Actuators

As you know, actuators are “movers” that play a key role when it
comes to moving and controlling mechanisms in various industrial
applications, including conveyors. Actuators work by drawing
energy from some sort of source, and there are various types of
actuators that typically differ based on where they derive their
energy from.

Fig. 1 A typical actuating unit

At the core of an actuator's operation is the conversion of input

energy into mechanical motion. This energy can come in various
forms, such as electrical, hydraulic, or pneumatic, and is
transformed into linear movement or rotary motion to perform a
specific task. The type of motion generated depends on the
actuator's design and the application's requirements.
Control systems play a significant role in actuator operation,
ensuring that the motion generated is accurate, precise, and
responsive to the system's needs. These control systems receive
input signals, process them, and send output signals to the actuator,
dictating its movement. Feedback mechanisms are often employed
to monitor the actuator's position and performance, allowing for
adjustments to be made in real-time to maintain optimal operation.
The efficiency of an actuator is determined by its ability to convert
input energy into useful mechanical motion with minimal energy
loss. Factors such as friction, heat generation, and mechanical
resistance can impact an actuator's efficiency, making it essential
to consider these aspects when designing and selecting actuators
for specific applications. Additionally, the actuator's response time,
accuracy, and repeatability are critical performance metrics that
influence its suitability for various tasks.
The primary function of actuators is to control machines and allow
parts to move. This motion can be any one of hundreds of
operations such as lifting, clamping, blocking and ejecting.
Typically, actuators are key parts in industrial and manufacturing
operations where they activate valves, pumps, motors and
switches.

Classification of actuators

Three of the most common types of actuators found in industrial

applications are hydraulic, pneumatic and electric.

 Hydraulic actuators operate with compressed fluid and

control movement by managing the amount of fluid inside.
Adding fluid increases pressure while reducing it lessens the
force. These actuators are ideal for applications that need
more significant power such as trucks and heavy equipment
applications.
 The primary advantage of pneumatic systems is that they run
on compressed air or gas instead of fluid. As a result, they are
non-volatile. Pneumatic actuators are versatile and
 affordable, making them popular for braking systems and
pressure sensors.
 The function of an electric actuator is to generate mechanical
power from electricity input. Since the power source is
consistent and continuous, these actuator types offer easy
maintenance and are ideal for high-precision work. Electric
actuators are common in manufacturing, robotics and electric
vehicles.

Table 1
Advantages Disadvantages
Hydraulic High force capabilities Initial investment
Simple design Maintenance
Rugged construction Leakage
Affordable
Pneumatic Fast Limited power compared
Economical with hydraulic
Simple Shorter life cycle compared
with hydraulic
Pressure losses
Electric Fast: Electric actuators are Weak: You can’t get the
directly driven. same amount of strength
Precise and power with electrics
Clean: There is no potential that you can with hydraulics
risk for leakage. or pneumatics.
Complicated
Costly

Classification of actuators based on the motion

Rotary Actuator
Linear Actuators

The following table gives detailed classification for actuators.

