[PWM_CTL] Pulse width modulation control

PWM_CTL calculates the flow temperature setpoint corrections for heating or cooling cycles and controls the circulating pump accordingly. The duration of operation cycles can be set in the range of 1–24 hours. For 24-hour cycles, the pump switch-on time can be specified in the block configuration.

PWM_CTL provides the present operating mode [PrOpMod], calculated setpoint corrections [SpcHTFl] and [SpcCTFl], and the values for command [Cmd] and purge [Prg].

Present operating mode [PrOpMod]

[PrOpMod] is calculated at the start of every cycle based on the values at the inputs [EnH] and [EnC]. If [EnH] and [EnC] change during an ongoing cycle, their new values are considered at the beginning of the next cycle.

If [EnH] or [EnC] change during a cycle, the change is considered in the next cycle.

Calculating setpoint corrections

PWM_CTL calculates corrections for the given flow temperature setpoints [SpHTFl] and [SpcTFl]. With the higher or lower flow temperature, heating or cooling energy is introduced faster into the heating or cooling system.

PWM_CTL does not supply new flow temperature setpoints, only setpoint corrections [SpcHTFl] and [SpcCTFl].

The calculation of the setpoint corrections is done based on [PrOpMod].

[PrOpMod] = Heating

In the heating operation, energy is supplied with a higher flow temperature setpoint. The correction for the heating setpoint is provided at the output [SpcHTFl].

The setpoint correction is limited by:

[SpcHTFl] <= [SpcMax]

[SpcHTFl] <= [TFlMax] - [SpHTFl]

Key

[PrOpMod] = Heating/Cooling

In the heating/cooling operation it is not defined whether energy must be supplied or discharged. In such cases, the given flow temperature setpoint range [SpHTFl]...[SpcTFl] is narrowed down to [DSpHC] (neutral zone). The corrections for heating and cooling setpoints are provided at outputs [SpcHTFl] and [SpcCTFl].

Key

[PrOpMod] = Cooling

In the cooling operation, energy is discharged by a lower flow temperature setpoint. The correction for cooling setpoint is provided at [SpcCTFl] output.

Setpoint correction is limited by:

[SpcCTFl] <= [SpcMax]

[SpcCTFl] <= [SpcCTFl] - [TFlMin]

Key

[PrOpMod] = Off

No operation: PWM_CTL neither controls nor enables the recirculating pump ([Cmd] = 0 and [Prg] = 0).

[PrOpMod] = Transient

Disabled functionality: Calculation is not performed because function block is disabled or measurement for flow temperature is reported as not valid or PWM configuration for the corresponding operating mode is not provided ([Cmd] = 1 and [Prg] = 0).

Operation steps

Within a given operating mode, the function block performs the control in cycles. Each cycle contains the following steps.

Example for heating cycles

Example 1

The time period [TiPrdH] always starts with purge [Prg]. After expiration of the minimum purge time [TiPrgMin], a flow temperature [TFl] is measured below that for heating in continuous mode [SpHTFl]. Heating [Cmd] with setpoint correction [SpcHTFl] begins. Heating ends when the switch-off criteria of PWM_CTL is met. There is no change until the next period.

Example 2

The time period [TiPrdH] always starts with purge [Prg]. After expiration of the minimum purge time [TiPrgMin], a flow temperature [TFl] is measured over that for heating in continuous mode [SpHTFl]. Purge is extended until the flow temperature is below [SpHTFl]. Heating [Cmd] with setpoint correction [SpcHTFl] begins. Heating ends when the switch-off criteria of PWM_CTL are met. There is no change until the next period.

Example 3

The time period [TiPrdH] always starts with purge [Prg]. After expiration of the minimum purge time [TiPrgMin], a flow temperature [TFl] is measured below that for heating in continuous mode [SpHTFl]. Heating [Cmd] with setpoint correction [SpcHTFl] begins. During heating, the flow temperature for heating at cycle [SpHTFl] + [SpcHTFl] is not reached. Heating is extended until the switch-off criterion of the function is met. There is no change until the next period.

Time period configuration

Value 0 for time period configuration ([TiPrdH], [TiPrdC], [TiPrdHC]) can be used to disable the corresponding operating mode. If [EnH] and [EnC] request such disabled operating mode, PWM_CTL sets the outputs with the following values:

• [PrOpMod] = Transient
• [Cmd] = 1
• [Prg] = 0
• [SpcHTFl] = 0.0
• [SpcCTFl] = 0.0
• [TiOnEst] = 0.0

Time period configurations greater than 0 are limited in the range: 1h...24h.

When a time period is configured with value 24h, values at inputs [Prd24Mod], [SttTiOn], [CntrTiOn] can be used to configure the starting time (as time of day) for the corresponding cycle.

Thermal resistance ratio [RthRatio]

[RthRatio] denotes the ratio between two thermal resistances:

• Thermal resistance from flow media to heat buffer of heat emission or absorption system
• Thermal resistance of flow media to room air

For integrated TABS (concrete core activation), the following parameters are decisive:

• Pipe configuration (pipe distance TABS register)
• Water volume flow in the pipes of the TABS registers
• Structure of floor and ceiling construction

The following resistance ratios apply to concrete core activation:

Lower resistance ratios, e.g., [RthRatio] < 35%, generally result in shorter switch-on phases in cycles, high resistance ratios in longer switch-on phases.

Example

A narrow pipe configuration (15 cm) and a high water volume flow (15 kg/(m2 h)) have lower thermal resistance to the heat emission. A carpet and a blank ceiling tend to have less thermal resistance. Together, this results in [RthRatio] = 30%.

Maximum setpoint correction [SpcMax]

In heating or cooling cycles, [SpcMax] strongly influences the cycle ratio [TiOnEst]. [SpcMax] on the one hand must be as low as possible to avoid great differences of the flow temperature setpoint between cycle and continuous operation. On the other hand, [SpcMax] must be selected as high as possible to achieve short switch-on times of the recirculating pump. For TABS, guide values are between 1 and 3 K.

Special case
A very high [SpcMax] may be selected, e.g., 10 K, for cooling using only cooling tower operation, to take full advantage of maximum cooling potential (up to and including [TFlMin]).

Delta flow setpoint heating/cooling

In heating or cooling cycles, the flow temperature setpoint band [DSpHC] (neutral zone for heating/cooling) must be as small as possible to achieve short switch-on times. However, [DSpHC] must not be too small to avoid unnecessary changeover between heating and cooling. For TABS, guide values are between 0.1 and 1.0 K.

Minimum purge time

The minimum purge time is selected according to the water circulation period, e.g., longer than a complete water cycle.