Abstract: HGC (Hydraulic Cylinder Gap Control) is essentially a valve-controlled electro-hydraulic servo system. The servo valve is the core control element of the system. This paper introduces the working principle of the MOOGD791 servo valve, analyzes the control characteristics and fault analysis functions of the HGC system, and facilitates better use and maintenance of the system.
Keywords: servo system; HGC; servo valve; control characteristic analysis
Analysis on the Control Characteristic of Servo System of MPM ZHANG Yang - jun , ZHANG Zhi - guo , HOU Zheng - yu
Abstract: HGC is an electro-hydraulic servo system controlled by servo valves, the servo valve is the main control component. The paper introduces the working principle of the servo valve,analyses the control characteristics and failure analysis function of the HGC system,in order to conveniently use and maintain it.
Key words: servo system;HGC;servo valve;analysis on control characteristic
In 2007, Baogang Seamless Steel Plant upgraded its existing hydraulic cylinder control system to an HGC (Hydraulic Cylinder Roll Gap Control) servo system . This system controls the roll gap and pressure of the five-stand continuous rolling mill by controlling the position of the hydraulic cylinders of the rolls using servo valves, thereby controlling the wall thickness of the steel pipe online. The seamless steel plant's servo system is a position follow-up system, a type of control system. In this system, the output (mechanical displacement) can quickly and accurately reflect the changes in the input (position setpoint), therefore, it is a closed-loop control system with negative feedback. Because the transmitted torque and power are large, it has a signal power amplification effect, making it also a power amplification device. The D791 servo valve from MOOG Corporation used in this mill is itself a control valve with built-in closed-loop control. The HGC system compares the actual value of the electrical signal measured from the position sensor with the set value of the hydraulic cylinder position, and the deviation is output as the hydraulic cylinder position adjustment amount to the position adjuster, achieving accurate control of the hydraulic cylinder position. Since the power source of the HGC system is a hydraulic drive device (hydraulic pump), the HGC is essentially a valve-controlled electro-hydraulic servo system [1]. The servo valve is the core control element of the system, while the computer system that realizes closed-loop control is the main control mechanism of the HGC. The upgraded HGC has the characteristics of fast response speed, strong hardware versatility, powerful software functions, and high maintenance efficiency. Therefore, understanding the characteristics of the servo valve and its control system is essential for better use and maintenance of the servo system.
1. Application of D791 Servo Valve in HGC System
1.1 Working principle of D791 servo valve
Servo valves are the most important and fundamental component of servo systems. The valve core performance, rated flow rate, and frequency characteristics of servo valves are crucial selection criteria. The HGC system uses the D791 servo valve with an electrical feedback system and nozzle-flap mechanism. Its main valve core is not driven by an electromagnet, but by the hydraulic pressure output from the pilot valve. This pilot valve is a nozzle-flap valve with good dynamic characteristics. When no gap setting is performed, the main valve core should be in the center position. When setting the gap, an electrical signal (a setpoint value between ±10 mA) is applied to the integrated control amplifier within the servo valve. The amplifier drives the pilot valve, causing the nozzle-flap to deviate from the center position, resulting in a pressure change at both ends of the main spool valve. This changes the pressure at both ends of the main spool valve, driving its movement. The displacement sensor within the servo valve detects the position of the main valve core, rectifies this actual value using a demodulator, and feeds it back to the control amplifier for comparison with the command value. The control amplifier then drives the torque motor until the command voltage equals the feedback voltage. The displacement of the main valve core regulates the flow rate, thereby controlling the movement of the hydraulic cylinder. According to the control principle of the hydraulic system, the electromagnetic torque generated by the torque motor is: T = K × [i]I [/i] Where: T ——— electromagnetic torque, N·m; K ——— torque coefficient of the torque motor, N·m/A; I ——— input current, A.
