[CAS_CTR] Cascade controller

Cascade control

The supply air setpoints [SpLoSu], [SpHiSu], and [SpErcSu] are calculated as follows:

1. The reference controller is switched by the inputs [EnFnct] and [OoServ] (operating modes).
2. The lower and upper supply air setpoint [SpLoSu], [SpHiSu] are controlled by the controlled variable [XctrR] (upper, lower supply air setpoints).
3. The control action of energy recovery [ActgErc] determines the supply air setpoint for energy recovery [SpErcSu] (supply air setpoint for energy recovery).

Operating modes

Upper, lower supply air setpoints

The CAS_CTR block works as a reference controller for cascade control of the room supply air. The reference controller controls the control variable [XctrR] of the outer control loop to an interval value [SpLoR],[SpHiR]. To do this, the reference controller CAS_CTR determines the three setpoints [SpLoSu], [SpErcSu], and [SpHiSu], controlled by the inner control loop.

The sequence controller that controls the controlled variable [XctrSu] of the inner control loop to the setpoints supplied by the CAS_CTR block is part of the inner control loop ([CTR] Controller).

Calculation of the upper and lower supply air setpoint depends on the operating mode and the controlled variable [XctrR].

Supply air setpoint for energy recovery.

Calculation of the supply air setpoint for energy recovery [SpErcSu] depends on the operating mode, the setpoint selection for energy recovery [SelSpErc], and on the present direction of control action for energy recovery [ActgErc].

Integration in sequence control

CAS_CTR can be integrated in sequence control by connecting pins [ToLower] and [FmHigher] as well as [FmLower] and [ToHigher]. It can be reached directly or via SEQLINK. Integration of CAS_CTR in sequence control corresponds to integration of a PID sequence controller, where it is described in detail.

The following diagram shows control of a ventilation system. The air volume flow is increased when the min. or max. supply air setpoint is reached.

Temperature or humidity control

The reference controller is used for both temperature and humidity control. Change the following settings when using it as a reference controller for humidity control:

Humidity control by different physical controlled variables

If the supply air [XctrSu] is measured in absolute humidity [g/kg], and the room air [XctrR] in relative humidity [%hu], the integrator initialization value must be predefined; otherwise, the mean value calculated from [SpLoR] + [SpHiR] is used. If the room setpoints are defined in absolute humidity, the integrator initialization value starts at a high value and returns to the set integral action time [Tn]. As a result, humidification may be enabled during controller initialization until the integrator has reached the correct value, even if the room would need dehumidification.

To prevent this, the present measured value for the supply air humidity [XctrSu] is interconnected to the integrator initialization [IntgInit] or a fixed parameter value for the integrator is preset.

Requirement for high accuracy of control

If there are high requirements of the control accuracy (e.g. work without energy-free control zone), the present measured value for the supply air humidity [XctrSu] is interconnected to the integrator initialization [IntgInit] or a fixed parameter value is defined for the integrator.

Details for cascade controller operation: PID sequence controller

A sequence controller is used if several components control a controlled value in accordance with the predefined control sequence. The sequence controller switches the individual sequence controller elements according to the control sequences and coordinates control for the individual controller elements in an interrelated control behavior across all sequence controller elements or components.

The functioning of the sequence controller comprises all processes that influence each other:

• Enabling or disabling a sequence controller
• Control process of the sequence controller
• Enabling/disabling/commanding an individual sequence controller element
• Control process of the individual sequence controller elements. The control process corresponds to that of a universal PID controller.

Enabling or disabling a sequence controller

The sequence controller is disabled by disabling [EnFnct] = 0 or commanding [OoServ] = 1 all sequence controller elements. If then a selection of sequence controller elements is enabled with properly parameterized control action ([EnFnct] = 1 and [OoServ] = 0), the sequence controller elements are initiated within this selection: The control process is started with that sequence controller element of the enabled selection that is closest to the control action changeover and that contains the control action for the respective situation determined by the present actual values and setpoints.

If the sequence control process is to be initiated by a particular sequence controller element, the corresponding sequence controller element must be enabled prior to all other elements; else, the above described process is used.

Control process of the sequence controller

The sequence controller switches control of the individual sequence controller elements via the controller token. Further signals to coordinate control of the sequence controller elements are necessary in addition to the controller token. The associated signals are exchanged via the pins [ToLower] and [FmHigher] and the pins [FmLower] and [ToHigher].

In each process cycle, the individual sequence controller elements independently determine the sequence controller element that carries out control (independent of the operating mode of the sequence controller).

