Fundamental Analysis of DC Stepper VS Servo Motors
Are you stuck in a design rut and unsure which type of motor is best for your application? For example, do you ever feel like your robot lacks a certain aspect, say, power? Will you use a DC motor, stepper, or servo for your project?
Have you ever heard of motors? Do you know how manufacturers make motors or their operating principles? We often use motors in our daily activities. Motors run many machines we encounter and use while performing our day-to-day operations. We will break down the major differences of DC vs stepper vs servo motors. Moreover, we also look at the construction, the structure, and why a user would consider any of them. In addition, we will look at how each works and how we can apply each in real life.
What is a motor?
We can define a motor as a machine that converts other forms of energy into mechanical energy and initiates motion as a result. We have different types of motors. However, we will look at three types; DC, stepper, and servo motor.
What is a DC motor?
A direct current (DC) motor is an electric machine that takes direct current and converts it into a mechanical rotation. Earlier on, people all over the world used DC motors since they would easily power them using direct current from lighting power distribution systems.
The DC motor is the most common type of electric motors. They are useful in many applications, such as cars, trains, and other vehicles. DC motors can be present in both AC and DC electric circuits.
DC motors have been around for a long time, and they are still very popular today. They are present in many different types of devices, such as appliances, drills, vacuum cleaners, small devices, and even toys.
Structure of a DC motor
The figure below shows the structure of a DC motor.
The DC motor comprises an iron cylinder, a pair of brushes, a wire loop, a DC supply, and a commutator.
Classifications of DC motors
We classify DC motors as either brushed or brushless. We can also refer to the brushless DC motors as synchronous DC motors or electronically commutated motors. Brushless DC motors do not have a commutator. Instead, they have a servomechanism that detects signals and adjusts according to their meaning. The angle of the rotor changes depending on what you specify for the motor to do. The brushless DC motor will last longer since it does not have soft contact brushes like those in the brushed DC motors. Therefore, the brushless motor will not experience wear due to contact friction. As a result, we can term it a classically designed DC motor.
The operation of a brushless DC motor
The similarity in all-electric motors is that they develop torque through an alternating pattern of the polarity of a moving Magnet, the rotor, and a stationary magnet, the stator. One of these magnets must be an electromagnet. We can easily construct by winding a wire around a ferromagnetic material, say, an iron core. We then supply the motor with a direct current and create a magnetic field in the process. The direction of the north and south poles changes each time the armature rotates through an angle of 180 degrees. If we keep the magnetic field of the poles constant, the rotor will not rotate. To ensure a unidirectional torque, we must design the motor such that the direction of the current changes every time the armature rotates through 180 degrees.
Construction of a DC motor
In constructing a DC motor, the pivotal step is assembling the rotor. The rotor is the inner rotating
magnetic part of a motor. To achieve this, we need to use the commutator that converts the external applied DC into the alternating current that we need for the rotor. Alternatively, we can use the word armature to refer to the rotor of the DC motor. Finally, we use brushes to establish contact between the commutator and the external DC supply.
When constructing the commutator, we partition it into four parts. First, we now connect two loops, with an angle of 90º between them, each to two opposite commutator segments. We then connect two brushes to the DC supply. Consequently, one brush will have positive polarity, while the other will have negative. Finally, we now set the two brushes so that at any given time, they are in contact with two opposite commutator segments that are connected via a common loop.
How does the DC motor operate?
Applying the current will flow in the direction from the positive terminal to the negative terminal. As a result, we will have created an electromagnet with one loop. On turning the rotor, say clockwise, we contact the other two segments of the commutator and the brushes. Consequently, current will flow from the positive brush, through the loop, towards the negative brush. There is creation of an electromagnet in the process. The rotor will rotate clockwise due to the alternating contact between the brushes and the commutator segments. The contact phenomenon repeats after every 90º rotation.
We should note that the electromagnet’s north and south poles remain unmoved during rotation. Therefore, we can liken this to a stationary electromagnet, rotating in the opposite direction but with the same speed as the rotor. We can illustrate this using a diagram as shown below.
