X-Y-Z: Electric Cylinders Take the Air Out of Linear Motion
Pneumatic cylinders can be found throughout automated assembly systems in pick-and-place devices, clamps, pushers, elevators and linear actuators. Fast and inexpensive, pneumatic cylinders are used when a light load must be moved repeatedly between two fixed positions.
But, they're not the only game in town. In many cases, electric cylinders are a better choice. "If you want smooth motion control and multiple positioning points, you need an electric cylinder," says Chris Prior, applications support engineer at IAI America Inc. (Torrance, CA). "An air cylinder typically stops at only two points. With an electric cylinder, you can program up to 64 points, and you have variable control over acceleration and velocity."
Electric cylinders look like their pneumatic cousins. However, the rod is driven not by a piston and compressed air, but by a power screw and an electric motor. Electric cylinders are fast, accurate and powerful. For example, the velocity for IAI's Robo Cylinder ranges from 1 to 1,000 millimeters per second. Stroke length ranges from 50 to 1,200 millimeters, and repeatability is ±0.02 millimeter.
Compared with pneumatic cylinders, electric cylinders are cleaner, quieter, more efficient and more durable, says Alan Augello, director of the actuator business unit of SKF Linear Motion and Precision Technologies (Bethlehem, PA). "In the past, electric cylinders were quite expensive compared with pneumatic ones," he says. "Now, the prices are much closer, and if you consider total operating cost, electric cylinders are a better value."
The screw can be driven by a stepper motor or a servomotor. "For a pressing or squeezing application, use a stepper motor," says Prior. "For a high-speed application, use a servomotor. For an application like dispensing, use a servomotor, because it provides finer control over velocity."
The motor can be mounted in-line with the screw or parallel with it, depending on space requirements. If the motor is mounted to the side, it can be connected to the screw through a timing belt or helical gears. The screw can be a ballscrew, Acme screw or roller screw.
Another electromechanical alternative to the pneumatic cylinder is the tubular linear motor. This device consists of two cylindrical parts: a stator and a slider. The stator contains the windings, bearings to support the slider, and sensors for monitoring position and motor temperature. The slider is a stack of neodymium magnets laid out with alternating polarity. The stator's length and diameter set the force level, while the length of the slider determines the stroke.
Tubular linear motors are fast. Depending on the manufacturer, velocity can be as high as 180 ips, acceleration can be as high as 40 g, and peak force can be as high as 400 pounds. They're also accurate. Tubular linear motors can position loads with micron precision.
Because there is no electrical or mechanical connection between the stator and the slider, the slider can be connected to a rotary motor, so the load can be rotated as well as moved back and forth. In addition, tubular linear motors are available with hollow sliders, so that air and electrical lines can be fed through to grippers, vacuum cups and sensors.
When ordering electric cylinders, engineers should specify the payload, speed, stroke, duty cycle and life expectancy. Duty cycle is the ratio of operating time to total cycle time. If the cylinder cycles up and down in 2 seconds and rests for 4 seconds, the duty cycle is 33 percent. Specifying the duty cycle is important to ensure that the motor won't overheat. Life expectancy can be specified in total cycles or total inches moved during the operating life of the machine.
Engineers also need to specify the cylinder's orientation (horizontal, vertical or diagonal), and any special environmental requirements, such as clean room operation. As with pneumatic cylinders, an electric cylinder must be mounted so it is not subject to side loads. The load should be centered on the cylinder's axis.