Optimal start programmer

An optimal start programmer for starting a heating or cooling cycle for an intermittently occupied building includes a digital counter which is enabled at the start of a pre-occupancy "search" period and accumulates counts from a timing source. A digital to analog converter is responsive to the cumulative output count of the counter circuit for generating a staircase or ramp signal which provides a time-variable set point signal during the search period. In the heating mode, the reference signal to the digital to analog converter is a signal representative of the outdoor ambient temperature; and in the cooling mode the reference is a constant signal. The operating mode of the system during the search period is determined by a mode selection circuit which is responsive to a heating mass signal and a cooling mass signal to automatically determine whether the system is to operate in a heating mode, a cooling mode or an inhibit mode in which neither the boilers nor the chillers are actuated.

BACKGROUND AND SUMMARY 
The present invention relates to apparatus for programming the operation of 
heating, cooling and ventilating systems for buildings, particularly 
buildings which are intermittently occupied, such as office buildings, 
stores, shopping centers, and other commercial buildings. With the 
increased cost of energy, there have been great incentives to reduce the 
amount of energy used in heating and cooling buildings by way of 
scheduling lower temperatures in the buildings when they are not occupied, 
sometimes referred to as "night setback systems". 
One of the problems encountered in programming building temperatures is 
that the building must be brought to the desired temperature by the time 
the occupants arrive at the building for work. If, for example, during the 
winter the night setback temperature is 55.degree. F. and the building is 
scheduled for occupancy at 9:00 A.M., the system controller cannot wait 
until 9:00 A.M. before turning on the heating system because of the 
substantial time lag between the time the heaters may be turned on and the 
time the building will reach the desired temperature of occupancy. The 
same is true when the building is air conditioned and the outside air 
temperature exceeds the desired temperature of occupancy. 
Many systems and techniques have been suggested for controlling system 
operation during the pre-occupancy period in which the interior of the 
building and its contents are brought to the desired temperature. One 
system for achieving the desired temperature at occupancy is referred to 
as an "optimal start programmer". One such system, referred to as the 
C-7501 Optimal Start Programmer is sold by Johnson Controls, Inc., of 
Milwaukee, Wis. This system uses an outdoor sensor for sensing outdoor air 
temperature, an indoor sensor for sensing indoor space temperature, and a 
motor-driven potentiometer to generate a simulated "set point" signal 
which increases from the night setback temperature to the desired 
occupancy temperature in a predetermined manner. 
The length of the preheat period (that is, the period just prior to 
occupancy during which the simulated set point signal is generated) is 
determined by the outdoor temperature, as well as other variables, such as 
the heating mass of the building, the capacity of the heating plant, etc. 
In general, however, the preheat period is lengthened for colder outdoor 
air temperatures. When the set point signal becomes equal to or greater 
than the indoor space temperature, a "plant start" signal is generated to 
command the heating equipment to begin operation. Thus, the higher the 
indoor temperature during the preheat period and the higher the outdoor 
air temperature, the shorter will be the preheat period and the greater 
the savings in energy. 
In these systems which generate a set point signal mechanically, the 
maximum preheat period is determined by the ratio of a gear box. In order 
to change the maximum time for the preheat period, a major modification of 
the programmer is required. Earlier systems which used cam-driven 
potentiometers to generate the set point signal used the profile of the 
cam to determine the maximum preheat period. 
Because of the long periods required for preheating of buildings (eight 
hours or longer for a maximum preheat time is not unusual), conventional 
analog electronic circuit techniques are not used to generate the set 
point signal because of the prohibitive size of components and lack of 
accuracy. Further, prior art systems were not able to accommodate both 
warm up and cool down of the indoor temperature during the pre-occupancy 
period during which the energy source is actuated, without manually 
changing a mode switch on the programmer. 
The present invention is thus directed to an optimal start programmer in 
which a set point signal is generated electronically, and preferably using 
an oscillator circuit which may be synchronized with a line frequency 
signal and having its output coupled to a counter circuit. A standard 
timer and controller, which preferably is a controller oriented processor 
or COP, as it is referred to in the art, also receives the output of the 
oscillator as a clock signal. The timer generates time and calendar data 
for determining occupancy periods, non-work days, and the pre-occupancy 
period during which the temperature of the space is brought from the night 
setback temperature to the desired temperature at occupancy. The present 
system is adapted to accommodate both heating and air conditioning, and 
includes an automatic changeover circuit, (called a mode selection 
circuit) for determining the operating mode as a function of the mass 
temperature. For this reason, the preoccupancy period is referred to as a 
"search" period to indicate that it is generic to both the heating and 
cooling modes. 
