Biomedical Engineering Reference
In-Depth Information
FIGURE 3-6
BEI
Kimco LA10-12-27A
linear voice coil
actuator sensitivity
for a spring constant
k
s
= 2 N/mm.
Because the relationship between the current is slightly nonlinear, really accurate
positioning requires the addition of a linear displacement measurement sensor such as an
linear variable differential transformer (LVDT) or a linear potentiometer as discussed in
Chapter 2. However, for most applications the open-loop linearity is sufficiently good.
This description explains how the actuator can be positioned with good accuracy, but
it does not describe the dynamics of reaching that equilibrium point.
When a conductor moves through a magnetic field, a voltage is induced across the
conductor in a direction to oppose the motion (if current can flow); this is the back EMF
as defined by equation (3.5). A voice coil will continue to accelerate, if the stroke is
sufficiently long, until the back EMF is equal to the applied voltage, at which time the
voice coil will have reached terminal velocity. In general, however, the velocity is limited
to a value far below this.
In the case of the voice coil, the length of conductor is determined by the radius of
the coil,
a
(m), and the number of turns,
N
, making the back EMF,
ε
b
(V),
ε
b
=
2
π
NaB
v
(3.15)
When voltage is suddenly applied to a voice coil, the inductance of the windings
limits the rate of increase in the current, and the response of the device can be sluggish.
To overcome this, a copper sleeve fixed to the magnet structure surrounds the coil. Eddy
currents are induced in it by changes in the current through the coil. These currents cause
a magnetic field that opposes the original field and cancels much of the apparent coil
inductance, with the result that the response is much faster and more linear.
Should the high-frequency response of the voice coil be required when designing a
control system, the electrical and mechanical time constants as well as the inductance
must be considered.
Figure 3-7 shows a range of voice coil actuators including both linear and rotary types,
manufactured by USAS Motion.
