November 2, 2012
OMNI BEARING SELECTORS (OBS)
Electrical Zero (EZ)
Last month the operation of 30 Hz ORZ Omni Bearing Selectors was described. Early VOR converters were of this type, and were found on all types of aircraft – from those of General Aviation on up to the largest airliners. Gradually, the larger aircraft that had a readily available 400 Hz power source began to use the 400 Hz EZ type resolvers. The prevailing view is that the switch was made because eliminating the 30 Hz signals simplified the converter. The 30 Hz needed to be kept separate from the 400 Hz (when it was present) because there was always the potential for interaction, and signal conversions for RMIs were more complex. If you examine a Collins VIR-30/32 Nav receiver and complementary RMI you find that the 400 Hz reference provides all of the timing for driving the HSI and RMI bearing circuits. The same is true of the AlliedSignal KNR-634 Nav receiver with its corresponding RMI. Extracting VOR radial data with the 400 Hz EZ method, however, is radically different from the 30 Hz method.
A block diagram is shown below:
The 30 Hz ORZ and 400 Hz EZ converters produce the 30 Hz reference from the 9960 FM signals in a similiar manner. Likewise, they both process the 30 Hz variable signal from the composite and pass it through a 90° phase delay. But the commonality ends here. What happens next is the reference phase 30 Hz signal and the variable phase 30 Hz signal are converted into DC voltages proportional to the sine and cosine of the bearing from the aircraft to the VOR station. In the KNR-634, this is performed with amplifiers, limiters and phase detectors. In the VIR-32, an 8051 mP with D to A conversion does the trick. Both manufacturers use up to +/- 3 VDC for these sine/cosine signals, depending upon the bearing. Note that at this point you have all of the raw data needed to drive a RMI or bearing pointer.
The sine and cosine DC voltages are now converted (chopped) into 400 Hz square waves whose amplitudes are proportional to the sine and cosine, respectively, of the bearing from the aircraft to the VOR station. Further, these signals are either in phase or 180° out of phase with the reference 400 Hz signal depending upon the algebraic sum of the sine and cosine. Remember that the OBS resolvers can be zeroed at either 300° or 0° degrees and it is in this area of circuitry that 60° of phase difference is superimposed, as the installation dictates.
The signals exit the UUT and travel to the HSI/CDI where the 400 Hz cosine signal is connected to the OBS stator winding D/E. Similarly, the 400 Hz sine signal is connected to the OBS stator winding F/G. The OBS resolver in this configuration is acting as a vector adder. You are applying single frequency signals to the stators and when the rotor is turned until you have a null on one of the rotor windings the output of the other rotor winding is the magnitude of the resultant field. The physical position of the rotor is the direction to the VOR.
If you do not remember anything else from these OBS articles file deep in your brain the following: When troubleshooting a 30 Hz OBS problem, the rotor is excited by the converter and the stator signals are the resultant. When troubleshooting a 400 Hz OBS problem the stators are excited by the converter and the rotor signals are the resultant. In effect, the two methods of OBS conversion run signals both directions through the resolver.
The rotor winding A/C (with “C” grounded) is fed into the To/Fr discriminator where it is compared with the reference 400 Hz signal. The phase of this rotor signal determines whether you are “To” or “From” the station. From there the signal is filtered, amplified and sent out where needed in the aircraft.
The rotor winding B/C (again with “C” grounded) is fed into the deviation discriminator where it also is compared with the reference 400 Hz signal. The amplitude of this rotor signal is directly proportional to the difference between the OBS setting on the HSI or CDI and the VOR radial that the aircraft is flying on. The phase of this rotor signal indicates whether you are right or left of course.
The low and high level flags can be generated in many ways. Shown in the block diagram is a method that relies only upon the 30 Hz reference and variable signals; however, there is no standard method.
The demodulated signal from the receiver is applied to the converter where the two 30 Hz reference and variable signals are extracted. The reference signal then is applied to the rotor winding of the OBS resolver. We want to provide a phase shift out of the stators so that when the resultant reference 30 Hz signal is summed with the variable 30 Hz signal in the comparator, we will get a null at the proper radial of the VOR. Because the OBS resolver is mechanically connected to the readout, the pilot simply reads the face of his indicator.
I hope this primer on Omni Bearing Selectors helps you in troubleshooting VOR problems, whether on the bench or in the aircraft.
Next month: a test