Method and apparatus for calibration and controlling multiple heaters

• US 5,552,998 A
• Filed: 03/17/1995
• Issued: 09/03/1996
• Est. Priority Date: 11/05/1990
• Status: Expired due to Fees

• Current Assignee: Watlow Electric Manufacturing Company
• Original Assignee: Watlow Winona, Inc.
• Inventors: Datta, James R.
• Primary Examiner(s): Ramirez, Ellis B.
• Assistant Examiner(s): PEESO, THOMAS R
• Application Number:
  US08/406,126
• Time in Patent Office: 536 Days
• Field of Search: 364/557, 364/571.01, 364/571.04, 364/571.07, 219/662, 219/262, 219/395, 219/398, 340/825.08, 340/825.1, 324/601, 324/721
• US Class Current: 702/99
• CPC Class Codes: G05D 23/2401 using a heating element as a sensing element

Abstract

Multiple heaters are calibrated and controlled by a microcomputer, and a multiplexer for this system. The heaters are sequentially sampled and resistance calculated from voltage and current measurements. The operator enters a temperature for the calculated resistance to establish a calibrated resistance value. During run, the computer energizes and samples each heater to calculate its resistance. The calculated resistance if divided by the calibrated resistance to determine a resistance ratio. A look-up table is stored in the computer having resistance ratios for corresponding temperature. The computer compares the calculated ratio to that of the look-up table to determine actual heater temperatures. Based upon heater temperatures, the computer controls energization of the heaters until set points are achieved. Negative temperature coefficient and positive temperature coefficient transistors and semiconductors can be utilized.

Claims

1. A process of calibrating and running a multiple positive temperature coefficient electrical resistance heater system comprising:

a. energizing a first heater;

b. sampling the energized first heater to determine voltage across the first heater and current through the heater;

c. calculating the first heater resistance;

d. assigning a temperature to the calculated first heater resistance to establish a calibrated normalized resistance value;

e. repeating steps a-d for each of the heaters;

f. establishing set points for each of the heaters;

g. sampling the first heater for current and voltage;

h. calculating the resistance of said first heater;

i. dividing the first heater calculated resistance of step c by the calculated resistance determined in step h to determine a normalized resistance ratio;

j. comparing the normalized resistance ratio of step "i" to a table of normalized resistance ratios at specific temperatures for the first heater material;

k. controlling heater energization based on the comparison of step "j";

l. repeating steps a-k for each of the heaters;

m. continuously sampling each of the heaters in sequence and repeating steps g-k at each sampling; and

,

n. simultaneously measuring peak voltage Ev and selected heater current EcN for measuring heater resistance.

2. The process of claim 1 wherein the positive temperature coefficient resistance heater comprises a noble metal.

3. The process of claim 1 wherein the positive temperature coefficient heater comprises a metal alloy from the group platinum, chromium and nickel.

4. The process of claim 1 wherein the positive temperature coefficient resistance heater is a metal selected from the group copper, platinum, nickel, chromel A, B, C.

5. The process of claim 1 wherein the positive temperature coefficient material is a semiconductor selected from the group comprising silicon, germanium and gallium-arsenide.

6. A process of calibrating and running a multiple negative temperature coefficient electrical resistance heater system and comprising:

a. energizing a first heater;

b. sampling the energized first heater to determine voltage across the first heater and current through the heater;

c. calculating the first heater resistance;

d. assigning a temperature to the calculated first heater resistance to establish a calibrated normalized resistance value;

e. repeating steps a-d for each of the heaters;

f. establishing set points for each of the heaters;

g. sampling the first heater for current and voltage;

h. calculating the resistance of said first heater;

i. dividing the first heater calculated resistance of step c by the calculated resistance determined in step h to determine a normalized resistance ratio;

j. comparing the normalized resistance ratio of step "i" to a table of normalized resistance ratios at specific temperatures for the first heater material;

k. controlling heater energization based on the comparison of step "j";

l. repeating steps a-k for each of the heaters;

m. continuously sampling each of the heaters in sequence and repeating steps g-k at each sampling; and

,

n. simultaneously measuring peak voltage Ev and selected heater current EcN for measuring heater resistance.

7. The process of claim 6 wherein said negative temperature coefficient electrical resistance heaters comprise thermistor materials.

8. The process of claim 6 wherein the negative temperature coefficient electrical resistance heaters comprise semiconductor materials.

9. The process of claim 6 wherein the negative temperature coefficient electrical resistance heaters are thick film materials screened onto substrates for use as heaters.

10. The process of claim 6 wherein the negative temperature coefficient electrical resistance heaters are thin film materials placed on substrates as heaters.

11. Apparatus for calibrating and controlling operation of multiple positive temperature coefficient electrical resistance heaters having power supply means and switch means for controlling the application of power to each of said heaters comprising:

a. means for detecting current through and voltage across each of the heaters;

b. a multiplexer coupled to said means for detecting;

c. means including converter means coupled to said multiplexer;

d. a programmed microcomputer having memory means and calculating means;

e. means coupling said microcomputer to said multiplexer, converter means, and detecting means for control thereof;

f. means connecting said microcomputer to said switch means for control thereof;

g. a look up table, having data relating temperature to normalized resistance rations for the specific material of the heaters, stored in said memory means;

h. keyboard means connected to said microcomputer;

i. means for simultaneously measuring peak voltage Ev and selected heater current EcN for measuring heater resistance; and

,

j. wherein when power is applied to the system, the microcomputer through control of the switch means, detector means, multiplexer means, and converter means calculates normalized resistance ratios for each of the heaters and compares such ratios to the look-up table ratios for control of heater energization through said switch means based on said comparison.

12. The apparatus of claim 11 wherein the positive temperature coefficient electrical resistance heaters are of a type selected from the group comprising cartridge, tubular, radiant, cast-in film, cable, and silicon rubber.

13. Apparatus for calibrating and controlling operation of multiple negative temperature coefficient electrical resistance heaters having power supply means and switch means for controlling the application of power to each of said heaters comprising:

a. means for detecting current through and voltage across each of the heaters;

b. a multiplexer coupled to said means for detecting;

c. means including converter means coupled to said multiplexer;

d. a programmed microcomputer having memory means and calculating means;

e. means coupling said microcomputer to said multiplexer, converter means, and detecting means for control thereof;

f. means connecting said microcomputer to said switch means for control thereof;

g. a look up table, having data relating temperature to normalized resistance rations for the specific material of the heaters, stored in said memory means;

h. keyboard means connected to said microcomputer;

i. simultaneously measuring peak voltage Ev and selected heater current EcN for measuring heater resistance; and

,

j. wherein when power is applied to the system, the microcomputer through control of the switch means, detector means, multiplexer means, and converter means calculates normalized resistance ratios for each of the heaters and compares such ratios to the look-up table ratios for control of heater energization through said switch means based on said comparison.

14. The apparatus of claim 13 wherein the negative temperature coefficient electrical resistance heaters comprise thermistor materials.

15. The apparatus of claim 13 wherein the negative temperature coefficient electrical resistance heaters comprise semiconductor materials.

16. The apparatus of claim 13 wherein the negative temperature coefficient electrical resistance heaters are thick film materials screened onto substrates for heaters.

17. The apparatus of claim 13 wherein the negative temperature coefficient electrical resistance heaters are thin film materials put onto substrates as heaters.