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MULTISPEED SYNCHRO SYSTEMS The data to be transmitted is another important factor that we must consider when we discuss the accuracy of a synchro system. If this data covers a wide range of values, the basic synchro system is unable to detect any small changes in the data. When this happens, the accuracy of the system decreases. Because of this difficulty, multispeed synchro systems were developed. They handle this type of data very effectively and still maintain a high degree of accuracy. Multispeed synchro systems use more than one speed of data transmission. The speed of data transmission is, simply, the number of times a synchro transmitter rotor must turn to transmit a full range of values. For example, a system in which the rotors of synchro devices turn in unison with their input and output shafts is commonly called a 1-speed data transmission system. In this system, the transmitter's rotor is geared so that 1 revolution of the rotor corresponds to 1 revolution of the input. Until now, the discussion of synchro systems has dealt exclusively with this 1-speed system. In a 36-speed data transmission system, the rotor of the synchro transmitter is geared to turn through 36 revolutions for 1 revolution of its input. Units transmitting data at one speed are frequently called 1-speed synchros; a unit transmitting data at 36-speed would be a 36-speed synchro, and so forth. It is quite common in synchro systems to transmit the same data at two different speeds. For example, ship's course information is usually transmitted to other locations on a ship at 1-speed and 36-speed. A system in which data is transmitted at two different speeds is called a dual- or double-speed system. Sometimes a dual-speed system will be referred to by the speeds involved, for example a 1- and 36-speed system. In summary, the speed of data transmission is referred to as 1-speed, 2-speed, 36-speed, or some other definite numerical ratio. To indicate the number of different speeds at which data is transmitted, refer to the system as being a single-speed, dual-speed, or tri-speed synchro system. SINGLE-SPEED SYNCHRO SYSTEM If the data to be transmitted covers only a small range of values, a single-speed system is normally accurate enough. However, in applications where the data covers a wide range of values and the accuracy of the system is most important, the 1-speed system is not adequate enough and must be replaced by a more suitable system. Increasing the speed of a single-speed system from 1-speed to 36-speed provides greater accuracy, but the self-synchronous feature of the 1-speed system is lost. If primary power is interrupted in a 36-speed system and the transmitter is turned before power is reapplied, the synchros could realign themselves in an erroneous position. The number of positions in which the transmitter and receiver rotors can correspond is the same as the transmission speed. Thus, in the 36-speed system, there are 35 incorrect positions and only 1 correct position of correspondence. For accurate transmission of data over a wide range of values without the loss of self-synchronous operation, multispeed synchro systems must be used. Multispeed synchro systems use more than one speed of data transmission and, therefore, require more than one output shaft. DUAL-SPEED SYNCHRO SYSTEM A basic dual-speed synchro system consists of two transmitters and two receivers, as shown in figure 1-34. One transmitter receives the external input to the system and, through a network of gears, passes the effects of the external input to the second transmitter. The gear ratio between these two transmitters determines the two specific speeds the system will use to transmit the input data. The two speeds of this system are often referred to as fast and slow, high and low, or more often as fine and coarse. Figure 1-34. - Dual-speed synchro system.
If, for example, the gear ratio between the two transmitters is 36 to 1, 1 revolution of the rotor of the first transmitter causes 36 revolutions of the rotor of the second transmitter. The first transmitter-the one that accepts the external input-can be called the coarse transmitter, and the second one can be called the fine transmitter. Representative speeds include 1 and 36, 2 and 36, and 2 and 72. The output of each transmitter is passed through standard synchro connections to a receiver. One receiver receives the coarse signal and the other one receives the fine signal. The two receivers may or may not be connected by a network of gears similar to the network between the two transmitters. In some dual-speed applications, a double receiver is used instead of two individual receivers. The double receiver (fig. 1-35) consists of a coarse and a fine receiver enclosed in a common housing. It has a two-shaft output one inside the other. The coarse and fine receivers may be likened to the hour and minute hands of a clock. The coarse receiver corresponds to the hour hand, and the fine receiver corresponds to the minute hand. This double receiver has the advantage of requiring less space than two single receivers. However, it also has a disadvantage - when one receiver goes bad, both must be replaced. Figure 1-35. - Cutaway view of a double receiver.
In the dual-speed synchro system, data is transmitted by the coarse transmitter, while the system is far out of correspondence and then is shifted to the fine transmitter as the system approaches correspondence. This shifting from coarse to fine control is done automatically when the fine error signal exceeds the coarse error signal. The fine synchro transmitter then drives the system to the point of correspondence. When the dual-speed synchro system includes control transformers, there is always the possibility of a 180 error being present in the system. The reason is the rotor of the CT is not energized and its error-voltage output is zero both at its proper position and also at a point 180 away from that position. To prevent the CT from locking 180 out of phase with the rest of the system, a low voltage is applied across the rotor terminals of the coarse CT as shown in figure 1-36. Figure 1-36. - Dual-speed synchro system using a stickoff transformer.
This voltage is normally about 2.5 volts and is commonly termed "stickoff" voltage. It is obtained from the secondary of a small transformer. The voltage induced in the secondary of the transformer shifts the 0 position of the coarse CT To reestablish a new 0 position, the stator of the coarse CT must be turned through an angle that induces an opposing 2.5 volts in the rotor to cancel the stickoff voltage. Therefore, at 0 the two voltages cancel and no input exists to drive the servo amplifier. Should the rotor of the CT stop at 180, the same 2.5 volts would be induced in the rotor. However, it would be in phase with the stickoff voltage and no cancellation would occur. The end result is an error signal at 180 that drives the dual-speed synchro system out of any false synchronizations. TRI-SPEED SYNCHRO SYSTEM The advent of long-range missiles and high-speed aircraft has brought about the need for accurately transmitting very large quantities. This is best done by a tri-speed synchro system, which transmits data at three different speeds. These speeds are sometimes referred to as coarse, medium or intermediate, and fine. A typical weapons systems, for example, might transmit range in miles, thousands of yards, and hundreds of yards. By providing this range in three different scales, greater accuracy is obtained than would be possible with a dual-speed system. Representative speeds for tri-speed systems include 1, 36, and 180; 1, 36, and 360; and 1, 18, and 648. Q.53 What is the name given to the synchro system that transmits data at two different
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