Abstract: This paper proposes a novel method for online monitoring of hydraulic oil quality using a QCM sensor. The design schemes for the system's hardware and software are presented, and a QCM oscillation circuit suitable for hydraulic oil is designed. 1. Application Background In large-scale construction equipment, the hydraulic system is one of the main transmission methods. Hydraulic oil serves not only as the medium for transmitting power but also as a lubricant, sealant, and coolant for hydraulic components. Therefore, the condition of the hydraulic oil is crucial to the working condition and performance of the hydraulic system. Currently, the common method for changing hydraulic oil is periodic oil changes, which involves changing the oil at a predetermined cycle based on the structural characteristics, operating conditions, and oil quality of the hydraulic system. However, this method is prone to issues such as failing to change the oil when it should and performing oil maintenance when it is unnecessary, resulting in wasted manpower and resources and shortened equipment lifespan. The hydraulic systems of typical large-scale construction equipment (such as tower conveyor belt conveyors) are very large, with hydraulic oil volumes ranging from a few cubic meters to tens of cubic meters, making oil changes very expensive. Statistics show that the tower conveyor belt conveyors used in the Three Gorges Dam construction incurred hundreds of thousands of US dollars in oil change costs annually using a periodic oil change method. Therefore, irregular oil changes using a condition-based maintenance approach are more important. This helps reduce unnecessary parts replacements and allows for timely maintenance, minimizing further damage to hydraulic equipment and reducing downtime. This inevitably places higher demands on the quality analysis of hydraulic oil. This paper focuses on the quality monitoring of hydraulic oil. The tower conveyor operates in a harsh environment. To enable on-site quality testing, we developed an online hydraulic oil quality monitoring system based on the project requirements. This system primarily monitors changes in oil viscosity and temperature, reflecting the overall performance of the oil through these two parameters. Real-time monitoring of the oil is performed, and the AT89C52 microcontroller is used for on-site analysis, display, and alarm functions, greatly enhancing the sensor's capabilities. 2. Measurement Principle Quartz crystal microbalances (QCMs) were used for micro-mass monitoring in gaseous environments as early as 1964, but the application of quartz crystals in liquid environments has only been developed in the last decade. QCM is a quartz crystal plate with a metal layer on both sides as electrodes, driven by an external electronic oscillator. When an electric field of a certain frequency is applied to the electrodes, the QCM will oscillate at a certain frequency, which is determined by the mass of the electrodes and the viscosity and density of the liquid. The mechanism of using QCM for oil quality monitoring is based on a simple physical model established by the coupling of the shear wave of the quartz crystal and the reduced shear wave in the hydraulic oil. The derivation result is: ΔF=-F(-3/2)(ρ1η1)1/2(πUqρq)1/2 (1) Where, ΔF is the change in frequency of QCM after immersion in liquid, F is the natural frequency of QCM, Uq and ρq are the elastic modulus and density of quartz crystal, and ρ1 and η1 are the density and viscosity of liquid. Equation (1) can be simplified to equation (2), where K is a constant, that is, ΔF is proportional to (ρ1η1)1/2. ΔF=-K（ρ1η1）1/2 （2） When used on site, as long as the QCM sensor probe is connected to the monitoring point of the hydraulic system (oil pipeline or oil tank), the accuracy of the oil and the temperature change can be monitored online. In key large hydraulic equipment, multiple sensor probes can be installed at multiple monitoring points as needed to fully understand the oil deterioration status of the entire system. 3. Overall hardware design of the system The hydraulic oil quality online monitoring system is mainly composed of the following parts: single-machine system; quartz crystal (QC) sensor and its oscillation circuit; temperature measurement module; human-machine interface module (including keyboard input module, display module and alarm circuit); communication module. The overall structure is shown in Figure 1. It makes full use of the latest achievements of integrated circuits and low power consumption design ideas, so that the circuit board is small in size and low in power consumption. 