Introduction to Linear Actuators
- Saturday, 01 August 2009
Students trained in classic mechanical engineering are taught to construct a system using conventional mechanical components to convert rotary into linear motion. Converting rotary to linear motion can be accomplished by several mechanical means using a rotary motor, rack and pinion, belt and pulley, and other mechanical linkages, which require many components to couple and align. Although these methods can be effective, they each carry certain limitations.
Conversely, stepper motor-based linear actuators address all these factors and have fewer issues associated with their use. The reason? Rotary-to-linear motion is accomplished in the motor itself, which translates to fewer components, high force output, and increased accuracy.
A linear actuator is a device that develops a force and a motion through a straight line. A stepper motor-based linear actuator uses a stepping motor as the source of rotary power. Inside the rotor, there’s a threaded precision nut instead of a shaft; the shaft is replaced by a precision leadscrew. As the rotor turns (as in a conventional stepper motor), linear motion is achieved directly through the nut and threaded screw. It makes sense to achieve the rotary to linear conversion directly inside the motor, as this approach greatly simplifies the design of rotary-to-linear applications. This creates mechanical advantages, high resolution, and accuracy ideal for use where precision motion is required.
Why use a stepper motor instead of a conventional rotary motor? Unlike other rotary motors, steppers are unique in that they move a given amount of rotary motion for every electrical input pulse. This makes steppers a perfect solution for positioning applications. Depending on the type of stepper motor, they can achieve resolutions from 15 rotational degrees per step to 0.9 rotational degrees per step. This unique “stepping” feature coupled with the characteristics of the lead screw provides a variety of very fine positioning resolutions.
Permanent magnet stepper motors incorporate a permanent magnet rotor, coil windings, and a steel stator capable of carrying magnetic flux. Energizing a coil winding creates an electromagnetic field with a north and south pole. The stator conducts the magnetic field and causes the permanent magnet rotor to align itself to the field. The stator magnetic field can be altered by sequentially energizing and de-energizing the stator coils. This causes a “stepping” action and incrementally moves the rotor, resulting in angular motion.
“One-Phase On” Stepping Sequence
Figure 1 illustrates a typical step sequence for a simplified two-phase motor. In Step 1, phase A of the twophase stator is energized. This magnetically locks the rotor in the position shown, since unlike poles attract. When phase A is turned off and phase B is turned on, the rotor moves 90° clockwise. In Step 3, phase B is turned off and phase A is turned on but with the polarity reversed from step 1; this causes another 90° rotation. In Step 4, phase A is turned off and phase B is turned on, with polarity reversed from Step 2. Repeating this sequence causes the rotor to move clockwise in 90° steps.
“Two-Phase On” Stepping Sequence
A more common method of stepping is “two-phase on” where both phases of the motor are always energized. However, only the polarity of one phase is switched at a time, as shown in Figure 2. With two-phase on stepping, the rotor aligns itself between the “average” north and “average” south magnetic poles. Since both phases are always on, this method provides 41.4% more torque than “one-phase on” stepping.