The digital counter is enabled at the start of the search period by the 
timer and controller circuit, and it accumulates counts from the 
oscillator or timing source. 
The output signals of the counter are fed in parallel to a digital to 
analog converter which generates a staircase or digital ramp signal which 
is used as a time-variable set point signal during the search period. 
The mode control circuit actually defines three modes of operation during 
the search period. This circuit is responsive to a heating mass signal and 
a cooling mass signal. Each of these signals, theoretically, is 
representative of the temperature of the indoor space being controlled, 
although they may have different readings because of their different 
locations. They are referred to as temperature "mass" signals because in a 
dynamic situation, they represent not only the temperature of the space, 
but the temperature of the mass associated with the space which has an 
effect on the time required to bring the space to a desired occupancy 
temperature. Although a single mass temperature signal may be used, in 
some applications it is preferable to use both a heating mass signal and a 
cooling mass signal because of the desirability of locating the two 
sensors in different places. 
If the heating mass signal indicates a temperature less than a first 
predetermined temperature (for example, 65.degree. F.), the mode control 
circuit defines a heating mode of operation during the search period. If 
the cooling mass signal indicates that the temperature of the space is 
greater than a second predetermined temperature higher than the first 
predetermined temperature (for example, 75.degree. F.), then the mode 
control circuit defines a cooling mode of operation. The temperature band 
between the first and second predetermined temperatures defines a range in 
which the mode control circuit generates an inhibit signal so that neither 
the heating source nor the cooling source are energized. 
When the system is operating in the heating mode, the reference signal to 
the digital to analog converter which generates the time-variable set 
point signal is a signal representative of the outdoor air temperature, 
derived from an exterior sensor. In this manner, the starting point of the 
time variable set point signal in the heating mode is defined by the 
outdoor temperature. If the mode control circuit specifies operation in 
the cooling mode, a fixed voltage signal is used as the reference for 
digital to analog converter. The output signal of the digital to analog 
converter is a current, which is then converted to a voltage and 
subtracted from the reference signal to generate the actual set point 
signal. The set point signal is then compared with the mass temperature 
signal and, depending upon the mode of operation, the resulting signal is 
used to energize the "boilers relay" or the "chillers relay" to energize 
the heating or cooling source respectively, as conditions require. 
Thus, the present invention employs digital circuitry to generate the time 
variable set point signal and therefore takes advantage of the greater 
reliability and repeatability of electronic circuitry, particularly in 
relation to the cam-driven and motor-driven potentiometers of prior 
systems. By using solid state circuitry exclusively, no moving parts are 
incorporated in the optimal start programmer whatever. Further, the 
provision of a mode control circuit prevents inadvertent operation in the 
wrong mode, for example, as might be caused by failure to set prior 
systems manually into the desired mode, and it further permits the system 
to respond to wide changes in outdoor temperature as they are reflected in 
the mass temperature signal without having to effect a corresponding 
manual change on the programmer. 
Other features and advantages of the present invention will be apparent to 
persons skilled in the art from the following detailed description of a 
preferred embodiment accompanied by the attached drawing wherein identical 
reference numerals will refer to like parts in the various views.

DETAILED DESCRIPTION 
Referring first to FIG. 1, reference numeral 10 designates an oscillator 
circuit for generating a clock signal at a rate of 60 hertz. The 
oscillator may be a conventional 555 timer circuit wherein the period is 
determined by the values of an external resistor and capacitor, as is 
known in the art. The oscillator circuit 10 is synchronized with a 
conventional 60 hertz line signal each cycle for as long as power is 
available. Battery power may be used as a back up in the event of a power 
outage. 
The output signal of the oscillator 10 is fed to countdown circuitry 
generally designated 12 and to a Timer and Controller circuit 13. The 
countdown circuitry includes a flip flop 14, the output of which feeds the 
input of a divider circuit 15. The divider circuit 15 counts down the 
repetition rate of the input signal by 2560. Hence, by cascading the flip 
flop circuit and the divider circuit as shown, the repetition rate of the 
oscillator circuit 10 is divided by 5120, whereas by shorting out the flip 
flop, the repetition rate of the oscillator circuit 10 is divided by 2560, 
as will be explained more fully below. 