Additionally, if we increase the number of loops in this setup and partitions of the commutator, we will have lowered the rotation angle for each commutation.
How does the motor convert electrical energy to mechanical energy?
What will happen if we place the rotor near a permanent magnet? First, we will see that there will be an attraction between poles with different polarities. As a result, the rotor will start turning. Consequently, there will be a commutation after the rotor turns by a given angle. As a result, the electromagnet’s north and south poles will return. Next, there will be a force of attraction between poles with opposite polarity. Therefore, the motion of the rotor continues to align the poles with opposite polarity.
Commutation will continue as the rotation continues. A continuous rotation will occur since the resultant force always acts in the same direction. As a result, the motor converts electrical energy to mechanical energy. The nature of rotation, either clockwise or anticlockwise, will depend on the voltage polarity of the rotor’s brushes.
A DC motor as a generator
What do we do to generate current from our DC motor? During rotation, we need to cut the lines of force with a conductor. Consequently, we will have induced EMF in the conductor. Using Fleming’s left-hand rule, the induced EMF will generate a current flowing in the opposite direction. Therefore, we refer to the induced EMF that results from motor rotation as counterelectromotive force (CEMF). Additionally, the magnetic field’s armature speed and strength will determine the magnitude of the induced EMF.
Types of DC motors
There are three types of DC motors. These are; the shunt-wound, series-wound, and compound motors. First, let us briefly look at each motor.
In a shunt-wound motor, the field lines are parallel to the armature. Additionally, its torque decreases with an increase in speed. Therefore, its speed regulation is very good. For this reason, we term it as a constant speed motor. However, when you increase the load, its speed decreases.
The field lines are in series with the armature in a series-wound motor. Moreover, when you increase its speed, the torque increases rapidly. Also, if you remove the load from the motor, the speed will sharply increase. In addition, we can operate a series-wound motor at a low speed and develop a large torque. For this reason, the motor can easily start heavy loads.
The compounded DC motor has both a series and a shunt field.
Applications of DC motors
Here are some of these applications:
- Turning the propeller for electric ceiling fans. If we compare the cost of installing and maintaining a DC fan to that of an equivalent AC fan, the DC fan is more economical.
- Construction of electric cars due to durability and energy efficiency. In addition, series wound DC motors have a higher starting torque, and their speeds vary proportionally to the Input voltage.
- Almost all industries employ hydraulic pumps in their daily operation. We power the pumps using DC motors in steel, manufacturing, construction, and mining industries. Their easily variable speed control and great response during rotation make them best for such applications.
- Additionally, in the manufacture of robotics that we design to perform specific tasks, we apply the DC motors to activate such features as arms, cameras, or tracks. Due to the high torque and efficiency, hobbyists and engineers prefer DC motors to AC motors.
- When constructing a toy car, manufacturers use small DC motors that work very well with models such as remote-controlled cars and toy trains. Various voltages will help us achieve different speeds and natures of movement of these toy cars. We can also connect our motor to a controller and then program it to perform a task.
- Many people go for electric bikes since they do not require a license for speeds under 20 miles per hour. When constructing these bikes, we use brushless DC motors to achieve the power levels and torque that we need. We can mount the DC motor to the hub of any of its wheels or at the bike’s center. Next, we must connect the motor to the pedal sprocket to power the bike.
Stepper motors are much more expensive than DC motors, but they have fewer drawbacks. It is possible to control their speed in increments, making them ideal for applications that require precise movements, such as 3D printers or CNC machines.
What is a stepper motor?
We can define a stepper motor as an electromechanical system that transduces an electrical signal to a mechanical signal. Alternatively, we can define a stepper motor as a brushless electric motor that partitions a rotation into equal portions. The stepper motor is an electric motor that rotates in fixed increments or steps when powered by electricity or some other energy source. In technical literature, we can refer to the stepper motor as a step or stepping motor.
Stepper motors have a gearbox with several stages or steps that a microprocessor can control to provide precise control over the motor’s angle and speed. Stepper motors are commonly present in automation applications where the control must be precise and accurate. For example, they are in wide use in computer printers and scanners because they can move the print head or scanner’s scanning element with great precision at high speeds.
