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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Inertia. Fluid-flow inertia, which is the resistance to change in motion, is defined in fluid dynamics textbooks as momentum. This quantity can be obtained from the equation:
F = p × Q × ΔV
where:
F = Momentum, lb.
p = Mass density of the fluid, slug/ft3
(slug = lb-sec2/ft).
Q = Flow rate, ft3/sec.
ΔV= Change in velocity, ft/sec.
Momentum must be applied to a fluid circuit to initiate fluid motion as well as to stop it. There is no equivalent to this type of momentum in an electrical circuit.
The analogy between a hydraulic circuit and an electrical circuit centers on the medium creating the flow: fluid in a conduit or a stream of electrons within a wire. The density of the fluid is orders-of-magnitude higher than that of the electrons, so fluidflow inertia is orders-of-magnitude higher than electron-flow inertia.
Because electrons have negligible flow inertia, this inertia doesn’t cause problems in control applications. However, fluid-flow inertia causes many hydraulic control problems. Accelerating hydraulic fluid so that it can do work is a much more difficult task than accelerating electrons in a wire. A common method is to increase the pressure in a hydraulic circuit, up to 1,400 psi, and let a reservoir- accumulator act as a system buffer for pressure surges. When a valve opens, the fluid is quickly, but not instantly, accelerated by the accumulator (basically a hydraulic flywheel) to do work, and then empties into the reservoir. Because the fluid is essentially not compressible, the entire column of fluid in a hydraulic circuit must accelerate as a unit, and the inertia of that unit must be overcome by applying momentum.
After the fluid column is in motion, stopping it suddenly generates severe stresses in circuit components. When the momentum of a liquid changes in direction or magnitude, an external force acts on the liquid, and, by Newton’s laws No.1 and No. 3, the liquid exerts a reaction force.
In a home plumbing system, this phenomenon is called water hammer, and it can break pipe joints. Hydraulic systems control fluid hammer with diaphragmspring absorbers and proportional closing valves, yet it is still the primary cause of leaks.
Nonlinearity. Though hydraulic fluids are essentially not compressible, they still have a slight compressibility. This characteristic introduces nonlinearities into the fluid-flow system, acting somewhat like backlash in a mechanical system.
To illustrate, fluid flowing in a circuit can be considered to have stream lines that are tangent to the direction of flow. There is no velocity component perpendicular to the stream lines nor can any two of these lines cross each other. The flow is steady and linear as long as the fluid properties, such as pressure, density (compressibility), and velocity don’t change. But, the flow becomes unsteady and nonlinear if these characteristics change.
Considering the problems of getting a fluid to accelerate quickly, flow smoothly through piping and around corners in valves (without turbulence or cavitation), and stop quickly, the capabilities of hydraulic positioning servos are remarkable. However, the limitations of a hydraulic circuit (inertia and nonlinearity) make the electrical circuit a clear winner when it comes to linearity.
Flights of fancy
Electronic motion control systems from Moog Inc., East Aurora, N.Y., are challenging hydraulic motion control in one of its strongest markets — motion simulators. Charles Bartel, manager of market product development at Moog, credits the success of these electronic systems to the elimination of environmental concerns (oil disposal, leaks, and noise) with no loss of reliability, and the increased power and performance of brushless motors. Low maintenance requirements for these motors reduces the cost compared to hydraulic systems.
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Just as in pilot training simulators, motion simulators in the entertainment field give the user a sensation of motion, not just a moving picture. One of these simulators, called the Journey to Jupiter, is located at the U.S. Space and Rocket Center complex in Huntsville, Alabama. In this four-degree-of-freedom model, civilians experience take-off and flight-control sensations while on a hazardous course to and from (hopefully) the planet. This simulator, Figure 1, takes a “crew” of 30 people on each trip.
The simulator uses servo actuators, similar to Figure 2 but with longer strokes and higher force, to impart motion in four axes. Each servo actuator consists of:
• Three-phase brushless motor.
• Ball screw to provide linear motion.
• Spur or planetary gear section to reduce speed from motor to ball screw.
Small actuators, Figure 2, may include built-in sensors such as an LVDT to provide position feedback from the load, a rotor position sensor for sequencing the electronic commutation, and a tachometer to provide velocity feedback for damping control. Sensors for large actuators, as used in motion simulators, may include an external transducer to measure actuator position and a resolver for motor position and speed feedback. In other cases, the resolver may be used with limit switches to determine actuator position.
An electronic controller for the servo actuator contains:
• Servo electronic compensation to provide stable closed-loop control.
• Pulse-width-modulation (PWM) for proportional control.
• Power switches and fly-back diodes to provide a three-phase, full-wave motor drive.
• Current-limiting to protect the power switches and motor from heat damage.
• Electronic commutation to provide dc motor characteristics using a brushless motor.
This electromechanical actuation system is used in lieu of hydraulic actuators commonly found on other simulators. The electromechanical system eliminates many hydraulic components (pump, reservoir, accumulator, and filter), thereby reducing cost. In addition, the electromechanical system provides servo capabilities that control the motion profile (See Table 1 selection criterion), a function that is difficult to perform using hydraulic controls.
Into the future
Electronics — along with associated microprocessors — has had an impact on hydraulic systems. Electronically controlled proportional valves are claimed to raise the performance level of hydraulic motion control. It is said that this type of electronics can essentially mask the nonlinearities of fluids and help to reduce bothersome noise. Perhaps so, but the nonlinearities will probably require adjusting the control software for each installation. And, these nonlinearities may vary, requiring another tweak of the software. The same type of electronic “linearity fixes” or “system tuning” was proposed for nonlinear step motor systems in the past, but they had little success because of the ever-changing system variables.
The control infrastructure is growing towards electronics and away from hydraulics. Though engineering schools teach very little about motion control, they generally cover a lot more about electronic systems than hydraulic. This means that future engineers will prefer electrical motion control in most cases
Frank Arnold is president of Frank Arnold TechCom Ltd., a consulting firm in Oyster Bay, N.Y. that specializes in technical marketing communications. Prior to starting his own business, he was president of Portescap Inc.
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