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. 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.

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

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.

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).

7. Servoamplifier

• The dynamics 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.
• 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.
• Avoid placing the amplifier close to electric motor controllers