Which Motors are used in Robotics?

Robotics, the fascinating field where engineering intersects with automation, relies fundamentally on motion. And where there’s motion in a robot, there are almost certainly motors. These actuators are the muscles of a robot, converting electrical energy into mechanical work to make limbs move, wheels turn, and grippers grasp. But the world of robotics is vast and varied, and so is the landscape of motors used. Choosing the right motor is a critical design decision, impacting everything from a robot’s accuracy and speed to its power consumption and size.

This article delves deep into the various types of motors commonly employed in robotics, exploring their characteristics, advantages, disadvantages, and typical applications. We’ll go beyond a simple overview and provide specific details to give you a comprehensive understanding of the options available.

Table of Contents

1. Core Concepts: Torque, Speed, and Control
2. DC Motors: The Workhorses of Robotics
   • Brushed DC Motors
   • Brushless DC (BLDC) Motors
3. Stepper Motors: Precision Positioning Champions
4. Servo Motors: The Go-To for Precise Angular Control
5. AC Motors: Powering Heavier Industrial Robots
   • AC Induction Motors
   • AC Synchronous Motors
6. Other Motor Types and Considerations
7. Choosing the Right Motor: MoreThan Just the Type
8. The Role of Gearboxes and Transmissions
9. Conclusion: A Diverse Toolkit for Motion

Core Concepts: Torque, Speed, and Control

Before we explore specific motor types, it’s crucial to understand the key parameters that govern their performance in a robotic context:

• Torque: This is the rotational force a motor can produce. It’s the measure of how much “twist” or “turning power” the motor has. In robotics, high torque is often needed to overcome inertia, lift loads, or generate strong gripping forces. Torque is typically measured in Newton-meters (Nm) or ounce-inches (oz-in).
• Speed: This refers to how fast the motor’s shaft rotates, usually measured in revolutions per minute (RPM). A robot’s speed of movement is directly related to the speed of its motors, often reduced through gearboxes.
• Control: Robotic applications demand precise control over motor movement – whether it’s maintaining a specific speed, reaching a precise position, or applying a constant force. The ease and precision of control vary significantly between motor types.

These parameters are often inversely related. Generally, a motor producing high torque will operate at a lower speed, and vice versa. The choice of motor often involves a trade-off between these factors, depending on the robot’s specific requirements.

DC Motors: The Workhorses of Robotics

Direct current (DC) motors are perhaps the most ubiquitous motor type in robotics, especially in mobile robots and projects with simpler requirements. They are relatively simple to understand, inexpensive, and offer decent performance for a wide range of tasks.

Brushed DC Motors

Brushed DC motors are the most basic type of DC motor. They utilize physical brushes that make contact with a commutator to switch the direction of current in the rotor coils. This switching action generates the torque that rotates the shaft.

• Working Principle: Current flows through the brushes to the commutator, which is segmented. As the rotor turns, different segments of the commutator come into contact with the brushes, reversing the current direction in the appropriate coils. This continuous switching creates a rotating magnetic field that interacts with the magnets in the stator, causing the rotor to spin.
• Advantages:
  • Simple Control: Speed is typically controlled by varying the voltage applied to the motor (using Pulse Width Modulation – PWM).
  • Inexpensive: They are generally the most cost-effective motor option.
  • Good Starting Torque: They can produce significant torque from a standstill.
  • Reliable for basic applications: With proper maintenance, they can be quite durable.
• Disadvantages:
  • Wear and Tear on Brushes: The physical contact of the brushes causes wear, leading to reduced lifespan and requiring periodic replacement.
  • Commutator Arcing: Sparks can be generated at the commutator due to the switching action, creating electrical noise and potentially being a fire hazard in flammable environments.
  • Lower Efficiency: Friction in the brushes and heat generation contribute to lower efficiency compared to brushless motors.
  • Limited Speed Range: The maximum speed is limited by brush wear and commutator arcing.
  • More Difficult for Precise Positional Control: Achieving high positional accuracy without feedback (like an encoder) is challenging.
• Typical Applications:
  • Wheeled mobile robots (simple drive systems)
  • Educational robots
  • Simple actuators (e.g., opening and closing a gripper)
  • Toys and hobby robots

Brushless DC (BLDC) Motors

BLDC motors address many of the drawbacks of brushed DC motors. They eliminate the physical brushes and commutator, relying instead on electronic commutation. Sensors (like Hall effect sensors) or sensorless techniques are used to determine the rotor’s position and electronically switch the sequence of coils in the stator.

