Trends in Linear Actuators
| By: Kristin Lewotsky, Contributing Editor
Linear actuators put the load in motion, getting it from point A to point B at the speed and time desired. Today, the linear actuator market is in motion, as well. Electromechanical systems increasingly displace pneumatic and hydraulic actuators. As manufacturers look to improve flexibility and cost of ownership of their operations, they increasingly rely on integrated motor actuators. Within motion technology, linear motors offer functionality essential to a growing number of market niches.
The workhorses of automation, linear actuators are available in a range of technologies and configurations. The most common electromechanical actuator consists of a linear device driven by a rotary servo motor. It encompasses screw-type actuators, belt-drive actuators, and rack-drive actuators. For a detailed tutorial, including tolerances, click here.
The screw-type family of devices encompasses acme-screw actuators (often called lead screw actuators), ball screw actuators, and planetary roller screws.
In a lead screw actuator, a threaded nut travels up and down a threaded lead screw. The design is simple, economical, quiet, and less likely to be back driven. On the downside, screw whip limits speed, and friction from metal-to-metal contact between threads limits lifetime, speed, and efficiency. Sweet spots applications include cost-sensitive systems involving lighter loads and lower duty cycles.
Ball screw actuators conquer the friction problem using ball bearings in raceways between the nut and screw. This converts sliding friction to rolling friction, increasing lifetime. Drawbacks include higher cost, damage to bearings at high loads, screw whip, and higher levels of audible noise. Best suited to applications involving heavier loads and precision positioning.
Planetary roller screws consist of a planetary arrangement of threaded rollers surrounding a main threaded shaft. They feature a number of benefits such as high thrust, long lifetime, minimal maintenance, and high efficiency. The tradeoffs are the highest cost of screw-type actuators, larger size, easy to back drive, and screw whip. These actuators work particularly well for high-reliability applications that cannot tolerate failure.
These actuators can be configured as rod-style or rodless. In rod-style actuators the thrust element moves out of the end of the housing. They produce the most force and are sealed except for the end. Because the rod is not supported, it is subject to sagging in certain configurations. It also needs a carriage to support the load. Rod-style actuators work best for high-thrust applications, and high-contamination applications like wash-down environments.
In a rodless actuator the housing surrounds the screw, which moves the load on a platform that rides along the top of the actuator. On the upside, they support the load, have a smaller footprint, and the screw is fixed at both ends. On the downside, they can’t be sealed for wet environments. They work well for space constrained environments and loads that need to be supported.
Belt and Rack Drives
Belt-drives provide an alternative to screw-type actuators. They can operate at very high speeds for economical prices. There was a time disadvantages included stretching and need for regular tensioning. Today’s belts are highly structured and provide significantly better precision.
Rack-drives consist of a rotary motor that drives a gear, or pinion, along a precision-machined track. They deliver higher precision and accuracy than belt drives and offer higher speeds than screw-type drives. In theory, they are not distance limited, but past a certain point, cost becomes a factor.
In keeping with the trend toward integrated components and intelligent automation, integrated actuators have captured an increasingly large segment of the market. At their most basic, these devices combine motor and actuator such that the nut of the actuator acts as the rotor of the motor as it travels along the lead screw. Other versions with higher levels of integration may include the drive, sensors, and even the controller.
The integration brings multiple benefits. It removes the need for a coupler, reducing weight, part count, and points of failure. The overall device can be smaller.
Ease-of-use is another big selling point, notes Jim Mangan, Vice President of Sales at Nook Industries (Cleveland, Ohio). That holds for end-user, OEM, integrator, and vendor, alike. “If you sell it as a complete system, you know the capabilities of the motor and the actuator,” he says. “If you sold just the actuator and the customer supplied the motor, you don’t know what he may have done—it could be that they installed it wrong or programed it wrong, or something. If you sell the system, you know it’s done right, and so do they.”
That's particularly valued these days. Lean staffing means that maintenance departments may not have the expertise—or the time—to deal with home-grown solutions. Meanwhile, older machines typically need retrofitting or repairs. The original electronics may have gone obsolete, or perhaps it’s just time to replace the CAMs and pneumatic and hydraulic actuators with electromechanical systems. “Engineers need something plug-and-play so they can get their machine running with minimal startup time,” says Mangan.
