Machine Design

# Understanding mechanical limitations of linear actuators

In many cases it is essential to understand the mechanical limitations of an actuator to successfully apply linear-motion devices. The following information provides an overview to help in understanding the nuances and limitations of linear actuators.

Accuracy. Accuracy needs to be considered from two perspectives. First is the accuracy of the actuator itself. This relates to the actuator's moving-element ability to travel in an ideal single-axis path or straight line. This is primarily a function of the base or cylinder tube and the bearing or guidance system (ball bearings, slide bearings, etc.) Actuator motion can only be as true as the mechanical components permit.

Second is the actuator's ability to provide precise increments of motion along this axis. In a mechanical system this primarily concerns the lead screw or ball screw as well as the feedback device (encoder, linear scale etc.). Lead accuracy of the screw, resolution of encoders, and ability of the controller must combine to produce the desired accuracy. In a pneumatic or hydraulic system, accuracy depends on the cylinder, valve, and fluid, as well as controller and feedback devices.

Repeatability. Repeatability is the ability of a device to reach a specific location or position in successive attempts. Unlike accuracy, repeatability does not take into consideration the actuator's ability to travel in an ideal axis. Many times the actuator will follow a slightly bowed or twisted path due to imperfect construction.

Repeatability is influenced by the direction of approach to the target point. For example, approaching the target point from the same direction every time would indicate a unidirectional repeatability tolerance, whereas approaching from two directions indicates a bidirectional repeatability tolerance. Bidirectional repeatability includes the effects of leadscrew backlash in mechanical systems, which in some cases will dramatically influence the repeatability tolerance. It is possible that an actuator may be highly repeatable without being highly accurate.

Resolution. Resolution is the smallest positional increment which can be asked of a motion system. It involves how seals, bearings, motors, encoders, controllers and other mechanical components act together in defining system resolution. Engineers routinely construct systems which have extremely precise resolution, but there are practical limitations. Mechanical limitations such as friction and nut backlash in bidirectional moves can render very small move commands ineffective.

Mounting. Mounting considerations often influence a linear motion system's ability to attain the desired results. For example, actuators which are built from extruded aluminum profiles commonly exhibit bow or twist in addition to dimensional variances. If such an actuator mounts rigidly to a known true flat surface, the bow or twist generally will not affect the resulting accuracy. However, this typically has no effect on repeatability.

Conversely, mounting such an actuator at only one or two points will not straighten the actuator, and moves will typically follow the bow or twist of the extruded profile. Here accuracy is affected by bow or twist, while repeatability typically is not. Mounting a linear-motion system to machinery members which are not flat affects the outcome as well. For example, mounting to machinery framework that is not flat will tend to bend the actuator to match the framework. In cases where accuracy and repeatability are required it may be necessary to take extra steps when mounting the devices, such as using shims and jackscrews to adjust the actuator.