Multiplex heating system with temperature control

A multiplex heating system for measuring and regulating the temperature of a plurality of heating elements is disclosed. The heating system includes a plurality of heating elements, which also function as temperature sensing elements, that exhibit a known change in resistance as a function of temperature. The heating system further includes a power source, a control apparatus for regulating the respective heating power of each heating element, a measurement apparatus for measuring changes in resistance in the heating elements, and a plurality of switching arrangements. Each switching arrangement is coupled between one of the heating elements and, alternately, the control apparatus and the measurement apparatus, connecting the heating element to either the control apparatus or the measurement apparatus.

BRIEF DESCRIPTION OF THE INVENTION 
The present invention relates generally to a heating system which 
incorporates heating elements that also function as temperature sensors. 
More particularly, it relates to a heating system in which several heating 
elements are regulated by a central control unit. 
BACKGROUND OF THE INVENTION 
Conventional heating systems regulate the temperature of the heating 
element using two separate devices. A temperature sensor measures the 
temperature of the heating element, sending a signal to a temperature 
controller. The temperature controller evaluates the signal to determine 
when to supply power to the heating element, keeping the temperature 
within a predetermined range. Compensation wires connect the temperature 
signal to the temperature controller. 
In a conventional heating system, one temperature sensor with a 
corresponding compensation wire is needed for each heating element. If the 
system contains several heating elements, it will require several 
temperature sensors and compensation wires. While the conventional system 
sufficiently regulates the temperature of the heating elements, it is 
quite bulky. As a result, the number of possible applications for the 
system is limited. 
The actual temperature of a heating element is not uniform throughout the 
device. The temperature sensor detects the temperature of that portion of 
the element to which it is attached. The measured temperature will vary 
with the positioning of the temperature sensors. The measured temperature 
also deviates from the actual temperature even at that single location 
because of a propagation delay in the interface between gas, liquid, and 
solid. Some situations require precise temperature measurements to closely 
regulate a heating system. 
U.S. Pat. No. 3,789,190 issued to D. Orosy et al on Jan. 29, 1974 discloses 
an electrical heating device which incorporates heating elements that also 
function as temperature sensors. The Orosy heating element uses materials 
in which the resistance changes linearly with respect to a change in 
temperature. One heating element serves as the fourth leg of a Wheatstone 
bridge. A change in temperature causes a change in resistance which 
results in a bridge imbalance. The system uses the bridge imbalance to 
regulate the power supply to the heating element. This regulation is 
accomplished by turning the power supply to the heating elements on and 
off for variable periods of time dependent upon the temperature of the 
heating element. The Orosy temperature control system offers more 
precision when regulating a single heating element than the conventional 
heating system. 
Another prior art heating systems controls the temperature of the heating 
elements by connecting or disconnecting the power supply once the measured 
temperature leaves the desired range. When the temperature drops below the 
lower limit, the power supply is connected to the heating element, causing 
the temperature to increase. When the measured temperature reaches the 
upper limit, the power supply is disconnected and the temperature 
gradually decreases. As a result, the temperature oscillates back and 
forth between the upper and lower limit, spending little time at a single 
desired temperature. A heating system which regulates a heating element by 
adjusting the power supply level would have better control over the actual 
temperature of the element. Instead of supplying full power over variable 
time periods as in the prior art, the system could supply power in 
increments, thus preventing the extreme oscillation of the conventional 
systems. 
Certain conditions require a heating system which regulates several heating 
elements with a central control unit. In such multiple element heating 
systems, the use of heating elements which function as temperature sensors 
is less bulky and more accurate than the conventional system. Such a 
heating system which controls several heating elements with a central 
control unit is even more accurate since each element can be regulated 
with respect to the others. A central control unit eliminates the need for 
a controller for each heating element, reducing the number of parts needed 
for the system. This further reduces the overall bulk of the heating 
system. A central control also improves the reliability of the system. 
OBJECTS AND SUMMARY OF THE INVENTION 
The general object of the present invention is to provide a heating system 
to regulate heating elements, wherein the heating elements also function 
as temperature sensing elements. 
A further object of the present invention is to provide a heating system in 
which a central controller regulates a plurality of heating elements. 
A further object of the present invention is to provide a heating system 
which continuously varies the amount of power supplied to the heating 
element. 
A particular object of the present invention is to provide a real time 
multiplex heating system wherein the temperature is maintained by 
plurality of heating elements which are regulated by a central controller. 
