Voice Coil Actuators

In the simplest form, a linear voice coil actuator consists of a current carrying tubular coil of wire placed in a radially oriented magnetic field produced by permanent magnets. Linear voice coil actuator allows direct, cog-free linear motion 1).

Structure of Linear Voice Coil Actuator (Blck, Lopez & Morcos, 1993))

Rotary voice coil actuator is a flatten version of linear voice coil actuator. Force is generated along the circumference of an arc, where torque equals to force times radius. Rotary voice coil actuator provides smooth motion and good for applications that require quick response, limited angle actuation 2).

Structure of Rotary Voice Coil Actuator
Source: http://machinedesign.com/site-files/machinedesign.com/files/archive/motionsystemdesign.com/images/rotary-voice-coil-actuator1095.jpg

Principles of Operation

Electromechanical conversion mechanism of a voice coil actuator is governed by Lorentz Force Principle:

F = kBLIN,

where F is force, k is constant, B is magnetic flux density, L is the length of a conductor, I is current and N is the number of conductors. Direction of the force depends on the cross-product of current and magnetic field vectors. When current flow is reversed, the direction of the force will reverse. In a voice coil actuator, the magnetic field and the conductor length are constant, so the generated force is directly proportional to the input current.

According to the Faraday’s law, when a conductor (a coil of wire) moves through a magnetic field, voltage is generated:

E = kBLvN,

where E is the magnitude of the voltage, and v is the velocity of the conductor. Combining this equation with the equation of Lorentz Force Principle, in a voice coil actuator, the force is proportional to current and a voltage proportional to velocity of the voice coil. If pulse-width modulation of voltage is sent, direction and input of current flow will vary over time. This can control the voice coil movement 3)

Movement of Voice Coil
Source: http://www.spikenzielabs.com/SpikenzieLabs/Laser_Display.html

While they are in some ways similar to solenoids, they have several useful differences. First, reversing the current flow in the coil causes a reversal in the interaction with the field of the permanent magnet. This allows for the voice coil to move in both directions. Second, the displacement of the voice coil is proportional to the current in the coil. These two properties allow for the production of positive and negative air pressure variations of varying amplitude, as in a loudspeaker. The proportionality of the movement of the voice coil allows its use for the accurate positioning necessary for computer hard drives4).

Voice Coil Applications

Loudspeakers

Structure of Loudspeaker
Source: http://copyright.lenardaudio.com/laidesign/images/a05/a05_spkpic.jpg

Voice coil actuators received their name due to their common use in audio loudspeakers. Some of the earliest designs of voice coils also came from loudspeakers. The majority of loudspeakers are still constructed with voice coils 5). The design of a speaker system relies as much upon the speaker enclosure as the speaker driver 6).

When a voice coil is attached to a cone, the motion of the voice coil will make the cone reproduce sound waves according to input audio signals. The larger the magnet and the voice coil are, the greater the power and efficiency will be, if the loudspeaker is well made. The energy of the magnet is conducted through the pole plates and pole piece and concentrated (north - south) across the gap in which the voice coil is centered. The smaller the gap is, the more intense the magnetic field and the greater the efficiency are. However, normally, around 98% of the electrical energy becomes heat for cone speakers 7).

At bass frequencies, the voice coil has to move back and forth a long distance, especially at high power. During the movement, the percentage of voice coil in the gap must remain constant. So, either long voice coil plus thin pole plate or short voice coil plus thick pole plate can achieve the same purpose. At mid and high frequencies, movement of the voice coil is small, compared to the movement at bass frequencies. For this reason, the voice coil length and pole plate thickness are quite similar in mid and high frequency speakers cones 8).

Voice coil that has larger diameter has better control over the cone than voice coil with smaller diameter. This means that voice coil with larger diameter has better damping and linearity. However, it is more expensive to make a loud speaker with large voice coils 9).

Hard Disk Drive Voice Coil Motor

Hard Disk Drive Voice Coil Motor
Source: http://www.hitachi-metals.co.jp/e/eh2009/images/photo/p02_2.gif

A voice coil motor can serve as a servo motor, which is capable of moving to a precise and accurate angular or linear position according to a position feedback device such as hall-effect sensor (Link: Hall Effect). The voice coil motor keeps track on the pulse-wave width and move to the commanded position. When the PWM signal is sent repeatedly, the motor will hold the position and resist t external forces 10).

