Proportional Vs Servo Valves

Proportional Vs Servo Valves 1
  • The expanding application of electronics in today’s technology for governing various actuation systems such as hydraulic and pneumatic systems in process industries has naturally boosted interest in servo and proportional valves.
  • Both these Proportional & Servo Valves are Electro-Hydraulic and Electro-Pneumatic types.
  • These continuous-acting valves metamorphose the given dynamic analog input or digital input signal into a step-less static hydraulic output signal.
  • This step-less output signal could be either a flow signal or a pressure signal.

Servo Valves

Proportional Vs Servo Valves2
  • The phrase servo valve describes a valve design that encompasses a bushing spool assembly represented by the metering edge having a High-Precision value.
  • The phrase servo valve is derived from the term servo-mechanism, which represents constant and continuous monitoring of the valve’s motion.
  • The term proportional defines any kind of activity for which only one parameter changes in proportion to another.
  • It is a directional control valve with less than 3% center overlap that is constantly changeable and electrically modulated. 
  • Servo valves are implemented in advanced electronics along with closed-loop control systems.
  • A servo valve provides greater advantages such as high accuracy, very high repeatability, with minimum hysteresis rate, and the frequency response of this servo valve is very high.
  • Compared to proportional valves these servo valves are more expensive. 
  • A servo valve is also known as Pressure Control Valve (PCV).
  • Generally, in some cases, we use this valve to control the fluid pressure through a directional control valve.
  • The force or torque produced is sensed & measured by a load cell or pressure transducer.
  • The servo amplifier controls the valve by determining the feedback signal from the sensor.

Classification of Servo valves 

  1. Single-stage servo valve: It is a directly regulated valve. These valves have a single control stage, which means they have a single control orifice. Single-stage servo valves are relatively simple in design and provide basic flow control capabilities.
  2. Two valve stages servo valves:       It consists of a pilot stage and a final or main stage. Two-stage servo valves have two control stages or two sets of control orifices. These valves offer enhanced control and flow modulation compared to single-stage valves. The first stage controls the main flow, while the second stage modulates the fine control or pilot flow.
  3. Three-stage servo valves: These are similar to two valve stages servo valves, except that the pilot itself is a two-stage servo valve. Three-stage servo valves have three control stages, each with its own control orifice. The additional stage provides even more precise and fine control over hydraulic flow. These valves are capable of achieving high levels of accuracy and responsiveness.
  4. Multi-Stage Servo Valves: Multi-stage servo valves can have more than three control stages, typically four or more. These valves offer the highest level of control and are often used in applications that require extremely accurate and complex hydraulic control.

Proportional Control Valve 

Proportional Vs Servo Valves3
  • The term proportional illustrates a valve within a spool in a valve body. 
  • The term proportional valve refers to any type of action where only one variable fluctuates corresponding to another variable.
  • It is an electrically modulated and continuously variable directional control valve having a center overlap of more than 3%.
  • Proportional valves are designed to generate a smooth and continuous variation in flow or pressure in response to an electrical input signal. 
  • Connecting electronics to these valves requires precision.
  • These valves allow for limitless spool positioning, resulting in infinitely changeable flow volumes. 
  • Stroke-controlled actuators, which use pneumatic cylinders with proportional valves and solenoids to regulate the direction and speed of motion, can perform endless spool positioning.
  • Servo Motors      with electric cylinders are utilized on occasion to regulate the activation of stack-controlled solenoid valves.
  • The concept of a proportional valve refers to a type of valve that is independent of servo valves in the hydraulics segment.
  • Due to the variable placement of the valves, it may be done to build spools with metering notches enabling flow control, speed control, and directional control throughout a single valve.
  • Another significant benefit is that when the circuit permits more than one speed or maximum speed limit, it reduces the need to mount separate valves for direction and speed control. 
  • These different speeds are accomplished by adjusting the electrical signal level with the goal to spread the flow or speed as desired.
  • Auxiliary components are not necessary for these proportional valve types.
  • A DC power supply regulates or governs these proportional directional valves.
  • Proportional controls paired with similar electronic controls will enhance the acceleration and deceleration properties. 
  • This allows for an extensive collection of machine cycles that could be securely handled at greater speeds along with controlled start and stop characteristics. 
  • Increased machine cycles and production rates result from controlled acceleration and deceleration.
  • Proportional valves are most commonly used in open-loop applications.
  • Single Proportional valve unit replaces numerous Static Flow Valve and Pressure Valve for acceleration and deceleration under control for pressure and flow that are required to change continuously.

Comparison between Proportional Valves and Servo Valves 

Proportional ValveServo valve
Used in Open Loop Control SystemUsed in Closed Loop Control System
Cheaper than Servo valvesPrice is high compared to  Proportional valves
Requires  maximum power of about 50WRequires very less power less than 1W
Filtration is moderate about 30 micrometer     Filtration is high  about 1 to 5 micrometers
Spools  are over- lappedSpools are critically lapped
Flow characteristics are  nonlinearFlow characteristics are linear
Hysteresis  is large 0.5%Hysteresis  is low 0.1%
Used  as flow, pressure & directional control valvesPrimarily used in closed loops to create flow and pressure control
Used in closed loop control if high performance is not expectedUsed in the closed loop if high performance is expected.
Generally have faster response timesCan have slower response times
Relatively simpler designCan be more complex and sophisticated
Wide range of industrial applicationsAerospace, precision machinery, high-performance systems
Typically have lower control resolutionOffer higher control resolution
Provide moderate accuracy and repeatabilityProvide high accuracy and repeatability

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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