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Document Type: Prentice Hall
Author: Thomas E. Kissell
Book: Industrial Electronics
Copyright: 2000
ISBN: 0-13-012697-7
NI Supported: No
Publish Date: Sep 6, 2006


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Linear Stepper Motors

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Overview

The linear stepper motor has been made flat instead of round so its motion will be along a straight line instead of rotary. A picture of a linear motor and its amplifier is shown in Fig. 11-69, and the basic parts of the linear motor are shown in Fig. 11-70. In this diagram you can see the motor consists of a platen and aforcer. The platen is the fixed part of the motor and its length will determine the distance the motor will travel. It has a number of teeth that are like the rotor in a traditional stepper motor except it is passive and is not a permanent magnet. The forcer consists of four pole pieces that each have three teeth. The pitch of each tooth is staggered with respect to the teeth of the platen. It uses mechanical roller bearings or air bearings to ride above the platen on an air gap so that the two never physically come into contact with each other. The magnetic field in the forcer is changed by passing current through its coils. This action causes the next set of teeth to align with the teeth on the platen and causes the forcer to move from tooth to tooth over the platen in linear travel. When the current pattern is reversed, the forcer will reverse its direction of travel. A complete switching cycle consists of four full steps, which moves the forcer the distance of one tooth pitch over the platen. The typical resolution of a linear motor is 12,500 steps per inch, which provides a high degree of resolution. The typical load for a linear motor is low mass that requires high-speed movements.


FIGURE 11-69 A linear motor and its amplifier. (Courtesy of Parker Compumotor Division.)


FIGURE 11-70 The forcer is shown on top of the platen of a linear motor. The electromagnets are identified on the forcer. (Courtesy of Parker Compumotor Division.)

Theory of Operation

The forcer consists of two electromagnets that are identified in Fig. 11-70 as magnet A and magnet B and one permanent magnet. The permanent magnet is a strong rare-earth permanent magnet. The electromagnets are formed in the shape of teeth so that their magnetic flux can be concentrated. In the diagram you can see that the forcer has four sets of teeth and these teeth are spaced in quadrature so that only one set of teeth is aligned with the teeth on the platen at any time.

When current is applied to the coil (field winding) of the electromagnets, their magnetic flux passes through the air gap between the forcer and the platen, causing a strong attraction between the two. The magnetic flux from the electromagnets also tends to reinforce the flux lines of one of the permanent magnets and cancels the flux lines of the other permanent magnet. The attraction of the forces at the time when peak current is flowing is up to ten times the holding force.

When a pattern of energizing one coil and then another is established, the resulting magnetic field will pull the motor in one direction from one tooth to the next. When current flow to the coil is stopped, the forcer will align itself to the appropriate tooth set and create a holding force that tends to keep the forcer from moving left or right to another tooth. The linear stepper motor controller sets the pattern for energizing and de-energizing the field coils so that the motor moves smoothly in either direction. By reversing the pattern, the direction the motor travels is reversed.

Figure 11-71 shows a block diagram of the linear stepper motor controller. From this diagram you can see that it has a microprocessor that interfaces with a digital-to-analog converter, a force angle modifier, and a power amplifier. It also has a power supply for the amplifiers and it may have an accelerometer amplifier as an option. The microprocessor has ROM and EPROM memory to store programs.



FIGURE 11-71 A block diagram of a linear motor controller. (Courtesy of Parker Compumotor Division.)

Applications

The applications for a linear motor tend to be straight-line motion. These types of applications are slightly different from traditional stepper motor applications where the rotary motion is converted to linear motion with a ball and screw, rack and pinion, or other method. Figure 11-72 shows the linear motor used in a coil winding positioner application. The linear motor in this application is teamed with a servomotor that controls the speed of the coil winding mechanism. The linear motor determines the exact location of the next coil that is added to the spool. The speed of the linear motor can be increased or decreased when the machine is spooling larger-diameter or smaller-diameter wire. The ability of the linear motor to provide small incremental steps makes it a good match for this application.

Figure 11-73 shows a second application where the linear motor is used to transport a semiconductor wafer through a precision laser inspection station. The linear motor provides excellent locating ability for this application.

A Compumotor L-L20-P96 system acts as the traverse element to guide the wire, while a Z Series servo motor rotates the spindle. Both axes are coordinated by a Compumotor 4000 indexer preprogrammed to produce a number of different coil types. Precise position control and mechanical simplicity over a long length of travel are provided by the linear motor.


FIGURE 11-72 A linear stepper motor used in a coil winding application. The linear motor is used to control the position of the coil winder. (Courtesy of Parker Compumotor Division.)

In this application, the linear motor acts as a transport for semiconductor wafers. The L20 linear motor system offers increased throughput and gentle handling of the wafer.


FIGURE 11-73 A linear stepper motor used to transport a silicon semiconductor wafer through a laser inspection station. (Courtesy of Parker Compumotor Division.)

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