Introduction to Motors
Motors are electro-mechanical devices that convert electrical energy into mechanical energy. They come in a variety of types depending on their construction, working principle, and applications. The two most common types of motors used in industrial and automation applications are servo motors and stepper motors.
Both servo and stepper motors provide precise motion control, but they have some key differences in their design and operation. This article will compare servo and stepper motors in detail in terms of their construction, working principle, performance characteristics, applications, advantages and disadvantages.
Overview of Servo Motors
A servo motor is a rotary or linear actuator that allows for precise control of angular or linear position, velocity and acceleration. It consists of a motor coupled to a sensor for position feedback. It also requires a relatively sophisticated controller, often a dedicated module designed specifically for use with servo motors.
Servo Motor
The key components of a servo motor are:
- Motor – Provides the motive force for the actuator. Common motor types used in servo systems include DC brushed, brushless DC, AC induction, and AC synchronous motors.
- Gearbox – Used to increase the torque output from the motor. Often equipped with high ratio gears to allow slower motor speeds but higher torque.
- Encoder – Sensor that provides position and speed feedback to the controller. Resolvers and optical encoders are commonly used.
- Controller – Compares the actual position to the desired position and calculates the required drive signals for the motor. Uses PID control algorithms.
- Potentiometer – Measures position of output shaft and provides position feedback to controller.
The controller commands the motor to rotate to the desired position as fast as possible. Once the motor reaches the desired position, it stops. If an external force pushes against the motor, it will hold the position due to the servo control system.
Servo motors provide closed-loop control with high accuracy and fast response times. As a result, they are ideal for applications requiring precise position control such as robotics, CNC machines, pick-and-place equipment, and other factory automation machinery.
Overview of Stepper Motors
A stepper motor is a type of brushless DC electric motor that divides a full rotation into a number of equal steps. Unlike servo motors which rotate continuously, stepper motors move in discrete increments or steps.
Stepper Motor
The key components of a stepper motor are:
- Stator – The stationary part consisting of stacked steel laminations with winding coils around the poles.
- Rotor – The output shaft along with a magnetized part that interacts with the magnetic field from the stator coils.
- Driver – The driver energizes the motor coils in the proper sequence to cause rotation. Open-loop control.
Stepper motors operate on the principle of electromagnetism. The motor coils are energized in a specific sequence, creating magnetic poles that attract and repel the permanent magnets on the rotor. This causes the rotor to turn a precise angle or “step” each time a coil set is energized.
By controlling the sequence and coils energized, the motor can be turned in precise increments. No closed loop feedback is required, though often an encoder can provide position verification.
Stepper motors provide excellent low speed torque and position control without requiring encoders or servo control. They are commonly used in 3D printers, CNC machines, plotters, and other applications requiring high precision motion.
Construction and Design
Both servo and stepper motors have unique construction characteristics optimized for their particular applications and operating principles.
Servo Motor Construction
The construction and design of a servo motor can vary greatly depending on the motor power, torque, speed, and accuracy requirements. But in general servo motors consist of the following components:
- Stator – Usually made up of laminated steel with winding coils around the stator poles. The number of pole pairs determines the number of magnetic poles.
- Rotor – Made of permanent magnets alternating in polarity around a rotor shaft or core. Often high energy rare earth magnets are used.
- Shaft – Heavy duty output shaft with high stiffness for minimal deflection under load. Supported by precision ball bearings.
- Housing – Cast iron or steel housing provides structural support and protects internal components. Often thermally conductive to aid cooling.
- Feedback device – Resolvers, encoders or potentiometers are attached to monitor rotor position.
- Gearbox – High ratio gearboxes are often integrated to increase torque capacity. Spur, planetary, and harmonic gears are commonly used.
The stator, rotor, bearings, and housing make up the actual motor section. The feedback device and gearbox augment the motor capabilities for high performance servo applications.
Stepper Motor Construction
Stepper motors have a relatively simple electromechanical construction optimized for open-loop positioning control. The basic components include:
- Stator – Made up of stacked steel laminations with winding coils around the poles. The number of phases determines the number of coil sets.
- Rotor – A permanent magnet with alternating north and south poles arranged radially around a central shaft.
- Shaft – Usually an integrated shaft machined as part of the rotor. Supported by bushings or ball bearings.
- Housing – Lightweight aluminum or thermoplastic housings enclose the stator and rotor. Air or liquid cooling may be integrated.
- Bearings – High speed sleeve bearings, bushings or ball bearings support the rotor shaft.
Stepper motors do not require an encoder or gearbox for most applications. The simple stator and rotor design allows rapid manufacturing at low cost.