One of the applications where voice coil motor serves as servo motor is hard disk drive voice coil motor. Computer hard drives use voice coils for the positioning of read/write heads through microcontroller 11).

Voice Coil Actuator for Vibrotactile Feedback

Vibration of voice coils can provide vibrotactile feedback for simulation of acoustic events. There are several applications utilizing this vibrotactile feedback.

1. Touch Flute 12)

Touch Flute (Birnbaum, 2004)

Normally, when performers play the acoustic instruments like woodwinds and brass instruments, the surfaces of the instruments vibrate. In order to simulate the vibration, D. Birnbaum's Touch Flute uses voice coil actuators on finger-control keys and hall-effect sensors under the keys for getting the binary position of the keys. The voice coils have a magnetic property that can activate hall-effect sensors, so no extra magnet is needed. When the performer's fingers are pressing the keys, the hall effect sensors control the voice coils actuators to give haptic signals to the performer’s fingertips. Performers report that the vibration gives them warmth and familiarity with the instrument. This indicates that the vibrotactile feedback can improve players’ performance experiences.

2. Vibrotactile Chair 13)

Vibrotactile Chair (Karam et al., 2009)

Generally, through conductors like loudspeakers, it might not be possible to feel the physical vibration of sounds because the vibration generated by the conductors is not powerful enough. However, this kind of vibration can enhance musical experiences. In order to enhance the experiences, voice coils can be stored on a chair. When people sit on the chair, they will attach to the voice coils which are stored on the chair. When the voice coils vibrate, they can feel the vibration directly. The chair can help deaf people to experience music through haptic sensations as well.

3. Vibrotactile Stimulation for Performance Learning 14)

A guitar player using vibrotactile stimulation to learn a new piece (Giornado & Wanderley, 2011)

The idea is to use full-body vibrotactile display as a tool to help performers to learn new music, so learners can learn from listening and haptic feelings. The study shows the benefits of multi sensory learning. The device used by Giordana and Wanderley varies from the standard voice coil design in that the permanent magnet is the moving element and floats freely inside the coil.

Arrays of voice coils have been used for the creation of tactile displays as well 15).

External Links

References

1) , 2) , 3) Black, Bill, Mike Lopez, and Anthony Morcos. ”Basic of Voice Coil Actuators.” PCIM-VENTURA CA- 19 (1993): 44-44.
4) Brauer, John R. Magnetic Actuators and Sensors. John Riley and Sons, Hoboken, NJ:2006.
5) Gogue, George P. Voice-Coil Actuators, accessed March 23, 2012.
6) Dickason, Vance. Loudspeaker Design Cookbook. Audio Amateur Press, Vernon, CT:2006
7) , 8) , 9) Burnett, John. “Speakers.” Accessed April 8, 2016.
11) Georg Pelz, “Designing Circuits for Disk Drives,” Computer Design, International Conference on, p. 0256, 2001 IEEE International Conference on Computer Design (ICCD'01), 2001.
13) Karam, Maria, Frank A. Russo, and Deborah I. Fels. “Designing the model human cochlea: An ambient crossmodal audio-tactile display.” Haptics, IEEE Transactions on 2.3 (2009): 160-169.
14) Marcello Giordano and Marcelo M. Wanderley. “A Learning Interface for Novice Guitar Players using Vibrotactile Stimulation.” In Proceedings of the 8th Sound and Music Computing Conference, Padova, Italy, 2011.
15) Luv Kohli, Masataka Niwa, Haruo Noma, Kenji Susami, Kenichi Hosaka, Yasuyuki Yanagida, Robert W. Lindeman, Yuichiro Kume, “Towards Effective Information Display Using Vibrotactile Apparent Motion,” Haptic Interfaces for Virtual Environment and Teleoperator Systems, International Symposium on, p. 68, 2006 International Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems (HAPTICS'06), 2006.
actuators/voice_coil.txt · Last modified: 2016/04/08 10:38 by mirror
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