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Actuators, Actually
By: Kristin Lewotsky
(posted 01/27/2011)

When used properly, actuators integrated with motors and even drive electronics can simplify motion system design and assembly.

Figure 1 - Cutaway shot shows planetary roller actuator (center) integrated with a motor (right). (Courtesy of Exlar)Motion control is the ultimate enabling technology. For the end user, it’s not about the motion itself, it’s about using motion to accomplish some other task. Little surprise, then, that the single most dominant trend in motion control in the past decade has been integration: motors and drives, drives and encoders, drives and controllers, etc. Now, companies are taking the trend one step further, creating integrated motor-actuator combinations that offer varying degrees of simplicity and multifunctionality.

The degree of integration varies depending on the application and the environment. The most basic level involves integrated motor/actuator products that take the guesswork out of the coupling process (see figure 1). At the opposite end of the spectrum lies the total integrated package that incorporates motor, actuator, and electronics (see figure 2). Handled correctly, integrated actuators can provide big benefits. Handled incorrectly, they can generate frustration, at best, and unnecessary cost, at worst.

For starters, no matter what the level of integration, end-users need to properly specify their actuator (see sidebar). The application characteristics come first. "There are wide differences in what may be available for a mobile equipment application versus a typical factory application,” says John Walker, Vice President of Sales and marketing at Exlar Corp. What are your constraints on power, efficiency, size? Perhaps more important, are there any special environmental conditions? Factors like temperature, shock, vibration, moisture, or contact with corrosive chemicals can make a big difference in choice of materials, design, seals, and even lubricant.

Specifying An Actuator

Before you call your vendor to you should be clear on your specifications. They include:
  • Speed
  • Stroke length
  • Moment load and details of any other forces, including where they occur in the motion cycle.
  • Motion orientation—vertical or horizontal motion
  • Size
  • Mounting requirements
  • Coupling type
  • Any special environmental conditions, e.g. caustic chemicals, excessive heat, shock, vibration, moisture, high duty cycle, etc.
  • Lifetime

K.L.

Knowing lifetime is essential. "A lot of customers come in with a request for force and speed but they have no pre-determined life requirement," says Walker. "That's a big consideration because you can have multiple different actuators that can produce a certain amount of force and speed but there will be a wide range in terms of how long those actuators last. Without having a life requirement, it's difficult to make the right selection.” Lead-screw actuators provide economical power transfer but can be limited by friction, for example. Ball-screw actuators use ball bearings to lower friction but they introduce more points of failure, as well as noise. Planetary roller screws provide robust, long-lifetime performance but at higher cost. Belt drives supply low-cost, high-speed operation but performance varies over the lifetime of the belt and they're not suitable for vertical loads. There is no one perfect answer, just the right answer for your particular application.

With an integrated motor-actuator or motor-drive-actuator product, sizing takes on new challenges. One common pitfall arises during the replacement of pneumatic and hydraulic systems with electromechanical technology. On first blush, the process seems as though it should be straightforward: Start with the parameters for the current device, calculate pressure times area to derive force, then find an electromechanical actuator to provide that force. Easy, right? Not exactly. “That's really a bad mistake because you're going to end up over sizing the motor, drive, and actuator by a large margin,” says Aaron Dietrich, Product Manager of Electric Products at Tolomatic Inc. (Hamel, Minnesota). “In the servo world, people decide what they need and then add 20% or 30% as a safety factor. With a lot of fluid-power devices, they double or triple the size because going up in size doesn't cost a whole lot, but with an electric actuator, going up a size can actually cost quite a bit."

Figure 2 - Integrated motor actuators can include drive electronics, simplifying integration. (Courtesy of Exlar)As for switching from one line of motion control products to another, it's not as easy as laying two catalogs side-by-side to convert product numbers. "If you just cross-reference, you open up a whole can of worms," says Dietrich. “You want to be able to size the application appropriately and feel comfortable that your products will last.” Although a certain amount of over sizing is appropriate, past a certain point that can backfire. "If you oversize the actuator, you're going to have bigger bearings, maybe a Figure 3: Integrated motor actuator incorporates the windings of the motor directly into the actuator housing, as well as including an encoder (right). (Courtesy of Tolomatic  Inc.)bigger mass to move, probably a little more friction, more preloading. And when you oversize things, whether you like it or not, the system becomes less efficient because you're not running it in its sweet zone from an electrical standpoint.”

Motors, Actuators, and Electronics: The Total Package
The challenge of going with an integrated motor actuator product can be interfacing with a third-party drive. It may not necessarily be extraordinarily difficult, but it is one more task to perform. As a result, users may find themselves better off with a top-to-bottom package that already includes electronics. The approach can reduce cost and speed integration and assembly time, but gaining full benefits requires proper application.

In centralized architectures, sensitive electronics like drives and controllers stay in temperature-controlled enclosures or even in control rooms. If the design calls for a distributed architecture with integrated motor drives on all other axes, perhaps the best choice is to move to a higher level of integration with the motor/actuator. “One of the starting design choices is what else is on the machine,” says Walker. “If there aren’t a lot of things that need to be in large cabinets, it makes great sense to use an integrated actuator that has the drives and controls on board.”

The challenge is that electronics and the motor both generate heat on their own. Place a component in a high-temperature environment and both performance and lifetime may suffer. "On some of the highest performing axes, most integrated solutions that are out there today fall short because of heat issues," says Dietrich. In the end, it comes down to what the application requires. Figure 4: Integrated rod-style actuator combines a control electronics (green, right) with a motor (red, right) and a ball-screw actuator (center). (Courtesy of Tolomatic  Inc.)

For certain applications, instead of the choice between integrated motor-actuator and integrated motor-electronics-actuator, the best fit is a hybrid solution that lies somewhere in the middle. “You can have those same drive electronics available in a package that can be mounted off of, but close to, the actuator,” says Walker. “If you're in a foundry application, using actuators to release metal into molds, the environment right next to the actuator is very hot, but the overall environment 10 feet away is not, so we can move those electronics away and allow the desired performance.”

The next obvious step for integrated actuators lies in packages that integrate sufficient intelligence for preventative maintenance, or even include controls capabilities. For now, machine builders have a range of choices when it comes to satisfying their application, and integrated actuators are worth considering. “Integrated actuators are not going to take over the whole market,” Dietrich observes. “Right now it's more of a niche than anything.” It's an important niche, however. For the right application, the benefits can be substantial.


 
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