When the valve core is in a dynamic equilibrium position, its electromagnetic torque is balanced with the damping torque of the baffle assembly and the torque generated by the unbalanced pressure at both ends of the valve core. If the damping torque of the baffle assembly is ignored, the electromagnetic torque of the motor is balanced with the torque generated by the unbalanced pressure at both ends of the valve core. As can be seen from the above formula, the electromagnetic torque of the torque motor is proportional to the input current, so the displacement of the valve core is proportional to the input current, that is, the flow rate through the spool valve is proportional to the input current, and the polarity of the current determines the direction of the liquid flow, thus satisfying the functional requirements of the electro-hydraulic servo valve [2]. It should be noted that the actual position of the hydraulic cylinder is transmitted to the HGC system by the position sensor on the cylinder body, and is compared with the given value as the feedback link of the position regulator. This is something that maintenance personnel need to pay attention to. It can also be seen from the above that the D791 servo valve belongs to a multi-stage servo control element. The main spool valve, the pilot valve, and the displacement detector in the valve constitute the three-level hierarchical structure of the D791 servo valve.
1.2 Common Faults of Servo Valves
(1) Insufficient inlet pressure of the servo valve. Because the servo valve uses electronic control to throttle the flow, there is bound to be energy loss. Therefore, it needs a certain flow rate to maintain the operation of the pre-stage control circuit. The pressure of the pre-stage pilot valve comes from the inlet P of the servo valve. If the pressure at port P is insufficient, the pre-stage pilot valve cannot output enough pressure to drive the main valve core. Maintenance personnel can see the valve core position feedback value (expressed as a percentage) through the human-machine interface winHMI terminal in the HGC system. It reflects the degree of deviation of the valve core. At this time, the position of the valve core may not change or fluctuate greatly. This fault is generally caused by the locking valve in the control circuit not opening or the overflow valve core not returning to its normal position.
(2) The ±15 V power supply of the servo valve fails. At this time, the control amplifier cannot work and cannot adjust the valve core position. Its feedback value will increase rapidly (expressed as a percentage, up to ±100%). When there is no load, the valve core has seriously deviated from the middle position and is in a drift state.
(3) Wear and contamination of servo valve components and oil. This includes wear or contamination of valve cores, nozzles, etc., causing decreased control sensitivity or loss of control. In addition, when there are contaminants in the gap between the torque motor's magnetic conductor and the armature, it is equivalent to reducing the length g of each air gap when the armature is in the neutral position. According to the analysis conclusion of the hydraulic control system: when |x/g| > 1/3 (x is the displacement of the armature end from the neutral position), the armature is always unstable. This destroys the original static characteristics of the torque motor, which can easily cause difficulties in judgment for maintenance personnel.
2. Application characteristics of control systems
The upgraded HGC system retains the multiprocessor architecture based on the VME backplane bus. This system boasts good hardware compatibility and ease of upgrades, avoiding the shortcomings of the original system where control boards could not be procured due to failure. The target application system in the main control room uses the VxWorks embedded real-time operating system. It has the following characteristics:
(1) The system program occupies a small space, with a minimum of 8 kbyte. Its multi-task control mechanism (such as priority preemption and round-robin scheduling strategy) ensures reliable real-time performance for the entire system, making it suitable for process control, data acquisition and other applications in factory environments.
(2) Its strong stability has been proven in numerous applications. This is crucial for the stable operation of systems in complex application environments.
(3) Strong network support. It allows for quick access to other VxWorks systems and TCP/IP network systems. Whether it's a serial line, a standard Ethernet connection, or a tightly coupled backplane bus utilizing shared memory, all VxWorks network mechanisms adhere to standard Internet protocols. This facilitates front-of-house debugging and remote monitoring.
