SERVO COMPONENTS AND CIRCUITS

In this section we will discuss the circuits and components that make up a typical servo system. We cannot cover all possible servo applications here because of the vast number of servo system configurations. The circuits and components discussed in the following pages are the most commonly used and represent a broad view of the systems used in the Navy today. We have not attempted to put the units into any rigid classification system. We will mention some of the more common terms used by manufacturers and the Navy to classify the devices to familiarize you with the wide variety of nomenclatures.

We will be covering much of the electronic application without discussing the theory of the units. You may want to review some of the applicable NEETS modules or other sources before or during this discussion. You will find that much of the material necessary to understand these subjects is contained in the basic theory of electricity and electronics.

POSITION SENSORS

A position sensor is a device that changes a mechanical position into a voltage that represents that position. The output of a position sensor can be either ac or dc voltage. There are many different kinds of position sensors. In the last chapter you learned about the CX, a synchro device that represents the position of its rotor by a voltage on its stators. You saw a CX used as a position sensor in a servo system earlier in this chapter. Other devices can be used as position sensors. The potentiometer is one of these devices.

Potentiometers

Potentiometer position sensors are generally used only where the input and output of the servo mechanism have limited motion They are characterized by high accuracy and small size, and may have either a dc or an ac output voltage. Their disadvantages include limited motion and a life problem resulting from the wear of the brush on the potentiometer wire. Also the voltage output of the potentiometer changes in discrete steps as the brush moves from wire to wire. A further disadvantage of some potentiometers is the high drive torque required to rotate the wiper contact.

A potentiometer is one of the simplest means of converting mechanical positional information to a proportional voltage. A schematic representation of a potentiometer is shown in figure 2-11.

Figure 2-11. - A potentiometer.

A potentiometer is a variable voltage divider, with an output voltage that is a percentage of the input voltage. The amount of output voltage is proportional to the position of the wiper relative to the grounded end. For example, if the resistance from ground to the wiper is 50% of the total, the output voltage sensed by the load will be 50% of the total voltage across the potentiometer.

A basic, closed-loop servo system using a balanced potentiometer as a position sensor is shown in figure 2-12.

Figure 2-12. - Balanced potentiometer used In position sensing.

The command input shaft is mechanically linked to R₁, and the load is mechanically linked to R₂. A supply voltage is applied across both potentiometers.

The system is designed so that when the input and output shafts are in the same angular position, the voltages from the two potentiometers are equal and no error voltage is felt at the amplifier input. If the input shaft is rotated, moving the wiper contact of R₁, an error voltage is applied to the servo amplifier. This error voltage is the difference between the voltages at the wiper contents of R₁ and R₂. The output of the amplifier causes the motor to rotate the load and the wiper contact of R₂. This continues until both voltages are again equal. When the voltages are equal, the motor stops. In effect, the position of the output shaft has been sensed by the balanced potentiometer.

Q.18 When the input and output wipers of a balanced potentiometer are in the same angular position, what is the value of the error voltage?

ERROR DETECTORS

Electrical error detectors may be either ac or dc devices, depending upon the requirements of the servo system. An ac device used as an error detector must compare the two signals and produce an error signal in which the phase and amplitude will indicate the direction and amount of control, respectively, that are necessary for correspondence. A dc device differs in that the polarity of the output error signal determines the direction of the necessary correction. We will discuss in the following paragraphs various devices that are commonly used in servo systems.

Summing Networks

Summing networks, as we mentioned earlier, are used as error detectors in servo applications where the servo output must be proportional to the algebraic sum of two or more inputs. A typical circuit is shown in figure 2-13.

Figure 2-13. - Summing network as an error detector.

As in the case of potentiometers, the networks may use either ac or dc voltage, with the phase or polarity of the input voltage determining whether the signals are additive or subtractive. Refer to figure 2-13. If two input signals E₁ and E₂ are applied to the network, the network will provide an error voltage output that is proportional to the algebraic sum of the two signals. The servo motor drives the load and also a tachometer that supplies feedback voltage to resistor R_f. Resistor Rf nulls the error signal.

In some installations, the servo motor may position the wiper arm of a potentiometer instead of driving a tachometer to supply the feedback voltage.

E-Transformers

The E-transformer is a type of magnetic unit that is used as an error detector in systems in which the load is not required to move through large angles.

In the basic E-transformer shown in figure 2-14, an ac voltage is applied to the primary coil (2) located on the center leg of the laminated, E-shaped core. Two secondary coils (1 and 3) are wound series-opposing on the outer poles of the core. The magnetic coupling between the primary (coil 2) and the two secondaries varies with the position of the armature. The armature can be physically moved left or right in the magnetic circuit by mechanical linkage to the load. This changes the reluctance between either pole and the armature.

Figure 2-14. - Basic-E transformer.

When the armature is located in the center of the E-shaped core, as shown in the figure, equal and opposite voltages are induced in the secondary coils. The difference between them is zero. Thus, the voltage at the output terminals is also zero.

But, if the armature is moved, say to the tight, the voltage induced in coil 1 increases, while the voltage induced in coil 3 decreases. The two voltages are then unequal, so that the difference is no longer zero. A net voltage results at the output terminals. The amplitude of this voltage is directly proportional to the distance the armature has been moved from its center position. The phase of this output voltage, relative to the ac on the primary, controls the direction the load moves in correcting the error.

The basic E-transformer will detect movement of the armature in one axis only (either the horizontal or vertical depending upon the way the unit is mounted). To detect movement in both the horizontal and vertical axes, a CROSSED-E-TRANSFORMER is used.

If you place two E-transformers at right angles to each other and replace the bar armature with a dome-shaped one (fig. 2-15), you have the basic configuration of what is known as the crossed-E transformer, or pickoff. In most applications the dome-shaped armature is attached to a gyro, and the core assembly is fixed to a gimbal, which is the servo load.

Figure 2-15. - Crossed-E transformer.

The crossed-E transformer assembly consists of five legs (poles). Each leg is encased by a coil. The coil around the center leg is the primary, which is excited by an alternating voltage. The remaining four coils are the secondaries. From this view, you can see how it gets the name, crossed-E.

When the reluctance dome (armature) is moved to the left of center, more flux links the left leg with the primary coil, and the voltage induced in the left secondary increases. The right leg has fewer flux linkages with the center coil; therefore, the voltage induced in the right coil will be less than that in the left coil. Thus there will now be a net voltage out of the pickoff. The phase of the output will be that of the larger voltage. If the dome were moved to the right, the opposite condition would exist. From this brief description, you can see that the crossed-E transformer works on the same fundamental principle as the basic type described earlier. The major difference between the two is in shape and the number of secondaries, and in the fact that the armature has universal movement.

Control Transformers

A commonly used magnetic error detector is the synchro-control transformer, which is used as a control device in servo systems. Recall that we covered the CTs operation in depth in chapter 1 of this module, and discussed its application to the servo system earlier in this chapter.

As an error detector, the CT compares the input signal impressed upon its stator with the angular position of its rotor, which is the actual position of the load. The output is an electrical (error) signal taken from the rotor, which is the difference between the ordered position and the actual position of the load.

A primary advantage of the CT over other types of error detectors is its unlimited rotation angle; that is, both the input and the output to the synchro control transformer may rotate through unlimited angles. A disadvantage is that the output supplied to the servo amplifier is always an ac error signal, and must be demodulated if it is to be used in a dc servo system.

Q.19 In the output of an ac error detector, what indicates the (a) direction and (b) amount of control necessary for correspondence?
Q.20 What two basic types of magnetic devices are used as error detectors?