Page 401 - The Mechatronics Handbook

The system is servo controlled to maintain it at null position. Gravitational acceleration is balanced by
the centrifugal acceleration. The shaft speed is proportional to the square root of the local value of the
acceleration.

Electromechanical Accelerometers
Electromechanical accelerometers, essentially servo or null-balance types, rely on the principle of feed-
back. In these instruments, an acceleration-sensitive mass is kept very close to a neutral position or zero
displacement point by sensing the displacement and feeding back the effect of this displacement. A
proportional magnetic force is generated to oppose the motion of the mass displaced from the neutral
position, thus restoring this position just as a mechanical spring in a conventional accelerometer would
do. The advantages of this approach are better linearity and elimination of hysteresis effects, as compared
to the mechanical springs. Also, in some cases, electrical damping can be provided, which is much less
sensitive to temperature variations.
One very important feature of electromechanical accelerometers is the capability of testing the static
and dynamic performances of the devices by introducing electrically excited test forces into the system.
This remote self-checking feature can be quite convenient in complex and expensive tests where accuracy
is essential. These instruments are also useful in acceleration control systems, since the reference value
of acceleration can be introduced by means of a proportional current from an external source. They are
used for general-purpose motion measurements and monitoring low-frequency vibrations.
There are a number of different electromechanical accelerometers: coil-and-magnetic types, induction
types, etc.
Coil-and-Magnetic Accelerometers
These accelerometers are based on Ampere’s law, that is, “a current-carrying conductor disposed within
a magnetic field experiences a force proportional to the current, the length of the conductor within the
field, the magnetic field density, and the sine of the angle between the conductor and the field.” The coils
of these accelerometers are located within the cylindrical gap defined by a permanent magnet and a
cylindrical soft iron flux return path. They are mounted by means of an arm situated on a minimum
friction bearing or flexure so as to constitute an acceleration-sensitive seismic mass. A pickoff mechanism
senses the displacement of the coil under acceleration and causes the coil to be supplied with a direct
current via a suitable servo controller to restore or maintain a null condition. The electrical currents in
the restoring circuit are linearly proportional to acceleration, provided (1) armature reaction affects are
negligible and fully neutralized by a compensating coil in opposition to the moving coil, and (2) the gain
of the servo system is large enough to prevent displacement of the coil from the region in which the
magnetic field is constant.
In these accelerometers, the magnetic structure must be shielded adequately to make the system
insensitive to external disturbances or the earth’s magnetic field. Also, in the presence of acceleration
2
2
there will be a temperature rise due to i R losses. The effects of these i R losses on the performance are
determined by the thermal design and heat-transfer properties of the accelerometers.
Induction Accelerometers
The cross-product relationship of current, magnetic field, and force is the basis for induction-type
electromagnetic accelerometers. These accelerometers are essentially generators rather than motors. One
type of instrument, the cup-and-magnet design, includes a pendulous element with a pickoff mechanism
and a servo controller driving a tachometer coupling. A permanent magnet and a flux return ring,
closely spaced with respect to an electrically conductive cylinder, are attached to the pendulous element.
A rate-proportional drag force is obtained by the electromagnetic induction effect between the magnet
and the conductor. The pickoff mechanism senses pendulum deflection under acceleration and causes
the servo controller to turn the rotor to drag the pendulous element toward the null position. Under
steady-state conditions motor speed is a measure of the acceleration acting on the instrument. Stable
servo operation is achieved by employing a time-lead network to compensate the inertial time lag of

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