Simple PID Controller

Class modelling a simple PID controller.

Collaboration diagram for Simple PID Controller:

Author

Karl-Johan Reite

Date

13.08.2013 KJR: Initial version.

The sign of the gains

This class forms error = measurement - reference, which is the opposite of the usual convention, and sfh::filters::PID then forms -kP * error, and likewise for the integral and derivative parts. The two inversions cancel, so the composition is the ordinary

\[ \mathrm{Out} = k_P (r - y) + k_I \int (r - y)\,\mathrm{d}t + k_D \frac{\mathrm{d}(r - y)}{\mathrm{d}t}, \]

and each gain simply takes the sign of the plant it drives: positive when the measurement rises as the output rises, negative when it falls. An AUV heading loop on Auv.RudderYaw takes positive gains; an AUV pitch loop on Auv.RudderDive takes negative ones, because that port is named for its effect and a dive is a negative pitch. See tests/in/Auv/Auv_ClosedLoop_in.xml for all three worked out.

Neither inversion may be removed on its own, and removing both at once is pointless: the pair is already correct, and touching either would silently flip every gain in every existing trawl model. See FISH-0035 (resolved).

Example configuration excerpt

<Lib
   LibName           = "fishery"
   SimObject         = "Trawl/PIDController"
   Name              = "PID"
   InitOutput        = "0"
   ProportionalGain  = "1"
   IntegratorGain    = "0"
   DerivativeGain    = "0"
   MinOut            = "-1e10"
   MaxOut            = "1e10"
   MaxDerivativePart = "1e10"
   MaxIntegratorPart = "1e10"
/>

Input Ports

Name	Width	Description
Measurement	1	The measured value.
Reference	1	The desired value.

Output Ports

Name	Width	Description
Out	1	The output from the PID controller.

Configuration parameters

Name	Width	Description @addoption{InitOutput, 1, The value of the Out port before the first accepted step, and the initial value of the integrator part. }
ProportionalGain	1	Proportional gain on Reference - Measurement. Negative for a plant whose measurement falls when Out rises; see "The sign of the gains". @addoption{IntegratorGain, 1, Integral gain on Reference - Measurement. Takes the sign of the plant, like ProportionalGain. } @addoption{DerivativeGain, 1, Derivative gain on Reference - Measurement. Takes the sign of the plant, like ProportionalGain.}
MinOut	1	Minimum out value.
MaxOut	1	Maximum out value.
MaxDerivativePart	1	Maximum derivative part.
MaxIntegratorPart	1	Maximum integrator part.

Initial conditions

This simObject does not have initial conditions.

Full example file

<Contents>
  <OBJECTS>
   <Lib 
      LibName           = "fishery"
      SimObject         = "Trawl/PIDController"
      Name              = "PID"
      InitOutput        = "0"
      ProportionalGain  = "1"
      IntegratorGain    = "0"
      DerivativeGain    = "0"
      MinOut            = "-1e10"
      MaxOut            = "1e10"
      MaxDerivativePart = "1e10"
      MaxIntegratorPart = "1e10"
   />
  </OBJECTS>
  
   <INTERCONNECTIONS>
    <Connection
      PID.Measurement = "3"
      PID.Reference   = "4"
    />
  </INTERCONNECTIONS>
 
  <INITIALIZATION>
    <InitialCondition
    />
  </INITIALIZATION>
 
  <SIMULATION>
    <Timing TStart="0" TEnd="30"/>
    <Integrator Method="RK45_i" NumCores="1" LogStates="1" UseRSSNorm="0">
      <StepControl StepMax="0.002" StepMin="0.00000001" AbsTol="1e-3" RelTol="1e-3"/>
    </Integrator>
    <!-- TODO migrate_v3: FileOutput="objects:all" -> configure via <OBSERVERS> section -->
  </SIMULATION>
</Contents>

This SimObject is referred to as Trawl/PIDController