Control Device for a Synchro Receiver

The present invention relates to a control device for a synchro receiver for data supplied on the one hand by a synchro transmitter and on the other hand by a digital element, the said synchros comprising three stator windings.

It should be noted that the synchros involved here are predominantly used for the transmission of angular data. A description of the synchro apparatus can be found in the prior art.

By using this type of apparatus it is possible to control a display which depends on two quantities, these quantities being produced by apparatus of the same type. It increasingly happens that the quantities are processed by digital means such as, for example, microprocessor systems. This poses a difficult problem when one of the quantities is supplied by a digital element and the display must be effected by the synchro. This problem occurs more specifically in air radio navigation systems in which an aeroplane determines its position relative to a beacon. Aboard this aeroplane there is a more or less sophisticated equipment which supplies the bearing in digital form. This angle indicates, with respect to the aeroplane, the angular distance between the magnetic north pole and the beacon. A simpler equipment supplies the course. This angle is simply processed by a synchro which is coupled to a compass. The necessary remote display must be effected by a synchro and must indicate the position, the difference between the above-mentioned angles (the bearing and the course).

An object of the present invention is to provide a solution to this important problem.

To that end, a device of the type mentioned in the opening paragraph is characterized in that it comprises, on the one hand, four branches with the inputs of the first and the second branches being interconnected to receive a first voltage from the ends of the stator windings of the synchro transmitter. The inputs of the third and the fourth branches are interconnected to receive a second voltage from the ends of the stator of the synchro transmitter. In order to supply control voltages to the winding ends of the stator of the synchro receiver, the device comprises, on the other hand, two adders. Two inputs of the first adder are connected to the output of the first and the third branches, respectively and the two inputs of the second adder are connected to the outputs of the second and the fourth branches. Each of these branches have a transfer function of the form sin (.alpha.+.phi.1), sin (.alpha.+.phi.2), sin (.alpha.+.phi.3), sin (.alpha.+.phi.4), respectively, wherein .alpha. represents the information supplied by the digital element and .phi.1, .phi.2, .phi.3, .phi.4 the different phase shift angles.

The following description is given by way of non limitative example with reference to the accompanying drawings and will make it clear how the invention can be put into effect.

FIG. 1 shows a device in accordance with the invention.

FIG. 2 shows in detail an embodiment of a branch which forms a part of the device of FIG. 1.

The following description is based on the application of the invention which was already mentioned above by way of example, that is to say the case in which one wants to display the position as a function of the course and the bearing.

In FIG. 1, the device according to the invention is denoted by reference numeral 1. The synchro receiver used to display the position is denoted by reference numeral 5. The Figure shows its three stator windings 6, 7 and 8 and a rotor winding 9. The first end of the stator windings are interconnected. The second end of the winding 6 is connected to ground, while the second ends of the windings 7 and 8, respectively, are connected to the input terminals 10 and 11, so that the voltages for controlling the angular rotation of the rotor provided with the winding 9 occur between the terminals 10 and 11 and ground. The synchro transmitter which supplies the course information (.theta.) is denoted by reference numeral 15. The stator windings are denoted by reference numerals 16, 17 and 18, and the rotor winding by reference numeral 19. A voltage source producing an a.c. voltage of 26 Volts and having a frequency of 400 Hz is permanently applied to the ends of the rotors 9 and 19. The first end of the windings 16, 17 and 18 are interconnected. The second end of the winding 16 is connected to ground, while the second ends of the windings 17 and 18 are connected to the output terminals 20 and 21, respectively so that the voltages supplied by the synchro 15 appear between the terminals 20 and 21 and ground. The device 1 also has an input terminal 30 for receiving digital information which defines the bearing (.alpha.).

In the above-mentioned use, the device is provided to apply a voltage V10 between the terminal 10 and ground so that: ##EQU1## and a voltage V11 between terminal 11 and ground, ##EQU2## These voltages will cause the rotor of the synchro to rotate through an angle .gamma. which is the angular position:

This can be derived as follows. Associated with the angular course .theta. there is on the one hand a voltage V20 between the terminal 20 and ground and a voltage V21 between the terminal 21 and ground so that: ##EQU3## V0 being a constant which represents the maximum amplitude of the voltages.

