Rotary vs. Linear Electric Actuators
Industrial automation and many manufacturing processes rely on motion. Motion systems move parts into and out of machines, they bring tools into contact with workpieces, and they operate key parts of processing machinery.
This motion is provided by actuators. These are devices that, when supplied with power, move in a particular way. For engineers and others with responsibility for motion control systems it’s important to understand the various types of actuators available and when each is appropriate. Using the wrong industrial electric actuators in an application will result in motion control problems, equipment reliability issues, and downtime.
Previous blog posts have addressed topics such as appropriate uses for pneumatic actuators and why electric actuators are precise and repeatable. This blog post clarifies the differences between rotary and linear electric actuators.
A linear actuator provides back and forth motion in a straight line. Key performance parameters are distance traveled, velocity and acceleration. (Others include factors such as load-carrying capacity.) An important point about electric linear actuators, and one that sometimes causes confusion, is that the motion is provided by an electric motor, which rotates. Thus, an electric linear actuator converts rotary motion into linear.
As a side note, there are other forms of linear electric actuator that don’t use a conventional electric motor. These include piezo-electric and voice-coil actuators as well as linear induction motors. These have very short working ranges and are beyond the scope of this discussion.
A rotary actuator provides motion in a circular path around an axis. Most rotary actuators can travel an unlimited distance, from a single degree or even less to many revolutions. Key performance parameters are angular distance moved, torque applied, velocity and acceleration.
The conversion from motor rotation to linear movement is accomplished in one of two ways. Either the motor drives a belt fitted over a pulley on the end of the motor shaft or the motor is coupled directly to a leadscrew. (In both configurations it’s not uncommon to have some gearing in between.)
In leadscrew systems a nut is mounted on the screw. As the motor turns the nut moves along the thread. A platform or carriage is mounted to the nut and held in place by guide rails. Turning the motor moves the platform along the screw.
To stop the nut running into the end of the screw there are limit switches at each end. When activated these cut power to the motor, which prevents any damage.
Small, light load linear electric actuators use 12 or 24V DC motors. 120V AC motors are also common, but many industrial electric actuators use 230V or 460V 3-phase power. Such actuators can carry heavy loads and apply high forces.
A key difference between linear electric actuators and pneumatic cylinders is that the former can accurately stop at any point throughout its movement range. Pneumatic cylinders are difficult to stop accurately between the ends of their stroke.
Precise position control is achieved through the motor, and there are two options. A stepper motor counts pulses as it rotates. Each pulse corresponds to an angular movement that the leadscrew converts to distance. To move to a specified position the motor is commanded to move a number of pulses.
The alternative is to use a servomotor where an encoder and controller keep track of position. Servomotors are more expensive than steppers but provide more precise control. For applications requiring high precision, industrial electric actuators almost always use a servomotor.
As with the linear actuator, motion comes from an electric motor. Here though, the shaft rotation is taken through a gearset to turn a platform. The gear ratios dictate the torque and speed achieved with high levels of each being possible.
On some rotary actuators the motor is mounted within the housing or platform support; others have it outside. Motors may be DC or AC. Stepper and servomotor actuators are available to provide the level of precision needed. Unlike linear actuators, there’s no need for limit switches to protect the mechanism itself. However, the application may need limit switches to prevent overtravel.
Linear actuators are used to provide a translation from one location to another. Electric linear actuators have the advantage over pneumatics of being able to stop part way through the movement. This makes them a good choice in material handling tasks and more generally for moving things over a short distance in a straight line.
Rotary actuators are used in applications such as robot joints where they provide precise angular movement. Valve actuation and movement of conveyor gates are other examples.
When selecting industrial electric actuators, navigating through the specifications and options can be confusing. The specialists at JH Foster can help.