XPoint Wireless Daylight Control Methods


Daylight control settings for XPoint Wireless devices differ somewhat from nLight devices. The largest difference is where the setpoints are stored and evaluated. With nLight devices the photocell itself has setpoints and calculates what light level control devices should go to. With XPW devices, photocells only send out light level readings - each controller can be programmed with setpoints and control methods to respond to the light level from a photocell.
Methods: Open vs. Closed Loop
XPW devices support two basic control methods: Open Loop and Closed Loop.
Open Loop
Open Loop control makes use of a sensor that measures available daylight, usually pointing at a window or skylight. "Open Loop" refers to the idea that dimming the electric lights up or down does not significantly effect the photocell reading and so there is little to no feedback between the controlled lights and the measured light level. Open Loop control allows a single sensor to control many lights, even if those lights are in areas that receive relatively different amounts of daylight. For example, lights near a window or under a skylight can be programmed to dim more aggressively to measured light level than lights farther from the daylight source. Open Loop control requires a "calibration" step to determine what the response to a specific photocell location should be.
Closed Loop Control
Closed Loop control makes use of a sensor that measures the amount of light being delivered to the workspace. "Closed Loop" referes to the idea that the sensor measures the amount of light the work area receives, including the electric lights being controlled. As daylight increases and the photocell reading increases, the electric lights can be dimmed - which in turn will lower the reading at the photocell. Closed Loop control requires a photocell that is "seeing" the space controlled by the lights being controlled. Closed loop control can often be automatically configured by comparing the photocell reading with the electric light on and off and using that as the target level for control.
Continuous vs. Step control
XPW Daylight Control Methods
No daylight response.
Closed Loop Continuous
This method is analogous to nLight daylight control with dimming. The Set Point Dim level drives the output of the controller. When the light level is at our below the set point, the lights will be at 100%. As the level rises above the set point, the electric light will be reduced based on the Sunlight Discount Factor (SDF). Note that in release 2.5.0, SDF is fixed at 4. This means that lights will be reduced to the minimum output when the photocell reading reaches 4 times the setpoint with the lights off. The algorithm accounts for the amount of electric light in the photocell reading based on the output level of the light at the time of the photocell reading.
Closed Loop- 1 Step
This method is analogous to nLight daylight control without dimming. Rather then continuously dimming the electric lights to maintain a specified level, lights are switched off or set to a fixed dimming level once the setpoint (plus a deadband of 20%) is reached.
Open Loop Continuous
This method makes use of three set points:

  • Set Point Bright - the level above which the lights will start to dim
  • Set Point Dim - the level above which the lights will be at their lowest level
  • Set Point Off - the level above which the lights will turn off (automatically calculated based on (2*Set Point Dim - Set Point Bright)

And two light levels:

  • Daylight Level Bright - the level the lights will be at Set Point Bright (normally 100%)
  • Daylight Level Dim - the level the lights will dim to at Set Point Dim

open loop control
Open Loop - 1 Step
Operates very much like Closed Loop - Step, except that the electric light level is not compensated for in photocell reading.

Open Loop Control
Open Loop - 2 Step
Operates very much like 1 - Step, but two set points and levels can be specified.
Delays and Dimming Rates
The speed at which daylight control changes light levels is determined by the Dimming Rate parameter in the Dimming section of the settings. The following table shows the times associated with each dimming rate

RateRaise delayLower delay
Slowest 15 minutes 5 minutes
Slow 5 minutes 2 minutes
Normal 2 minutes 1 minute
Fast 1 minute 30 seconds
Fastest 30 seconds 15 seconds

Daylight Switching Lockout
In order to prevent lights from repeatedly cycling between levels, including on/off, controllers have a lockout feature which prevents lights from being switched repeatedly between high and low levels (including off). When lights are switched from a low level to a higher level, they will be prevented from being switched back to the lower level for 1 hour. If that cycle occurs, the subsequent cycle will use a 2 hour delay, then a 4 hour delay. This prevents constant, abrupt switching of lights which can be annoying to occupants. This lockout does not apply to continuous dimming control, with the exception of switching lights off if that feature is enabled.
Photocell Control Demo
Enabling Photocell Control Demo, disables the daylight lockout feature. This mode facilitates testing and demonstrations by allowing the lights to repeatedly turn off in response to photocell changes.
Commissioning Fast Response
If the Operating Mode is set to Commissioning Fast Response, all daylight delays are shortened to 5 seconds. This allows convenient testing and demonstration of daylight response. This mode will automatically revert to normal after 5 minutes.

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