The foregoing and other objects are achieved by coupling a microcomputer to 
thyristors, switching means, and a programmable peripheral interface. The 
switching means connects the heating element to a bridge circuit for 
temperature measurement, and then reconnects the element to the thyristor. 
An analog to digital converter converts the amplified voltage signal from 
the bridge circuit into a digital signal. The digital signal is sent to 
the microcomputer via the programmable peripheral interface. The 
microcomputer determines the actual temperature from the digital signal, 
compares it to a preset value, and converts the difference into a number 
of timing control pulses. The thyristor firing delay angle, which controls 
the power level, is determined by the number of timing control pulses.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, a heating system 10 of the present invention is 
depicted. Heating system 10 includes a microcomputer 100 and a plurality 
of heating elements 108 (from 108-1 to 108-N). Microcomputer 100 is the 
control center of heating system 10. It alternately samples the 
temperatures of each heating element 108 at predetermined time intervals. 
The sampling time depends on the response time of the system. A plurality 
of switching means 107-1 to 107-N are each coupled to their respective 
heating elements 108-1 to 108-N. 
When sampling one of the heating elements, for instance 108-N, 
microcomputer 100 actuates the corresponding switching means 107-N. 
Switching means 107-N then separates the heating power from heating 
element 108-N and connects heating element 108-N to a bridge circuit 109. 
Heating element 108-N becomes the fourth leg of a Wheatstone bridge 
circuit 109, as shown in FIG. 2. The other heating elements 108 remain 
connected to heating power source 20 (shown in FIG. 2). Switching means 
107 may be a relay. Heating elements 108 are constructed from a material 
in which the change in resistance as a function of the change in 
temperature is known. In the preferred embodiment, the resistance of the 
material varies linearly with the change in temperature. The change in 
resistance causes a change in voltage of bridge circuit 109. Since the 
relationship between changes in temperature and resistance is known, the 
temperature of heating element 108 can be easily obtained. 
In a preferred embodiment, heating system 10 utilizes an alternating 
current (A. C.) power source 20 during the heating procedure and a direct 
current (D. C.) power source 30 during the temperature measuring 
procedure. Connecting heating element 108 to bridge circuit 109 generates 
unwanted resistance transients from switching means 107 and heating 
element 108. To avoid interference from these transients, a short 
predetermined time passes before the voltage difference signal of bridge 
circuit 109 passes to an amplifier 110 which is coupled to bridge circuit 
109. The length of the time delay depends on the properties of switching 
means 107 and heating element 108. Generally, the delay is in the range of 
tens of milliseconds. 
Amplifier 110 amplifies the voltage difference signal of bridge circuit 109 
to facilitate processing and prevent interference. An analog-to-digital 
(A/D) converter 111 which is coupled to amplifier 110 receives the 
amplified analog signal and converts it into a corresponding digital 
signal. Switching means 107 then connects its respective heating element 
108 to the heating power source 20 as adjusted by its respective phase 
control circuit 106. 
Those skilled in the art will realize that the corresponding digital signal 
of A/D converter 111 is not the actual temperature value. In the present 
embodiment, the digital signal passes through a programmable peripheral 
interface 112 to microcomputer 100 where it may, if desired, then be 
converted to the actual temperature value. A preset value 113 which 
represents the desired temperature level of the heating element has been 
selected and preset by the user. Microcomputer 100 calculates the 
difference between preset value 113 and the actual temperature value, and 
then converts the difference into a number of timing control pulses. The 
number of timing control pulses may be obtained through use of software or 
circuitry. This number is sent to a decoder 104. Decoder 104 is coupled to 
a plurality of counters 105-1 to 105-N, each of which is connected to its 
respective phase control circuit 106. Counter 105 counts the timing 
control pulses and, dependent upon the count, sends a control pulse to 
phase control circuit 106. Phase control circuit 106 then adjusts the 
amount of power supplied to heating element 108, thus reducing the 
difference between the actual temperature value and preset value 113. 
Referring now to FIG. 2, the detailed operation of the control circuits 
used in the preferred embodiment will be described with respect to only 
one heating element 108. A zerocrossing signal circuit 102 is coupled to a 
programmable peripheral interface 103 as shown in FIG. 1. Zero-crossing 
signal circuit 102, which includes two photo couplers 211A, 211B, 
generates the interruption and timing signals. Photo couplers 211 may be 
photo transistors. Since the connection relationship of the circuit 102 is 
clearly depicted in FIG. 2, further description is deemed unnecessary. 
