1. Field of the Invention
This invention pertains to a resolver to digital converter and more particularly to a resolver to digital converter which uses digital signals to provide accurate control of the excitation voltages applied to the resolver windings so that the accuracy of the digital output representation is improved.
2. Description of the Prior Art
In many types of equipment it is desirable to reliably and accurately measure the position of a rotatable shaft and to convert the resulting measurement into a digital signal which is representative of the shaft position. One such type of equipment where it is desirable to digitally measure angular position is a weapons delivery system where the elevation and azimuth pointing angles must be accurately measured. Some of the prior art converters make use of the conventional resolver or synchro transmitter having a rotor winding and a plurality of stationary windings. A voltage source and a phase shifter supply multiphase AC voltages to the stator windings to induce a magnetic flux which rotates at the frequency of the applied voltage and induces an AC voltage in the rotor winding. The AC voltage in the rotor winding has a phase angle which varies with the angular position of the rotor. Thus, the angular position of the rotor can be determined by measuring the difference in phase angle between the voltage induced in the rotor winding and a fixed reference voltage. Such difference can be measured by determining the time which elapsed between the zero passage of the reference voltage and the zero passage of voltage from the rotor winding, and this time can be expressed as a digital representation of the rotor angle. A serious disadvantage of the foregoing type of equipment is that a very high degree of accuracy must be maintained between the phase of the voltages applied to the various stator windings. Aging of the components used in the circuitry may cause a change in phase between the voltages applied to the various stator windings and therefore cause inaccuracies in the measured angle between the reference voltage and the voltage produced by the rotor winding. These inaccuracies lead to an inaccurate output reading of the angular position of the rotor.
One type of prior art apparatus as shown in prior U.S. Pat. No. 3,634,838, uses a high-frequency signal source to develop signals for the stator windings of a resolver or goniometer having three stator windings and a rotor winding. The signal source develops high-frequency pulses which are reduced to low-frequency pulses by a frequency divider. The output of the frequency divider is coupled to a filter which converts the pulses into a sinusoidal voltage that is coupled to one of the stator windings. The sinusoidal voltage from the filter is applied to a first stator winding and to a 90.degree. phase shifter. The phase shifter develops a second sinusoidal voltage shifted 90.degree. from the first voltage, the second sinusoidal voltage being applied to a second stator winding. A counter which is coupled to the signal source is used to count the high-frequency periods. In addition, a phase detector is provided for comparing the phase of an output voltage from the counter with that of a reference voltage from the goniometer. The phase of the pulses from the frequency divider is controlled relative to the phase of the voltage from the counter by varying the number of pulses supplied to the frequency divider. This prior art apparatus has no provision for correcting any variation in the phase angle between the voltages applied to the first and second stator windings of the goniometer.