3.1 Microcontroller system The microcontroller adopts the AT89C52 controller with 8K bytes of electrically erasable memory, which has a high performance-price ratio. To ensure long-term stable and reliable operation, this system employs the high-performance μP monitoring chip X25045, which integrates a reset controller, watchdog timer, and 4K serial E2PROM, enhancing the system's integration and reliability. 3.2 Temperature Measurement Module: The system uses the DS1820 temperature sensor as the temperature sensor device. It is a one-wire digital sensor that directly outputs data signals, eliminating the need for an A/D converter in the circuit, simplifying wiring, and improving system reliability. 3.3 QCM Sensor and its Oscillation Circuit: The AT-cut quartz crystal oscillator has a low zero-temperature coefficient; therefore, a 5MHz AT-cut quartz crystal oscillator was selected to fabricate the sensor probe. To prevent electrode oxidation in the oil, gold-plated electrodes were used, with a crystal diameter of 14mm and an electrode radius of 7mm, employing double-sided oil contact. To ensure the QCM oscillates in the oil, an oscillator circuit suitable for the oil environment must be used. A self-excited oscillator typically consists of three parts: a basic amplifier circuit, a positive feedback network, and a frequency selection network. In the implementation circuit, the positive feedback network and the frequency selection network are often the same, as shown in Figure 2. Here, K(S) and F(S) are the transfer functions of the basic amplifier circuit and the positive feedback network, respectively. Oscillation can only occur when K(S) × F(S) = 1. In the quartz crystal oscillator circuit, the quartz crystal, as the main component of the positive feedback network, is also a frequency selection network. This condition can only be met at the inherent resonant frequency of the quartz crystal oscillator. Based on this principle, we use an oscillator with the AMXIM913 chip as its core. Its output is TTL level, which is convenient for microcontroller acquisition. This circuit solves the shortcomings of poor driving capability of previous oscillator circuits, enabling the QCM to oscillate stably in liquids. The specific circuit is shown in Figure 3. Quartz crystals and their oscillator circuits exhibit temperature drift and time drift during use. After a period of use, changes in measured values ​​caused by the sensor and circuit itself lead to a significant error in the measurement system. We employed a reference quartz oscillator to eliminate this error and ensure data accuracy and reliability. Two square wave signals output from the measuring QCM oscillator circuit and the reference QCM oscillator circuit are respectively input to the D and CLK terminals of the 74LS74 differential frequency converter. The resulting differential frequency signal is then input to the T0 port of the microcontroller for counting. 3.4 Display and Alarm Circuit This system uses the highly integrated PS7219 display driver module, a new type of multi-digit LED display driver module. It features a simple three-wire SPI interface, an internal clock circuit, requires no external components, and offers diverse display functions. Compared to previous display driver circuits, it simplifies the hardware structure, saves microcontroller system resources, and improves functionality. The alarm circuit mainly consists of a buzzer and LEDs, used for system parameter exceeding limits alarms. 3.5 Communication Module To store the measured temperature and viscosity changes of the hydraulic oil in the host computer database for trend analysis, this system uses an AMX232 to achieve communication between the AT89C52 microcontroller system and the industrial control computer. 4. System Software Design The software design adopts a modular design approach. The system mainly consists of the following modules: main program module, display subroutine module, temperature measurement subroutine module, filtering subroutine module, QCM frequency measurement module, calibration subroutine module, X25045 serial E2PROM read/write module, and communication subroutine module. Figure 4 is the main program flowchart. This monitoring system can monitor the viscosity and temperature changes of the hydraulic oil in real time. Based on field experiments, the following conclusions can be drawn: using the MAX913-based oscillation circuit enables the QCM to oscillate well in the hydraulic oil with good stability; using the QCM sensor for online monitoring of hydraulic oil quality ensures the reliability of the hydraulic system and significantly reduces maintenance costs, which is of great significance for further automation of hydraulic system oil quality analysis in the future.