How to chose between hydraulics and electromechanics

The case for electromechanical motion

by  Christopher Griffin, Product Sales Manager-Electromechanical Parker Hannifin Corp., Automation Actuator Div.

The idea of controlled force and motion has long been the domain of the fluid power industry, in particular hydraulics. Servo motor technology, however, has reached a stage of development where it is both cost effective and relatively easy to learn and apply. Servos, step motors, ac and dc motors are all well suited to drive lead screws, ball screws, belts and pulleys and racks and pinions. Where they excel is in the speed, repeatability and positional accuracy they can achieve.

The generally smaller form factor of electromechanical systems has opened doors to opportunities in many industries. On the smallest scale, servo-driven linear and rotary actuation systems have been important to the development of semiconductor manufacturing and testing, medical manufacturing, packaging and testing, and small electronics manufacturing. In addition to being small, these systems may be manufactured to meet clean room specifications, low noise requirements and micron-level positioning requirements.

On a larger scale, electromechanical solutions have increased the throughput and quality of a feast of packaging applications, from palletizing and box-forming to product marking, labeling and filling. The key to success in the packaging industry is a combination of the speed, torque and accuracy of electromechanical systems. This success is augmented by the growing sophistication of control systems, which extend from simple hard switch logic to PLCs and microprocessor-based multi-axis motion controllers.

The automotive industry has integrated electromechanical systems to automate many of its manufacturing and material handling processes. These systems have typically improved the achievable part or process quality.

Electromechanical solutions typically do not produce much more than 20 hp, which ultimately limits the thrusts and payloads that can be positioned. There are, however, emerging technologies that will broaden the application envelope.

Comparing hydraulics and electomechanics. We need to understand that these are two fundamentally different disciplines. Hydraulic systems produce linear motion naturally, while electromechanical systems produce rotary motion naturally. The application overlap does not enclose the extent of either technology's capabilities.

When comparing these two technologies, two aspects stand out: cost and complexity. Hydraulic systems are generally purported to be lower cost, while electromechanical systems are thought to be prohibitively expensive and require a control systems degree to figure out. For the application designer to be most effective, he or she really needs to gauge the needs of the application and select the most suitable technology. Both technologies can move loads from position to position. The differences lie in cost, load, environment and accuracy.

Cost. Hydraulic systems, in fact, can be more expensive in lower power applications, due mainly to infrastructure costs, such as power units, hoses and valves. Electromechanical systems, obviously, require only a power supply, although the wiring involved is no less complex than a hydraulic circuit. As horsepower increases, hydraulic systems become more cost-effective, as the infrastructure doesn't change. This is often referred to as "power on demand." Electromechanical systems require increasingly expensive power electronics and mechanical components, such as ball screws and bearings.

Load. The power density of hydraulic systems makes them extremely efficient. Hydraulics can produce tremendous amounts of thrust and generate considerable power without transmitting power from one mechanism to another. Electromechanical systems must by design transmit power from device to device, such as rotary motor to lead screw. The reduction in efficiency, the need for a coupling, and the prohibitive cost of high power mechanical components limit the feasibility of the electromechanical solution in high power applications.

Complexity. Multiple position applications, where a position feedback device is necessary, require a competent understanding of the control system and the mechanics it is tied to. Hydraulic position control normally centers itself on a position transducer and proportional valve. Producing thrust and hence motion in a hydraulic system is non-linear, due to fluid dynamics. Tuning such a system to reach a position quickly and in a repeatable fashion requires experience. Tuning an electromechanical system, where thrust ( in the form of motor torque ) is linear, might require a half day of training and a good grasp of the particular needs of the application. Step motor systems in fact require no tuning, although they still provide reliable and repeatable positioning. Newer servo system products almost invariably include an 'auto-tuning' feature that accepts system parameters and calculates tuning gains automatically.

Accuracy. Digital position feedback in the form of optical encoders and scales makes micron-level positioning possible in electromechanical systems. With variable displacement transducers onboard, hydraulic control systems typically cannot achieve much more than 0.003-inch repeatability.

Environment. Pressure vessels, such as hydraulic products, are inherently rugged. They find themselves in the harshest environments: extreme temperatures, outdoors, and in mobile applications. Unlike electromechanical systems, they are unaffected by dirty environments and can operate where explosion-proof motion is needed. Electromechanical systems run quieter, require less infrastructure, and can be designed to operate in clean room environments.

As servo technology costs fall, certain products are becoming more accessible. Brushless servo motors are becoming more common in applications typically held by brush motors and step motors. Linear servos are viable alternatives to screw-driven positioners, offering higher speeds and higher throughput without the lead error of the screw. While screw technology isn't new, there have been advances in design that enhance performance. High lead ball screws allow for high linear speeds without increasing the screw diameter. Roller screw technology, in which a set of planetary screws replaces the ball nut, can produce thrusts well over 1,000,000 lb-ft.

The case for hydraulic motion

by  Richard H. Woodring Electrohydraulic Business Unit Manager, Parker Hydraulic Valve Div.

Today's controls engineer enjoys the benefit of a variety of methods for generating mechanical motion and/or force in his machine design. Included in these are pneumatic, hydraulic, and a number of electromechanical actuator devices. The options available to a designer can create confusion over which provides the optimal solution for a given application. Recently the industry has given significant publicity to advances in electro-mechanical control technology. The author has encountered many machine designers who select electromechanical options because they believe they're smaller, quieter, cleaner, more reliable, more controllable or simply more "state of the art."

