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The Governor

The Governor is an experiment state manager: it manages the transitions between different positioners positions in a beamline endstation in a safe and reliable way. This is especially useful if such positioners share motion envelopes and, therefore, might collide with each other if not driven carefully. To achieve this goal, the Governor implements a fully configurable state machine.

The Governor is controlled via EPICS PVs. The devices controlled by the Governor, in turn, must be controllable through EPICS PVs.

This document starts with an example and then expands on the Governor concepts.

Example Instance

This example is a trimmed down and modified version of actual configuration files used in NSLS-II's FMX and AMX beamlines. These beamlines have a very tight endstation and instruments that can collide with each other.

They have two modes for loading samples: Human and Robot. Human mode Transitions are slower because the instruments are moved a long distance to to ease the mounting of a sample by a person. Robot mode, however, can have a tighter mounting space due to the precision and repeatability of the sample mounting robot.

In this hypothetical example, let's assume that there are three instruments: a sample backlight (li, Light), a beam stop (bs, Beam Stop) and a detector cover (dc, Detector Cover). Let's also assume that the beam stop In Target collides with the backlight's Up Target. Therefore, whenever we want to move the light Up, we have to first take the beam stop Out. Conversely, whenever we want to move the beam stop to the In Target, we have to first make sure that the light is Down. Here are the configuration files:

Configuration Files

File human.yaml:

name: Human

devices:
  bs:
    type: Motor
    name: Beam Stop
    pv: FMX{BS:1-Ax:Z}Mtr
    tolerance: 5
    timeout: 5
    positions:
      In: 40.0
      Out: 15.0
  li:
    type: Motor
    name: Light
    pv: FMX{Light:1-Ax:Y}Mtr
    tolerance: 5
    timeout: 5
    positions:
      Up: 8.0
      Down: -100.0
  dc:
    type: Valve
    name: Detector Cover
    pv: FMX{Det:FMX-Cover}
    timeout: 5

states:
  M:
    name: Maintenance
  SE:
    name: Sample Exchange
    targets:
      bs: {target: Out, limits: [0, 0.0]}
      li: {target: Up, limits: [0, 0]}
      dc: {target: Closed, limits: [0, 0]}
  SA:
    name: Sample Alignment
    targets:
      bs: {target: In, limits: [0, 0]}
      li: {target: Down, limits: [-101.0, 1.0], updateAfter: True}
      dc: {target: Open, limits: [0, 0]}

init_state: M

transitions:
  M:
    SE: [dc, li, bs]
  SE:
    SA: [[dc, bs], li]
  SA:
    SE: [[dc, li], bs]

File robot.yaml: same as human.yaml but with a different name and different Target values.

Let's break the human.yaml file down:

Setting the State Machine's name

First, the name of the State Machine is set. This is what will appear in the Config-Sel PV.

name: Human

Declaring the Devices

devices:
  bs:
    type: Motor
    name: Beam Stop
    pv: FMX{BS:1-Ax:Z}Mtr
    tolerance: 5
    timeout: 5
    positions:
      In: 40.0
      Out: 15.0
  li:
    type: Motor
    name: Light
    pv: FMX{Light:1-Ax:Y}Mtr
    tolerance: 5
    timeout: 5
    positions:
      Up: 8.0
      Down: -100.0
  dc:
    type: Valve
    name: Detector Cover
    pv: FMX{Det:FMX-Cover}
    timeout: 5

Here, three Devices are declared. Note that the dc Device is a Valve-like device and the pv field is actually a prefix. The actual PVs that will be used for this particular device are:

FMX{Det:FMX-Cover}Pos-Sts
FMX{Det:FMX-Cover}Cmd:Opn-Cmd
FMX{Det:FMX-Cover}Cmd:Cls-Cmd

A tolerance of 5 for bs means that, for example, when in In, the Governor will allow bs to go between 35.0 and 45.0 before complaining.

The timeout specifies the time that the Governor will wait for the device to reach the desired position in a transition. A slow (or sputtering) motor can still use a short timeout value because the timeout timer only starts if the motor is standing still according to its readback value.

Here both li and bs have two Targets each. Any number of targets can be specified.

Note how dc doesn't specify any Target. Valve-like Devices have two implicit Targets: Open and Closed.

