Clear-cut advantages can be shown for pneumatic and electric linear actuators. But where pneumatics once reigned supreme, the role in applying force over distance has now been challenged by electric actuators
Both pneumatic and electric linear actuators answer a wide and sometimes overlapping range of requirements for linear positioning of components and load movement in a wide range of machinery. Pneumatic actuators, Figure 1, provide defined stroke, quick response, and typical forces from 20 to 4,000 lb. Electric actuators, Figure 2, provide a wide range of control options for the motion profile. Besides giving the motion designer wide flexibility, both types are clean, compact, and cost effective. But neither is ideal all the time.
Basic factors for choosing one style over another
Once an application is defined, you need to determine factors such as load weight and pressure, speed, acceleration/ deceleration, positioning, mid and end-of-stroke load support, system orientation, external force or bending moments, and actuator stroke length.
All these are important but the prevailing two are force and speed. It’s an exercise in over-simplification to say pneumatic actuators are best for providing lower cost muscle and speed while electric is best for controllable speed and positioning accuracy for intermediate stops. The system designer must consider the advantages of one over the other within a given application. The tradeoffs are many, including obvious ones such as cost, access to air supply, and operating environment. There is a justright answer to almost every situation. The system designer need only make a logical examination of options and tradeoffs to find the answer that is correct for his application.
Classifying types of actuator configurations helps to understand their application. Here are four common major types of rodless actuators.
The cable cylinder, Figure 3, uses a piston within a cylindrical tube to drive the cable. The cable wraps around end pulleys and provides motion within the actuator stroke length.
These earliest pneumatic “rodless cylinders” date back to the 1950s and continue today to enjoy a growing market, because they do certain things better than their newer pneumatic rodless or electric actuator counterparts. From large wrapping and packaging applications to moving theater stage sets, cable cylinders are preferred. Besides being economical and reliable, they provide motion around corners — motion not practical with other straight-line actuators. Cable cylinders are cost effective in longer strokes. Sheer size of application also gives cable cylinders the edge. Cylinder bores to 8 in. and custom lengths to 60 ft are available.
The band (rodless) cylinder, Figure 4, incorporates a slotted cylinder tube with inner sealing band and outer dust band. The pneumatically driven piston connects to the carrier bracket through the tube slot. Self-lubricating bearing rods on the carrier bracket transfer the load directly to the cylinder tube, making external guides unnecessary in many applications.
Consider rodless pneumatic cylinders when motion is repetitive and reciprocating. Examples include material handling, pick-and-place operations in product assembly, packaging, and paint spraying.
Rodless pneumatic cylinders are desirable where you have little mounting space and can’t tolerate cylinder-rod extension. Because of the space saving design, rodless cylinders serve well in tight work envelopes.
Rodless pneumatic cylinders can be prelubricated and provide millions of maintenance-free cycles. They are clean and nearly contaminant-free. Thus, they suit many medical, electronic, food, and pharmaceutical applications.
Magnetically coupled cylinders and slides, Figure 5, are actuators with totally enclosed stainless steel tube bodies. External magnets in the actuator block couple the block with the internal piston magnets through the cylinder tube. The load attaches to the actuator block. These are especially desirable to prevent or withstand contamination. Food, medical, and electronic applications are the most frequent users, especially where no external leakage is tolerable. Other design features that make it appealing in some product lines for certain applications include field repairability to minimize downtime and three magnetic coupling strength options
Electric powered screw or beltdriven actuators, Figure 6, are actually “smart” actuators when coupled with dc step-motor or servo drives and controls.
Electric actuators offer several advantages over pneumatics, beginning with those applications where air just isn’t available. In office machines, for example, electricity is the only practical power source. And some military applications are exclusively electric powered. Most importantly, electric actuators can provide controllable, consistent speed over the stroke length, accurate and repeatable positioning for end-of-stroke and intermediate stops, and programmable flexible-motion profiles. Also electric actuators need only one electrical supply and require power only when in action.
Though component cost is moderately higher for electric actuators, strong argument can be made that installation and maintenance costs are sometimes lower than for pneumatic actuators.
