VOICE COIL ACTUATORS VS MOVING MAGNET ACTUATORS

Voice coil actuators have been around for many years, it is a type of direct drive linear motor. It consists of a permanent magnetic field assembly (permanent magnets and ferrous steel) and a coil assembly. Actuators are used for a wide range of applications. They compete most directly against moving-magnet rotary motors and pneumatic drives. Voice Coil Actuators can be designed in different ways. The type that most professionals are familiar with are the moving coil type actuators. But another type of actuator is winning in popularity, the moving magnet design. With this actuator the coil is fixed and magnet assembly moves. In this article we will compare the Voice Coil Actuator with a Moving Magnet Actuator.

VOICE COIL ACTUATOR (VCA)
Voice Coil Actuators (VCA) are non commutated direct drive devices with a limited stroke. They utilize a permanent magnetic field and a current carrying conductor (coil) to produce a force (Lorentz). Application areas for voice coil actuators are: loudspeakers, HDD drive, optical storage (DVD, CD technology), beam steering, linear pumps, pressure and flow control.

Typical modern hi-fi loudspeaker Voice Coils employ materials which can withstand operating temperatures up to 150°C, or even 180°C.

Advantages:
Low moving mass (eg coil and coil holder)
High bandwidth possible
Zero hysteresis (usually cables for connection add hysteresis however!)

Disadvantages:
Eddy current damping of coil holder (value usually not specified)
Dissipated heat in coil will heat up payload
Small thermal time constant (>10 °C/sec heating of coil)
Moving wires (power connections)
Difficult coil cooling (especially for semi static operation)
Single side force exit from actuator
Low steepness (N2/W)

Figure 1: Voice Coil Actuator (VCA)​

VOICE COIL ACTUATORS VS MOVING MAGNET ACTUATORS

VOICE COIL ACTUATORS VS MOVING MAGNET ACTUATORS

MOVING MAGNET ACTUATOR (MMA)
Moving Magnet Actuators (MMA) are non commutated direct drive devices with a limited stroke, that utilize a moving permanent magnet and a static current carrying conductor (coil) to produce a force (Lorentz). Application areas; mechatronic systems, linear pumps, pressure and flow control, vacuum applications, automotive systems.

The MMA technology can be taylored to the specific use of the customer. OEM devices have been designed and built for peak forces ranging from 1 to 10.000 N and stroke levels between 1 and 50+ mm.

Magnet materials employed which can withstand operating temperatures >150°C, or even >200°C if required.

Advantages:
Heat dissipation on static part (no direct heating of payload)
Hollow shaft version allows more flexible use and compact design
Improved reliability, no risk of mechanical coil or connection wire damage
Can be operated in dirty environment (full encapsulation possible)
Increased thermal time constant allows high peak force
Reluctance force could be tuned to compensate leaf spring guide/Gravity
Force exit from both sides of actuator
High steepness (N2/W), reduced electrical power requirements
High peak force to moving mass ratio (very high G accelerations possible)

Disadvantages:
Eddy current damping of back iron
Increased (≈ factor 2) moving mass
(Reduced) cogging force

Figure 2: Moving Magnet Actuator (MMA)

COMPARISON COMMERCIAL VCA AND MMA

Parameter Unit Commercial
VCA
Moving Magnet Remark
OD [mm] 60 60
Max. length [mm] 55 33 Length at maximum stroke.
Stroke [mm] 8.3 8
Moving mass [g] 106 130
Thermal resistance [°C/W] 7.1 2.0
Peak Force [N] 111 150 10 s/Tcoil=155°C/Tamb=25°C
Continuous Force [N] 26 45 Tcoil=155°C/Tamb=25°C
Steepness [N²/W] 57 65
Max acceleration G 107 117 MMA achieves higher accel.
Specific Peak Force [N/cm³] 0.77 1.61
Specific Cont.Force [N/cm³] 0.18 0.48
Specific Steepness [N²/W/cm³] 0.39 0.70

CONCLUSION VCA VERSUS MMA COMPARISON
With the same electrical power more mechanical power can be created with the MMA due to the higher steepness
The lower thermal resistance and the higher motor steepness of the MMA results in
2.1x higher specific peak force
2.7x higher specific continuous force
1.1x higher Max acceleration (max force divided by the moving mass)
The peak force can be applied longer due to a larger thermal time constant of the MMA (Voice coils overheat easily).
The higher forces that are possible with the MMA outweigh the (sometimes) larger moving mass of the MMA. This becomes even more clear when the load mass (usually factor 10 to 50 times higher than the moving actuator mass) is taken into account.

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