Capital Avionics Tech Time Article

Omni Bearing Selectors – May 1999

November 1, 2012

OMNI BEARING SELECTORS (OBS)
Omni Range Zero (ORZ)

The Omni Bearing Selector (OBS) is an integral part of the VOR navigation system. With its attendant VOR converter it allows a pilot to select and determine the radial that he is on and provides an accurate error signal that is proportional to the offset from the intended track.

The ORZ method of deriving VOR information is based upon 30Hz Resolvers. RTCA paper 209-54/DO-62 defines the OBS setting as using a 300° reference. Before describing the resolver operation, a brief summary of VOR operation is in order.

The transmitted VOR signal is made up of the RF carrier that has been amplitude modulated (AM) by a 9960 Hz signal which has itself been frequency modulated (FM) by a 30 Hz signal. The 9960 Hz reference varies between 9480 Hz and 10,440 Hz depending upon the phase of the 30Hz signal. It is important to remember that the transmitted signal from the ground station is constant in amplitude. The antenna is directional, however, and it is electrically rotated at exactly 1800 times per minute, or 30 times per second. The direction that the antenna points at any instant is carefully aligned with the phase of the 30 Hz frequency modulated reference and the resulting amplitude modulation perceived by the receiver is the variable signal. It is this careful alignment between the 30 Hz frequency modulated signal and the 30 Hz amplitude modulated signal that makes it possible to determine the position of the aircraft. When the two signals are in phase, you are on the 0º radial, when the two signals are 180º out of phase you are on the 180º radial and so forth. Figure 1 below is a block diagram of a typical VOR system:

 

Figure 1

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.

 

Refer to figure 2. Because the resolver is basically a transformer, the stator windings will always be either in phase or 180º out of phase with the input. What we want is a constant amplitude output with a variable phase delay that is dependent upon rotor position. By adding a resistor – capacitor (RC) network to the stators, we cause one stator to lead by 45º and the other stator to lag by 45º. These are now called quadrature voltages because they are 90º out of phase with one another. They can now be added as vectors, one representing the X  or horizontal axis, the other representing the Y or vertical axis.

Figure 2

Example: When you have a converter that will align at 90º and 270º but not 0º or 180º, one of the stators may not be shifted 45º. Usually changing the components in the RC network will fix it. To check the resolver itself, look for the same maximums and minimums from the stators at their corresponding 90º points. When one stator is at its maximum, the other stator should be at its minimum. And vise versa. A broken stator wire in an aircraft will cause large errors and the deviation needle will not center.

The phase shifted, constant amplitude reference 30Hz signal is now applied to the phase comparator where it is summed with the variable 30 Hz signal. The output of the phase comparator drives the deviation needle  and flag in the indicator.

In the late 1950s and early 1960s, avionics manufacturers were experimenting with 30 Hz and 400 Hz driven resolvers for their converters. They were trying to use the two signals interchangeably with one resolver and documented the errors when this was done. They could not find a way for one resolver to provide the necessary phase delay at two frequencies. One solution was for Collins to add a choke in series with a rotor winding i.e. HSI models 331A-3F/3G/6P. (Note: The original ARINC resolver standard called for seven wires A through G. A, B and C were rotor wires, D, E, F and G were stator wires. Collins added the wire designation H for their series choke.) AlliedSignal’s answer has been to add a capacitor in parallel with a rotor winding i.e. HSI models KI525/KPI552/523. When troubleshooting a VOR problem or performing an installation ensure that you know which OBS method you are using otherwise there can be errors in excess of 3º.

There are several ways to calibrate 30 Hz ORZ resolvers but the most common appears to be the constant voltage rotor feed method. The instructions for constructing this test set and its operation are found in RTCA Paper 209-54/DO-62. Closer to home the same instructions are found in the RCA AVN-220 Navigation Installation/ Maintenance Manual IB8029013, or schematic information only is found in the Collins PN101 Instruction book CPN 523-0755824. The Collins 479X-2 Resolver Zeroing Panel can be purchased for those who don’t want to construct it themselves.

Next Month: EZ Resolvers