- SPECIAL REPORTS
Most assembly operations don’t require complex motion. A simple up-and-down or side-to-side move is all that’s required.
A linear actuator might be used to:
Extend and retract a dispensing valve or a set of fixtured screwdriving spindles.
Apply sealing fixtures to an assembly in a semiautomatic leak testing station.
Move a pallet in and out of a standalone semiautomatic assembly cell.
Move a machine-tending robot from one processing cell to another.
Snap a stack of parts together or clamp parts in place while an assembly operation is performed.
Or, consider a standard pick-and-place operation: A linear actuator extends and retracts a gripper to retrieve a part from an escapement. A second linear actuator then moves the assemblage to a rotary indexing dial, and the first actuator extends and retracts again to place the part in a fixture.
Despite the simplicity of the motion, engineers have multiple options and many factors to think about when choosing a linear actuator. There’s speed, distance, load and accuracy, to name a few. Do you need an electric or pneumatic actuator? A belt drive or a ballscrew?
“You need to consider what loads you’re moving and how fast you want to move them,” says Frank Langro, director of marketing and product management at Festo Corp., which supplies both pneumatic and electric actuators. “Do you need to accelerate or decelerate the load? Will the motion be constant and repetitive? Are you always going to move from point A to point B? Or do you need a machine that’s more flexible?”
Pneumatic actuators have long been the standard technology for linear motion. They are simple, fast, lightweight and inexpensive.
“A big advantage of pneumatic actuators is that they provide a high force density,” says Langro. “So, for a smaller package size, pneumatics can typically produce more force at the end-stroke than electromechanical actuators.”
For the most part, pneumatic actuators are designed to move to only two positions. “If all you need is two positions, there’s no point in dealing with servos and controls,” advises R.J. Ruberti, team leader for linear systems at SCHUNK Inc., which offers both pneumatic and electric actuators. “You may as well use a pneumatic actuator and keep it simple.”
Maximum stroke length with a rodded cylinder is typically 400 millimeters or less. Rodless cylinders can provide longer stroke lengths, but less force.
While pneumatic actuators have been around a long time, the technology has not remained static. Today’s actuators are superior to their predecessors, offering better seals, valves and control sensors. Many actuators are lubricated for life, which eliminates the need for lubricated air and its associated maintenance costs.
If the actuator must move to more than two positions, some units can be equipped with proportional flow valves to modulate air on both sides of the piston. “It’s not as precise as electromechanical actuators, since air is compressible, but it does give you some flexibility,” says Langro.
Festo recently introduced pneumatic actuators with self-adjusting air cushioning. To reduce the impact energy at the end of each stroke, pneumatic actuators are typically equipped with cushioning, either a bumper made from an elastomeric material or an air cushion regulated by a needle valve. The former can only absorb small amounts of energy. The latter can be tricky to set up and requires frequent manual adjustments.
Festo’s new design automatically adjusts the amount of air cushioning depending on the impact energy. In principle, it works the same way as manually adjustable air cushioning. However, the cushioning chamber is not exhausted via a needle valve but rather through notches in the cushioning piston. These notches make it possible to exhaust the air cushion independent of the cushioning length. The geometry of the channels provides phased venting of the cushioning air.
“That saves setup time, and it gives you more consistency from cylinder to cylinder,” explains Langro. “That can be important in a machine with a lot of cylinders.”
The chief advantages of electric linear actuators are flexibility and control.
“With electric actuators, you can move to hundreds of different points,” says Chris Prior, senior technical support engineer at IAI America Inc. “You have control over velocity and acceleration at each position, and you can apply varying amounts of force. Electric actuators allow you to optimize your process for greater efficiency.”
Electric actuators are also more energy efficient. “A pneumatic actuator might be 15 percent energy efficient,” says Prior. “Electric actuators are 80 percent to 90 percent efficient.”
Another advantage of electric actuators is product range. They are available in myriad sizes, shapes and configurations to suit any application. They can be equipped with stepper motors or servomotors to drive leadscrews, ballscrews or toothed belts. Alternatively, they can dispense with rotary motion entirely and be driven directly by a linear motor.
“We have small actuators that can move 1 or 2 pounds, and we have large actuators that can move thousands of pounds,” says Ruberti. “We have belt drives with a top speed of 8 meters per second, and we have screw drives that provide 8,000 pounds of thrust.”
The choice of motor depends on the application and the control technology. “Servos have a flat response to payload,” explains Prior. “So whether the actuator is unloaded or fully loaded, its performance doesn’t change.
“A stepper motor, by its nature, will have a curve. With more payload, you get less speed.”
“Stepper motors are best for applications with low speed (less than 2,000 rpm) and acceleration requirements, and where cost is a major consideration,” adds Nathan Davis, product specialist at Bosch Rexroth Corp. “Servomotors are ideal for highly dynamic applications, because they can supply several times their rated torque for short periods.”