Displacing Hydraulics and Pneumatics
End-users aren't just taking advantage of integrated actuators to replace obsolete electronics. More and more, electromechanical technology is superseding hydraulic and pneumatic cylinders. There was a time manufacturing centered around these technologies, along with fixed-speed motors, gears, and cams. Changeovers were laborious and required hours or even days. As a result, multi-purpose machines were the exception rather than the rule. Manufacturers focused on single-use machines or cycling through a small handful of products, filling up the warehouse with each cycle before the dreaded changeover.
One of the key growth areas for motion control is displacing the hydraulic and pneumatic cylinders in a variety of applications from packaging to military and aerospace. Integrated actuators with Ethernet connectivity can make changeovers as easy as choosing a new recipe on the HMI or changing the program on the controller. It’s a far cry from the days of wrenches and toolkits. “With a pneumatic cylinder or hydraulic cylinder, it was either dedicated or somebody would have to adjust one of the hard stops with a hand crank or a slide lock so that it would bottom out at the right position,” says Aaron Dietrich, Director of Marketing at Tolomatic Inc. (Hamel, Minnesota). “Now, a high-performance axis can adjust automatically to the next product size because it’s fully programmable.” The technology generates a lot of interest from users-at least until the discussion turns to numbers.
Everyone is on a tight budget these days, but none so much, perhaps, as the industrial sector, where fierce competition rages. That has only worsened the disconnect that exists between engineering, which focuses on total cost of operations (TCO), and purchasing, which targets total cost of acquisition (TCA). Over the lifecycle of industrial equipment, TCO dwarfs TCA in terms of issues like maintenance, downtime, power consumption, consumables, etc.
This is important because the number one misconception regarding electromechanical technology as it stacks up to hydraulics and pneumatics is that motion control is always more expensive. The key is to look at it from a TCO—and even a TCA—point of view. “We help a lot of people who want to switch from hydraulics to electromechanical actuators,” says Mangan. “They’ll see the price point and say, ‘I can buy a hydraulic cylinder for $250 and you’re charging me $2000?’ Well, yeah, but you have to look at it from the perspective of cost of ownership of the entire system. It’s programmable. It can take the load to multiple positions. It’s not just in and out [like a hydraulic or pneumatic cylinder]. You don’t have to buy servo valves, you don’t have to do a lot of stuff that takes time and costs money. People think that electromechanical is drastically more expensive than hydraulics. It’s really not.”
When it comes to replacing a pneumatic or hydraulic cylinder with an electromechanical actuator, the most common mistake is improper sizing. “Customers give us specs based on the bore and pressure of their system,” says Dietrich. “They’ll say, ‘I have a 3-in bore pneumatic cylinder running at 80 PSI, I need 550 lbs. of force.’ That’s not the way to go from fluid power to a motion-control system. They should look at how quickly they are trying to move the load, and the size of the load and how it is supported. Then we can point them to a much more economical motion-control solution. “
Lining Up for Linear Motors
The previous examples focus primarily on hydraulic actuators but electromechanical actuators deliver advantages over pneumatics, as well. Air cylinders typically shift between full extend and full retract. Now, customers want to be able to run multiple products on the same machine. To get an idea of the kind of advantages programmable actuators can deliver, let’s look at another style of linear actuator, the linear motor.
Although the combination of rotary motors and mechanical actuators dominates, linear motors have captured a number of niche markets. A linear motor can be thought of as a rotary servo motor unrolled. Instead of a rotor surrounded by a stator, a linear motor features a forcer moving with respect to a line of magnets. That’s a gross oversimplification, of course. In reality, a number of different configurations exist, including iron core, coreless, versions with the forcer moving in a U-shaped channel of magnets, motors with stationary coils and moving magnets, and rod-style forcers that are completely enclosed within magnets.
Linear motors offer a number of performance benefits. They operate to sub-micron accuracies and high accelerations. “If you need the combination of high speed and high accuracy, that’s where linear motors win the day,” says Dietrich. “A belt drive system can do super high speed but its moderate accuracy and repeatability.” This level of performance makes linear motors a good fit for applications like semiconductor inspection and nanotechnology. In theory, a linear motor can be made any length required, although issues of magnet cost impose practical limits. Strictly speaking, there is no contact between the forcer and the magnets, although they typically need a bearing to support the load.
Linear motors do have some drawbacks, of course. They require external support, which must be held to very tight tolerances for flatness. They can’t generate as much force as a rotary motor, although certain configurations such as multiple motors ganged together can still produce a fair amount of muscle.
