Electronic gearing and camming, and when you need either

Both of these replace something that used to be made of metal, and understanding what they replaced explains what they do.

Electronic gearing

A mechanical gear train forces two shafts to turn in a fixed ratio. Turn the master one revolution, the slave turns exactly the ratio, forever, with no drift, because they are connected.

Electronic gearing does the same thing in software. The slave's position command is the master's position multiplied by a ratio.

The important word is position. Commanding both axes to the same speed looks equivalent and is not: any difference in following error integrates into a position offset that grows without bound. Gearing on position corrects rather than accumulates.

This is the whole reason to use a motion controller rather than two drives with a shared analogue reference. The shared reference is speed matching, and speed matching drifts.

Electronic camming

A mechanical cam is a shaped disc; a follower rides on it, and the follower's position is an arbitrary function of the cam's angle.

An electronic cam is that function as a table: master position in one column, slave position in the other, interpolated between points.

It does everything gearing does, since a straight line in the table is a fixed ratio, and everything gearing cannot: dwell while the master keeps moving, accelerate through part of the cycle and decelerate through another, reverse briefly.

Where each is used

Gearing. Two rollers that must stay in step. A conveyor and an indexer. Anything that used to be a shared line shaft.

Camming. A flying shear that accelerates to web speed, cuts, and returns. A rotary filler where the nozzle follows the bottle. A packing machine where a flight bar must dwell at the load point. Anything that used to be a barrel cam with a mechanic adjusting it.

The virtual master

The design decision that makes a machine maintainable.

Rather than every axis following a physical encoder on one of them, generate a master position in software. Every axis, including what would have been the master, follows the virtual one.

The benefits are immediate. The whole machine is jogged by moving one number. Stopping is one deceleration profile rather than a negotiation. And nothing depends on a physical axis that might be the one that has failed.

Registration

The reason cams alone are not enough on anything involving a web.

Printed film stretches. Over a hundred metres the print drifts relative to any fixed count of encoder pulses, and a cut positioned purely by the cam lands progressively further from the mark.

A registration sensor sees the mark, the controller compares where it arrived with where it expected, and applies a small correction to the cam offset. The correction is deliberately limited per cycle, so a missed mark produces a small error rather than a lurch.

Where this leaves ladder

Worth being honest about the boundary. This is motion controller territory, programmed in the vendor's motion environment or in PLCopen motion function blocks, not in ladder.

Ladder's job around it is the machine sequence: when to engage the cam, when to disengage it, what to do when a fault occurs mid-profile. That part is ordinary logic and belongs where the rest of the sequence lives.