blockLimPID

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 = Buildings.Types.Reset.Disabled, which is the default, then the controller output is never reset.
• If reset = Buildings.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 = Buildings.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 = Buildings.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.

Parameters

Connectors

Components

Revisions

• August 4, 2020, by Jianjun Hu:
  Moved the block to Buildings.Obsolete.Controls.OBC.CDL.Continous.
  This is for issue 2056.
• June 1, 2020, by Michael Wetter:
  Corrected wrong convention of reverse and direct action.
  This is for issue 1365.
• April 23, 2020, by Michael Wetter:
  Changed default parameters for limits yMax from unspecified to 1 and yMin from -yMax to 0.
  This is for issue 1888.
• April 7, 2020, by Michael Wetter:
  Reimplemented block using only CDL constructs. This refactoring removes the no longer use parameters xd_start that was used to initialize the state of the derivative term. This state is now initialized based on the requested initial output yd_start which is a new parameter with a default of 0. Also, removed the parameters y_start and initType because the initial output of the controller can be set by using xi_start and yd_start. This is a non-backward compatible change, made to simplify the controller through the removal of options that can be realized differently and are hardly ever used. This refactoring also removes the parameter strict that was used in the output limiter. The new implementation enforces a strict check by default.
  This is for issue 1878.
• March 9, 2020, by Michael Wetter:
  Corrected unit declaration for gain k.
  See issue 1821.
• March 2, 2020, by Michael Wetter:
  Changed icon to display dynamically the output value.
• February 25, 2020, by Michael Wetter:
  Changed icon to display the output value.
• October 19, 2019, by Michael Wetter:
  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.
• November 13, 2017, by Michael Wetter:
  Changed default controller type from PID to PI.
• November 6, 2017, by Michael Wetter:
  Explicitly declared types and used integrator with reset from CDL.
• October 22, 2017, by Michael Wetter:
  Added to CDL to have a PI controller with integrator reset.
• 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.