PLCopen State Machine

This diagram normatively defines according to PLCopen the behavior of the axis at a high-level when multiple motion control function blocks are simultaneously activated.

• This combination of motion profiles is useful in building a more complicated profile or in handling exceptions within a program.
  • In real implementations there can be additional states defined at a lower level.
• The basic rule is that motion commands are always taken sequentially, even if the PLC has the capability of real parallel processing.
  • These commands act on the state diagram of the axis.
• The axis is always in one of the defined states.
  • A change of state is reflected immediately when issuing the corresponding motion command.
  • The response time of immediately is system dependent.

The defined seven states are:

• Continuous Motion
• Discrete Motion
• Error Stop
• Homing
• Stand Still
• Stopping
• Synchronized Motion

Figure 1: Motion State Machine (PLCopen)

Notes

• In the ErrorStop or Stopping state, all function blocks can be called, although they are not executed.
  The exceptions are MC_Reset and Error which generate the transition to StandStill or ErrorStop respectively.
• MC_Power FB is called with Enable=TRUE and there is an error in the Axis.
• MC_Power FB is called with Enable=TRUE and there is no error in the Axis.
• MC_Stop.Done and not MC_Stop Execute.

State Machine for Multi-axes Motion Control

The diagram is focused on a single-axis.

• The multi-axes function blocks (e.g., MC_CamIn, MC_GearIn, or MC_Phasing) can be looked at, from a state diagram point of view, as multiple single-axes all in specific states.
• Example: The CAM-master can be in the state Continuous Motion.
• The corresponding slave is in the state Synchronized Motion.
• Connecting a slave axis to a master axis has no influence on the master axis.

A normal procedure starts in StandStill.

• In this state the power can be switched on per axis (via the MC_Power command).
  • Access the Homing state (via the issue of the command Home per axis).
    This returns to StandStill after normal completion.
  • From here, transfer an axis to either Discrete Motion or Continuous Motion.
  • Via the Stopping state you can return to StandStill.
  • ErrorStop is a state to which the axis transfers in case of error.
  • Via a (manual) MC_ResetError command, return to StandStill.
    • From this state, the machine can be moved to an operational state again.

The States define the functionality of the function blocks.

• Function Blocks not listed in the State Diagram do not affect the state of the axis.
  • This means that, whenever they are called, the state does not change.
  • They are:
    • MC_CamTblSelect
    • MC_ReadActPos
    • MC_ReadAxisErr
    • MC_ReadBoolPar
    • MC_ReadParam
    • MC_ReadStatus
    • MC_WriteBoolPar
    • MC_WriteParam

Disabled State

The Disabled state describes the initial state of the axis.

Disable means power off without error.

• In this state, the movement of the axis is not influenced by the FBs.
  • The axis feedback is operational.
  • If the MC_Power FB is called with Enable=TRUE while being in Disabled, this leads to either:
    • Standstill if there is no error inside the axis.
    • ErrorStop if an error exists.
  • Calling MC_Power with Enable=FALSE in any state, the axis goes to the state Disabled, either directly or via any other state.
  • If a motion generating FB controls an axis while the MC_Power FB with Enable=FALSE is called, the motion generating FB is aborted (CommandAborted).

ErrorStop State

The ErrorStop state is the axis goes to a stop, if possible.

• No further FBs are accepted until a reset has been done from the ErrorStop state.
• The transition Error refers to errors from the axis and axis control, and not from the function block instances.
• These axis' errors are reflected in the output of the function blocks FB instances errors.
• Issuing MC_Home in any other state than StandStill goes to ErrorStop, even if MC_Home is issued from the Homing state itself.
• ErrorStop is valid as highest priority and applicable in case of an error.
  • The axis can have either power enabled or disabled, and can be changed via MC_Power.
  • However, as long as the error is pending, the state remains ErrorStop.

From StandStill to Stopping

StandStill is power on without an error.

• Calling the FB MC_Stop in state StandStill changes the state to Stopping and back to Standstill when Execute = FALSE.
• The Stopping state is kept as long as the input Execute = TRUE.
• The Done output is set when the stop ramp is finished.