Controlling Blinds with KNX
When it comes to controlling blinds and curtains there are a few different approaches depending on the motor type used.
230V AC Motors
The most common type, 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 can be 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.
Most curtains and blinds will have an integrated limit switch which stops movement when it reaches the limit of travel. This can be relied on to stop movement, however the feed should be isolated a short time after. This is achieved by defining the run time of the fitting in the parameters of the actuator. Alternatively, blind control actuators are available which 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.
230V motors are the simplest and often the cheapest, so they are the most common. They are also able to have a high torque output and so will be required on larger fittings or where the fabric is heavy.
24V DC Motors
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 24V DC KNX blind actuators will do this automatically but require a separate power supply to provide the 24V DC feed.