Rotary motors and linear motors can, in many cases, be utilized to accomplish the same goal. Linear motors leverage the same basic magnetic theory as rotary varieties, but in an open and flattened form. As with rotary varieties, a myriad of linear motors exist: Linear steppers, linear AC induction, permanent magnet, and brushless. Linear motors also utilize drives, motion positioners, and feedback devices such as linear encoders.
Linear motor benefits include faster speeds, maximum possible acceleration, and much higher accuracy than their rotary counterparts. Consider that replacing a rotary open-loop stepper with a rotary closed-loop servo can improve accuracy by a factor of 80 times; substituting a linear motor can improve accuracy by a factor of 500 times.
Likewise, a typical servomotor and ball screw with a pitch of 5 rev/in. can move a load at 20 to 40 in./sec; in contrast, a linear motor can provide speeds to 400 in./sec. The same servomotor may accelerate at up to 2 g, while the linear motor accelerates at 10 g.
Finally, the typical servomotor-ball screw actuator provides accuracy ranging from 0.001 to 0.0001 in., while the linear motor provides 0.0007 to 0.000008-in. accuracy. Note that these figures don’t account for coupling and ball screw backlash factors. One of the only disadvantages of a linear motor is its initial cost.
Common uses for linear motors
Linear motors are used in short-move pick and place and inspection equipment (to 60 in./sec), longer moves and flying shear applications (to 200 in./sec), and roller coasters, people movers, and vehicle launching systems (2,000 in./sec). They are also used in semiconductor and electronics markets, laser cutting and water etching machines, material handling, component insertion, and bottle labeling and inspection equipment.
When investigating rotary and linear motors, application considerations include speed and accuracy. Comparing the relative price (whether rotary or linear), steppers are the least expensive, followed by induction, permanent magnet, and finally brushless motors.
When comparing the costs of linear and rotary motors, keep in mind that the latter requires motor mounting and possibly a gearbox, ball screw, or belt drive, plus bearings, a slide, and cabling. The linear system requires a bearing system and cabling.
Linear motors are easily configured into multi-axis stages — typically most expensive, as they encompass either a single or multiple-axis mechanical system to position the payload, plus linear motor, bearings, encoder, limit switches, cable carrier, and bellows.
Troy Hardy is a field applications engineer at the Dallas branch of Innovative-IDM. You can reach him at firstname.lastname@example.org