FUNCTIONAL SERVO LOOPS

Servo systems are also classified according to their functions:

POSITION, VELOCITY, and ACCELERATION. We will cover the most common, POSITION and VELOCITY, in detail.

The Position Servo Loop

The primary purpose of the POSITION SERVO is to control the position of the load it is driving. It can be used to position a great number of devices (for example, valves, control surfaces, weapons, etc.). The basic servo loop we just explained using the block diagram in figure 2-2 is that of an ac position servo system. In the ac position servo system, the amplitude and phase of the ac error signal determine the amount and direction the load will be driven.

In a dc position servo system, the amplitude and polarity of a dc error signal respectively are used to determine the amount and direction the load will be driven.

Figure 2-3, view A, is a block diagram of a closed-loop de position servo. Note the Greek letter Sigma (S), meaning summation, surrounded by a circle.

Figure 2-3A. - Block diagram of a position servo.

This is the summation, or "sum point" where the input signal, and the response signal (feedback) are summed to produce the error signal.

View B shows a more in-depth illustration of view A. With the wiper arms of R₁ and R₄ at the midpoint of travel, the voltage from the wiper arm to ground is zero volts. Therefore, zero volts would also be measured at the connection point between R₂ and R₃ (the summation point). This means that the error signal is zero. With no input signal, the amplifier output is zero; therefore, the motor shaft remains stationary.

Figure 2-3B. - Block diagram of a position servo.

For the purpose of explanation, imagine that the R₁ wiper arm is mechanically moved upward to a new position where a voltage between the wiper arm and ground measures +10 volts. Further measurement shows zero volts between the wiper arm of R₄and ground. Since R₂ and R₃ are of equal values, +5 volts is measured between the sum point and ground because 5 volts is dropped across each resistor. The +5 volts at the sum point is the "error" signal.

As shown in figure 2-4, (view A,, view B, and view C),when no error is present, the voltage at the sum point is zero. This is because the network composed of R_1, R₂, R₃, and R₄ is balanced. When the wiper of R₁ is moved toward +45 volts, the network becomes unbalanced as shown in view B. The left-hand side of R₂ becomes positive. This causes current to flow from +45 volts through R₃ and R₂ to the +10 volts at the left side of R₂. Because R₂ and R₃ are of equal value, the voltage drops then will be equal; therefore, the voltage at the sum point will equal +5 volts.

Figure 2-4A. - Development of the error signal.

Figure 2-4B. - Development of the error signal.

Figure 2-4C. - Development of the error signal.

The +5 volt error signal is fed into the amplifier. The amplified output starts driving the motor. The mechanical feedback from the motor drives the R₄ wiper arm down when the R₁ wiper is moved up, as shown in view C. This causes the right-hand side of R₃ to go negative. When the R₄ wiper travels far enough toward negative, it causes the right-hand side of R₃ to equal the voltage, but of opposite polarity, of the left-hand side of R₂. Simply stated, the voltage at the sum point will be zero again, and the motor will stop. This is true because R₂ and R ₃ are of equal ohmic value, and when the left-hand side equals +10 volts, the right-hand side equals -10 volts. The point between the two resistors becomes zero volts at this time. At the instant that this occurs, the output shaft will have positioned the load to the new position.

Figure 2-5 shows the basic operation of a typical position servo having wide application in Navy equipment. Remember that in a position servo, an input order indicates a position in which a load is to be placed. The load in figure 2-5 is a gun turret. The purpose of the system is to position the gun by means of an order from a remote handcrank. The load is mechanically coupled through a gear train to the rotor of a CT so that the turret's position is always accurately represented by the position of the CT's rotor. An order signaling the desired position of the gun turret is fed into the servo by positioning the rotor of the CX with the handcrank. A corresponding signal immediately appears across the CT stator. This signal differs from the actual position of the gun turret, causing an error voltage to be developed across the CT rotor. The error voltage is fed from the CT rotor to the servo amplifier. At this point it is converted into power with a polarity or phase relationship that drives the motor in the direction necessary to bring the load into the desired position. As the turret moves, mechanical feedback turns the CT rotor toward agreement with the CX rotor. As the load approaches the proper position, less and less power is supplied to the motor because of the decreasing error voltage developed in the CT. When the electrical position of the CT rotor agrees with the position of the CX rotor, the error voltage reaches zero and power is removed from the motor. The turret is now in the desired position.

Figure 2-5. - Typical position servo.

In the actual system, the heavy gun turret's momentum tends to carry it past the desired position. This overshoot causes the rotor of the CT to move out of correspondence with the CX rotor. This, in turn, develops a new error signal that is opposite in polarity to the original input signal. The new error signal causes the turret to drive back toward the desired position - but the turret's momentum once again causes an overshoot, making the system drive in the opposite direction again. If this oscillation of the load around the desired position is allowed to go unchecked, a condition known as HUNTING results. Figure 2-6 shows graphically the result of a series of overtravels of the correspondence point (hunting). In most servos an electronic network known as an ANTIHUNT or DAMPING system is used to minimize this undesirable effect. We will cover antihunt and damping systems in depth later in this chapter.

Figure 2-6. - Overtravels of the correspondence point (hunting).

Q.6 What are the three relatively common classifications of servo systems by function?
Q.7 The output of the sum point must contain information that controls what two factors of load movement in a position servo?
Q.8 What term is used for a series of overshoots in a servo system?