Since the 1970s, linear motors have directly driven high-speed motion systems. Their operation is similar to that of their cylindrical cousins, as they physically resemble flattened rotary motors, split open along the axis of rotation. But linear motors convert electrical power directly into translational movement — eliminating the mechanical transducers such as ballscrews and belts that complicate accurate positioning.

All linear motors make smooth strokes, but the steppers are unique in that they use pulses of excitation current for precisely defined linear position increments. Input is not dc or ac; it is pulsed current that flows to motor phases for metered steps of motion. Even with this basic open-loop control using simple controller electronics, linear steppers position with high resolution — to 5 mm and better.

Linear steppers fall into two categories: Variable reluctance and hybrid types. Mechanically, the two are similar, consisting of a primary (or forcer) and secondary or platen. Their platens are identical, made of low-cost ferromagnetic steel plate. Force-producing magnetic flux travels in transverse yoking paths, consisting of alternating teeth and slots. However, as we'll soon explore, the forcers of linear variable reluctance and hybrid steppers differ in both construction and electromagnetic operation.

Variable reluctance types

Variable reluctance is the original linear motor design. The forcer consists of phases (with teeth) and laminated stacks with no permanent magnets. The phase coils are nestled into the slots; a phase is associated with one or more teeth. To boost electromagnetic coupling between the forcer and platen (and minimize core losses) the stack material is made of laminated, low-carbon magnetic silicon steel. It's cut in either a U or E profile. The platen consists of photochemically etched teeth on a steel bar, filled with epoxy and ground smooth.

How do variable reluctance linear motors work? Without permanent magnets, the only control on variable reluctance types is input current. It's fed into the motor in different phases as pulses of current. Input current switches on and off at angles to different motor phases to generate thrust force by electromagnetic reluctance. The forcer is constantly moving from positions of high to minimum reluctance. The latter positions occur where the tooth of the forcer aligns with the tooth of the platen. In variable reluctance linear steppers, the physical distance between phases must be:

Where M = Number of motor phases

τ_t = Tooth pitch

n = Integral number

The fluxes generated by a two-phase reluctance motor in phase A and B are:

Where Λ_A, Λ_B = constant component of magnetic permeance in the relative poles of phases A and B

Λ_A1, Λ_B1 = fundamental component of magnetic permeance in the relative pole of phases A and B, respectively.

Reluctance force generated by a pole in the motor is:

Where mmf = magnetomotive force required by the motor

Λδ = airgap permeance

Then, the thrust reluctance force generated by the pole 1 in phase A is:

The reluctance force generated by the other poles can calculated in the same way. To enhance the force density of these motors, airgap permeance is increased; this maximizes the difference between maximum and minimum permeances of the motor's flux loops.