When designers need a modest amount of motion and power, they naturally think of a motor. That makes sense since there’s often a brushed, brushless, or stepper motor that can meet the objectives, sometimes with some necessary add-on gearing. Or they may think of a solenoid for impact-type linear motion.
This FAQ looks at another type of motor, generally known as the voice coil actuator (VCA) and more formally as non-commutated DC linear actuator. This linear (and sometimes rotary) motor is an effective solution for many applications and can be driven and controlled with fairly good precision and repeatability.
Q: What is a VCA?
A: It consists of a very simple design and construction, in principle. There’s a permanent-magnetic field assembly comprised of permanent magnets and ferrous steel, plus a coil assembly. When current flows through the coil assembly, it interacts with the permanent magnetic field, thus generating a force vector perpendicular to the direction of the current. This force vector can be easily reversed by reversing current flowing through the coil.
Q: Why is it called a voice coil actuator?
A: This is another case of a primary historical, mass-market application establishing the path. The VCA’s first application was to move the paper (now often plastic) cone of a loudspeaker to create sound. Most people don’t associate this action with a “motor” but it is just that.
Q: What’s the operating range of the VCA?
A: A typical VCA provides displacement of two to three inches (five to 7.5 cm), and they can be designed to reach twice that distance. Depending on size, they can develop just a few ounces of force to as much as several hundred pounds of force.
Q: What is the level of control that can be achieved with a VCA?
A: A VCA can be controlled to move in either direction and can provide a constant force over the stroke (except for slightly reduced force at the ends, due to “fringing” effects). They can also be used in simple open-loop mode, or closed loop for greater precision in critical applications. The closed-loop mode can use either position or force feedback, depending on what the application requires.
Q: How is the VCA physically constructed?
A: The VCA can be built in one of two ways: so that the coil assembly moves, or the permanent magnetic-field assembly moves.
Q: What are the implications of these two types of construction?
A: The moving coil design is the one with which most engineers are probably familiar – it’s what is used in loudspeakers, Figure 1a and 1b. The coil is wound around a non-magnetic bobbin, and this bobbin moves in and out of the permanent-magnetic field assembly. That assembly is constructed with a steel housing having a concentric permanent-magnet assembly in the middle. Since the coil moves, the wires which are attached to it must be thin, lightweight, flexible, and arranged so they do not fatigue or break from the constant flexing. Although that’s a challenge, it can be done, as billions of loudspeakers have proven.
In the moving-magnet approach, Figure 2a and 2b, the coil does not move; instead, the magnet itself moves. As a result, there is no issue with lead flexing or breaking. The moving magnet has a permanent-magnetic field assembly – somewhat like a piston — which moves as directed by the current flow within a cylindrical coil tube.
Q: Is linear-motion operation of the VCA the only option?
A: Not at all, although it the most common one and the one with which most engineers are familiar. A VCA can be built in an arc shape, thus allowing it to provide rotary over a limited sweep instead of linear motion. This can solve some challenging design problems
Part 2 of this FAQ will look at VCA specifics and applications, and compares the VCA to a solenoid.
[…] Part 1 looked at the basic of the VCA. Part 2 looks at operation in more detail along with applications, as well as a comparison to the solenoid. […]