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 |
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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 |
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Stop transmission of input signal | OoServ | Out of service | • | • | • | • | • | • | • | • | • |
Priority mechanism | DefVal | Default value |
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| PrioArr | Priority array |
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Local override | Ovrr | Override |
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OvrrVal | Override value |
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Alarm value monitoring - Limits - Reference values - Monitoring periods | EnAlm | Alarm enable | • | • | • | • | • | • | • | • | • |
HiLm | Upper limit | • | • | • |
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LoLm | Lower limit | • | • | • |
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Nz | Neutral zone | • | • | • |
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RefVal(s) | Reference value | • |
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TiMonOn | Monit. time switch-on |
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TiMonOff | Monit. time switch-off |
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TiMonDvn | Monit. time deviation | • | • | • | • | • | • | • | • | • | |
Switching delays | DlyOn | Switch-on delay |
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DlyOff | Switch-off delay |
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TbTiDly | Time delay table |
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Switching action Normal (motor) Release command Trigger Switch Switch with delay | SwiKind | Switch kind |
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Function | Outputs | Description | AI | AVAL | AO | BI | BVAL | BO | MI | MVAL | MO |
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Feedback monitoring | PrVal | Present value | • | • | • | • | • | • | • | • | • |
FbVal | Feedback value |
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Reliability monitoring | Rlb | Reliability | • | • | • | • | • | • | • | • | • |
Fault monitoring | Dstb | Fault | • | • | • | • | • | • | • | • | • |
Status monitoring | TraSta | Transitional state |
| • | • |
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Priority monitoring | SftyActv | Safey priority Active |
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CritActv | Critical active |
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PgmActv | Program active |
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PrPrio | Present priority |
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Function | Inputs | Description | AI_RED | AO_RED | BI_RED | BO_RED | MI_RED | MO_RED |
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Stop transmission of input signal | OoServ | Out of service |
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DefVal | Default value |
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Priority mechanism | PrioArr | Priority array |
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Local override | Ovrr | Override |
| • |
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OvrrVal | Override value |
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Alarm value monitoring - Limits - Reference values - Monitoring periods | EnAlm | Alarm enable |
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HiLm | Upper limit |
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LoLm | Lower limit |
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Nz | Neutral zone |
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RefVal(s) | Reference value |
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TiMonOn | Monit. time switch-on |
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TiMonOff | Monit. time switch-off |
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TiMonDvn | Monit. time deviation |
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Switching delays | DlyOn | Switch-on delay |
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DlyOff | Switch-off delay |
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TbTiDly | Table for time delay |
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Switching action - Normal - Release command - Trigger | SwiKind | Switch kind |
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Function | Outputs | Description | AI_RED | AO_RED | BI_RED | BO_RED | MI_RED | MO_RED |
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Feedback monitoring | PrVal | Present value | • | • | • | • | • | • |
FbVal | Feedback value |
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Reliability monitoring | Rlb | Reliability | • | • | • | • | • | • |
Fault monitoring | Dstb | Fault | • | • | • | • | • | • |
Status monitoring | TraSta | Transitional state |
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Priority monitoring | SftyActv | Safety priority Active |
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CritActv | Critical active |
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PgmActv | Program active |
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PrPrio | Present priority |
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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 |
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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.