Two common features found in robotics projects include the ability to navigate and avoid obstacles, and to interact with or manipulate objects. To implement this functionality, robotics developers rely on actuators, which are components that turn electronic commands into real-world movements.
Of course, it should be no surprise that there are many different types of actuator hardware options for different applications. In this article we’ll review the different types of actuators commonly used in robotics, discuss some of the basic considerations for choosing them, and explore how to interface with them.
Types of Actuators
There are five general types of actuators used in robotics projects:
- DC motor: a motor with an output shaft driven by DC voltage at various levels, and used primarily with drivetrains for mobility (e.g., a crawler). A DC motor’s output shaft is often mounted to a pinion, spur, or other type of gear, and is generally wired to an electronic speed controller that sets the motor’s speed and direction of rotation. DC motors, like other actuators, are available in a variety of sizes and torque handling characteristics.
- Servo: a component with a DC motor, output shaft, and control circuitry all packaged into one unit. Servos usually provide rotational movement, except for continuous-rotation servo variants that can rotate 360 degrees. There are hobby-grade and robotic-grade servos which we’ll discuss later in this article.
- Stepper: a hybrid between a DC motor and a servo. Steppers are often used in 3D printers and CNC machines for high accuracy and repeatability, often where lower torque output is sufficient.
- Linear actuator: similar to a servo but provides linear motion, often through linear mechanisms such as worm drives.
- Solenoids: a specific type of linear actuator that can assume binary positions (i.e., on/off, open/closed, etc.). Solenoids are typically used for applications like valves, latches, and locks, or for pushing buttons, and are usually controlled by an external microcontroller.
Aspects and Considerations of Actuators
Robots can be built for a variety of purposes, functions, and operating environments, so there are a lot of aspects to consider when choosing actuators such as:
Purpose and intended functionality
The type of actuator required for a given application will depend on the robot’s purpose and intended functionality. For example, a DC motor will likely be used in conjunction with a drivetrain to allow for mobility, while a servo is often used to provide articulation such as that found in a robotic arm.
Physical requirements and constraints
Once the type of actuator has been decided upon, developers should look at the physical requirements and constraints. The first aspect to look for is the physical size and weight of the actuator itself to see if it will fit where it is needed, and if the combined weight of the actuator and mechanism on which it’s mounted is appropriate. For example, placing a heavy actuator on a small, weak robotic arm may cause the arm to fail under its own weight.
Mounting space and options
Similarly, developers must determine the mounting space available on or in the robot, and the mounting options provided by the actuator itself. Some actuators include separate mounting hardware allowing you to mount the unit in different orientations, while others come with integrated mounting points, requiring them to be installed into a specific position and orientation. Developers may also need to adapt the actuator to the mechanism that it’s controlling (e.g., control linkage). For example, a servo may include different types of servo horns that attach to the actuator’s output shaft, to provide different options for mounting control linkage.
Another aspect to look at are the digital interfaces available on both the actuator and the microcontroller responsible for controlling it. For example, servos generally have three wires including ground, power, and a control signal. Depending on the power requirements of the actuator, you may be able to power it directly from the microcontroller, while in other cases, power may need to be supplied from elsewhere (e.g., the microcontrollers are often limited to around 5VDC). The power consumption is often dictated by the amount of torque provided by the actuator and the amount of load it handles while in operation. So be sure to gather the specifications on the actuator’s power requirements, as this will affect how long the robot’s batteries will last.
Strength and Power
Depending on their intended usage, developers should ensure that a given actuator is strong enough to get the job done. For example, a DC motor must be selected such that it provides enough power to the drivetrain to move the robot and the robot’s load in the operating environment. If the environment is difficult to navigate (e.g., muddy terrain that causes slippage in the drive train), and/or the robot’s load becomes overly heavy, these worst-case scenarios need to be considered up front during actuator selection.
Finally, the communication protocol must also be considered. Many actuators support communications using pulse width modulation (PMW) while some may support serial communications.
Hobby Versus Robot Grade Servos
There are two general grades of servos: hobby grade and robot grade. This distinction exists primarily because the radio control (RC) hobby industry has had decades to develop small, consumer/hobby-oriented servos (e.g., for RC cars), while the robotics industry is still relatively new.
Hobby-grade servos are relatively low cost and usually handle lower payloads than their robot-grade counterparts. They tend to have a limited rotational range and often only support communications via PWM.
Robot-grade servos offer a number of benefits over hobby-grade. Physically, they often provide more torque and include multiple mounting options. From an electronics standpoint, they can often be daisy chained and may be individually addressable through serial packet communications (e.g., via UART). In addition, they often provide two-way communication to send telemetry data back to the microcontroller such as temperature readings, and load monitoring, and provide the ability to vary torque.
Another nice option to look for on some units is a sophisticated microcontroller to which tasks and logic can be offloaded, freeing up your main control board for more general tasks. In some cases, the more sophisticated microcontrollers also support firmware upgrades.
Controlling it All
There are a number of platforms out there on which you can build your robots and control your actuators. One notable platform is the Qualcomm® Robotics RB3 Platform, and corresponding Qualcomm Robotics RB3 development kit, which is based on the Qualcomm® Snapdragon™ Mobile Platform for developing intelligent and power-efficient robotics, such as that demonstrated in our seeing and hearing robotic arm project.
Figure 1: Qualcomm Robotics RB3 Development Kit
Developers can interface this kit with their actuators using the board’s expansion connectors. For example, the expansion connector’s GPIO pins can be used to send PWM signals to an actuator, while the UART pins can be used for serial communications with actuators that support serial packets. In addition, the board supports numerous other connection types (e.g., SPI, I2C, etc.) for interfacing with other systems.
As you can see, there are a number of important decisions to be made when choosing actuators for your robotics projects. Making the correct choices will create a robot that lives a long and useful life.
Rajan Mistry is a Sr. Applications Engineer at Qualcomm Technologies, Inc. with the Qualcomm Developer Network team. His role is to help grow the developer community and work on the next generation of solutions that utilize our technologies.