RATE GENERATOR (TACHOMETER)

As we mentioned earlier, the tachometer in the velocity servo system is the heart of the feedback loop. It is used to sense the speed (velocity) of the load. The tachometer is sometimes referred to as a RATE GENERATOR. Whatever the name, it is a small ac or dc generator that develops an output voltage (proportional to its rpm) whose phase or polarity depends on the rotor's direction of rotation. The dc rate generator usually has permanent magnetic field excitation. The ac rate generator field is excited by a constant ac supply. In either case, the rotor of the tachometer is mechanically connected, directly or indirectly, to the load.

The AC Rate Generator

One type of ac rate generator used widely in the past is the drag-cup type.

The tachometer generator shown in figure 2-16 has two stator windings 90° apart, and an aluminum or copper cup rotor. The rotor rotates around a stationary, soft-iron, magnetic core. One stator winding is energized by a reference ac source. The other stator winding is the generator output, or secondary winding the voltage applied to the primary winding produces a magnetic field at right angles to the secondary winding when the rotor is stationary, as shown in view A. When the rotor is turned by mechanical linkage from the load, it distorts the magnetic field so that it is no longer 90 electrical degrees from the secondary winding. Flux lines cut the secondary winding, and a voltage is induced in the output winding as shown in views B and C. The amount of magnetic field that will be distorted is determined by the speed of the rotor. Therefore, the magnitude of the voltage induced in the secondary winding is proportional to the rotor's velocity (speed).

Figure 2-16. - Ac drag-cup rate generator.

The direction of the magnetic field's distortion is determined by the direction of the rotor's motion. If the rotor is turned in one direction, the lines of flux will cut the secondary winding in one direction. If the motion of the rotor is reversed, the lines of flux will cut the secondary winding in the opposite direction. Therefore, the phase of the voltage induced in the secondary winding, measured with respect to the phase of the supply voltage, is determined by the direction of the rotor's motion. The phase relationship is shown in views B and C at the output winding.

The frequency of the tachometer generator output voltage is the same as the frequency of the reference voltage. The output voltage is generated by the primary alternating flux field cutting the secondary winding; therefore, the output voltage must have the same frequency as the supply voltage.

Other types of ac tachometer generators have a squirrel-cage rotor. Otherwise their construction and principles of operation are identical to the drag-cup type.

The DC Rate Generator

The dc rate generator uses the same principles of magnetic coupling as the ac rate generator. The dc rate generator, however, has a steady (nonfluctuating) primary magnetic field. This magnetic field is usually supplied by permanent magnets. The amount of voltage induced in the rotor winding is proportional to the number of magnetic flux lines cut. The polarity of the output voltage is determined by the direction in which the rotor cuts the lines of magnetic flux.

The physical makeup and theory of operation of the dc rate generator (tach) is very similar to the dc generator (NEETS, Module 5, Introduction to Generators and Motors). The only major differences are size and the prime mover. The tach is much smaller and is linked mechanically to the servo motor or load instead of to a prime mover.

Tachometer generators are used in servo systems to supply velocity or damping signals and are sometimes mounted on or in the same housing as the servo motor.

Q.21 What is the basic difference between the primaries of ac and dc rate generators?

MODULATORS IN THE SERVO SYSTEM

Because of problems associated with dc amplifiers, such as drift (where the output varies with no variation of the input signal), the ac amplifier is more widely used in servo applications. This creates a need for a device to convert a dc error signal into an ac input for the servo amplifier. Such a device is referred to as a MODULATOR.

Modulator and modulating techniques vary with different types of electronic equipment. The modulator in the servo system performs a completely different function than its counterparts in radar or communications systems.

The servo modulator converts a dc error signal into an ac error signal. The modulator uses two inputs to produce the ac error signal. One input is the dc error signal (for example from an input potentiometer); the other input is an ac reference voltage from some other source, such as the swp's ac supply system. The ac output error signal must contain the same control information that is contained in the original dc error signal. This is done in the following manner:

The phase between the ac output and the ac reference signal is determined by the polarity of the dc input signal. The phase of the ac output indicates the direction of error (direction of the load movement). The amplitude of the ac output is proportional to the amplitude of the dc input signal and indicates the amount of error signal (speed or angular displacement of the load).

These relationships of phase and amplitude must be maintained to ensure that the load will move the desired amount, or the proper speed, and in the right direction.

A typical modulator that you will see in a servo system is the CRYSTAL DIODE MODULATOR. The following paragraphs provide a brief explanation of how this modulator works.

Crystal Diode Modulators

The crystal diode modulator (fig. 2-17) consists of a diode bridge and a transformer network. When the ac reference voltage is applied to transformer T₁, diodes CR₂ and CR₃ conduct during the negative half-cycle. Conversely, diodes CR₁ and CR₄ conduct on the positive half-cycle. The diodes will conduct under these conditions because of the 180° phase reversal across T₁. Current flow during the positive and negative half-cycles is represented by dotted arrows and solid arrows, respectively. Suppose a positive, dc error signal is applied during the negative-going ac input half-cycle at the primary of T₁. Current will flow from ground, through the upper half of the primary winding of transformer T₂, through diode CR₂, and through the upper half of the secondary winding of transformer T₁ to the dc source. This produces a positive-going voltage (error signal) across the secondary of T₂ (the first half-cycle of the output signal).

Figure 2-17. - Crystal diode modulator.

On the positive-going ac input reference voltage half-cycle, current will flow from ground, through the lower half of the primary of transformer T₂, through diode CR₄, and through transformer T₁ to the dc error signal source. This produces a negative-going voltage (error signal) across the secondary of T₂ (completing the cycle of the ac input reference). Notice that the error signal is 180° out of phase with the reference signal.

If a negative dc error signal is applied to the modulator, under the same conditions of ac reference signal, current flow through the circuit will be reversed. Keep in mind that this occurs, for example, when the load approaches the desired position from an opposite direction. This circuit will work with either a positive or a negative dc input signal, but only one condition will exist at any given time.

With a negative dc error applied, current will flow from the dc error signal source through diodes CR₃ and CR₁ (on different half-cycles of the ac reference) to ground. This causes an ac voltage to be produced across the secondary of T₂ in the same manner as previously described with the positive dc error signal input.

The only difference is that current will flow through the upper and lower halves of T₂ in a different direction (toward ground) and cause the output to be in phase with the ac reference signal.

In summary, the modulator produced an ac output, either in phase or 180° out of phase with the ac reference signal, depending upon the polarity of the dc input signal. The amplitude of the output will be proportional to the dc input signal amplitude and at the frequency of the ac reference voltage.

Q.22 What is the purpose of a modulator in a servo system?