Overload Capacity
Both electric and hydraulic motion platforms are typically designed for a specific application where the payload is clearly defined. Should the payload “increase” appreciably, say by 20%, an electric system may not function adequately due to overloading. Electrical systems are highly dependent on the sizing of the electrical motors, either DC or brushless AC systems and their ability to be over-loaded. It is quite simple, electric platforms are “sized” for a much narrower band of operation; beyond that band and the platform will fault. Typically, the only solution in these cases is to order a new, larger motion platform.
A hydraulic system, on the other hand, can accommodate a much wider range of operating parameters. Typically, a properly designed hydraulic system can be “overloaded”, up to 50% or more and still operate effectively. Certainly, you do not get something for nothing; if you increase the operating pressure to handle larger pay loads, then you must re-stroke the HPU so as to not over load the motor. In such a case you would give up some “dynamic performance” for the ability to handle the larger payload. Additionally, by simply replacing the HPU with a larger unit, it is possible re-obtain the performance criteria. The main point to be made here is that if all of the components are designed and sized correctly, it is possible to operate a hydraulic system far beyond its original “customer defined” specification limits. For manufacturer’s of motion platforms, this is unfortunately a common problem; i.e.; the customer asks for a system to be designed for an 8,000 lb payload and ultimately puts 12,000 lbs on it. An electrical system would, in this case, need to be replaced, while a hydraulic systems wide operating latitude can allow operation with proper adjustment.
Performance
In terms of obtainable accelerations and velocities, assuming equal design criteria; both hydraulic and electric system are comparable. Conceptually electrical systems should have higher resolution capability since most of these systems utilize encoder, resolver or similar feedback systems. In reality, however, manufacturing tolerances coupled with design and assembly tolerances relegate the true resolution to that comparable to hydraulic systems which use both digital and analog feedback systems depending on the manufactures design. Theoretical and actual accuracy, and, resolution for electrical systems is misleading. Digital accuracy relative to computer programming and theoretical computation can be far different from the actual accuracy once machining and fabrication tolerances are taken into account. Hence, at best, hydraulic and electric systems accuracies are comparable.
A second issue that is almost never mentioned, is the problem associated with “loosing” communications and/or interruptions with the control system and its affect on the physical hardware. Where digital systems are used; typically electrical systems, loss of power or interruptions in communications result in a system that looses it orientation unless more sophisticated and complicated control logic is required. Hydraulic systems, which typically use analog feedback systems always have “one foot on the ground”. The actuator extension, hence, attitude of the analog system is always available and known; it does not lose its spatial orientation. This is important for installations where frequent power outages are the norm. Loosing orientation is of major concern particularly in applications where very heavy payloads are elevated and power is lost or an E-STOP condition is initiated.
Durability / Reliability
Hydraulic systems, by far, have proven to be significantly more durable. Many of the older hydraulic motion platform systems have been in operation for over sixty years with virtually all of the same equipment in place. Control and computer systems have changed and been upgraded, but the physical hardware in most instances is original. Other than servo-valves, there is essentially only one physical moving part in each actuator; the piston/rod combination. Essentially there are no other moving parts. Baring the HPU, which is typically located remotely, noise from the actuators amounts to a very low level smooth “swishing” sound. In virtually all applications, any sound emanating from the system is masked by surrounding noise levels. Developments in servo-valves over the years have made them very reliable and durable; with operational lives greater than thirty years in most instances.