When it comes to linear motion, engineers often face two choices: voice-coil actuators (VCAs) or solenoids. Both are simple electromechanical devices that can accomplish the task. But there are definite differences between the two, and it’s important to select the component most suitable for the task at hand. Comparing the design and capabilities of the two components makes it easy to determine which is more appropriate for a given application.
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Solenoids normally consist of a coil with no magnet attached to a soft magnetic housing, an iron or steel core, and, often, a spring. These on/off components are controllable via simple switches. When current flows through the coil, the electromagnetic field created by the coil attracts the iron core (Fig. 1). When the coil is de-energized, the spring pushes the iron core back to its original position (Fig. 2).
In some applications, designers employ solenoids without springs to move a common iron core back and forth. In all cases, it’s possible to design solenoids to achieve a certain speed and a certain amount of force. But once set, alterations for making adjustments to the necessary movement are impossible, i.e., it cannot provide any controllable changes in speed, force, or position.
Essentially, solenoids meet simple linear-motion requirements at a low cost in a simple on/off control mechanism. Suitable solenoid applications include car starters and ticket machines. If the application requires more than simple on-off control with precise positioning, then consider VCAs.
VCAs come in many shapes and sizes and in two types: VCAs with a moving coil and VCAs with a moving magnet. The first type consists of the usually stationary field (magnet) assembly and the moving coil assembly. In contrast, moving magnet VCAs have the coil attached to a stationary soft magnetic housing, which also serves as a conductor of the magnetic flux. The field assembly typically consists of an axially magnetized, permanent cylindrical magnet and two soft magnetic pole pieces attached to both ends of the magnet.
Applying a voltage across the terminals causes the VCAs’ moving part, magnet or coil, to travel in a given direction. Reversing the polarity of the applied voltage will change the direction of the moving magnet or coil. The generated force is proportional to the flux crossing the coil and the current that flows through this coil.
By design, VCAs support a specific or set stroke. Typically, for a specific current, the force created by a VCA at mid-stroke is approximately 15% higher than the force created at both ends of the stroke.
VCAs shine in applications where more precise control is necessary, primarily because they are available with position feedback devices. Also, with low moving mass, VCAs are ideal for many oscillatory applications such as stabilization platforms and shaker tables. And because moving magnet VCAs consist of a stationary coil and a moving permanent magnet assembly, as opposed to a coil assembly and a piece of steel in a solenoid, VCAs can typically create more force than solenoids for a specific size, stroke, and input power.
VCAs with moving coils are ideal for many limited-angle rotary applications that require high acceleration. The fast acceleration capability is achievable by minimizing the moving mass, in this case, the moving coil. Since it is separate from the magnet, it is customizable to be more lightweight than the heavy permanent magnet field assembly, allowing extremely fast speeds.
Rotary VCAs are common to gimbal applications where fast yet controllable moves are necessary for azimuth and elevation axes. Gimbal application examples include antennas, pan-and-tilt security cameras, target acquisition, laser pointing, scanning, and stabilization.
The physical characteristics of a VCA also make it a first choice in medical, aerospace, and military applications in which size and weight are as important as its functionality. Many medical equipment applications require high mobility, and every component must meet a specified dimension. A VCA can weigh as little as half a pound, making it a perfect solution for such requirements.
There are definite differences between VCAs and solenoids and, obviously, application requirements will determine which one to employ. Chosen for their technologies and performance, VCAs are high-power-density components. Therefore, in applications with a short stroke or excursion angle, a VCA can do the job where other technologies such as motors or gear motors will be bigger and heavier.
Although solenoids are suitable for on/off linear movement and intermittent duty, VCAs are the obvious choice to control force, speed, travel, and acceleration/deceleration for continuous performance and accurate positioning (Fig. 3).
- Basics of Voice Coil Actuators
- Solenoid Actuator
- Piezo-electric alternative to conventional step-motors, actuators, transducers, and solenoids