The Desigo control concept is a set a rules that determine in general terms the principles governing all control, reporting and monitoring operations and the switching interventions in the Desigo system. The rule applies to block-internal control (priority array) and to functional interactions among participating blocks.

This specifically deals with:

  • Structure and design of control as function blocks
  • The hierarchical assignment of the function blocks among themselves
  • The function hierarchy within the control chain for the function blocks
  • Processing operational and fault messages
  • Interventions in monitoring functions
  • Impact of emergency switching

The open loop control strategy is based on the exchange of predefined signals between functional units. Each functional unit is an image, or memory map, of an actual element of the plant, e.g., ventilation or boiler plant.

Control functions

The open-loop control functions required for a given element are locally an integral part of the functional unit (e.g., the increase, after a time delay, in the speed of a multi-stage fan, or the demand-based switch-on of a boiler). In each functional unit, various possible requirements are prioritized and evaluated. The resulting operating mode is then passed on to the elements or subordinate functional units. The functional unit already incorporates the I/Os needed for the physical data points.

Structure control functions

In this way, complex control and monitoring functions of a plant can be logically subdivided to allow for clear assignment of the function unit or the real element of the plant. The higher-level control concentrates on the control and monitoring of the overall plant, while the sub-control function units assume internal control and monitoring of the given elements for the function unit.

Standardization of control functions

Moreover, plant security and available was increased through standardized control and monitoring functions which would result in considerable expense using conventional methods.

Standardized control and monitoring functions:

  • Unambiguous selection of operating mode
  • Uniform fault-related shutdown
  • Comprehensive status monitoring
  • Switching sequence for ventilation systems
  • Output stage control for heat generating plant
  • Reporting of local intervention
  • Avoidance of unnecessary attempts at switching
  • Prevention of inadmissible switching operations
  • Protection of plant by preventing switch-on or switch-off

Blocks bound by the control concept

Function

Block name

Task in the control concept

Prioritization of influencing variables

ENSEL_BO

ENSEL_MS

Collect information for the selection of the resulting plant operating mode. All superposed information are processed by priority resulting in the plant being turned on or off, e.g., smoke extraction switch, frost protection, scheduler program.

The blocks are primarily used on the hierarchy level plant/partial plant, but may also be reasonably used, e.g., in aggregates.

Command control

CMD_CTL

Superposed control block for sequence control. The block ensures that individual plant aggregates are switched on or off sequentially in a certain order. The block monitors aggregates and can send alarms. It is optimized for controlling air handling plants, but can be used for other applications. The block is used on the hierarchy level plant/partial plant.

Power control

PWR_CTL

Superposed control block for power control. The block is used for control and monitoring of the performance of a number of energy producers (multiple boiler systems, refrigeration machines). Depending on the request power demand, energy produces are switch on or off in stages. PWR_CTL is optimized for controlling heating and refrigeration plants. The block is used on the hierarchy level plant/partial plant.

I/O blocks with control functionality

BO

MO

AO

Output blocks implemented per BACnet standard and therefore including a priority mechanism (priority array) that is well suited for control tasks. The priority array [PrioArr] be used through data flow interconnections and BACnet commanding. Moreover, the block integrate the following control functionality:

- Motor control (pump, burner, etc.), one- to four-speed [BO, MO]

- Fan control, one- to four-speed [BO, MO]

Value blocks with control functionality

BVAL

MVAL

AVAL

Value objects or value blocks are implemented per BACnet standard and therefore includes output blocks via the priority mechanism. These blocks are referred to as data points that can communicate within the system with the I/O modules via BACnet. These blocks are primarily used as the communication interface between superposed control [CMD_CTL, PWR_CTL] and the aggregates.

Rotation block

ROT_8

The Rotation block switches the operating mode on and off for a maximum of eight functional units in accordance with a selected mode of rotation (sequence or hours run). The change of operating mode is based on demand, hours run, occurrence of a fault or manual intervention (override).

The block is used to process the functional units (e.g., aggregates or components) as a function of run-time or faults. These blocks are used, e.g., for double pumps, that are changed over based on runtime.

Control hierarchy

Control hierarchy is the map of the functional assignment and linking of those function blocks included in the control concept for a plant. The structure of the control hierarchy is subject to certain rules. A distinction is drawn between higher-level plant control and local control of the functional units.

Superposed control

Within the hierarchical structure, higher-level control functions are typically assigned to the partial plant level. All the variables which are influencing factors on the overall plant are weighted and combined to give the effecting plant operating mode. In respect of each of the possible plant operating modes, a control strategy can be defined for each underlying functional element. This makes it possible to develop specific plant scenarios, such as fire control, smoke extraction, frost control, on/off-switch control.

Local control

Within the hierarchical structure, local control of the function elements is typically assigned to the partial plant level. The main function of local control is to respond to faults. The functional unit itself determines how the outputs are to be controlled in the event of a fault. Interlocks between functional units (e.g., damper/fan) must be implemented locally. Local control prevents the risk of damage to plant, in the event that the command control parameters are set incorrectly.

