Engineers designing off-highway vehicles are always under pressure to reduce fuel consumption by making vehicles lighter and more efficient. They also have to avoid environmentally damaging leaks and spills, and, most significantly, reduce installation and maintenance costs with simpler systems. One way to accomplish all three is to replace hydraulic systems with electric actuators.
There are still situations that demand hydraulics, such as when loads exceed 2,000 lb or when moving loads with short strokes at 100% duty cycle. But for most off-road applications, electric systems eliminate bulky, heavy hydraulic power packs in favor of the existing battery power, replace cumbersome and dangerous hoses with wires, and use actuators that are usually smaller, lighter, and faster than hydraulic cylinders with the same force and stroke. Moreover, electric actuators run quieter than noisy hydraulic power supplies.
Sizing electrical actuators
It takes only three steps to determine the size of an actuator for an application: measure the load, determine the duty cycle, and specify stroke and retracted length. The first step -- know the load -- may not be straightforward. Often, the precise load on an actuator (whether hydraulic or electric) is not immediately known because intermediate linkages are generating mechanical advantages or worse, disadvantages. Software tools can simulate mechanical systems and precisely calculate loads. However, most designers measure loads with a load cell on the mechanism or select worst-case loads, then find an actuator that can lift that load using trial and error.
Trial and error might be a little costly for a hydraulic system, but it's relatively inexpensive and simple for electric actuators. For example, when an electric actuator does not lift the load with the stroke and velocity required, often all that's needed is a different internal clutch and gears with a different ratio in the same size package.
An alternative method of determining load is to install a calibrated actuator large enough to move the unknown load. Calibrated actuators come with curves that plot load against motor current. Working in reverse, measure motor current while moving the load over a specified displacement, record the maximum current, enter that value on the graph, and read out the load. An actuator that will handle that load is just enough to move it over all necessary duty cycles.
The third major design consideration is the full-stroke and retracted lengths of the actuator, especially when it is being considered as a drop-in replacement for a hydraulic cylinder. Cylinders often have smaller retracted lengths than actuators, so mounting hardware might have to be modified.
Another concern is the system's operational temperature range. Actuators have overload switches that reset after exposure to high temperatures, regardless of whether the heat comes from inside or outside the actuator. Furthermore, electric actuators run more efficiently than hydraulic cylinders at low ambient (-40°F max) temperatures where hydraulic fluid ends to become more viscous. The thicker fluid makes the cylinders operate more sluggishly.
One last consideration deals with the motion. Actuators push and pull, raise and lower, roughly position, shake, and rotate loads with constant force and displacement in one direction. But they experience a gear lash when reversing direction. Cylinders are not quite as stiff, and tend to be cushy in both directions.
Why go electric
Electrical systems are generally much simpler than their hydraulic counterparts. A basic electrical system consists of an actuator, three-position DPDT switch, fuse, and some wiring, and is simple to assemble. Moreover, electric wires can be installed anywhere, even in places not as easy to reach or where cumbersome hydraulic lines might not be as safe. Plus, electrical components are generally smaller and lighter than their fluid-power counterparts. Hydraulic systems, on the other hand, require technical expertise to fabricate hoses and lines and install components so they leak only minimally. They also need more components, including pumps, valves, hose fittings, a pressure regulator, and a control valve.
Electric actuators are easy to control when applications need more than simple two-way manually operated motion. Some systems, for example, use actuators that cycle for a specified number of times at various strokes and then halt. This is easy for programmable-logic controllers (PLC), and it's just as easy to interface to an electric system. PLCs can also handle more complex motions. And electric actuators offer the flexibility of control through IR and RF handheld switches. It's not quite as easy or inexpensive to interface hydraulic or electric controllers with fluid-power systems and run the same profiles.
In most systems, the cylinder or actuator must sometime simply remain motionless. Actuators inherently maintain loads in any given position with or without power, making them safer than one-way cylinders. Hydraulic cylinders need safety check valves to hold loads in case a hose breaks. In comparison, electric actuators will not drop a load despite loss of power or a damaged wire.
Electric actuators can use a variety of power supplies. The most common power used on off-highway vehicles is 12 Vdc, but 24, 48, and 90 Vdc, along with 115-Vac supplies will do just as well, considering the wide range of motors available that fit the same actuator envelope. In fact, moving up to 24 or 48 Vdc often helps because current draw goes down without decreasing the peak torque delivered to the load. Lower current at a higher voltage translates into smaller-diameter wires handling the same power and lower installation cost.
Finally, electric actuators are safer because they reduce operator fatigue with simpler, easier-to-use control. They also eliminate risks of maintaining hydraulic power supplies and high-pressure fluid hoses.
The rating game
A major difference between hydraulic cylinders and electric actuators is the way they are rated. Designers cannot meaningfully compare operating parameters on a one-to-one basis. That's because cylinders operate under continuous ratings while actuators operate intermittently.
Hydraulic power supplies run continuously to ensure there's a reservoir of pressure with enough fluid flow for any possible move. So system pressure must always be at its full rated value, and the pump is usually overrated so it will handle peak loads. Consequently, hydraulic systems are continuously consuming energy. Actuators, on the other hand, only use power when activated, and are normally rated to handle full loads at a minimum of 25% of full-rated load capacity for all duty cycles. (Operating at 100% rated load can be done only under a small-duty cycle whose value varies with the specific actuator. In other words, the actuator is intended for intermittent duty. Continuous duty requires that it operate at 25% of maximum rated capacity.) Furthermore, power consumed at the rated duty cycle is so low that OEM batteries are usually large enough to handle the load.