RESOLVERS

The last device we will discuss is the resolver. Physically, resolvers are similar to synchros, and are used to perform mathematical computations electrically. They are rotary electromechanical devices that provide outputs that are trigonometric functions of their inputs. As you may know, the branch of mathematics that deals with the quantities and angles of a right triangle is known as trigonometry. Many "trig" problems that can be solved with paper and pencil can be solved by applying the proper electrical or mechanical quantities to a resolver. The resolver has the advantage of giving instantaneous solutions if the input quantities are changing continuously.

Resolvers are classified according to size (diameter) in the same manner as standard synchros and may be mounted with most standard synchro mounting hardware. A cutaway view of a resolver is shown in figure 4-5.

Figure 4-5. - Cutaway view of a resolver.

Notice that the stator of the resolver is a cylindrical structure of slotted laminations on which two coils are wound. The rotor is composed of a shaft, laminations, two windings, and four slip rings. Compensator components, which improve the angular accuracy of resolvers, may consist of resistors or additional windings in the stator and rotor winding circuits. Compensator windings, which increase the accuracy of the resolver, are located inside the stator. Compensating (calibrating) resistors, which compensate for voltage inaccuracies and phase shifts, may be mounted either inside or outside the resolver housing.

A cylindrical frame with a standardized mounting flange houses the assembled resolver. External and internal connections can be made to an insulated terminal board on the rear of the housing. Miniature resolvers often have lead wires brought out through the rear of the resolver, eliminating the need for a terminal block. A reference line is scribed on the face of the housing for alignment with a similar line on the end of the rotor shaft. These are used in determining coarse electrical zero.

Basically, a resolver is a transformer in which the secondary windings can be rotated with respect to the primary windings. Consequently, the amount of magnetic coupling between the primary and the secondary is variable. In the most common form, a resolver consists of a stator and a rotor, each having two separate windings placed precisely at right angles to each other as shown in figure 4-6.

Figure 4-6. - Resolver schematic.

Since the two stator windings are physically and electrically at right angles to each other, there is no magnetic coupling between them. The stator windings are mounted on the resolver housing and are stationary with respect to it.

Similarly, the rotor windings of the resolver are wound at right angles to each other. Hence, there is no magnetic coupling between the two windings. The rotor windings are mounted on the rotor shaft and turn with it. The rotor is capable of unlimited rotation. Thus, the rotor windings can be set at any angle with respect to the stator windings.

Because of the 90 physical and electrical relationships, the resolver has the ability to separate a quantity into its two right-angle components. This is called RESOLUTION.

Figure 4-7 illustrates the use of a resolver in solving a resolution problem. Assume that a voltage, E, and an angle, R, represent the magnitude and direction of a known quantity. To determine the two right-angle components of the quantity, feed the magnitude of the quantity to one stator coil and physically turn the rotor through angle R. The input voltage (E) induces voltage E1 and E2 in the two rotor coils. The values of these rotor voltages represent the vertical and horizontal components of the known quantity and depend on both the value of E and the angle (R) through which the rotor was turned.

Figure 4-7. - One example of resolution.

Similarly, the resolver has the ability to add two vectors that are at right angles to each other and produce the resultant vector (hypotenuse) at the resultant angle. This is called COMPOSITION.

Figure 4-8 illustrates one use of a resolver in solving a composition problem. Assume that we have two known quantities, vertical and horizontal components, that are represented by E1 and E2, respectively. Each of these is fed to a stator coil. These two voltages induce a voltage, ET, in one of the rotor coils. ET represents a voltage that is proportional to the hypotenuse. The voltage induced in the other rotor coils is fed to a closed-loop servo, which positions the rotor shaft to the angle (direction) of the hypotenuse.

Figure 4-8. - One example of composition.

Other mathematical solutions are possible to designers who apply resolvers in equipment. Typical naval problems solved by resolvers involve distances, speeds, angular quantities, etc.

In most cases, as in figures 4-7 and 4-8, only resolver coils actively used in solving a particular problem are shown in schematics.

Resolvers can also perform a third function COMBINATION. This is the process of resolution and composition taking place simultaneously.

The resolver is a precision component, whose electrical characteristics are critical, and any deviation may result in excessive errors in the system. Before working on or replacing resolvers, you should check the associated equipment technical manual.

Q.4 What type of mathematical problem is solved by resolvers?