Traditionally, linear motion system designers implementing high-duty-cycle motion had to choose among hydraulic, pneumatic, and electromechanical systems. Each technology did the job but came with specific limitations relative to the support infrastructure required to drive/control them. This can include frequent maintenance or high upfront costs for things like servo drives/encoders, compressors, and hydraulic power units as well as accessories and piping installations.
Now, a new generation of long-life electric linear actuators has emerged to give designers more flexibility for high-duty-cycle/lower-accuracy motion in applications for which traditional actuators were previously cost-prohibitive or substandard in their duty cycle ratings/abilities.
Actuator life is a function of its duty cycle, i.e., the percentage of time it can apply to its work, as calculated by the following equation:
Duty cycle (%) = (actuator on-time)/(on-time + off-time)
So, if the actuator operates for 15 seconds and then rests for 45 seconds during a 1-minute cycle, its duty cycle is calculated as: (15 s.)/(15 s. + 45 s.) = 15/60 = 0.25 or 25%.
Manufacturers issue duty cycle guidelines to help keep the actuator at rest long enough to prevent the motor from overheating and damaging actuator components. Duty cycles for hydraulic and pneumatic actuators tend to be in the 40% to 80% range or even higher with enough power. But that functionality is accompanied by a higher cost of operation.
Fluid power systems like those required to operate pneumatic or hydraulic actuators are complicated and can be messy, labor-intensive, space-consuming, and expensive to own/operate.
Even the smallest system would require multiple support components backed by a system of tubing/hoses, valves, and other major components. These systems are also typically far less energy efficient when compared to electromechanical equivalents.
Conventional electromechanical actuators that are commonly used to support higher-duty-cycle applications likewise are not cheap. Requiring stepper- or servo-motor-driven prime movers means that this equipment comes at a high cost that can only be fully justifiable if high precision is also needed. Otherwise, they are somewhat overpriced for lower-precision industrial applications, which may cause some users to avoid automation entirely. Stepper- and servo-driven actuators also require connection to AC power, further limiting their use in mobile/robotic applications.
Thanks to recent advances in linear motion control, a new generation of long-life, DC-driven electric actuators is available at a price point that makes high duty cycle affordable for a new range of applications.
Inside a New-Generation Long-Life Actuator
Long-life actuators take advantage of a unique combination of design features in order to perform for high-duty-cycle applications. First, instead of using servo, stepper, or brushed electric motors, long-life actuators use brushless DC motors. Unlike a brushed DC motor, which would overheat at high duty cycles, the brushless DC motors generate essentially no friction. This enables a duty cycle of up to 35% or, depending on the load and ambient temperature, up to 100%.
Other features contributing to the actuator’s durability are double-circuit nuts and larger screws. They reduce another common point of actuator wear and extend cycles from 30,000 or 40,000 to about one million, improving travel time by orders of magnitude. Additional life-extending design features include a strengthened screw and extra-angular contact bearings.
Together, these features result in an actuator that lasts ten times longer than the traditional brushed motor-driven actuator that might otherwise have been deployed in transport-grade industrial applications while still at a fraction of the cost of servo- or stepper-driven styles. When compared to stepper- or servo-driven solutions, brushless DC motor-driven actuators tend to be more compact because they don’t need gearing configurations. Long-life actuators are also typically sealed for high-ingress protection and lubricated for life in the factory, thereby requiring zero additional maintenance.
Long-life actuators are best suited for applications in which the capability to perform frequent back and forth or transport-type motion extensively is more important than complex, high-precision movement. This includes clamping, positioning, blocking, diverting, or any basic application where linear motion can be deployed to improve/automate a process. For example, fixturing the weldment in place for a robotic welder often requires frequent manual positioning, clamping, or rotation on multiple axes. The more often the fixturing needs adjustment, the more potential benefit can be gained from putting actuators on those axes.
Long-life actuators also contribute to improved ergonomics for operators. Just as electrically controlled seats on automobiles optimize seat comfort for drivers, assembly line operators may use an actuator to adjust a conveyor or workstation height to improve ergonomics and reduce workplace injuries.
In an increasing number of factories, conveyors are being supplemented with automated guided vehicles (AGVs). In an automotive plant, for example, AGVs might deliver doors, hoods, hinges, bolts, and other components to an automated assembly line on a 24/7 basis. Actuators could be deployed in numerous lifting, loading, and positioning applications that previously could not have been actuated cost-effectively. The high-duty-cycle option on mobile applications such as AGVs, which are often powered by contact with magnetic floor strips, also opens new possibilities for motion designers.
As the cost, reliability, and flexibility benefits of long-life actuators become more apparent, designers will likely consider automating axes/functions that previously had not been feasible. Just as automobile seat controls were once available only for driver’s seats on top-of-the-line models, some newer models have electronic controls on every seat, front and back.
Freedom to Actuate
McKinsey Global Institute (MGI) estimates1 that more than half of all current manufacturing activity is spent in “predictable physical” activity that can benefit from automation, with some applications such as welding, cutting, and soldering having automation potential exceeding 90%. Taking advantage of this emerging opportunity will no doubt require some complex, high-precision actuation, while other opportunities may not. The emergence of long-life actuators gives motion system designers more freedom to choose actuation strategies that are best for their application.
- McKinsey Global Institute. (2017) A FUTURE THAT WORKS: AUTOMATION, EMPLOYMENT, AND PRODUCTIVITY. MGI-A-future-that-works-Executive-summary.ashx.
This article was written by Travis Gilmer, Product Line Specialist – Linear Actuators, at Thomson Industries (Radford, VA) and Ian Miller, National Services Business Development Manager at Motion Industries (Calgary, AB, Canada).