Heating and cooling with a dead band

On Mars it is freezing in the morning and too warm in the greenhouse at noon. A room then needs heating and cooling. If both run at the same time, they work against each other and eat power without anything changing. A dead band prevents that.

Four limits, one target#

Between 20 and 22 °C lies the dead band: neither device switches on there. Whoever switches the heater on never gets near the cooling limit and vice versa.

Setting it centrally#

Four separate defines quickly become inconsistent – whoever changes the target forgets a limit. Better: one target, two offsets, and the limits are computed once before the main loop. A define cannot compute with another define, so they end up in registers:

define TARGET 294.15   # 21 C
define BAND 1          # K: half the dead band
define HYST 1          # K: hysteresis per side
alias heatOff r4
alias heatOn r5
alias coolOff r6
alias coolOn r7
sub heatOff TARGET BAND      # 20 C
sub heatOn heatOff HYST      # 19 C
add coolOff TARGET BAND      # 22 C
add coolOn coolOff HYST      # 23 C

Now a single number moves everything: define TARGET 296.15 turns a 21 °C room into a 23 °C one.

Two hystereses#

The heater gets the pattern with slt, the cooler the one with sgt:

slt tmp temp heatOff
and heat heat tmp
slt tmp temp heatOn
or heat heat tmp
sgt tmp temp coolOff
and cool cool tmp
sgt tmp temp coolOn
or cool cool tmp

The interlock#

With properly ordered limits the two never run together. Still, a second safeguard belongs in there – for the day someone sets BAND to −3:

seqz tmp heat
and cool cool tmp            # cooler only while the heater is off