Working with Open Loop Constant Light

		Working with Open Loop Constant Light

What is Open Loop Constant Light Control?

Open Loop Constant Light control is a method of automating the operation of the system’s lamps. Its use can provide a great deal of convenience for the building’s occupants, but perhaps the main reason for using it is that it can provide a considerable increase in energy efficiency. It ensures that the output of the lamps is no higher than is required to maintain the desired light level.

For buildings fitted with air conditioning systems, there is an additional saving associated with reducing the need to remove waste heat.

How Open Loop Constant Light Works

The available light in a room is a mixture of ambient (daylight) and artificial light.
The External Light Sensor measures the amount of daylight.
The system is provided with a scene level for the amount of light required in the absence of daylight.
A Constant Light scene is recalled.
The Light Sensor measures the daylight and sends proportion commands that raise or lower the output level of the lamps corresponding to the Source Level Direct Proportion profile table. This process operates continuously to maintain the programd proportionality profile.

The effect of this is that the lamps should only output sufficient light to maintain the desired level of light in the room. This means that the lamps can be operating at minimum when the room is filled with sunlight. The lamp’s output will gradually increase as night falls, or the sky becomes overcast. Since the lamps will only provide the amount of light that is necessary, they will use the absolute minimum amount of energy, in order to achieve the desired light level.

Open Loop Constant Light in Operation

The art of designing a successful Open Loop Constant Light control system is to position the external sensor carefully, profile the controlled areas to understand daylight contribution under various sky conditions and to suitably match the Source Level for Direct Proportion levels.

It is essential that the light sensor measurement accurately represents the lighting conditions if predictable and consistent results are to be achieved and for this there are two critical effects that ideally should be avoided, shadowing and hot-spotting.

1) Shadowing

In the ideal a building would be isolated and removed from other structures with a sensor mounted on the roof and away from obstructions.

In practice there will be adjacent buildings and obstructions that can give strong shadows at various times of the day and during the course of the year. One solution is for the sensor to be mounted in association with the controlled area and thereby minimise compromise. In all cases careful thought to sensor siting will save programming and response compromises.

Where a single sensor is to be used with multiple controlled areas or if simply there are practical restrictions to mounting location and height then compromises will be inevitable and these must be understood and accommodated. Typically the control system could be programd to ignore light changes at certain times of the day or month, etc.

When complex shadowing cannot be avoided it can be beneficial to locate the sensor in permanent shadow such that it measures the skylight only. This will give a consistent measurement but will not reflect the local effects of strong sunlight interspersed with shadow. Here more careful consideration is required to The Source Level for Direct Proportion table values.

2) Hot-spotting

Hot-spotting is a consequence of an unwanted reflection giving additional light to the sensor. Predicting a problem can be difficult as for example the effect of opening a nearby window can easily be missed.

There are important practical factors to be aware of:

A) Setting the Source Level for Direct Proportion

The external sensor functions on an open loop principle, and has no means of knowing either the capabilities of the lamps or the effect of modifying their output. This means that it is possible to set the source level correctly for one type of light whilst being totally inappropriate for another. Correct profiling, that is understanding the effect of modifying the artificial light level for the controlled area with respect to the contributing daylight is essential in achieving the intended operation.

B) Interacting with other controls

If there are methods of controlling the daylight locally, such as blinds, then this must be factored into the design strategy. For example, if the blinds are controlled as part of the lighting control system then conditions can be applied to the Open Loop Constant Light routing entry to effect a change in response.

C) Setting Specific Levels

The light level measured by the external light sensor is the LUX value at its location and can not be calibrated to correspond to a specific LUX level produced by the lighting system.

Attaining specific LUX levels for your installation requires accurate profiling to determine the appropriate Direct Proportion reduction in the artificial lighting in response to available daylight. The contributing daylight of the controlled area relative to the external LUX level and thereby the supplementary lighting, if any, required from the artificial lighting needs to be measured and the results used to create the Source Level for Proportion Control profile table.

One method you can obtain an approximate setting is as follows:

1. Place a LUX meter in the working plane to be controlled.

2. Adjust the lamps until the LUX meter shows the desired value.

3. Record the Current Light Level reported by the external sensor.

4. Compare the % light against the default Direct Proportion for the daylight and intended starting constant light scene.

5. Modify, if necessary, the Source Level for the Direct Proportion needed then scale other source levels accordingly.

Note:

Accuracy will improve with the greater number of variations in daylight measured.
Scaling using the typical LUX value against sky condition, see External Light Sensor, provides a useful starting point profile. This assumes pro-rata daylight contribution with changes in sky condition that may not be the case with low sun angles.
Although this method will provide an approximation of the desired level, it is dependent on the exact conditions at the time of profiling. You may not be able to repeat the measurement unless you are able to exactly reproduce the conditions that existed at the time the setting was made.
If lamps are already at full level, and the LUX level is still not achieved, then, obviously, you will not be able to achieve your desired LUX level. This is why it is good to know your system capabilities.

Exploiting Scaling with Multiple Output Groups

Open Loop Constant Light operation may require the use of two groups. The first of these, known as the Input Group (Light Sensor), contains the controllers that are to be used to set up and control open loop constant light operation. The second, the Output Group (Controlled Loads), contains the LIUs (Lamp Interface Units, or loads) for the lamps that are to be subject to open loop constant light control.

Tip:

In the simple case, with a sensor controlling Constant Light in only one group of loads, the Input Group and Output Group may be the same. For more complex situations, where more than one Output Group is required, the Input Group and Output Groups must be different..

The same external light sensor (Input Group) can be used to control as many output groups as desired each simply requiring a separate routing entry.

A useful feature for numerous areas serviced by the same windows is to group the lighting in sections corresponding to the distance from the windows. This allows scaling from the window group(s) to the more distant groups but with the same Source Level profile.

Recall of an Open Loop Constant Light scene in the Input Group results in that scene being called in the Output Group, and the Scaling of the Source Level for Direct Proportion being applied. After a settling period, the lights in the Output Group are automatically adjusted.