accurate test of the servo drive. However, due to the use of two sets of servo drive – motor systems, this test system is large in size, cannot meet the requirements of portability, and the measurement and control circuits of the system are also relatively complex and the cost is very high.
2. Test platform with adjustable simulated load
This test system consists of three parts: the tested servo drive – motor system, the adjustable simulated load, and the upper computer. The adjustable simulated load, such as magnetic powder brakes and power dynamometers, is connected coaxially with the tested motor. The upper computer and the data acquisition card control the load torque through the adjustable simulated load, and simultaneously collect the operation data of the servo system, and save, analyze and display the data. For this test system, by controlling the adjustable simulated load, it can also simulate the dynamic and static performance of the servo drive under various load conditions, and complete a comprehensive and accurate test of the servo drive. However, this test system is still relatively large in size and cannot meet the requirements of portability, and the measurement and control circuits of the system are also relatively complex and the cost is very high.
3. Test platform with an actuating motor but no load
This test system consists of two parts: the tested servo drive – motor system and the upper computer. The upper computer sends speed command signals to the servo drive, and the servo drive starts running according to the instructions. During the running process, the upper computer and the data acquisition circuit collect the operation data of the servo system and save, analyze and display the data. Compared with the previous two test systems, this test system has a relatively smaller volume because the motor does not carry a load. However, this system still cannot simulate the actual operation situation of the servo drive. Usually, such test systems are only used for the speed and angular displacement tests of the tested system in the no-load condition, and cannot conduct a comprehensive and accurate test of the servo drive.
4. Test platform with the actuating motor driving the inherent load
This test system consists of three parts: the tested servo drive – motor system, the system inherent load, and the upper computer. The host computer sends the speed command signal to the servo driver, and the servo system starts to operate according to the command. During the operation process, the host computer and the data acquisition circuit collect the operation data of the servo system and save, analyze and display the data.
For this testing system, the load adopts the inherent load of the tested system, so the testing process is close to the actual working situation of the servo driver, and the test results are relatively accurate. However, since some of the inherent loads of the tested systems are not convenient to be removed from the equipment, the testing process can only be carried out on the equipment, which is not very convenient.
5 Testing platform using online testing method
This testing system only has a data acquisition system and a data processing unit. The digital acquisition system collects and processes the real-time operating status signals of the servo driver in the equipment, and then sends them to the data processing unit for processing and analysis. Finally, the data processing unit makes the test conclusion. Due to the use of online testing method, this testing system has a relatively simple structure and does not need to separate the servo driver from the equipment, making the testing more convenient. Such testing systems are completely tested based on the actual operation of the servo driver, so the test conclusion is closer to the actual situation. However, due to the characteristics of many servo drivers in manufacturing and assembly, the installation positions of various sensors and signal measurement elements in this testing system are difficult to select. Moreover, if other parts of the equipment malfunction, it will also have an adverse effect on the working state of the servo driver, ultimately affecting its test results. [2]

The mainstream servo drives all adopt digital signal processors (DSP) as the control core, which can implement relatively complex control algorithms and achieve digitalization, networking, and intelligence. The power components generally adopt drive circuits designed with intelligent power modules (IPM) as the core. The IPM integrates the drive circuit internally and also has fault detection and protection circuits for overvoltage, overcurrent, overheating, and undervoltage. A soft-start circuit is also added in the main circuit to reduce the impact on the drive during startup. The power drive unit first rectifies the input three-phase electricity or mains electricity through a three-phase full-bridge rectifier circuit to obtain the corresponding direct current. The rectified three-phase electricity or mains electricity is then transformed to frequency by a three-phase sinusoidal PWM voltage-type inverter to drive the three-phase permanent magnet synchronous AC servo motor. The entire process of the power drive unit can be simply described as an AC-DC-AC process. The main topology circuit of the rectification unit (AC-DC) is the three-phase full-bridge uncontrolled rectifier circuit.
With the large-scale application of servo systems, the use, debugging, and maintenance of servo drives are all important technical topics of servo drives in today’s era. More and more industrial control technology service providers have conducted in-depth technical research on servo drives.
Servo drives are an important component of modern motion control and are widely used in industrial robots and CNC machining centers and other automated equipment. Especially, servo drives used for controlling AC permanent magnet synchronous motors have become a research hotspot at home and abroad. Currently, in the design of AC servo drives, the 3-closed-loop control algorithm based on vector control, which includes current, speed, and position control, is commonly used. Whether the speed closed-loop design is reasonable plays a key role in the entire servo control system, especially in the performance of speed control.