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. 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
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of new RealPars videos. This way you never miss another one! 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. Make sure that you head
over to realpars.com. To find even more training material
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