When most engineers hear the term motor they most-naturally think of rotating arrangements such as brushed DC, brushless DC, stepper, or variable-frequency prime movers. But a motor doesn’t have to be rotational, and there are many times when the design needs linear motion. One solution is to add some sort of gear or belt arrangement to transform the rotary motion, and that’s a solution which works well in many cases. However, it may be too complicated, too costly, too heavy, have backlash issues, or simply not be a good match.

One direct way to get the desired linear motion is to “unroll” the stator of a conventional rotary motor, an approach which has been used successfully for long-excursion requirements. But for travel distances ranging to a few inches, there are two effective, inherently linear alternatives: the voice-coil actuator (VCA), often called a voice-coil motor (VCM) and the piezoelectric motor. They operate on very different principles from each other and so offer different performance attributes.

As its name implies, the VCA was developed for driving the cone of loudspeakers, which also provided considerable mass-market design and manufacturing experience. The VCA has a very simple design and construction, with a field assembly of permanent magnets and ferrous steel, plus a coil assembly. Current flowing through the coil assembly interacts with the permanent-magnetic field and generates a force perpendicular to the direction of the current; this direction of force can be reversed simply by reversing the current flow.

The VCA can be constructed so that the coil assembly moves, or so that the permanent magnetic-field assembly moves (Figures 1 and 2). For the former, the wires attached to the coil must be thin, lightweight, flexible, and arranged so they do not fatigue or break from the constant flexing – something perfected through production of billions of loudspeakers. Typical VCA displacement is two to three inches, but units can be built to reach twice that distance. They can provide force of just a few ounces to as much as several hundred pounds, and the position of the moving element can be precisely controlled.


Figure 1
The schematic and cross section of the moving coil VCA show its simplicity; this arrangement is used in loudspeakers to control the speaker cone. Source: Group Six M, LLC


Figure 2
The schematic and cross section moving magnet VCA shows how it is the physical complement of the moving-coil arrangement. Source: Group Six M, LLC

Even if you have never worked directly with VCAs, you have undoubtedly benefited from them: they are the standard for positioning the head of the disk drive. Given the drive’s tight track density and the positioning requirements of that application, you get a good idea of the precision, control, speed, and motion-control profile that a VCA offers.


Bu why restrict linear motor ideas solely to electromagnetic principles? The piezoelectric motor, as its name implies, is based on the well-known symmetrical principle of the consequences of an applied electric field to a crystal’s dimensional stability and vice versa: when a crystalline material is subject to mechanical stress (squeezed), it generates a voltage; and when a voltage is applied to the same crystal, the material expands by a very small amount.

In the piezoelectric motor, a voltage across the crystal creates an electric field and the material elongates very slightly, on the order of 0.001 to 0.1 percent for typical applied voltages. The crystal is usually physically small (10 mm in each dimension is common) and the resultant motion can be controlled down to the order of microns, yet with newtons of force. Piezo-based motion is used in infusion pumps, microscope stages, optical positioning systems, instrumentation, inkjet nozzles, semiconductor processing machines, and more, plus they are non-magnetic (a major virtue in many situations) and have no bearings needing lubrication (which may cause contamination).

The piezo material can be used to move ahead like a worm, by having it held in tiny clamps that are alternately held and then released (Figure 3), or one end can be fastened in place while allowing the other end to move back and forth as the voltage is applied and removed, yielding a micro-sized piston-like motion (Figure 4).


Figure 3
The piezo motor can crawl ahead in tiny increments like an inchworm, with appropriate timing of clamping and unclamping with respect to energizing and de-energizing (1: housing, 2: moving crystal, 3: locking crystal, 4: rotary part). Source: Wikipedia user LaurensvanLieshout



Figure 4
The piezo motor becomes a precise, highly controllable piston with one end fixed in place as it is energized and de-energized. Source: Wikipedia user Inductiveload

These two linear-motion sources differ in much more than their size, travel range, force, speed, and other performance parameters, as they also need very different drive signals. As an electromagnetic-based unit, the VCA needs an analog current drive, although it can be at low voltage. In sharp contrast, the piezo-motor is a voltage-based device and needs anywhere from 50V to thousands of volts at low to modest currents to deliver enough power; ICs and modules are available specifically for this situation. Thus, designers of VCM drivers often focus on IR drop and use of heavier wires, while designers of piezo drivers must be concerned about safety issues related to presence in high voltage – that’s a big difference in perspectives.

So, when you are looking for a linear-motion solution for a unique application, consider the alternatives to the well-known rotary motors and associated motion “transformers,” as an inherently linear motor may be a better option. or the only one that works.

Have you had any experience with non-rotary motors of either type, or perhaps some other very different motor technology? What were the biggest surprises you encountered with the electrical and mechanical issues?

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.


References

  1. Voice Coil motors and actuators, Group Six
  2. Fundamentals of driving piezo motors, PiezoMotor Uppsala AB
  3. How It Works: Piezo-Ceramic Motors, Nanomotion/Johnson Electric
  4. Driving high voltage piezoelectric actuators in microrobotic applications, Harvard University

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