The amplifiers for DC servomotors are slightly different from the push-pull amplifier and the chopper amplifier in that the power transistors can have a constant bias on their base rather than a pulsed signal. Figure 11-79 shows an example of a two-transistor amplifier for a DC servomotor. The power supply for this amplifier is AC voltage. The first part of this circuit is the bridge rectifier that provides a DC voltage at the DC bus. The output stage of this amplifier uses two transistors and two capacitors that are connected across the DC motor armature. 

The base of each of the power transistors is controlled by a switching circuit. This circuit can be controlled by an analog circuit or from a microprocessor. When the direction signal indicates the motor should run in the forward direction, the top transistor is biased on so that positive voltage is provided to the right-side terminal of the armature. The amount of bias voltage to the transistor base will increase or decrease to change the speed of the motor. When the direction signal indicates the motor should run in the opposite direction, the bottom transistor will be biased on and negative voltage is applied to the right side of the motor armature. A diode is connected in reverse bias across the emitter-collector terminals of each power transistor to limit the effects of voltage transients on the transistors. When a transient occurs, the diode provides a path to route the excess voltage and current back into the motor winding where it will be dissipated harmlessly. 

Four-transitor Amplifier for DC Servomotor 

One of the drawbacks of a two-transistor amplifier is that the transistors must handle large amounts of current. Figure 11-80 shows an example of a four-transistor amplifier for a DC servomotor. The four-transistor amplifier is commonly called a bridge driver. In this diagram you can see that the bridge rectifier is drawn as a rectangle but its operation is identical to the one shown in the two-transistor amplifier circuit. You should remember that it is easier to see the operation of a bridge rectifier in this configuration when three-phase power supply is used.

The base of each transistor is controlled by a switching circuit. Again the bias of each transistor is a continuous signal that can be varied from minimum to maximum. When the amplifier is set to run the motor in the clockwise direction, transistors Q2 and Q3 are biased on so that positive voltage is applied to the right side of the motor armature. When the motor is set to run in the counterclockwise direction, transistors Ql and Q4 are biased on so that positive voltage is directed to the left side of the armature. The amount of bias voltage will determine the amount of voltage each transistor passes to the armature, which will in turn change the speed of the motor. 

The output stage of all servo amplifiers is an analog circuit. The analog circuit provides a means to allow the voltage and current for the motor to be adjusted to control position, velocity, and torque. The feedback and comparator stages can be any mixture of digital and analog devices. For example, if the feedback section uses a resolver, the output of this device is analog, so the section it works with is generally also analog. If the feedback device is an encoder, its output is digital, and the digital signal can be converted through a frequency-to-voltage converter so that the signal is usable in an analog circuit. Or it can be filtered and can use a digital value. The advent of microprocessors has allowed the digital values to be used through every part of the servo controller except the final output stage. 

In conclusion, when selecting a drive for a servo motor, the current specification is the determining factor. You should not exceed the nominal current rating for the motor. Next, look at the effects of ripple current on the motor. Larger motor winding resistances is bad. Small motor inductance is likewise bad. Depending on the mass of your motor, you probably don't want your heat dissipation due to ripple current to be larger than 5-10 Watts.