This lesson is a continuation of a series of lessons involved
with learning about Stepper Motors and Servo Motors.
If you haven’t already, please review our previously released lesson on,
what is a Stepper Motor and How it works.
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In this lesson we will discuss the Stepper Motor Advantages
and Disadvantages compared to Servo Motors.
Have you ever thought about how a robotic arm moves with precision
or how graphic printers create accurate replications of images,
or how consumable products are moved precisely around a plant floor
or any other type of motion control positioning system?
Together with the recently RealPars published video,
What is a Stepper Motor and How it Works, and this lesson,
you will learn about Stepper Motor Advantages in motion control
when using different types of motors available,
primarily stepper and servo motors.
Selecting between a stepper motor and servo motor
can be quite a challenge with the balancing of several design factors in cost considerations,
torque, speed, acceleration, and drive circuitry
all play an important role in selecting the best motor for your application.
In this lesson we will discuss the many advantages
along with the disadvantages of Stepper Motors,
so let’s first define a few definitions used with Stepper Motors
before we begin to discuss the Stepper Motor Advantages and disadvantages.
What is torque? Torque is the tendency of a force
to rotate an object about an axis, fulcrum, or pivot.
Just as a force is a push or a pull,
a torque can be thought of as a twist to an object
or the rotation of the Stepper Motor rotor.
What is Open Loop versus Closed Loop motion systems?
An open loop system has no feedback or mechanisms
for error regulation to determine if its output has achieved
the desired goal of the input.
What is repeatability and Accuracy?
Repeatability is a drive mechanism’s ability
to return to the same position multiple times under identical conditions.
Accuracy however, is the degree in measurement for motion
can be determined to which the final position matches the commanded position.
For example, in an application that will allow a result of 535 mm ±2 mm,
then as long as the ball screw achieves a position between 533 and 537 mm,
it will be considered to be accurate.
What is an overload condition?
Motor overload will occur when a motor is under excessive load.
Overload symptoms are excessive current draw,
insufficient torque and overheating.
What is motor efficiency?
Motor efficiency is the comparison of the amount of mechanical work
the motor performs and the electrical power it consumes to do the work
using the interaction between a magnetic field
and current in its winding to produce and generate force or torque in a motor.
What is resonance and does it affect the Stepper Motor?
This can be best defined when the motor moves a single step
it tends to overshoot the final resting point
and oscillates round this point as it comes to rest.
This undesirable ringing is exhibited as motor vibration
and is more pronounced in unloaded motors and will cause the motor to stall.
Top Stepper Motor Advantages:
Now let’s look at some of the top Stepper Motor Advantages.
One of the biggest advantages of stepper motors
is their relatively low cost and availability.
Stepper motors offer flexibility in application
for a wide range of applications because the design of the stepper motor
provides a constant holding torque
without the need for the motor to be powered.
The torque of a stepper motor of that of the same size of a servo motor,
at lower speeds is greater than that of a servo motor.
The Stepper Motor is often used in an Open Loop System
that does not require positional or torque feedback
making the Stepper Motor simpler and less costly to control.
The Stepper Motor is in itself the position transducer,
as we have learned in our previous,
what is a Stepper Motor and How it works lesson.
We learned that Stepper motors have a large number of poles,
magnetic pairs of north and south poles
generated either by a permanent magnet or an electric current,
typically 50 to 100 poles.
We learned each pole offers a natural stopping point
for the motor shaft and with the greater number of poles,
a stepper motor is able to move accurately
and precisely between each pole.
Because of this natural stopping points
a stepper can be operated without any position feedback for many applications.
One of the biggest advantages of Stepper Motors
are their relativity low cost and availability.
Stepper Motors are essentially plug and play,
easier to setup and use overall.
The Stepper Motor offers a better bottom line for your budget
and are inexpensive when compared to Servo Motor motion control systems
for their better suited applications when considering lower acceleration
and high holding torque requirements.
Stepper Motors are considered to be safer,
where as if anything breaks, the Stepper Motor will stop.
Many of the moving parts are frictionless,
so it is considered that Stepper Motor offer a longer life
when considering that the bearings are essentially the only part that will wear-out.
And Stepper Motors offer excellent low speed torque,
which means, the motor will drive many loads
without having to utilize any additional gearing or gearbox mechanisms.
As we discussed earlier, when commanded to do so,
the inherent capabilities of the Stepper Motor allow the shaft of the motor
to return to the same location accurately
offering excellent repeatability.
