What Is a Servo Unit?
A servo unit is a single unit that combines the mechanical elements required to run the servo motor.
Specifically, two elements are added in addition to the servomotor. These are a programmable controller (PLC) that commands the servomotor to achieve the desired motion and a servo amplifier that supplies the AC current necessary to move the servomotor as commanded by the PLC.
The “servo” in servo unit is said to be derived from the word “servant.” In other words, servo unit means a unit that moves as commanded.
Uses of Servo Units
Servo units are used in a wide variety of industrial robots and industrial production machinery. In industrial robots, servo units are used in automobile plants to perform movements such as welding and painting car bodies and picking parts.
In other industrial machinery, servo units are incorporated to move and control machines in motion, such as injection molding machines, press machines, and label packaging machines. Servo units are often used for reciprocating motion as well as rotary motion.
A typical servo unit for reciprocating motion includes a unit in which the servomotor is connected to a ball screw, and a table connected to the nut section of the ball screw on a slide regulated by a linear guide is used as an optional positioning table.
Such a system is designed and manufactured as a one-of-a-kind product, but depending on the slide length and its thrust, a lineup may be created and used universally. A system that can be programmed to move back and forth, left and right, like the table surface of a machining sensor, is a similar system used.
Principle of Servo Units
Most servomotors today are AC servomotors, which run on AC current. AC servomotors consist of a rotor made of permanent magnets and a stator made of multiple electromagnets arranged to surround the rotor. An alternating current is applied to the stator, which sequentially switches the N and S poles of the electromagnets to produce the rotational motion of the rotor.
The servo amplifier sends this alternating current to the servomotor, and the servo controller commands the servo amplifier what motion to make. The movement of the servomotor is detected by an encoder attached to the rotor of the servomotor, which sends a feedback signal to the servo amplifier.
Other Information on Servo Units
Servomotor Control Method
There are three ways to control servomotor motion: position control, speed control, and torque control.
1. Position Control
Position control moves and stops servomotors with high precision in relation to a commanded position. It is used in positioning systems for numerically controlled lathes and machining center tables.
2. Speed Control
Speed control moves the servomotor to achieve a target speed. Speed control responds quickly to changes in rotational speed caused by external influences and corrects the command.
3. Torque Control
Torque control controls the magnitude of torque output by a servomotor. Torque control ensures accurate operation at the commanded torque even when the load varies. This control method is used in screw tightening machines that tighten screws at a constant torque and in press machines that convert the rotational torque of the servomotor into propulsive force via a ball screw.
Servo units are classified as a closed-loop control system. Closed-loop control is characterized by the addition of feedback commands, making it less susceptible to external disturbances. In other words, servo units are capable of high-precision positioning.