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Part D Amplidyne follow-system
10D1. General
An amplidyne follow-up system in its simplest form consists of the four units shown in
figure 10D1:
1. The synchro control transformer.
2. The amplifier.
3. The amplidyne motor-generator.
4. The follow-up DC motor.
The synchro control transformer receives the order signal which indicates electrically what the position of the load should be. The rotor of the synchro control transformer is turned by the response shaft, which is geared to the load and so indicates what the position of the load actually is. The synchro compares the actual load position with the ordered position; and, if the two do not agree, it generates an alternating-current signal which is transmitted to the amplifier. The angular difference between the two positions is called the error, and the signal to the amplifier is the error signal. The error signal indicates by its electrical characteristics the size and direction of the error. If no error exists, the system is said to be in correspondence and the error signal is zero.
The amplifier receives the alternating-current error signal, amplifies it, and converts it into direct current suitable to energize the field windings of the amplidyne generator.
The amplidyne generator supplies direct current to operate the follow-up motor. The direction of rotation of the motor depends on the polarity of the output of the amplidyne generator, which in turn depends on the direction of the error as indicated by the error signal. As a result, the motor moves the load in the proper direction to reduce the error.
The amplidyne generator is a power amplifier on a large scale. Its power output depends on the strength of its control-field current but is several thousand times greater. The additional power is supplied by the motor which drives the amplidyne generator. The strength of the control-field current from the amplifier depends on the size of the error as indicated by the error signal. The power applied to the follow-up motor, therefore, is greater for a large error than for a small one.
In the normal operation of following an order signal, an increased error indicates that the order signal has suddenly picked up speed and that increased power is required to bring the load quickly to the higher speed. In response to an increased error, the amplidyne generator promptly supplies the necessary added power. If the order signal suddenly slows down, the load may overrun the signal and reverse the direction of the error. As a result, the polarity of the amplidyne output is reversed; the motor tries to run in the reverse direction, and so applies a retarding force to the load.
When the order signal moves at a uniform speed, the motor must supply only enough power to overcome the friction in the system. The power output of the amplidyne is then much less than when the speed is increasing or decreasing, and the error will be correspondingly smaller.
Because of the immense power amplification available in the amplidyne generator and amplifier, an extremely small error signal supplies enough power to control the mount. In following the usual gun-train or gun-elevation order, the errors should not be more than a few minutes of arc under the most adverse conditions
10D2. Synchro control transformer
The functioning of a synchro control transformer was described earlier in this chapter. As in the system described in article 10B3, the synchro control transformer’s output in an amplidyne system produces an AC output signal which depends on the position of the rotor with respect to the stator’s magnetic field. The stator receives a gun-order synchro signal from a synchro transmitter, and the synchro control transformer’s rotor is driven by the gun mount. As the gun mount is driven toward gun-order position by the amplidyne power drive, the control transformer’s rotor approaches the null position. At null, the gun mount is in gun-order position.
Because of the sensitivity of the system, the synchro control transformer’s output very closely controls the operation of the power drive, increasing acceleration of the mount’s movement as the rotor signal increases, and minimizing overshooting.
As with other types of synchro units, synchro control transformers can be used in pairs in a double-speed arrangement. Identical synchros are used, but they are geared at 36: 1. Both synchros are connected to the input of an electronic amplifier, but a relay, switching circuit, or electrical network automatically selects the output of either the coarse or the fine synchro. The coarse synchro control transformer’s signal is switched to the amplifier input when the gun mount is more than about 3° out of synchronism. As the gun mount approaches synchronism with the gun-order signal, the fine synchro signal automatically switches into the circuit to furnish the controlling input to the amplifier and continue gun mount movement until it is fully matched with gun order.
10D3. Amplifier
The function of the amplifier is to supply two control-field currents for the amplidyne generator. In following an order signal in automatic control, these currents must be varied in accordance with changes in the error signal. When the error signal is zero, the two control currents should be equal. When the error signal calls for movement of the mount in one direction, one control current must increase and the other must decrease. When the mount is to move in the opposite direction, the unbalance in the control currents must be reversed.
The amplifier has two stages. The first stage is primarily a rectifier stage in which two direct currents are produced whose magnitudes are controlled by the error signal. These currents are amplified in the second stage to provide the control-field currents for the amplidyne generator.
10D4. Amplidyne generator
The construction and operation of the amplidyne generator can best be understood by following through the steps necessary to convert an ordinary direct current generator into an amplidyne generator.
When a coil of wire is rotated in a magnetic field, voltage are induced in the coil, and, if the ends of the coil are connected together, these voltages cause electric currents to flow in the coil. This is the basic principle of a generator.
