Servo- and stepper-driven electrical linear actuators are an efficient and economical way to provide accurate motion control for a wide variety of machine applications.

Chip McDaniel 
AutomationDirect 

When it comes to automated machinery, there’s no doubt that observing a high-speed robotic arm zoom through intricate movements can be impressive. But although the price and complexity of robotics has been decreasing somewhat, the reality is that a significant segment of automated machines rely on more straightforward linear actuation methods.

Linear actuators can be electrically, pneumatically, and even hydraulically driven depending on the force, speed, accuracy, and cost requirements, and there are specific use cases where each technology is preferred. However, there are many reasons why electrically-driven linear actuators in particular—especially those operated by servo or stepper motors—can be the preferred solution for a wide variety of applications. This is especially the case when these devices are integrated with compact programmable logic controllers (PLCs) to provide a complete internet of things (IoT) capable motion solution.

Getting motion in line
Why is linear motion so important when designing such a wide range of machinery? Consider some of the most common motion tasks:

• Moving a product-holding tray or fixture for machining or assembly operations
• Positioning manufacturing functions like drilling, inserting, gluing, or taping
• Picking-and-placing products or parts into and out of trays and packaging containers

The simplest way to perform each of these actions is often to control the linear movement of a physical mechanism. Properly designed, linear motion can be easy to control and monitor, and it provides reliable force and accuracy.

Sometimes, linear motion only needs to be in a single-line direction, such as an “X” axis to move a cutter knife on a stationary product. Other times, users need the equipment to navigate over a plane, such as an “X-Y” axis to direct a laser over a surface to be engraved. In other cases, the equipment must move up and down in the “Z” axis, perhaps to apply a cutting tool. Whether the equipment motion is single- or multi-axis, users can develop grippers and end tooling to handle almost any application.

Mechanical systems moving in one or more axes are sometimes called Cartesian equipment, or perhaps a gantry. Depending on the application, designers will need to consider and balance the following physical details:

• The nature and magnitude of the load, as this will determine the support for carrying it and the force needed to move it
• Overall travel distance
• How to handle end-of-travel (overtravel) and intermediate stops
• Whether adjustable positioning is needed, or if position switches are suitable
• Travel speed
• Accuracy

Here, we’ll address motion distances and sizes of about a meter or less which can be implemented with readily available linear actuators and slides, as opposed to larger equipment which might require custom engineering. Off-the-shelf linear actuators, especially when driven by servos or steppers, provide an ideal way for creating the necessary motion, and they are easily integrated with compact PLCs to provide a complete control solution, with IoT connectivity included.

Linear actuator basics
OEMs, systems integrators, and other personnel designing machinery would usually prefer to use off-the-shelf hardware, electronics, and software when at all possible—instead of performing custom development.

Motor-driven electrical linear actuators, and the mechanical slides associated with them, represent a readily available form of off-the-shelf equipment to help designers rapidly create machine motion solutions. Linear actuators use mechanical methods to convert the rotational motion of a motor into linear motion. There are three major types of off-the-shelf linear actuators:

• Lead-screw driven – Uses a threaded shaft with a simple nut. The motor spins the shaft within a fixed base, while a nut rides along the threads to move the carriage. This technology can be used for light or heavy loads, and it is a relatively low-cost option compared to ball-screw driven.
• Ball-screw driven – Much like a lead-screw driven actuator, but with ball bearings that are continuously circulated in between the ball nut and the shaft to provide heavy-duty load handling with reduced friction and a long life. This technology is suitable for both light and heavy loads, and it provides high performance, even at fast speeds and with frequent duty cycles.
• Belt driven – Consists of a toothed belt (often called a timing belt) mounted between two pulleys within the base to provide simple and dependable motion of a carriage over longer distances than the other two types, although with less precision than is possible with a screw-driven arrangement.

Some linear actuators come ready to accept a variety of motor sizes, while others are complete packages including the motor. Designers can arrange linear actuators as needed to drive equipment and mechanisms, but they also must make other provisions for guiding the equipment with suitable low-friction support along the length of travel. Sometimes this takes the form of wheels and rails, while other times they may choose to use undriven (passive) linear slides, which are typically available from the same linear actuator suppliers.

Controlling the drive
Linear actuators are available with various hardware and couplings to mount small NEMA frame stepper and servo motors as needed.

Stepper motors move in small increments when triggered with a pulse, one “step” for each pulse. A typical example may be 200 steps per a 360-degree revolution, but finer-resolution microstepping modes are available. Most stepper motors do not incorporate built-in-encoder feedback, so extra provisions are required to periodically check their position status. Especially when oversized enough to overcome heavier loads, steppers are a useful and economical way to accurately move and position a linear actuator.

Servo motors are a higher-performance solution than stepper motors for precisely controlling position, velocity, and torque over complex motion profiles. A complete servo system consists of a motor, drive, and feedback elements, and therefore is more costly and complex than other technologies. Because industrial servos incorporate constant position feedback, they can immediately determine when disturbances occur and can either apply more torque to overcome a disturbance, shut down and alert upstream equipment and/or the operator that a problem has occurred. The system designer has wide flexibility to determine how exceptions will be handled.

The performance required by the application will dictate which motor control type is best. The basic idea is that each linear actuator type will have a specific travel distance for each whole rotation of the motor. With this knowledge, users can command the stepper or servo motor as needed to accomplish the linear move. For more advanced applications, users can specify motion profiles that accelerate and decelerate the actuator as needed to protect the payload, provide optimum positioning accuracy, and perform other functions.

Most linear actuation systems will require some other signals, provided by overtravel and homing position sensors. Overtravel sensors operate if an actuator has moved too far in a direction, providing a failsafe method of stopping the equipment before damage occurs. Homing sensors allow the automation to periodically—sometimes once a cycle—run the equipment to a “known” position to verify the calibration of the system. This is important for both stepper and servos, but steppers are likely to require more frequent homing in case they become stalled and “miss” or “skip” a step.

In years past, all this motion control would have typically required the stepper/servo motor controls, one or more dedicated motion controllers, and a supervisory PLC. But today, some of even the most compact PLCs include built-in software instructions and high-speed input/output provisions to command stepper and servo controls directly.

Another benefit of using a PLC as the all-in-one motion control solution regards how well it works with industrial human-machine interfaces (HMIs) for location visualization, and the extensive remote visualization and IoT connectivity natively available with these automation systems. Users can create a machine that works well, while keeping operators informed via web browsers and mobile devices.

Summary
Linear actuation is useful for many types of applications. For example, in pick-and-place applications (with a multitude of gripper types) for handling products and payloads. Also, industrial-grade automation of cutters, routers, CNC machines such as the Thermal Dynamics a80, and more, as well as commercial-grade control of 3D printers, laser engravers, vinyl cutters, and other similar applications.

By choosing off-the-shelf linear actuators and pairing them with stepper or servo motors commanded by modern PLCs, OEMs and designers have a reliable way of rapidly creating efficient, accurate, and powerful machine automation with IoT connectivity built in.

AutomationDirect
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