Servo Components

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 servo 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 military to classify the devices to familiarize you with the wide variety of nomenclatures.

A position sensor is one of the many types of servo components used to changes 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 section 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 section. Other devices can be used as position sensors. The potentiometer is one of these devices.

Potentiometers

A position sensor is another type of servo components 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 the illustration below.

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 the illustration below.

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.

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