Yaskawa
eLearning
The Technical Training Department of
Yaskawa America Incorporated Presents...
Servo Basic Concepts
An Overview of Applications and Guidelines for Successful Use
Hi, I'm Steve Koehler.
Now most people who are involved
in the world of technology
have heard the word servo before.
And while most have heard the word,
many aren't familiar with exactly what it means
or how a servo functions.
If you've seen an industrial automation system
in operation you've seen servos at work.
To fully understand automation
it's essential to understand servo operation and application.
And that's what you'll get out of this training program...
A practical overview of AC servo systems
and how they are used.
We'll define what a servo is,
examine its components
and explore how it works with a motion control system.
We'll examine some of the most important concepts
used when applying a servo
including Axis of Motion,
System Power Up,
Directional Movement,
Over-Travel,
Distance and Position
Homing and Actuators.
Here's a basic definition of a servo.
It's a device that produces motion in response to a command,
then regulates the speed and direction of that motion in response to feedback.
You'll find servos in use on automation systems that require motion
with precise control and especially in systems that need
very swift response to commands
and exact fidelity to position requirements.
The world of industrial servos is a very diverse one.
Some servos are hydraulic some are pneumatic
and others are electromechanical.
In recent years industry has moved more and more
toward the use of AC servo systems.
AC, of course, refers to the alternating current waveform
in the motor during operation.
The AC servo system involves two basic components: the servo amplifier and the servomotor.
The servo amplifier takes in low voltage commands for torque, speed or position
then amplifies them into a high-powered format that the servomotor can use.
A servo motor is much more powerful
than other motors of the same size.
Powerful magnets make servomotors exceptionally quick and accurate.
A feedback device called an encoder is built into the servomotor at the back end.
Now this is an example of an optical encoder.
It's a disc mounted on the motor shaft.
As the motor rotates,
small photo sensorsdetect lines on the disk.
The electrical pulses from the photosensor are sent back to the amplifier.
The amplifier uses the data to control the motor's position, speed and torque.
Now we have some insight on what a servo is,
but we haven't yet described how a servo gets things done.
We put a servo to work by making it part of a motion-control system.
At the input to the servo amplifier is a controller.
The controller provides the commands
that dictate when and how the servomotor will move.
The controller sends commands
to the amplifier in the form of an analog signal or data.
The controller and the amplifier also exchange position feedback from the encoder.
Connected to the controller
you'll find an operator interface.
The interface is used to start, stop and adjust
the various functions of the machine.
Interfaces can be simple buttons
and indicator lamps or they may be a
more complex human machine interface or HMI.
On the other end of the process the servo motor is connected to a mechanical
system that it moves - the servomechanism.
This is a common example.
A simple linear actuator
that translates the servomotor's rotary motion
into linear motion.
In this example using a screw.
This is only a simple example.
The mechanisms used in
industry can be complex and diverse
depending on the job that needs to be done
and the imagination and expertise
of the machine builder.
While these five components exist in every motion control system,
some manufacturers have designed units
that combine one or more of these components.
This is an example of a unit that includes
both a controller and an amplifier.
Combining components means fewer parts,
fewer connections and a smaller footprint
and greater convenience.
The term axis is one you'll hear very often in this industry.
Each servo and the mechanism it moves counts as one Axis of Motion.
This word Axis is drawn from mathematics
and the concept of axes in a Cartesian
coordinate system.
Here's another way of looking at it.
Imagine that an Etch-a-Sketch toy was
controlled by two servos.
One server would control the up-and-down direction,
the other the left and right.
Now that would be two axes of motion.
In this example the two axes are mechanically
connected together to create two
dimensional motion,
but even if there's no connection
each servo is considered an axis.
Now this distinction is important because in
the world of industrial automation a single machine may have several servo
axes, sometimes even hundreds of axes.
Regardless of the number, there are some fundamental ideas that apply to each axis.
And here's one of the most basic.
An axis will provide no motion
unless you turn the servo on.
Turning on the servo isn't as simple as switching on a light bulb.
It's important to know that there are three stages to enable a servo.
First, the amplifier itself must be turned on.
We refer to this as Control Power.
The system is typically wired to power up the
controller and the operator interface at the same time
servo control power is established.
The second stage is to apply the high
voltage and high current power source that the amplifier will use to drive the motor.
We refer to this as Main Power.
The main power input allows flexibility and
wiring to the servo amplifier.
The third stage is to energize the servomotor.
We refer to this as Servo Enable.
This signal to enable the motor comes from the controller.
Servo enable doesn't mean the motor is moving,
it only means that it's ready to move when commanded.
Before the servo is enabled,
the shaft will easily move
but if you manually try to turn the shaft
while the server was enabled,
it will hold its position.
You may hear a hissing sound as the servo works to maintain this position.
After the servo motor is energized the controller
typically requires the operator
to press some type of a start button.
At this point, the controller will command motion according to its program.
This particular controller is programmed to move the
axis in both forward and reverse direction.
To do so, it needs to define
what forward and reverse means.
What is considered forward for the servo
may turn out to be reversed with respect to the machine.
