Electric actuators explained

» The basics

How do electric actuators contribute to productivity in today's motion-centric automation environment?

John • Exlar: The flexibility provided by electric actuators greatly enhances a design engineer's ability to provide performance not otherwise available. Electric actuators, as opposed to fluid power devices, offer the ability to rapidly change machine setups, travel length, speed, and acceleration.

Yoshi • THK: An actuator generally consists of a drive — whether a ballscrew, belt, or linear motor — and some type of linear guide. Choosing the right drive system for the application is the key to productivity. Ballscrews offer a large mechanical advantage (input torque vs. the axial force they are capable of generating), but are limited in stroke length and speed. Belt drives, on the other hand, are capable of long stroke lengths and high traverse speeds, but are not appropriate for precise positioning. Linear motors achieve both speed and precision, but are more expensive. Still, if a high accuracy, high speed electric actuator is needed for a light load, then a linear motor is probably the best choice.

Jim • Tolomatic: Machine automation depends on the ability of a power transmission system to provide controlled motion. The key word is “controlled.” Early 20th century systems used belts, pulleys, and gearing based on simple ratios to control speed and torque. With the advent of fluid power, rotary and linear motion gained a significant increase in controllability to include pressure and flow compensation, and feedback loops that allow better control, which translates into better machining capabilities.

One limitation of fluid power systems, when it comes to control, is the compressibility of the fluid itself. Compressed air has been likened to a coiled spring, and even hydraulic fluid, under higher pressures, exhibits some degree of compressibility. In addition, environmental conditions greatly affect the performance of fluid power systems. Heat, water, fluid condition, and fluid type contribute a degree of variability that affects control, even though advances in fluid power feedback mitigate these conditions.

Enter electromechanical systems, especially those with closed-loop servo and micro-stepping motors. The goal in any automation system is to match, as closely as possible, the exact speed and torque/force requirements as demanded by the application: The “tighter” the controls, the better the ability to respond and react, thereby increasing accuracy compared to intent. Although designers must contend with the physical limitations of leadscrews, belts, and mechanical stack-up tolerances, advances in feedback devices help minimize output errors, achieving near-perfect response.

» Tried-and-true tips

What's your best advice on specifying, sizing, and applying electric actuators where productivity is the goal?

John • Exlar: Size for the worst possible case. For example, think about pressing applications. Assume that the machine may be left in the pressing condition continuously, even if it's unintentional. As a result, the motor driving the actuator must be sized for continuous rather than intermittent duty to maintain pressing force without burning up. Downtime is the number one productivity killer, and undersizing an actuator will eventually cause it.

Get good information before you make a decision. As the saying goes, garbage in means garbage out. Failing to have accurate information before the selection process can easily lead to oversized or undersized actuators. One common error is neglecting to account for the inefficiency of hydraulics when replacing them with electric actuators. Hydraulic systems are typically about 50% efficient. Often, when an electric replacement is chosen, it is sized to provide a force equal to the pressure times the piston area of the hydraulic cylinder. The result is an electric actuator much bigger (and potentially less responsive) than it needs to be.

Consider the environment. Chemicals, liquids, and contaminants can affect how an actuator works. Electric actuators typically contain sealing elements such as rod seals and o-rings. These devices are resistant to some chemicals, but not to others. If an electric actuator is going to be in contact with different solutions and chemicals, it's important to discuss the sealing materials with the manufacturer to ensure compatibility.

Jim • Tolomatic: Keep actuator life factors in mind. Dynamic loading calculations are necessary to determine the life of the drive mechanism. In the case of a rolling contact leadscrew, reducing the load by 50% increases screw life by a factor of eight; increasing the load by 50% reduces life by 30%.

Select the correct leadscrew. Two general types include rolling contact devices such as ballscrews or planetary roller screws, and sliding contact devices such as Acme leadscrews. The wear on rolling contact devices is much more predictable than for sliding contact types. It is best to base life expectations for sliding contact screws on empirical data.

Don't forget about side loads. This is especially important with rod style actuators. Screw life is inversely proportional (and not always linear) to side loading. Be sure to check actuator specs and be prepared to guide and support the load.

Don't believe everything you read from suppliers. Some suppliers may refer to a “high capability of life and loads” — until you read the fine print. For example, one manufacturer may report life based on using a recirculation oil bath; when used as-is, actuator life is actually reduced by 30 to 50%. The lesson here is: Qualify spec by referring to a known load for its calculated life, or specify a minimum life and then determine maximum allowable load.

