Electric motion systems have been in operation for approximately ten to fifteen years. During that time, several have been replaced with hydraulic systems for many dead weight bearing applications. The number and type of components in electrical actuator designs is of major concern. The present primary designs use either lead screw, ball screw or roller screw mechanisms. All of these utilize a multitude of components to effect the rotary to linear motion. Combinations of multiple planetary rollers, a generous number of roller balls and wearing lead screws comprise the design features for the various mechanisms. All of these need proper and typically generous amounts of lubrication. Also, at high speeds the roller screw and ball screw designs result in significant noise emanating from the number of loaded moving parts, unless, significant measures are taken to dampen the inherent noise. Component wear is of major concern. The combination of multiple small heavily loaded mechanical parts is a recipe for metal-to-metal contact and wear. These designs, although clever, are not “simple” in their approach to converting rotary motion to linear translation.
Electrical systems require high voltages to be brought to the motion platform system actuators, within the personnel space. Special precautions need to be taken to make sure such systems are safe and meet national and code requirements. Hydraulic systems, however, only require the high voltage service to be brought to the remote HPU location only. It is common for the hydraulic HPU to be located in a remote location under tight control. Hence, only low control voltages are required at the motion platform directly.
Hydraulic systems require pressurized hoses run to each of the actuators under servo-valve control. Typically system pressures between 1000 psig and 1500 psig are used. These pressures are easily and safely handled by two-wire-braid hoses commonly rated for pressures in excess of 3000psi. Leaks are virtually a thing of the past with modern SAE’ O-ring, JIC and other fitting configurations. Similarly, hydraulic oils can be replaced with “food grade” mineral oils in situations where required. The depiction of modern high performance hydraulic systems being comparable to that used in commercial and industrial equipment is misleading. Modern hydraulic systems do not have the leaking and rupture problems associated with low cost implement equipment.
Simplicity / Complexity
Hydraulic systems are quite simple: actuators, servo-valves, controller and computer. Hydraulic actuators are designed to absorb the total system energy, in case of a “worse case” run away conditions. This is accomplished via the use of built-in cushions at either end of the actuator stroke. This very simple cylinder design feature is built into the cylinder to protect against all control, electrical and hydraulic failures. Essentially, when any condition of failure occurs, i.e.; computer, controller, electrical, servo-valve and even operator error, results in failure; the cushions built into the cylinders will protect both the motion platform and the payload from damage.
Electrical systems, on the other hand, typically rely on a combination of “switches” and control logic to protect the system. Many electrical systems rely on a “braking” system to violently stop the motion. Although these can be effective in their implementation, programming and additional complexity results in a much higher installed cost and potential for system failure that can result in damage to either the motion platform or payload. It should be noted that if all of these backup electrical systems fail, the actuator will simply slam into one of the hard stops with instantaneous lock-up of the actuator. This is typically a binding jam that requires mechanical means to unlock unless more complicated systems and equipment are added to limit the deceleration to reasonable limits.
Hydraulic systems typically require maintenance approximately every 1000 hours of operation. This usually amounts to checking the HPU filter, lubrication (depending on the manufacturer) and general clean up and verifying performance responses. These “low tech” system checks are quite easy to maintain with standard maintenance techniques and personnel. Such systems have been in operation for many years with minimal maintenance.
Electric systems, on the other hand, typically require lubrication approximately every 80 hours of operation, along with checking of power wiring and a multitude of limit switches, control logic verification and encoder/resolver operational checks. Their reliability is predicated on the proper functioning of a combination of components arranged in a “series” arrangement. Any one of the monitored components will result in the system being halted, hence increasing down time and its associated expense.
Hydraulic actuators utilized in the equipment space are extremely quiet with sound levels under 50dbA. Of significant importance is the issue of hydraulic system noise associated with the HPU. This is typically addressed by locating it remotely from the motion platform. This is advantageous for two main reasons: Reduced noise, and, controlled and reduced heat rejection.
Electric motion platform systems can be noisy due to the many small moving parts in the actuators as the result of their particular design. There is a significant “rushing” sound that is very distinct and may or may not interfere with the programmed content on the moving deck. This can be of particularly concern in “flight simulation” applications.
By J.F. Samicola and G.P. Kokalis