Declaring the States

states:
  M:
    name: Maintenance
  SE:
    name: Sample Exchange
    targets:
      bs: {target: In, limits: [0, 0.0]}
      li: {target: Down, limits: [0, 0]}
      dc: {target: Closed, limits: [0, 0]}
  SA:
    name: Sample Alignment
    targets:
      bs: {target: Out, limits: [0, 0]}
      li: {target: Up, limits: [-101.0, 1.0], updateAfter: True}
      dc: {target: Open, limits: [0, 0]}

init_state: M

Note that the first State is special. When things go wrong, the Governor will immediately move to this State, and stay there while the fault condition persists. There should be NO incoming Transitions into this State. However, this State can always be reached from any other State immediately, without moving any Device.

Each State takes a short name, a long name and the list of Targets that define the State. The limits entry defines the allowed range of motion for that particular Device when in that particular State. For example:

li: {target: Up, limits: [-101.0, 1.0], updateAfter: True}

When in the SA State, the li Device will be in the Up Target (which is the position 8.0). Now, the tolerance for this particular Device is 5, which means that li has an allowed range of motion of [3.0, 13.0] without complaints by the Governor. Furthermore, since the limits are set to [-101.0, 1.0], the final allowed range is [-98.0, 14.0].

The last interesting bit on this line is the updateAfter setting. When True, it means that the Target will be updated after leaving this State. For example, if we are in the SA State (Target Up: 8.0) and we make a small adjustment to the motor position, say, to 7.0 (which must be in the allowed range [-98.0, 14.0]), then when the Governor leaves the State SA, the Target Up will be set to 7.0 instead of 8.0.

Declaring the Transitions

transitions:
  M:
    SE: [dc, li, bs]
  SE:
    SA: [[dc, bs], li]
  SA:
    SE: [[dc, li], bs]

This set of Transitions is the last necessary piece of information to fully define the State Machine:

M → SE ↔ SA

The first transition, M → SE is declared as follows:

  M:
    SE: [dc, li, bs]

This means that first dc (Detector Cover) will be moved (to the Closed Target), then, after it is done, li will be moved to Down and finally, after li is done, bs will be moved to In.

Sometimes more than one Device can be moved in parallel, and the Governor allows for that. The next lines illustrate the syntax:

  SE:
    SA: [[dc, bs], li]

In this transition, SE → SA, dc and bs are moved in parallel. Only when both reach their targets, li is moved. The transition SA → SE is similar, but li is moved before bs to satisfy physical constraints.

Synchronization File

Assuming that we want to keep all li Targets and one of the bs Targets in sync between Human and Robot State Machines, we can use a synchronization file (sync.yaml):

li: [Up, Down]
bs: [In]

If this file is loaded, whenever one of these three Targets is changed in the Human State Machine, its counterpart in the Robot State Machine will reflect the changes. The converse is also true. bs's Out, however, will have potentially different values in Human and in Robot.

Command line invocation

python3 main.py -c human.yaml robot.yaml -s sync.yaml --prefix FMX

Resulting PVs

The following PVs will be created after the previous invocation:

FMX{Gov}Active-Sel                    # enum values: "Inactive", "Active". "Inactive" prevents all changes
FMX{Gov}Config-Sel                    # enum values: "Human", "Robot"
FMX{Gov}Sts:Configs-I                 # list of all state machines: ["Human", "Robot"]
FMX{Gov}Cmd:Abort-Cmd                 # when written to, abort the current transition in active Governor instance
FMX{Gov}Cmd:Kill-Cmd                  # when written to, forces the IOC to exit

FMX{Gov:Human}Sts:States-I            # a sorted list of all state names: ["M", "SA", "SE"]
FMX{Gov:Human}Sts:Devs-I              # a sorted list of all device names: ["bs", "dc", "li"]
FMX{Gov:Human}Sts:State-I             # the name of the current state: "M" (at first)
FMX{Gov:Human}Sts:Reach-I             # a list of all currently reachable states: ["SE"] (at first)
FMX{Gov:Human}Sts:Busy-Sts            # 1 if performing a transition, 0 otherwise

FMX{Gov:Human}Cmd:Abort-Cmd           # when written to, abort the current transition
FMX{Gov:Human}Cmd:Go-Cmd              # write the name of the desired state to start a transition
FMX{Gov:Human}Sts:Status-Sts          # current state machine status (Idle/Busy/Disabled/FAULT)
FMX{Gov:Human}Sts:Msg-Sts             # message from the Governor