If one critically evaluates the variety of motion control requirements available, he will see that there is no "one-size-fits-all" solution. Every technology has advantages and limitations. The intent of this article is not to focus on the weaknesses of any competing motion control technology, but rather to explore the attributes that make hydraulic motion control an attractive option for the controls engineer.

Hydraulic control systems have gained a reputation for being noisy, dirty, and--in some circles--old-fashioned. In recent years, the hydraulic control component industry has made numerous advances in addressing these issues. A properly designed and maintained hydraulic system can be quiet, clean, efficient and relatively easy to maintain.

Without a doubt, the most significant advantage of hydraulic motion control is its ability to concentrate high levels of horsepower, torque and/or force in a small package. As a rule of thumb, a hydraulic motor will deliver 1 hp per lb of weight compared to only 1/16 hp per lb for an electric motor. The rotary actuator--a simple device that converts the linear motion of a hydraulic cylinder into a rotary motion via an integral rack and pinion arrangement--can deliver 900 inch-lb of torque in an 11-lb package compared to 30 inch-lb for a dc motor drive of similar weight.

Hydraulic control's advantages become readily apparent when one looks at linear motion or the generation of force or thrust. In today's market, it's very common to find hydraulic pumps, control valves and actuator components that are rated at operating pressures of 5000 psi (350 bar). At this pressure level, a relatively compact 2-inch diameter hydraulic cylinder will generate over 7 tons of force. By the laws of geometry, this force generation capability grows by the square of the increase in its diameter, leaving the electromechanical solution in the dust.

In addition to the raw force generation of the hydraulic actuator, the hydraulic actuator also exhibits linear velocity capabilities that approach 2 meters/second compared to less than one meter per second for its electromechanical cousin. The actuator's high thrust-generation capability allows it to accelerate and decelerate heavy loads much more effectively than can an electromechanical actuator.

The positioning capabilities of an electrohydraulic actuator system vary significantly from application to application. Factors like load mass, frictional characteristics, load velocities, machine frame stiffness, control valve bandwidth, hydraulic plumbing compliance and sophistication of the control electronics all interact to affect the application's positioning accuracy. We are, however, aware of electrohydraulically controlled machine tool feed stations that are maintaining positional tolerances of ± 0.0002 inches, which is comparable to typical electromechanical solutions with a similar stroke capability.

The hydraulic actuator is also self-lubricated by the hydraulic fluid used in the system, another inherent advantage. When proper filtration techniques are used to maintain fluid cleanliness, one can expect the actuator to provide tens of thousands of hours of wear-free, trouble-free service. The electromechanical actuator typically incorporates a lead-screw or ball-screw mechanism that is susceptible to lack of lubrication, external contaminant ingression and ball-screw loading.

In the past decade there have been numerous advances in motion profile control capability for electrohydraulic actuators. The advent of high bandwidth proportional valves and solenoid operated servovalves has significantly advanced the application of electronic motion control for hydraulic actuator systems. Admittedly, developments in the electromechanical controls industry have had a significant, positive impact in improving control capability levels of electrohydraulic systems. Today's state of the art high-bandwidth solenoid servo proportional valves use high-speed electronic switching solenoid drive techniques developed and perfected by the electric servomotor industry. In fact, the sophisticated microprocessor based motion and force controllers used today for electrohydraulic profiling can trace their roots to devices designed for the control of servo and stepper motors.

Yet, having acknowledged that the electromechanical industry has provided some tips on how to "fight," we might put the gloves back on and go a couple of more rounds in our friendly match. In addition to the advances in electronic- and hydraulic-control technology discussed above, the industry has made other improvements to remain in fighting form. Designers have continued to improve the hydraulic pump's control features, efficiency and sound levels. Variable volume pumps are now available with a full compliment of electronic flow and pressure controls. Volumetric efficiencies are in the mid- to upper-90 percentile range, and well-designed hydraulic power units operate in the 65-70 dB sound level range. The hydraulic control system now also has an added advantage when heat and noise are an issue. The power unit can be located remotely from the hydraulic actuator. This effectively isolates hydraulic noise and also allows for any heat generated at the actuator location to be removed through the exchange of fluid in the actuator. By contrast, electromechanical actuator installations require control electronics to dissipate any heat generated by the actuator's braking action.

The hydraulic controls industry has also addressed the problem of external fluid leakage. The commonly used tapered pipe thread is gradually being replaced with straight thread fittings fitted with elastomeric seals. Wherever feasible, dynamic seals are being eliminated in hydraulic component designs. Effective use of electrohydraulic control devices is reducing the hydraulic shock that often results in external leakage at fittings and hydraulic line connections. Finally, expanding use of the popular hydraulic integrated circuit has significantly reduced the number of potential leak points in the systems. In addition, we must credit the machine designers and manufacturers; in recent years, we have seen a rededication to the design of clean, leak-free, hydraulically operated machines.

You should realize, however, that we do not advocate hydraulic actuation as the optimum solution for all motion control applications. It is, however, the solution of choice in those environments subject to temperature extremes and high levels of dirt and/or moisture. In such environments, the hydraulic solution is equal to the electromechanical in terms of motion profiling and position control. Throw in a requirement of providing high levels of torque, force, power or thrust in a small, reliable package, and hydraulic control technology wins hands down. Moreover, a properly designed system can overcome the frequently cited problems of external fluid leakage and operating environment noise pollution.