Abstract: This paper starts with the speed control principle of viscous speed-regulating clutches, and adopts an anti-integral saturation PID control algorithm for the core of the speed control system—the electronic controller. A specific PID algorithm implementation method is given, and key technologies such as a chatter signal generation circuit, V-I conversion, and current amplification circuit are used in the hardware design for the electro-hydraulic proportional valve. Experimental verification shows that the designed speed controller can meet the speed regulation time and accuracy requirements of viscous speed-regulating clutches. Keywords: Viscous clutch, electronic controller, speed regulation Introduction Viscous speed-regulating clutches are a new type of transmission device developed and widely used in the 1970s. They rely on the viscosity of the liquid and the shearing action of the oil film to transmit torque and regulate speed. By adjusting the control oil pressure to change the oil film thickness (i.e., the degree of compression) between the driving and driven friction plates, stepless speed regulation of the driven shaft is achieved while the driving shaft speed remains constant. Hydraulic speed regulation and constant speed control technology are key technologies that directly affect the speed regulation and constant speed capabilities of viscous speed-regulating clutches, and the electronic control system is the core of the hydraulic speed regulation and control system. 2. Hydraulic Speed ​​Regulation and Control System Analysis and Scheme Design The main functions of the viscous speed regulation clutch control system are twofold: first, speed regulation, which adjusts the control oil pressure on the friction plate assembly according to the target value (i.e., set value) of the output speed; and second, speed stabilization, which automatically suppresses fluctuations in output speed caused by various disturbances (such as load fluctuations, input speed fluctuations, etc.). Existing domestic and international viscous speed regulation clutch control systems can be broadly divided into two categories. One type is a control system centered on an Omega valve. This type of control system has an Omega valve mounted on the output drum. This is a centrifugal speed regulating valve that can improve the stability and accuracy of the speed. Control oil pressure is applied to both the inner and outer ends of the Omega valve core. When the drum rotates with the valve core, the valve core generates centrifugal force, which is balanced by the spring force and hydraulic pressure acting on the valve core. When the rotational speed suddenly fluctuates slightly, the centrifugal force of the valve core increases, the spring extends, the throttle opening of the Omega valve increases, the control pressure decreases, and the output speed decreases, returning to the originally set speed. Conversely, if the output speed decreases for some reason, the centrifugal force of the valve core decreases, the spring compresses, the throttle opening decreases, the control pressure increases, and the output speed returns to the originally set speed. If continuous changes in output speed are required, the control pressure causes continuous changes in the throttle opening of the Omega valve, ensuring continuous increases or decreases in output speed. This type of closed-loop speed control system has a simple structure, omitting the electronic feedback part of the speed; the extraction, comparison, and processing of its feedback signal are all performed by hydraulic conversion. Because the Omega valve is very sensitive, it can effectively suppress speed fluctuations and reduce the fluctuation rate of speed. However, the control of the control oil pressure applied to the Omega valve core in this control system is relatively simple, resulting in weak speed regulation function and low speed regulation accuracy. Another type of control system uses an electronic controller and an electro-hydraulic proportional servo valve as its core, which constitute an electronic speed feedback closed-loop control system. The electronic controller (ECU) is the main control unit and the hub for achieving speed control; the electro-hydraulic proportional valve is the actuator, and the magneto-electric speed sensor serves as the feedback element. By changing the control current output from the ECU to the electro-hydraulic proportional valve, the system oil pressure is altered, which changes the pressure of the pressurizing piston and the thickness of the oil film between the friction pairs, thereby adjusting the output speed of the viscous speed-regulating clutch. Thus, a larger command current results in a higher output speed, and vice versa. This type of control system uses electronic speed feedback, offering sensitive speed control, fast dynamic response, and ease of operation. It can also be networked with a computer for control. Both types of control systems have their own characteristics. The former, due to the presence of an Omega valve in the oil circuit system, has better speed stability but inferior speed regulation performance compared to the latter; the latter has better speed regulation performance but inferior speed stability compared to the former. In designing the hydraulic speed regulation and control system, the advantages and strengths of two types of control systems are combined, and Omega valves, electronic controllers, electro-hydraulic proportional valves, etc. are comprehensively applied (as shown in Figure 1) to form two closed-loop feedback control loops: one is a centrifugal hydraulic speed stabilization feedback loop formed by the centrifugal speed regulating valve itself (such as the Omega valve); the other is a speed regulation feedback loop composed