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 po-sition 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 cor-respond 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 11-89 Diagram of a pulse-width modulator (PWM) amplifier with a brush-type DC servomotor. (Courtesy of Electro-Craft, A Rockwell Au-tomation Business.)

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

11.11.8.1 An Example of a Servo Controlled In-Line Bottle-Filling Application

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 dis-pense 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 Applica tion of a beverage-filling station controlled by a servomotor. (Courtesy ol Electro-Craft, A Rockwel] 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. 11.11.8.2 An Example of a Servo Controlled 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 mate-rials 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 ma-terial can be different from recipe to recipe.

FIGURE 11-92 Applica-tion of a precision auger filling station controlled by a servomotor. (Cour-tesy of Electro-Craft, A Rock-well Automation Business.)

The speed of the auger can be adjusted so that it runs at high speed when the con-tainer 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.

11.11.8.3 An Example of a Label Application Using Servomotors

The fourth appli-cation 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.

11.11.8.4 An Example of a Random Timing Infeed System Controlled by a Servomotor

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 con-nected 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.

FIGURE 11-93 Example of a labeling application controlled by a servomo-tor. (Courtesy of Electro-Craft, A Rockwell Automa-tion Business.)