In accordance with the invention, four branches 40, 41, 42 and 43 are provided. The inputs of the branches 40 and 41 are connected to the terminal 21 via an attenuator 50 which produces an attenuation by a factor 1/2. The inputs of the branches 42 and 43 are connected to the terminal 20 via an attenuator 51 which also produces an attenuation of a factor 1/2. A first adder element 61 adds together the voltages V1 and V3 which appear at the outputs of the branches 43 and 41. A second adder element 62 adds together the voltages V2 and V4, which appear at the outputs of the branches 42 and 40. The output voltages of the elements 61 and 62 are applied to the terminals 10 and 11 via amplifiers 71 and 72 having a gain (-4/.sqroot.3). The branches 40, 41, 42, 43 have the following respective transfer function: ##EQU4## To explain the operation of the device in accordance with the invention it is sufficient to write: ##EQU5## Therefore: ##EQU6## Hence: ##EQU7## Similarly: ##EQU8## Therefore: ##EQU9## Hence: ##EQU10## wherein k is a proportionality factor. It is possible, when the gain of the amplifiers 71 and 72 is chosen to be equal to A, to obtain that k=1, that is to say: ##EQU11##

FIG. 2 shows a detailed embodiment of the branches 40, 41, 42, 43. These branches have identical structures, except for the angular value present at an input of a digital adder 90, whose other input receives the digital quantity .alpha.. The displayed angular value corresponds to the different phase deviations 0, .pi./3, 2.pi./3 and 3.pi./3. The digital value at the output is applied to a "sine table" 92 in the form of, for example, a read-only memory. This memory supplies only the positive sine and this digital value is applied to the digital input 93 of a digital-to-analog converter 94 (for example the circuit AD 7524). The reference signal input 96 of this circuit is connected to the output of the circuit 50 for the branches 40 and 41 and to the output of the circuit 51 for the branches 42 and 43. The digital-to-analog converter is of the resistor type so that the signal at its output 97 is the product of the voltage applied to the input 96 and the code applied at its digital input 93. In order to change the sign of the voltage at the output 97 as a function of the angle at the input of the table 92 a digital comparator 98 is used which detects whether this angle is comprised within the interval 0 and .pi., or is outside this interval. As a function of this information supplied to the comparator 98, a change-over switch 99 is acted on to multiply by +1 or by -1 the signal at the output 97. This may be effected by two amplifiers 100 and 101 having a gain +1 and -1, respectively, which are connected between the output 97 and the input of the change-over switch 99. The four circuit branches 40, 41, 42, and 43 operate as four look-up tables. The output signal of each of the branches is dependent upon the input signal (.alpha.), a predetermined and fixed phased shift angle (0, .pi./3, 2.pi./3, 3.pi./3), and a scale factor (a so-called "reference signal") which is the output of either circuit 50 or circuit 51 (FIG. 1). For example, the output signal V.sub.3 of circuit branch 41 provides a signal which is the product of the signal at terminal 21, attenuated by a factor of 1/2 in circuit 50, and a factor sin (.alpha.+.pi./3). The factor sin (.alpha.+.pi./3) is generated by the circuit shown in FIG. 2.

In FIG. 2, reference numeral 92 designates the look-up table for the trigonometric function sin (.alpha.) for values of .alpha. between 0.degree. and 180.degree. (or .pi. radians). The input signal (.alpha.) is increased by the appropriate phase shift angle (.pi./3 for the branch 41) and the resulting angle is used as an address for look-up table 92. The digital output signal of look-up table 92 is fed to the digital input of the digital-to-analog converter 94. For reasons of memory economy, we use only half the table of sin (.alpha.), i.e. 0.degree.-180.degree.. The comparator circuit 98 in combination with circuits 99, 100 and 101 are used to effectively expand the table to 0.degree.-360.degree. by inverting the value of sin (.alpha.+.pi./3), where appropriate.

For a person skilled in the art it will be obvious that by the use of time-division multiplexing techniques one single sine table may be used for the several branches 40, 41, 42 and 43.