Bionic underwater electrolysis algae removal robot system

Abstract

The bionic underwater electrolysis algae removal robot system comprises a hardware part and a software part, the hardware part comprises a bionic shell, a modular sealed cabin, a shore power-standby battery dual-mode power supply device and a floating communication relay, and the sealed cabin, an electrolysis cabin, a salt storage tank and a buoyancy material are arranged in the bionic shell; the sealed cabin protects internal components from water invasion, the sealed cabin internally comprises a main control system, the main control system selects Arduino and an integrated module and is responsible for controlling operation of the robot, the electrolysis cabin is located in the center of the robot, an anode titanium wire, a cathode titanium wire, a nail iridium titanium anode and a titanium cathode are arranged in the electrolysis cabin and used for electrolytic algae removal, and the salt storage tank is connected to the electrolysis cabin. The software part comprises a motion control and navigation module, an electrolysis tank intelligent regulation and control module and a safety monitoring and emergency processing module, efficient and accurate removal of algae in a water source reservoir is achieved, and the problems that a traditional algae removal method is low in efficiency, large in pollution, poor in mobility and the like are effectively solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of water ecological environment treatment, and particularly relates to an underwater autonomous operation algae removal device based on the principle of electrochemical oxidation. Background Art

[0002] In waters such as water source reservoirs, excessive reproduction of algae can cause water quality problems. Traditional algae removal methods mostly use chemical agents, which are prone to cause secondary pollution and are cumbersome to operate. Existing physical algae removal equipment is often bulky and has poor mobility, making it difficult to operate flexibly in reservoirs. The algae removal efficiency is limited, and at the same time, there is a lack of precise intelligent control of the algae removal process, unable to meet the algae removal requirements in different water environments.

[0003] The existing patent CN112875820A discloses a mobile underwater algaecide intelligent spraying device, which has the advantages of safe operation, uniform spraying, precise regulation, and flow control, etc. However, it relies on a carrier ship, has poor flexibility, uses chemical algaecides that may cause harm to aquatic organisms, and has a complex structure, high cost, and difficult maintenance. Another existing patent CN112090526A discloses a mechanical device for rapid underwater algae removal, which has the advantages of high automation, environmental protection of mechanical algae removal, and compact structure, but has a limited operation range, low algae removal efficiency, and poor adaptability.

[0004] Combined with the new type of algae removal device, its advantages lie in improving flexibility through bionic design, intelligent regulation to adapt to different environments, environmentally friendly and efficient electrolysis technology, ion separation technology to ensure stable operation, and strong self-adaptive ability. These characteristics enable the new type of device to surpass the existing technology in terms of flexibility, intelligence, environmental protection, and adaptability, and better meet the design objectives and functional requirements of the new type of algae removal device. Summary of the Invention

[0005] The present invention aims to provide an efficient, environmentally friendly, and intelligent bionic underwater electrolytic algae removal robot system. Through the bionic design of the boxfish shape and the coordinated work of various internal sensors and components such as the electrolysis chamber, it realizes the precise removal of algae in the water source reservoir, and at the same time has good mobility and adaptability, overcoming the deficiencies in the existing technology.

[0006] The present invention is realized through the following technical solutions:

