ABB Busch-priOn Series Technical Reference Manual page 15

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Continuous controller
A continuous controller has a continuously changing control value which can accept values between 0 and 100%. For the
KNX this control value signal is converted to a 1-byte value, which means that control value 0% corresponds to value "0" and
control value 100% to value "255".
Continuous controllers with a 1-byte control value, for example, can be used to activate electromotive actuating drives. They
translate the value received directly into the valve position via an installed motor. This results in optimum control.
The 1-byte control value of a continuous controller can also be sent to KNX heating actuators which convert the 1-byte signal
into a PWM size. This allows electrothermal valves to be activated. Here it is practical to limit the dynamic range since
electrothermal valves require time to open and close. This takes place via parameters "Minimum control value" or "Maximum
control value". If, for example, a maximum control value of 80% is specified, the control will always automatically send the
value 255 when the control value of 204 has been exceeded.
To protect the bus from unnecessary loads the change of the control value that is permitted to be sent to the bus can be set.
The setting is in percent. The control value sent, unless it has changed, is specified by means of a cycle time. The cycle time
selected should not be too small (e.g. every 10 min.).
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PWM controller
The PWM controller has the same continuous control as the continuous controller. The difference is that with a PWM
controller the 1-byte control value (0...255) is converted into an On/OFF switching relationship (0 and 1). If, for example, a
control value of 70% is to be issued, at a pre-set cycle time of 10 minutes the switch-on time will be 7 minutes and the
switch-off time 3 minutes.
This transfers the advantages of the continuous control (control at the desired setpoint, no overshooting) to drives which are
designed only for On/Off switching signals, such as electrothermal drives.
To optimise the controlling characteristics of the heating/cooling system, the "PWM control value cycle time" can be set. To
set a practical cycle time, the type of heating or cooling as well as the actuating drive used should be taken into
consideration. The following recommendations can be used:
• Electrothermal actuating drive
To fully open an electrothermal control valve takes approximately 2-3 minutes. That is why a cycle time of less than 15
minutes is not practical.
• Floor heating
The time constant of floor heating is rather large. That is why a cycle time of 20 minutes is sufficient.
• Hot water heating
Her electrothermal drives are generally used. A cycle time of 15 minutes will produce excellent control results.
• Electro-convector heating
Cycle times of between 10 and 15 minutes are recommended, depending on the electric heating system and the spatial
circumstances.
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Fan coil
With the selection of fan coil for "Control types" the control value output takes place in the same way as described under
Continuous control.
With 'fan coil' there is the additional option of activating fan stages via a 1-byte or three 1-bit communication objects.
The added connection of the fan stages heats or cools the room correspondingly faster.
Which fan stage is to be active at which control value is specified on a separate tab "Fan coil heating" or "Fan coil cooling".
Here it should be ensured that threshold value stage 1 must always be smaller than threshold value stage 2, which in turn
must be smaller than threshold value stage 3.
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KNX Technical Reference Manual | 0073-1-7388 | 15