Closed Eyes: Open Loop
Open Eyes: Closed Loop
Understanding the fundamental principles of control systems and the critical role of the hydraulic servo valve in modern automation.
1. Open Loop vs Closed Loop
On a typical soccer field, walking from one side to the goalpost on the other side is a simple task for most people when their eyes are open. However, attempting the same feat with eyes closed, without any other assistance, would almost certainly result in failure to reach the target accurately.
Why is this the case? The answer lies in the difference between open and closed loop control systems. When walking, the length of each step is never perfectly consistent. With eyes closed, there's no way to perceive the actual position or adjust step length and direction accordingly. This is what control engineers refer to as an open loop system. Conversely, with eyes open, we continuously compare our actual position with our intended path and make necessary adjustments—this is a closed loop system, as illustrated in Figure 4-94.
Open Loop System
Figure 4-94a: Open loop control system
Controller → Command → Actuator → Result
Closed Loop System
Figure 4-94b: Closed loop control system
Controller → Command → Actuator → Result → Feedback → Controller
The walking example demonstrates manual control, where the brain acts as the controller, the legs as actuators, and the eyes as feedback sensors. Closing one's eyes creates an open loop scenario—the brain issues commands based on expectation but lacks information about the actual situation, making corrections impossible. Opening one's eyes establishes a closed loop system, where continuous feedback allows the brain to compare actual position with desired position and adjust commands accordingly.
To automate this process, implementing a closed loop system with instruments requires a structure similar to that shown in Figure 4-95. A sensor acts as the feedback mechanism, providing information about the actual situation to the controller. The controller compares this feedback with the desired outcome and issues corrective commands to achieve the desired result. This principle is fundamental to the operation of a hydraulic servo valve, which relies on precise feedback to maintain accurate control.
Figure 4-95: Instrument-based closed loop system
Feedback enables adjustments that minimize the effects of disturbances—both external and internal, unexpected, uncertain, and variable resistances. Early controllers simply calculated the difference between the desired value and the feedback value (effectively using negative feedback) to issue corrections. Modern digital and AI-based controllers, however, can perform much more complex calculations and judgments based on feedback before issuing commands, a capability that significantly enhances the performance of systems utilizing a hydraulic servo valve.
The term "servo" derives from the Latin word "Servo," sharing the same root as "servant." Just as a servant attends to their master, observing and responding to their needs, a servo system monitors and reacts to actual conditions through feedback mechanisms. Thus, feedback is essential to the definition of a servo system, making it indispensable in industrial automation. This principle is perfectly embodied in the hydraulic servo valve, which relies on continuous feedback to maintain precise control over fluid flow and pressure.
2. Closed Loop Control of Electro-Proportional Valves
As previously discussed, various influencing factors (disturbances) make it difficult for the spool of a conventional electro-proportional valve to achieve the desired displacement (opening) under open loop control. Implementing closed loop control, as shown in Figure 4-96, allows a position sensor to inform the controller of the actual spool displacement. The controller can then adjust the current to the proportional solenoid accordingly to achieve the required displacement, much like how a hydraulic servo valve operates with enhanced precision.
Symbol
Figure 4-96a: Graphical symbol
Structure
Figure 4-96b: Structure schematic
Component
Figure 4-96c: Proportional solenoid with position sensor
- Proportional solenoid
- Position sensor
- Controller
When an appropriate control method such as PID (Proportional-Integral-Derivative) correction is employed in the controller, the response speed of the valve can be significantly improved. This enhanced performance brings the valve closer to that of a traditional hydraulic servo valve, leading some manufacturers to market closed loop controlled electro-proportional valves as industrial servo valves. The key distinction lies in the precision of feedback and response time, where a high-quality hydraulic servo valve still maintains an advantage in critical applications requiring exceptional accuracy.
The integration of feedback mechanisms into proportional valves represents a significant advancement in fluid power control, bridging the gap between simple on/off valves and sophisticated hydraulic servo valve systems. This evolution has enabled more precise control in applications where a full hydraulic servo valve might be unnecessarily complex or costly, while still providing the benefits of closed loop operation.
Key Advantages of Closed Loop Valve Control
- Reduced sensitivity to supply pressure fluctuations, a critical improvement over open loop systems
- Minimized effects of temperature variations on valve performance
- Compensation for wear over time, maintaining accuracy throughout the valve's lifespan
- Faster response times compared to open loop counterparts
- Improved repeatability, approaching the precision of a dedicated hydraulic servo valve
- Enhanced stability across varying operating conditions
These advantages make closed loop controlled proportional valves suitable for a wide range of industrial applications, from material handling to precision machining. While they may not match the extreme performance specifications of a high-end hydraulic servo valve, their cost-effectiveness and versatility have made them a popular choice in modern automation systems. The hydraulic servo valve remains the gold standard in applications requiring micron-level precision and sub-millisecond response times, such as in aerospace and advanced manufacturing.
