Advantages of Electric Linear Actuators

Reduced downtime. Electric linear actuators (whether screw- or belt-driven) are very low-maintenance. Regreasing may be the only regular maintenance necessary, and many screw-driven models are lubricated for the life of the actuator.

Electric (left) and pneumatic actuators

Electric actuators commonly use stepper motors or brushless DC (BLDC) servo motors to generate torque. Because both motors are brushless, there is no contact between the rotor and stator other than the load bearings. This eliminates motor maintenance, and allows the motor life to equal the life of the bearings.

An air cylinder, on the other hand, must be taken out of service regularly to be rebuilt or replaced. At full duty-cycle operation, pneumatic cylinders may need rebuilding or replacing as often as once per month, depending on the cylinder style. Rebuilding includes cleaning; inspecting worn or scratched items; replacing seals, cushions, and bushings; and reapplying grease. Air leaks in lines and fittings must also be identified and repaired. Such maintenance requires several hours of downtime, which increases costs due to lost production.

Reduced energy consumption. The “green machine” trend of energy savings pushes manufacturers to think about energy use over time. Servo-driven electric linear actuators consume only the energy required to perform the programmed motion. When idle, little energy is consumed.

A stepper motor may run at 100 percent of its rated current during a motion, and have a substantial steadystate holding current as well. This holding current can waste energy because it locks the rotor to a position regardless of whether any work is being performed. However, many stepper drivers can reduce the current to minimal levels.

Figure 1. The many teeth inside a stepper motor.

Pneumatic cylinders are commonly oversized for the job because a larger-bore cylinder may not cost much more. Pneumatic actuator systems consume a lot of energy when incorrectly sized to the compressor, or if air leaks are not monitored and contained. Pneumatic systems are only efficient when sized correctly for the number of air cylinders in use, and when the duty cycle of the application is high.

Increased precision and speed. As technology improves, engineers and end users demand more capability at lower costs. In the case of industrial automation, this means product must flow faster through production lines, and meet increasingly tighter specs with minimal machine downtime and energy consumption.

Electric linear actuators can meet these demands because they offer more control over position, speed, and force. High-resolution feedback devices are common today and easily allow for micrometer-scale precision. Advanced drive tuning can compensate for things like changing loads, inertia, and preventing mechanical resonance. Electric actuator manufacturers often provide sizing software to accurately size an electric actuator to the application, and select a motor with enough torque to reach the desired speeds.

Choosing a Stepper or Servo

When selecting a motor for an electric linear actuator, motor type is a major consideration. Two common motor options for electric actuators are stepper motors and BLDC servo motors, each with advantages and limitations.

Stepper motors. A stepper motor has the advantage of being controlled accurately in open-loop position control. Open-loop control means there is no feedback information needed to position the motor. This provides a cost savings over BLDC servo motors as no position feedback sensor or associated cabling is required.

The stepper motor’s position is known by the number of input pulses or steps commanded. A common stepper motor can have 1.8 degrees mechanically per step, which results in 200 steps per revolution. Manufacturers will spec a step angle accuracy of 3 to 5 percent of one step noncumulative in a typical stepper motor.