In this lesson we will continue the discussion on the topic of motion control.
All of us at RealPars, hope you have been following our previous lessons
motion control and have begun to benefit from these lessons.
Now that we have learned about motion control motors,
we can begin to explore some advantages and disadvantages when selecting,
primarily between, the Servo Motor and Stepper Motor.
In this lesson we discuss Servo Motor Advantages
and Disadvantages when compared to Stepper Motors.
Before we get into today’s video, if you love our videos,
be sure to click the like button below,
and make sure to click subscribe and the bell
to receive notifications of new RealPars videos.
This way you never miss another one!
First let’s remember what we have learned about servo motor basics.
The Servo Motor has the same construction of DC motor,
it has a stator, rotor and controlling assemblies.
It also has feedback for controlling the speed & torque.
A Servo Motor is a self-contained electrical device,
that rotate parts of a machine
with high efficiency and with great precision.
The output shaft of this motor can be moved to a particular angle,
position and velocity that a regular motor does not have.
The Servo Motor utilizes a regular motor
and couples it with a sensor for positional feedback,
the controller is the most important part of the Servo Motor
designed and used specifically for this purpose.
The Servo Motor is a closed-loop mechanism
that incorporates positional feedback
in order to control the rotational or linear speed and position.
The motor is controlled with an electric signal,
either analog or digital, which determines the amount of movement
which represents the final command position for the shaft.
A type of encoder serves as a sensor providing speed and position feedback.
This circuitry is built right inside the motor housing
which usually is fitted with gear system.
One of the main differences between Servo Motors
and stepper motors is that Servo Motors,
utilize a control loop and this requires position feedback.
The Servo Motor control loop uses feedback from the motor
to help the motor arrive at a desired position and velocity.
These control loops come in a variety of electro-mechanical types.
The controller or PLC software will generally use a PID
(Proportional, Integral, Derivative)
control loop for Servo Motors to achieve accurate speed, acceleration, and distance.
When using a control loop such as PID,
you may need to tune the Servo Motor.
One of the disadvantages of a Servo Motor
is tuning a motor can be a very difficult and a troublesome process,
but the PID loop is also an advantage
providing more control over the behavior of the motor.
Since Servo Motors have a control loop
to monitor the current velocity and distance,
they are for the most part more reliable than stepper motors.
For example, when a stepper motor loses a step for whatever reason,
there is no control loop to compensate for the error in movement.
The control loop in a Servo Motor
are constantly monitoring the critical control attributes of the path
and will make the necessary adjustments when movement is in error.
Like stepper motors, Servo Motors are available as conventional rotary motors
and linear Servo Motors and have several advantages
that are leading a growing numbers of machine builders
to favor Servo Motors to boost overall performance.
In general, Servo Motors run more smoothly
than a stepper motor except when microstepping are used.
A Servo Motor will typically provide 2-3 times the speed
of a typical stepper motor as the Servo Motor speed increases,
the torque (the twisting force that tends to cause rotation)
of the Servo Motor remains constant,
thus performing better than a stepper motor
at higher speeds usually above 1000 RPM.
It is important to note that the continuous speed rating of the Servo Motor
exceeds that of the comparable stepper motor.
When we compare the performance of the stepper motor
to that of the Servo Motor performance,
the observer would notice a slight torque advantage
in favor of the stepper motor at low RPMs, less than 1000.
However, once the RMPs cross over this 1000 RPM limit,
the torque performance of the stepper motor decreases drastically,
and speed-torque performance of the Servo Motor has the advantage.
The Servo Motor can only operate in this peak torque range for a short interval
and must allow generated heat to be dissipated,
but the Servo Motor operating in this range of operation is a Servo Motor Advantage.
This prominent torque advantage of Servo Motors running at high speeds,
can easily achieve speeds of 3,000 RPM
and it is not uncommon for some Servo Motors
to achieve speeds of 5,000, 6,000 and even 8,000 RPM
while sustaining usable torque.
Servo Motors have an advantage
with inherent sophisticated control algorithms,
providing some control features that are simply
not available in most stepper motor controllers.
With the advantages of torque control
many applications accomplish all the required pressing,
holding, pushing and twisting motions at very precise torques.
If the torque applied is too high,
crushing of the widget and similar damage can result.
If the torque is too low, not enough holding force is created
and the end result could be slippage resulting in defects or wasted product.
Many servo drives provide torque parameters
to allow one to quickly and easily control
the needed torque for a particular application.
Accuracy is a Servo Motor advantage.
There is always a chance that the stepper motor
may experience a misstep or stall that could go undetected.
In these situations, there is no indication from the stepper motor
and by the time the error is noticed
much time may have passed with undesirable results.
Servo Motors utilize position and speed feedback devices
that may encompass the use of encoders to sense positional error in a system
and provide path correction for this error
to achieve the desired performance of the machine.