[0007] Bionic underwater electrolytic algae removal robot system, including a hardware part and a software part. The hardware part includes a bionic shell, a modular sealed cabin, a shore power-backup battery dual-mode power supply device, and a buoy communication relay. Inside the bionic shell are a sealed cabin, an electrolysis cabin, a salt storage tank, and buoyancy materials. The sealed cabin protects the internal components from water intrusion. The sealed cabin contains a main control system. The main control system selects Arduino and integrated modules and is responsible for controlling the operation of the robot. The electrolysis cabin is located in the center of the robot and is equipped with an anode titanium wire, a cathode titanium wire, a ruthenium-iridium-titanium anode, and a titanium cathode for electrolytic algae removal. The salt storage tank is connected to the electrolysis cabin to provide a high-concentration salt solution. A GPS antenna is installed in the center of the top of the bionic shell for real-time positioning and navigation to ensure precise control of the robot underwater. Signal lines and power lines extend from the sealed cabin and run through the entire robot, responsible for transmitting control signals and power to ensure the normal operation of each component. At the rear of the robot, a vertical thruster is close to the bionic tail swing and is responsible for controlling the vertical movement of the robot to maintain depth stability. Horizontally distributed horizontal thrusters and horizontal pushes on both sides are responsible for horizontal movement to ensure that the robot can flexibly adjust its position underwater. The robot is also equipped with a pH / ORP sensor for monitoring water body parameters, a camera for underwater monitoring, and an attitude / water depth level sensor for monitoring the robot's attitude and water depth. The software part includes a motion control and navigation module, an electrolysis cabin intelligent regulation module, and a safety monitoring and emergency handling module. The modular sealed cabin includes a functional cabin, a separated anode and cathode chamber, and an equipment cabin. The shore power-backup battery dual-mode power supply device includes a shore power supply device and a backup power supply. The buoy communication relay selects a buoy type GPS-485 communication relay, including a buoy structure, a signal transmission device, and an anti-interference part. The motion control and navigation module includes a thruster device, an electronic speed control module, a multi-sensor fusion positioning module, and an ICM-20689 attitude sensor. The electrolysis cabin intelligent regulation module includes a current closed-loop control module, a brine automatic replenishment module, and an intelligent regulation algorithm module. The safety monitoring and emergency handling module includes a multi-level safety trigger mechanism module and an operation log module.

[0008] As a preferred embodiment, the bionic shell adopts a bionic boxfish streamlined shell design with a smooth surface, reducing turbulence and resistance, lowering the resistance during underwater movement, and enhancing mobility and energy efficiency. The functional cabin integrates an electrolysis cabin, a salt storage tank, and a peristaltic pump. The electrolysis cabin uses a CMI-7000s cation exchange membrane to separate the anode and cathode chambers to achieve ion-selective permeation and improve electrolysis efficiency. The anode chamber uses an NF7 nanofiltration membrane to intercept chloride ions, saving the amount of sodium chloride replenishment. The equipment cabin is equipped with an Arduino Mega 2560R3 main control board, a water depth sensor, an ICM-20689 attitude sensor, an ORP sensor, and 6 electronic speed control modules to ensure the waterproofness and stability of the equipment.

[0009] As a preferred embodiment, the onshore power supply device is connected to the onshore power source (220V AC) through a waterproof cable and converted to 12V DC by a voltage stabilization module for system use. A 24V / 2500mAh lithium battery is integrated in the sealed cabin as a backup power supply, which automatically switches when the onshore power is interrupted to maintain communication between the thruster device and the buoy.

[0010] As a preferred embodiment, for the buoy structure, a GPS module and an RS-485 signal repeater are built into the surface buoy, which is connected to the robot main control through a waterproof cable. For the signal transmission device, shore-end instructions are transmitted to the buoy via RS-485 and then forwarded by the buoy to the underwater Arduino main control to avoid underwater signal attenuation. Anti-interference design: The RS-485 communication protocol has strong anti-interference ability and supports long-distance communication of more than 500 meters.

[0011] As a preferred embodiment, the electrolysis tank includes an anode chamber and a cathode chamber. The anode chamber is located on the left side of the electrolysis tank and contains anode titanium wires and ruthenium-iridium-titanium anodes. The water inlet and outlet of the anode chamber are located at the bottom and top of the anode chamber respectively; the cathode chamber is located on the right side of the electrolysis tank and contains cathode titanium wires and titanium cathodes. The water inlet and outlet of the cathode chamber are located at the bottom and top of the cathode chamber respectively. The anode chamber and the cathode chamber are separated by an NF7 diaphragm and a cation exchange membrane. The NF7 diaphragm intercepts chloride ions from entering the water outlet to save the replenishment of sodium chloride, and the cation exchange membrane ensures that sodium ions are allowed to pass through from the anode chamber to the cathode chamber during the electrolysis process to ensure the electrolysis efficiency.

[0012] As a preferred embodiment, a pH / ORP sensor is installed on the top of the electrolysis tank to monitor the acidity and redox potential of the electrolyte. The liquid storage tank is located outside the electrolysis tank to store the electrolyte. The thruster device includes 2 vertical thrusters for adjusting the depth and 4 horizontal thrusters for achieving movement in the XY directions.

