How does a linear motor move in a straight line?

How the parts work together

1. Read the track

   The magnets alternate north and south, each one pole pitch wide, so a north-south pair (the grey bracket) is 2 pole pitches. On the back panel the dark curve is the magnets' field along the track: a peak over each north magnet, a trough over each south magnet and zero at every boundary. The red and blue strip under it repeats the magnets. Straight above each coil stands a bar in its colour showing its current, and the arrows along the top show the push on each coil.

2. Current follows the slope

   A coil's two sides carry opposite currents, so it is pushed only by the difference in field between them: the slope of the dark curve at the coil. Making each current follow that slope gets the most push for the current and keeps the total steady. Here a phase C coil sits over a boundary, where the curve is steepest: full current and the biggest push. Phases A and B carry half the opposite current and get small pushes the same way; the three currents add to zero. While the forcer heads right, the dashed teal curve through the bar tops is the dark curve's slope; heading left, every current is reversed and the curve is that slope turned upside down.

3. Zero beside full

   Now a phase C coil sits over the middle of a magnet, where the curve is flat: no current and no push. Its neighbours sit where the curve is steep in opposite directions, so they carry opposite currents, 0.87 of full. Opposite current times opposite slope gives a push the same way, so every arrow still points right. As the forcer moves, each coil's push rises and falls, but in the smooth field drawn here the six always add to the same total; real magnets leave some ripple, which drives can reduce by reshaping the currents.

4. The coils' field stands still

   Each current follows the slope at its coil, so the dashed teal curve is also the pattern of the coils' own field, drawn as a smooth, idealised wave through the six coils: where it is high, a tooth's field points the same way as a north magnet's. It sits half a pole pitch behind the magnets' curve, so each hump is pulled toward the matching magnet hump ahead and pushed by the opposite one behind. Press Play: the coils slide forward but the teal curve stays put along the track, which is why each coil's current rises and falls. At each end of its stroke the drive reverses every current (drawn as an instant reversal; a real drive brakes the forcer and speeds it up again). The teal curve flips over and now sits half a pole pitch behind on the other side, so the push and the travel reverse; an amber U-turn mark flashes on the carriage plate and the Direction box follows by itself.

5. Set the speed

   The drive is told a speed. Its control loop sets how strong the currents are, so that at steady speed the total thrust just balances friction and the load, while the encoder sets their timing. Because each current depends only on where its coil is, the currents complete one cycle each time the forcer passes a north-south pair. At 0.6 m/s with a 30 mm pole pitch that is 10 cycles a second, and doubling the speed doubles the frequency. Here the motion is prescribed and shown 10 times slower. Because the coils move, a real forcer also trails its power and encoder cables in a flexible cable carrier, which is not drawn.

6. Look into the air gap

   This close-up magnifies the front of the forcer 6 times (9 times on a phone) and cuts the windings at the iron's front face. The coloured blocks on either side of each tooth are the coil sides in section. Below the wider tooth tips, the pale band marked by the small black bracket is the air gap, drawn 1.5 mm. The magnets pull the iron teeth across this gap all the time, even with no current, so the guides carry that pull as well as the forcer's weight; the thrust acts across the same gap, along the track. Turn off Air gap close-up to see the whole machine again.

The main parts

Base bed

A flat aluminium bed that carries the magnet track, the guide rails and the encoder scale. It never moves.

Magnet track

Eight permanent magnets bonded to a 6 mm steel back plate, north (red) and south (blue) alternating. Each is one pole pitch wide, 30 mm in this model, so a north-south pair is 2 pole pitches. The track has no wires and needs no power.

Guide rails and stops

Two rails on wider risers carry four low bearing blocks that straddle them and carry the moving frame. The dark stops at each end limit the stroke.

Forcer iron

Six laminated iron teeth, each with a wider tip, face the magnets across the air gap, and a back plate on top joins them. The slots between the teeth are deeper than they are wide, and the windings fill them. The air gap, drawn 1.5 mm, sits under the coils; turn on Air gap close-up to see it as a pale band under the teeth. The front of the back plate is drawn cut away, its cut face shaded orange and hatched, so the coils and the tops of the teeth stay visible. Each coil is wound around one tooth.

Phase A coils

Two coils of phase A. Colour strength shows how large the current is right now: a pale tint means little current, full colour means full current. The arrows on the coil's two sides show which way it flows, toward you on one side and away on the other, and grow longer with more current. On the graph, a bar in the coil's colour stands above the coil and shows its current, and an arrow higher up shows the push it gets. Selecting the phase outlines its coils in blue and fades the other phases on the graph.

Phase B coils

Two coils of phase B, a third of an electrical cycle (120 degrees) from phase A along the forcer. Their current peaks at a different position from A and C.

Phase C coils

Two coils of phase C, another third of a cycle along. The three phase currents always add up to zero.

Moving frame

A blue-grey carriage plate bolted along the top of the back iron and resting on spacers on the rear bearing blocks, and end plates that tie it, the iron's end faces and all four blocks together. The plate is drawn cut away at the front, like the iron under it, and the end plates stay low in front of the cut. Whatever the frame carries moves with the forcer along the rails.

Position encoder

A read head on a bracket from a front bearing block reads the marks on a scale along the bed. The drive uses this position to set each coil's current, so it always knows where every coil sits over the magnets.

Field and push graph

Drawn on a panel behind the machine, not a machine part. The dark curve is the magnets' field along the track. The dashed teal curve is the field pattern the coils' currents make, drawn only where the forcer is. Bars in the coil colours stand above their coils, show the coil currents and end on the dashed curve. The arrows along the top are the push on each coil. The red and blue strip under the curves repeats the magnets below it, and the grey bracket marks one north-south pair, 2 pole pitches. The panel is shifted sideways so that, from the starting view, its marks line up with the parts below; orbiting breaks that line-up.

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