The box arrives late, although the axis says it is there
A pusher transfers a carton from a conveyor into a fixture. The PLC asks for 250 mm. The drive reports 250 mm. The carton still sits a few millimetres short of the fixture. Before changing the program, identify where position is measured: a motor encoder measures the motor shaft. Backlash, a loose coupling or a slipping belt can leave the load somewhere else.
An axis is the motion controller's representation of that mechanism. It combines a coordinate, motion limits, feedback and a state. A virtual axis has no physical motor; it can provide a timing reference for several real axes. A physical axis connects to a drive and feedback arrangement. The same PLCopen command vocabulary can be used in different systems, but the hardware configuration and library options still belong to the vendor.
Follow the command to the load
The application requests a destination and dynamics. A trajectory planner produces successive position or velocity demands. The drive follows those demands through its control loops and power electronics. Motor torque acts through a transmission, and the encoder returns a measurement. Depending on the architecture, some loops run in the drive and others in the controller. EtherCAT, PROFINET or another motion-capable network transports the relevant cyclic data; the network name alone does not establish synchronized motion performance.
The position loop corrects position error. The velocity loop controls shaft speed. The current or torque loop controls the motor's electrical effort. These loops have different update rates and tuning parameters. A PLC task calling a function block is not the same thing as the drive's current-control loop.
| Quantity | What it tells you | What it cannot prove alone |
|---|---|---|
| Target position | Where the application wants to finish | That the planner accepted it |
| Command position | The planner's demand at this instant | That the mechanics followed it |
| Actual position | Feedback translated into axis units | Product location after a slipping coupling |
| Following error | Command minus actual position | The root cause of poor tracking |
| Velocity | Rate of position change | That a clamp or guard is ready |
Compare timestamps as well as units. Subtracting an old feedback value from a new demand can exaggerate following error during fast travel.
Choose the coordinate around the machine
Give zero a physical meaning: the fixture datum, a reference switch position or an established encoder coordinate. Record the positive direction. In this course's single-axis bench, positive travel moves the carriage to the right and units are millimetres. Its rail drawing is illustrative; it does not implement physical end stops.
A ball-screw feed might use millimetres. A rotary index table might use degrees. Both accept a number called Position, so the number is meaningless without the axis configuration. A target of 90 is a quarter turn only on an axis scaled in degrees.
An AXIS_REF is a reference to the configured axis, not the axis position itself. Every command intended for the same mechanism must bind to the same reference. Declaring two move instances does not create two motors.
Bench investigation
Enable the drive, raise Move Execute and run scans. Observe the carriage, state and velocity. Lower Execute while it is moving. The accepted command continues; completion belongs to the function-block lifecycle, not to the duration of a pushbutton signal. Reset the bench and issue a move before enabling the drive. Record the command error instead of treating its target as accepted.
For a real transfer station, separate three acceptance conditions: the motion command completed, the carriage feedback is plausible and a product sensor confirms transfer. A carton can fail the third condition even when the first two pass.
A fault to investigate
Suppose following error stays small, but the carton repeatedly lands short. Inspect the coupling, belt tension, gripper slip and where feedback is measured. Increasing gain may make the motor track its demand more closely while doing nothing for lost motion after the encoder. If both motor and load feedback exist, compare them at low speed before changing tuning.
Draw the command path for a horizontal screw and a belt conveyor. Mark the measurement point and one fault that point would miss. Then specify the sensor that would detect the failed transfer.
Ordinary control examples are not safety functions. Physical testing requires a risk assessment, an appropriate safety system and validated procedures; a software interlock or MC_Power signal does not make access to moving equipment safe.
References
PLCopen motion-control specifications define the common vocabulary. For the drive and feedback arrangement, use the exact installed controller and drive manuals.