[0013] As a preferred embodiment, the electronic speed control module receives the Arduino PWM signal to precisely control the rotation speed of each thruster. For the multi-sensor fusion positioning module, the buoy GPS provides the water surface coordinates, and the three-dimensional position is calculated in combination with the water depth sensor. The ICM-20689 attitude sensor real-time feeds back the roll / pitch angles, and the Arduino corrects the thrust direction of the thrusters through the PID algorithm.

[0014] As a preferred embodiment, for the current closed-loop control module, the Arduino adjusts the current of the electrolysis tank (0 - 10A) through the MOSFET module, and the ORP sensor monitors the redox potential of the anode chamber to dynamically adjust the current intensity.

[0015] As a preferred embodiment, the automatic brine replenishment module: The salt storage tank quantitatively injects high-concentration brine into the anode chamber through a peristaltic pump. When the ORP value is lower than the threshold, dosing is triggered (e.g., when ORP < 600 mV, the pump speed is increased by 20%). The drainage rate of the cathode chamber outlet is controlled by a peristaltic pump to maintain the pressure balance of the electrolysis cell. The intelligent control algorithm module dynamically adjusts the electrolysis parameters based on the fuzzy control algorithm to ensure efficient and stable operation.

[0016] As a preferred embodiment, in the multi-level safety trigger mechanism module, when the humidity sensor in the sealed cabin detects water leakage, the power supply of the electrolysis cell is immediately cut off and the backup battery is activated to float. When the RS-485 communication is interrupted for more than 10 seconds, it switches to the attitude holding mode to avoid out-of-control drift. The operation log module uploads data to the shore end in real time for easy fault diagnosis and maintenance.

[0017] Beneficial effects: Through the boxfish-shaped design and the integration of various advanced sensors, electrolysis cells, and intelligent control systems inside, the present invention realizes the efficient and precise removal of algae in water source reservoirs. The device can move flexibly in the reservoir, autonomously adjust the algae removal operation parameters according to the distribution of water body algae and water quality conditions, and effectively solves the problems of low efficiency, high pollution, and poor mobility existing in traditional algae removal methods. At the same time, its intelligent operation mode reduces the manual operation intensity, improves the safety and reliability of the algae removal operation, and has significant environmental and social benefits. Description of the Drawings

[0018] Figure 1 It is a schematic diagram of the overall external structure of the bionic underwater electrolytic algae removal robot.

[0019] Figure 2 It is the composition of the bionic underwater electrolytic algae removal robot.

[0020] Figure 3 It is the composition of the electrolysis cell of the robot.

[0021] Figure 4 System startup and initialization flowchart.

[0022] Figure 5 Remote control and process navigation flowchart.

[0023] Figure 6 Electrolytic cell control and automatic dosing flowchart.

[0024] Figure 7 Safety monitoring and mission termination flowchart. Detailed Implementation Modes

[0025] The following is a detailed description of the embodiments of the present invention in conjunction with the accompanying drawings: These embodiments are implemented on the premise of the technical solution of the present invention, and detailed implementation manners and specific operation processes are given. However, the protection scope of the present invention is not limited to the following embodiments.

[0026] As Figure 1 shown, it shows the external shape of the bionic underwater electrolytic algae removal robot, presenting the structural layout at the top of the robot. The streamline design of the overall bionic shell 21 of the robot imitates the boxfish, aiming to reduce the resistance of underwater movement and improve maneuverability. A GPS antenna 22 is installed in the center of the top of the bionic shell 21 for real-time positioning and navigation to ensure precise control of the robot underwater. A signal line and a power line 19 extend from the sealed cabin 20, and the signal line and the power line 19 run through the whole robot, responsible for transmitting control signals and electricity to ensure the normal operation of each component. At the rear of the robot, the vertical thruster 24 is close to the bionic tail swing 27, responsible for controlling the vertical movement of the robot and maintaining depth stability. The horizontally distributed thrusters 25 and 26 on both sides are responsible for horizontal movement to ensure that the robot can flexibly adjust its position underwater.

