By Steve Meyer, Contributing Editor
Pneumatic actuators and electric actuators are increasingly being used in the same system to achieve results that one technology, by itself, would have difficulty solving.
Most often we tend to think of air cylinders as direct competition for electrically powered actuators. But that’s not the whole story. Electronic controls and actuators are routinely combined with pneumatic systems to solve problems in a variety of applications. Pneumatics gives you brawn, tremendous motive power and speed—while electronics provides finesse and control that is hard to achieve otherwise.
Pneumatic technology is capable of high speeds and high force. As you can see from the following table, as cylinder diameter increases, power goes up exponentially with the square of the radius.
As air pressure increases, the force available increases proportionately. With enough flow to support the volume, high actuation speeds are possible. Typical industry values for linear actuator speeds are 8-10 fps, which is moving quite fast.
Once a load is accelerated to speed, bringing it to a stop can be difficult. In some pneumatic applications, this is done with hard stops, hence the term “bang-bang” control. More typically, shock absorbers are used, but even better are double-acting or rodless cylinders, where there is air on both sides of the piston and stopping the load is better controlled.
Electrically powered linear actuators come in two primary families depending on which mechanical approach is used, screw driven or belt driven. Speeds of 16 fps are possible with belt drives, while screw actuators are typically in the range of 3 to 6 fps. While high speeds are possible with screw actuators, the limitations are based on the bending forces generated during acceleration, so each case is different. Belts, however, don’t have the positioning accuracy of a screw. High precision screws can reach +/-0.0002-in. precision, which is difficult to duplicate with a belt.
The families that work together
Pneumatics and electronics work well together when properly applied. The combination of electronic controls and air power has been around for decades, which is what gives pneumatics its incredible flexibility. Extremely precise timing based on electronic controls, provides pneumatic systems with precise, repeatable performance. Sensors integrated into the actuators ensure accurate positioning, and servopneumatic valves offer precise air flow, adding to the precision motion of pneumatic actuators.
There are ranges of applications where the technologies work together well. In many machines, the two are combined where the respective advantages are complementary. In wood and metal saws, cutting and measuring are done with electronic controls and power—but for clamping and fixturing of parts, air power has the advantage. Once the clamps are closed, the parts can be secured indefinitely by closing a control valve. If there are no leaks, the parts stay put with no additional energy expended.
In many food and beverage applications, vacuum pickups are used to handle complex shapes. The simplicity of this approach leads to the integration of vacuum pickups at the end of 6-axis robots.
In recent years, as industrial control networks and information systems have matured, the use of electronics has allowed pneumatics users to keep pace with the changing environment. Integration of the network and control electronics directly into air control manifolds permits the pneumatic hardware to interface with all the major networks, including like Ethernet, DeviceNet, and Fieldbus.
Extruded aluminum is a great material for machine construction. With dozens of vendors making ever more sophisticated offerings, it also makes sense to see how actuators can be easily integrated into the frame technology. Festo Corp. is a leading supplier of both air and electric motor powered motion systems. Within the array of belt drive, screw drive and air powered actuators there are models that use the extruded aluminum package to simplify integration of the actuator into a complete machine design.
The parallel between electric and pneumatic technology is significant. Speed in an electric motor is analogous to flow in a pneumatic system, and pressure in pneumatic systems is comparable to torque in electrics. Both technologies will handle just about any kind of mechanical load. Both technologies have significant advantages in the right applications.
Festo pneumatic actuator housed in an aluminum extrusion. Identical format is available in electrically powered version.
Pneumatic mechatronics at the extreme
Where extreme force or extreme precision is required, extreme solutions are necessary. Pneumatic systems, like hydraulics, can generate high forces. But unlike hydraulic systems, pneumatics are also capable of extremely high speeds.
Acceleration forces of 200 to 400 gs are required to simulate the severe shock and recoil imparted when a bullet is fired from a rifle barrel. Generating an impact force that precisely simulates the severe shock of gunfire is difficult enough, but when you have to do it five times per second with full cycles of motion to simulate the bullet firing and the chambering mechanism, that is a real mechatronic challenge. The engineers at Ascendant Engineering Solutions in Austin, Texas, developed a pneumatically powered solution that combines accelerometer sensors, high-speed data acquisition and high-speed pneumatic controls to finely tune the shock load imparted to the test fixture with incredible precision.
The combination of air bearing surfaces and vacuum zones results in a controllable 40 micron air gap that enables precise movement of the glass through various processes.
Non-contact material handling
Pneumatics reaches another high point with non-contact pickups where air pressure is applied in rigid “cup” shaped fixture. This employs the Bernoulli principle to accelerate the airflow in a funnel-shaped pattern, producing a vacuum pickup in one direction while the escaping airflow produces an air cushion at the edges of the circle. Thus, you achieve lifting forces that don’t require the tool to be in actual contact with the target part. For semiconductor wafer handling and some food industry applications, this is an ideal combination and is an off-the-shelf technology from pneumatics suppliers such as Festo and Bosch Rexroth.
Air bearing conveyors
Another extreme solution is non-contact conveyors for flat panel display and solar cell manufacturing. As flat panels (FPD’s) get bigger, the unit of handling is increasing to further drive costs down. The difficulty here is that the glass sheet is only 0.0276 thick, which is thin enough for the glass bend and flex freely as it is handled.
By combining large air bearing surfaces with vacuum zones, a controllable 40 micron air gap can be provided over large surface areas to allow precision movement of the glass through various processes.
Filed Under: Actuators, Factory automation, Material handling • converting, Electronics • electrical, Fluid power, Mechatronics, Pneumatic equipment + components