The Timer and Controller circuit 13 may be a conventional integrated 
circuit such as the MM57160 Standard Timer and Controller produced and 
sold by National Semiconductor Corp., of Santa Clara, Calif. It is a chip 
which contains a controller oriented processor (COP) and is designed for 
use in repetitive timing applications. It has a 24 hour real time clock 
and is capable of generating up to four output control signals at four 
different set points or programmed times of the day. It is also capable of 
skipping certain days so that different weekend conditions may be 
implemented. A four-digit digital display 18 is associated with the Timer 
and Controller 13 for displaying time-of-day information. The time-of-day 
data is generated from the periodic signal received from the oscillator 
circuit 10. 
The Timer and Controller 13 is programmed to generate a first output signal 
on a line 19 to energize a ventilation relay 220 during the search period, 
preferably 10-15 minutes before occupancy, although this may vary 
depending on the application. A second output signal is generated on a 
line 21 to enable a counter circuit 22, also at the beginning of a search 
period. The programmed time for beginning a search period will depend upon 
whether the flip flop circuit 14 is employed or not. Assuming that the 
flip flop circuit 14 is in circuit with the divider 15, the approximate 
time of the search period is 6.06 hours. That is, the flip flop 14 divides 
the repetition rate of the oscillator 10 by two, and the divider 15 
divides it again by 2560, thereby generating an output signal to the 
counter circuit 22 every 85.3 seconds. Since the counter 22 is an eight 
bit counter, in this example, it is capable of accumulating 256 counts, 
each count representing 85.3 seconds, thereby comprising a period of 364 
minutes or 6.06 hours. If the flip flop 14 is not employed, the search 
period is approximately three hours and two minutes. 
The eight outputs of the counter 22 are fed in parallel by means of a bus 
25 to the inputs of a digital to analog converter circuit 26. In the 
illustrated embodiment, the counter 22 is a "countup" circuit but the 
invention is not so limited. That is, with suitable modification the 
system will work equally well with a "countdown" circuit. 
The output signal of the digital to analog converter 26 is a staircase or 
digital ramp which increases by a fixed increment each time the contents 
of the counter 22 is incremented. This signal is converted to a voltage 
signal by the I/E converter circuit 27, the output of which feeds the 
minus input of a summing junction generally designated 29. The reference 
voltage for the digital to analog converter 26 as well as the signal to 
the positive input of the summing junction 29 is received from a reference 
signal generator 30 which, in turn, is controlled by a Mode Selection 
Circuit 31. If the Mode Selection Circuit 31 determines that the system is 
operating in the heating mode, as will be explained further below, then 
the output signal of the block 30 is representative of the outdoor air 
temperature (OAT). On the other hand, if the system is operating in the 
cooling mode, as determined by the Mode Selection Circuit 31, the 
reference signal generated by the block 30 is a fixed voltage reference 
signal. 
Turning now to the summing junction 29, it has a fixed reference signal at 
its plus input, although obviously in the heating mode, over an eight hour 
period, when the outdoor temperature signal is used as a reference, it may 
vary. At the negative input of the summing junction is the digital ramp 
signal generated by the digital to analog converter 26. The output signal 
of the summing junction 29 is thus a signal which begins at a positive 
level and is then decremented as the output signal of the digital to 
analog converter 26 increases. The positive level at which the output 
signal starts depends upon the outdoor air signal when operating in the 
heating mode, as diagrammatically illustrated in the inset graph 
designated 31. As the outdoor air temperature increases, the output signal 
of the generator 30 (namely, the bridge circuit in which the sensor is 
connected) decreases. Thus, the characteristic curve defining the output 
signal of the summing junction 29 will, for example, change from the line 
designated 33 to that designated 34 for an increase in outdoor air 
temperature. When the system is operating in a cooling mode, the voltage 
reference is constant, and the characteristic curve relating the output 
signal of the summing junction 29 to time is defined by the graph 
generally designated 35. Thus, the output signal of the summing junction 
29 is a time variable set point signal, designated E.sub.V. That signal is 
coupled to the negative terminal of a comparator circuit 37 having its 
positive input connected to the output of a mass temperature sensor 
designated by the block 38. In the illustrated embodiment, two separate 
mass temperature sensors are used, one for heating and one for cooling. 
