Stepper Motors Advantages and DisadvantagesIn this lesson we will discuss the Stepper Motor advantages and disadvantages compared to Servo Motors.
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? 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 (millimeter) ±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.
2. Greater Torque
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.
3. Cheaper than Servo Motors
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.
4. Stepper Motors are Considered to be Safer
Stepper Motors are considered to be safer, where as if anything breaks, the Stepper Motor will stop.
5. Stepper Motor Offer a Longer Life
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.
6. Excellent Low Speed Torque
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.
7. Excellent Repeatability
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.
8. Stepper Motors are Overload Safe
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.
9. The Rotation Angle of the Stepper Motor is Proportional to the Input Pulse.
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.
10. Stepper Motors Provide Full Torque at Standstill
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.
11. Very Reliable
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.
12. Low-speed Synchronous Rotation
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.
13. Good Choice for Applications Requiring Low Speed with High Precision.
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
1. Low Efficiency
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.
2. The Torque of a Stepper Motor Declines Rapidly with Speed
The torque of a Stepper Motor declines rapidly with speed as torque is the inverse of speed as defined in the previous graph.
3. Low Accuracy
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.
4. No Feedback is Used to Indicate Potential Missed Steps
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 incorporate encoders and closed-loop controllers.
5. Low Torque to Inertia Ratio
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.
6. Stepper Motor can be very Noisy
Stepper Motor can be audibly very noisy at moderate to high speeds and have low output power for size and weight.
7. Holding Torque has to be Overcome
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.
We at RealPars hope that you found it interesting, and that you will come back for more of our educational blogs.
With so much love and excitement,
The RealPars Team
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