To further understand application of electric actuators for highly controlled linear motion, see the box, “Typical electric linear applications.”
Selecting an electric actuator: special considerations
Designing and building an electric actuator system involves some of the criteria for pneumatic actuators, plus knowledge of the electric drive and control components needed and how they work together. A basic electric system consists of a motor, a drive, and a controller, as well as an operator interface to input the required moves or move sequences. The system can be servomotor or step-motor driven.
Step-motor systems, because of their ease of programmable point-to-point positioning, precise speed control, and integration with external events, are growing in popularity in many linear positioning applications. Advantages include zerospeed holding torque for positive, stable holding; brushless design that minimizes service requirements; inherently digital operation for accuracy and repeatability; velocity control without feedback; open-loop functionality for tuningfree setup; and tolerance of repeated stalling without motor or drive damage. The following discussion of rotarystep- motor-driven systems refers mainly to those of Axidyne linear actuators by Tol-O-Matic.
Step motors. For supplying discrete and controllable shaft power to a load such as a linear actuator, the step motor is a 2 or 5-phase motor using permanent magnets in the rotor in conjunction with windings in the stator to produce high torque in a small package. Step motors, in effect, divide each revolution into discrete steps (for a 2- phase drive, 200 steps per revolution; for a 5-phase drive, 500 steps). The number of power pulses received determines the angle through which the motor moves; the pulse rate determines angular velocity.
Step motors have generally been standardized so that motors of a given frame size from different manufacturers all use the same mounting pattern. Common sizes include industry standard NEMA 17, 23, and 34 frames for flange mounting. A flexible coupling between the motor shaft and the load is recommended to isolate the motor from vibration and compensate for slight shaft misalignment.
Step-motor drives. In response to input signal pulses from the controller, the step-motor drive converts 110V, 60 Hz supply power to the power pulse waveform that the motor requires to produce torque. A step-motor drive is matched to the motor it is to drive. The drive can be set to provide half-step power pulses to the motor, resulting in a twofold resolution improvement.
Controller. The controller converts speed and position demands from operator inputs into pulse rates and pulse counts that produce the required motion when fed through the drive and motor.
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The controller can communicate with external devices through inputs and outputs. For example, Axidyne controller modules can accept external device inputs through a serial port or a thumb-wheel-dedicated (parallel) port as well as programmable and dedicated inputs. Because the controller is programmable, it can make decisions on those inputs, provide outputs through the serial port or programmable outputs, and ultimately provide motor motion outputs to the step-motor drive. The microprocessor is the interpreter in the controller, processing high level commands such as acceleration, velocity, distance, and direction and modulated output ramp and direction signal to the step-motor drive, Figure 7.
Operator interface. Any device that provides or translates inputs to or outputs from the step-motor controller is part of the operator interface. These are some interface options: Pushbutton interface — a basic interface which includes start, stop, I/O, and jog switches.
Thumbwheel interface — similar to pushbutton interface but with thumbwheel switches for more flexibility in adjustment.
Hand-held programmer — a remote programming device using RS-232C communication.
Other interface options include a host computer or a programmable logic controller (PLC).
Shrinking engineering staffs in many large companies have brought a corresponding need for pre-engineered linear motion products that are simple to assemble and use. The building-block approach to creating custom automation is the logical way to go, but it’s equally important that components match up and function properly. With that in mind, more suppliers of one or just a few linear motion components are becoming fullline actuator suppliers to provide everything a company with limited engineering resources will need.
Typical electric actuator applications
Electric actuation can help where controllable and programmable linear motion is needed, in applications such as:
Doug Moore is a vice president of sales at Tol-O-Matic Inc., Minneapolis. He has been there over 21 years. He was behind all of its linear motion product introductions and helped build a worldwide distribution network. Mr. Moore graduated from the University of Minnesota in business and science. Derrick Alcock is a vice president of Tol-OMatic, heading up electric actuator product lines. He managed the development of these lines after a long term as president of a large automation distributorship. Mr. Alcock has an MS degree in electrical engineering, University of Nottingham, England.