The control hierarchy in the following figure considers only the example application for ventilation.

I/O block functions and interfaces

The I/O blocks are the most important blocks in the Desigo system. In addition to controlling the hardware, they are responsible for numerous control and monitoring functions. They enable otherwise complex functions to be implemented with just a small number of blocks.

Main functions and interfaces of I/O blocks

Function

Inputs

Description

AI

AVAL

AO

BI

BVAL

BO

MI

MVAL

MO

Stop transmission of input signal

OoServ

Out of service

Priority mechanism

DefVal

Default value

 

 

 

 

PrioArr

Priority array

 

 

 

Local override

Ovrr

Override

 

 

 

OvrrVal

Override value

 

 

 

Alarm value monitoring

- Limits

- Reference values

- Monitoring periods

EnAlm

Alarm enable

HiLm

Upper limit

 

 

 

 

 

 

LoLm

Lower limit

 

 

 

 

 

 

Nz

Neutral zone

 

 

 

 

 

 

RefVal(s)

Reference value

 

 

 

 

TiMonOn

Monit. time switch-on

 

 

 

 

TiMonOff

Monit. time switch-off

 

 

 

 

TiMonDvn

Monit. time deviation

Switching delays

DlyOn

Switch-on delay

 

 

 

 

 

DlyOff

Switch-off delay

 

 

 

 

 

TbTiDly

Time delay table

 

 

 

 

 

 

Switching action

Normal (motor)

Release command

Trigger

Switch

Switch with delay

SwiKind

Switch kind

 

 

 

Function

Outputs

Description

AI

AVAL

AO

BI

BVAL

BO

MI

MVAL

MO

Feedback monitoring

PrVal

Present value

FbVal

Feedback value

 

 

 

 

 

 

Reliability monitoring

Rlb

Reliability

Fault monitoring

Dstb

Fault

Status monitoring

TraSta

Transitional state

 

 

 

Priority monitoring

SftyActv

Safey priority Active

 

 

 

CritActv

Critical active

 

 

 

PgmActv

Program active

 

 

 

PrPrio

Present priority

 

 

 

Function

Inputs

Description

AI_RED

AO_RED

BI_RED

BO_RED

MI_RED

MO_RED

Stop transmission of input signal

OoServ

Out of service

 

 

 

 

 

 

DefVal

Default value

 

 

 

Priority mechanism

PrioArr

Priority array

 

 

 

Local override

Ovrr

Override

 

 

 

OvrrVal

Override value

 

 

 

Alarm value monitoring

- Limits

- Reference values

- Monitoring periods

EnAlm

Alarm enable

 

 

 

 

 

 

HiLm

Upper limit

 

 

 

 

 

 

LoLm

Lower limit

 

 

 

 

 

 

Nz

Neutral zone

 

 

 

 

 

 

RefVal(s)

Reference value

 

 

 

 

 

 

TiMonOn

Monit. time switch-on

 

 

 

 

 

 

TiMonOff

Monit. time switch-off

 

 

 

 

 

 

TiMonDvn

Monit. time deviation

 

 

 

 

 

 

Switching delays

DlyOn

Switch-on delay

 

 

 

 

 

 

DlyOff

Switch-off delay

 

 

 

 

 

 

TbTiDly

Table for time delay

 

 

 

 

 

 

Switching action

- Normal

- Release command

- Trigger

SwiKind

Switch kind

 

 

 

Function

Outputs

Description

AI_RED

AO_RED

BI_RED

BO_RED

MI_RED

MO_RED

Feedback monitoring

PrVal

Present value

FbVal

Feedback value

 

 

 

 

 

 

Reliability monitoring

Rlb

Reliability

Fault monitoring

Dstb

Fault

Status monitoring

TraSta

Transitional state

 

 

 

Priority monitoring

SftyActv

Safety priority Active

 

 

 

CritActv

Critical active

 

 

 

PgmActv

Program active

 

 

 

PrPrio

Present priority

 

 

 

Priority mechanism

Within the Desigo PX system, the BACnet priority mechanism is used for the I/O output blocks and in the value blocks. This priority mechanism provides a series of prioritized levels at which intervention is possible, for use with the control functions in HVAC plant and the associated components.

The following priority levels are available with blocks AO, BO, MO (and blocks AO_RED, BO_RED, MO_RED) and AVAL, BVAL and MVAL.

Level

Application

Description

Safety level

Life safety

The safety level is assigned the highest priority and is used for the protection of people and equipment. This is where local safety switches and emergency OFF buttons are wired or superimposed commanded, e.g., smoke extraction control or frost control.

Plant safety

Operator level

Local manual intervention

The operator level is where components are overridden manually. Here the operator unit may be used to force the output of an I/O function block. This operation overrides all operations at a lower priority level.

Superposed manual intervention

Automatic level

Local control

The automatic level is used for local control functions and for superposed BACnet commanding.

General BACnet commanding

The following figure illustrates the structure of [PrioArr] and the influence of local and higher-level control.