And so, with a Servo Motor, the inherent use of an encoder
does not allow for position errors to go undetected.
A servo drive continuously reads in current position data
and compares this feedback with the commanded position
and calculates the error,
then provides an updated command to the Servo Motor
proportional to the degree of the error.
Therefore, a Servo Motor will continuously try to correct
for any errors and will drive the motor as necessary
to move the load to the desired position.
This error correction is accomplished
for Servo Motor torque, velocity, and/or position.
In addition, Servo drives have the added advantage
of including a position fault error limit.
If the position error feedback by the encoder exceeds a limit,
the Servo Motor controller will fault the motor,
thereby preventing any erratic movement
that could lead to potential damage to equipment, product or people.
And, the positioning error fault limit can be adjustable allowing for tighter,
and better-defined control.
Properly tuned Servo Motors are extremely quiet.
The biggest source of noise in servo-driven applications
are often the drive train or bearing.
In comparison, stepper motors will typically emit around 68 dB during operation.
In the past, the desire to reduce costs
has often dictated the use of lower-cost technologies such as stepper motors.
However, with growing pressures on machine and equipment designers
to maximize the efficiency of machines,
and now with the availability of lower-cost Servo Motors
and positional feedback devices,
are giving more thought to the advantages offered by Servo Motor technologies.
When purchasing Servo Motors
(or stepper motors) standard designs help
competing and will offer the OEM (Original Equipment Manufacturer)
and end-user an advantage in competitiveness and serviceability.
NEMA (an acronym for the National Electrical Manufacturers Association)
and IEC (an acronym for the International Electro Technical Commission)
include member companies that manufacture products used in the generation,
transmission, distribution, control and end-use of electricity.
The goal is to promote competitiveness of the international
electrical product industry through the development of standards.
The advantage for using standard sized motors
is so customers can reliably replace and ensure
a compatible and dimensionally fit when a motor is sourced from one company
and replaced by a motor of another company.
You can easily visualize how standards will help to promote competitiveness
in the electrical Servo Motor industry.
A Servo Motor and drive solution is more expensive than a stepper motor solution
because of the complex encoder feedback device
found in the servo drive hardware
and the algorithms required to drive the motor
with the appropriate control signals.
The added performance capability found in Servo Motors
comes at a premium price
when compared to a properly sized stepper motor solution.
Another disadvantage is the complexity involved
in configuring a Servo Motor solution.
Servo Motors require tuning as mentioned earlier.
Tuning is a method by which the user optimizes the internal algorithms
and electronics for physically controlling the torque,
velocity and position of the motor.
The tuning process requires a continuous modification
and adjustment of the servo drive output signals to the motor
so the end result is achieved for the best smoothness,
accuracy, and efficiency.
Servo Motor tuning is an arduous task
that is often difficult and time consuming
and becomes an art in performing the perfection required
to obtain the accuracy required.
Research indicates that tuning a servo drive
can take anywhere up to 6 hours or more to complete.
While many Servo Motor manufacturers and providers
continue to improve upon the tools available
for performing the tuning task,
it is always recommended to select a Servo Motor drive combination
that offers tools to help minimize the work required to tune the servo.
An example could be selecting a matched Servo Motor
with a controller that has been designed to function together.
Now let’s summarize the top Servo Motor Advantages and Disadvantages.
Top Servo Motor advantages are:
High output power relative to motor size and weight.
Encoder determines accuracy and resolution.
High efficiency. It can approach 90% at light loads.
High torque to inertia ratio.
Servo Motors can rapidly accelerate loads.
Has 2-3 times more continuous power for short periods.
Has 5-10 times more rated torque for short periods.
Servo motors achieve high speed at high torque values.
Quiet at high speeds.
Encoder utilization provides higher accuracy
and resolution with closed loop control.
The top Servo Motor disadvantages are:
Servos Motors requires tuning to stabilize the feedback loop.
Servo Motor will become unpredictable when something breaks.
So, safety circuits are required.
Complex controller requires encoder and electronic support.
Peak torque is limited to a 1% duty cycle.
Servo Motors can be damaged by sustained overload.
Gear boxes are often required to deliver power at higher speeds.
Higher overall system cost
and the installation cost of a Servo Motor system
may be higher than that of a stepper motor
due to the requirement for feedback components.
This concludes the video,
Servo Motor Advantages and Disadvantages.
I hope you have learned what’s required to move forward
in creating your own motion control project.
Make sure that you head over to realpars.com.
To find even more training material for all of your PLC Programing needs.
We offer many videos to assist you in learning PLC Programing
and landing that job in a high-paying,
highly thought after field of automation and controls engineering.
Go to realpars.com and subscribe to our highly effective training series now!