Designing Motion-Control Systems With Electric Cylinders

Ross Goluba
Product Manager
Industrial Devices Corp.
Petaluma, Calif.

Edited by Miles Budamir

Motion-control systems that require linear movement can be built around a variety of different actuators. The most common types fall into linear actuators, linear motors, and precision positioning tables. Linear actuators, generally the lowest cost approach, can be hydraulic, pneumatic, or electromechanical. Each comes with advantages and drawbacks, so their use depends on the specific application.

Hydraulic cylinders, for example, can handle high loads and generate forces up to several tons. But they're not so good at accurate load positioning. This task requires relatively expensive position sensors and electrohydraulic servovalve controls. Hydraulic systems also tend to leak which contaminates the work area.

By comparison, pneumatic cylinders are primarily used when light loads must be repeatedly moved between fixed positions. Pneumatics have high-speed capabilities, reaching speeds to 200 in./sec. Because of these qualities, they are generally used for simple bang-bang type applications where multiple positioning or accurate velocity control is not required.

To overcome the limitations of pneumatics and hydraulics, more applications are using electromechanical linear actuators. They consist of an electric motor mounted to either a screw or belt-drive system. These actuators generally come with rods or without. Rodless actuators guide and support a load with a carriage that moves along the actuator body. A built in bearing system lets them function as the primary load support. They can serve a single axis or be assembled into mulitaxis cartesian systems. They are not suited for high thrust applications because the carriage is offset from the linear bearing and drive system of the actuator. Where high thrust is needed, rod-type actuators are applied.

Rod-type actuators, commonly referred to as electric cylinders, are similar in configuration and appearance to hydraulic or pneumatic cylinders. They have no internal linear bearings, so the load must be externally supported or the cylinder must be mounted on a pivot to keep the force in line with the thrust tube. Small side loads can be tolerated. An advantage of electric cylinders over rodless actuators is their ability to extend into a work area and then retract, freeing the area for subsequent operations. Also, protective boots mounted on thrust rods keep contaminants out of the cylinder body. This allows assigning an IP65 rating, making them suitable for reliable operation in dirty or wet environments.

Electric-cylinder advances
More machine builders are moving away from hydraulics and pneumatics in favor of more versatile electromechanical solutions. This has encouraged manufacturers to further improve the load-handling capability and accuracy of their products, while making them smaller and easier to size and install. Manufacturers generally offer several basic models to cover a wide range of thrusts with options such as rod ends, mounting supports, linear potentiometers, and environmentaloriented options. The options let machine designers choose the cylinder that fits in their machine, rather than designing the machine around the cylinder.

Electric cylinders generate the highest thrust among the various types of electromechanical linear actuators. This is because the thrust tube is directly in line with the screw and nut. Some cylinders can handle high loads which rival the capabilities of hydraulic actuators. Although at high thrust ranges, hydraulics tend to be more cost effective.

The most common applications for electric cylinders are in the range of 6,000 lb or less. Electric cylinders also combine high speed with high accuracy, repeatability, and reliability. Maximum speeds can be as high as 50 in./sec and controlled accurately.

Cylinders are usually driven by stepper motors, servomotors, and dc motors. Each has benefits. Selecting one depends on the load, duty cycle, repeatability, and flexibility requirements. The cylinders' accuracy and repeatability depend on the mechanics and motor. Repeatability can be as low as ±0.0005 in. Controls for electric cylinders range from simple pushbuttons to more complex programmable multiaxis motion controllers. These can interface easily to a variety of operator and machine interfaces such as simple manual thumbwheel switches or more sophisticated programmable-logic controllers and personal computers. This control flexibility gives designers a wide choice, allowing them to weigh the cost and benefits of each solution.

Anatomy of an electric cylinder
An electric cylinder consists of an electric motor, gear reduction or timing-belt reduction, coupled to an Acme or recirculating ball screw. The motor can be attached directly to the screw, in an in-line configuration, or mounted on the side of the housing and coupled to the screw through a gear or belt drive. The latter mounting is called a parallel configuration. The thrust tube attaches to the nut on the screw and is kept from rotating with a guide bearing. The thrust tube attaches to the load by the rod ends, which could be a male or female thread, spherical joint, or a clevis.

The operating principle of a belt-drive electric cylinder is fairly straightforward. In the example illustrated, power applied to the electric motor turns a belt and pulley system, which in turn rotates the ball screw. The rotating ball screw moves the ball nut, which cannot rotate, to move forward or backward along the screw axis. The thrust tube either extends forward from the cylinder housing or retracts inside the housing.

Electric cylinders can be mounted rigidly or on a pivot. In the pivoting style, the actuator mounts with a clevis or trunnion. The load also mounts on a pivot and moves along an arc. Application examples include lid lifters, conveyor diverter gates, pivoting rollers, and other applications requiring arc-path motion. In rigid mounts, end angle brackets or side lugs secure the cylinder. Threaded rod ends are preferred when the load is rigidly mounted to the thrust tube. By comparison, rodless actuators, positioning tables, and linear motors can only be rigidly mounted because they move and position the payload along the body of the mechanics.

Manufacturers of electric cylinders typically offer designers a choice of permanent-magnet brush type dc motors, stepper motors, or brushless servomotors. Permanent magnet, brush-type dc motors cost the least. They have modest controllability, and only position to switches or hard stops.

More expensive stepper motors offer programmable positioning. They are repeatable and can be programmed in microsteps, typically in the range of 25,000 to 50,000 steps/rev. Acceleration, velocity, and position can be programmed by the motion controller. Steppers are open loop, which means the motion controller does not sense a stalled motor. However, an encoder can be installed on the rear of the motor to make it a closed-loop system.

The most costly, but highest performance motor is the brushless servomotor. Servos have the same programmability as stepper motors, but are inherently closed-loop systems. They produce higher torque and speed, so they can accelerate faster and handle larger loads than any other type of electric motor. Moreover, they are compact for their power and run cooler than a stepper motor.

When choosing between an Acme or a recirculating ball screw, consider such factors as duty cycle, backdriving, cost, and speed. The efficiency of ball screws is around 90% which lets them turn most of the motor's torque into actual force. Also, ball screws are 100% duty-cycle rated making them the only drive choice in designs that require high duty cycles or continuous operation. Their high efficiency gives them a low back-drive force. This means an external brake is normally required where loads must be held in place after removing power from the motor. Ball screws are more expensive than Acme screws, but typically only add about 5% to the cost of an electric cylinder.

With efficiency only in the 50 to 60% range, Acme leadscrews are not suitable for continuous operation. Friction and the heat generated in the leadscrew and drive-nut assembly are the main sources of energy loss. The duty cycle must be limited to 60% or less to avoid overheating. However, Acme screw drives are self-locking. When the power is removed from the motor, the Acme screw holds the load in place. Generally, an Acme-screw drive can hold up to 20 times greater load without backdriving than a similar ball-screw drive.