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.
To demonstrate the characteristics of a motion control system, we will examine test results of a valve-controlled cylinder in a closed-loop, positional servomechanism, represented below. Otherwise known as a torque cell, the mechanism was designed for special electrohydraulic motion-control training programs.
Analogies exist between hydraulic flow and electrical flow, and the molecules of fluid in a hydraulic circuit behave much like the electrons in an electrical circuit. Let’s examine analogies between pressure and voltage and between ground and the hydraulic reservoir.
System design normally calls for a specific load to be overcome and propelled at some required velocity. This is called the design pointor design target. A further reality is that most machines are required to operate at an essentially unlimited number of operating conditions as the actuator accelerates, decelerates, and stops. When the system is designed, the design point must accommodate the absolute worst-case operating point expected over the entire lifetime of the machine.
System design requires that components supply pressure adjusted so that the operating envelope encompasses the worst case force-speed operating point. An infinite number of combinations exists that will accomplish this, so some other strategies must be applied to reduce the number of possibilities.
The hydromechanical resonant frequency (HRF) of a valve-cylinder circuit is an interesting concept and an important value to know. If a cylinder is stroking, and its control valve suddenly shifts to block flow, the cylinder and its load will vibrate, usually with considerable noise and sometimes with considerable violence. This is HRF in action. The noise arises from the resonance that exists when the kinetic energy of the load mass and the potential energy stored in the hydraulic fluid’s compressibility are exchanged.
The term bus is extremely general in the electrical engineer’s parlance. It is a synonym for wires or conductors. The wires that carry power around your factory, office, or house are a power bus. The wires that carry your telephone conversation are a communication bus. Most of the peripheral devices connected to a PC communicate with the processor through an internal, digital, parallel data bus. That bus has about 50 different conductors. A data bus provides a universal means of two-way communication among machines.
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.