[0027] As Figure 2 shown, it includes Figure 2 a, 2b, which shows the composition structure of the bionic underwater electrolytic algae removal robot, presenting the layout and functions of each component of the robot. Inside the bionic shell 21 are the sealed cabin 20, the electrolytic cabin 13, the salt storage tank 14, and the buoyancy material 23. The sealed cabin 20 protects the internal components from water intrusion. The sealed cabin contains a main control system. The main control system selects Arduino and the integrated module 16, responsible for controlling the operation of the robot. The electrolytic cabin 13 is located in the center of the robot, with an anode titanium wire 5, a cathode titanium wire 6, a ruthenium-iridium-titanium anode 7, and a titanium cathode 8 built-in for electrolytic algae removal. The salt storage tank 14 is connected to the electrolytic cabin to provide a high-concentration salt solution. The robot is also equipped with a pH / ORP sensor 12 for monitoring water body parameters, a camera 17 for underwater monitoring, and an attitude / water depth sensor 18 for monitoring the attitude and water depth of the robot.

[0028] As Figure 3 shown, it shows the composition of the electrolytic cabin, including Figure 3a, 3b, 3c, 3d. The structure of the electrolysis cell 13 is mainly composed of an anode chamber and a cathode chamber. The anode chamber is located on the left side of the electrolysis cell and contains the anode titanium wire 5 and the ruthenium-iridium-titanium anode 7. The water inlet 3 and the water outlet 1 of the anode chamber are respectively located at the bottom and the top of the anode chamber. The cathode chamber is located on the right side of the electrolysis cell and contains the cathode titanium wire 6 and the titanium cathode 8. The water inlet 4 and the water outlet 2 of the cathode chamber are respectively located at the bottom and the top of the cathode chamber. There is an NF7 diaphragm 9 and a cation exchange membrane 10 separating the anode chamber and the cathode chamber. The NF7 diaphragm 9 intercepts chloride ions from entering the water outlet to save the replenishment of sodium chloride. The cation exchange membrane 10 ensures that sodium ions can pass through from the anode chamber to the cathode chamber during the electrolysis process to guarantee the electrolysis efficiency. A pH / ORP sensor 12 is installed on the top of the electrolysis cell to monitor the acidity, alkalinity and redox potential of the electrolyte. The liquid storage tank 14 is located outside the electrolysis cell for storing the electrolyte.

[0029] As Figure 4 shown, for the complete process of the system from startup to entering the main control loop, the process starts from "Startup". First, shore power is connected to power on the buoy system, and then the initialization of the main control Arduino is performed. After the initialization is completed, the system enters the device self-check phase to check the status of all sensors and actuators. If all devices are normal, the system will directly enter the main control loop and start normal operation. If a device anomaly is detected, the system will feedback an error code through the 485 bus and trigger an alarm at the shore end to prompt the operator for manual intervention. After the manual intervention is completed, the system can perform a device self-check again and attempt to resume normal operation. The entire process ensures that the system can operate stably and safely during startup, and at the same time provides a timely feedback and processing mechanism in case of device anomalies.

[0030] As Figure 5 shown, it shows a flowchart of remote control and process navigation, describing the complete process of the system receiving remote instructions and performing corresponding operations in the main control loop. The process starts from "Main Control Loop". The system first receives remote instructions and then performs different operations according to the instruction type. If the instruction is a movement instruction, the system will perform positioning and navigation, read the data of the GPS and depth sensors, calculate the three-dimensional target position, read the ICM attitude angle, calculate the position deviation, and allocate thruster control according to the deviation to adjust the depth of the vertical thruster and the direction of the horizontal thruster. Finally, a PWM signal is output to the electronic speed controller. If the instruction is electrolysis control, the system will jump to the electrolysis cell control and automatic dosing. If the instruction is to terminate the task, the system will jump to safety monitoring and task termination. The entire process ensures that the system can flexibly adjust operations according to remote instructions to achieve precise navigation and control.

[0031] As Figure 6As shown, an electrolysis control flow chart is presented, which describes a series of operations performed by the system after receiving an electrolysis control instruction. The process starts with "receiving the electrolysis control instruction". The system first sets the target current value and then reads the salt water volume in the storage tank. If the salt water volume is sufficient, the system will maintain the current output; if the salt water volume is insufficient, the system will trigger a low-salt alarm and start a peristaltic pump to supplement the salt water or perform manual supplementation. Next, the system adjusts the digital potentiometer or MOSFET to adjust the current and reads the data of the ORP sensor. If the ORP is normal, the system will return to the main loop; if the ORP is abnormal, the system will reduce the current and alarm. The entire process ensures that the electrolysis process operates stably when the salt water volume and ORP value are normal, and provides a timely feedback and adjustment mechanism in case of abnormalities.