“The downside to the technology for some of the simple applications is that it’s relatively high tech,” says Dietrich. “The motors can be harder to size because they’re direct drive. They’re more complex to deploy. You have to really get it right. There is not a whole lot of room for variation and change in the system.”
As far as the question of cost goes, the technology is becoming more competitive. “There are more and more manufacturers getting into the linear motor business so the cost is coming down,” says Mangan.
“Cost is a big deal. You can get an actuator with a built-in controller and amp for around $750,” says Edward Neff, President of SMAC Moving Coil Actuators (Carlsbad, California). “Now you’re competitive with ball screw actuators. You might be a factor of three or four times more than an air cylinder but if that cylinder is running at around five cycles per second, it’s going to wear out in six months. With a linear motor actuator, you’re good for 10 years.”
Integrated Linear Motors
Linear motors have followed the trend toward higher levels of integration and intelligence. The addition of position feedback enables devices to track their exact location and speed, while connectivity allows them to send the data to the supervisory control and data acquisition (SCADA) or enterprise networks. There, it can be used for everything from process control to business analytics.
Intelligent, integrated actuators promise significant improvements across a range of manufacturing sectors. “If you know what you’re doing when you put things together, then you really improve the quality,” says Neff. Consider a beverage bottling line. Screwing on a lid requires positioning it until the threads align with those on the bottle, then rotating it to tighten. That works just fine when you’re trying to recap the rest of that two-liter bottle of Mountain Dew. It’s harder to accomplish on a 300-part-per-minute bottling line.
The standard solution is to use a pneumatic actuator to put the lid in contact with the bottle, then spin it with a rotary motor. If the threads don’t meet properly, however, the bottle becomes scrap. In a low-margin industry like food and beverage, that’s a problem. Here, linear motor actuators provide a solution.
Instead of just using a rotary servo motor to screw on the cap, the approach combines a rotary servo motor with a high-resolution linear motor to mate the threads together without damage. “We can bump into a surface and recognize it immediately,” Neff says. “The cap moves up about 100 to 200 µm and when the threads match, it will drop. Now you know you’ve matched. You’ll go clockwise and check the torque as you’re rotating, check the distance, calculate the number of turns.” Armed with this information, the system applies torque, first lightly, with a pause for data collection, then tightly. At the end of the process, the system reports the data to the end-user for every cap put on.
He cites a customer who built lenses made of four separate elements screwed together. They positioned each component using pneumatic slides. The problem was that the slides were unable correct for misalignment so that the threads would mate up. As a result, up to 30% of production was scrapped as rejects. The manufacturer replaced the pneumatic actuator with a moving-coil linear motor with feedback, which could correct the misalignment. As a result, scrap dropped to 2%, the bulk of which was attributed to faulty parts and not to assembly.
Applications for this type of solution abound. An automotive manufacturer doesn’t want to discover an inaccurately tapped hole in an engine block by cross threading the spark plug. Mount a thread gage on the end of the linear motor and the controlled touch of the linear actuator enables it to quickly and efficiently perform acceptance testing on all tapped holes. The ability of the system to acquire position and speed data is essential to the process. The combination of positioning and data acquisition doesn’t just provide a tool to improve quality and yield, it supports traceability and safety, and potentially even regulatory compliance. The latter is an important benefit for high-reliability applications like automotive and medical.
The Road Ahead
The trends discussed above are seeing a strong level of uptake. As for more sophisticated technologies like the intelligent factory and the Industrial Internet of Things, the industry remains in the early adopter stage. End-users understand the value of functionality like additional sensors, but that doesn’t necessarily mean they’re ready to pony up just yet. “There is chatter in the market about getting better diagnostics, which would require temperature sensors or other types of sensors in an actuator, but by and large customers don’t want to pay for it right now,” says Dietrich. “Customers will come in with a laundry list of wants. Then you give them the big price tag where we have met everything, and they choke. That’s when we have the conversation about what they really need versus what they want.”
For now, the paradigm he sees apply to both automation vendors and end-users is one of incremental improvements rather than radical shifts. “People see the value of change but when it comes down to brass tacks, they have plants that are done this way or that way,” says Dietrich. “They might change some but not lock, stock, and barrel. They might bring in the system to speed changeovers and make them flexible.”