After the fix, each index move completes on measured position and mechanism-ready feedback before drilling or tool change begins. A PLC with integrated motion is the practical starting architecture for three linear axes, a four-position rotary index, and a six-station tool indexer because the process is sequential and does not require CNC-style path interpolation.
Which control approaches fit the machine?
Follow the command from the sequence program to the actuator and back through feedback. The main architectural choice is where trajectory generation and coordination occur.
| Approach | Best fit | Coordination | Engineering cost | Main limitation |
|---|---|---|---|---|
| PLC with integrated motion | Sequential positioning, fixed indexing, drilling cycles | Motion instructions run in a scheduled motion context; sequence logic waits for completion | Lowest of the three when one project contains sequence, safety interface, I/O, and motion | Becomes restrictive when the process needs tightly coupled multi-axis trajectories |
| Dedicated motion controller | Axes that must maintain a defined position relationship while moving | Central trajectory planner synchronizes axes with deterministic updates | Additional controller, interface, software, and diagnostics | Adds architecture without improving a machine whose axes move and settle sequentially |
| Drive-centric motion | Fast local control, specialized axis functions, or distributed coordination | More planning or application logic executes near the drives | Requires clear ownership of sequence, interlocks, recovery, and diagnostics | Can split machine state across PLC and drive projects |
Use PLC-based integrated motion first. Keep a migration path by selecting a platform whose motion facilities cover the required axis count and whose drives expose position, velocity, torque, fault, homing, and move-status data. Siemens and Beckhoff were identified as PLC-plus-motion candidates; Elmo represents the more drive-centric direction. Select among them only after writing the motion requirements.
Where does each motion request travel?
The sequence task requests a move; the motion subsystem plans it; the controller-to-drive link carries cyclic commands; the drive closes its inner control loops; the motor moves the mechanism; and encoder plus mechanism feedback returns status. A machine can stop at any hop even while every program instruction appears valid.
| Hop | Command or feedback | What to verify |
|---|---|---|
| Sequence to motion subsystem | Target position, direction, move request | Request is issued once, accepted, and not retriggered every scan |
| Motion subsystem to drive | Trajectory setpoints and control state | Axis is enabled, referenced, and owned by the intended controller |
| Drive to motor | Motor current and commutation | Power, motor wiring, brake release, limits, and direction |
| Encoder to drive/controller | Position and velocity feedback | Counts change in the commanded direction without discontinuities |
| Mechanism to sequence | Clamp, index, tool, and process-ready states | Physical confirmation agrees with the axis position before the next operation |
Layer one first. Inspect power, grounding, shielding, connectors, encoder wiring, brake circuits, and mechanical limits before changing program timing. Then confirm device identity and configured network settings. Record the controller address, each drive address, network port, update period, and motion-task period from the selected project; the evidence supplies no values to copy safely.
How much synchronization does indexed drilling need?
Synchronization matters only while two or more axes must preserve a defined relationship during motion. If the rotary index moves a bar to one of four fixed positions, stops, clamps, and then permits drilling, positive completion feedback decouples the motions. The same principle applies to the six-station tool indexer: request one station, verify that station mechanically, and only then release the process step.
A normal PLC scan does not by itself define servo quality. The drive closes fast internal loops, while the controller's scheduled motion function supplies trajectory commands. Sequence logic should supervise motion states instead of generating position increments with ordinary scan logic.
| Observed requirement | Coordination method | Controller implication |
|---|---|---|
| Move, settle, then drill | Command/complete handshake | Integrated PLC motion is appropriate |
| Two axes start together but finish independently | Grouped start plus independent completion checks | Integrated motion may be sufficient; test start and finish variation |
| Axis position must follow another axis continuously | Electronic gearing or synchronized group motion | Use a platform with explicit synchronized-motion functions |
| Tool must follow a geometric path while cutting | Coordinated interpolation | Evaluate a dedicated motion architecture |
Lack of coordination appears as variable relative position during movement, inconsistent registration, collisions at handoff points, or lost cycle time from excessive settling margins. Static final-position accuracy alone will not reveal a synchronization problem; trend commanded and actual positions for every participating axis on the same time base.
What requirements decide the controller size?
