.AixLib.Controls.Continuous.LimPID

Information

PID controller in the standard form

y = k   ( e(t) + 1 ⁄ T_i   ∫ e(s) ds + T_d de(t)⁄dt ),

where y is the control signal, e(t) = u_s - u_m is the control error, with u_s being the set point and u_m being the measured quantity, k is the gain, T_i is the time constant of the integral term and T_d is the time constant of the derivative term.

Note that the units of k are the inverse of the units of the control error, while the units of T_i and T_d are seconds.

For detailed treatment of integrator anti-windup, set-point weights and output limitation, see Modelica.Blocks.Continuous.LimPID.

Options

P, PI, PD, or PID action

Through the parameter controllerType, the controller can be configured as P, PI, PD or PID controller. The default configuration is PI.

Direct or reverse acting

Through the parameter reverseActing, the controller can be configured to be reverse or direct acting. The above standard form is reverse acting, which is the default configuration. For a reverse acting controller, for a constant set point, an increase in measurement signal u_m decreases the control output signal y (Montgomery and McDowall, 2008). Thus,

• for a heating coil with a two-way valve, leave reverseActing = true, but
• for a cooling coil with a two-way valve, set reverseActing = false.

Reset of the controller output

The controller can be configured to enable an input port that allows resetting the controller output. The controller output can be reset as follows:

• If reset = AixLib.Types.Reset.Disabled, which is the default, then the controller output is never reset.
• If reset = AixLib.Types.Reset.Parameter, then a boolean input signal trigger is enabled. Whenever the value of this input changes from false to true, the controller output is reset by setting y to the value of the parameter y_reset.
• If reset = AixLib.Types.Reset.Input, then a boolean input signal trigger and a real input signal y_reset_in are enabled. Whenever the value of trigger changes from false to true, the controller output is reset by setting the value of y to y_reset_in.

Note that this controller implements an integrator anti-windup. Therefore, for most applications, keeping the default setting of reset = AixLib.Types.Reset.Disabled is sufficient. However, if the controller is used in conjuction with equipment that is being switched on, better control performance may be achieved by resetting the controller output when the equipment is switched on. This is in particular the case in situations where the equipment control input should continuously increase as the equipment is switched on, such as a light dimmer that may slowly increase the luminance, or a variable speed drive of a motor that should continuously increase the speed.

References

R. Montgomery and R. McDowall (2008). "Fundamentals of HVAC Control Systems." American Society of Heating Refrigerating and Air-Conditioning Engineers Inc. Atlanta, GA.

Revisions

• June 1, 2020, by Michael Wetter:
  Corrected wrong convention of reverse and direct action.
  Changed default configuration from PID to PI.
  This is for issue 1365.
• March 9, 2020, by Michael Wetter:
  Corrected wrong unit declaration for parameter k.
  This is for issue 1316.
• October 19, 2019, by Filip Jorissen:
  Disabled homotopy to ensure bounded outputs by copying the implementation from MSL 3.2.3 and by hardcoding the implementation for homotopyType=NoHomotopy. See issue 1221.
• September 29, 2016, by Michael Wetter:
  Refactored model.
• August 25, 2016, by Michael Wetter:
  Removed parameter limitsAtInit because it was only propagated to the instance limiter, but this block no longer makes use of this parameter. This is a non-backward compatible change.
  Revised implemenentation, added comments, made some parameter in the instances final.
• July 18, 2016, by Philipp Mehrfeld:
  Added integrator reset. This is for issue 494.
• March 15, 2016, by Michael Wetter:
  Changed the default value to strict=true in order to avoid events when the controller saturates. This is for issue 433.
• February 24, 2010, by Michael Wetter:
  First implementation.