FMX{Gov:Human-Dev:bs}Sts:Tgts-I       # value: ["In", "Out"]
FMX{Gov:Human-Dev:li}Sts:Tgts-I       # value: ["Up", "Down"]

FMX{Gov:Human-Dev:bs}Pos:In-Pos       # value: 40.0
FMX{Gov:Human-Dev:bs}Pos:Out-Pos      # value: 15.0
FMX{Gov:Human-Dev:li}Pos:Up-Pos       # value: 8.0
FMX{Gov:Human-Dev:li}Pos:Down-Pos     # value: -100.0

FMX{Gov:Human-Dev:bs}SE:LLim-Pos      # value: 0.0, lower limit of the bs device when in state SE
FMX{Gov:Human-Dev:bs}SE:HLim-Pos      # value: 0.0, upper limit of the bs device when in state SE
FMX{Gov:Human-Dev:bs}SA:LLim-Pos      # value: 0.0
FMX{Gov:Human-Dev:bs}SA:HLim-Pos      # value: 0.0
FMX{Gov:Human-Dev:li}SE:LLim-Pos      # value: 0.0
FMX{Gov:Human-Dev:li}SE:HLim-Pos      # value: 0.0
FMX{Gov:Human-Dev:li}SA:LLim-Pos      # value: -101.0
FMX{Gov:Human-Dev:li}SA:HLim-Pos      # value: 1.0

FMX{Gov:Human-St:M}Sts:Reach-Sts      # if 1, the state M is reachable from the current state
FMX{Gov:Human-St:M}Sts:Active-Sts     # if 1, M is the current state
FMX{Gov:Human-St:SE}Sts:Reach-Sts
FMX{Gov:Human-St:SE}Sts:Active-Sts
FMX{Gov:Human-St:SA}Sts:Reach-Sts
FMX{Gov:Human-St:SA}Sts:Active-Sts

FMX{Gov:Human-Tr:M-SE}Sts:Active-Sts  # when 1, the transition M->SE is in progress
FMX{Gov:Human-Tr:M-SE}Sts:Reach-Sts   # when 1, the transition M->SE can be requested
FMX{Gov:Human-Tr:SE-SA}Sts:Active-Sts
FMX{Gov:Human-Tr:SE-SA}Sts:Reach-Sts
FMX{Gov:Human-Tr:SA-SE}Sts:Active-Sts
FMX{Gov:Human-Tr:SA-SE}Sts:Reach-Sts

# Also, corresponding PVs will be generated with the
# prefix "FMX{Gov:Robot"

State Machines

The Governor can manage several State Machines at the same time. However, only one State Machine can be enabled at any given time. This is useful if one or more State Machines that share devices and setpoints are to be used.

Entities

There are three kinds of entities in the Governor:

1. Devices

A Device entity represents, quite simply, a positioner that will be driven by the Governor. Currently a Device can be of one of two types:

  1. Motor: a motor that is available as an EPICS PV for a motor record.
  2. Valve: a valve-like device that is available as a few PVs that follow the NSLS-II Naming Standard.

A third type exists, called simply Device, which represents a dummy Device. Dummy devices always move immediately and successfully to their Targets. No communication with a real device is done, even if the pv parameter is specified. This is useful for debugging, for temporarily disabling a device without having to modify the entire State Machine and for testing new devices before issuing real "move" commands.

2. States

A State entity represents a particular experiment state with the specific positions the positioners are expected to be in. States are the vertices in the State Machine.

The initial State is special: it is a fallback that is moved into when a transition fails.

3. Transitions

A Transition specifies which positioners will be moved between States and in which order.

Command line arguments

This is a list of command line arguments accepted by the Governor.

usage: main.py [-h] -c CONFIG [CONFIG ...] [--check_config]
               [-l {DEBUG,INFO,WARNING,ERROR,CRITICAL}] [--prefix PREFIX]
               [-s SYNC]
  • -h: Shows the help message and exit
  • -c CONFIG [CONFIG ...]: Configuration files to be loaded. Each configuration file will result in a different State Machine being instantiated. At least one configuration file must be passed in.
  • --check-config: Exits right after checking that the configuration files passed in the -c option are valid. Any errors in the configuration files are reported.
  • -l, --log-level: The log level to be used. One of DEBUG, INFO, WARNING, ERROR or CRITICAL. The default log level is INFO.
  • --prefix: The prefix to be prepended to the Governor's own PVs. Default is an empty string.
  • -s SYNC, --sync SYNC: The name of the Synchronization Configuration file to be used.

