1. Hydraulic control system
Hydraulic control system consists of
Power source (electric motor or combustion engine + pump)
Control component (typically valve, sometimes pump)
Actuator (cylinder or motor)
Transducers (in valve, in actuator)
Control electronics and control (feedback) loops (one or more)
System output is, e.g., position, speed, force, of the actuator.

2. Control systems – open and closed loop
Control system with feedback

3. Fluid power control systems – feedback control
1. Electric Motor Control
2. Pump Control
DFCU
Digital Flow Control Unit
3. Valve Control
4. Digital Hydraulic (Valve) Control
Figures: Rydberg (1, 2, 3) and Linjama (4)

4. Electric control components in hydraulics
Majority of continuously controlled systems are realized either with proportional or servo valve technology, where the actuator is controlled with a single valve. This valve is in turn controlled with an electric actuator that is either or
Bosch Rexroth AG
Linear magnet – Proportional solenoid
Torque motor
Output of electric actuator is proportional to its command value and thus also the output of the valve is proportional to it.

5. Proportional solenoid
Electric actuator for controlling spool force or displacement
Stroke-controlled proportional solenoid
Force-controlled proportional solenoid
Due to position feedback position …
Hysteresis small
Repetition error small
Force-stroke curves for force-controlled solenoid

6. Valves for control systems (1)
Separate control electronics
Valves for control systems (1)
Proportional Solenoid operated proportional control valves
Spool valve
Proportional solenoid
LVDT position sensor
Proportional Directional Control Valve
Rexroth 4WRPH6
Step response at 100 % step < 10 ms

7. Valves for control systems (2)
Control electronics
Voice coil
Voice Coil operated proportional control valves
Size:
NG06 / CETOP 03 / NFPA D03
Spool valve
Nominal flow up to bars
Step response at 100 % step < 3.5 ms
Measured with load (100 bar pressure drop/two control edges)
Voice Coil operated Proportional Directional Control Valve
Parker DFplus

8. Torque motor – core of servo valve
Valves for control systems (3a)
Torque motor – core of servo valve
Flapper-jet system
First stage

9. Servo valve – two stages and mechanical feedback
Valves for control systems (3b)
Servo valve – two stages and mechanical feedback
Torque motor (1) electric current changes
Displacement of torque motor and flapper plate (6)
Unbalance of pressure losses in flapper/nozzle
Pressure difference acts on spool (5) ends
Spool (5) moves
Feedback spring (3) bends and pulls flapper plate (6) back
New equilibrium is achieved due to feedback (spring)
Certain torque motor current
Certain spool position
Feedback spring

10. Electric control components in hydraulics
Traditionally the use of torque motors has resulted into significantly higher boundary frequencies of valve output, but the recent development of linear magnets has diminished the frequency gap between these technologies.
Dynamic characteristics of valves depend greatly on the characteristics of the electric actuator, on the feedback system and on valve size. Examples of boundary frequencies:
- proportional magnet  70 Hz
- servoproportional magnet  120 Hz
- voice-coil-magnet  220 Hz
- torque motor  300 Hz
Example diagram, does not represent any certain valve.

11. Valves for control systems (4)
Solenoids
Electronics
Solenoid and pilot valve operated digital hydraulic valve
Aalto University – Tapio Lantela
32 Poppet valves (ON/OFF)
8 valves per control edge
NG06 / CETOP 03 / NFPA D03 sub plate  can replace NG6 proportional control valve
Nominal flow up to 8*9 35 bars
Step response at 100 % step < circa 1 ms
Enhanced 3D printed version also ready and tested

12. Servo system stiffness and nominal angular velocity1 2 M
A piston area
Kf bulk modulus
V fluid volume
m inertia mass 3 4
Differential cylinder connection, hydraulic spring only in chamber A
Asymmetric cylinder (differential cylinder), asymmetric springs
Symmetric cylinder (symmetric cylinder), symmetric springs
Hydraulic motor + transmission (gear ratio) kA kB kA

13. Linearized P controlled position servo
Control valve orifice (flow gain), cylinder and mass load
- Second order system + integrator
Control valve dynamics
- First order system
P gain
Flow gain
Open loop gain
K=KAKVKqKh/A
𝐾 𝑞 = 𝜕 𝑞 v 𝜕 𝑥 v
Position sensor
Cylinder+load
Nominal angular velocity
Bulk modulus
Piston area
Mass
Fluid volume
Cylinder+load
Damping ratio
Mass
Valve opening
Cylinder leakage
Cylinder viscous friction
Cylinder volume
Bulk modulus
Piston area

14. Stability? Definitions (many!)
Stable
Unstable
Marginally stable
Definitions (many!)
A system is stable if its impulse response approaches zero as time approaches infinity (Asymptotic stability).
A system is stable if every bounded input produces a bounded output (BIBO: Bounded Input, Bounded Output).
Marginally stable
The output does not go to zero, but it does not grow infinite either.
Stable

15. Stiffness and stability?
By increasing gain the output can follow the command signal rapidly and accurately.
However there is a risk for loosing system stability.
Analytical means (for a linear/linearized system) give us that stabilty criteria for a P controlled position servo is
K < 2hh
Which means that we can increase the system gain (K) and enhance the performance if we can have a
higher system damping h and/or
nominal angular velocity h.
Open loop gain
K=KAKVKqKh/A

16. Fluid power system nonlinearities?
Cylinder volume  cylinder stiffness
Bulk modulus can vary as a fuction of pressure
Control valve (Flow Gain) is dependent on pressure difference
Seal forces vary as a function of
Velocity
Pressure
Mode
Bending
Sliding
𝐾 𝑞 = 𝜕 𝑞 v 𝜕 𝑥 v V q
𝑞= 𝐶 𝑞 𝐴 2∆𝑝 𝜌

17. Simulation and testing
In addition to using analytical tools in hydraulic servo systems simulation and testing could be recommended for
Reliability
Safety
Performance