Biomedical Engineering Reference
In-Depth Information
FIGURE 3-5
Voice coil actuators.
(a) Moving coil.
(b) Moving magnet.
magnetic field is generated across a fixed gap using a powerful permanent magnet. The
coil lies within this space and is held in neutral position by a spring mechanism.
When current is applied to the coil, a force is generated that is proportional to the
product of the flux density, the length of the conductor in the magnetic field, and the
current, as described in equation (3.2). It can be seen that, unlike solenoids, there is a
linear relationship between the current and the force and hence between the current and
the displacement against a spring. An additional characteristic of voice coil actuators that
differs from solenoids is that the direction of motion is determined by the direction of the
current flow.
Linear voice coil actuators range from the minute, with peak forces from 0.7 N and
strokes of 1 mm, to huge with forces of 2000 N and strokes of up to 50 mm. A typical
voice coil actuator in the middle of the range is the LA10-12-027A manufactured by BEI
Kimco. Its specifications are shown in Table 3-1.
Because of end effects, the force sensitivity,
K
f
(N/A), is not completely constant
over the stroke and will therefore result in some nonlinearity in the displacement when
restrained by a spring, as shown in Figure 3-6.
TABLE 3-1
Specifications of the BEI Kimco LA10-12-027A Linear Voice Coil
Actuator.
Winding Constants
Units
Tolerance
Symbol
Value
DC resistance
Ohms
+
/
−
12.5%
R
11
Voltage
Volts
Nominal
V
p
25.1
Current
Amperes
Nominal
I
p
2.28
Force sensitivity
Newton/amp
+
/
−
10%
K
f
5.87
Back EMF constant
V per m/s
+
/
−
10%
K
b
5.87
Inductance
Milli Henry
+
/
−
15@
L
3.05
Actuator parameters
Units
Symbol
Value
Peak force
N
F
p
13.3
N W
−
0
.
5
Actuator constant
K
a
1.77
Electrical time constant
Microsecond
τ
e
277
Mechanical time constant
Millisecond
τ
m
4.43
Power from
I
2
R
Watt
P
p
57.2
Stroke
mm
-
4.57
Notes:
DC, direct current. EMF, electromotive force.