• Working Principle: The stator contains coils, and the rotor is typically a permanent magnet (in outer rotor configurations) or contains magnets (in inner rotor configurations). By electronically switching the current flow through the stator coils in the correct sequence, a rotating magnetic field is created that pulls or pushes the rotor’s magnets, causing it to spin.
• Advantages:
  • Longer Lifespan: No brushes mean no wear, significantly increasing the motor’s lifespan and reducing maintenance.
  • Higher Efficiency: Reduced friction and arcing lead to higher efficiency and less heat generation.
  • Higher Speed Capabilities: Not limited by brush wear, they can achieve much higher speeds.
  • Less Electrical Noise: The absence of brush arcing reduces electromagnetic interference.
  • Easier for Precise Speed Control: Electronic commutation allows for more accurate speed regulation.
• Disadvantages:
  • More Complex Control: Requires a dedicated electronic speed controller (ESC) to perform the commutation, making the system more complex and expensive.
  • More Expensive: The motors and their controllers are generally more costly than brushed DC motor systems.
  • Potential Cogging: Can sometimes exhibit a jerky motion at very low speeds or when starting due to the magnetic poles “locking” into specific positions.
• Typical Applications:
  • Drones and UAVs (for propulsion)
  • High-performance mobile robots
  • Robotic arms (for smooth and powerful movement)
  • Industrial robots (for high speed and precision)
  • Actuators requiring high torque and efficiency

Stepper Motors: Precision Positioning Champions

Stepper motors are a unique type of DC motor known for their ability to rotate in precise, discrete steps. This makes them ideal for applications where accurate positioning is paramount, without the need for continuous feedback mechanisms (like encoders in some cases).

• Working Principle: Stepper motors have a series of electromagnet coils in the stator and a toothed rotor (often a permanent magnet or soft iron). By energizing the stator coils in a specific sequence, the rotor is attracted to the closest energized coil, causing it to move a fixed angular increment, or “step.”
• Types of Stepper Motors:
  • Permanent Magnet (PM): The rotor is a permanent magnet. Offers higher torque compared to variable reluctance motors but can experience detent torque (resistance to turning when de-energized).
  • Variable Reluctance (VR): The rotor is not magnetized but is made of soft iron with teeth. Torque is generated by the rotor’s tendency to align with the energized stator coils. Less torque than PM motors but no detent torque.
  • Hybrid Synchronous: Combines features of both PM and VR motors, offering higher torque and smaller step sizes. This is the most common type in robotics.
• Advantages:
  • Excellent Positional Accuracy (Open-Loop Control): Can achieve precise positioning by simply stepping the motor a specific number of times without feedback. This simplifies the control system.
  • Good Holding Torque: Can maintain a position when energized without the need for continuous power input to hold position (unlike some other motor types that might drift).
  • Simple Control (Basic): Basic step-and-direction control is relatively straightforward to implement.
  • Reliable (Mechanical Simplicity): Compared to brushed motors, they have fewer wearing parts.
• Disadvantages:
  • Lower Efficiency (Compared to BLDC for Continuous Motion): They consume power even when holding a position.
  • Poor Torque at High Speeds: Torque typically decreases as the stepping speed increases.
  • Can Miss Steps: If the load inertia is too high or the motor is driven too fast, it can miss steps, leading to position errors (unless a closed-loop system with a sensor is used).
  • Limited Continuous Torque and Speed: Not ideal for applications requiring continuous, high-speed rotation under load.
• Typical Applications:
  • 3D printers (for axis movement)
  • CNC machines
  • Pan and tilt mechanisms in cameras and sensors
  • Robotic arms (for joints where discrete movements are sufficient)
  • Automated dispensing systems

Servo Motors: The Go-To for Precise Angular Control

While the term “servo motor” is often used broadly, in the context of hobbyist and certain industrial robotics, it often refers to a specific type of geared DC motor with integrated control electronics and a position sensor (typically a potentiometer for hobby servos, or encoders for more industrial variants). These motors are designed for precise angular positioning.

