Actuators: Where the REAL Action Takes Place!
Fred DonelsonKevin McKone
What is an Actuator?
• It is a mechanism that converts some type of energy into motion in order to do work (move a force over a distance)
• The three common types of energy used in ROV work are electrical current, hydraulic pressure, or pneumatic pressure
Most Common ROV Actuators:
• Motors• Solenoids• Pneumatics and/or hydraulics
Let’s start with motors:
A motor might turn some gears:
Or a propeller
Two important factors/variables to control with motors
• Direction that the motor axle turns• Speed at which the motor axle rotates
Here is a simple circuit with a power source and the motor as a load:
The red arrows show the direction of the current flow
Spin Direction of Axle
By flipping the battery terminals, the direction of the motor rotation can be
changed
Motor
Spin Direction of Axle
Of course, continually flipping the terminal
connections would get old very fast!
Another way to change directions is to use a
double pole/double throw switch
First, we make these connections
When the switch is in the center
position (B & E), no current flows
(off)
When the switch is in the up position (connecting B to A & E to D), notice the current (red
arrows) flows through motor
from left to right.
When the switch is in the lower
position (B to C & E to F), current
flows from right to left through the
motor, thus changing the
direction of spin
Many ROVs use three of these DPDT switches to
easily control the direction
of three motors
However, there is one drawback:The DPDT switch only has ONE
SPEED!
It is either all the way on, or all the way off!
And after a while, your fingers get tired of pushing those switches too!
And even if we didn’t get tired, there’s a limit as to how fast we can mechanically flip the switch
each second.
Fortunately, there are ELECTRONIC DEVICES called transistors that can act as switches for
us!
One type is known as aMetal Oxide Semiconductor Field
Effect Transistor:MOSFET
These electronic switches can turn on and off several thousand times
a second
When connected to a controller/computer, one can vary the speed as well as the direction
of a motor!
A very fast switch can turn the motor on (12V) or off (0V), and the controller can control the average time the motor is on each second, affecting its speed
This is known as Pulse Width Modulation (PWM)
Each MOSFET and controller can control how long the motor is on
(which is called it’s duty cycle)
But it takes 4 MOSFETS arranged in a special arrangement called an
H – Bridge to actually change the direction of a motor
It works similar to a DPDT switch
Basically two MOSFETs lead
from the positive
terminal and two from the
negative
If P1 and N2 are turned on, the current travels
from left to right through
the motor (black arrows)
If P2 and N1 are turned on, the current travels from right to
left through the motor (red
arrows)
You can learn more about how the H-Bridge works by checking out
page 519 in your textbook!