3. Closed Loop Control in Hydraulic Systems
Hydraulic systems face numerous disturbing factors, including load fluctuations and changes in control element characteristics. Under open loop control (illustrated in Figure 4-97), it becomes difficult to ensure that the actuator achieves the desired motion, even when using precisely manufactured components. This limitation underscores the importance of closed loop control in hydraulic systems, where maintaining consistent performance is critical to operational efficiency and safety.
Hydraulic System Open Loop Control
Figure 4-97: Hydraulic system open loop control block diagram
In an open loop hydraulic system, the controller sends signals to the electro-hydraulic converter based solely on the desired value, without any information about the actual system output. This approach works adequately in controlled environments with minimal disturbances but fails to maintain accuracy when conditions change, highlighting the need for more sophisticated control methods involving components like the hydraulic servo valve.
The solution to improving the motion accuracy of hydraulic systems lies in implementing closed loop control, as shown in Figure 4-98. By incorporating feedback from sensors that monitor the actual system output (such as position, velocity, or pressure), the controller can make continuous adjustments to counteract disturbances and maintain the desired performance. This is the operating principle behind the hydraulic servo valve, which uses feedback to precisely control fluid flow and pressure.
Closed Loop Control Principle
Closed loop hydraulic control systems introduce feedback mechanisms that create a complete control loop:
- The controller receives a desired value signal
- Simultaneously, it receives feedback about the actual system output
- The controller calculates the difference (error) between desired and actual values
- Corrective signals are sent to the hydraulic components, often via a hydraulic servo valve
- The process continues in real-time, maintaining precise control
System Components
Key components of a closed loop hydraulic system include:
- Controller (often a PLC, microprocessor, or dedicated control module)
- Electro-hydraulic converter (solenoid, torque motor, or voice coil)
- Hydraulic control element (hydraulic servo valve or proportional valve)
- Actuator (cylinder or hydraulic motor)
- Feedback sensor (position, velocity, pressure, or force transducer)
- Load (the external system being controlled)
Hydraulic System Closed Loop Control
Figure 4-98: Hydraulic system closed loop control block diagram
The integration of feedback into hydraulic systems represents a significant advancement in fluid power technology. By continuously comparing actual performance with desired outcomes and making instantaneous adjustments through devices like the hydraulic servo valve, these systems can maintain precise control even in the presence of varying loads, temperature changes, and other disturbances. The hydraulic servo valve plays a crucial role in this process, acting as the interface between the electronic control signals and the hydraulic power elements, translating small electrical signals into precise fluid flow control.
Modern hydraulic closed loop systems often employ advanced control algorithms that go beyond basic PID control, incorporating adaptive control, fuzzy logic, or even machine learning techniques to optimize performance. These intelligent systems can learn and adapt to changing conditions, further enhancing the precision and efficiency of hydraulic systems utilizing the hydraulic servo valve.
Applications of closed loop hydraulic systems are widespread across various industries:
Industrial Machinery
CNC machine tools, injection molding machines, and robotic arms utilize closed loop hydraulic systems with precise hydraulic servo valve control for accurate positioning and repeatability.
Mobile Equipment
Construction equipment, agricultural machinery, and material handling vehicles employ closed loop hydraulic systems to maintain performance under varying load conditions.
Aerospace & Defense
Flight control systems, missile guidance, and simulation equipment rely on high-performance hydraulic servo valve systems for extreme precision and reliability in critical applications.
The evolution of closed loop control in hydraulic systems has been closely tied to advancements in sensor technology, electronics, and control algorithms. Modern sensors provide accurate, high-bandwidth feedback, while powerful microprocessors can perform complex calculations in real-time. These developments, combined with improvements in hydraulic servo valve design and manufacturing, have resulted in hydraulic systems that offer exceptional precision, responsiveness, and efficiency.
One of the key advantages of hydraulic closed loop systems is their ability to deliver high power density with precise control. This unique combination makes them indispensable in applications where both strength and accuracy are required. The hydraulic servo valve is the critical component that enables this performance, precisely regulating the flow of high-pressure fluid to actuate cylinders or motors with exceptional accuracy.
As industries continue to demand higher performance, greater efficiency, and enhanced automation, the role of closed loop control in hydraulic systems will only grow in importance. The hydraulic servo valve, as a key enabler of this technology, will remain central to these advancements, evolving to meet the increasingly stringent requirements of modern industrial applications.
In summary, the transition from open loop to closed loop control represents a fundamental advancement in hydraulic system technology. By incorporating feedback mechanisms and intelligent control strategies, modern hydraulic systems can achieve levels of precision and performance that were once unimaginable. The hydraulic servo valve stands at the heart of this technology, enabling the precise control of fluid power that drives industrial automation, mobile equipment, and advanced manufacturing processes around the world.