• img1
  • img2
  • img3
  • img4
  • img5
  • img6
  • img7
  • img8
  • img9
  • img10
  • img11
  • img12
  • img13
  • img14
  • img15
  • img16
  • img17
  • img18
  • img19
  • img20
  • img21
  • img22
  • img23
  • img24
  • img25
  • img26
  • img27
  • img28
  • img29
  • img30
  • img31
  • img32
  • img33
  • img34
  • img35
  • img36
  • img37
  • img38
  • img39
  • img41
  • img42
  • img43
  • img44
  • img45
  • img46
  • img47
  • img48
  • img49
  • img50
  • img51
  • img52
  • img53
  • img54
  • img55
  • img56
  • img57
  • img58
  • img59
  • img60
  • img61
  • img62
  • img63
  • img64
  • img65
  • img66
  • img67
  • img68
  • img69
  • img70
  • img71
  • img72
  • img73
  • img74
  • img75
  • img76

Practical Considerations

 
PRACTICAL CONSIDERATIONS WHEN LAYING OUT
ELECTROHYDRAULIC CONTROL SYSTEMS
  1. Power Units
    Pumps:
    Constant supply pressure is preferred with minimum variation. Use accumulators with variable displacement pressure compensated pumps. Fixed displacement pump: constant pressure with use of accumulator is an option.

    • If more than one critical system is fed from one pump, isolate each system with check valves and accumulators (avoids cross-talk).
    • Reservoir breather: 3 to 5 micron air filter preferred with capacity appropriate to fluid displacement.
    • Temperature and pressure should be closely controlled if good long term control accuracy is critical.
    • Fluid flowing over a relief valve represents wasted energy.
  2. Piping and Fittings
    Do not use pipe dope. (It contains fine, hard to filter, particulate.) Use TFE tape when necessary. Do not use pipe or pipe fittings.

    • Use only correct tube cutting tools, no hacksaw. Deburr if necessary.
    • Cold bending preferred.
    • Descale after hot bending and welding. Rotating joints can generate contamination.
    • Flexible lines: if unavoidable use teflon, nylon or thermoplastic lined hoses rather than rubber (neoprene) which eventually shed particles. Place flex lines before filter, not after.
    • Use O-ring fittings rather than tapered pipe type. If pipe fittings cannot be avoided, use Teflon tape.
  3. . Filtration
    The filtration philosophy is summarized as follows:

    • Use a 10 to 15 micron absolute non-bypass high pressure filter just before the Servo or Proportional Valve.
    • Use a 3 to 5 micron low pressure filter in an off-line filtration loop.
    • Recirculate oil in reservoir more than 5 times per hour. This is justified on the bias that:
    • (i) The Servo or Proportional Valve can accept the odd particle up to 25 microns.
    • (ii) It is neither practical nor economical to try to clean the oil with a small, relatively expensive, high pressure element.The cheaper, low pressure element is many times larger and has the potential to filter continually and under more ideal conditions. (Steady flow and lower velocities increase filtration efficiency.)
    • In the case where large changes of oil volume in the reservoir occur, as with a single ended hydraulic cylinder, it is suggested that a 3 micron low pressure element be used as an air breather.
    • Always use dirt alarms/pressure switches to enable changing of elements at correct intervals. Never use elements for more than 6 months.
    • Use cheaper low-pressure flushing elements to flush the system on start-up – remember that new oil is “dirty oil,” having picked up contaminant in transit and packaging.
    • The tank volume should be flushed through the filter at least 50 times, changing the element when indicated by the pressure switch (contaminate alarm), or until the system has operated 6 to 8 hours without the need for a flushing element change.
  4. Servo and Proportional Valve
    Characteristics of Major Importance:

    • Frequency response (time constant)
    • Threshold (resolution)/hysteresis

    4.1 Placement:

    • Mount as near as possible to the actuator to reduce the entrapped oil volume. Oil is compressible and can often limit servo response.
    • Flexible lines between valve and actuator can be rarely justified. As a rule of thumb they decrease stiffness to one-third of the volume that they contain. Additionally, they produce contamination which must pass through the valve. Use only nylon, teflon or thermoplastic lined hose.

