Electrohydraulic Control Systems

( 11 )
230 pages
Fluid power is used in a vast range of applications and power levels. This book specifically considers the application of electrohydraulic valves in control systems.
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John Watton BSc PhD DSc FREng FIMechE CEng

The author has taken a broad approach to the study of fluid power control, covering a period of almost forty years. He started his career in industry working on heat exchangers and then obtained BSc and PhD degrees in Mechanical Engineering. He retu...


Fluid power is used in a vast range of applications, often with fast response times and for power levels that can be up to several MW and where no other form of actuation is feasible. Fluid power control systems may be placed in environmentally-difficult applications and increasingly with alternative fluids to pure mineral oil.

This book specifically considers the application of electrohydraulic valves in control systems, an extremely important part of fluid power. The aim is to bring together various key aspects and up to a design level beyond a basic text but without an overload of fundamental derivations. Some background theory is still considered essential but only at a level that allows the reader to get a ‘feel’ for how it is used in practice. The layout of the book is such that the reader may progress through it from basics of steady-state operation to essential concepts of dynamic design and control.

  1. An introduction to the operation of electrohydraulic valves
    1. Aim
    2. Electrohydraulic valves, direction and pressure relief types
    3. Electrohydraulic valves, the servodrive concept
    4. Servovalve operation and design
  2. Servovalve flow characteristics
    1. Aim
    2. Servovalve flow characteristics, critically-lapped spool
    3. Servovalve flow characteristics, under-lapped spool
    4. Valve rating
    5. Servovalve specification by the manufacturer
    6. Maximum power transfer to the load and selection of supply pressure
  3. Connecting a servovalve to an actuator, steady-state behaviour
    1. Aim
    2. Connecting a servovalve to a cylinder, the open-loop steady-state behaviour
    3. Connecting a servovalve to a motor, the open-loop steady-state behaviour
    4. Connecting a servovalve to a motor, the closed-loop steady-state behaviour
    5. Connecting a servovalve to a cylinder, the closed-loop position transient response
    6. Servodrive damping, leakage and friction losses, efficiency
    7. Improving the steady-state performance of an open-loop motor servodrive using a Programmable Servo Controller
  4. System dynamics and computer simulation
    1. Aim
    2. Fluid compressibility and its effect on flow rate
    3. Force and torque equations for actuators with moving mass or rotary inertia
    4. Undamped natural frequency of an actuator
    5. Actuator equations with losses and dynamics
    6. Linearisation of the system equations
    7. Pipe resistance, compressibility and inertia
    8. Servovodrive open-loop linearised differential equation with losses and dynamics
    9. Servovalve dynamics
    10. The role of computer simulation
  5. Laplace transforms, transfer functions, block diagrams, transient and frequency response
    1. Aim
    2. Laplace transforms
    3. Transfer functions and block diagrams
    4. Undamped natural frequency with connecting line effects
    5. Transient response and its specification
    6. Frequency response
    7. The effect of a pure delay
  6. Closed-loop stability
    1. Aim
    2. Closed-loop stability
    3. The use of frequency response
  7. Improving closed-loop behaviour
    1. Aim
    2. Some preliminary comments
    3. The use of servoamplifier dither to improve steady-state error drift for a position control system
    4. The effect of spool under-lap on the steady-state error for a position control system
    5. Steady-state tracking error in response to a velocity demand for a position control system
    6. Optimising the closed-loop transient response
    7. Velocity sensing or Derivative computation
    8. Additional acceleration, or pressure, feedback
    9. Proportional+Integral+Derivative (PID) control
    10. Gain scheduling using a PSC
    11. Digital control algorithms using a PSC
    12. Improving the dynamic performance of an open-loop motor servodrive using a PSC and pressure derivative feedback
  8. Pressure and force control
    1. Aim
    2. Pressure control of a fixed-volume container
    3. Force control of a servoactuator
  9. Closing comments and further reading guidance