of a magnetoelectric speed sensor, electronic controller, electro-hydraulic proportional valve, and Omega valve. The viscous speed regulating clutch is mainly used in applications such as fans, water pumps, belt conveyors, and special marine power systems. Based on the load characteristics, a bypass throttling pressure regulating loop is adopted in the control oil circuit of the pressure cylinder to increase its speed stiffness under high load conditions (see Figure 1). The throttling valve (i.e., the electro-hydraulic proportional valve in the oil circuit) is installed on a branch connected in parallel with the hydraulic cylinder inlet. Adjusting the overflow of the electro-hydraulic proportional valve can achieve the purpose of regulating the control oil pressure. Therefore, to achieve rapid and stable speed regulation, controlling the overflow of the electro-hydraulic proportional valve is the key to the electronic controller design. The electronic controller (ECU) is the core of the speed control system. It compares the speed feedback signal with the speed setpoint signal, processes the error value, and then amplifies it to control the overflow of the electro-hydraulic proportional valve, so that the control system obtains the corresponding oil pressure, thereby obtaining the required output speed. The electro-hydraulic proportional valve is the object directly controlled by the ECU. The valve's overflow is proportional to the input current, and the pressure of the control oil circuit can be continuously and steplessly adjusted. In the design of the ECU, a combination of hardware and software is used to optimize the control current output from the ECU to the electro-hydraulic proportional valve, achieving both sensitive speed regulation and fast response, while reducing impact and speed fluctuations. 3. Control Algorithm Design of the Electronic Control System Modern servo drive systems generally require high speed regulation accuracy, fast response, and small overshoot during speed regulation. Therefore, adopting a reasonable control strategy is crucial. Currently, PID control is still the most widely used in industrial process control, with a proportion exceeding 95%. Even in developed countries such as Japan, the usage rate of PID control has reached 84.5% [j]. This is because the PID control algorithm is simple, robust, reliable, and easy to implement. In addition, since there are often nonlinear elements such as dead zone and hysteresis in electro-hydraulic servo systems, it is difficult to achieve good control effect using unoptimized PID algorithms. Therefore, it is necessary to analyze the specific characteristics of the electro-hydraulic circuit and optimize and improve the digital PID algorithm. The discrete form of the position PID controller control algorithm is: The functions of P, I, and D control parameters are: (1) P control only changes the amplitude of the deviation signal without affecting its phase. Increasing the proportional gain K can improve the open-loop gain of the system, reduce the steady-state error of the system, and thus improve the control accuracy of the system. However, when Kt is too large, the stability of the system deteriorates, and it may even cause instability of the closed-loop system. (2) I control can memorize and integrate the deviation, which is beneficial to eliminating steady-state error. Therefore, using an integral controller is beneficial to improving the steady-state performance of the system. However, the integral reciprocating effect adds an open-loop pole located at the origin, causing the signal to lag by 9°, which is not conducive to the stability of the system. (3) P control and I control are adjusted according to the direction and magnitude of the current and past deviation signals, while D control is very sensitive to the trend of signal change and has a certain predictive ability. However, D control only affects dynamic processes and has no effect on steady-state processes, and it is very sensitive to system noise. The characteristics of the P, I, and D control elements are different, and in practical applications, these three control laws are often optimized and combined in different ways to meet the comprehensive requirements of the system for dynamic and steady-state performance. The response speed of valve-type controlled objects in hydraulic circuits is much slower than that of electronic circuits. Therefore, when deviations occur in actual control processes, integral saturation often occurs without certain measures. That is, if the system consistently deviates in one direction, the output of the PID controller increases due to the continuous accumulation of integral action, causing the actuator to reach its limit position +X... (i.e., the valve opening of the electro-hydraulic proportional valve reaches its maximum) or -X (i.e., the valve of the electro-hydraulic proportional valve closes), as shown in Figure 2. If the controller output u(k) continues to increase, the valve opening cannot increase further. At this point, the computer output control quantity is said to have exceeded the normal operating range and entered the saturation region. Once the system experiences a reverse deviation, u(k) gradually exits the saturation region. The deeper the system enters the saturation region, the longer it takes to exit. During this period, the actuator remains in its limit position and cannot immediately respond to the deviation. At this time, the system appears to be out of control, leading to a deterioration in control quality. This phenomenon is called integral saturation or integral runaway. Considering the characteristics of the controlled object (electro-hydraulic proportional valve), and taking into account both the rapid speed regulation during dynamic processes and the speed stability in steady state, using an anti-integral saturation PID control algorithm can achieve better control results. When calculating u(k), first determine whether the control quantity u(k-1) from the previous moment has exceeded the limit range, and limit u(k): If u(k-1) > u, only accumulate the negative deviation; if u(k-1) > u, then only accumulate the negative deviation. 