Designing Motion Control Systems with Electric Cylinders

by Ross Goluba, Product Manager, Industrial Devices Corporation

Motion control systems that require linear movement can be designed with a variety of different technologies. The most commonly used can be grouped into three categories: linear actuators, linear motors and precision positioning tables. Linear actuators solve a wide range of applications and are generally the lowest cost solution. They can be hydraulic, pneumatic or electromechanical, each with advantages and drawbacks that depend upon the application.

"hydraulic systems have a tendency to leak..."

Hydraulic and pneumatic limitations

Hydraulic cylinders, for example, are used for handling extremely high loads. While they have force capabilities as high as several tons, hydraulic systems have a tendency to leak which can cause maintenance issues, contamination and environmental concerns. Further, accurate positioning of the load requires expensive position sensing and electro-hydraulic servo-valve controls. By comparison, pneumatic cylinders are primarily used when a light load must be repeatedly moved between two fixed positions. Pneumatics have very high speed capabilities, reaching speeds upwards of 200 in/sec. Because of these characteristics, they are generally used for simple "bang-bang" type applications where multiple positioning or accurate velocity control is not required.

Electromechanical solutions

Because of the inherent limitations of pneumatics and hydraulics, more and more designers are specifying electromechanical linear actuators, which consist of an electric motor mounted to either a screw or belt drive system. In general, these actuators come in two versions. Rodless actuators guide and support the load with a carriage which moves along the actuator body. Since they have a built-in bearing system, they are used as the primary source of load support. They can be used as a single-axis or assembled into multi-axis Cartesian systems. Because the carriage is offset from the linear bearing and drive system of the actuator, they are not designed for high thrust applications.

"electric cylinders...extend into a work area and then retract..."

In cases where high thrust is needed. rod-type actuators need to be used. Commonly referred to as "electric cylinders," they are similar in configuration and appearance to hydraulic or pneumatic cylinders. Since there are no internal linear bearings, the load must be externally supported or the cylinder must be mounted such that it can pivot to keep the force in line with the thrust tube. Small side loads, however, can be tolerated. An important 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 can be mounted on the thrust rod to keep contaminants from entering the body of the cylinder, which could damage the screw and nut. This allows them to have an IP65 rating, so that they reliably operate in bad or wet environments.

Electric cylinder advances

Manufacturers of electromechanical devices have been improving the load handling capability and accuracy of their products, while making them smaller and easier to select and install. Several basic models are generally offered that cover various thrust ranges, plus have a variety of options such as rod ends, mounting supports, linear potentiometers and environmental options.

Electric cylinders generate the highest thrust among the various types of electromechanical linear actuators -- even rivaling the hydraulics -- although at the very high thrust ranges, hydraulics tend to be more cost effective. This is because the thrust tube is directly in line with the screw and nut. The most common applications for electric cylinders are in the range of 6000 lbs. or less. Electric cylinders combine high speed with high accuracy, repeatability and reliability. Their maximum speed can be as high as 50 in/sec, which can be accurately controlled.

The cylinder's leadscrew is typically driven in-line, in-line through a gearbox or offset via belt-drive, with stepper, servo or DC motors. Motor selection depends on the load, duty cycle, repeatability, and flexibility requirements. Accuracy and repeatability depend both on the mechanics and the motor. Controls for electric cylinders can range from simple pushbuttons to more complex programmable multi-axis motion controllers. These controls can interface easily to a variety of operator and machine interfaces such as simple manual thumbwheel switches, the more sophisticated PLCs and the newer PC-based systems.

Selecting an electric cylinder

Several key parameters -- thrust, duty cycle, peak speed, and stroke length -- need to be determined when selecting an electric cylinder. The maximum thrust that the cylinder produces must overcome gravitational, frictional, acceleration and applied forces of the load:

Tm = Fg + Ff + Fac + Fap

where Tm = maximum thrust, lb; Fg = gravity, lbs.; Ff = frictional force, lb; Fac = acceleration force, lb; and Fap = applied force, lb. Usually, a 10% to 30% safety factor is added to the required thrust rating. The value depends on the type of motor employed.

Cylinder peak speed rating depends on whether the motion profile is triangular or trapezoidal. In the case of the trapezoidal profile, the peak speed equals to 1.5 times average speed. The average speed is found by dividing the distance the load must travel by the time it takes to complete the motion.

Speed-thrust curves provided by cylinder manufacturers can help in selecting the proper device. Close attention should be paid to the required accuracy, repeatability and other known parameters. The types of lead screw and the motor available for a cylinder family come as part of the selection process.

"...leadscrew is typically driven in-line, ...or offset via belt-drive..."

Several stroke-length options are available for each electric cylinder. However, before selecting the stroke, designers should add safety areas at each end of travel to exceed the cylinder stopping distance over the distance that the load must travel. The higher the speed, the higher the stopping distance, and the higher the safety area must be. After verifying that the speed and thrust are not limited by the actuator stroke length, mounting and other options are selected.

Control system options

Several control system architectures can be used, depending on the required positioning accuracy, speed, degree of flexibility, and budget.

DC motor controls provide the most economical option for the applications where only one or two speeds are required for each direction and the load stops at the same point in each cycle. The user interface can be as simple as a push-button switch. A wide range of limit switches or analog positioning packages are readily available. Two industry-standard control signals are used for the interface: 0 to 10V or 4 to 20 mA.

Servo- and stepper-motor controls add flexibility to the design. For example, numerous stopping points can be pre-programmed, force can be controlled, motion profiles can be customized, and keypads and displays can be used as an interface with a computer to control the system. When high accuracy, repeatability, and resolution are required, a servo- or stepper-motor control system should be used.

"The more sophisticated the control, the more options can be added..."

The more sophisticated the control, the more options can be added to the motion control system to satisfy the needs of the overall machine. It is common practice to install an end-of-travel limit switch to prevent damage to the load or the cylinder, when the cylinder could extend beyond the safe-operating zone. The switch signals the controller that the cylinder is approaching the set point so the controller can stop the cylinder motion before possible impact. Position sensors used in these limit switches contain mechanical reed or Hall-effect switches. Both types are available in a normally-open or normally-closed configuration. The Hall-effect switches are available with either current sinking (NPN transistor) or current sourcing (PNP transistor) output stages.

The choice of the control system goes hand-in-hand with the selection of the motor. The limit-switch control, the analog position control, and the edge-guide control are examples of widely-used DC motor controls suitable for many applications requiring a few stopping positions and a simple motion profile. More sophisticated stepper-motor controllers, micro-stepping drives and controllers are available for systems requiring higher levels of accuracy, repeatability and programmability. Analog and digital servo systems are available that include a variety of servo drives, programmable servo drives, and motion controllers that offer an unparalleled degree of performance and flexibility for the most demanding applications.

For more information:

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