Not all servo motor types are the same, as they come in a wide range of shapes and sizes, with each type being suited to different applications.
Well, perhaps the range of shapes is not so wide, as they all tend to be approximately cylindrical or rectangular in shape, depending on if they are rotary or linear.
But servo motors definitely come in a huge variety of sizes and types, and more importantly, they differ in how they operate from an electromechanical point of view.
So, what type of servo motor is best suited for your particular use case? Read on for more information.
What Is A Servo Motor?
Regular electric motors (such as AC motors found in a desktop fan) operate by energizing a set of coils located in a stator, which produce a magnetic field that interacts with a rotor, producing rotary motion in a shaft.
In terms of control, they are not particularly smart devices, and are generally open-loop, with the most basic of controls (such as a potentiometer) for speed adjustment.
Generally speaking, the average electric motor of this type has no capacity for position control, which is required in advanced motion control systems.
If you want a high level of controllability in an electric motor, you will be wanting to invest in a servo motor of some type.
While the principles of how the motion in a servo motor is generated remain largely the same as a regular electric motor, the addition of feedback systems allow for the system to monitor its outputs in terms of speed, torque, and position.
Feedback is achieved by means of a sensor (such as an encoder or resolver) within the motor, which detects the state of the hardware, and sends a signal back to a servo drive or controller, which checks that the measured output is matching the commanded value. The controller then processes the signal, corrects for any deviation (error), and then adjusts the motor’s operation accordingly.
The correction is performed using control algorithms such as PID (Proportional-Integral-Derivative) control, which allows for stable and precise operation of the motor (and whatever the motor is driving).
This is referred to as a closed loop control system, and this is the crux of what differs a servo motor from a traditional motor with open loop control.
What Are The Different Servo Motor Types?
Servo motors can broadly be divided into two categories based on their motion. These are linear servo motors and rotary servo motors. Linear and rotary servo motors come in many flavors.
Specifically, these flavors can further be subdivided according to current type (DC or AC), commutation type (brushed or brushless), or by control type (stepper or induction).
Let’s take a look at what defines these types of motor in more detail.
Linear Servo Motors
Linear servo motors, as the name suggests, provide a translational motion (back and forth or side to side) without the need for additional hardware, such as lead screws, gears or belts.
They achieve this motion thanks to a component referred to as a “primary” (also known as a “forcer” or “stator”) containing coil windings aligned on a straight path, which magnetize when current is applied.
The magnetized coils then interact with an array of permanent magnets located in the “secondary” (referred to as a “track”, or “platen”), producing a Lorentz force and thus driving the forcer along the track. This is the linear motion.
Characteristics:
- Linear servo motors are highly precise, capable of positional accuracy down to sub-micron levels.
- Thanks to their direct drive operations, they are highly efficient with minimal energy losses.
- They are capable of rapid response in terms of acceleration and deceleration, making them ideal for dynamic applications.
Advantages:
- Linear servo motors are capable of direct linear motion, require no additional hardware. This makes them lightweight, and requiring minimal maintenance.
- They are smooth and silent, thanks to the lack of contact between moving parts. This also means that there is no backlash, as you would find in a lead screw-based setup.
- Linear servo motors have a high degree of scalability, meaning they can be designed to provide motion over long and short distances.
Applications:
- Pick and place systems, coordinate measuring machines, optical systems, laboratory automation.
Rotary Servo Motors
The majority of servo motors employed in commercial and industrial applications are rotary.
They are available in both AC and DC varieties, and generally make use of an encoder for providing feedback to the controller / drive. The motion is created by use of a stator and rotor system.
Rotary servo motors can also be used as part of linear systems, by use of lead screws or balls screws, to convert rotary motion to linear motion.
But for the purpose of this article, we will consider linear servo motors in terms of stand-alone units that create linear motion without the need for additional hardware.
Characteristics:
- Rotary servo motors are notable for their high torque-to-inertia ratio, which enables fast acceleration and deceleration.
- They have high efficiency, which aids in reducing heat, and energy losses.
- Rotary servo motors have excellent dynamic response, enabling them to deal with rapid changes in speed under varying load conditions.
Advantages:
- Rotary servo motors are highly versatile, and can be used in a multitude of applications. With additional hardware (lead/ball screws), they can also be converted to provide linear motion.
- They are highly adaptable, and are well suited for both heavy duty, and light duty applications.
- This type of motor is capable of incredibly precise operations, allowing position control down to sub-degree, and even arc-second levels of precision.
Applications:
- Rotary servo motors are commonly found in aerospace, industrial automation, semiconductor manufacturing, medical devices, CNC machines, 3D printers, and more.
AC Servo Motors
Servo motors can further be categorized in terms of the type of current they use (AC or DC).
