Jack Johnson is an electrohydraulic specialist, fluid power engineering consultant, and president of IDAS Engineering Inc., Milwaukee. Contact him at email@example.com, phone (414) 236-5350, or visit www.idaseng.com.
Last month’s edition of “Motion Control” introduced the VCCM equation:
fL = PS APE –v2(APE3 / KVPL2) (1+ρv2/ρc2)
where fL is the load force that must be overcome,
PS is supply pressure,
APE is the cylinder size,
v is the speed of cylinder propulsion,
KVPL is the degree to which the
valve is open,
ρv is the symmetry of the valve, and
ρc is the cylinder area ratio (cap side of piston area/rod side area).
The leakage input parameter was set to 2.75 in.3/sec for all the previous investigations. Now it is going to be seen how this value is only an estimate of the peak leakage that results from the simulation. Furthermore, the simulated peak leakage is affected by the amount of overlap that is used for any specific computer run.
The ability to set the overlaps at each valve land individually offers an opportunity to gain insight that would not be practical with real valves. Whereas previous simulations have all dealt with perfect symmetry in all the lands, such an idealized condition can never exist in any practical way, due to manufacturing (such as flow grinding) tolerances.
This is the first textbook that is devoted to the design and analysis of hydraulic circuits and systems that use feedback control of hydraulic pressure. The early chapters are written at about a sophomore to junior engineering/technologist theoretical level, however, the later chapters do make use of calculus, transform methods and state variable diagramming methods to present the control problem and strategy.
This Glossary of Terms is a compendium of words and expressions that has been collected over the years of teaching the principles of electrohydraulic motion control to students who have come mostly from industry.
This book covers all the basic theories and concepts needed to know how electronic equipment works, how to use it and even how to design it. Many of the necessary design formulas are given and discussed.
To see the null zone of a valve clearly, a simulation was run for a range of ±6% of spool shift. Performance parameters from previous simulation runs were not changed, so Figure 1 and several that follow show a magnified look at the null zone.
Figure 1 gives a detailed look at pressure metering because it covers such a small amount of spool shift. Again, it confirms the rule of thumb that pressure metering is confined to the overlap (0.85% in this case) plus about 5% of spool travel.
When modeling a proportional or servovalve for subsequent simulation, output data for flow metering and leakage are steady state. As such, they are arranged similar to those that would be collected during testing a real valve, such as an automated data acquisition system. Figure 1 contains the flow metering and land-to-land leakage data over the spool position range of ±100% for the valve model from previous discussions.
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