Jack Johnson is an electrohydraulic specialist, fluid power engineering consultant, and president of IDAS Engineering Inc., Milwaukee. Contact him at firstname.lastname@example.org, phone (414) 236-5350, or visit www.idaseng.com.
Lenz’s Law explains more than just the “speed voltage,” which also happens to be another moniker for that law. It says an induced voltage will occur anytime there’s relative motion between the conductor and the magnetic field.
An open electrical switch blocks current, whereas a closed switch conducts. A closed hydraulic valve blocks flow, but an open valve allows flow. These are clearly contradictory analogies that are often confounding to those who are seeking to cross-fertilize, so to speak, and learn the other language and concepts. There is a method to this madness, or at least an explanation, that belongs in the pantheon of “No wonder we don’t understand each other.”
Last month’s discussion illustrates how voltage and pressure provide the motivating forces for their respective fundamental elements — electrons and molecules of fluid. Voltage is a measure of the difference in potential energy per unit of charge between two points in a circuit or some other electrical space. The unit of voltage is the volt. Voltage is the energy per unit of electrical charge, measured as Newton-meter per coulomb. What’s important is the force element (Newton).
Most technical people working in the fluid power industry have roots either in the mechanical realm or the electrical realm. Those who consider themselves “mechanically inclined” often struggle with understanding of electrical concepts. Likewise, those well versed in electrical science are often faced with terminology in fluid power that is inconsistent with their knowledge.
Large positional errors can result when a programmable logic controller (PLC) selects the deceleration point. The error is caused by the scan-time delay in the digital controller, and it is directly proportional to the speed at the instant that the deceleration decision is made. To reduce this error, many open-loop motion controllers decelerate the system to a creep speed to approach the target position slowly. Thus, positioning error is made at creep speed instead of at maximum speed, so the ultimate position will be acquired more accurately.
Most motion-control applications are of a critical nature — they must meet accuracy, bandwidth, or some other performance demand. The most sensible and expedient way to design such systems is to use performance requirements as the design goals at the very outset of the design process. The techniques are analytical in nature, so they require mathematical descriptions of all elements of the system. Only then can synthesis and simulation methods be applied to direct the design process toward the end result without undue trial-and-error techniques.
To understand the function of and need for integral control, you must understand the shortcomings and limitations of the proportional electrohydraulic positioning servomechanism. A simplified, combined cutaway and block diagram is shown in Figure 1.