• Working Principle: A servo motor receives a control signal (typically a PWM pulse of a specific width). The internal electronics interpret this signal as a desired shaft angle. The motor then drives the geared output shaft until the position sensor (feedback) indicates that the desired angle has been reached. The control system continuously adjusts the motor’s power to maintain this position.
• Components:
  • DC Motor: Provides the rotational force.
  • Gearbox: Reduces the motor’s speed and increases its output torque.
  • Position Sensor: Measures the current angle of the output shaft.
  • Control Electronics (Servo Amplifier): Interprets the input signal and drives the motor based on the feedback from the position sensor.
• Advantages:
  • Excellent Positional Control: Designed specifically for accurate and repeatable angular positioning.
  • Simple Interface (Hobby Servos): Controlled by a simple PWM signal, readily available from microcontrollers.
  • Maintains Position: Actively holds its commanded position.
  • Compact and Integrated Solution: The motor, gearbox, sensor, and control electronics are typically integrated into a single unit.
• Disadvantages:
  • Limited Range of Motion (Hobby Servos): Typically limited to 180 degrees or 360 degrees of rotation.
  • Lower Efficiency (Internal Electronics): The continuous operation of the control electronics consumes power.
  • Can be Noisy: The gearbox can generate audible noise.
  • Stall Current: Can draw significant current when stalled against a load.
• Typical Applications:
  • Robotic grippers
  • Joints in small robotic arms
  • Steering mechanisms in wheeled robots
  • Actuating levers and linkages
  • Model airplanes and RC vehicles

Important Note: While “servo motor” is commonly used as described above in hobby robotics, in industrial contexts, the term “servo system” or “servo drive” is often used to refer to a closed-loop motor control system that can utilize various motor types (including BLDC and AC induction motors) combined with high-resolution feedback sensors (like encoders or resolvers) for extremely precise control of position, velocity, and torque. These are significantly more sophisticated and expensive than hobby servos.

AC Motors: Powering Heavier Industrial Robots

Alternating current (AC) motors are typically found in larger, more powerful robotic systems, particularly in industrial settings. They are known for their robustness and ability to handle heavy loads and continuous operation.

AC Induction Motors

AC induction motors are the most common type of AC motor and are widely used in industrial applications. They derive their name from the fact that the current in the rotor is induced by the magnetic field produced by the stator.

• Working Principle: The stator contains coils that are energized with AC current, creating a rotating magnetic field. This rotating field induces a current in the rotor, which itself becomes an electromagnet. The interaction between the stator’s rotating field and the rotor’s induced field causes the rotor to rotate. The rotor always “lags” behind the stator’s field speed (synchronous speed) – this difference is called “slip.”
• Advantages:
  • Robust and Durable: Simple construction with no brushes or commutators makes them very reliable.
  • Inexpensive (for basic operation): Relatively low initial cost for constant speed applications.
  • High Power Density: Can deliver significant power for their size.
  • Reliable for Continuous Operation: Suitable for heavy-duty, continuous tasks.
• Disadvantages:
  • More Difficult for Precise Speed and Torque Control (Without VFDs): Traditionally controlled by line frequency, making precise speed and torque control challenging without variable frequency drives (VFDs).
  • Lower Starting Torque (Compared to DC Motors): Can have difficulty starting under heavy loads.
  • Efficiency Drops at Low Loads: Not as efficient when operating at significantly reduced loads.
  • Requires AC Power Supply: Needs access to an AC power source.
• Typical Applications:
  • Conveyor belts in automated systems
  • Large pumps and fans in industrial robots
  • Driving heavy machinery integrated with robotic systems
  • Applications where constant speed and robustness are the primary concerns.

