Hydro-Mechanical vs Electro-Hydraulic Solutions

By Lauren Schmeal, Editor, Fluid Power Journal
As presented by Dr. Medhat Khalil, Director of Professional Education at the Milwaukee School of Engineering

Dr. Khalil’s book may be found on compudraulic.com/textbooks

Understanding Through Analogy

Fluid Power vs. Electric Control

I am honored to delve into the fascinating world of electrohydraulic systems. This article is based on a chapter from our professional education course, “Electrohydraulic Components and Systems,” aiming to bridge the understanding between mechanical and electrical engineering perspectives.

Our goal is to facilitate a mutual understanding between professionals from mechanical and electrical engineering backgrounds through comparative analysis. By exploring the analogies between fluid power and electric control, we can appreciate the synergy that leads to advanced electrohydraulic systems. The marriage of fluid power and electric control results in electrohydraulic systems that are smarter, more compact, and more reliable.

Key Analogies

A Comparison

There are many analogies between fluid power and electric control systems, along with the benefits of combining them into electrohydraulic systems. In fluid power, logic functions are carried out by valves, energy conductors are pipes, and the energy medium is oil or air. In contrast, electric control utilizes relays for logic functions, wires for energy conduction, and electrons as the energy medium. Fluid power systems commonly use pumps or compressors as energy producers, whereas electric systems employ electrical generators.

Linear actuators (cylinders) dominate fluid power, while rotational actuators (motors) are more prevalent in electric systems. While fluid power systems tend to have lower energy transmission efficiency and potential for leakage, electric systems are relatively more efficient and cleaner. Despite higher initial and running costs, fluid power excels in applications requiring high load (force or torque) or precise control. The pressure difference in fluid power is analogous to voltage in electric systems, and oil flow corresponds to electric current. Both systems exhibit power distribution patterns with sources and consumers, experiencing losses in components like pumps, valves, and conductors.

There are several parallels between components like generators/pumps, flow/pressure control vs. voltage/current control, and control valves vs. switches. It also touches on capacitive elements like accumulators/air reservoirs vs. capacitors, resistive elements like needle valves vs. resistors, and check valves vs. diodes. Hydraulic systems exhibit both linear and non-linear resistance, analogous to electrical resistance. Series and parallel arrangements of hydraulic resistances mirror electrical circuits. Finally, the piece stresses that electrohydraulic systems offer more dynamic and precise control, citing pressure control as an example, enabling adjustments on the fly and optimizing performance for varying loads and applications.

Cost Considerations

Initial vs. Running

The initial cost of components in fluid power systems is generally higher compared to their electric counterparts. Additionally, the running costs, including maintenance and conditioning of oil or air, can be significant. However, this doesn’t imply that fluid power is inferior. There are numerous applications where fluid power excels over electric systems, particularly in situations requiring high load or precise control.

Power Distribution

Similarities in Energy Flow

In electric systems, voltage is essential for current to flow through an electric motor, generating torque. Similarly, in fluid power systems, a pressure difference is required for flow through a hydraulic motor. This analogy highlights that pressure difference in fluid power systems is akin to voltage in electric systems, and oil flow corresponds to electric current. Both systems exhibit power distribution patterns, featuring a power source section, power control section, and a power consumer section.

Energy Losses

Tracing the Decay

Energy transfer from the source to the consumer involves inevitable losses. In electric systems, these losses occur in the generator armature, the motor, and through conductor leakage. Similarly, fluid power systems experience losses in the pump, valves, conductors, and actuators. It’s estimated that fluid power systems can experience at least 30% losses from power generation to consumption.

System Parallels

A Deeper Dive

A closer examination reveals striking similarities between hydraulic and electric control systems:

• Power Source: Generators vs. Pumps/Compressors
• Regulation: Flow and Pressure Control vs. Current vs. Voltage Control
• Directional Control: Control Valves vs. Switches
• Capacitive Elements: Accumulators/Air Reservoirs vs. Capacitors (batteries for long-term energy storage)
• Resistive Elements: Needle Valves vs. Resistors
• Check Valves: Equivalent to Diodes in Electric Systems

Hydraulic Resistance

Linear and Non-Linear

Hydraulic systems exhibit two types of resistance:

• Orifices or Valves: These offer non-linear resistance, with a complex relationship between flow and pressure drop governed by a valve coefficient.
• Hydraulic Lines: These offer linear resistance, determined by the line’s geometry and fluid properties. Electrical resistance, provided by resistors, mirrors the linear resistance in hydraulic lines.

Series and Parallel Arrangements

Pressure Drop and Resistance

When hydraulic resistances are arranged in series, the total pressure drop is the sum of individual pressure drops. Conversely, in a parallel arrangement, the total resistance is calculated using a reciprocal formula, mirroring the behavior of electrical resistances in series and parallel circuits.

Capacitive Elements

Energy Storage

The accumulator in hydraulic systems, analogous to a capacitor in electric circuits, stores energy for later use in short term. The size of the accumulator, air reservoirs, or capacitor dictates the amount of energy that can be stored and the time required for charging.

Power Calculation

A Common Thread

The formula for calculating power involves multiplying the effort variable by the flow variable. In fluid power, this translates to pressure multiplied by flow rate. Similarly, in electric power calculations, voltage is multiplied by current to determine wattage.

Electrohydraulic Systems

A Marriage of Strengths

The benefits of combining electrical and hydraulic systems are numerous. Let’s consider pressure control as an example. Conventional hydraulic pressure control often involves pressure relief valves or variable displacement pumps. However, these solutions may be limiting when frequent adjustments are needed. Electrohydraulic systems offer more dynamic and precise control, enabling adjustments on the fly and optimizing performance for varying loads and applications.

By integrating electronic controls with hydraulic power, we create systems that are not only efficient but also highly adaptable to complex operational requirements. This fusion marks a significant advancement, paving the way for smarter, more responsive fluid power solutions.