[0032] As Figure 7 shown, the operation process of the system during task termination and safety monitoring is presented. In the task termination process, after receiving the termination instruction, the system sequentially shuts down the electrolytic cell, stops all thrusters, starts the vertical thruster to make the device float, records the position of the float, and finally ends the task. In the safety monitoring process, the system continuously monitors the leakage of water, communication interruption, and abnormal shore power in the main loop. If water leakage is detected, the system will cut off the power supply and emergently float; if communication is interrupted, it will standby at the position; if the shore power is abnormal, it will maintain the GPS through a capacitor and trigger an alarm. If no abnormality is detected, the system returns to the main loop to continue monitoring. The entire process ensures that the system can safely conclude the task during task termination and take emergency measures to ensure safety in case of abnormalities.

[0033] The above shows and describes the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. What is described in the above embodiments and the specification only illustrates the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A bionic underwater electrolysis algae removal robot system, characterized in that: It includes hardware and software parts. The hardware part includes a bionic shell, a modular sealed cabin, a shore power-backup battery dual-mode power supply device, and a floating communication relay. Inside the bionic shell are a sealed cabin, an electrolysis cabin, a salt storage tank, and buoyancy materials. The sealed cabin protects the internal components from water intrusion. The sealed cabin contains a main control system. The main control system selects Arduino and integrated modules, which are responsible for controlling the operation of the robot. The electrolysis cabin is located in the center of the robot, with built-in anode titanium wire, cathode titanium wire, iridium titanium anode and titanium cathode for electrolysis of algae. The salt storage tank is connected to the electrolysis cabin to provide a high-concentration salt solution. A GPS antenna is installed in the center of the top of the bionic shell for real-time positioning and navigation to ensure precise control of the robot underwater. Signal lines and power lines extend from the sealed cabin, which run through the entire robot and are responsible for transmitting control signals and electricity to ensure the normal operation of each component. At the rear of the robot, the vertical thruster is close to the bionic tail, which is responsible for controlling the vertical movement of the robot and maintaining depth stability. The horizontal thrusters and horizontal thrusters symmetrically distributed on both sides are responsible for water The robot can move in the horizontal direction to ensure that it can flexibly adjust its position underwater. The robot is also equipped with a pH / ORP sensor for monitoring water parameters, a camera for underwater monitoring, and a posture / water depth and water level sensor for monitoring the robot's posture and water depth. The software part includes a motion control and navigation module, an electrolysis chamber intelligent control module, and a safety monitoring and emergency processing module. The modular sealed cabin includes a functional compartment, an anode-cathode isolation chamber, and an equipment compartment; the shore power-backup battery dual-mode power supply device includes a shore power supply device and a backup power supply; the buoy communication relay selects a buoy GPS-485 communication relay, including a buoy structure, a signal transmission device, and an anti-interference part; the motion control and navigation module includes a thruster device, an electric adjustment module, a multi-sensor fusion positioning module, and an ICM-20689 posture sensor; the electrolysis chamber intelligent control module includes a current closed-loop control module, a brine automatic replenishment module, and an intelligent control algorithm module; the safety monitoring and emergency processing module includes a multi-level safety trigger mechanism module and an operation log module.

2. The bionic underwater electrolysis algae removal robot system according to claim 1 is characterized in that: The bionic shell adopts a streamlined shell design imitating boxfish, with a smooth surface, which reduces turbulence and resistance, reduces resistance during underwater movement, and improves maneuverability and energy efficiency. The functional warehouse integrates an electrolysis chamber, a salt storage tank and a peristaltic pump. The electrolysis chamber uses a CMI-7000s cation exchange membrane to separate the anode and cathode chambers to achieve ion selective permeation and improve electrolysis efficiency. The anode chamber uses an NF7 nanofiltration membrane to intercept chloride ions and save the amount of sodium chloride supplement. The equipment compartment has a built-in Arduino Mega 2560R3 main control board, a water depth sensor, an ICM-20689 attitude sensor, an ORP sensor and 6 electric adjustment modules to ensure the waterproofness and stability of the equipment.