This permits placement of the sensors in preferred locations. For example, 
the mass sensor for heating should be located on an interior surface in an 
exterior zone of the space being heated. A wall perpendicular to an 
exterior wall or a concrete pier would be suitable places for its 
location. The cooling sensor might be located in the southwest or west 
sides of a building because they are the most likely places to be 
influenced by solar energy. 
When the mode selection circuit 31 dictates operation in the heating mode, 
the heating mass temperature signal is coupled from the block 38 to the 
comparator 37; and when the time variable set point signal falls below the 
heating mass temperature signal the comparator 37 generates an output or 
command signal to energize a relay 40 to actuate the boilers. Similarly, 
when the system is operating in a cooling mode and the set point signal 
falls below the cooling mass temperature signal, the comparator 37 
generates a command signal to energize a relay 41 to actuate the chillers. 
Turning now to FIG. 2, some of the functional blocks identified in FIG. 1 
are repeated, such as the divider circuit 15 (shown in fragmentary form in 
the right central portion of the drawing), the counter 22, digital to 
analog converter 26, and output comparator 37. In addition, the circuitry 
associated with the reference generator 30, mode selection circuit 31 and 
mass temperature signal generator 38 are shown in more detail. 
Turning first to the reference signal generator 30, a bridge circuit 45, 
referred to as the outdoor air temperature bridge includes, in one branch, 
an outdoor temperature sensor 46. This temperature sensor, as well as the 
indoor temperature sensor to be described subsequently may have a nominal 
resistance of 1000 ohms at 70.degree. F. and a positive temperature 
coefficient of approximately 3 ohms/.degree. F. Such sensors are 
commercially available. As the sensed temperature increases, the 
resistance of the sensor 46 also increases, and it is connected in circuit 
in the bridge 45 such that as the outdoor temperature increases, the 
output signal of the bridge 45 also increases. The output signal from the 
bridge circuit 45 is connected to a terminal of a potentiometer generally 
designated 47, the other fixed terminal of which is grounded. The 
potentiometer 47 is set to determine earlier or later starting, as will be 
described. The signal from the movable arm of the potentiometer 47 is 
connected to an input terminal designated H of an analog switch 50. The 
output 51 of the analog switch 50 is connected to the H input terminal 
when the system is operating in a heating mode as determined by the Mode 
Selection Circuit 31. The cooling reference signal to the switch 50 is 
derived from a potentiometer 53, the power end of which is connected to a 
fixed reference voltage. The potentiometer 53 also determines earlier or 
later settings for operating in the cooling mode, as will be described. 
The output signal of the analog switch, taken on line 51 is connected to 
an amplifier 54, the output signal of which is the reference voltage to 
the digital to analog converter 26, being coupled to it through a 
reference resistor designated R.sub.r. The output of the digital to analog 
converter 26 is taken at terminal 56 and connected to one terminal of a 
resistor R.sub.d, the other terminal of which is connected to the 
reference voltage from amplifier 54. Terminal 56 is also connected to the 
negative input of the previously described comparator 37, the input 
impedance of the comparator 37 is very much greater than the impedance of 
the resistor R.sub.d. Further, the value of the resistor R.sub.d is chosen 
to be equal to the value of the reference resistor R.sub.r. 
The digital to analog converter is an eight bit multiplying converter in 
which the output signal is a current signal. The output current is given 
by the following equation: 
EQU I.sub.Out =V.sub.Reference /R.sub.Reference .times.n/255 Eq.(1) 
where 
n=number of counts since beginning of search cycle 
V.sub.Reference =Outdoor Air Temperature Signal (Heating Mode); Constant 
Voltage (Cooling Mode). 