• If a sequence controller element satisfies the following conditions, the controller token adopts [TknSta] = CntrTkn.
• The sequence controller element must be enabled ([EnFnct] = 1) and not commanded ([OoServ] = 0).
• The sequence controller element controls the requested demand, e.g., the heating demand: [Actg] = Reverse and [Xctr] < [Sp].
• The sequence controller element controls the responsible component in the operating sequence, e.g., the previous components have been removed from control or cannot be controlled.
• The sequence controller must be enabled.
• The sequence controller element featuring the controller token controls the plant [Yctr]. All other sequence controller elements are constant.
• [TknSta] = CntrTkn
• [CtrSta] = CntrOn
• Exception: Enabling/disabling/commanding individual sequence controller elements
• If the control range of the controlling sequence controller element is exhausted, [Yctr] > [YctrMax] or [Yctr] < [YctrMin], the controller token is passed on to the following sequence controller element (according to the operating sequence).
• [TknSta] = NoTkn
• [CtrSta] = CntrMin or CntrMax
• Exception: Sequence controller elements with [EnFnct] = No or [OoServ] = On are not switched.

If all controller outputs of the enabled sequence elements are set to their limit values [CtlSta] = CntrMin or [CtlSta] = CntrMax, the sequence controller element does not have a controller token. This may occur in different parameter settings of the sequence controller elements, e.g., within the energy-free zone due to different setpoints.

Enabling/disabling/commanding an individual sequence controller element

The sequence controller considers the influence on control and integration if a sequence controller element is enabled, disabled or commanded via its operating mode (e.g. via operator intervention).

Details for cascade controller engineering: Engineering - PID sequence controller

The sequence controller is formed by interconnecting and parameterizing CTR function blocks (sequence controller elements). The sequence controller functionality is generated automatically via information channels. The sequence of the individual components is determined by the interconnection sequence of the CTR function block.

Engineering comprises the following steps:

Determining the order of the sequence controller elements

Essentially, the sequence controller consists of individual CTR function blocks. Each PID_CTR block acts as a sequence-controller element for a component.

The interconnection sequence of the CTR function blocks (from low to high) corresponds to the order of the control sequences (1..n) of the sequence controller. Accordingly, both the intended operating range (e.g. heating) and the switching sequence must be considered when interconnecting the CTR blocks.

For example: Components
 1 = Reheater, 2 = Preheater, 3 = Air dampers, 4 = Cooler

Control sequence for heating: 1 ---> 2 ---> 3.

Control sequence for cooling: 4 ---> ....

• The lowest sequence controller element corresponds to control sequence 1, the highest to control sequence n.
• The lowest sequence-controller element controls a reverse-acting component (if used)

Interconnecting sequence controller elements

There are two ways to interconnect the CAS_CTR function blocks with a sequence controller:

1. Direct interconnection

The individual CAS_CTR function blocks are interconnected with each other.

Interconnection is made between the pins [ToLower] and [FmHigher] and the pins [FmLower] and [ToHigher].

This type of interconnection is used if the PID_CTR function blocks are located on the same chart.

2. Interconnection to SEQLINK

The individual PID_CTL function blocks are interconnected via the SEQLINK block.

Interconnection occurs between the pins for function block CTR and a place on the SEQLINK. The sequence of CTR must match the order of the places. However, free places are possible on the SEQLINK. Several SEQLINKs can be interconnected in series.

This type of interconnection is used if the CTR function blocks are located on different charts or if the individual sequence controller elements or components may not be interconnected (CAS library).

The sequence elements must be parameterized in accordance with the interconnection.

The pins [ToLower] and [FmHigher], [FmLower] and [ToHigher] are interconnected only on a sequence controller.

Parameterizing the sequence controller element' control action

The set operating range (e.g. heating, cooling) determines the direction of control action [Actg] for a sequence controller element. Direction of control action changeover during operation is possible (e.g. energy recovery). If component control demands an inverted signal, inversion [Inv] of the controller output [Yctr] helps solve this problem.

In the sequence controller, the direction of control action must feature the following response:

• The first or lowest sequence controller elements have reverse direction for control action [Actg] = Reverse.
• The first or highest sequence controller elements have direct direction of control action [Actg] = Direct.
• A sequence controller element with changing direction of control action (e.g. ERC) can only be in between.

Deviations generate an error signal [ErSta] = Yes. Troubleshooting

Parameterizing setpoints and an energy-free zone

In the sequence controller, the setpoints [Sp] of the sequence controller elements (1...n) must increase monotonously:

[Sp]₁ <= [Sp]₂ <= [Sp]₃ <= ... <= [Sp]_n

Modulating control on transition from one control sequence to another is ensured if the control sequences with the same direction of control action have the same setpoint.

The energy-free zone is defined by the setpoints on changeover of the direction of control action (e.g. heating setpoint, cooling setpoint).