The principal parts of a generator are the stator, or stationary part, and the armature, or rotating part. In a common form of generator, a coil of wire is wound on a part of the stator and is supplied with a small exciting current which magnetizes the iron in the stator and armature to provide the necessary magnetic fields. The armature carries other coils which are rotated in the magnetic field as the armature is turned. As a result, voltages are induced in the armature coils.
The ends of the armature coils in a DC generator are connected to copper bars on a commutator which rotates with the armature. The voltages induced in the coils are taken off by stationary carbon brushes engaging the commutator as it turns. If the brushes are connected together through an external circuit, current will flow in the circuit and through the armature coils.
The connections to the commutators are such that the maximum voltage appears across two points on opposite sides of the commutator. The positions of these points depend on the direction of the magnetic field and do not change as the commutator rotates. The brushes are located at or near these points to take advantage of the maximum voltage.
In
figure 10D2, the upper view represents an ordinary direct-current generator such as the one just described. The inner circle is the commutator, with brushes at top and bottom. The next circle represents the armature, and the outer structure is the stator with a coil carrying the exciting current wound on its pole piece. Other conditions being equal, the power out put of the generator will be proportional to the power input to the excitation winding, within the limits of normal operation. This generator is assumed to be a 10-kw machine (10,000 watts output), and the excitation required is about 100 watts. The amplification, therefore, is 100 to 1.
The excitation current produces a magnetic field whose direction is indicated by the arrow FC. It is this magnetic field which induces the 100 volts which appears across the brushes. At the same time, the 100-amp load current flowing in the armature coils creates another magnetic field FS at right angles to FC. It has about the same strength as the field FC. This second magnetic field, called armature reaction, does no useful work in the ordinary generator and is, in fact, a source of trouble.
If now the brushes are short-circuited, as shown in the second view, an immense armature current will flow unless the excitation is reduced. If the excitation is cut down to about 1 watt, FC is reduced accordingly, and the normal full-load current of 100 amperes flows through the short-circuit path. This current produces the same armature reaction FS as before.
The armature reaction FS induces a voltage in the armature in the same manner as flux FC but this voltage appears on the commutator at 90 degrees from the voltage induced by FC. A voltmeter connected to points on the commutator, as shown in the second view, will indicate approximately full-load voltage.
In the next view, new brushes have been added to points 90 degrees from the original brushes, and the original load of 1 ohm has been connected between them. The high voltage formerly existing between these points has almost disappeared. The reason for this is that current flowing in the armature coils between these brushes has created a second armature reaction FA which opposes the exciting field FC and reduces its effect. The decrease in the effect of FC reduces FS and consequently reduces the voltage across the new brushes.
The lower view shows the last modification necessary to produce an amplidyne generator. The armature current from the new brushes has been taken through a compensating field winding and creates a magnetic field FB opposed to FA. This field may be adjusted to balance out FA and thus restore the full effect of the exciting field FC. FS is restored to normal, and full-load current may be drawn from the new brushes. Since both FA and FB depend on armature current, they will always be approximately balanced and the output voltage is nearly independent of the armature current. Full-load output has been obtained with only 1-watt excitation instead of 100. The amplification is 10,000 to 1 instead of 100 to 1.
Other refinements are necessary to produce the fast, stable operation necessary in a follow-up system, but the machine shown in the lower view of
figure 10D2 is the basic form of all amplidyne generators. In the equipment now in use, excitation is supplied to two control windings which are oppositely wound. The direction of the magnetic field FC and the polarity of the output of the generator depend upon which winding receives the stronger current. Thus, the direction of rotation of the follow-up motor, which receives its power supply from the amplidyne generator, can be controlled at will by supplying the stronger current to one or the other of the control fields. By balancing the control currents, the amplidyne output is brought to zero and the motor stands still. The difference between the two control currents determines the amount of power supplied to the motor.
10D5. 5”/54 amplidyne train drive
An example of amplidyne power drives in the Navy is the Train Power Drive Mark 14 on the 5”/54 single mount.
The main units in this system at the mount are (1) gun train indicator-regulator, (2) 40-hp train motor, (3) brake unit, (4) train-selector switch, (5) master switch, and (6) shifting clutch; those located in the amplidyne control room below deck are (7) train amplidyne motor-generator and (8) amplifier unit. See figure 10D3.
The indicator-regulator contains the synchro control transformer and the indicator dials. The brake unit is a safety mechanism which locks the drive and holds the mount stationary if power supply fails during power operation. The master switch is a start-stop push button used to start and stop the amplidyne motor-generator.
The shifting lever has two main positions, MANUAL and POWER. A middle position of STOP acts as a safety feature in shifting the mount between power and manual.
The selector switch has four positions: AUTO, LOCAL HIGH, LOCAL MEDIUM, and LOCAL LOW.
It is used to select any of these four means of power operation of the mount.
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