Control Laws:
Terms Used:
Tmax: The goal is to not exceed this temperature inside the SGH
Tmin: The goal is keep the inside temperature above this value
RHmax: Maximum desirable inside Relative Humidity
AHins: The inside Absolute Humidity (AH)
AHouts: The outside Absolute Humidity
The control system is of the type commonly referred to as a “bang-bang” control system (there is no proportional, integral or derivative feedback used). The Control Laws are quite simple:
1. If the inside temperature (T) is above Tmax, the ventilation system shall be “on” and the heaters shall be “off”.
2. If inside T is below Tmin, the ventilation system shall be “off” and the heaters shall be “on”.
3. If inside T is between Tmax and Tmin, the heating and ventilation systems control shall be a function of RHmax:
3a. If the inside RH is below RHmax, both the heating and the ventilation systems shall be “off”.
3b. If the inside RH is above RHmax, the heating and ventilation systems control shall be a function of the difference between AHins and AHouts:
3ba. If AHins is greater than AHouts the ventilation system shall be “on” and the heaters shall be “off”.
3bb. If AHins is less than AHouts, the ventilation system shall be “off” and the heaters shall be “on”.
These control laws cause temperature and relative humidity control to be interdependent. Heating to increase T will cause RH to decrease. This is almost always desirable in a vegetable greenhouse on the Pacific Northwest (PNW) coast. Ventilating to decrease T can, in theory, cause RH to increase. In practice however, even on the high rainfall PNW coast, outside AH is almost always lower than the inside AH of a vegetable-growing hydroponic greenhouse.
The most important trade-off in the control system design is the decrease in T when the ventilation system is turned “on” to reduce inside RH. This can, and often does, reduce T to below Tmin, causing Control Law 2 to take over.
In reviewing measured data, one sees oscillations due to the above-described interdependencies. For the parameters of the Research SGH, the oscillations observed to date are considered acceptable. This is based on what is suspected to be a nearly a worst-case situation of many consecutive days of 100 percent overcast and rain.
The operating philosophy used to date is that first priority is to keep the inside RH less than 100%. (Condensation on plant leaves is a well-known cause of plant disease.) Second priority is to keep T at or above 60 deg F (for good fruit-setting of tomato plants). The simple (thermostats only) control system has trouble achieving these priorities. To date, the computer-based control system is doing better.
Terms Used:
Tmax: The goal is to not exceed this temperature inside the SGH
Tmin: The goal is keep the inside temperature above this value
RHmax: Maximum desirable inside Relative Humidity
AHins: The inside Absolute Humidity (AH)
AHouts: The outside Absolute Humidity
The control system is of the type commonly referred to as a “bang-bang” control system (there is no proportional, integral or derivative feedback used). The Control Laws are quite simple:
1. If the inside temperature (T) is above Tmax, the ventilation system shall be “on” and the heaters shall be “off”.
2. If inside T is below Tmin, the ventilation system shall be “off” and the heaters shall be “on”.
3. If inside T is between Tmax and Tmin, the heating and ventilation systems control shall be a function of RHmax:
3a. If the inside RH is below RHmax, both the heating and the ventilation systems shall be “off”.
3b. If the inside RH is above RHmax, the heating and ventilation systems control shall be a function of the difference between AHins and AHouts:
3ba. If AHins is greater than AHouts the ventilation system shall be “on” and the heaters shall be “off”.
3bb. If AHins is less than AHouts, the ventilation system shall be “off” and the heaters shall be “on”.
These control laws cause temperature and relative humidity control to be interdependent. Heating to increase T will cause RH to decrease. This is almost always desirable in a vegetable greenhouse on the Pacific Northwest (PNW) coast. Ventilating to decrease T can, in theory, cause RH to increase. In practice however, even on the high rainfall PNW coast, outside AH is almost always lower than the inside AH of a vegetable-growing hydroponic greenhouse.
The most important trade-off in the control system design is the decrease in T when the ventilation system is turned “on” to reduce inside RH. This can, and often does, reduce T to below Tmin, causing Control Law 2 to take over.
In reviewing measured data, one sees oscillations due to the above-described interdependencies. For the parameters of the Research SGH, the oscillations observed to date are considered acceptable. This is based on what is suspected to be a nearly a worst-case situation of many consecutive days of 100 percent overcast and rain.
The operating philosophy used to date is that first priority is to keep the inside RH less than 100%. (Condensation on plant leaves is a well-known cause of plant disease.) Second priority is to keep T at or above 60 deg F (for good fruit-setting of tomato plants). The simple (thermostats only) control system has trouble achieving these priorities. To date, the computer-based control system is doing better.
posted by Larry Nash at 5:14 PM
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