AC Synchronous Motors

AC synchronous motors rotate at a speed that is exactly synchronized with the frequency of the applied AC current. This characteristic makes them suitable for applications requiring precise speed control and when connected to a grid, maintaining grid frequency.

• Working Principle: The rotor is typically a permanent magnet or an electromagnet. The stator’s rotating magnetic field “locks in” with the rotor’s magnetic field, causing the rotor to spin at the same speed as the stator’s field (synchronous speed).
• Advantages:
  • Precise Speed Control (When Tied to AC Frequency): Rotates at a constant speed directly proportional to the AC frequency.
  • High Efficiency: Generally more efficient than induction motors, especially at high power factors.
  • Can Improve Power Factor: Can be used to improve the overall power factor of an electrical system.
• Disadvantages:
  • Require an AC Excitation Source (for Wound Rotor Types): If the rotor is an electromagnet, it needs a DC power supply.
  • Not Self-Starting (Typical): Often require an external starting mechanism or drive system.
  • Can Fall Out of Synchronization: If the load exceeds the motor’s capacity, it can lose synchronization and stop.
• Typical Applications:
  • Applications requiring precise timing and constant speed.
  • Driving generators.
  • Used in some high-performance servo systems in industrial robotics, combined with VFDs and encoders for precise control.

Other Motor Types and Considerations

While the above types are the most common in robotics, other motors are used in specific niche applications:

• Linear Motors: Produce linear motion directly without the need for rotating to linear conversion mechanisms (like lead screws). Used in high-precision positioning systems, gantry robots, and linear actuators.
• Piezoelectric Motors: Utilize the piezoelectric effect to generate small but precise movements. Suitable for micro-robotics, highly precise positioning stages, and applications requiring very fine adjustments.
• Hydraulic and Pneumatic Actuators: While not strictly electric motors, hydraulic and pneumatic cylinders and rotary actuators are also used in robotics, particularly for high force applications or in hazardous environments where electrical systems are undesirable.

Choosing the Right Motor: MoreThan Just the Type

Selecting the appropriate motor for a robotic application involves considering several factors beyond just the motor type:

• Torque and Speed Requirements: Will the motor have enough power to move the robot or its components as intended? Is the speed sufficient for the desired operation?
• Accuracy and Precision: How accurately does the robot need to position itself or its end effector? Stepper motors and servo systems with high-resolution feedback are crucial for high precision.
• Weight and Size Constraints: Robots often have limited space and weight capacity. The size and weight of the motor and its associated gearbox and control electronics are important considerations.
• Power Consumption and Efficiency: Battery-powered robots are heavily influenced by motor efficiency. Industrial robots need to consider energy costs.
• Control Complexity: How easy or difficult is it to control the motor for the desired motion profile?
• Cost: Budget constraints are a significant factor in any robotic project.
• Environment: Will the robot operate in a harsh environment (dust, moisture, temperature extremes)? The motor’s enclosure and durability are important.
• Lifespan and Maintenance: How often will the robot operate, and what is the expected lifespan? Will the motor require regular maintenance?

The Role of Gearboxes and Transmissions

It’s crucial to remember that motors are often used in conjunction with gearboxes or other transmission mechanisms. These components modify the motor’s output torque and speed to match the robot’s requirements. A gearbox can significantly increase the torque output of a small motor, albeit at the cost of reduced speed. Without gearboxes, it would be challenging to achieve the necessary forces and controlled movements in many robotic applications.

Conclusion: A Diverse Toolkit for Motion

The world of robotics relies on a diverse range of motors, each with its own strengths and weaknesses. From the fundamental simplicity of brushed DC motors to the precise control of servo systems and the industrial power of AC motors, engineers and hobbyists alike have a rich toolkit to choose from. Understanding the principles behind these different motor types and considering the specific requirements of the robotic application are essential steps in building effective and capable robots. As robotics continues to evolve, so too will the motors that power them, pushing the boundaries of automation and intelligence.