Steppers provide precise positioning and repeatability of movement
since good stepper motors have an accuracy of 3 – 5% of a step
and this error is non-cumulative from one step to the next.
The Stepper Motor is essentially overload safe.
The motor cannot be damaged by mechanical overload,
but positional loss may occur effecting accuracy and repeatability
when the load increases beyond design.
This is why Stepper Motors are better suited for constant load applications.
Another advantage is that the rotation angle of the Stepper Motor
is proportional to the input pulse.
Once again we learned in the previous lesson,
what is A Stepper Motor and How it Works,
that moving to an accurate position is simply
a matter of sending the correct number of pulse commands.
When considering from that lesson,
that Stepper motors may have up to 200 rotor teeth,
or 200-400 full steps per revolution of the motor shaft.
And so as mentioned in our earlier lesson,
to determine the resolution of rotation we can perform a little math.
If a stepper motor has 200 incremental steps
and we know a full rotation is equal to a circle or 360-degrees
then we can divide 360-degrees by 200.
This equates to 1.8-degrees of a full step angle rotation
and 400 steps divided into 360-degrees
provides 0.9-degrees of full step angle rotation.
Outputting one digital pulse from the controller driver
is equivalent to one step of rotation.
One of the many Stepper Motor advantages
is a feature of providing full torque at standstill while the windings are energized
and the rotor is stationary or in other words a benefit we call holding torque,
which means the motor can hold the load in place when the rotor is not rotating.
But a stepper motor can also hold a load in place
when there is no current applied to the windings or in a power-off condition.
This is commonly known as the detent torque or residual torque.
In the lesson, what is a Servo Motor and How it works,
we discussed that some motors use mechanical brushes
to deliver current to the motor.
Stepper Motors are very reliable
since there are no contact brushes in the motor.
Therefore, the life of the motor is simply dependent
on the life of the shaft bearing.
With Steppers it is possible to achieve very low speed synchronous rotation
with a load that is directly coupled to the shaft
and a wide range of rotational speeds can be realized
as the speed is proportional to the frequency of the input pulses.
Normal DC motors do not have very much torque at lower speeds
however, a Stepper Motor has maximum torque at low speeds,
so this makes Steppers a good choice for applications
requiring low speed with high precision.
Stepper Motors provide ruggedness, high reliability,
simplicity of construction, allow for low maintenance,
offer excellent response to starting/stopping/reversing
and will work in many environments.
Top Stepper Disadvantages:
Some of the top Stepper Motor disadvantages
are in that the motors provide a low efficiency.
The Stepper Motor draws substantial power
(remembering holding torque) regardless of load.
The torque of a Stepper Motor declines rapidly with speed
as torque is the inverse of speed as defined in the previous graph.
When compared to Servo Motors,
Stepper Motor are considered to have low accuracy.
However, there are three methods to improve the accuracy of a stepper motor:
One is to use a stepper motor with more teeth.
Another is to use gears to reduce the amount of movement per step
or by increasing the lead screw number of threads per inch
and the last is to use micro stepping.
Micro stepping allows the motor to move more smoothly
and decreases being prone to resonances.
As we discussed earlier, one of the advantages in using a Stepper Motor
is it will operate in Open Loop Control,
but this two under varying load scenarios become a disadvantage when no feedback
to the control system is used to indicate potential missed steps.
And while speaking of Open Loop systems,
a drawback of open-loop control using steppers
is that if the machine load is too high,
or the motor attempts to move too quickly
then positional steps may be skipped.
The controller has no means of detecting this
and so the machine continues to run
slightly out of adjustment until the position is reset.
For this reason, more complex robots and machine tools
will utilize Servo Motors that incorporates and closed-loop controllers.
Stepper Motors have low torque to inertia ratio,
which means they cannot accelerate loads very rapidly
and the motor can get very hot in high performance configurations.
Stepper Motor can be audibly very noisy at moderate to high speeds
and have low output power for size and weight.
And finally, detent or holding torque
has to be overcome in order for the motor to move,
it reduces the ideal torque that the motor can produce when it’s running.
Overcoming the detent torque requires more power from the motor,
and the amount of power needed is proportional to speed.
So, the faster the motor turns,
the greater the effect that detent torque
will have on the motor’s actual torque output.
This concludes the video, Stepper Motor Advantages and Disadvantages.
I hope you have learned what’s required to move forward
in creating your own motion control project.
Please stay tuned to RealPars for more Motion Control lessons.
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