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!