Friction Clutch Damping

The friction clutch damper uses a friction clutch to couple a weighted flywheel to the output drive shaft of the servo motor. As the servo motor rotates, the clutch couples some of this motion to the flywheel. As the flywheel overcomes inertia and gains speed, it approaches the motor speed. The flywheel, in turning, absorbs energy (power) from the servo motor. The amount of energy stored in the flywheel is determined by its speed (velocity). Because of inertia, the flywheel resists any attempt to change its velocity.

As the correspondence point of the system is approached, the error signal is reduced and the motor begins to slow down. In an attempt to keep the output shaft turning at the same speed, the flywheel releases some of its energy into the shaft. This causes the first overshoot to be large. When the servo system drives past the point of correspondence, a new error signal is developed. The new error signal is of opposite polarity and causes the servo system motor to drive in the opposite direction. Once again the flywheel resists the motor movement and absorbs energy from the system. This causes a large reduction in the second overshoot and all subsequent overshoots of the system. The overall effect is to dampen the oscillations about the point of correspondence and reduce the synchronizing time.

The motor rotation is transmitted to the flywheel through the friction clutch. The inertia of the flywheel acts as an additional load on the motor. The friction clutch is designed to slip with a rapid change of direction or speed. This slipping effectively disconnects the flywheel instantaneously, and thus governs the amount of power the flywheel draws from the motor.

Magnetic Clutch

Another type of damper is the MAGNETIC CLUTCH. This type is similar in function to the friction-clutch damper. The main difference between the two is the method used to couple the flywheel to the shaft of the servo motor. There are two distinct types of magnetic clutch dampers. The first uses a magnetic field to draw two friction clutch plates together to produce damping. The action is similar to the friction clutch we just described.

The second version of the magnetic clutch uses the action of a magnetic field generated by two sets of coils, or one set of coils and the induced eddy currents, which result from rotation of the single set of coils near a conducting surface (the flywheel).

Coupling in this type of clutch is made by the interaction of two magnetic fields without a physical contact between the two. The two-coil or eddy-current type of magnetic clutch offers smoother operation than a pure friction clutch and has no problem of wear because of friction.

In summary, a smooth, efficient operating servo system can only be achieved by a system of compromises. As you recall, earlier we increased the gain of the amplifier to reduce time lag. This had the drawback of increasing hunting or oscillations about the point of correspondence. We overcame this difficulty through friction damping. This solved the problem of hunting and smoothed out servo operation but acted as part of the servo load. It caused a large first overshoot and increased the time lag. Some form of damping that can be used with high amplification to obtain smooth servo operation and minimum time lag is needed. The answer lies with the use of ERROR-RATE damping.

Q.14 Why is damping needed in a practical servo system?

Error-Rate Damping

Error-rate damping is a method of damping that "anticipates" the amount of overshoot. This form of damping corrects the overshoot by introducing a voltage in the error detector that is proportional to the rate of change of the error signal.

This "correction" voltage is combined with the error signal in the proper ratio to obtain the desired servo operation with reduced overshooting and minimum time lag.

The advantages of error-rate damping are as follows:

Maximum damping occurs when a maximum rate of change of error signal is present. This normally would occur as the servo load reverses direction. Since a CHANGE in the signal causes damping, there is a minimum amount of damping when no signal, or a signal of constant strength, is present.

Error-rate voltages are generated by either electromechanical devices or electrical networks in the equipment. One electromechanical device widely used to generate an error-rate voltage is the tachometer generator. As you learned previously, its output voltage is proportional to the output velocity of the servo. Hence, the output voltage of the tachometer can be used to anticipate sudden movement changes of the load.

The compensating electrical network used for error-rate damping consists of a combination of resistors and capacitors forming an RC, differentiating or integrating network. You should recall that a differentiating circuit produces an output voltage that is proportional to the rate of change of the input voltage and that an integrating circuit produces an output proportional to the integral of the input signal.

Figure 2-9 shows a basic RC INTEGRATOR.

It can be recognized by the output voltage being taken across the capacitor. R₁ is added in this circuit to develop the transient error signal (small variation in the signal from the error detector). The RC integrator is sometimes referred to as an INTEGRAL CONTROL CIRCUIT and will be used to explain electrical error-rate damping.

Figure 2-9. - Error rate stabilization network using an RC integrator.

The network consists of a capacitor and two resistors connected in series with the servo amplifier. The components of this circuit are designed to work with a constant or very slowly changing error signal.

Initially, all of the error voltage is divided between R₁ and R₂. But the longer the error voltage is applied, the more C₁ charges, and the greater the voltage at the input of the amplifier. Because of the RC time of the circuit, it takes time for the capacitor to charge to the value of the error input signal. Because of the long charge time of C₁, the circuit can not respond instantaneously to a rapid change in error signal. What this means is that all error signals will be integrated (or smoothed out). The load will not respond as quickly. The inertia of the load will be reduced, and the system will be damped.

The capacitor, by not responding instantaneously to the error signal, causes the damping action. This action is used to stabilize the servo system at the new velocity. By tailoring the stabilization network (through the proper selection of the RC components) to the system's performance requirements and the type of load to be driven, undesirable load or performance characteristics can be minimized.

The various compensating networks that you will encounter will depend on the design of the individual servo system and will be covered in the associated system's technical manual.

In summary, the key to understanding compensating networks is to realize that components are chosen so the capacitor does not have time to charge and discharge in response to large, rapid fluctuations.

Q.15 Error-rate damping is effective because the circuitry has the capability of ______________the amount of overshoot before it happens.

Frequency response

The Frequency response of a servo is the range of frequencies to which the system is able to respond in moving the load. It is a characteristic of the system, chosen by the designers so the system will be able to respond to whatever frequencies are expected to be present in the input signal for the particular application.

Oscillating Input Signal

At first, we considered the input order to a servo as being suddenly put at a fixed desired value. Later, we studied the case where the order slowly increased to the desired value. Actually, the input order to a servo in a given application may accelerate, start, stop, or oscillate about a fixed point. We will now consider the actions of a servo while the order oscillates. When the order is constant, oscillations of the load are undesirable. When the order oscillates, the load must oscillate in a similar manner.

Let's assume that an oscillating input signal (order) is applied to a servo. The load may behave in several ways. Ideally, it would respond in perfect sync with the order. Actually, the amplitude and phase of the load are different from those of the order, figure 2-10. As we noted above, the Frequency response of the system is normally designed so the load is able to respond to the order.

Figure 2-10. - Frequency response.

A servo may follow the order in amplitude and differ in phase; it may follow the order in phase, and differ in amplitude; or it may differ in both phase and amplitude.

Bandpass Frequencies in a Servo system

Servos are plagued by noise signals that ride through the system on desired electrical signals. These noise signals cause roughness in the servo system and must be eliminated to obtain smooth servo operation.

By examining the different signals in a servo system, we can determine which frequencies are related to the movement of the load and which ones are from noise sources, such as static, motors, harmonics, and mechanical resonances.

Filters in the signal circuit can be used to shunt some of the unwanted frequencies away from the amplifier, and allow only those frequencies that represent load movement to enter the amplifier. This can also be accomplished by designing the BANDWIDTH of the servo amplifier to accept only the range of frequencies that represents valid servo signals and to reject all others. This smooths servo response, but has the drawback of reducing amplifier gain. Reduced amplifier bandwidth is another compromise in achieving optimum servo operation.

Q.16 In a properly designed servo system that has an oscillating input (order), what should be the response of the load?
Q.17 What is the advantage of designing a limited bandwidth into a servo amplifier?