Bob • Bimba: Know your load. Identify axial and radial loads, take measurements, and make calculations so that the right actuator may be identified the first time.

Use safety factors. An electric actuator that has headroom will run cooler and last longer.

Understand your environment. Reliability problems can be avoided if it is known initially whether the actuator will be exposed to dust, filings, shavings, moisture, or other contaminants.

Plan for surprises. Choose an actuator that will accommodate potential variations in your production process. Consider the operating range — speed, actuation distance, load force — over which the actuator can safely function, understanding how the motor, actuator, driver/controller, and encoder play into it.

Be flexible. Enable your machine to be flexible and adaptable to changes in product.

Develop a maintenance schedule. Thermal imaging can be used to identify stressed components before they fail.

» Worst-case scenarios

What's the worst that can happen if an electric actuator is not specified or installed correctly?

John • Exlar: In one case, someone was using large electric actuators on a punch press protected by holding brakes. The brakes were designed to keep vertical loads from falling during power interruptions. However, the system was improperly set up, and the holding brake would come on every cycle with the motor at full speed in response to an operator breaking a light curtain. The inertia and speed were more than the brake could handle, equating to a torque load about 10 times larger than the brake capacity. Needless to say, the brakes were rapidly being destroyed; the actuators were literally tearing the splines out of the brakes.

In another case, someone returned a small actuator with the seals and o-rings removed, and the unit had been contaminated with a liquid, causing it to fail. We contacted the user to find out what the liquid was, and why the seals and o-rings had been removed. It turned out they hadn't been removed — they were completely dissolved by the chemical mixture with which the actuator was regularly doused. After some research, special seal and o-ring materials were sourced that were resistant to the chemical; the unit was refurbished with the new materials, which were also used in future units.

Yoshi • THK: Friction is still the main enemy of industry. In an assembly line, objects are in motion and the more friction present in the system, the more energy it takes to move an item from point A to point B. Unfortunately, if there is physical contact between two rolling or sliding elements, sooner or later they will wear out: Ball bearing raceways will experience surface cracking and belt drives will lose their pretension and experience tooth wear. Applied loads, moments, accelerations, and other forces that resist a change in motion can compound these effects.

Assessing the application and what is truly required is essential to choosing a bearing support system and drive mechanism that satisfies those requirements. Productivity will then follow. Don't pay for performance you don't need; use only what is necessary to satisfy the critical constraints of the application. For example, a positioning system for a stop-saw woodworking application need not attain 10µm positioning accuracy. Likewise, an engineer who's designing a DNA scanner cannot choose a belt-driven actuator just because he's looking to cut cost.

Bob • Bimba: One of the risks with standard products is that end-users will order out of the catalog or website and not ask for application consultation beforehand. Sometimes a brief phone consultation with the manufacturer's technical team can prevent misapplication of products and ensuing headaches.

For example, a slide was once returned to us because bearings seized. It was obvious that the slide was being used to move a saw blade (trimming filters). The screw and guide rods were unprotected, and the fine dust produced from cutting formed sludge with dripping oil, which eventually hardened and seized the bearings. If a screw wiper had been added and the slide inverted, the user could have circumvented the problem. If only they would have asked for some upfront advice.

» Future perfect

What would the ideal electric actuator look like?

Bob • Bimba: I can't envision a single ideal. Speed, thrust, stroke, end play, precision, repeatability, cost, radial load capacity, sealing, and software compatibility all vary in importance depending on the application. For MRO applications, the ideal actuator might be an electromechanical chameleon of sorts that requires no additional hardware and interfaces flawlessly with a variety of systems, equipment, and operating software. In OEM applications, the ideal might be a flexible mechanical platform that accepts a wide variety of motors and electronics and is adaptable to various environments.

Jim • Tolomatic: The ideal actuator works in an ideal system that shares these characteristics:

Truly plug and play
Universal and wireless communication, from encoders to drives and controls
Ability to read and interpret any motor parameter on-the-fly
Ability to compensate in real time for system performance variations
Integration in every sense of the word: drives/controls, actuators/motors, connectors, encoders, diagnostics, force feedback, and software

The laws of physics will always impact performance, but efforts need to be directed toward reducing mechanical limitations and increasing seamless integration regardless of brand name.