The spirited competition between electromechanical motion control and hydraulic (or electrohydraulic) motion control technologies has been active since the early 1970’s when electronic methods were just getting a start in the computer peripheral field. But electrical systems have steadily gained ground due to environmental factors and improved capabilities.

In the beginning, users sorted the two technologies by the amount of power needed — higher power applications were handled by hydraulic control and lower power by electromechanical control (see Table 1 selection criteria). Positioning accuracy was a second sorting regime. Where high-resolution positioning was required, users chose electrical motion control, regardless of the power range. On the other hand, if positioning was controlled manually, such as in earth moving equipment, the choice was, and is, hydraulic.

Cost was also a sorting factor. When both technologies were suitable for an application, the deciding factors often depended on user experience and equipment characteristics. Thus, if the designers and maintenance workers had hydraulic experience, then changing to electronics was difficult. Similarly, if the facility already had hydraulic pumps, accumulators, and reservoirs, changing to electronics was less cost-effective.

Another sorting factor cited by designers is power density of the drive motor — its capacity in horsepower per cubic inch (or pounds). A hydraulic motor delivers more power density than an electric motor, and this factor is important in many applications (if size and weight of the remotely located hydraulic pump, reservoir, and accumulator is not considered).

New factors tilt playing field

In the 1980s, two new sorting factors became important — environmental cleanliness and work-place safety. When hydraulic systems leaked, someone had to clean up the fluid with rags or absorbent granules to keep the work area neat and safe. Disposing of these oilsoaked materials became a cost factor. Also, periodic maintenance of the oil supply and filters meant replacement and costly recycling.

To ensure work place safety, high-pressure hoses and plumbing needed safety enclosures to protect against breakage — an added cost. In addition, hydraulic pump noise and high-pressure fluid-flow valve noise (which was tolerated in the past) required sound-proof enclosures in some applications. This was especially true in relatively quiet areas where the hydraulic components produced most of the noise (assembly and robotics areas).

By the late 1980s and 90s, environmental concerns and the performance advantages of electrical motion control prompted users to chose electrical systems in many high-power applications where hydraulics formerly reigned. With electrical systems, the power switching devices used in pulse-width-modulated drives permitted the use of higher-power electric motors. Also, the electric motors were often brushless, which meant high reliability plus increased power density.

Other advantages of electrical control are high positioning resolution, the ability to easily and continuously control applied torque through current limiting, and the ability to precisely control motion through combined velocity and torque control.

Physical laws

Though competition between the two technologies has been waged on the basis of performance, cost, environmental, and safety factors, the basic laws of physics (Newton’s laws No. 1 and No. 3) favor electrical motion control.

The source of power for each technology is electrical. With hydraulics, however, the electrical energy used to power the high-pressure pumps must be converted to fluid-flow energy. Though this method provides high power density at the point of force application, it has the inherent disadvantages of fluid-flow nonlinearities, and, most importantly, fluid inertia.

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