Comparison of Construction
| Comparison | Servo Motor | Stepper Motor |
|---|---|---|
| Stator design | Laminated steel with concentrated windings | Stacked laminations with distributed windings |
| Rotor design | Salient pole or permanent magnet | Permanent magnet |
| Housing | Heavy cast iron or steel | Light aluminum or plastic |
| Bearings | High precision ball bearings | Bushings, sleeve or basic ball bearings |
| Feedback device | Encoder, resolver or potentiometer | Often none, sometimes encoder |
| Gearbox | Often integral high-ratio gearbox | Rarely used |
In summary, servo motors have a robust stator, rotor and housing designed for high torque, speed and continuous duty operation. Stepper motors use lightweight materials and simple construction aimed at low cost and open-loop control.
Working Principle
While both servo and stepper motors produce rotational motion, their internal operating principle and control methods differ significantly.
Servo Motor Working Principle
Servo motors operate on the basis of a closed-loop control system. A controller provides a drive signal to the servo that is proportional to the difference between the commanded position and actual position.
Servo Motor Working Principle
The working principle involves:
- An input command to the controller indicating the desired position. This is compared to the position feedback.
- Error between actual and desired position is calculated. The controller sends a proportional drive signal to the servo amplifier.
- Motor is driven proportionally faster or slower to minimize position error.
- Encoder or resolver provides continuous position feedback to complete the control loop.
This closed loop control allows a servo motor to quickly and precisely rotate to a commanded angular position. Servo motors use PID control algorithms to optimize the dynamic response.
Stepper Motor Working Principle
Stepper motors operate on the principle of electromagnetism and discrete stepping of the rotor. Rotation is achieved by energizing coil windings in a specific sequence.
The stepper motor working principle:
- Applying current to a motor coil creates an electromagnetic pole. Opposite poles attract, similar poles repel.
- Alternating current to the motor coils moves the rotor. As the rotor teeth align with the energized stator pole, they lock in position.
- The driver sequentially energizes the motor coils to rotate the rotor in increments or steps. No closed loop control is used.
- Step angle is determined by rotor construction. Number of steps per revolution depends on step angle.
The open loop stepping operation allows the motor to move in controlled discrete increments without requiring a position sensor.
Comparison of Working Principles
| Comparison | Servo Motor | Stepper Motor |
|---|---|---|
| Control method | Closed loop PID control | Open loop stepping |
| Feedback device | Encoder or resolver | Not required |
| Accuracy | Very high with closed loop control | Limited by step resolution |
| Speed | Wide speed range with rapid acceleration | Typically lower speed operation |
| Torque | High and continuous torque capability | High holding torque, lower power torque |
In summary, servo motors use closed loop control for high accuracy while stepper motors operate open loop in discrete steps. Servo systems are more complex but offer better dynamic performance.
Performance Characteristics
The design and operating principles of servo and stepper motors impart distinct performance characteristics.
Servo Motor Performance Metrics
Key performance characteristics of servo motors include:
- Speed – Servo motors can operate over a wide speed range, often exceeding 6000 rpm for brushless servo motors. Rapid acceleration and deceleration is possible.
- Torque – Servo motors produce high torque, especially at lower speeds. Peak torque ranges from 10 Nm to over 1000 Nm depending on motor size.
- Power – Motor output power ranges from under 100 Watts to over 15 kW for large industrial servos. Power density is high.
- Accuracy – Position accuracy is very high, reaching 0.01 degree or better with high resolution encoders. Helped by tuned PID control gains.
- Repeatability – Positional repeatability is excellent, with consistent positioning under 0.01 degree. Benefits from closed loop control.
High torque density, acceleration, accuracy and repeatability make servo motors well suited for demanding automation applications. Advanced control algorithms allow optimized motion.
Stepper Motor Performance Metrics
The performance characteristics of a stepper motor include:
- Speed – Maximum speed ranges from 100 to 2000 RPM. Higher speeds require reduced torque and incremental microstepping.
- Torque – Stepper motors have very high holding torque but weaker power torque at higher speeds. Holding torque can exceed 3 Nm for small motors.
- Power – Typical motor power output ranges from below 100 Watts up to around 750 Watts. High speed operation results in lower power.
- Accuracy – Positioning accuracy depends on stepper resolution. Full step resolution is typically 1.8 degrees. Microstepping divides steps further.
- Repeatability – Excellent repeatability thanks to inherent step resolution. Positional error is limited to less than one step.
Stepper motors excel at slow speed, high precision positioning applications. Their relatively simple construction provides robustness at low cost.
Comparison of Performance Metrics
| Comparison | Servo Motor | Stepper Motor |
|---|---|---|
| Speed range | Very wide, thousands of RPM | Narrow, typically below 2000 RPM |
| Torque density | Very high | High holding torque, lower running torque |
| Power output | Up to 15 kW+ | Typically below 750 Watts |
| Accuracy | 0.01 degree or better | 1.8 degrees full step, higher with microstepping |
| Repeatability | Excellent, less than 0.01 degree | Very good, limited to step resolution |
In summary, servo motors are designed for high speed and power operation with very high accuracy. Steppers trade off speed and power for high precision motion in an open loop control system.
Applications and Uses
The performance characteristics and capabilities of servo and stepper motors make them suitable for different applications.
