For hydraulics, control generally includes manual control, mechanical control, hydraulic control, and electronic control:
Manual control: Humans control the hydraulic system through hydraulic valves, limited by the operator’s proficiency, reaction sensitivity, and attention concentration.
Mechanical control: Using equipment components to drive trip dogs to open and close valves, with extremely poor flexibility.
Hydraulic control: Using oil pressure to transmit control information, divided into two types:
Automatic response to working conditions: such as relief valves, constant pressure difference elements (pressure compensating valves), displacement control of negative flow/positive flow/load sensing pumps, etc.; characterized by simplicity and reliability, but intelligence level far below computers.
As an auxiliary means for manual control: such as in construction machinery, where operators control pilot hydraulic valves to change pressure, then control main valves (hydraulic multi-way valves).
(2) Analog Control and Digital Control
Analog control: The original form of electronic control, using resistors, capacitors, potentiometers, operational amplifiers, etc. to form non-digital control; operational amplifiers can perform PID (proportional, integral, differential) processing on the difference between feedback signals and expected values, and were the only automatic control method before digital control. In the 1960s and 1970s, analog computers were once used in domestic universities/research institutions, but have now been phased out.
Digital control: The anti-interference capability, accuracy, transmission speed, etc. of (binary) digital circuits gradually surpassed analog circuits in the 1960s with breakthroughs in electronics/computers/chip technology; digitalization means digital control completely replaced analog control.
After the invention of PLC (Programmable Logic Controller) in the 1970s, hydraulic supporting controllers gradually changed from analog to digital control starting in the 1980s.
China’s “mechatronics” originates from the European and American portmanteau word “mechatronic” (mechanic+electronic), where “electronic” refers to digital control.
Electronic control now is almost entirely digital control; however, there are claims that AI requires massive logical operations and does not require high precision, so to reduce energy consumption, analog circuits may be partially reused in very large-scale integrated circuits.
(3) Electronic Control Expands from Fixed Hydraulics to Mobile Hydraulics
Electronic control for hydraulic drive was widely adopted in fixed equipment early on, but application in mobile equipment was later, because:
Mobile equipment mostly works outdoors, and early electronic products had poor environmental adaptability;
Early electronic products were relatively expensive.
Starting from the 1980s, mobile equipment complexity increased (demand for electronic control strengthened) + modern electronic components became more reliable (resistant to harsh environments) and prices decreased → electronic control application became widespread in mobile hydraulics. A new word “hytronic” (hydraulic+electronic) emerged in Europe and America, emphasizing “hydraulic drive + computer control + sensors,” which can be translated as “hydro-electrical integration” or “hydraulic drive with electronic control” (electronic control = computer control + sensors).
Currently, computer intelligence far exceeds hydraulic control: the new generation of tablet chips in 2022 contains 40 independent CPU cores and 14 billion diodes (while a hydraulic check valve is equivalent to only one diode); combined with software science and AI development, electronic control is the “commander/superior/leader” of hydraulic drive.
2. Advantages of Electronic Control
Using electronic control for hydraulic drive can leverage the intelligent characteristics of computers, with the following advantages:
(1) Achieving Complex Actions
In the linkage articulation mechanism of excavators, a single (group of) hydraulic cylinder can only drive components to rotate around a fulcrum; if the bucket tip is to move in a straight line, coordinated action of the boom cylinder, stick cylinder, and bucket cylinder is required.
With manual control: Requires coordinated operation of at least three multi-way valve openings with both hands simultaneously, depending on operator proficiency, with the throttle opening shape of multi-way valve spools determined by experienced operators.
With electronic control: After establishing a mathematical model of the mechanism and inputting geometric parameters, the movement of each hydraulic cylinder can be decomposed from the motion trajectory, and multi-way valve opening commands can be given (as easy as pie).
Case: At the 2019 Munich Construction Machinery Exhibition, the Hydac excavator model’s electronic control could automatically coordinate three hydraulic cylinders to make the bucket tip draw a five-petaled rose curve; Hitachi and Volvo’s excavator electronic control can simplify operation: move the bucket to the starting point, select the desired trajectory (plane, slope, etc.), then only need the left hand to control the stick speed, and the computer automatically coordinates bucket and boom speeds to scrape out the target trajectory.
(2) Helping Save Energy
There are many R&D projects for energy saving with electronic control:
Medium pressure network: In single-pump, multi-hydraulic cylinder machinery, traditional load sensing circuits (pump port pressure varies with highest load) will throttle through constant pressure difference elements in low-pressure branches, consuming excess pressure; establishing a medium pressure layer can reduce pressure differential consumption at throttle openings. A 2022 report stated that excavators using medium pressure layers (STEAM) reduce energy consumption by 30% compared to traditional types (relying on electronic control to continuously monitor load and medium pressure layer pressure, switching in time).