It is important to match the servo direction with the machine direction
not only for programming the controller
but also for any other features in the
servo amplifier that are sensitive to direction.
For example, the server may use
a protective torque limit in one direction but not in the other.
I can manually over power the motor in one direction
but I can't overpower it in the other direction.
So you can see why it's important to understand which direction is forward
and which direction is reverse.
In some circumstances this could create catastrophic damage to your machine
if not properly configured,
Another direction sensitive feature found in amplifiers is Over-Travel.
Some people refer to this as the limit switch function.
Over-travel is an amplifier input that stops the motor from moving in one direction.
One input for forward and another input for reverse.
We establish the boundaries of over-travel by placing
a sensor at each end of the machine's range of motion.
A properly programmed controller should prevent the machine from ever getting
close enough to reach these sensors, but experience dictates the wisdom of
planning for the unexpected.
Over-travel inputs can prevent costly damage
without it, moving parts of the machine could be allowed to crash into one another.
Now the positive over-travel sensor is connected to the amplifier input that
stops motion in only the forward direction.
The negative over-travel sensor
is connected to the amplifier input that stops motion in only the reverse direction.
So you can see that if these sensors get switched around or if the direction of
the servo doesn't match the direction of the machine
the motor might not stop at the over-travel sensor.
Damage or even injury might result.
It pays to remember that an over-travel stop
is NOT an emergency stop.
An emergency stop would prevent all motion in all directions.
An over-travel stop is simply a quick way
to stop motion in one direction.
In an over-travel condition
the controller can be programmed to recover
and the servo can still be moved
in the opposite direction.
While the servo amplifier and servo motor are the devices at the heart of
the motion control system
let's move our discussion upstream to the controller
where all the commands originate.
The topic of controller programming
is beyond the basics
but we can cover some fundamentals like Distance and Position
the program may require the servomechanism
to travel a certain distance relative to its current location
this is called a Relative move
and this approach is helpful when programming a
move that needs to repeat itself several times
on the other hand the program may have a preset table
of positions to which the machine is to move.
In this case an Absolute Positioning move
to a certain position can make programming
the controller a lot easier
Now here's an example,
if you order me to take three steps forward
well that's a Distance command
a Relative move.
If you tell me to go to the table
that's a Position Command - an Absolute Move
it's the same result
I'm at the table but the type of command is different
modern industrial motion controllers
allow the programmer the option to
define the move either relatively or absolutely
whichever makes the most sense for that particular part of the control program
for this reason we really can't tell which method was used by simply looking
at the machine in motion.
Absolute positions and moves only makes sense when compared to the absolute
position which the controller has identified as zero.
The machine requires a repeatable method
to find this zero point.
This method is often called Homing
or Zero Point Return
To reorient an axis that may have been moved
while the power is off
the controller needs to Home
each axis every time the machine powers up
a very simple way to accomplish homing
is for the operator to visually align the axis
to its zero position.
In actual practice however
most machines rely on a fully automated sequence
called a Homing Routine.
Here is a typical homing routine in a controller
the axis first move slowly in Reverse then stops when it detects a dedicated
sensor known as the home sensor.
We could use this sensor as the home position
but if we did, we'd need to physically move
the sensor to calibrate the system.
Instead the axis moves a programmable
offset distance to stop at a home position.
Our final stopping point is then defined as Position Zero
for the controller's coordinate positioning system.
Now as long as the sensor remains in place
this homing routine will always put
the axis back to the same starting point.
Different types of machines require
different types of Homing Routines.
Programming a successful Homing Routine
in the controller for non-standard
machine configurations can be a considerable challenge for a controls engineer.
Up to now in our discussion of components
we have focused on linear actuators
they are common they're also the easiest to understand
but there are many other types of mechanisms used in industrial automation
we can put the whole range of mechanisms
in two categories: those producing Linear Motion and those producing Rotary Motion
For Linear Motion, the movement is defined
in linear units commonly inches or millimeters.
A Rotary Actuator defines movement in rotary units usually degrees
now within these two categories there are
two subtypes: Finite and Infinite
for example a linear actuator has a finite
distance it can move
but a pinch roll can pull material virtually forever
a rotating fixture with cables has a fixed distance it can
move without twisting or breaking the cables but a rotary table can keep
spinning endlessly in the same direction.
These machine characteristics are very important when you are programming
because they influence how
and if homing is performed
how relative and absolute moves are implemented
and whether or not over travel inputs are used.
There's a reason we call this program Servo Basics
it's only the beginning of the things you want to learn about servos and the
extensive role they play in industrial automation
and a great way for you to expand your knowledge
is with a visit to yaskawa.com
it's a rich resource for technical information and product documentation
all searchable and ready to download
and put to use.
Well we've come to the end of this training program but it definitely isn't
the end of our commitment to make
Yaskawa drives and motion products
the best in the industry.
The commitment to quality continues in the way
we work with our customers and with our vendors.
It's in the way we train our associates
it means we deliver product on time we
we answer questions quickly and we never say we can't
Yaskawa quality is reflected in the effort our associates bring to work every day
to us
quality means doing everything we can to make our customers, partners and
employees experience a great one.
We commit to that,
we make it happen,
we can because to us
it's personal.