When the input A.C. voltage is not at the zero level, the output of one 
photo coupler 211A is low while the output of the other photo coupler 211B 
is high. Thus, the output of NAND gate 212 is high. When the input A.C. 
voltage is at the zero level, both photo couplers 211A, 211B have high 
outputs. Thus, the output of NAND gate 212 is low. For example, if the 
frequency of the A.C. power source 20 is 60 Hz, zero crossing signal 
circuit 102 can generate one hundred and twenty zero crossing signals per 
second. 
As mentioned above, microcomputer 100 generates a number of timing control 
pulses from the difference between preset value 113 and the actual 
temperature value. The number of pulses determines the firing delay angle 
of a thyristor or triac 215. After measuring the temperature of heating 
element 108, microcomputer 1? 0 sends the number of timing pulses together 
with the address of the particular heating element measured to decoder 
104. Decoder 104 identifies which of heating elements 108 is to be 
adjusted by the timing control pulses and sends the pulses to its 
respective counter 105. Counter 105 counts the number of timing pulses and 
sends a control pulse signal to phase control circuit 106 dependent upon 
that number of pulses. The control pulse signal turns on a transistor 213 
of phase control circuit 106 which activates a pulse transformer 214. 
Pulse transformer 214 sends a firing pulse to fire triac 215 at a firing 
delay angle which is determined by the firing pulse. 
The power supplied to the system is regulated by controlling the firing 
delay angle. For example, the available heating power varies from 0% to 
100%, and the firing delay angle varies from 0.degree. to 180.degree.. 
When the firing delay angle is 0.degree., the power output is at 100%. 
When the firing delay angle equals 180.degree., the heating power is at 
0%. The relationship between the heating power and the firing delay is not 
linear. The microcomputer 100 calculates the amount of heating power which 
should be supplied to heating element 108 so as to bring the actual 
temperature value closer to the preset value 113 and using the 
relationship between the heating power and firing delay angle, determines 
the number of timing pulses. This allows the present invention, except for 
the temperature sampling time, to continuously control the temperature of 
the system providing power to the heating element during every cycle of 
the alternating current electrical power source. Instead of switching the 
power on during some cycles of the power source and off during other 
cycles, the system varies the amount of power supplied by changing the 
firing delay angle in response to any change of temperature in the heating 
element. 
Referring to FIG. 3, a flow chart of the control program that is stored and 
run in microcomputer 100 is depicted. The first step in using heating 
system of the present invention is presetting the desired temperature 
levels (procedure 301) and sampling times (procedure 302) of heating 
elements 108 into microcomputer 100. Different heating elements have 
different sampling times. To enable the heating system to count time, zero 
crossing signal circuit 102 generates interruption signals and timing 
signals (procedure 303). When the sampling time of one of the heating 
elements 108 is reached (procedure 304), the microcomputer calls the 
switching and analog to digital converting subroutines into play 
(procedure 305). Switching means 107 connects heating element 108 to 
bridge circuit 109. The voltage difference signal of bridge circuit 109 is 
detected and sent to amplifier 110, and the amplified signal is then sent 
to A/D converter 111. Once the signal has passed through A/D converter 
111, switching means 107 reconnects heating element 108 with the heating 
circuit. Microcomputer 100 corrects the signal after it has passed through 
A/D converter 111 to obtain the measured temperature value (procedure 
306). The corrected measured temperature value is then compared to preset 
value 113. Proportional control means 101 converts the difference between 
the two values into a number of timing control pulses (procedure 307). 
Counter 105 sends a control pulse signal to fire the thyristor, thus 
regulating the temperature of the heating element (procedure 308). 
The heating system of the present invention contains the multiplex, time 
division, and temperature proportional control means as described above. 
The information of the heating element temperature may be displayed on a 
monitor connected to the microcomputer (not shown in the drawings). This 
information may also be stored in a memory file. The response speed of the 
system is faster and the measured temperature is closer to the actual 
value than in the conventional heating system. The present invention can 
be applied to a variety of uses, including hair dryers used in salons, 
hand-driers, various heating devices used in the home, electric chafing 
dishes, electric solder irons, thermostatic tanks, and heating furnaces. 
While the invention has been described in terms of what is presently 
considered to be the most practical and preferred embodiments, it is to be 
understood that the invention need not be limited to the disclosed 
embodiments. On the contrary, it is intended to cover various 
modifications and similar arrangements included within the spirit and 
scope of the appended claims, the scope of which should be accorded the 
broadest interpretation so as to encompass all such modifications and 
similar structures.