Q: What equation governs the behavior of a VCA?
A: As with most magnetic-coil devices, it’s the Lorentz force equation. This shows that the force generated is proportional to the product of the current in the coil and the magnetic flux in the permanent magnetic field: F = B × I, where F is the force (newtons or pounds); T is the magnetic flux density in Tesla (often informally referred to as the magnet’s “strength”; and I, of course, is the current. This equation shows that the force is proportional to the current and is constant over the stroke, except (as noted precisely) at the ends of the coil where the field tails off and the force is reduced by 10-20%.
Q: When does using a VCA makes sense versus a “conventional” motor?
A: It makes sense when the capabilities and strengths of the VCA match the needs of the application. That’s not news. If it is a possible fit with respect to its underlying capabilities for precision motion, the VCA can produce controlled back-and-forth action without resorting the gear assemblies and other mechanical elements that add cost, weight, and complexity, factors which engineers strive to minimize. By eliminating gearing, the VCA also eliminates the possibility of mechanical cogging. Further, the VCA has very low hysteresis, another problem in many motion assemblies.
Q: Where and why would a VCA be used?
A: Among the top applications are optical systems, for focusing, mirror adjustment, platform tilting and gimbaling, and beam pointing; also for oscillatory systems, to implement mirror scanning, and small-range position control. They are also used in hard disk drives to position the arm and read/write head assembly.
Virtues of the VCA include simple design and drive/control requirements; low hysteresis (often a negative aspect of associated mechanical linkages; inherent reliability (little to wear, fail, or go wrong); ease of sizing and installation; attractive ratio of force to both footprint and volume; and high acceleration (critical in many applications) makes it a viable alternative to conventional motors. It is also attractive when there is a need for rotary motion over a limited angle since a linear-rotary linkage or mechanism is not needed.
Q: Is the Lorentz force equation all that is needed to size and use a VCA?
A: No, but it is a big step in that direction. Fortunately, the “magnetics” principles are well understood and analytical models are available which take into account many of the real-world adjustments and corrections for fringing, self-heating of the coil’s windings (which increases coil-wire resistance and thus current flow), back EMF of the moving coil, and other factors that high-end, critical applications such as spacecraft assemblies may need.
Q: What are typical minimum/maximum values for VCA stroke, diameter, peak force, power dissipation, and operating frequency?
A: The number differ somewhat for moving-coil and moving-magnet versions:
For the moving-coil VCA, stroke can range between just 0.1 inch and 5 inches, with coil diameters from under 0.5 inches to 10 inches for the largest units. Peak force can be as low as 0.1 pounds to nearly 2,000 pounds, while power consumption can be as low as 1 W and up to several kW; frequency can be up to 500 Hz.
The moving-magnet design has the same 0.1-inch minimum stroke, but the maximum stroke is somewhat less, about 4 inches; it also has the same minimum diameter (just under 0.5 inches) but a smaller maximum diameter, up to about 5 inches. Similarly, the peak force can range from about 0.1 pounds, but it can develop about 10% higher peak force on the high end, and power consumption ranges between 1 W (as with the moving-coil design) but can reach about 3.5 kW; frequency is the same at 500 Hz.
Q: Isn’t the VCA just a fancy version of the solenoid?
A: At first glance, they look similar, but there are major differences. The solenoid, Figure 1, is activated by a simple on/off current which energizes the coil to “slam” the armature at maximum force; there is no control or modulation of the position of that moving armature. Solenoid applications include engaging a gear or starter, unlocking a lock mechanism, or punching a ticket.
The solenoid armature returns to its energized position only when the current is removed, but a spring must provide the needed return force, whereas the VCA returns by reversing and controlling the current in the coil. (Some solenoid controls reduce or modulate the current to hold the armature in the active position but with lower power, but this does not move the armature away from its final position.)
The solenoid is not superior alternative to the VCA, nor is the reverse the case. Each is a better fit for a specific application, depending on the position, control, and force requirements. Although they have a similar construction at first appearance, they are different in the details and in their drive.