Structure of a stepper motor
The figure below shows a stepper motor:
The stepper motor consists of stationary coils (stators) that produce a magnetic field once we supply them with the current. In addition, it has a rotating part (rotor) made of magnets or ferromagnetic materials. Therefore, we will have a varying magnetic field when we supply the stator with impulsive energy. Moreover, if we place a permanent magnet in the varying magnetic field, we achieve equilibrium and initiate motion as a result.
We have three different structures of a stepper motor. These are:
- Permanent magnet design
- Hybrid design
- Variable reluctance design
Permanent magnet design (PM)
To construct the rotor, we position permanent magnets to obtain alternate North and South poles. Consequently, the permanent magnets interact with the resulting stator’s varying magnetic field. To achieve rotation, we apply energy alternatively to the stator coils and then attract any of the rotor’s poles to the magnetized stator. If we apply no current, we will need a small torque to displace the rotor from equilibrium since there will be a strong interaction between the stator and the permanent magnets. In technical literature, we can refer to this torque as the residual torque, the cogging torque, or the detent torque. The permanent magnet design is the most suitable design for tiny stepper motors, with a diameter of below 20mm.
Additionally, they provide a large torque. The resolution of these motors is normally under control to 20 or 24 steps for every revolution. The precise stepper motor is an example of a permanent magnet design.
Variable reluctance design
To construct a motor with the variable reluctance design, we need to have a ferromagnetic rotor with several teeth, e.g., an iron rotor. We use this to achieve attraction to the stator poles. We then cyclically apply energy to the stator coils. As a result, the stator poles will attract the rotor’s teeth. Consequently, these teeth will align with the magnetized stator poles and initiate a rotation. The number of steps in every resolution will be higher, and we will achieve higher resolutions as a result. However, the variable reluctance stepper motors will produce a small dynamic torque. As a result, we cannot use motors with this design for tiny motors.
Additionally, since the rotor has no permanent magnets, we can use motors with variable reluctance in strong magnetic fields. For example, we can use them in MRI devices. However, besides their low torque, motors with the variable reluctance design do not maintain their position once there is no current flowing in them. As a result, we can say that they have no residual torque.
The hybrid design
We aim at combining the rotation of the stator poles and the attraction of the stator’s teeth during alignment. To achieve this, we should have a multiple teethed rotor made of a permanent magnet and a stator with multi-toothed poles. We construct the hybrid design stepper motors by combining the high torque of the permanent magnet stepper motors and the high resolution of the variable reluctance design. Additionally, these motors have a resolution of 200 steps in every revolution. Moreover, the smallest hybrid stepper motor has a diameter of 19mm.
Unique features of stepper motors
Do you know that a stepper motor consumes current even when at rest? However, they cannot convert this current to motion. Consequently, the motor heats to a temperature that is 100ºC hotter than that of the surrounding. Due to this reason, manufacturers advise the users of stepper motors to practice temperature monitoring.
A stepper motor operates irregularly with different speeds. Due to this reason, we can conclude that they can operate at their resonance frequency. Additionally, this type of operation is dangerous since it can make the motor stop. We can differentiate between low, medium, and higher frequency resonances. If the frequency range is approximately 250 Hz, the resulting resonance is a low-frequency type.
Additionally, medium and high-frequency resonance results from electrical parameters such as capacitance in the supply lines and inductance in the motor’s windings. However, these parameters have no tangible effect on the torque. We can control them easily by appropriate timing. The motor’s mechanical parameters cause the lower frequency resonance. Besides the irregular motion, there is a substantial decrease in torque. Consequently, this interferes with the functioning of the motor.
A stepper motor is a typical oscillating system, like the spring/mass system. The rotor moves with a moment of inertia, with the magnetic field acting as the restoring force acting on the rotor. Although the movement and release of the rotor result in a damped oscillation, every current pulse results in a transient phenomenon about the position of the rotor. Therefore, we superimpose the transient phenomenon with a subsequent pulse by increasing the frequency. As a result, we get a smooth speed curve. Furthermore, we amplify the oscillation if the control frequency rhymes with the resonance frequency. Consequently, the rotor moves randomly, off the steps, and oscillates between two positions.