    4.2 Sizing:

    • Select the valve size to obtain between 1/4 and 1/3 system pressure (PS) drop across the valve at maximum velocity. If the drop across the valve is too small, then a flow change will not take place until the valve is nearly closed.
    • Remember: to control flow the valve must drop pressure across itself.Too large a valve is a waste, or worse than that, it lowers system resolution.

  5. Actuator(cylinder/motor)

    • Size the area for dynamic and static forces(remembering the 1/4 to 1/3 PS requirement of the Servo or Proportional Valve in the dynamic case).
    • Calculate the resonant frequency and adjust the actuator areas and valve size, if necessary, to optimize accuracy. (Increased area plus increased natural frequency improves accuracy.)
    • Recognize the 2% to 20% breakout friction of different seals and their effect on position resolution.
    • Manifolds should not contain air pockets. If they do, you cannot flush the air out of the manifold, leading to a “soft” system.
    • Keep the cylinder full area/rod end area ratio ² 2:1 to avoid greatly differing extend and retract velocities.

    5.1 Actuator Connection to Load and Frame:

    • There should be no free play (a practical limit in a position loop would be 3 to 10 times less than the required position accuracy).
    • The mechanical stiffness should normally be 3 to 10 times higher than hydraulic stiffness to avoid degrading performance.
    • Gearing down decreases inertia felt at actuator and hence increases natural frequency (and with it system response and accuracy). However, gearing down could lower stiffness and introduce play/backlash due to the gears.
  6. Feedback transducer
    Closes the loop and its characteristics are of paramount importance, e.g.:

    • Linearity
    • Threshold (resolution) and hysteresis
    • Drift with temperature or time
    • Frequency response (it must be 3 to10 times faster than the slowest element in the system).

    6.1 Transducer Placement:

    • Placing at the actuator output eliminates many control problems (by excluding secondary spring-mass systems and play), but may not provide accuracy at the point required.
  7. Servoamplifier

    • Thedynamics of the analog electronics are always better than the Servovalve and spring-mass system.Therefore, they can be neglected.
    • Some digital systems, however, lack the level of dynamics that are needed. In order to see if this is a problem, check the following:
      (i) That the update rate of the PLC is a maximum of 20 times faster than the frequency of the valve.
      (ii) That the update rate of a digital-to-analog converter, which is required for Electric Feedback Servovalves, is faster than the valve. A rule of thumb is that the converter should be a minimum of 20 times faster, and preferably 100 times faster than the 90° frequency of the valve.
      (iii) Use of 12 and 16 bit digital-to-analog converters. Anything slower could compromise the valves resolution.
      (iv) The last stage to the Servovalve is a current output.
    • Use of compensation techniques (Proportional, Integral or Derivative) can be reviewed when selecting the Servoamplifier. (Note that 90% of position loops can be handled by a straight ‘P’ controller, and the simplicity of set up and troubleshooting a ‘P’ controller is invaluable).
    • Avoid placing the amplifier close to electric motor controllers or other components that generate high electromagnetic fields – consider shielding if necessary.
    • Interconnection to the command signal and feedback transducer should use shielded cables to minimize interference. (Ground only the chassis end to prevent ground loops.)
  8. Conclusion
    To lay out a design for a Servosystem means taking care of minimizing lags in the control chain. (In addition to the usual design requirements of strength, fatigue life, ease of maintenance, ease/cost of manufacture, etc.) Lags may be caused by:

    • Free-play/backlash/stick-slip.
    • Free-time constants of components.
    • The time constant of the valve can be selected. However, the time constant of the actuator-mass system is dependent upon the control of hydraulic and structural stiffness and the mass of moving parts.
 
         Related Links…