For industrial applications, AC servo motors are the most common, due to the wide range of voltages and power levels they can handle.
AC servo motors are available in both linear and rotary types.
Low to Medium Voltage AC Servo Motors
Just as there are high voltage servo motors, there are also “low” voltage variants which for the benefit of this article, we will class as an AC servo motor with a voltage of less than 480v. Yes, by low voltage, we mean “relatively” low. Naturally, a 480 volt electrical shock would be quite harmful to the human body, but in terms of industrial machines, it’s quite low.
These servo motors come in small form factors, and are ideal for applications requiring a smaller footprint. Despite their diminutive size, they are still capable of providing reasonable amounts of power in the kW range.
Generally, they come in 3-phase varieties, offer a high torque density for the size, and are sturdy, reliable, and offer a high degree of precision and efficiency.
Characteristics:
- Lower voltages available, ranging from 12v up to 480v.
- They are available in a smaller volume, for applications with space constraints.
- Low voltage servo motors can provide power in excess of 20kW.
Advantages:
- They offer a good balance of performance and affordability when compared with higher voltage systems.
- Due to the lower voltage, there is a lower operational cost that is associated with higher voltage servo motors, thanks to reduced electrical infrastructure needs (transformers, etc.)
- There is a reduced risk of high voltage hazards (such as arcs and equipment failure) when dealing with lower voltage servo motors.
Applications:
- Industrial robots, cobots, material handling systems, packaging machines, printing presses
High Voltage AC Servo Motors
Higher voltage AC servo motors typically operate in the range of 300V to 480V and are well-suited to industrial applications such as CNC machining, where high forces are required.
These motors, along with specialized high-voltage servo drives, can deliver high power output (in the kW range) without requiring a significant increase in motor size, making them ideal for compact, high-performance machinery.
Characteristics:
- High voltage AC servo motors tend to be very rugged and durable, which are required in heavy-duty industrial environments.
- Higher voltage systems often come equipped with active cooling systems, such as liquid-cooling systems or fans.
- They may require higher upfront and operational costs due to the need for advanced electrical infrastructure and power electronics requirements.
Advantages:
- Operating at higher voltages means lower currents are required for the same power output. This is especially beneficial to reduce losses
- Lower current requirements result in reduced heat generation. Lower guage wires can be used, reducing costs.
- High voltage AC servos are capable of producing extremely high torque.
Applications:
- Heavy-duty robotics, aerospace actuators, industrial CNC, textile machinery, wind turbine pitch control, solar tracking systems`
Synchronous AC Servo Motor
The majority of AC servo motors are of this type, meaning that the rotor turns at the exact same speed as the stator’s rotating magnetic field (RMF), meaning there is no lag (known as “slip”) between the RMF and the rotor. They are synchronous (hence the clever name).
- Characteristics: Synchronous AC servo motors are known for their high torque density, and consistent speed.
- Due to the lack of slip, they have excellent positioning accuracy using regular control means.
- Advantages: Efficiency, reliability under varying loads, and precise speed control.
- Applications: Widely used in robotics, packaging machines, and industrial automation requiring consistent performance.
Asynchronous AC Servo Motors
Most regular AC servo motors are synchronous, as discussed in the previous section. They also tend to be either permanent magnet based, or brushless DC (BLDC) types.
However, asynchronous AC servo motors are available, and are essentially induction motors with control capabilities added to them. Being asynchronous, they experience a difference in speed between the stator’s synchronous speed and the rotor speed. This difference is usually represented as a percentage, and is referred to as “slip”
Induction motors are a subset of AC motors, that use induction generated from induced currents in the rotor. This is opposed to permanent magnet motors, which generate torque from the magnets and the stator’s electromagnetic field.
In order to achieve the same levels of precision as traditional AC servo motors, asynchronous AC servo motors must be capable of complex vector control, such as field oriented control (FOC), in combination with encoders or resolvers for feedback. This extra control allows for dynamic control of the asynchronous AC motor’s slip.
Characteristics:
- Asynchronous AC servo motors might feature a squirrel cage, or wound rotor with slip rings.
- They generally require FOC control to achieve any kind of precision.
Advantages:
- They are not prone to demagnetization, unlike permanent magnet motors.
- Asynchronous AC servo motors can be cheaper to manufacture compared to permanent magnet motors.
Applications:
- Conveyor systems, pumps, material handling, elevators.
DC Servo Motors
DC servo motors generally operate on the same principles as AC servo motors, albeit they use direct current rather than alternating current to power them. They are typically lighter and smaller than their beefier AC siblings, and are generally speaking, of lower cost as well.
Despite the low(er) cost, they are still very capable in terms of efficiency and precision, and have low inertia, allowing for rapid directional changes and changes in speed. They are inherently bidirectional, allowing shaft rotation in both clockwise and counterclockwise directions, with equal efficiency.