Advantages of stepper motors
- Even at low speed, stepper motors maintain high holding torque.
- Stepper motors are easy to control.
- Stepper motors do not need an encoder for basic positioning tasks within performance limits. Therefore, this makes them cost-effective for simple positioning tasks.
Applications of stepper motors
The permanent magnet stepper motors are good in building automation. Additionally, they are a simple drive solution that requires low power to operate.
Due to their good performance, hybrid motors greatly apply in the manufacture of robotics. Moreover, the construction of printers and scanners depends on stepper motors.
Servo Motors are the most expensive type of motor, but they provide the highest precision and power of all three types. As a result, they can be useful for any application where high precision is necessary, such as robotics or machine tools.
What is a servo motor?
We can define a servo motor as an electrical device that converts electrical energy into mechanical energy with servomechanism. In addition, we can classify servo motor based on the nature of the controlled motor. For example, if it is a DC motor, we refer to it as a DC servo motor. On the other hand, if an alternating current (AC) drives the controlled motor, we refer to the motor as an AC servo motor.
Alternatively, we can define a servo motor as a linear or rotary actuator that facilitates the control of the linear or angular position. We achieve this by coupling a motor to a sensor and reading the position feedback. We also require a classy controller whose design is for application in servo motors.
Servo Motors are useful for controlling rotational positions with accuracy. They work by sending pulses to the motor, which then moves according to the sent pulses at what frequency, direction, and duration of each pulse.
Here is the structure of a servo motor:
The theory of a servo motor
We may encounter some electric motors that specify rotation angles during their application in our daily activities. We, therefore, require a special motor that we can program to rotate a certain angle once we feed a certain electrical input. With the aid of an additional servomechanism, we can control a DC servo motor to make a specific angular displacement. In this case, the servomechanism will be a closed-loop feedback control system.
Applications of servo motors
Nowadays, we apply servo motors in various industrial activities. Here are some of these applications:
- A good example is the toy car. We can control the direction of motion of the car using a remote-control system. In this case, the car rotates as we direct it and stop after the commanded rotation is over.
- We also apply servo motors in the movement of the CD or DVD player. When we instruct it to open the tray, the motor rotates and stops when the tray is out. On instructing it to close the tray, the servo motor will make a rotation so that the tray goes in. Additionally, the motor stops accomplishing what we have instructed it to do.
Why would we prefer a servo motor to other types of motors?
A servo motor rotates to our precision and stops when the stated rotation is over. Moreover, it waits for the next signal before it takes further action. Unlike other electric motors that start immediately when we have supplied it with power and stop when we cut the power supply, the servo motor waits for a signal after which it starts. Therefore, we cannot control the rotation of a normal motor. Due to this factor, we can prefer the servo motor for rotations with high precision. Also, we can easily control the speed of a servo motor, which is not the case for the other motors. Since servo motors are easy to operate, we can often include them while designing Arduino starter kits for beginners.
It is possible to understand DC vs Stepper vs Servo Motors from the above details. DC motors are an inexpensive and uncomplicated way to power a variety of projects that need rotation but don’t require precise positioning or speed control. They’re also great for basic motion applications in robotics, like scooting around autonomously. The downside of these motors is that they can’t provide fine control or strong torque.
Stepper motors overcome some of the limitations of DC motors but require more specialized electronics. They are useful in both rotational and linear motion applications, with a range of resolutions and torque options. These are great for more complex robotics projects like 3d printers and CNC mills. However, stepper motors don’t have as much momentum as DC ones and are less efficient because there is powering of only one phase at a time.
Servo motors are position memory and feedback devices useful for anything from robotics to Nerf guns. They’re great for fine-tuned motion applications like computer mice or RC cars. Unlike DC motors, servos are geared to provide a lot of torque, but they’re usually less accurate than stepper motors.
If you want precise control, the stepper motor is the best option. The DC motor is your best bet if you don’t mind sacrificing some accuracy. Both of these are great for basic motion applications like wheels.
Servos are the way to go if you need to fine-tune a complex machine.