Replacing constant pressure difference elements: The constant pressure difference elements + throttle openings in traditional load sensing circuits continuously consume at least 2MPa pressure; some research abandons constant pressure difference elements, using electronic control and fast-adjusting electronically controlled multi-way valves to avoid consumption, and entered trial phase in 2020.
Bosch Rexroth “Connected Hydraulics beyond Limits”: Hydraulic components exchange information through electronic control (IO-Link) to better perform their functions.
(3) Other Advantages of Electronic Control for Hydraulic Drive
Solidifying experienced operator experience into control programs, improving operational friendliness;
Achieving operation automation, shortening training time;
Predicting faults based on early information (“health management”), improving planned maintenance capability.
Hydraulic components (valves) have been developing for over 200 years and are mature; new structures are difficult to commercialize; however, combined with electronic control and grasping actual needs (selecting appropriate circuits/valves/performance), breakthroughs in the industry are easy; the author’s patent inventions at German companies are mostly based on electronic control.
3. Possible Approaches for Electronic Control
The core of hydraulic technology is regulating “oil volume” (flow rate). There are three approaches for electronic control of hydraulic systems:
Valve control: Using electro-hydraulic converters (electro-proportional directional throttle valves, electro-proportional flow control valves, etc.) to regulate valve spool displacement (opening); control types include switching solenoids, proportional solenoids (currently most widely used, domestically mostly reliant on imports, a bottleneck for hydraulic equipment automation), servo valves, stepper motors, servo motors, etc. Multi-way valves in mobile hydraulics are mostly hydraulically controlled and can be upgraded to electronic control after being equipped with electro-hydraulic converters.
Pump (displacement) control: Variable displacement pumps in actual application are piston pumps (single-acting vane pumps are only suitable for low pressure), among which axial piston pumps (swash plate pumps, bent axis pumps) are more commonly used; the swash plate inertia of swash plate pumps is less than the cylinder block inertia of bent axis pumps, with better dynamic response, so swash plate pumps are mostly used in electronic control.
Variable displacement pump displacement control is divided into internal control and external control: all external control (manual control, mechanical control) can be replaced by electronic control; all internal control (negative flow/positive flow/constant pressure/constant pressure difference/constant power pumps, etc.) is hydraulic control, which in principle can also be achieved with electronic control:
Pressure sensors transmit pump outlet pressure and load pressure to the controller;
The controller sends signals to the electro-proportional valve according to function commands;
The electro-proportional valve controls the swash plate tilting cylinder pressure, thereby controlling the swash plate angle;
The swash plate angle sensor feeds back the actual angle to the controller, which adjusts the signal.
Case: Bosch Rexroth eOC pump (electronic Open Circuit) can switch control functions (constant pressure/constant power/angle control, etc.) through software, and has been applied in A10VO, A11VO, A15VO pump models; after small multifunctional excavator-loaders adopt eOC pumps, control functions can be selected as needed in the operation room.
Speed control: Regulating the speed of the prime mover driving the pump, thereby regulating flow rate.
Internal combustion engine speed regulation range is small (maximum to minimum speed ratio <6, economical speed ratio <2), not suitable for speed control; only servo motors can undertake this task.
4. Tasks of Hydraulic Engineers
Mechanical structure is the foundation of hydraulic systems, and hydraulic systems are the foundation of electronic control; after functions are determined, hydraulic engineers need to design/select appropriate hydraulic systems:
Select electronic control method: Choose valve control, pump control, or speed control according to needs; note: valve spool mass (tens of grams) < swash plate (hundreds of grams) < motor rotor (thousands of grams), the greater the mass, the more difficult rapid response, requiring development of measures according to needs.
Coordinate application requirements with control software programming: In scaled main equipment enterprises, design departments are divided into mechanical design, hydraulic design, electrical design, and software design; hydraulic engineers have better understanding of main equipment movement and hydraulic system/component characteristics, and need to make requirements to software programmers:
Number and signal types of input components (knobs, buttons, sensors, etc.);
Number and signal types of output components (indicator lights, alarms, switching valves, electro-proportional valves and driving currents, etc.);
Control logic (rules for input corresponding to output);
Adjustable parameters (such as delayed closing time of pump port unloading valve).
Hydraulic engineers need to understand basic programming and establish a “common” language with software programmers.