Characteristics:
- DC servo motors are highly efficient with low heat losses.
- They can be powered by batteries, such as Li-poly or lead-acid batteries.
- They are capable of efficient bidirectional rotation.
Advantages:
- DC servo motors can operate efficiently over a wide range of speeds making them well-suited for variable speed applications.
- They have less complexity when compared to AC servo motors, and this is reflected in the lower cost.
- DC servo motors are ideal for low power operations.
Applications:
- Camera systems, gimbals, electric vehicles, prosthetics, optical media players (DVD / Blu-ray, etc)
Brush Servo Motors
DC servo motors come in brushed and brushless varieties, which differ in how they handle commutation. Commutation is the process of switching current direction in the motor windings to maintain continuous rotation and torque generation.
Brushed DC servo motors use mechanical commutation, where brushes physically contact a commutator (a segmented ring attached to the rotor). As the rotor spins, the commutator segments sequentially switch the current flow in the armature windings, ensuring that the magnetic field interaction continues producing torque in the correct direction.
In a servo system, feedback mechanisms such as encoders or potentiometers enable precise control of speed, torque, and position, distinguishing DC servo motors from regular DC motors.
Characteristics:
- Brush DC servo motors have a simple design, and a relatively lower cost than AC servo motors.
- Brushed DC motors deliver high starting torque since armature current is maximized without back EMF, overcoming inertia effectively.
- Brushes are seen as consumable, and will need replacing periodically.
Advantages:
- Brush DC motors are a cost-effective solution for low-speed, high-torque applications.
- Speed and torque can be easily controlled by adjusting the applied voltage or current.
Applications:
- Robotics, conveyor belts, small industrial equipment requiring precision.
Brushless Servo Motors
Brushless DC servo motors eliminate mechanical brushes and commutators, using electronic commutation to control the current in the stator windings. A feedback mechanism, (typically a Hall-effect sensor or encoder), monitors rotor position to synchronize the stator’s magnetic field with the rotor’s motion.
The torque is generated by the interaction between the stator’s electromagnetic field and the rotor’s permanent magnets, allowing for precise and efficient control of speed and position.
As a side-note, most AC servo motors are also “brushless” by definition.
Characteristics:
- High efficiency, low maintenance, reduced noise, and compact size.
Advantages:
- Long operational life, higher speed range, and better reliability.
Applications:
- Ideal for CNC machines, drones, and medical devices requiring high-performance motion control.
Stepper Servo MotorRegular stepper motors are brushless, synchronous DC motors that move in discrete steps, allowing precise position control without needing a feedback system. However, since they operate in an open-loop system, they cannot detect errors if they overshoot or miss steps.
Stepper servo motors, on the other hand, integrate stepwise motion with a feedback mechanism (typically an encoder), enabling closed-loop control for higher accuracy, improved torque efficiency, and correction of missed steps.
Stepper servo motors, therefore, are basically stepper motors, but better!
Characteristics:
- Stepper servo motors have closed-loop control by definition.
- They are brushless DC motors deep down, although they exhibit some AC characteristics (they are synchronous, for example).
- Some models use smoother sinusoidal control for quieter, smoother motion (also similar to AC motors).
Advantages:
- Stepper servo motors can maintain torque at higher speeds and reduce resonance, enabling better Speed and torque performance compared to a regular stepper.
- They can also maintain torque at higher speeds, unlike regular stepper motors.
- The feedback allows for real-time corrections, improving accuracy.
Applications:
- Industrial automation, CNC machines, 3D printers, robotics, aerospace, satellite systems.
Conclusion
In this article, we have looked at how servo motors can be categorized in broad categories, such as by input current (AC or DC), motion type (rotary or linear), voltage (high or low) and the type of commutation (in the case of DC servo motors).
Of course, there are further sub-categories of specialized servo motor than we can divide these into even further, but this is for another article.
Understanding the distinctions between various types, even at this high level, can ensure optimal selection for specific use cases.
Naturally, knowing what type of servo motor is best suited for your needs is only one part of the equation. Knowing where to procure reliable motion hardware from is equally important. Choosing a trusted brand can help to reduce downtime and long-term costs, particularly in critical operations. Well, don;t worry, because we have you covered there also.
Top brands like ABB, Ametek, Allied Motion, and others consistently deliver high-quality rotary servo motors, ensuring reliable performance across various industries.
And if linear motion is your thing, then you can check out companies such as Geeplus, H2W Technologies, Tolomatic, or HyperCyl for more information.
All of these companies manufacture hardware that has proven compatible with AMC servo drives. For a more exhaustive list of 3rd parties manufacturing compatible servo motors, head on over to this link.
