Autosyn
This system uses 26 V AC 400 HZ power from the aircraft’s instrument power supplies. The term Selsyn is often used to describe this self synchronous system, which can be used to measure and indicate such things as fuel flow, oil pressure, and flap position. The name autosyn is derived from automatic synchronism. The units are of similar construction, the transmitter and indicator being variable transformers, the rotors being the primaries and the stators being secondaries. They are connected in parallel. Figure 3.1 shows the connections of an autosyn.
Operation
In this system the transmitter has its rotor physically positioned by the medium to be measured, whilst the rotor of the indicator moves because of magnetic action. When power is applied, the current in the rotors sets up an alternating flux that induces a voltage into the stators. The position of the rotors determines the value of voltage induced into each segment of the stators.
Whenever two rotors have the same physical position, both stators will have the same voltages induced into their corresponding segments, and since they are connected in parallel, no potential difference exists, and no current will flow between the units. This position is called the in correspondence condition and no pointer movement will take place.
When the two rotors do not have the same physical position, the voltages induced into the stators will not be the same. This will now cause a potential difference to exist and current will flow through the connecting wiring. The current flow will create a motor action that moves the indicator rotor until both rotors are again aligned. Whenever the two rotors are out of alignment, their voltages differ, and they are said to be out of correspondence.
Figure 3.1: Autosyn system
Summary
From the above, we find that as the transmitter rotor is moved due to a change in the value of the medium being measured, the stator voltages would not be the same in both the transmitter and indicator. This causes the out of correspondence condition to occur, and the current that flows sets up a magnetic action within the indicator stator. This interacts with the rotor field causing the indicator to turn until the in correspondence condition occurs.
The main disadvantage of this system is that the pointer will remain on scale when the power fails, which could give the crew misleading information about the system being monitored. Many of these systems incorporate a power off flag to alert the crew to a power failure situation.
Magnesyn
This system makes use of 26 V AC 400 HZ single phase AC power from the
aircraft’s supply. It can be used wherever a mechanical movement is available. The system consists of a transmitter and indicator connected electrically and is more compact, lighter, and simpler than an autosyn.
The transmitter consists of the mechanical actuating mechanism and the transmitter, which can be either a rotary type or a linear type. The theory of operation is the same for both, and the rotary type is described here.
The rotor is a permanent magnet attached to, and positioned by, the actuating shaft. The stator consists of a circular laminated core, upon which is wound the excitation coil, and a tapping is made at each 120 degrees on the coil away from the input. Outer laminations within the housing encircle the outside of the stator, and provide a return path for the magnetic flux.
The indicator is of the same construction, except that the rotor is attached to a pointer which indicates the medium which is being measured.
Operation
When the permanent magnet rotor is placed inside the ring or stator of soft iron, the flux lines will establish a flux within the ring. If a coil is wound around the ring and connected to an AC supply, the ring will become magnetically saturated twice each cycle when the current reaches its peak. The rotor flux is forced out of the ring because the ring now has a higher reluctance than the air surrounding the rotor. When the excitation current is at zero, the rotor flux cuts across the excitation coil inducing an EMF, this is generated in all three sections of the stator windings. The amount and phase of the EMF in each section is dependent on the position of the permanent magnet rotor.
When the indicator rotor corresponds with the transmitter rotor, identical changes to the EMF’s will take place to their respective stators. There will be no difference in potential at each tapping, and therefore no current flow.
Figure 3.2 shows the layout of a magnesyn system.
When the mechanical mechanism moves the transmitter rotor, the EMF will differ in the stator windings creating a difference in potential, and current will flow in the interconnecting wires. As the transmitter is mechanically held, the receiver rotor will turn to align itself with the transmitter rotor thus moving the pointer around the scale.
Ratiometer
In this system, the measurement of pressure is obtained by measuring the ratio of two alternating values of current. A pressure sensitive element (bellows) causes the linear movement of two armatures, positioned inside two stator coils in such a way, that an increase of current is produced in one stator, and a decrease of current in the other. A small change of inductance at the stators, results in a relatively large ratio of current between the stator coils which is measured on an AC ratiometer.
Operation
The circuit is arranged in such a way that when the transmitter and indicator are connected together they form an AC bridge. Any change in pressure will cause the bellows to move the armature cores within their stators, which will result in a change of inductance in the stator coils. This, in turn produces a differential change of current in the coils of the electro magnetic elements of the indicator.
The current flowing in these coils produce an alternating flux in the cores on which they are wound. The shading ring on each core causes the flux in that section to lag behind the main field flux thus producing a sweeping flux action across the pole faces. This will produce a torque into the cam shaped discs due to the interaction of the sweeping flux and the eddy currents induced into the discs.
The turning motion is such that the disc moves to reduce the effective radius in the air gap. When the effective radius is reduced, the disc impedance increases, thus reducing the torque. Conversely, an increase in radius creates an increase in torque because of a decrease in impedance through the disc.
When a change in position of the transmitter armature causes an increase in current at one arm of the bridge and a decrease in the other, the moving element rotates in a direction determined by the coil having the increased current. The movement of the indicating element is designed so that the torques produced in both coils are in opposition, and therefore as the element rotates, the torque that produces rotation is decreasing while the opposing torque is increasing.
This will mean that rotation will stop when the torques are balanced. Figure 3.3 is a schematic of ratio system.
Two capacitors are included in the circuit to reduce the effect of changes in phase displacement of the induced currents, brought about by an increase in frequency, and the impedence of the discs. These changes will cause an increase in the ratio of the currents in the bridge. Errors resulting from changes in temperature in the laminated iron cores are reduced by means of a high temperature coefficient resistance connected across the bridge.
Cam - shaped discs Bellows Armature Stator 26 V AC 400 Hz Transmitter Indicator Indicator Transmitter 26 V 400 Hz Lamination & bobbin assy Damping magnet & disk N S