2.1 Application of PDA Data Acquisition System in Production
The HGC system's client utilizes a PDA (pensoninal data acquist) from the German company iba. This is a high-performance PC-based data acquisition system that runs on Windows NT 4.0 and Windows XP. Signal input is handled by the dedicated interface module SM128, which provides 128 analog channels and 128 digital channels. Measurements acquired on-site are transmitted to the PDA's desktop system via the SM128 module's dual-port memory. Through the PDA, the HGC system achieves high-speed (1ms) data acquisition, recording, and analysis of production process data.
Compared to the old system, HGC's data acquisition system has the following characteristics:
(1) Large storage capacity. The PDA can store the rolling data of 100 steel pipes in fault condition and the rolling data of 10,000 steel pipes in normal condition. It can also perform hard disk backup.
(2) A large number of signals are collected, and configuration can be selected from nearly a thousand signals as needed.
2.2 Online Diagnostic Tools
The WinHMIHGC software system provides the online diagnostic tool WinHMI, which has the following functions:
(1) All measured values and control parameters can be accessed remotely. The diagnostic tools can monitor the signal changes of pressure transmitters and position sensors in real time, detect abnormalities and deal with them in time to avoid equipment accidents.
(2) Test the circuitry of the control system and certain functions of the hydraulic system components. By applying a given signal to the servo valve through winHMI, the current signal value at the valve head can be measured on-site, which can be used to determine certain faults in the circuit. In addition, its offline testing function can be used to test the frequency response and step response of each servo valve, and servo valves with poor performance can be replaced as early as possible.
(3) Online observation and recording of accident information. When the roll gap is in a faulty state, winHMI can display the frame with alarm, hydraulic cylinder number and alarm type, so that maintenance personnel can make timely judgments.
(4) Hardware diagnostic function. When testing I/O modules, winHMI provides three modes: Normal mode, Online test mode, and Diagnostic mode, which can be easily selected through the user interface. Mode switching can only occur between Normal mode and the other two modes. Normal mode only displays numerical values; Online test mode allows forcing the value of a single channel without affecting other channels; while Diagnostic mode is an offline test. In this mode, winHMI disconnects all I/O module channels and can only accept forced input values. For example, offline testing of the responsiveness of a servo valve uses this method.
2.3 Application of Fault Analysis Software (iba Analyzer)
On the HGC system client, the iba Analyzer analysis software is used in conjunction with the PDA data acquisition system. This software can analyze and edit various types of data recorded by the PDA. The real-time nature of PDA acquisition means that the acquired data is unprocessed, "flattened" massive data. After being compressed by a specific algorithm, the PDA data is stored in a 3.dat file. iba Analyzer's decompression function allows maintenance personnel to easily access PDA records offline.
Some of the main features of iba Aanalyzer are as follows:
(1) Template session function for signal analysis. The system has defined some commonly used analysis templates. If maintenance personnel frequently analyze some signals that are of reference value, they can create their own signal analysis template and save it in the system in 3.pdo file format. This will allow for more targeted analysis of the data during faults.
(2) Graphical filtering function. Using it, we can analyze the frequency curve of the servo valve. By filtering the low-frequency band, we can analyze its steady-state performance, while by filtering the high-frequency band, we can determine its anti-interference ability.
3. Conclusion
Since its launch a year ago, the HGC system in the seamless steel plant has maintained stable operation. Utilizing the HGC's powerful signal acquisition and fault analysis capabilities, fault locations can be quickly identified, significantly shortening incident handling time. It should be noted that the stable operation of the HGC system relies on the reliable operation of field control components such as servo valves. Servo valves are precision-manufactured mechatronic products with extremely high accuracy, and are highly sensitive to signal changes and oil contamination. Therefore, they require a high level of expertise in maintenance, cleaning, and understanding of the importance of maintenance.
References
[1] Wang Chunxing. Hydraulic Servo Control System [M]. Lanzhou: Gansu University of Technology Press, 1989.
[2] Zhang Liping. Quick Reference Handbook of Hydraulic and Pneumatic Technology [M]. Beijing: Chemical Industry Press, 2007.
