When controlling the brightness of luminaires we have two general ways to proceed, with or without a control loop. Both ways have advantages and disadvantages, so there is no general solution for every scenario available. But with the examples below we can manage about 90% of ocurring scenarios.

1 Control loop

In a loop the luminance output of the luminaires is taken into account and can offer a more precise output value. Especially the aging of the luminaires as well as other light sources like sunlight are considered. Here the sensor monitors the light output as well as the disturbance, the light from outside. But in large installations or with high ceilings, dark ground and walls or multiple individually controlled groups these systems might start oscillating or just do not work. Then refer to the next major section "2 Value Tracking".

1.1 Measure Compare Adjust (PID)

This is the easiest way of bringing daylight harvesting to work.

First create a "Measure Compare Adjust (PID)" block.

Second set the fixtures to the level of luminance, that is needed in the scenario.

Then check the sensor value. This is the target value, you will have to set in the PID block.

After that connect the sensor on the Input side and on the output side connect the opacity of the sequence or the luminance of the DALI group.  

Now you are done and your control in the loop is up and running.

(1) The Sensor value
(2) The PID block with the target value, set
(3) The DALI group that is controlled

If you have more than one scenario, like several steps of luminance, then use the "Target Value" as port instead of property. Just change the target value to the appropriate value, the PID will do the rest.

When the control loop is too slow or too quick, you can change the settings. In the expert mode you can directly set proportional, integral and derivative gain of the control loop.

2 Value Tracking

When indoor sensors do not provide reliable and wide ranged values, or more than one control loop would be needed and those would have an impact on each other, then we just track the outside sensor value and calculate the luminance value for the fixtures.

The outside luminance can be recorded by an external outside sensor.

The control is done in the workflow. When trying to control the brightness of DMX luminaires, the Math block offers a way to map values. The DALI Ballast Group block can directly take a formula as a dynamic scene.

The biggest disadvantage is, that aging of the luminaires is not considered, here.

The easiest mapping is the linear mapping. Here are two examples, how you can do this.

2.1 Luminance Tracking in general

You as a customer request for a formula or "the formula". When we think of a linear system, then the following formula might be a good approach:

Formula: target = yMax - ( x - xMin ) * ( yMax - yMin ) / ( xMax - xMin) + yMin
yMax: Target max
yMin: Target min
xMax: Sensor max*
xMin: Sensor min*
x: act. Sensor value
*) The sensor values are measured under the real conditions.

xMax - xMin is the number of steps, where we can control the target. This range should be as large as possible.
If this value range is too small, you cannot control the luminance in an effective way. Then try to use a different sensor with higher sensitivity, use more sensors, or place them somewhere else, where lighting conditions are better.

When you insert real values you will see, that you can simplify this formula. You can also recognize, where problems may arise when the interval is too small or too big.

Two examples:

(1) and (2) are sensor values.

(3) and (4): The formula is used in the Math formula block.

(5) There is an additional Sensor Average block, that will calm rapid changes. (6) The Fade Time should be set accordingly.

(7) There is an additional formula to clamp between a min and max value. Of course you could put the formulas together in one Math Block, but here you can see better what happens.

2.2 Luminance Tracking for DALI

Here is another solution:

The following formula is used:
((( xOutMin - xOutMax ) / 100 ) * xSensor ) + xOutMax

You will have to paste the formula below, because only x<number> is allowed as a variable.
((( x0- x1) / 100 ) * x2) + x1

xSensor: sensor input value range 0..100
xOutMin: min Light 0..100
xOutMax:max Light

(1) The sensor value

(2) In order to start the value tracking, set the sequence first.

(3) The DALI Group block. Now the formula is set here. You see the properties and port.

(5) When clicking on the cog wheel icon, the configuration dialog opens.

(4) In the Properties view you can see, how the variables are configured.

The Ballast should have a Fade Time of several seconds, otherwise it could start to oscillate bright and dark, when the sky is cloudy. The attached Template  61 Sensor Average V5.0.slwt could reduce this effect, by approaching slower to a target value.

Conclusion

The ideas above should give you a good and easy start in lighting control and daylight harvesting.

Of course not all questions can be answered here. On one hand the internet will offer more information when searching for PID control or control in the loop and on the other hand our staff might help regarding problems with daylight harvesting. Then raise your questions to support@ecue.com.