Tutorial on the Rotary or Swing Arm Voice Coil Actuator used on modern Hard Disk (Disc) Drives

Robert Thibadeau, Ph.D.

Another Saturday Afternoon on the Internet

Borrowed Heavily from http://www.consult-g2.com/course/chapter11/chapter.html

The Voice Coil Actuator

A Voice Coil Actuator is a very simple positioning device that utilizes a coil of wire in a permanent magnetic field.  In speakers, the magnet is cylindrical North-South Pole in-out (or top-bottom of the cylinder) and the coil goes North-to-South Pole (or top-bottom of an inner cylinder).  Changing the amplitude and polarity of the current in the coil causes an in-out force that 'plays' the diaphragm on the speaker.  The spring tension on the diaphragm keeps the voice coil actuator centered when no current is applied.  You know the voice coil and speaker ... here's Alexander Graham Bell's original invention drawing from 1876 (courtesy U.S. Library of Congress):

The hard disk voice coil actuator is used to position disk heads across the platter of the disk.  It is a 'voice coil actuator' in that it only uses a coil and a permanent magnet in a simple push-pull fashion, but doesn't work like a speaker voice coil actuator.  The geometries are completely different. 

The voice coil actuator looks like this:

A big question!  Why is the coil shaped like a 'trapezoid'?  There's a reason.

The "steel poles" (blue in diagram below is the nickel plated iron container) and "magnets" (red) create a captured magnet field (through the iron) but between the magnets the lines of flux are nearly vertical.  (In the example below, the flux is vertical pointing down on the left and vertical pointing up on the right). Or, as Kevin Gomez from Seagate Research says: "The magnets should be made out of neodymium and coated with nickel. The blue part of your diagram - nickel plated iron is the keeper and guides the flux round the back of the magnets from the top-left N to the bottom-left S of the other magnet to complete the magnetic circuit and helps to keep the flux lines in the air gap (where the coil is) parallel as well as minimize flux leakage from the structure."

Or, simpler still, the wire gets pushed to the left or right depending on whether the north pole is on the top or bottom and which direction the current is coming:

Figure 1. Force on a current carrying wire in a magnetic field.

This explains the odd shape of the coil that has two long straight runs on the left and right (to create the left-right rotational force) and two arc runs (which contribute nothing to the rotational force but are a source of annoying friction and probably some instability particularly when the actuator is more 'in than out' or more 'out than in').

Want more detail?

Now: from http://www.consult-g2.com/course/chapter11/chapter.html :

"When electric current flows in a conducting wire which is in a magnetic field, as shown in Figure [1], a force is produced on the conductor which is at right angles to both the direction of current and magnetic field,

If the conductor (wire) is at right angles to the direction of travel and magnetic field then the force in the travel direction, in usual US engineering units, is:

The design of voice-coil motors for accurate performance is far more challenging than it appears at first. The very simple linear relationship above is complicated in practice by 

  1. variations in the static magnetic field,
  2. flux-leakage
  3. nonlinearities in the B-H curve of the pole steel
  4. field variations caused by DC coil current
  5. other effects caused by 
    1. the rate of change of flux, 
    2. effects on the drive electronics caused by coil motion
    3. changing resistance 
      1. due to heating, 
      2. changing inductance 
      3. and other problems."

 See http://www.consult-g2.com/course/chapter11/chapter.html for the math needed to characterize most of these other complicated, but secondary, effects.   You need to know these if you ever want to create servo control electronics and firmware.

Positioning the Head using the Rotary Voice Coil Actuator

Servo control electronics and firmware vary the currents and voltages through the coil based on closed loop feedback.  The source for the feedback comes from servo bursts patterned into the platter media.    The feedback source wastes platter media surface dedicated to the task of reading information from a platter for dynamically correcting the position of the voice coil actuator. 

The voice coil actuator in the disk generally has no 'natural position' that it springs to as is the case with speaker voice coil actuators.  All movement and positioning is dynamic.  However, there is a safe position of the head (either fully in or out -- left or right most -- depending on the disk drive design), which locks the head position when the heads are not in use.  Often there is a solenoid that unlocks the head when it is needed, Fig 2.

Fig 2. Disk (Disc) Drive Component Parts including Heads, Discs, Voice Coil Disk Head Actuator, Steel Poled Magnets, Drive Motor, and Head Unlock Solenoid with Spring Lock

By the way, is a solenoid just a voice coil actuator with the coil on the outside and the magnet on the inside?   If that's true (and it appears to be), then why isn't a voice coil actuator just an inside-out solenoid? But that can't be right. Bell's original telephone, certainly the first instance of a 'voice' coil and a 'voice' coil actuator, was two solenoids...