Assuming the output current of the digital to analog converter 26 is in the 
direction of the arrow 57, as it flows through the series dropping 
resistor R.sub.d, it generates a voltage which is subtracted from the 
reference voltage generated by the amplifier 54. Further, because of the 
relatively low impedance of the dropping resistor, the signal is converted 
to a voltage to comprise the current to voltage converter 27. Thus, the 
resulting signal taken at junction 58 is the output of the summing 
junction 29--namely, the time varying set point voltage E.sub.V ; and it 
is fed to the negative input terminal of the comparator 37. In 255 equal 
steps, the output of counter 22 reduces E.sub.V to 0 volts as follows: 
EQU E.sub.v =V.sub.Ref -I.sub.Out .times.R.sub.d Eq. (2) 
and from Eq. (1), since R.sub.d =R.sub.Ref 
EQU E.sub.v =V.sub.Ref [1-n/255] Eq. (3) 
As soon as E.sub.v drops below the mass temperature signal from switch 65, 
the command signal to start heating or cooling is given. 
The circuitry of the mass temperature signal generator 38 of FIG. 1 is 
enclosed within the dashed line 38 of FIG. 2; and it includes a mass 
heating bridge 60 in which a temperature sensor 61 is included, and a mass 
cooling bridge 62 which includes a second temperature sensor 63. As 
explained above, it is desirable to have two separate temperature sensors 
for the space being heated, one for heating and one for cooling. This 
permits the two sensors to be located more advantageously, depending upon 
the operating mode of the system. The output signal of the bridge 60 is 
fed to a terminal designated H of an analog switch 64, and the output of 
the bridge 62 is fed to an input designated C of the analog switch 64. The 
analog switch 64 also has a control input and an inhibit input designated 
I, both of which are received from the mode selection circuit 31, to be 
described presently. The output of the analog switch 64 is a signal from 
one of the mass temperature bridges 60, 62, depending upon the operating 
mode, and it is fed to the positive input of the comparator 37. 
Turning now to the Mode Selection Circuit 31, it includes a first 
comparator 65 having its negative input connected to a negative reference 
voltage by means of a resistor 66, and its positive input connected to the 
output signal of the mass heating bridge 60. It also includes a second 
comparator 68 having its positive input connected to the negative 
reference signal by means of a resistor 69, and its negative input 
connected to the output signal of the mass cooling bridge 62. The output 
signal of comparator 65 is coupled via a resistor 70 to a signal output 
junction 71. The output signal of the comparator 68 is coupled via a 
resistor 73 and a diode 74 to an inhibit output junction 75. A diode 76 is 
connected between the junctions 71 and 75 in the polarity shown. The 
junction 71 is connected directly to the control inputs of the analog 
switches 50 and 64. The junction 75 is connected directly to inhibit input 
of the analog switch 64. 
Referring now to FIGS. 2 and 3, the sensors 61 and 63 are located within 
the space being heated, as already disclosed. When the signal from the 
mass heating bridge 60 is below a first predetermined temperature 
(65.degree. F. in the illustrated embodiment), derived from the resistor 
66, the output of the comparator 65 is a logic "0" and when the signal 
from the bridge 60 exceeds the reference, the comparator 65 switches to 
generate a logic "1", see curve 78 of line L1 of FIG. 3. When the output 
signal of the mass cooling bridge 62 is below a second predetermined 
temperature (preferably higher than the first predetermined temperature 
and 75.degree. F. in the illustrated embodiment), the output signal of 
comparator 68 is a logic 1; and as the temperature rises above 75.degree. 
F., the output signal of the comparator 68 becomes a 0, as illustrated by 
the curve 79 on line L2 of FIG. 3. The signal on line L1 of FIG. 4 is 
present at junction 71 of the Mode Control Circuit 31; and the signal on 
line L2 is present at the output of comparator 68. Thus, when the interior 
temperature is below 65.degree. F., as sensed by sensor 61, the signal at 
junctions 71 and 75 is a logic 0 because the diode 74 is connected in a 
polarity to oppose the output signal of comparator 68. When the signal at 
junction 71 is a logic 0, and the signal at the input to the inhibit 
terminal of the analog switch 64 is a 0, the Mode Selection Circuit 31 
defines an operating mode of heating. When the temperature rises above 
65.degree. F., but is below 75.degree. F., as indicated by the bank 80 on 
line L3 of FIG. 3, both comparators 65 and 68 generate 1 output signals, 
so that the signal at junction 75 is a 1, and analog switch 64 is 
inhibited. This inhibits operation of the circuitry because comparator 37 
has no input signal at its positive input terminal and is therefore 
inhibited. As the temperature rises above the higher predetermined value, 
75.degree. F., the output signal of comparator 68 becomes a 0 to force the 
signal level at junction 75 to 0, thereby enabling the analog gate 64. The 
signal at junction 71 remains a logic 1. This causes the system to operate 
in a cooling mode. 