The choice of belt or screw drive depends on the application. “Belt drives can provide long strokes—24 to 100 feet—and the actuator can move very quickly, up to 200 ips,” says Paul Kuczma, marketing manager at Raco International.
“The downside is that they cannot handle the loads that screw drives can. Our belt drives have a maximum thrust of about 800 pounds, whereas our electric cylinders can produce up to 225,000 pounds of thrust.”
When specifying linear actuators, engineers should provide as much information about the application as possible. What will the actuator be doing? Where will it be located? Will it be mounted horizontally or vertically? Will it be operating by itself or will it be part of a multiaxis positioning system? How often will it operate? How far will it travel? Will it need to stop at multiple points? What is the payload? How much thrust is required? How fast will it move? How accurately will it need to position the load? What control system and fieldbus will it be working with?
“One factor that engineers often overlook is the motion profile,” says Ruberti. “Customers often say they want an actuator that can move at 10 ips, but they might really mean that the actuator should move 10 inches in 1 second, which is different. You have to account for acceleration and deceleration.”
What Do You Recommend?
To get a better idea of the choices involved in specifying linear actuators, we presented motion control experts with various application scenarios and asked for their recommendations.
Scenario 1: Raising and lowering a set of fixtured screwdriving spindles in an in-line automated assembly system.
“We’d typically look to an electric actuator because you can control both velocity and force,” says Langro. “You can slow down the actuator as the screwdriver approaches and contacts the assembly. Then, once the screws have been tightened, you can retract the actuator very quickly.
“Now, if it were a pressing station, where you needed a high force density to push a part into place, we might recommend a pneumatic actuator.”
“If the actuator is just moving between two points, I’d think about pneumatics, particularly if the stroke requirement is less than 2 feet,” adds Ruberti. “However, I’d want to know more about the screwdrivers. How big are they? How much do they weigh? Our electric actuators are available with much wider bodies, which might be necessary to carry multiple screwdriving spindles. If the screwdrivers were heavy, I might go with an electric ballscrew actuator, just because of the load.”
Whether an electric or pneumatic actuator is chosen, Bosch’s Davis advises engineers to pay attention to the linear support system. “A stiff housing and guide are important where the actuator may be exposed to reaction torque from a tightening system,” he points out. “A preloaded ball rail guide will eliminate backlash and minimize deflection from external forces.”
Scenario 2: A pick-and-place unit for a multistation automated assembly system.
“That could be accomplished either way,” says Langro. “If it’s a consistent motion, we would typically look to a pneumatic system first. The components are a less expensive, and, if you don’t need the flexibility, pneumatics are pretty quick.
“The Z axis, in particular, would be ideal for a pneumatic actuator. In fact, we often design systems that combine both electric and pneumatic actuators. The gantry system might be electromechanical, because of the size, but the Z axis might just be pneumatic, since it doesn’t need to be programmable.”
“The first question I’d ask is, how many points are there?” adds Ruberti. “Is there one pick point and one place point? Or is there one pick point and 10 place points? What are the requirements for stroke length and cycle time?
“With pneumatics, you’re limited to 24 inches of travel. For anything longer than that, we’d look at an electric actuator. Also, electromechanical systems are generally faster than pneumatics. An electromechanical system can move in an arc, as opposed to a pneumatic system, which has rectangular motion profile—up, then over, then down.”
Scenario 3: Moving a touch probe up and down to activate buttons on an electronic product in a semiautomatic functional test cell.
“That could be done both ways. It depends on the test,” says Langro. “If you’re testing the robustness of the buttons, you might want to vary the force applied by the probe. You could do that pneumatically by controlling the air pressure, but you could also do it electromechanically.
“If it’s a high-speed, high-duty application, a pneumatic cylinder might be more cost-effective.”
“Electronics testing applications normally require minimum size and accurate positioning,” adds Davis. “An actuator that has the guide and housing integrated into one piece, along with the ballscrew drive, will provide the best combination of small size and accuracy.”
“That application definitely sounds like an all electric system, especially if there’s an array of test points,” says Ruberti. “You’d want the linear axes for X and Y to be electric, because of the multiple positions, but you’d also want the Z axis to be electric, too, even if it’s just going to two positions.
“The reason for that is control. You want to control how much force you’re applying to each button, and you may want some feedback as to how much force it took to press the buttons. A servo-driven axis would give you that control.”
In fact, SCHUNK has supplied servo-driven linear actuators and grippers for testing touch screens on smartphones. The company has also supplied servo-driven linear actuators to a company that makes articulating arms for flat-screen televisions. The company uses the actuators in life-cycle testing equipment for its products.