Servo Valve를 이용한 제어시스템의 정밀도는 Servo valve 성능만이 중요한 것이 아니라 Pump unit, Cylinder 설계, Feedback sensor, Controller 등이 모두 다 Matching이 되어야 합니다.

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PRACTICAL CONSIDERATIONS WHEN LAYING OUT ELECTRO-HYDRAULIC CONTROL SYSTEMS
1. Power Units

Pumps:
Constant supply pressure preferred with minimum variation. Variable displacement pressure compensated pumps: recognise response of pump and add accumulator to avoid limiting servo response. Fixed displacement pump: constant pressure with use of accumulator is an option.

If more than one critical system is fed from one pump, isolate them with check valves and accumulators (avoids cross-talk).

Reservoir breather .3 to 5 micron air filter preferred with capacity appropriate to fluid level displacement.

Temperature and pressure control should be tight if good long term control accuracy is critical.

2. Piping and Fittings

Use only correct tube cutting tools, no hacksaws. 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 rather than rubber (neoprene) which eventually shed particles. Where possible place before filter, not after.
Use O-ring fittings rather than tapered thread type. If tapered fittings cannot be avoided, use liquid hydraulic sealants rather than teflon tape.
(Globules of sealant are less insidious than slivers of teflon tape.)

3. Filtration

The Moog filtration philosophy is summarised as
follows:

-Use a 1.0 to 15 micron absolute high pressure filter just before the servovalve.
-Use a .3 to 5 micron absolute low pressure filter on return or bypass line

This is justified on the basis that:

(i) the servovalve can accept the odd particle up to 25 microns. The Beta 10/Beta 15> 75 high pressure element (without bypass check valve) ensures this even when contamination causes the pressure drop across the filter to exceed the 7 bar indicator level.

(ii) it is neither practical nor economical to try to clean the oil with the 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 can draw in airborne contaminant, it is suggested that a 3 micron low pressure element be used as an air breather.

-Using high-flow 3 stage valves may make full flow filtration prohibitively expensive. In this case use an external supply to the pilot valve and full-flow filter this only, the third stage being much less contamination sensitive.

-Always use dirt alarms/pressure switches to enable rational changing of elements.

-Use cheaper low-pressure elements to flush the system on start-up - remember that new oil is perhaps filtered to 40 microns and is thus "dirty oil".

-The tank volume should be flushed around through the filter at least 100 times, changing the element when indicated by the pressure switch (dirt alarm).

4. Servovalve -- Characteristics of major importance:

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

4.1 Placement

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

4.2 Sizing

Select the SV size to obtain between 1/4 and 1/3 system pressure (Ps) drop across the valve at max. velocity. Lithe drop across the SV is too small, then a flow change will not take place until the valve is nearly closed (nearly closed in fact to the SV size that should have been chosen!). Remember, to control flow the SV must drop pressure across itself.

Too large a valve is a waste and 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 juggle areas and SV size if necessary to optimise this (increasing area = increased natural frequency = increased accuracy).

Recognise the 2% to 20% breakout friction of different seals and their effect on position resolution.

Any manifolding should not contain air pockets which will not be flushed automatically.
(Air + Oil = a sponge).

Keep the cylinder full area/annulus area £ 2:1 to avoid greatly differing extend and retract velocities.



Note: retract velocity = annulus area

5.1 Actuator Connection to Load and Frame

This should be free of play (a practical limit in a position loop would be 3 to 10 times less than the required position accuracy).

The 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), but could lower stiffness and introduce play/backlash due to the gears.

6. Load - define in the following terms

Constant/changing? (velocity dependent?)

Uni/bidirectional?

Stick-slip friction?

These will affect accuracy and may also determine whether backlash in the system will be `felt`.

7. 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 (no reason normally not to be 3 to 10 times higher than slowest other element).

7.1 Placement

Placing directly at the actuator output reduces many control problems (by excluding secondary spring-mass systems and play for example) but may not provide accuracy at the point required.

8. Setpoint/Command Signal

The four properties listed for the transducer can be relevant.

9. Servoamplifier

The dynamics of analogue electronics are usually several orders of magnitude better than the SV and primary spring-mass system. They therefore can be neglected.

The dynamics of a digital system however can be significant check.

The update time of a PLC will invariably limit the closed loop.

Modern high resolution, high response electrical feedback SVS can follow DAC Steps. Consider a low pass analogue filter outside the system band width to extend valve life.

Use 10 to12 bit DACS to avoid compromising the SV resolution. Too coarse a DAC resolution can cause jumps in flow and pressure.

The end stage to the SV is to be a current amplifier to minimise the inductive lag of the coils.

Necessity for compensation techniques (Proportional, lntegral or Derivative) can be reviewed when selecting the servoamplifier. (Note (a) that 90% of position loops can be handled by a straight `P` controller, (b) the simplicity of set up and troubleshooting of 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 setpoint and Feedback Transducer should similarly be shielded cables to minimise interference. (Grounded only at chassis end.)

10. Conclusion

To lay out a system for a servosystem really only means taking care to minimise lags in the control chain. (This naturally in addition to the usual design requirements of strength, fatigue life, ease of maintenence, ease/cost of manufacture etc.) These lags are due to:

1. play/backlash/stick-slip

2. time constants of components

The time constant of the servovalve can be selected while the time constant of the actuator-mass system can be kept under control by control of stiffness (oil and structure) and mass of moving parts.