Solid state repeater for three wire synchro with selectable phase and frequency adjustment

The instant invention relates to a repeater for processing signals from a three wire synchro to control carrier phasing and frequency, while retaining identity of the angular information. The solid state repeater consists of an arrangement in which the synchro signals bearing the angular information are first demodulated. The demodulated signals are thereafter remodulated in a closed loop modulating network. The phase and frequency of the carrier which is modulated in the closed loop modulating network may be controlled as desired. The modulated carrier is impressed on a negative feedback path in which the signal is demodulated utilizing the selected carrier to produce a variable DC signal which is fed back to the input of the modulating loop so that at null balance the output signal from the modulating path duplicates the angular information from the three wire synchro except that the carrier phase and frequency is controlled as desired. In this fashion, a signal from a three wire synchro may be repeated and the phase and frequency controlled to complement the output equipment without requiring the use of mechanically servoed synchro repeaters.

The instant invention relates to a synchro repeater circuit, and more 
particularly to a solid state repeater in which the carrier of the 
repeated output signal is selectively controlled as to phase and/or 
frequency. 
In many control or indicating systems such as are commonly used in aircraft 
systems, a mechanical angular sensor, such as a three-wire synchro, is 
used to obtain primary indication of angular shift position. For example, 
a synchro may be coupled to the aircraft gyroscope to obtain an output 
signal which represents the gyroscope angular position. The output signals 
from such a primary sensor, i.e., the three wire synchro, are used to 
perform various control and indicating functions in the aircraft. Thus, 
the angular position signals may be utilized as inputs to auto-pilots, 
heads-up displays, in-flight data processing equipment as well as in 
directional and other displays. Often such a single sensor is not capable 
of interfacing with all of these equipments. 
One limitation is related to limit on the power output from a synchro. If 
too many systems are driven from the output from the synchro the accuracy 
of the synchro is degraded and also there is a risk of damaging the 
synchro if it is loaded too heavily. Another limitation is incompatible 
grounding and AC phasing conditions between the synchro and the various 
control and indicating equipments which use the synchro signals. Each of 
the different equipments may have power supplies and circuitry which are 
not necessarily compatible with grounding and the phasing of the output 
from the synchro. It is desirable to lock the synchro signal to the 
electrical supply system for each of the different displays and control 
equipments which utilize this signal. As a result, in systems as they are 
known today, it is necessary that a synchro buffer repeater or a number of 
repeaters be utilized. This requires the use of mechanically servoed 
synchros whose reference or supply voltages can be manipulated as desired. 
However, mechanically servoed synchros are expensive, cumbersome, have 
slow response time and suffer from all of the reliability limitations that 
are inherent in any mechanical design. 
Applicant has found that signals from a three wire synchro may be repeated 
and the phase and frequency of the carrier output selectively controlled 
in a completely solid state electronic system thereby eliminating the 
expensive, cumbersome, etc., mechanical repeaters with all the favorable 
cost and size improvements attendant thereto. 
It is therefore a principal objective of the instant invention to provide a 
solid state synchro repeater with selectable carrier output. 
Another objective of the invention is to provide a synchro repeater with 
selectable carrier output which contains no moving parts. 
Still another objective of the invention is to provide a solid state 
repeater for a three-wire signal from a synchro which is simple, compact, 
smaller and less expensive than existing arrangements. 
Still other objectives and advantages of the invention will become apparent 
as the description thereof proceeds. 
Briefly, in accordance with one aspect of the invention, the three-wire 
synchro signals are converted in a Scott-Tee transformer or the like to 
two phase signals representative respectively of the sin and cos of the 
mechanical or shaft angle represented by the synchro output signals. The 
two phase signals are respectively demodulated to produce a varying DC 
output signals which represent the angular information i.e., the sin and 
cos of the synchro shaft angle .theta. . The DC signals are applied as the 
modulating signals to closed loop modulating networks in which a carrier 
signal having the desired phase and frequency is modulated by the angular 
information. The modulating networks also contain negative feedback paths 
in which the output signals from the modulators are demodulated using the 
reference carrier of selected phase and frequency. The demodulated signal 
in each negative feedback path is fed back to the input of the modulating 
path until, at null balance, the output of the modulated signal accurately 
represents the angular information from the three wire synchro but with a 
carrier of any selected frequency and phase.

