How to select brushless dc motors for robotic vehicles

Several types of motors are used in building robots. These include brushed dc motors, brushless dc motors, stepper motors, servo motors, geared dc motors, ac motors, pneumatic motors, hydraulic motors, and linear motors. Each type of motor serves a specific purpose in a robotic application and has advantages and disadvantages.

Brushed dc motors are the most affordable and simplest type of motors used in robots. They’re suitable for applications that require low torque and speed. Although they may be cost-effective, they have a relatively short lifespan due to wear and tear on the brushes.

As the name suggests, brushless dc motors are regular dc motors sans brushes. They’re more costly but also quite durable and efficient. Brushless motors also provide higher torque and speed than brushed motors.

Stepper motors are used in auxiliary applications where precise positioning is required. They’re neither fast nor powerful. Servo motors are specific types of stepper motors with a built-in feedback mechanism for accurate positioning.

Geared dc motors are brushed dc motors with a gear mechanism that varies the rotation speed. Ac motors are only used in some industrial robots where high power is required. Pneumatic motors are also used in applications that require high power. Hydraulic motors are typically used to deliver high torque. Linear motors are used in specific applications requiring precise positioning along a straight line.

In mobile robots, dc and servo motors are used. The servo motors control specific positioning applications while dc motors drive the robot. Brushless dc motors are widely used for driving rovers and all types of mobile robots.

In this article, we’ll discuss how to select a brushless dc motor for driving a mobile robot.

Brushless motors
Brushless DC (BLDC) and brushed DC motors differ in how they communicate and switch electrical currents to produce rotation. Brushes, commutator, stator (stationary part with permanent magnets), and rotor (spinning part with electromagnets) are the components of a brushed motor.

Physical carbon brushes grind against the rotor’s segmented commutator in brushed motors. A brushless motor consists of a rotor that rotates with permanent magnets and a stator, a stationary element with electromagnets. Neither brushes nor commutators are present. When sensors (typically Hall effect sensors) detect a change in the rotor’s position, the brushless motor uses electronic controllers to alter the current flowing through the stator windings.

Mechanical brushes are the weakest component of a motor. Motors used in robotics applications must be durable, light, and efficient. Getting rid of the brushes is advantageous. In real-life situations, brushed motors are prone to fail when subjected to high shock loads due to their mechanically intricate assembly. The spring-loaded brushes also bounce, causing arcing.

Typically composed of delicate carbon alloys, the brushes tend to deteriorate rapidly and require regular replacements. They also limit the speed of rotation and reduce the motor’s efficiency.

So, the brushless motors without brushes or commuters are more efficient than the brushed motors. Brushless motors can also take higher peak currents and rotate faster. They’re half the weight of their brushed counterparts and are more durable and reliable. The speed and direction of brushless motors can be more precisely controlled as they are electronically controlled. They also have no risk of sparking because there’s no commuter. Brushless motors are expensive but suitable for all applications, whether rovers, battle robots, or prosthetics.

Where to use brushless motors in robots
Brushless motors offer several advantages in robotics applications, particularly in terms of safety. They’re well-suited for the demanding tasks that robots perform. BLDC motors are more power efficient and can run for extended periods on a battery supply with minimum heat dissipation. They pack a lot of power in a compact size.

In fact, they’re almost half in weight compared to brushed motors and much smaller due to their different design and operation. The electronic control also provides excellent speed controllability, which is a key requirement in robots that need to perform delicate maneuvers or maintain consistent speeds during operation.

One major advantage of the brushless design is it makes motor operation spark-free, a crucial safety feature. The BLDC motors can be used in potentially more risky applications, such as near flammable materials or explosive atmospheres. They also deliver higher torque and speed compared to brushed motors, making them efficient in tasks like grasping objects, climbing obstacles, or maneuvering heavy loads. Due to their different design and operation, BLDC motors offer greater uptime at lower maintenance.

Due to high current, torque, and speed, and precise electronic control, brushless motors are commonly used in industrial robots. For the same reasons, these motors are also used in drones and unmanned aerial vehicles. Their spark-free operation and lightweight, high-power features make them suitable for use in search and rescue robots. Medical robots also rely on BLDC motors because they offer precise control and maneuverability.

In mobile robots, BLDC motors are primarily used for locomotion. They power the wheels of the rovers, as their high torque output allows rovers to navigate uneven terrain, climb obstacles, and handle inclines. They’re also used to steer and maneuver the rovers. By precisely controlling the speed of the motors, rovers can make controlled turns, adjust speed for different terrains, and maneuver in tight spaces.

Brushless motors are primarily used to drive and steer the wheels of a mobile robot — but their application might extend beyond it. Many mobile robots use low torque and low-speed brushless motors to control arms, pick-and-place objects, interact with their environment, and manipulate payloads. Many robots also employ BLDC motors for braking as either part of regenerative braking or as actuators for mechanical braking. In many rovers, brushless motors assist the suspension system in adjusting height or provide active damping.

Many robotic designs also use BLDC motors for auxiliary applications like shutting and opening doors, driving pumps for hydraulic systems, or driving fans for thermal management.