 Table 2
Actuator Features
Electrical
Thyristor, transistor, power MOSFET, solid  Electronic type
state relay, etc.  Very high frequency response
 Low power consumption
Electromechanical
DC motor Wound field Separately Speed can be controlled either by the
excited armature voltage or by varying the field
current
Shunt Constant-speed application
Series High starting torque, high acceleration
torque, high speed with light load
Compound  Low starting torque, good speed
regulation
 Instability at heavy loads
Permanent Conventional High efficiency, high peak power, and
magnet fast response
Moving coil Higher efficiency and lower inductance
than conventional dc motor
Torque Designed to run for long periods in a
motor stalled or a low rpm condition
Electronic commutation  Fast response
(Brushless motor)  High efficiency, often exceeding 75%
 Long life, high reliability, no
maintenance needed
 Low radio frequency interference and
noise production
AC motor Induction  The most commonly used motor in
industry
 Simple, rugged and inexpensive
Synchronous  Rotor rotates at synchronous speed
 Very high over a wide ranges of
speeds and loads
 Needs an additional system to start
Universal  Can operate in dc or ac
 Very high horse power per pound ratio
 Relatively short operating life
Stepper motor Hybrid  Change electrical pulses into
mechanical movement
 Provide accurate positioning without
feedback
 Low maintenance
Variable reluctance
Permanent magnet
Electromagnetic
Solenoid-type devices Large force, short duration
Electromagnets, relay On/off control
Hydraulic and pneumatic
Cylinder Suitable for linear movement
Hydraulic motor Gear type Wide speed range
Vane type High horsepower output
Piston type High degree of reliability
Air motor Rotary type No electric shock hazard
Reciprocating Low maintenance
Valves Directional Control Valves
Pressure Control Valves
Process Control Valves
Smart material actuators
Piezoelectric and electrostrictive -High frequency with small motion
-High voltage with low current excitation
-High resolution
Magnetostrictive -High frequency with small motion
-Low voltage with high current excitation
Shape memory alloy -Low voltage with high current excitation
-Low frequency with large motion
Electrorheological fluids  Very high voltage excitation
 Good resistance to mechanical shock
and vibration
 Low frequency with large force
Ultrasonic piezo motor Intrinsic steady-state auto-locking
capability, no servo dithering and heat
generation
Micro- and Nano actuators
Micro motors  Suitable for micro mechanical system
 Can use available silicon processing
technology
MEMS thin film optical switches Reduced size, low power requirements,
high frequency
MEMS mirror deflectors Low power consumption, high frequency
MEMS fluidic pumps and valves Ideal for very low volume and precise
manipulation of fluids. Typically both
force and stroke are small
MEMS drug dispensers  Physiological- stimuli based.
 Accurate, precise, and typically
dispensed directly into blood stream
 ELECTRICAL ACTUATORS DEVICES
Actuators take low power signals transmitted from the computer
and produce high power signals which are applied as input to the
process.
Electrical actuators convert electrical command signals into
mechanical motions.
Selection of actuator device for mechatronic applications are
affected by;
1- Precision
2- Accuracy and resolution
3- Power required for actuation
4- Cost of the actuation device

1. Direct Current (DC) Motors

DC motors are the most popular actuators in mechatronic systems.
DC motors, which are electromechanical devices, provide precise
and continuous control of speed over a wide range of operations by
varying the voltage applied to the motor.
The desirable features of DC motors are their high torque, speed
control ability over a wide range, speed-torque characteristics, and
used in various types of control applications.
DC motors are suitable for many applications such as;
manufacturing equipment, CNC systems, servo valve actuators,
tape transport mechanisms, and industrial robots.
It converts DC electrical energy into rotational mechanical energy.
Its working principle is that wire carrying current in magnetic field
experiences a force.
Mathematical Model of a DC Motor
The behavior of DC motors can be explained by two fundamental
equations:

𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑒𝑞𝑢𝑎𝑡𝑖𝑜𝑛: 𝐸𝑏 = 𝐾𝑒 𝜃 ′ 𝛷
𝑡𝑜𝑟𝑞𝑢𝑒 𝑒𝑞𝑢𝑎𝑡𝑖𝑜𝑛: 𝑇 = 𝐾𝑡 𝐼𝛷
Where;
𝑇: motor torque in N-m,
𝐸𝑏 : induced voltage in V,
𝐼: armature current in A
Ɵ: motor angular displacement in radians
Kt: torque constant in Nm/A, and
Ke: voltage constant in V/(rad/sec)

𝑑𝐼𝑎
𝑉𝑎 = 𝑅𝑎 𝐼𝑎 + 𝐿𝑎 + 𝐸𝑏
𝑑𝑡
Where;
𝑉𝑎 : Armature voltage in V
𝑅𝑎 : Armature resistance in 
𝐿𝑎 : Armature inductance in H
𝐼𝑎 : Armature current in A
2. Brushless Dc Motors
As their name implies, brushless DC motors do not use brushes. So
how does a brushless motor pass current to the rotor coils? It
doesn’t—because the coils are not located on the rotor. Instead, the
rotor is a permanent magnet; the coils do not rotate, but are instead
fixed in place on the stator. Because the coils do not move, there is
no need for brushes and a commutator. (See Figure. 2.)