The modulated three phase output signals from a three wire synchro which 
represent angular position are applied as one input to a Scott-Tee 
transformer shown at 10. Scott Tee 10 has Y connected synchro type primary 
winding, not shown, to which the three phase signals from the three wire 
synchro are coupled. The secondary of the Scott-Tee transformer, also not 
shown, has a pair of orthogonally wound windings so that the three phase 
output signal from the synchro representing the angular information is 
converted to a pair of output signals which are respectively 
representative of the sin and cos of the synchro shaft angle .theta. . 
Scott-Tee transformers are well-known devices for transforming either a 
two-phase input to a three phase output or conversely, a three phase input 
to a two phase output as is the case Scott-Tee 10 in FIG. 1. Reference is 
hereby made to the text book "Alternating Current Machinery"-LV Bewley, 
MacMillan Company, N.Y. (1949) and particularly to pages 89-91 which 
describe the basic characteristics of the so-called Scott-Tee connection. 
The modulated sin and cos .theta. signals are applied to a pair of signal 
processing networks 11 and 12 in which the sin and cos .theta. signals are 
first demodulated and then remodulated in a closed loop modulating network 
to produce a modulated output signal which duplicates the angular 
information in the output from the three wire synchro but with the phase 
and/or frequency of the carrier being selected to complement the phase, 
frequency and power requirements of the various equipments which utilize 
the repeated signals from the three-wire synchro. The modulated signals 
are then applied to a further Scott-Tee network 13 coupled to the signal 
processing networks to convert the two phase sin and cos .theta. signals 
to three wire signals which are utilized in the output equipments. 
The sin and cos .theta. output signals from Scott-Tee transformer 10 are 
respectively applied as inputs to phase sensitive demodulators 14 and 15 
in channels 11 and 12. The other input to demodulators 14 and 15 is a 
square wave reference carrier wave which has the same frequency as the 
excitation voltage to the three-wire synchro. To this end, a sinusoidal 
voltage from the excitation voltage source for the three wire synchro is 
also applied to isolation transformer 16 to isolate the repeater from the 
three wire synchro. The signal from isolation transformer 16 is applied to 
comparator amplifier 17 which has its other input terminal grounded. 
Amplifier 17 saturates at very low positive and negative voltage levels 
relative to ground so that the incoming sine wave is clipped producing a 
square wave voltage at its output which has the same frequency and phase 
as the excitation voltage for the three wire synchro. A square wave 
reference carrier signal is preferred in the event that the phase 
sensitive demodulators 14 and 15 are phase sensitive switching type 
demodulators since the switching devices respond more accurately and more 
rapidly to a square wave then they would to a sinusoidal reference 
carrier. The output of demodulators 14 and 15 are therefore varying DC 
voltages which are respectively proportional to the sine and cos of the 
shaft angle .theta. . 
The output from demodulators 14 and 15 is utilized as DC modulating 
voltages to modulate a carrier signal of selected phase and frequency to 
produce a modulated output signal which duplicates the angular information 
from the three wire synchro but which has the desired carrier phase and 
frequency characteristics. 
Modulation networks 18 and 19 are of the closed loop, null balanced type in 
which the incoming DC signal is utilized to modulate a carrier of selected 
phase and frequency. The modulated output signal is also applied to a 
negative feedback path in each of the modulating networks. The negative 
feedback paths contains suitable demodulators driven by the carrier of 
selected phase and frequency to produce a negative DC feedback signal 
which is compared with the varying DC output voltages from the 
demodulators 14 and 15. If any difference exists between the D.C. output 
from the demodulators 14 and 15 and the D.C. output from the demodulators 
in the negative feedback paths, modulating networks 18 and 19 are driven 
to force their equalization at null balance thereby enhancing the accuracy 
of the output signals from the solid state repeater. 
The DC outputs from demodulators 14 and 15 which represent the sin and cos 
.theta. information are applied as one input to summing nodes 20 and 21 
respectively. The demodulated D.C. signals from the negative feedback 
paths in networks 18 and 19 are the other inputs to those nodes. The 
signals are compared to drive the system to a null balance. The 
differential outputs from summing nodes 20 and 21 are applied as one input 
to summing nodes 22 and 23 respectively. The output from nodes 22 and 23 
are applied to high gain amplifiers 24 and 25 in the modulating networks 
18 and 19. Low pass filters 26 and 27 are included in negative feedback 
paths around amplifiers 24 and 25. The other input to summing nodes 22 and 
23 at the input of amplifiers 24 and 25 is from the low pass filters. Low 
pass filters 26 and 27 are provided to prevent the loop from oscillating. 