Still referring to FIG. 2, by adjusting the potentiometers 47, 53 to 
generate a greater signal (that is, towards the "later" position), the 
reference voltage V.sub.Ref generated by the circuit 30 is increased. This 
signal continues to be a function of the outdoor air temperature when the 
system is operating in the heating mode, but by increasing the reference 
signal, the time at which the set point signal E.sub.V falls below the 
mass temperature signal from the analog switch 64 is delayed. Thus, these 
potentiometers are used to independently adjust the start time of the 
boilers and chillers in the heating and cooling modes respectively, 
compare FIGS. 5 and 6 for the heating mode adjustment. 
The outdoor air temperature bridge which includes the sensor 46 has a 
second adjustment which is referred to as the "outdoor authority" 
adjustment, and it has the effect of reducing the ability of the outdoor 
temperature sensor 46 to change the output signal from the bridge 45. That 
is, the error signal from the outdoor air temperature bridge becomes a 
constant signal, compare FIG. 4 with FIG. 7, for example. 
OPERATION 
Referring now to FIG. 4, the operation of the system will be described by 
way of example. Assuming that the search period is 8.5 hours, and the flip 
flop 14 is in circuit with the divider 15, and occupancy is scheduled for 
9:00 A.M., if the outdoor air temperature is at 0.degree. F., the time 
variable set point signal is generated according to the characteristic 
curve or line 85. Assuming that the outdoor air temperature authority is 
100% and the earlier/later setting is 5 (an arbitrary number based on a 
scale of 0-10), if the mass heating temperature is 55.degree. F., the time 
variable set point signal E.sub.V will fall below the mass heating 
temperature signal at approximately 6:30 A.M., as indicated at 86. For a 
lower outdoor air temperature, the start time will be earlier, and for a 
higher outdoor air temperature it will be later. 
Referring now to FIG. 5, the effect on the characteristic operating curves 
of a setting of the earlier/later time is shown. By reducing the signal 
from the potentiometer 47, the start time is advanced, as can be seen by 
the position of the characteristic curve 85A, corresponding to the curve 
85 discussed in connection with FIG. 4. Setting an earlier start time has 
the effect of rotating the characteristic curves clockwise. 
Correspondingly, by increasing the signal from the earlier/later 
potentiometers, the characteristic curves are rotated counterclockwise as 
illustrated in FIG. 6, to thereby delay start up time (see curve 85B). 
By reducing the outdoor air temperature authority, for the same 
earlier/later setting, the effect is to cause all of the characteristic 
curves to come closer to one another. In the limit condition, for a 0% 
outdoor air temperature authority, as illustrated in FIG. 7, a single 
characteristic curve is defined as at 89, for all outdoor air 
temperatures--that is, the characteristic is independent of outdoor air 
temperature. Again, for FIG. 7, it is assumed the system is operating in 
the heating mode. 
Referring now to FIG. 8, when the system is operating in the cooling mode, 
there is no reliance on the outdoor air temperature; however, the 
potentiometer 53 does permit an adjustment of the start time of the search 
period. By increasing the signal from the potentiometer 53, the operating 
characteristic is rotated clockwise in FIG. 8 to yield a later start time 
for a given mass cooling temperature (represented on the abscissa of the 
graph of FIG. 8). 
It will thus be appreciated that the present invention provides a solid 
state circuit for generating a time variable set point signal for an 
optimal start programmer of the type used in heating and air conditioning 
of a large, intermittently occupied building. The circuit also provides 
for automatic selection of the mode of operation as a function of the 
indoor temperature, and accounts for differences in outdoor temperature 
when operating in the heating mode. Adjustments are permitted for 
weighting the effect of the outdoor air temperature in the heating mode, 
as well as for delaying or advancing the start time of the equipment. 
Having thus disclosed in detail a preferred embodiment of the invention, 
persons skilled in the art will be able to modify certain of the structure 
which has been illustrated and to substitute equivalent elements for those 
disclosed while continuing the practice the principle of the invention; 
and it, therefore, intended that all such modifications and substitutions 
be covered as they are embraced within the spirit and scope of the 
appended claims.