There are two types of voice coil actuators: moving coil and moving magnet. The materials of construction are similar, since they both use rare earth magnets, steel, copper wire, and basic insulation materials. There is a tendency to want to say one type is better suited for certain applications; however, there are many different sizes and shapes of voice coil actuators, making it difficult to make blanket statements about which type of actuator works better, and where.
Voice coils were originally developed for loudspeakers for the conversion of electrical energy into sound waves through the use of a diaphragm. Loudspeakers were used to amplify or create a louder voice; thus the name “voice coil.” Today they are called audio speakers, and their ability to achieve the desired frequency response is where this technology excels. Voice coil actuators (VCAs) use similar technology to speakers, but they are designed to produce higher forces over a larger distances or strokes. The increased force and stroke capability does come at a price with regard to frequency response, i.e. most voice coil actuators do not operate above 200 Hz unless the stroke is extremely small (.5 mm or less).
Since their inception, VCAs have found wider use in applications where proportional or tight servo control is necessary. By virtue of their high acceleration, VCAs offer numerous advantages in applications such as medical valve actuators, semiconductor manufacturing Z-axis applications, and instruments used in spectroscopy and chromatography. In VCA applications where position accuracy is important, position resolutions are only limited by the position feedback device and control electronics, not by the VCA itself. With control electronics and feedback sensors, VCAs can perform a wide variety of motion profiles. They can accelerate quickly then come to a gentle stop at a very precise position. They can also push with a precision amount of force in both directions. High-speed wire bonding is a good example of an application where VCAs are used in a high-speed position loop. Voice coil actuators are ideal for life and stress testing applications.
Moving Coil Voice Coil Actuators
Typical voice coil actuators consist of a stationary field (magnet) and a moving coil winding (conductor) that produce a force proportional to the applied current (see Figure 1). Compared to other electric linear actuators such as motors with gearboxes and motorized lead screws, a voice coil actuator’s key advantage is its ability to accelerate quickly because of its low moving mass. The conventional voice coil actuator, like the one shown in Figure 2, has zero cogging, zero detent forces, and zero backlash.
Moving Magnet Voice Coil Actuators
Moving magnet VCAs are similar to moving coil VCAs because they have a permanent magnet field in the middle, surrounded by a coil with soft magnetic return path at the outermost diameter. The main difference is what gets attached to the outer soft magnetic return path. In moving coil actuators, the permanent magnet field assembly is attached to the outer soft magnetic return path. This is called the field housing. In moving magnet VCAs, the coil is attached to the outer soft magnetic return path, so now it is in the stationary portion of the actuator, and the permanent magnet field assembly is free to move axially (see Figure 3).
With many moving coil actuators, the designer’s primary concern is keeping the mass of the coil low so it can accelerate quickly; this is where VCAs typically shine. These devices require thermal dissipation management. Design considerations to make the coil assembly lighter tend to be tradeoffs with thermal conductivity. Keep in mind the coil is surrounded by air, which is a poor thermal conductor. In high-duty-cycle applications, standard moving coil VCAs are limited by how much heat they can dissipate. If high performance is a requirement, adjustments need to be made to prevent the coil from overheating. Because moving magnet VCAs have their coils in contact with the stationary outer housing, they expel heat much better. As a result, they are more conducive to high-duty-cycle applications where high accelerations may be less important.
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, Click Here .