It provides sufficient damping to prevent the modulated output signal from 
undergoing rapid preturbations. That is, the synchro output may in many 
cases be subject to "jitter." However, because of the presence of the low 
pass filters in the negative feedback paths for amplifiers 24 and 25, the 
output from these amplifiers is damped and as a result, the output from 
the solid state synchro repeater will be similarly damped and will not be 
subject to rapid preturbations due to jitter in the output of the primary 
sensor, i.e., the three wire synchro. 
The amplified, damped, DC modulating voltages representative of the sin and 
cos of the shaft angle .theta. are applied as one input to multipliers 28 
and 29. The other input to multipliers 28 and 29 is a carrier of selected 
phase and frequency. This carrier is provided from a sinusoidal reference 
source, not shown, which is applied to isolating transformer 30. The 
output from isolating transformer 30 is applied over lead 31 as the other 
input 10 multipliers 28 and 29. The outputs from multipliers 28 and 29 are 
therefore linear, amplitude modulated signals which are proportional to 
the sin and cos of the shaft angle .theta. but with the carrier signal 
having the desired phase and frequency. Modulated output signals from 
multiplers 28 and 29 are applied to power amplifiers 32 and 33 in which 
the signals are amplified and then applied to Scott-Tee transformer 13 
where the two phase signal is converted to a three phase, three wire 
signal which may then be coupled to the control or indicating equipments 
as desired. 
The outputs from amplifiers 32 and 33 are also applied to negative feedback 
loops 34 and 35 which are coupled between the output of the amplifier and 
summing nodes 20 and 21. The negative feedback paths contain phase 
sensitive demodulators 36 and 37 which demodulate the signals to produce a 
varying DC voltage proportional to sin .theta. and cos .theta. 
information. The DC voltages are applied as the other input to summing 
nodes 20 and 21. The carrier applied to phase sensitive demodulators 36 
and 37 is a square wave having the same phase and frequency as the 
sinusoidal carrier applied to multipliers 28 and 29. To this end, the 
sinusoidal reference carrier from isolation transformer 30 is also applied 
to a comparator amplifier 38 which has it other input terminal grounded. 
Amplifier 38 saturates at very low positive and negative voltage levels 
relative to ground so that the sinusoidal signal is clipped and a square 
wave having the same phase and frequency as the sinusoidal carrier is 
produced at the output of the amplifier. This square wave is applied as 
the other input to demodulators 36 and 37 in which the amplified, 
modulated signals from multipliers 28 and 29 are demodulated. As pointed 
out previously the closed loop drives the output from demodulating 
networks so that the output from the networks duplicate the angular 
information from the three wire synchro but with the phase and frequency 
of the remodulated signal being that of the new carrier from the new 
reference signal source. 
By virtue of the Scott-Tee transformers that are utilized at the input and 
output of the networks and the isolation transformers complete electrical 
isolation is maintained between inputs and outputs as well as between the 
carrier supplies. 
It will also be appreciated that the accuracy of the system is greatly 
increased over straightforward demodulation, amplification and modulation 
because the modulator and the amplifiers are all within a closed loop. 
Consequently, the primary sources of error are the demodulators 14 and 15, 
36 and 37, and amplifiers 24 and 25. However, these components can be 
designed for accuracy with a minimum of complexity. Modulators 28 and 29 
and the output power amplifiers 32 and 33 which normally would require 
high precision devices and circuitry no longer need to have the same 
degree of accuracy because the null balancing loop corrects errors that 
may be due to these components. It may also be seen that by providing the 
power output amplifiers 32 and 33 in the loop the power level of the 
repeated signals may be increased so that many equipments may be driven 
without in any way affecting accuracy by the repeated signals. This 
eliminates or obviates one of the problems associated with trying to drive 
too many equipments from the output of the synchro namely loss of accuracy 
and overloading of the synchro. 
While a number of specific embodiments of this invention have been shown 
and described above, it will, of course, be understood that the invention 
is not limited thereto since many modifications, both in the circuit 
arrangement and in the instrumentalities employed therein, may be made. It 
is contemplated by the appended claims to cover any such modifications 
which fall within the true spirit and scope of this invention.