Using its two outputs (one that controls heating, the other one cooling), ETH-2 is able to perform basic air conditioning using meteorological data obtained from its inputs. For each of the two channels, operator can define up to four conditions that cause its activation. Each “condition” consists of the chosen measurement channel (temperature, humidity of dew point) and one of its previously defined limits (absolute or relative). If at least one of the four conditions belonging to a particular output channel is satisfied, that output activates.

Please note that both output channels can be active at the same time as they are not necessarily meant to control chilling and heating of air inside the dome. Rather, while chilling is usually applied to the inside air, heating output often serves to turn on primary mirror heating with the aim of preventing dew formation on its reflective surface. In case that chilling and heating should not be active simultaneously, a simple AND-OR type hardware logic is expected to be added by the dome air conditioning system designer to one of the ETH-2 outputs in order to prevent its activation when the other output chanel is on.

All that has been said up untill now might sound a bit complicated, but this example should help in clarifying things up. Let’s imagine that raw temperature channels t5 - t8 are combined into a “CT8“ composite temperature channel which measures average primary mirror temperature, t1 and t3 measure air temperature at two heights inside the dome, while t2 and t4 measure temperature of the air outside. Also, let’s imagine that rH1 measures relative air humidity inside the dome, while rH2 that of air outside of it. Consequently, dp1 and dp2 are calculated dew points for air inside and outside the dome (this setup is schematically depicted in the diagram under the previous tab). Finally, let’s declare “FT” the forecasted temperature for the following night.

Problem

Now let’s imagine that the telescope operator wants to control the heat pump so that air temperature inside the dome is:

In addition, let’s imagine that operator also wants to control the primary mirror heater so that mirror temperature is:

Solution

The first step in finding the solution is picking key measurement channels and defining their limit thresholds such that they will provide defining adequate actuation conditions for control functions:

After that, the operator sets states of both air conditioning control outputs to “active” and dedicates appropriate conditions to each of them (4 former conditions to the chilling-, and 3 latter conditions to the heating output channel). This is represented schematically in the diagram below:

In addition to this, operator is able to define several more limit conditions, but they will not be taken into account by ETH-2 firmware control functions when deciding whether to activate its two control outputs. Those limits will instead trigger ELM-4 Central controller module audio and visual alarms (with the help of AS2 Communicator software), and will be able to influence activation of additional AS2 Communicator software control functions that are able to perform advanced air conditioning of the dome using the additional output hardware, but will not influence the operation of the two ETH-2 control outputs directly.

We remind the reader that ELM-4 data bus specification and AS2 Communicator PC application software permit easy integration of up to 15 input-output controllers into the system, each of them with 8 input and 8 output channels that can be used to control various equipment around. Similarly, if a single ETH-2 device per dome is insufficient, up to 15 of them can be connected to the same ELM-4 system.