BME Systems Inc. Vibratactile Stimulator is a linear
motor for delivering precise, single-point vibratory
stimuli. Two modes of operation are possible. Position
mode permits control of the motor's probe tip to within
1 micron over a 2 mm range. Force mode enables the application
of up to 200 grams to a specimen with sinusoidal modulation
flat (* 1 dB) to 200 Hz. The motor features completely
new design principles that extend its performance and
operating envelope well beyond that of previous models.
It is characterized by its flat response ( < 1 dB
deviation from flat, 1 to 300 microns peak-to-peak travel
from 0 to 300 Hz) and minimal distortion through its
stimulator is centered around a cylindrical, moving-coil,
type of linear actuator governed by the Lorentz Force
Principle. Its permanent magnet has been developed with
a flux-focused design to increase the flux density within
the air gap in which the coil moves. The BME Systems
Vibratactile Stimulator motor design incorporates no
suspension. Instead, the coil and its attached moving
components are guided by a connected shaft, captured
by a low-friction bushing which extends through the
center of the motor magnet body. The end of the shaft
opposite the coil form is attached to a solid cylinder,
the upper surface of which is used as a target for an
inductive position sensor with 0.2 micron static resolution.
The shaft extends through the coil form and is joined
to a load cell through a temperature-insulating coupling.
A light-weight Delrin* probe is attached to the active
end of the load cell.
of the BME Linear Motor design are:
associated with large movements in spring suspension
designs are eliminated. This permits a high degree of
spectral purity for large dynamic displacements. Only
a small amount of viscous friction (linearly related
to velocity) impedes free movement of the motor's parts.
relatively light components used in the moving-coil
design increase the force/mass ratio which permits an
extended frequency response.
increased efficiency of the stator/coil combination
(force sensitivity, 1.6 lb/A) contributes to a low mechanical
time constant which enables higher gain for critically
damped control at high frequencies. In addition, integral
compensation is implemented to increase stiffness and
virtually eliminate static position error.
force required to overcome the acceleration and deceleration
of the moving mass is related to the square of the frequency
of oscillation. The high force capacity (8.0 lb. for
10 sec.) of the motor also extends the usable frequency
stimulator uses a primary method for sensing force very
close to the specimen. This yields highly accurate control
of both static and dynamic force stimulation. The
motor is powered by a unique controller. Command inputs
are given as analog voltages through rear panel BNCs.
The power supply and motor drive components are specifically
designed to provide power that is free of harmonic and
crossover distortion with minimal phase lag. The
controller monitors its RMS current output and gives
indication to the user (via a front panel LED) when
the maximum continuous value for the motor has been
reached. The controller will supply up to 8 amps peak-to-peak
(current is automatically limited) while operating above
the maximum continuous RMS value for up to 10 seconds.
This enables the user to produce intense stimuli for
short periods. If operation continues for more than
10 seconds above the maximum continuous value, the controller
will automatically shut-down the motor for 20 seconds.
Indication of system shut-down is given via a front
panel LED and a rear panel logic output. The mode
in which the motor operates (Position or Force) is normally
controlled via a rear panel BNC. However, this can be
overridden with a front panel switch to enable static
control through front panel potentiometers. Both force
(0 to 199.0 grams) and position (0 to 1.999 mm) are
displayed through two front-panel LED indicators. In
Position mode, Proportional-Integral-Lead compensation
is implemented to provide damping and improved steady
state accuracy. Force mode is centered around
the application of a highly sensitive load cell. The
load cell is separated from the specimen by a probe
tip weighing less than 0.25 grams. Primary measurement
of force is, therefore, achieved with minimal error
from both gravity and mass acceleration/deceleration
forces of the probe itself. Force commands are applied
via a rear panel BNC (20 grams/volt). If the probe should
extend to within 5% of its maximum travel, the closed
force loop is broken and between 40 and 160 grams of
force (depending on the orientation of the motor) is
applied to its mechanical stop. The loop is restored
by retracting the probe either by command or by physically
countering the default force for a distance of one millimeter.
A similar default force is applied, in the opposite
direction, to a retraction stop for any negative force
us for more information.