By Cale Reese, Electromechanical Products Manager, PHD, Inc.
Single axis actuation devices have proven to be the MVP in many successful machine designs, using the fundamentals of pneumatic, hydraulic, and electric components.
Performance of a fluid power actuator’s physical dimensions and properties of its driving medium (pressure, flow and compressibility) have a similar correlation. Pneumatic and hydraulic actuators use pressurized fluid to manipulate a piston within a sealed chamber. The piston is typically a rod or saddle in which external motion is achieved. Operating pressures for pneumatics and hydraulics are commonly seen in excess of 100 psi and 5000 psi, respectively. Pneumatic actuators are capable of very high speeds and can take advantage of various built-in features for limited control of speed and deceleration at end of travel. For this, flow controls and end-of-travel cushions are options the end user can adjust for optimum performance and acutator life.
Valves and control
Valves are the primary means of control for fluid power actuators. Either electrically, pilot, or manually operated, valves direct the flow of fluid to the actuator for the desired motion. In the case of hydraulic systems, valves capture volumes of non-compressible fluid in the actuator allowing it to achieve low positional precision in a multitude of positions throughout its stroke.
Conversely, in pneumatics, actuators are typically two-position devices as multi-positioning is limited due to the compressibility of the driving medium. Pneumatic multi-positioning is achievable mechanically through internal or external stopping mechanisms.
For fluid powered actuators, positional feedbacks typically use proximity or magnetic sensing. An external target or an internal magnet signals a sensing device attached to the unit. These signals can indicate discrete positions have been reached and indicate to the PLC or operating system additional operations to perform. An advantage to fluid power is pressure fluid, which is responsible for very high power density.
Plumbing and fittings
For pneumatic and hydraulic systems, plumbing and fittings are required for the transmission of fluid power. High-pressure hydraulic systems use fixed tubing, heavy-duty hoses, and fittings capable of withstanding extremely high pressures, while pneumatics use lighter duty, flexible tubing and fittings.
A fluid powered actuator’s life is measured in travel length. These actuators are capable of reaching tens of millions of linear inches before relatively inexpensive dynamic seals and other wear components require replacement. When applied correctly, the life of the actuator is independent of thrust or speed. However, it is important to note that side-loads and excessive impacts can result in premature failure.
Fundamentals of electric actuators
The fundamentals of electric actuators begin with the power source: the motors and controls. Motors and controls come in many combinations depending on the requirements of the system. The use of a controller enables the motor and control system within the actuators to communicate. The function of a controller is to tell the motor and control system driving the actuator what to do and how to do it. One function of the controller could be a circuit telling a motor to turn clockwise until an external signal tells it to stop, or as advanced as a networked multi-axis controller synchronizing the motion of multiple actuators and managing I/O.
Though the motion of a motor in an electrical actuator is similar to its fluid powered counterpart, the method of operation for a single axis electrical actuator is different. Just as the hydraulic or pneumatic actuator transforms stored energy from a pressurized medium into motion, the electric actuator uses motors to create motion from an electrical power source.
Another advantage of electromechanical systems is the ability to constantly monitor feedback directly from the motor and adjust its performance accordingly. Though not necessary for every application, closed-loop operation is the method that offers constant control of the actuator. Closed-loop control has the ability to adjust and correct variances in the operation, resulting in repeatable and accurate motion with every move. The savvy controls engineer may manipulate nearly every element of the motors motion and power output to the actuator.
Motor motion into actuator motion
Turning the motor’s power into linear motion is the job of the mechanical transmission of the device. For single-axis electromechanical actuators, this typically comes by one of three means: Linear motor, belt, or screw drive.
Linear motors are the most technologically advanced and efficient method of directly transmitting the power of the motor into the motion of the actuator. Instead of the rotor rotating in the stator, the rotor travels in a linear, flat-array fashion along the stator. Typically found in base slides, any additional mechanics exist for support of the external load. Base slides powered by linear motors can exhibit extreme speed, stroke, and accuracy with moderate to high thrust capability. In addition, with appropriate controls, two completely independent saddles can share the same base slide axis. Maintenance is low, but due to advances in technology, it has a high level of initial investment.
The use of a belt drive is another alternative for high-speed linear actuation. An efficient mechanical means of transmission, a pinion attached to the output shaft of a traditional motor or gear head drives a timing belt. Often used in rodless cylinders and base slides, this timing belt directly transmits power to the saddle of the actuator. Much simpler in technology than the linear motor, these actuators require less investment and can still obtain extreme linear speeds. Because the motor is separate from the drive, the mechanical advantage can be used to increase thrust speed for actuator optimization. The disadvantage to belts is that they wear over time and require maintenance.
Screw drives, commonly found in rod-style actuators and rodless cylinders, are the last group of power transmission mechanics. With screw drives, a motor transmits power through a coupler or pulley arrangement to rotate the screws and translate a nut along the axis of the screw. Attached to this nut is either the rod or saddle of the actuator.
Screw drives come in three types: Roller screws, ball screws and lead screws. Roller screws are the most expensive to produce and most advanced of the group. These screws are used in applications demanding extreme thrust and life.
Ball screws are less complex using recirculating ball bearings to transmit moderate to high thrusts and speeds with high efficiency and duty cycles.
Lead screws are the simplest in design and complexity, relying on material properties to overcome the friction inherent in their nature. Although not as efficient or able to operate at high duty cycles, lead screw simplicity offers a cost effective method of driving an actuator.
All screw drives exhibit some stroke and speed limitation due to the inherent physics of their design (rod deflection and screw whip experienced at high rotational velocities), but mechanical gearing between the motor and screw, in addition to the choice of the screw lead, makes this method of drive attractive. Maintenance for the screw drives is moderate; yet, they require re-lubrication at specified intervals dependent on the application.
A major difference between a fluid power actuator and its electromechanical counterpart is its size for an application. Thrust and speed are the major factors in determining selection of a pneumatic actuator.
Investment in Motion
It is important to consider the costs associated with each type of single axis actuation. Performance is generally a function of investment, which can range from minimal to extreme. This is why it is important to thoroughly understand the application and invest in appropriate actuators, remembering that oversizing and undersizing can prove costly.
Also consider both short-term and long-term expenses associated with the motion. Controls (whether a motion controller or valve bank) and communication (in the form of tubing and fittings or I/O cables) must be considered. Other considerations include long-term maintenance and support cost of each device.
These motion devices can often complement each other, offering an efficient combination of technology. While the newest technology is attractive, many times it is best to concentrate it only where necessary. Identifying where to focus appropriate technology and finding the balance is vital to creating a successful integrated solution.
Filed Under: Ball screws • lead screws, Design World articles, Fluid power, Hydraulic equipment + components, Pneumatic equipment + components, Valves