Brushless motors specifications
Brushless motors are better than brushed motors when driving mobile robots. Like any electronic component, brushless motors also come tagged with some specifications. These specifications are critical in selecting a motor for a given robotic application. The key specifications of a brushless motor that must be considered are as follows:

Voltage (V): This is the voltage the motor is designed to operate on. The BLDC motors designed for use in rovers and mobile robots have a voltage rating of 12 V, 24 V, or higher. The motor’s voltage must match the battery’s voltage.
Current (A): This is the amount of current that the motor draws from the power source. The motor’s current rating should be significantly lower than the maximum current the battery can supply.
Power (W): This indicates the overall power output of the motor, which is calculated by multiplying the motor’s voltage and current rating. A higher-wattage motor can deliver more power and torque. The motor’s power output must be greater than the maximum power required to drive, steer, or maneuver the robot in any situation. If the motor is used to manipulate payload or operate robotic arms, the power requirement should be accordingly calculated.
Speed (RPM): This is the rotational speed of the motor expressed as revolutions per minute at a specific voltage. Rovers generally need high RPM for driving over flat surfaces, while they require high torque when need to cross difficult terrain. The desired speed range for the rover must be first calculated. The RPM of the motor should be greater than the maximum speed the rover will drive.
Torque (Nm): The twisting force that the motor produces is known as torque. High torque is essential for rovers to handle uneven terrain, climb inclines, and overcome obstacles.
KV rating: The motor’s KV rating indicates how many revolutions per minute (RPMs) it produces per volt. For example, a motor with a KV rating 1000 will run at 1000 RPMs on 1V of battery power. While a lower KV rating provides greater torque, a higher KV rating produces more speed. A battery with a lower voltage is needed for a low KV rating, while one with a higher rating calls for a higher voltage battery. The KV rating also impacts performance and motor lifespan. Higher KV-rated motors have more heat dissipation than lower KV-rated motors. The KV rating is also called the motor constant (Km).
Size and weight: The motor’s weight and dimensions should match the space allotted in your rover’s design. Smaller motors may be better for reducing weight, but their torque and output may be limited.
Features: Some BLDC motors have extra features that improve their functionality. Some motors have built-in encoders for precise speed and position control. Some motors have an integrated gearbox to boost torque output at the price of speed.

How to verify specifications
There can be two situations: you already have a brushless motor and have to build a robot with it, or you are fabricating a robotic design, and you have to finalize the motor. In the latter case, it’s important to review the motor information online. Now, suppose you already have a motor and need to check its specifications. In this case, the motor might have a label with printed specifications like the voltage, current, and KV ratings.

Otherwise, you can identify the motor’s brand and model number and search the manufacturer’s website for a datasheet. The datasheet will contain detailed specifications like voltage, current, power output, speed (RPM), torque, KV rating, and the other relevant information about the motor.

If you’re looking at a BLDC motor online from a retailer, the retailer includes a product description that details the motor’s size, voltage, current draw, and KV rating. An alternative is to do an internet search using the motor model number. By clicking on this link, you can find the manufacturer’s website, online retailers with thorough product descriptions, or even discussion groups where people can talk about the details of the motor. To ensure consistency, compare the specifications if you find the motor from different suppliers.

How to select a brushless motor for your robot
Selecting a brushless motor for a rover or mobile robot definitely involves some brainstorming. There are so many types of brushless dc motors available that it can be a daunting task to select the right one for your rover or RC car. It’s important to consider many factors, such as size, weight, KV rating, voltage, current, brand, quality, budget, and compatibility with other components.

First of all, you must define your rover’s requirements. Estimate the total weight of the rover, including components and any payload it might carry. Determine the size available for motor installation. You need to consider the type of terrain your rover will navigate, like flat surfaces, obstacles, or inclines.

Accordingly, you need to determine the motor’s speed and torque requirements. You will probably need to bargain between the motors’ speed and torque deliverability. High speed should be preferred if you are designing the rover for flat surfaces. High torque is necessary if you are developing the rover to climb inclines or drive through challenging terrain. In specific situations, motors with integrated gearboxes can solve the speed vs torque dilemma.

In any situation, you will still need to prioritize between speed and torque. Higher torque output should be your priority if your rover needs to overcome obstacles, climb inclines, or handle rough terrain. In that case, you should search for motors with a lower KV rating (motor constant), which indicates higher torque capability. If your rover prioritizes high speeds for smooth surfaces, a motor with a higher KV rating and potentially lower torque will be suitable. Low-KV rating motors require batteries with lower voltages, while high-KV rating motors require high-voltage batteries.

The voltage rating of the motor should be compatible with the battery voltage. Once you are certain you need a 12V motor, or 24V motor, or higher voltage, you should calculate the current rating according to the power requirements of the rover. The motors with higher power or torque output tend to be larger and heavier.

So, finally, you can shortlist the motor options available according to their size and weight. While satisfying the power requirements of the rover, a motor with the smallest design and lightest weight should be selected. In certain situations, the physical design of the rover might need to be altered to suit the motors’ size, weight, and dimensions.

For RC car designs, motors are sized by different RC car scales. The scale refers to the size of the car compared to a real-world vehicle. It’s typically expressed as a ratio. For example, 1:10 means the RC car is 1/10th the size of a conventional car, and 1:8 means the RC car is 1/8th the size of a real car.

Motor size in RC cars is typically determined by its physical dimensions and power output. The motor size is expressed as a four-digit number, where the first two digits indicate the diameter of the motor in millimeters, and the last two digits indicate the length of the motor in millimeters.

For example, a 3650-size motor is 36 mm in diameter and 50 mm in length. The 28XX size motors are smaller motors suitable for smaller-scale RC cars or applications requiring low-power delivery. The 36XX motors suit mid-sized RC cars like 1/10th-scale RC cars. The 42XX motors are ideal for heavier RC cars like 1/8 scale RC cars or applications requiring more power or torque.

The selected motor should also be compatible with the battery, electronic speed controller, and other components of the RC car. Brushless motors from reputed brands with suitable warranties must be preferred. The price of the motors may also be a key factor in the final selection of a motor model for your RC car or Rover. The selection of the motors must not compromise the Rover’s requirements while constraining the budget.