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Needs

Sometimes you cannot quantify a requirement and must state something qualitatively. For example, you may want to refer to an externally defined standard. This can be specified using a need, which should be used with caution. Needs serve a documentation purpose, but are mostly ignored by the compiler as far as any checks can go.

Let's say our drive-mechanism that lives in the world definition needs to be waterproof up to IP68 compliance. There are no further constraints on need lines than that it needs to start with a variable or a component name as in [variable | component-name] [free text], which is why they are some form of 'escape' in the specifications. Use this wisely and responsibly!

You can add the IP68 need to the world like so:

world.esl
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world
  # Earlier content is visible in File tab.
  need
    IP68: drive-mechanism must be IP68 compliant
world.esl
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define type
  MechanicalEnergyFlow is a real with unit Nm
  ElectricalEnergyFlow is a real with unit W
  LiquidMaterialFlow is a real with unit L/s
  EnergyPotential is a real with unit Wh
  ControlSignal is a boolean
  Distance is a real with unit m
  SpatialMeasure is a Distance of at least 0.0 [m]


define verb
  provide to
  convert into


world
  variables
    torque is a MechanicalEnergyFlow
    water-flow is a LiquidMaterialFlow
    drive-length is a SpatialMeasure
    pump-length is a SpatialMeasure

  design-requirements
    min-water-flow: water-flow must be at least 1.0 [L/s]

  design-constraints
    dc-drive-length: drive-length must be equal to pump-length

  components
    pump is a CentrifugalPump
    drive-mechanism is an ElectricalDriveMechanism

  goal-requirements
    provide-torque: drive-mechanism must provide torque to pump

# ---8<--- [start:highlight]
  need
    IP68: drive-mechanism must be IP68 compliant
# ---8<--- [end:highlight]


define component CentrifugalPump
  variables
    torque is a MechanicalEnergyFlow
    water-flow is a LiquidMaterialFlow

  transformation-requirements
    convert-torque: must convert torque into water-flow


define component ElectricalDriveMechanism
  variables
    power-potential is an EnergyPotential
    power is an ElectricalEnergyFlow
    torque is a MechanicalEnergyFlow
    motor-control-signal is a ControlSignal


  transformation-requirements
    convert-power-potential: must convert power-potential into torque

  components
    motor is a BrushlessMotor
    power-source is a Battery
    power-switch is a Switch

  goal-requirements
    provide-power: power-source must provide power to motor

  behavior-requirements
    toggle-power:
      case on:
        when
          * c1: motor-control-signal is equal to True [-]
        then
          * r1: power must be at least 300 [W]
      case off:
        when no other case applies
        then
          * r2: power must be equal to 0 [W]


define component BrushlessMotor
  variables
    power is an ElectricalEnergyFlow
    torque is a MechanicalEnergyFlow

  transformation-requirements
    convert-power: must convert power into torque


define component Battery
  empty

define component Switch
  empty

Tip

When client specifications are too vague (yet) to be implemented in a quantitative manner, it may be wise to document them using a need for the time being. These can serve as action points to reduce the neediness of the specification and support the transition from qualitative to quantitative requirements later on.

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