DRAWING TABLES DRIVEN BY ELECTRONIC SERVOS, ETC.
9
The tachymeter part, set up coaxially to the biphase motor, is formed by two
stator windings with axes at 90 0 from one another, and of a rotor, rigidly connected
to the « squirrel cage » of the motor. One of the stator windings (excitation winding)
Avvolgimento Avvolgimento
di controllo di eccitazione
Schema di un moto-tachimetro
is fed by the network voltage (115 volts, 400 c/s), the other (<output winding) is fed
as stated hereinafter. The rotor of the tachymeter, instead, consists of a cjdindrical
aluminium surface {bell) wherein we induce the currents of the magnetic field of
the stator excitation winding. These induced currents create in the rotor cavity
an alternate magnetic field, with a stationary direction in space ; this field may
be considered as the result of two fields of half the amplitude, rotating in a con
trary sense and the same frequency as that of the input.
As long as the rotor does not move, the said magnetic field does not induce
any voltage into the output winding, as the two axes of the stator windings are at
right angles. But if the rotor turns, (because the rotor of the biphase motor, with
which it is rigidly connected, turns) we reduce the rotation speed of one of the
two components in which we imagined the said magnetic field to be decomposed,
and at the same time we increase the rotation speed of the other component :
therefore their resultant is not an alternate field, fixed in space, but a (no longer
alternate) field which rotates and can induce into the output winding a voltage
proportionate to the rotation speed of the rotor. Obviously the sign of such a voltage
depends on the rotation sense.
If we now’ connect the input of the servo-amplifier with the output of the
tachymeter (fig. 7), so that the latter voltage is deducted from that coming from
the synchro-receiver we obtain the tachymetric reaction : thus, in fact, the voltage
error which is being amplified acquires also a damping component.
In addition to the advantage stated at the beginning, we also obtain a strong
torque in the coincidence position which makes it insensible to any mechanical
troubles in the controlled member. In fact, any outside disturbance, causing a
small displacement of the synchro-receiver from the coincidence position may cause
an appreciable y signal at the tachymeter output, the stronger the faster