BOOK IN SERIES
1.1.3- Power Consumption (Actuation)
Each of the systems has linear and rotational actuators. A
Rotational Actuator reconverts the corresponding input
power (hydraulic or electrical) back to the mechanical form to
drive a rotational torque T with an angular speed w. A Linear
Actuator reconverts the corresponding power to a mechanical
form to drive a translational force F with a speed v.
1.1.3.1- Rotational Actuators
Figure 1.16, shows constant speed electrical motor (1), fixed
displacement hydraulic motor (2), variable speed electrical
motor (3), and variable displacement hydraulic motor (4). The
following section explores the main features of Rotational
Actuators in both systems:
Similarities:
• Both systems have rotational actuators referred to
“Motors” of different sizes.
• Both systems have constant speed motors (1 & 2) and
variable speed motors (3 & 4).
Density than electrical motors.
• Saturation: Hydraulic motors are not saturated as electric
motors. A Hydraulic motor generates torque proportional
to differential pressure across the motor and its size. The
only saturation limits of a hydraulic motor are maximum
working pressure and maximum torque of the motor’s
shaft.
• Starting Torque: Hydraulic motors have better starting
torque and better dynamic response as compared to the
electrical motors.
• Cost: Electric motors are commonly less expensive for the
same power.
1
2
Differences:
• Types: Electrical motors could be of AC or DC type.
• Nmax: Maximum speed of electrical motors is higher than
hydraulic motors. Fluid friction limits the maximum speed
of a hydraulic motor.
• Nmin: Minimum speed of electrical motors is less than
hydraulic motors because the hydraulic motors are
inefficient at low speed.
• Power Density: Hydraulic motors have larger Power
1.1.3.2- Linear Actuators
Figure 1.17, shows a linear electrical actuator (1), hydraulic
cylinder (2), servo-controlled linear electrical actuator (3),
and servo-controlled hydraulic cylinder (4). The following
section explores the main features of Linear Actuators in both
systems:
3
4
Figure 1.16 - Rotational Actuators
• Cost: Hydraulic cylinders are commonly less expensive
than the equivalent electrical linear actuators that have
the same size.
2
1
Similarities:
• Both systems have linear actuators of different sizes. They
are called “Cylinders” in hydraulic systems.
• Both systems have servo-controlled linear actuators.
4
3
Differences:
• Mechanisms: A hydraulic cylinder produces the linear
motion without the use of any intermediate mechanism
as in electrical linear actuators.
• Strokes: Hydraulic cylinders offer longer strokes than
electrical linear actuators.
• Power Density: For the same physical size actuator,
hydraulic cylinders carry higher loads than electrical linear
actuators.
Figure 1.17 - Linear Actuators
may 2018 Global MDA Journal
59