3. The bionic underwater electrolysis algae removal robot system according to claim 1 is characterized in that: The shore power supply device is connected to the shore-based power supply (220V AC) through a waterproof cable, and converted into 12V DC for system use by the voltage stabilizing module. A 24V / 2500mAh lithium battery is integrated in the sealed cabin as a backup power supply. It automatically switches when the shore power is interrupted to maintain communication between the thruster device and the buoy.

4. The bionic underwater electrolysis algae removal robot system according to claim 1 is characterized in that: The float structure, the surface float has a built-in GPS module and RS-485 signal repeater, which is connected to the robot main control through a waterproof cable. The signal transmission equipment, the shore-side command is transmitted to the float via RS-485, and then forwarded by the float to the underwater Arduino main control to avoid underwater signal attenuation. Anti-interference design: The RS-485 communication protocol has strong anti-interference ability and supports long-distance communication of more than 500 meters.

5. The bionic underwater electrolysis algae removal robot system according to claim 1 is characterized in that: The electrolysis chamber includes an anode chamber and a cathode chamber. The anode chamber is located on the left side of the electrolysis chamber and contains an anode titanium wire and a ruthenium-iridium-titanium anode. The water inlet and outlet of the anode chamber are respectively located at the bottom and top of the anode chamber; the cathode chamber is located on the right side of the electrolysis chamber and contains a cathode titanium wire and a titanium cathode. The water inlet and outlet of the cathode chamber are respectively located at the bottom and top of the cathode chamber. The anode chamber and the cathode chamber are separated by an NF7 diaphragm and a cation-exchange membrane. The NF7 diaphragm intercepts chloride ions from entering the water outlet to save the replenishment of sodium chloride. The cation-exchange membrane ensures that sodium ions are allowed to pass from the anode chamber into the cathode chamber during the electrolysis process to ensure the electrolysis efficiency.

6. The bionic underwater electrolysis algae removal robot system according to claim 1 is characterized in that: A pH / ORP sensor is installed on the top of the electrolysis chamber to monitor the pH and redox potential of the electrolyte. The liquid storage tank is located outside the electrolysis chamber to store the electrolyte. The thruster device includes 2 vertical thrusters for adjusting the depth and 4 horizontal thrusters for achieving movement in the XY directions.

7. The bionic underwater electrolysis algae removal robot system according to claim 1 is characterized in that: The electric control module receives the Arduino PWM signal to accurately control the rotation speed of each thruster. The multi-sensor fusion positioning module uses the floating GPS to provide water surface coordinates and calculates the three-dimensional position in combination with the water depth sensor. The ICM-20689 attitude sensor provides real-time feedback on the roll / pitch angle, and the Arduino corrects the thrust direction of the thruster through the PID algorithm.

8. The bionic underwater electrolysis algae removal robot system according to claim 1 is characterized in that: In the current closed-loop control module, Arduino adjusts the electrolytic chamber current (0-10A) through the MOSFET module, and the ORP sensor monitors the redox potential of the anode chamber to dynamically adjust the current intensity.

9. The bionic underwater electrolysis algae removal robot system according to claim 1, characterized in that: The brine automatic replenishment module: The salt storage tank quantitatively injects high-concentration brine into the anode chamber through a peristaltic pump, and the dosing is triggered when the ORP value is lower than the threshold (such as ORP < 600mV, the pump speed is increased by 20%). The drainage rate of the cathode chamber outlet is controlled by a peristaltic pump to maintain the pressure balance of the electrolysis chamber. The intelligent control algorithm module dynamically adjusts the electrolysis parameters based on the fuzzy control algorithm to ensure efficient and stable operation.

10. The bionic underwater electrolysis algae removal robot system according to claim 1, characterized in that: The multi-level safety trigger mechanism module immediately cuts off the power supply of the electrolysis cabin and starts the backup battery to float when the sealed cabin humidity sensor detects water leakage. If the RS-485 communication is interrupted for more than 10 seconds, it switches to the attitude holding mode to avoid uncontrolled drift. The operation log module is uploaded to the shore in real time to facilitate fault diagnosis and maintenance.

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