Configuration files

Each Governor State Machine is generated from a YAML configuration file.

Configuration name

Devices

States

Transitions

Initial State

Exposed PVs

The Governor exposes the following PVs (here shown without prefix). The PVs follow the NSLS-II Naming Convention.

Base PVs:

  • {Gov}Active-Sel: R/W, enumeration. Allowed values: Active, Inactive. Controls whether the Governor will accept any command to any State Machine.

  • {Gov}Config-Sel: R/W, enumeration. Allowed values: names of the loaded State Machines. Controls which State Machine is active at the moment.

  • {Gov}Sts:Configs-I: R/O, list of string. Holds the list of existing State Machines.

  • {Gov}Cmd:Abort-Cmd: W/O. Writing any value to this PV will abort the current in progress Transition.

  • {Gov}Cmd:Kill-Cmd: W/O. Writing any value to this PV will cause the Governor to exit.

Per State Machine PVs

Assuming a State Machine named "SM".

  • {Gov:SM}Cmd:Go-Cmd: W/O, string. Write the name of a State to this PV to start a Transition.

  • {Gov:SM}Sts:Status-Sts: R/O, enumeration. Possible values:

    • Idle: when holding a state.
    • Busy: when transitioning between states.
    • Disabled: when a State Machine other that SM is enabled.
    • FAULT: when at least one device is either Disconnected or Not Homed.

  • {Gov:SM}Sts:Msg-Sts: R/O, string. A descriptive string for the State Machine status:

    • When status is Idle or Disabled, this PV will display the name of "SM"'s current State.
    • When status is Busy, this PV will display the names of the states involved in the Transition in progress.
    • When status is FAULT, this PV will display the conditions that are causing the State Machine to be in a Fault state.

  • {Gov:SM}Sts:Busy-Sts: R/O, enumeration. Possible values:

    • No: a Transition is not in progress.
    • Yes: a Transition is in progress.

  • {Gov:SM}Sts:State-I: R/O, string. Holds current State name.

  • {Gov:SM}Sts:Devs-I: R/O, list of string. Holds all Device names.

  • {Gov:SM}Sts:States-I: R/O, list of string. Holds all State names.

  • {Gov:SM}Sts:Reach-I: R/O, list of string. Holds all State names that are reachable from the current state.

Per State PVs

Assuming a State Machine named "SM" and a State named "A".

  • {Gov:SM-St:A}Sts:Reach-Sts: R/O, binary. Indicates whether the "A" State is reachable from the current State.
  • {Gov:SM-St:A}Sts:Active-Sts: R/O, binary. Indicates whether "A" is the current State of the "SM" State Machine

Per Device PVs

Assuming a Device named "dev".

  • {Gov:SM-Dev:dev}Sts:Tgts-I: R/O, list of string. Holds all Target names of this Device.

Per Device and State PVs

Assuming a State Machine named "SM", a State named "A" and a Device named "dev".

  • {Gov:SM-Dev:dev}A:LLim-Pos: R/W, number. Holds the lower limit, relative to the target position, for the device "dev" when in state "A".
  • {Gov:SM-Dev:dev}A:HLim-Pos: R/W, number. Holds the upper limit, relative to the target position, for the device "dev" when in state "A".

Per Device and Target PV

Assuming a State Machine named "SM", a Device named "dev" and a Target named "Tgt".

  • {Gov:SM-Dev:dev}Pos:Tgt-Pos: R/W, number. The actual position for the Target "Tgt" of the Device "dev".

Per Transition PVs

Assuming a transition between states A and B.

  • {Gov:SM-Tr:A-B}Sts:Reach-Sts: R/O, binary. Indicates whether the Transition A → B can be performed.
  • {Gov:SM-Tr:A-B}Sts:Active-Sts: R/O, binary. Indicates whether the Transition A → B is currently under way.

Synchronization Configuration

When more than one State Machine is loaded into the Governor, it is sometimes desirable to keep some Targets for the same Device across State Machines in sync. In order to achieve this goal, a Synchronization Configuration file can be used. The syntax for this YAML file is very simple: it is a list of entries of the form dev_name: [List of Targets]. For example:

device1: [Up, Down]
device2: [In]

If this synchronization file is loaded, then whenever one of the specified Targets is changed in one State Machine the change will be propagated to all other State Machines currently loaded.