Write the sequence before selecting hardware. Separate every actuator into a controlled axis, a discrete indexer, or a mechanism confirmed by sensors. A rotary device with fixed stations may still require servo motion, but fixed positioning does not automatically require interpolated motion.
- List the three linear axes, four-position rotary index, and six-station tool indexer as distinct functions. State whether the two indexers are servo axes, drive-controlled indexers, or discrete mechanisms.
- For each move, define target position, direction rules, permissible travel, homing method, completion condition, clamp state, and fault response.
- Mark motions that may overlap for cycle time. Distinguish concurrent independent moves from axes that must remain synchronized.
- Define accuracy and repeatability at the tooling point, not only at the motor encoder. Include backlash, compliance, fixture movement, and clamp repeatability in the measurement.
- Define recovery from interrupted motion: loss of power, drive fault, open guard, failed clamp, and an indexer stopped between stations.
- Count axes, I/O, feedback channels, safety interfaces, and network nodes. Compare those totals with the selected platform's published limits and supported motion functions.
How should the PLC sequence control each index?
Use a state machine with explicit requests, acknowledgements, completion checks, and fault exits. Do not advance because a timer expired. A timer may detect a failure to finish, but measured feedback decides success.
- Verify the axis is referenced, enabled, fault-free, and permitted by machine interlocks.
- Verify the destination station is valid and no conflicting mechanism occupies the travel path.
- Release or engage clamps in the required order and confirm their feedback.
- Issue one move request with the selected station target.
- Wait for command acceptance, then monitor busy, error, and completion states.
- At completion, compare actual position with the configured in-position criterion and confirm the physical station sensor or locking mechanism where fitted.
- Permit drilling, transfer, or tool use only after both motion and mechanism-ready conditions are true.
- On timeout or fault, inhibit the next operation, preserve diagnostic state, and require a defined recovery path.
Avoid unlatching motion requests from scattered routines, using a dwell as the sole index confirmation, or treating a drive's move-complete bit as proof that a clamp engaged. Keep station commands and station confirmations separate so diagnostics can show requested, reported, and physically verified states.
How do you commission and verify the recommendation?
Commission one hop at a time before running the automatic cycle.
- With motive power controlled, verify motor, feedback, brake, limit, grounding, shielding, and network connections against the selected hardware documentation.
- Establish controller-to-drive communication and confirm that each configured device maps to the intended physical axis.
- Jog at reduced operating conditions. Confirm direction, encoder polarity, limit action, brake release, and emergency or guard response under the machine's risk-control procedure.
- Home each axis and repeat homing to measure return consistency at the mechanism.
- Command every linear endpoint, all four rotary positions, and all six tool stations individually. Record requested position, actual position, station feedback, completion state, and fault state.
- Run each process transition in manual step mode. Deliberately withhold a completion or clamp signal and confirm that the sequence stops before the next operation.
- Run the intended overlapping moves while trending commanded position, actual position, motion state, and mechanism feedback on one time base. Escalate to tighter synchronized motion only if the trend shows unacceptable relative error or cycle variation during the overlap.
FAQ
Why does an indexed drilling machine usually not need a dedicated motion controller?
The axes move to fixed destinations and the process can wait for position plus mechanism feedback before continuing. Dedicated coordination becomes valuable when axes must maintain a defined relationship while they are moving.
Why does PLC scan time not determine servo accuracy?
The drive closes its internal control loops, while a scheduled motion subsystem supplies trajectory commands. Ordinary sequence scans should request and supervise moves, not calculate servo setpoints.
Why does the indexer report complete before the machine is ready?
Move completion confirms the axis criterion, not necessarily the clamp, lock, tooling, or workpiece state. Gate the next operation with both axis completion and independent mechanism-ready feedback.
Why does synchronization trouble appear only at higher cycle rates?
More overlap exposes variation in start time, acceleration, settling, and mechanism response. Trend commanded and actual positions on the same time base to distinguish network or motion timing from mechanical compliance.
How do I verify PLC motion is sufficient before production?
Run every endpoint and station, test failed feedback paths, then trend all overlapping moves under the intended operating cycle. Accept the PLC architecture only after relative position, completion order, mechanism confirmation, and cycle variation meet the written machine requirements.