Digital Twin

To reference data from other Things from a given Thing’s calculations and alerts, the following keys provide a way of traversing your Digital Twin hierarchy. From any of the following keys, you can define a role-path to the role which you are interested in.

Available Keys

$self

Retrieve a reference to another component with respect to “myself”.

Usage

// Get Reference to My Child's Child
$self.child_A.childs_child

// Get Reference to My Child's Child
// **Note** this is illustrative only, as you can access sensors / 
// calculations / directly without this
$self

// Use reference to access the aggregated value of `sensorA`
$self.child_A.childs_child.sensor.sensorA

$parent

Retrieve a reference to another component with respect to “my parent”.

Usage

// Get Reference to My Sibling
$parent.sibling_A

// Get Reference to My Parent directly
$parent

// Use reference to access the aggregated value of `sensorA`
$parent.sibling_A.sensor.sensorA

$root

Retrieve a reference to another component with respect to “my top-level root”.

Usage

// Get Reference to My Family Member
$root.someRole.subassembly

// Get Reference to My Root directly
$root

// Use reference to access the aggregated value of `sensorA`
$root.someRole.subassembly.sensor.sensorA

$context

Retrieve a reference to another component by referencing an External ID defined in a Context Field. When using the $context key, you must supply a context field name. Formula processing will assume the referenced field is the External ID to another Thing and attempt to access its data.

Usage

// Get Reference to an arbitrary Thing's child
$context.contextFieldName.child_A

// Get Reference to an arbitrary Thing directly
$context.contextFieldName

// Use reference to access the aggregated value of `sensorA`
$context.contextFieldName.sensor.sensorA

Examples

Digital Twin Definition

Let’s assume we have the following Digital Twin Role Hierarchy:

(role : thingType)
bike : Bicycle
  ├ rearWheel : Wheel
  │   ├ brake : Brake
  │   └ tire : Tire
  │
  ├ frontWheel : Wheel
  │   ├ brake : Brake
  │   └ tire : Tire
  │
  └ bottomBracket : BottomBracket
      ├ crank : Crank
      ├ bearing : Bearing
      ├ leftPedal : Pedal
      └ rightPedal : Pedal

Digital Twin Instantiation

Let’s then assume the following instance of this Digital Twin:

(externalID : role)
BIKE001 : bike
  ├ WHEEL001 : rearWheel
  |   ├ BRAKE001 : brake
  |   └ TIRE001 : tire
  |
  ├ WHEEL002 : frontWheel
  |   ├ BRAKE002 : brake
  |   └ TIRE002 : tire
  |
  └ BRACKET001 : bottomBracket
      ├ CRANK001 : crank
      ├ BEARING001 : bearing
      ├ PEDAL001 : leftPedal
      └ PEDAL002 : rightPedal

Referencing Data

On the Bicycle thing type let’s write the formula for a Calculation named tirePressureDifference:

$self.frontWheel.tire.sensor.pressure - $self.rearWheel.tire.sensor.pressure

Then in the Tire Thing Type (which will execute for each tire) let’s add the following formula for an Alert named tirePressureAbnormal:

if ($root.calculation.tirePressureDifference > 10) {
  // if parent is 'rearWheel', sibling is 'frontWheel' else sibling is 'rearWheel'
  const siblingPressure = ($parent.twin.role === 'rearWheel') ?
    $root.frontWheel.sensor.pressure :
    $root.rearWheel.sensor.pressure;

  // check if I'm the "low" one
  return (sensor.pressure < siblingPressure)
} else {
  return false;
}

Some further examples of traversing this Digital Twin:

• From PEDAL001

$parent // BRACKET001

$parent.bearing // BEARING001

$parent.$parent.frontWheel // WHEEL002

$root.frontWheel // WHEEL002

• From WHEEL002

$root.bottomBracket // BRACKET001

$self.tire // TIRE002