Servo Motors

Servo motor - This is a small motor in a C-band dish feed assembly. This motor selects whether vertical or horizontal polarization is to be received.

Servo motor A Servo is a small device that has an output shaft. This shaft can be positioned to specific angular positions by sending the servo a coded signal. As long as the coded signal exists on the input line, the servo will maintain the angular position of the shaft. As the coded signal changes, the angular position of the shaft changes. Three basic types of servo motors are used in modern servosystems: ac servo motors, based on induction motor designs; dc servo motors, based on dc motor designs; and ac brushless servo motors, based on synchronous motor designs.

Figure 9 - Typical dc servo motor system with either encoder or resolver feedback. Some older servo motor systems use a tachometer and encoder for feedback.

Servomotor Applications 45 ratings | 3.49 out of 5

Table of Contents

1. Press Feed 2. In-line Bottle Filling 3. Precision Auger Filling System 4. Label Applications 5. Random Timing Infeed System 6. Buy the Book

Press Feed

You will get a better idea of how servomotors and amplifiers operate if you see some typical applications. Figure 11-90 shows an example of a servomotor used to control a press feed. In this application, sheet material is fed into a press where it is cut off to length with a knife blade or sheer. The sheet material may have a logo or other advertisement that must line up registration marks with the cut-off point. In this application the speed and position of the sheet material must be synchronized with the correct cut-off point. The feed-back sensor could be an encoder or resolver that is coupled with a photoelectric sensor to determine the location of the registration mark.

An operator panel is provided so that the operator can jog the system for maintenance to the blades, or when loading a new roll of material. The operator panel could also be used to call up parameters for the drive that correspond to each type of material that is used. The system could also be integrated with a programmable controller or other type of controller and the operator panel could be used to select the correct cutoff points for each type of material or product that is run.

FIGURE 11-90 Application of a servomotor controlling the speed of material as it enters a press for cutting pieces to size. (Courtesy of Electro-Craft, A Rockwell Automation Business.)

In-line Bottle Filling

A second application is shown in Fig. 11-91. In this application multiple filling heads line up with bottles as they move along a continuous line. Each of the filling heads must match up with a bottle and track the bottle while it is moving. Product is dispensed as the nozzles move with the bottles. In this application 10 nozzles are mounted on a carriage that is driven by a ball-screw mechanism. The ball-screw mechanism is also called a lead screw. When the motor turns the shaft of the ball screw, the carriage will move horizontally along the length of the ball-screw shaft. This movement will be smooth so that each of the nozzles can dispense product into the bottles with little spillage.

The servo drive system utilizes a positioning drive controller with software that allows the position and velocity to be tracked as the conveyor line moves the bottles. A master encoder tracks the bottles as they move along the conveyor line. An auger feed system is also used just prior to the point where the bottles enter the filling station. The auger causes a specific amount of space to be set between each bottle as it enters the filling station.

The bottles may be packed tightly as they approach the auger, but as they pass through the auger their space is set exactly so that the necks of the bottles will match the spacing of the filling nozzles. A detector is also in conjunction with the dispensing system to ensure that no product is dispensed from a nozzle if a bottle is missing or large spaces appear between bottles.

FIGURE 11-91 Application of a beverage-filling station controlled by a servomotor. (Courtesy ol Electro-Craft, A Rockwell Automation Business.)

The servo drive system compares the position of the bottles from the master encoder to the feedback signal that indicates the position of the filling carriage that is mounted to the ball screw. The servo drive amplifier will increase or decrease the speed of the ball-screw mechanism so that the nozzles will match the speed of the bottles exactly.

Precision Auger Filling System

A third application for a servo system is provided in Fig. 11-92. In this application a large filling tank is used to fill containers as they pass along a conveyor line. The material that is dispensed into the containers can be a single material fill or it can be one of several materials added to a container that is dumped into a mixer for a blending operation. Since the amount of material that is dispensed into the container must be accurately weighed and metered into the box, an auger that is controlled by a servo system is used.

The feedback sensor for this system can be a weighing system such as the load cell discussed in earlier chapters. The command signal can come from a programmable controller or the operator can enter it manually by selecting a recipe from the operator's terminal. The amount of material can be different from recipe to recipe.

FIGURE 11-92 Application of a precision auger filling station controlled by a servomotor. (Courtesy of Electro-Craft, A Rockwell Automation Business.)

The speed of the auger can be adjusted so that it runs at high speed when the container is first being filled, and the speed can be slowed to a point where the final grams of material can be metered precisely as the container is filled to the proper point. As the price of material increases, precision filling equipment can provide savings as well as quality in the amount of product used in the recipe.

Label Applications

The fourth application has a servomotor controlling the speed of a label-feed mechanism that pulls preprinted labels from a roll and applies them to packages as they move on a continuous conveyor system past the labeling mechanism.

The feedback signals are provided by an encoder that indicates the location of the conveyor, tach generator that indicates the speed of the conveyor, and a sensor that indicates the registration mark on each label. The servo positioning system is controlled by a microprocessor that sets the error signal, and the servo amplifier that provides power signals to the servomotor. This application is shown in Fig. 11-93.

FIGURE 11-93 Example of a labeling application controlled by a servomotor. (Courtesy of Electro-Craft, A Rockwell Automation Business.)

Random Timing Infeed System

The fifth application is presented in Fig. 11-94, and it shows a series of packaging equipment that operates as three separate machines. The timing cycle of each station of the packaging system is independent from the others. The packaging system consists of an infeed conveyor, a positioning conveyor, and a wrapping station. The infeed conveyor and the wrapping station are mechanically connected so that they run at the same speed.

The position of the packages on the wrapping station must be strictly controlled so that the packages do not become too close to each other. A piece of metal called a flight is connected to the wrapping station conveyor at specific points to ensure each package stays in position. A sensor is mounted at the beginning of the positioning conveyor to determine the front edge of the package when it starts to move onto the positioning conveyor.

A second sensor is positioned at the bottom of the packaging conveyor to detect the flights. Both of these signals from the sensors are sent to the servomotor to provide information so the servo can adjust the speed of the positioning conveyor so that each package aligns with one of the flights as it moves onto the packaging conveyor. This application shows that the servo positioning controller can handle a variety of different signals from more than one sensor because the controller uses a microprocessor.

[pic] FIGURE 11-94 Example of a packaging system with random timing functions controlled by a servomotor. (Courtesy of Electro-Craft, A Rockwell Automation Business.)

Some of the advantages of servo motors over stepper motors are as follows: • High intermittent torque • High torque to inertia ratio • High speeds • Work well for velocity control • Available in all sizes • Quiet Some of the disadvantages of servo motors compared with stepper motors are as follows: • More expensive than stepper motors • Cannot work open loop - feedback is required • Require tuning of control loop parameters • More maintenance due to brushes on brushed DC motors