Solenoids consist of a coil (no magnet) attached to a soft magnetic housing, an iron or steel core, and often a spring. When the current flows through the coil, the core is attracted by the electromagnetic field created by the coil. Solenoids are simple on/off devices controlled by switches (see Figure 4). When energized, the active part of the solenoid makes a full stroke. When turned off, a spring typically pushes it all the way back, or in some applications, another solenoid is used to push it the other way. Solenoids can be designed to achieve a certain speed and a certain amount of force. But these devices are either on or off — once set, they cannot be altered to provide any controlled changes in speed, force, or position. Solenoids meet simple linear movement requirements at a low cost in a simple on-off control mechanism. Typical solenoid application examples include car starters, ticket machines, and where a machine needs to only be turned on or off. If the application requires more than simple on-off control with precise positioning, then VCAs would be a better choice.
Piezo actuators are gaining popularity due to their ability to deliver extremely high accelerations, generally with shorter stroke lengths when compared to VCAs. This type of actuator converts an electrical signal into a precisely controlled physical displacement or stroke through the piezoelectric effect, which manifests as a direct relationship between mechanical deformation and electrical charge in the piezoelectric material. This effect is most often found in crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins. Piezo devices are available as stacked actuators and stripe actuator types (see Figure 5). Common applications include production and detection of sound, generation of high voltages, electronic frequency generation, microbalances, and ultrafine focusing on optical assemblies, as well as more everyday uses such as acting as the ignition source for cigarette lighters and push-start propane barbeques.
For applications where rapid movement and fine position control is called for, a VCA is the ideal solution. Where only simple on/off requirements are needed, these can be fulfilled by a solenoid, provided that the abrupt force spike at the end of travel is not excessive for the application. Finally, for fast movement with a short stroke, a piezo actuator could be considered. For general purposes, typically the goal is to accelerate and get to a position as fast as possible, but when it gets to the end, it needs to stop softly. The voice coil actuator, with some smart control electronics, is a good solution. For applications with a higher duty cycle and lower speed requirements, a moving magnet design could be a better choice, but when extremely high accelerations are required for a short duty cycle or even momentary use, moving coil VCAs are ideal.
This article was written by Jim McNamara, Senior Applications Engineer for BEI Kimco Magnetics, a brand of Custom Sensors and Technologies (CST), in Vista, CA. For more information, visit http://info.hotims.com/49742-321.