In the first installment of this series, we covered the reasons why off-highway applications are primed for electrification — especially those vehicles with lots of axes that deliver relatively slow yet high-torque (or force) motion.
Where the axes on a piece of off-highway or other equipment are linear, electric actuators have in some cases replaced those based on fluid power. One reason is because electromechanical systems consume energy only when actually moving an axis, and they’re 82% efficient on average at converting input power into output work. 45% end-to-end efficiency is common for hydraulics. In addition, utility electricity tends to be cheaper than fossil fuels in many areas.
So, what are the options for linear motion if electrifying an application? Except for pneumatic, solenoid, piezo, electrohydraulic, and linear-motor designs, the core of all linear-motion systems is a rotary electric motor paired with some mechanical rotary-to-linear device. The latter converts the high-rpm rotary motor output into linear stroke.
More common options include ballscrews, roller screws (including planetary and differential types), jack screws, belt and chain drives, and rack-and-pinion.
If a ballscrew-based actuator is used in an electrification application, what form does it take — carriage or rod? Carriage-type actuators are the common and quintessential variation; motion stays within the limits of a housing and load mounts to a carriage taking the form of a saddle or table. In contrast, rod-type actuators deliver motion via a rod that extends out of and retracts back into a housing. The load may mount to the rod end … or the rod can be used to push the load.
Carriage-type actuators can be guided by recirculating or plain bearings, depending on the load for which they’re designed. In contrast, rod-style versions are not typically designed for radial loads from downward or sideways force vectors.
Instead, they usually include use simple plain bearings to provide guidance to the rod without significantly contributing to load-carrying capacity.
Ballscrew rod-type electric actuators more often replace fluid-power cylinders due to their similar form factor, force transmission, and mounting. Rod-type actuators output thrust or plunge in extension or retraction — a lot like fluid-power cylinders. So, they can be easy to retrofit into legacy machine designs.
High axial force and stiffness are other benefits … especially over short to moderate strokes. Plus, rod actuators support clevis, trunnion, or foot mounting just like fluid-power cylinders do. Plus, during the design-engineering phase, their strokes are specified in comparable ways.
Related article: Automation in agriculture and off highway
Many rod-style electric actuators are also IP-rated and sealed to satisfy the same requirements as fluid-power cylinders to withstand challenging environments.
To be clear, traditional carriage-type ballscrew actuators are applied in electrification efforts and even to replace fluid power. But more often they’re used in previously manual applications or in totally new designs needing accurate and repeatable positioning such as pick-and-place tasks.
Out of all the pneumatic actuator types, rodless cylinders (complete with linear bearings to address moment loads in X, Y, and Z) are most likely to be a competing technology to electromechanical solutions. They have higher power densities than size-equivalent electric actuators so can output more force than comparable electric actuators. Plus, rodless pneumatic cylinders are extremely fast — capable of meters per second.
That said, compressed-air power characteristics mean upsizing pneumatic cylinders for high force generally diminishes their speed and vice versa. Long strokes also diminishes cylinder top speed … though ballscrew whip imposes similar speed limits on extra-long axes.
Electromechanical designs (especially ballscrew-based actuation of linear axes) also offer the two benefits of efficiency and simple physical architecture. Pneumatic cylinder needs clean air via hose, compressor, valves, filter, regulator, lubricator, and fittings. Electromechanical designs only need an electrical supply and control. The efficiency differences can in many cases be considerable.
So how to know whether a ballscrew-type electric actuator (of any subtype) is the correct choice for a given electrification design? What if it’s more suitable to specify a roller-screw-based electric actuator — a component type we’ll cover in a moment?
An industry rule of thumb us that ballscrews generally compete against pneumatics and roller screws compete against hydraulics. But no such rule of thumb should ever be followed blindly — especially given the wide variety of adaptations of all four of these technologies.
What’s more, especially with electrification of very established designs such as farm equipment, quantification of the machine axes’ performance values may be needed. Typically, an application’s required forces, speeds, and precision are more or less known. But sometimes full definition involves reverse engineering the function and feel of existing equipment in real-world environments.
Only after such investigations and analysis in simulation software can sizing and specification happen to ensure the electric drive, motor, and mechanical componentry satisfies power and speed requirements especially.
Or consider ballscrews’ somewhat rarer cousin — roller screws. Planetary, recirculating, differential, and inverted roller screws excel where high speeds and loads are present … even while maintaining efficiencies of 80% or better. These mechanical rotary-to-linear devices work in an increasing number of electromechanical actuators. Satellite roller-screw geometry especially includes far more internal contact between load-bearing subcomponents than their ballscrew cousins, and that boosts rigidity and dynamic load capacities. What’s more, fine pitches of their subcomponents’ grooves or threads often impart unbeatable mechanical advantage … so less input torque is needed to move a given load.
Roller screws often compete with ballscrews and fluid-power-based cylinders for linear motion. Forces are comparable to those of similarly sized hydraulics — typically from 20,000 lbf to 800,000 lbf depending on size and design — and high thrust capacity thanks to the numerous and relatively large (line) contacts between nut rollers and screw. In addition, actuators based on roller screws maintain efficiencies to 90% compared to hydraulic-cylinder efficiencies to 50% or so.
Speeds and accelerations are comparable to those of pneumatics … exceeding 40 in./sec and 3g in some cases. Such output is possible because the load-carrying rollers in roller screws don’t contact one other. That’s unlike the balls in ballscrews that collide and strike recirculation end caps which in turn unwanted generates forces and heat. So even where the high forces of roller screws aren’t required, their high speeds and acceleration is useful: Roller screws (instead of ballscrews) to replace pneumatics gives designs heightened ability to withstand continuous-duty and frequent cycling conditions.
Two drawbacks of roller screws include their higher cost (even to 100% more than comparably sized ballscrews) and their being less common than other technologies.
Final note about motors for electrification
As covered in a recent Design World podcast at designworldonline.com/podcasts, one might assume that for battery-powered vehicles, dc-driven motors are the only viable option. In fact, power electronics and advanced control algorithms are now letting engineers use ac-based systems for mobile equipment. Such ac systems offer better controls and efficiencies than many dc systems.
The electric linear actuators (and other electromechanical solutions) here need ruggedized and sealed housings to withstand the challenges of operating outdoors. As of now, most all actuators in these designs also have at their core a dc motor for easy integration into battery-powered and onboard power-generation systems. But change is coming.
Consider garbage-collection trucks driven slowly and subject to lots of stops. Regenerative braking (and the electric systems to leverage its benefits) is perfect here. Certain ac electric-motor systems on container handlers at shipping ports are another example.
Instead of direct current, the drive motors on these vehicles might better employ permanent-magnet ac motors. In contrast with sinusoidal input to three-phase ac motors, the motors accept the finessed square waves of pulse-width modulation adaptations generated by specialty inverters. Efficiency is highest when such drives feature semiconductors to optimize magnetic flux to the motor windings in realtime.
IGBT transistors dominate but silicon-carbide transistors for drive switching could one day take over — especially for electrified off-highway equipment — because they’re slightly more efficient … by 5 to 10% or so. Even such tiny gains help a battery-powered vehicle.
Leave a Reply
You must be logged in to post a comment.