< > [Motion](../../motion) > Voice Coil Actuators
## Voice Coil Actuators
#### High bandwidth, moderate stresses
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Voice coil on the principle of lorentz force where a current carrying wire in a magnetic field creates a mechanical force.
This is what a basic cross section of a voice coil actuator looks like:
The name "voice coil" comes from their use in speakers:
But they are also used in hard disks for their high speed, precision actuation:
Because the moving coil elements of these actuators don't require any iron to define flux paths, they can be made very light. This suits them well for high-frequency actuation.
Their energy and force density are moderate for their size/weight. The increased air gap in the magnetic circuit (required to fit the coil), reduces the potential flux density compared to an equivalently sized more conventional iron-core linear motor.
We're using them to power millimeter-scale robots that are assembled from a small set of discrete parts:
<video controls src="https://gitlab.cba.mit.edu/assembledAssemblers/assembledActuation/raw/master/measurement/high_speed_capture_021318/video/highSpeedActuatorClip.mp4" width="640"></video>
<a href="https://langfordw.pages.cba.mit.edu/middleOutMachines/actuation/walking_motor.html"><img src="https://langfordw.pages.cba.mit.edu/middleOutMachines/actuation/images/walking_motor1.jpg" width="500"></a>
> #### Fun aside...
>Since this actuator produces a very linear relationship between force and applied current, the resolution of this actuator can essentially be made arbitrarily high.
>It's for this reason that it's being used to redefine the unit of the kilogram as part of a [Watt balance](http://iopscience.iop.org/article/10.1088/0026-1394/53/5/A46/pdf). Here is a "NIST approved" design for a Watt balance constructed from lego bricks:
><a href="https://phys.org/news/2014-12-nist-physicists-watt-lego-blocks.html"><img src="https://3c1703fe8d.site.internapcdn.net/newman/csz/news/800/2014/vdfvdfgf.jpg" width="500"></a>
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