Shading Control with KNX
Automating blinds and curtains offers a range of benefits from making the building more comfortable to reducing energy and even time saving.
KNX actuators are referred to as blind actuators however they can also be used for all types of shading and window control including curtains, louvers, window motors, venetian and roller blinds
Most curtain and blind motors will have an integrated limit switch which stops movement when the limit of travel is reached. This can be relied on to stop movement however the KNX actuator will still need switch off a short time after. This is achieved by defining the run time of the fitting in the parameters of the actuator via ETS. Some control actuators also use current detection to determine when the fitting has reached its limit. This is a good option if the fitting is likely to be changed by the customer or if slightly more accurate positional control is required.
KNX offers a control solution for every type of curtain and blind motor however as the amount of control can vary it is important to understand the difference so you can guarantee the end result.
230V motors have a phase (live) for each direction of travel along with a common (neutral). Only one of the directions should be powered at any time which is achieved with a software interlock on two standard relays or by using a dedicated blind controller which will have a relay that provides the interlock.
Although it’s convenient to put multiple blinds on the same channel, this needs to be avoided due to the potential for feedback from one motor once the end stop is reached. This can cause each motor to feedback in a cycling loop, causing irrevocable damage.
DC motors can be smaller and quieter than the equivalent AC motor so they are particularly common in high end residential applications. To control DC motors the polarity of the DC feed to the motor is reversed. Most 24VDC KNX blind actuators will do this automatically, but require a separate Power supply to provide the 24VDC feed.
In the same way as 230v it is important to wire each blind to an individual channel ensuring there is no erroneous feedback or operation of the blinds.
Certain manufactures have a controller embedded into the motor or fitting. In this case, control is via contact closure which triggers the on-board controller to start the motor in the appropriate direction. Most of these blinds require a short pulse on an open or close input and a pulse on both to stop.
KNX blind controllers are designed to directly control the motor and so it is advisable to use standard relays with blinds or curtains that have their own on board controller. This makes it possible to close both outputs simultaneously for the stop command.
Like contact closures, some blind motors have an embedded controller that can receive the move and positional commands via a digital interface. This could be a standardised solution such as an SMI (Serial Motor Interface) or a propriety solution such as Somfy’s Digital Network. With this method, it is possible to control the blinds more accurately as the on-board controller can determine the positioning instead of guessing with the run time. Environmental conditions and individual motor performance can affect the run time of fittings, but as a digital control will overcome this, it is the only solution if accurate positioning or hem alignment is required. It is also a more cost-effective solution when multiple fittings are to be controlled simultaneously, since multiple fittings can be controlled.
Some solutions, like Somfy RTS wireless, can only be interfaced using a proprietary gateway which will have an interface such as RS485. In this case a KNX RS485 gateway can be used between the two systems.
When using blinds as part of the wider building control system it is very easy to realise real energy savings and increase the comfort of the living environment. For example if the internal temperature exceeds 25º, no one is home and the sun is shining, the blinds can be automatically closed to prevent solar gain. If someone is home the system will tilt the blinds to maintain the natural light and prevent direct sunlight.
Integrating a weather station with solar tracking will allow the blinds to protect internal surface or furniture or just close to prevent heat build up. Some such as the Theben weather station have three separate lux sensors, GPS and inertial logic to track sunlight from east to west.
The full range of shading control actuators can be found here.