4. Hardware Design of the Electronic Control System The electronic controller uses a high-performance 196k series microcontroller as its core, including peripheral 12I modules for speed signal acquisition, temperature and pressure analog signal acquisition, digital I/O, and current output for the electro-hydraulic proportional valve. The current output module for the electro-hydraulic proportional valve plays a crucial role in speed regulation. It consists of sub-modules for D/A conversion, chatter signal generation, signal conditioning, V-I conversion, and current amplification. The chatter signal and V-I conversion sub-modules are discussed in detail below. Proportional electromagnets typically exhibit significant electromagnetic hysteresis. To improve their dynamic performance and reduce hysteresis, a chatter signal of a certain frequency and amplitude is usually superimposed on the stable signal of the coil during normal operation (see Figure 4). Superimposing a chatter signal of a certain frequency on the operating signal of the electro-hydraulic proportional valve can effectively prevent the spool valve from jamming. The valve core of the electro-hydraulic proportional valve continuously generates chattering motion during operation, which on the one hand converts the static friction between the valve core and the valve sleeve into dynamic friction, improving the sensitivity of the electro-hydraulic proportional valve. On the other hand, the relative movement between the valve core and the valve sleeve can also remove large and small particulate impurities that accumulate in the gap between the valve core and the valve sleeve due to long-term static state. The jamming of the spool valve has a certain process, which is closely related to the accumulation of impurities in the oil on its surface. If the spool valve frequently makes small-amplitude vibrations to frequently remove large and small particulate impurities from its surface, the probability of jamming will be greatly reduced. At the same time, to avoid the vibration signal affecting the unit's load, the frequency of the vibration signal should be reasonably selected. Usually, the frequency of the vibration signal is selected in the range of 1.2 to 2 times the undamped natural frequency of the electromagnet core. Therefore, although the electro-hydraulic proportional valve moves continuously due to the vibration signal, the normal operation of the viscous speed-regulating clutch will not be affected [2]. The frequency of the vibration signal emitted by the electronic controller can be set by software. According to the characteristics of the electro-hydraulic proportional valve, the input frequency of the vibration signal can be flexibly changed on-site through software. The amplitude of the frequency can be adjusted by changing the voltage division ratio of the voltage divider resistor (see Figure 4). The electro-hydraulic proportional valve requires a control current of 0-800mA. Such a large control current must be achieved by setting up VI conversion and current amplification. There are three basic configurations for amplifier circuits: common-emitter circuits have relatively large voltage, current, and power gains; common-base circuits have good stability over wide bandwidths or at high frequencies; and common-collector circuits have very high input resistance and very low output resistance, and are often used in input stages, output stages, or buffer stages. Because a high-frequency dithering signal is superimposed on the control voltage to increase the valve's sensitivity, using a common-base circuit is more reasonable (see Figure 5). In addition, commonly used V/I converters can be divided into two types: one with a common power supply to the load and the other with a common ground to the load, as shown in the figure below. In this design, we use the load-supply method, and the following describes its working principle using the load-supply method as an example; assuming the amplifier's open-loop gain is large enough, the base current I_id = 0. Where, ———— the common-base current method coefficient of TIP142. RL——- the equivalent input impedance of the electro-hydraulic proportional valve. The Zener diode IN4001 is used to protect the Darlington transistor, while the TIP142 is a high-power Darlington transistor capable of outputting a sufficiently large current. The use of a high-power Darlington transistor overcomes the drawback of increased gain and error that occurs in transistors under high current conditions. 5. Experiment and Verification Based on the above control algorithm and hardware circuit, a TSQA-type electronic controller was developed and tested on TY16 and TY10 type viscous speed-regulating clutches. The test results show that the adopted PID control algorithm and hardware circuit can meet the requirements of rapid and stable speed regulation of the viscous speed-reducing clutch. Under the specified operating conditions of a rated speed of 1500 r/min and a rated torque of 1.6 kNm (TY16 type) and 9.6 kNm (TY10 type), the stable speed regulation range reaches 0.2–0.1, the output speed fluctuation rate is <5%, and the speed regulation time is within 10 seconds. When the load or input speed fluctuates, the clutch can recover to the output speed setpoint in a very short time, maintaining a constant output speed, fully meeting the requirements of high-performance speed regulation.