Some applications that require a linear motor are best served with a linear step motor. One example is an application that requires moving a tray of radioactive fluid held in containers. The tray must move in small, precise, smooth motions to prevent tipping and spilling the fluid and the resulting contamination. After evaluating several solutions, engineers chose a linear step motor, mounting the tray on the motor. It smoothly moves the tray in small increments, 12,500 steps/in., meeting the application’s throughput requirements. The motor runs open-loop, which made this a low-cost solution as no feedback device is needed.

Another application involves welding, Figure 1. Parts constantly move on a conveyor belt that varies in speed. A weld head must adjust its position to spot weld parts at precise locations while the parts are in motion. A linear step motor can make the quick adjustments in position and velocity to compensate for the conveyor’s speed fluctuations sensed by an encoder.

Linear step motors generally operate to travel lengths of about 10 feet. Beyond that length, run-out error becomes significant and it is difficult for manufacturers to hold flatness tolerances.

A linear step motor, like a linear servo motor, provides quick, precise moves in short increments. With a linear step motor, though, once peak current is set, it does not require parameter tuning. Thus, system setup time is low.

As mentioned earlier, these motors can operate open-loop, eliminating the cost of encoder feedback. This makes it simpler to change the length of travel by simply changing the platen. A new feedback device is unnecessary.

Like rotary step motors, linear step motors:
• Have a high pole count. Industry standard is 25 poles per inch. With microstepping drives these motors can give 10,000 or more steps per in.
• Do not accumulate position error over time (because of the pole structure), enabling good repeatability.
• Offer brushless design.
• Can be repeatedly stalled without damaging the motor.
• Are easy to construct and are one of the least expensive types of linear motors.

Also like rotary step motors, linear step motors have a few constraints. These motors may experience position and velocity oscillations during movement. External damping can combat these disturbances, as can modifing the normal sinusoidal phase currents to actively control resonance, which is available on newer drives. When internal sensors sense the resonance, additional current is commanded to create a torque to mitigate the oscillation. One drive, for example, uses accelerometers mounted in the forcer to sense vibrations. The drive then closes an acceleration loop to create a smooth linear motion.

Friction, whether coulomb or viscous, is another constraint that can cause error in the final position as the motor comes to rest.

And, unlike rotary step motors, there are no commercial standards for linear step motors. Rotary motors have standard frame and shaft sizes. By contrast, each linear motor has a different mounting configuration, requiring custom interfacing. This can add cost and development time.

Operation

A Sawyer motor, Figure 2, helps illustrate the operation of a linear step motor. The Sawyer motor is one of the simpler linear step motor configurations; more complex motors build on the same physical characteristics. As with other linear motors, the stationary element (stator) is frequently called the platen. It is a series of equally spaced teeth machined from one piece of steel. In a rotary step motor, the stator teeth are used repeatedly during rotation. In a linear step motor, each platen tooth is used only during a portion of the move.

The moving element of a linear step motor, the forcer, contains the permanent magnet and the phase windings. For step motors, it is generally impractical (and expensive) to insert the windings in the platen, although it is done for other linear motor types. The length of the platen, less the forcer length, determines the maximum travel distance for the motor.

The platen provides a passive return path for the magnetic flux. With windings on the forcer, multiple units can operate from the same platen. Thus, engineers can design machines with linear motors to perform parallel operations. Since the forcers usually operate open-loop, the same drive can power multiple units.

Cabling is carried along during movement. It must be rugged enough to withstand constant flexing, and positioned so that it does not interfere with the machine operation.

In the forcer, the flux from the permanent magnet passes through Phase A, the platen, and then Phase B. The electromagnets steer the flux into the correct air gap, Figure 2. First, a positive current is applied to Phase A with Phase B unenergized. The electromagnets guide the flux to the A1 pole face, causing it to align (for an unloaded motor). The A2 pole face is 180 degrees out of phase from A1, so no flux crosses the air gap at that point. If Phase A is relaxed and Phase B energized, the flux is then guided to the B2 pole face. When this pole face aligns, the forcer has moved one-quarter of a tooth pitch to the left.

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