Since the rotor is a permanent magnet, it needs no current,

eliminating the need for brushes and commutator. Current to the
fixed coils is controlled from the outside.

With a BLDC motor, it is the permanent magnet that rotates;

rotation is achieved by changing the direction of the magnetic
fields generated by the surrounding stationary coils. To control the
rotation, you adjust the magnitude and direction of the current into
these coils.

Brushless direct current electric motors, or BLDC motors for short,

are electronically commutated motors powered by a dc electric
source via an external motor controller. Unlike their brushed
relatives, BLDC motors rely on external controllers to achieve
commutation. Put simply, commutation is the process of switching
the current in the motor phases to generate motion. Brushed motors
have physical brushes to achieve this process twice per rotation,
while BLDC motors do not, hence the name. Due to the nature of
their design, they can have any number of pole pairs for
commutation.

BLDC motors provide significant advantages over traditional

brushed motors. They typically offer an efficiency increase of 15-
20%, require less maintenance without brushes to physically wear
out, and deliver a flat torque curve at all rated speeds. While
BLDC motors are not a new invention, widespread adoption had
been slow due to the need for complicated control and feedback
circuitry. However, recent advancements in semiconductor
technology, better permanent magnets and the growing demand for
greater efficiency has led to BLDC motors replacing brushed
motors in many applications. BLDC motors have found their niche
in many industries, including white goods, automotive, aerospace,
consumer, medical, industrial automation equipment and
instrumentation.

3. Permanent Magnet Stepper Motor

Nowadays, stepper motor has appeared as a cost-effective
alternative to DC motors in motion-control applications.
Stepper motor translates electrical pulses into precise, equally
spaced, angular movements of the rotor in the form of steps.
The rotor is positioned by magnetically aligning the rotor and
stator teeth, which occur when the air gap between the two sets of
teeth is minimized and aligned.
Two basic types of stepper motors are;
1. Variable reluctance (VR) stepper motors; step angle is 15°,
with torque ranges up to 14 N-m.
2. Permanent magnet (PM) stepper motors; step angle is 1.8°or
0.9°, with torque ranges up to 3.5 N-m.
This limits the range of applications for PM motors to a lower
torque region than that of VR motors. PM motors are available in
smaller standard sizes (commercially known as size 23 or size 34).
The four-phase, size 23 motor produces under 0.7 N-m torque with
a speed range of up to 30,000 steps per second (sps), While size 34
motor produces roughly three times the torque at one third of the
speed.
They are extremely well suited for use in open-loop applications
due to their accuracy and noncumulative position-error
characteristics.
Compared to DC servo motors, stepper motors produce less torque,
lower speeds, and higher vibrations.

Stepper motor positioning system

360
𝑆𝑡𝑒𝑝 𝑎𝑛𝑔𝑙𝑒; Ɵ=
𝑛𝑠
Ɵ = step angle, degrees
𝑛𝑠 = number of step angles

𝐴𝑛𝑔𝑙𝑒 𝑜𝑓 𝑟𝑜𝑡𝑎𝑡𝑖𝑜𝑛; 𝐴 = 𝑛𝑝 Ɵ

𝐴 = Angle of lead screw Rotation, degrees

𝑛𝑝 = number of pulses received by the motor
 𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑚𝑜𝑣𝑒𝑑; 𝑆 = 𝑝𝐴/360
𝑆 = position relative to the starting position, mm
𝑝 = pitch of the lead screw, mm/rev.
𝐴/360 = number of revolutions (and partial revolutions) of the
lead screw
𝑅𝑜𝑡𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝑠𝑝𝑒𝑒𝑑; 𝑁 = 60𝑓𝑝 /𝑛𝑠

𝑁 = rotational speed, rpm

𝑓𝑝 = pulse frequency pulses/sec.

𝑓𝑒𝑒𝑑 𝑟𝑎𝑡𝑒; 𝑓𝑟 = 𝑁. 𝑝