Servo Motor Applications
Servo motors are designed for high power, high precision motion control applications including:
- Industrial robotics – Multi-axis articulated robot arms require high torque servo motors to handle dynamic payloads.
- CNC machines – High precision machining relies on fast, accurate servo motor control of linear and rotary axes.
- Medical equipment – MRI scanners, CT scanners, and lab automation use servo positioning.
- Semiconductor manufacturing – Integrated circuit production involves extensive servo-controlled motion.
- Printing presses – Servo systems position printing plates and control tension.
- Packaging – High speed coordinated motion is needed for packaging equipment.
Servo motors handle the most demanding motion control applications where speed, accuracy, and flexibility are critical performance requirements.
Stepper Motor Applications
Stepper motors are ideal for applications requiring high precision open loop positioning:
- 3D printers & CNC machines – Stepper motors precisely position print heads and machine tools.
- Textile manufacturing – Stepper motors control threading and weaving patterns.
- Medical syringes – Stepper motors provide metered dispensing of fluids in analyzers.
- Wafer scanners – Precise positioning of optics for integrated circuit photolithography.
- Telescopes – Stepper motors orient telescope mounts to track celestial objects.
- Robotics – Used in cost-sensitive robots requiring modest position control performance.
Stepper motors may not match servos in speed and power, but they provide excellent control in open loop positioning machines.
Comparison of Applications
| Comparison | Servo Motor | Stepper Motor |
|---|---|---|
| Performance reqs | High speed, power, torque | Modest speed with high precision |
| Control principle | Closed loop feedback | Open loop stepping |
| Example apps | CNC machines, robotics, packaging | 3D printers, telescopes, medical dispensing |
| Industries | Manufacturing, robotics, industrial | Manufacturing, medical, aerospace |
In summary, servo motors meet the demands of high power, high speed automation applications while steppers excel at slower speed open-loop positioning tasks.
Advantages and Disadvantages
Both servo and stepper motors provide excellent motion control capabilities. But they each have certain advantages and disadvantages to consider.
Servo Motor Pros and Cons
Servo motor advantages include:
- Extremely accurate position, speed and acceleration control
- High power density and torque-to-inertia ratio
- Fast dynamic response and tuning through PID gains
- Advanced control capabilities and flexibility
- Wide speed range operation
- High efficiency
Disadvantages of servo motors:
- More complex requiring closed loop control
- Higher costs than stepper motors
- Requires maintenance of brushes and contacts (for brushed DC servos)
- Encoder or resolver feedback device adds cost
- Control electronics are complex and expensive
In summary, servo motors provide high performance and flexibility but at higher complexity and costs than alternatives like stepper motors.
Stepper Motor Pros and Cons
Stepper motor advantages:
- Excellent position control and repeatability without feedback device
- Low cost and simple construction
- Open loop control eliminates tuning complexity
- Extremely reliable with long service life
- High holding torque prevents loss of steps
- Available in a wide range of sizes and torque levels
Stepper motor disadvantages:
- Lower power and torque during running operation
- Limited high speed capability compared to servo motors
- Open loop nature limits control accuracy
- Can lose steps at high speeds or accelerations
- Torque ripple can cause vibrations in some cases
- Audible noise during operation
In summary, stepper motors are simple, inexpensive, and reliable, but lack the speed, power, and accuracy of closed loop servo systems.
Comparison of Advantages and Disadvantages
| Comparison | Servo Motor | Stepper Motor |
|---|---|---|
| Accuracy | Extremely high with encoder feedback | Limited by open loop control |
| Complexity | High due to closed loop control | Very simple open loop operation |
| Cost | Expensive; electronics, encoder | Low component cost |
| Reliability | Brush and contact wear over time | Very reliable and long service life |
| Speed/power | High speed, power and acceleration | Lower speed and power capabilities |
In summary, servo motors provide unmatched performance while stepper motors excel at simple, low-cost, open-loop positioning tasks.
Servo Motors vs. Stepper Motors
| Servo Motors | Stepper Motors | |
|---|---|---|
| Construction | Robust mechanical design, precision gears and bearings, encoders for feedback | Simple lightweight construction, open-loop control |
| Principle | Closed loop PID control with encoder feedback | Open loop stepping operation without feedback |
| Performance | High speed and power output, very high accuracy and repeatability | High precision but lower speed and power, modest accuracy |
| Applications | High performance tasks like CNC machines and robotics | Simpler positioning tasks like 3D printers or telescopes |
| Advantages | Extreme precision control, fast dynamics, advanced control capabilities | Low cost, simple operation, excellent repeatability, high reliability |
| Disadvantages | Complexity, high cost, maintenance | Lower power and speed, limited accuracy |
Frequently Asked Questions
1. What is the main difference between a servo motor and a stepper motor?
The main difference is that servo motors use closed loop control with position feedback while stepper motors operate open loop without requiring position sensors. This allows servo motors to achieve very precise, high speed control while steppers offer simpler operation at lower speeds and accuracy.
