Linear motors are synchronous motors: Current is applied to the coil to form an electromagnet. The coil then synchronizes itself to the magnetic field generated by the permanent magnets in the magnet track. Force in a linear motor is generated due to the relative strength of these magnetic fields and the angle of their intentional misalignment.

One type, linear shaft motors, consists of stationary, cylindrically wound coils and a smooth, nonferrous moving rod; inside the latter is a series of rare earth-iron-boron (NIB) permanent magnets propulsed by attractive and repulsive coil forces. Some of these motors are available with shaft diameters from 4 to 115 mm, and output of less than 1 to more than 36,000 N.

Parallel drives basics

Parallel-drive systems are most commonly utilized in Cartesian or gantry robots. More specifically, these robots use twin, side-by-side drives for a variety of applications, such as: pick-and-place work; glasscutters and sealant applicators; assembly operations; laser engravers and arc welding; and for handling machine tools.

Parallel-drive systems are also found in other motion-control applications:

• High-precision (and ultra-high-precision) single-axis robots are used in optics, microscopes, semiconductor, and machine tool applications. These require resolution and position accuracy in the sub-nanometer to high-picometer range.

• Parallel drives are used in actuators demanding very high force — including material testing equipment and punches.

Linear shaft motors with solidly packedd rods output more force than typical cylindrical linear motors, which typically incorporate spacers inside the rods to fill in gaps created by repulsion between like magnetic poles. The twin linear shaft motors shown here are of the former design, and produced in larger volumes than any other cylindrical linear motor.

Building a traditional parallel drive creates certain design challenges: All parallel drives (or gantries) require orthogonal alignment — the ability to keep the parallel axis square. In machinery that utilizes traditional mechanical drives (whether screw, rack and pinion, belt, or chain drive) misalignment or stacked and significantly accumulated mechanical tolerances can cause the motion system to bind and even lockup entirely.

Direct drives suffer from another issue: Problematic sine error (which we'll fully define in the next section) can be introduced if installation is inaccurate or the linear motors exhibit internal variances.

The common solution to these issues is to drive and control each side of the parallel system and electronically synchronize them. This approach, called tracking control, has drawbacks: Cost is higher, as twice the electronics are required (including drivers and feedback) compared to a single-axis system. Tracking control can also introduce synchronization and tracking errors, which degrade performance.

In contrast, linear shaft motor responsiveness simplifies their integration into parallel systems. As with all parallel drive systems, the motors require physical connection with the mechanism, which in turn allows the axis to only move in one degree of freedom. However, dynamic motion generated by two identical linear shaft motors that receive the same control signal is inherently synchronous — so the parallel linear shaft motors act as one.

Linear shaft motors also eliminate force differences that other linear motors exhibit as performance loss and synchronization and tracking errors.