Digital temperature sensing conditioning and safety system with current control

• US 5,705,792 A
• Filed: 03/18/1996
• Issued: 01/06/1998
• Est. Priority Date: 03/18/1996
• Status: Expired due to Fees

• Current Assignee: Robertshaw Controls Company (Schneider Electric SE)
• Original Assignee: Robertshaw Controls Company (Schneider Electric SE)
• Inventors: Graham, Donald W.
• Primary Examiner(s): Paschall, Mark H.
• Application Number:
  US08/617,172
• Time in Patent Office: 659 Days
• Field of Search: 219/492, 219/413, 219/497, 219/411, 219/501, 219/412, 219/499, 219/505, 219/504, 307/117, 307/119
• US Class Current: 219/497
• CPC Class Codes: F24C 15/022 Latches
• CPC Class Codes: G01K 7/21 for modifying the output characteristic, e.g. linearising

Abstract

A temperature sensing system that includes a temperature probe, a signal conditioning circuit, and a microprocessor. The conditioning circuit incorporates a novel dual voltage range measurement technique that improves the accuracy of such systems. The safety circuitry is provided in order to protect against thermal runaway and to inhibit or enable a door lock.

Claims

1. A temperature sensing system, comprising:

a signal conditioning circuit connected to a temperature sensing probe;

wherein said signal conditioning circuit creates a first signal to control a heater device in response to an input from said temperature sensing probe based on an external variable temperature measurement;

a safety circuit connected to said signal conditioning circuit;

wherein said safety circuit creates a second signal that operates to disable an oven door lock device above a first maximum temperature level and creates a third signal that operates to disable an oven heater device above a second maximum temperature level; and

,

a microprocessor connected to said signal conditioning circuit and said safety circuit.

2. A temperature sensing system according to claim 1, wherein said signal conditioning circuit further comprises:

a first voltage source;

a first operational amplifier stage connected to said first voltage source;

wherein said first operational amplifier stage is a voltage-to-current converter;

wherein said temperature sensing probe further comprises a resistance temperature detector;

wherein said first operational amplifier stage converts the resistance change of said resistance temperature detector into a proportional first voltage signal;

a second operational amplifier stage connected to said first operational amplifier stage and a second voltage source;

wherein said second amplifier stage is a difference amplifier that compares said second voltage source to a second voltage signal to create said first signal;

wherein said second voltage signal is created by dual resistor network voltage divider circuits which include a first switch device to select one circuit at a time and divide said second voltage source.

3. A temperature sensing system according to claim 2, wherein said safety circuit further comprises:

a third operational amplifier stage connected to said first voltage source;

wherein said third operational amplifier stage is a comparator that compares said first voltage source to a third voltage signal to create said second signal;

wherein said third voltage signal is created by dual resistor network voltage divider circuits which divide said first voltage signal; and

a fourth operational amplifier stage connected to said first voltage source;

wherein said fourth operational amplifier stage is a comparator that compares said first voltage source to a fourth voltage signal to create said third signal;

wherein said fourth voltage is created by dual resistor network voltage divider circuits which include a second switch device to select one circuit at a time and divide said first voltage signal.

4. A signal conditioning circuit, comprising:

a voltage source;

a first operational amplifier stage connected to said voltage source;

wherein said first operational amplifier stage is a voltage-to-current converter;

a temperature sensing probe connected to said first operational amplifier stage;

wherein said temperature sensing probe further comprises a resistance temperature detector that is located in a feedback loop of said first operational amplifier stage;

wherein said first operational amplifier stage operates to convert a resistance change of said resistance temperature detector into a proportional first voltage signal; and

a second operational amplifier stage connected to said first operational amplifier stage and a second voltage signal;

wherein said second operational amplifier stage is a difference amplifier that compares said second voltage signal to said proportional first voltage signal to create an output signal to control an oven heater device;

wherein said second voltage signal is created by a second voltage source and resistor network voltage divider circuits which include a switch device to divide said second voltage source.

5. A safety circuit, comprising:

a first operational amplifier stage connected to a first voltage source;

wherein said first operational amplifier stage is a comparator that compares said first voltage source to a first voltage signal to create a first digital signal to control an oven door lock device;

wherein said first voltage signal is created by a resistor network voltage divider circuit which divides a second voltage source; and

a second operational amplifier stage connected to said first voltage source;

wherein said second operational amplifier stage is a comparator that compares said first voltage source to a second voltage signal to create a second digital signal to control an oven temperature limiting device;

wherein said second voltage signal is created by dual resistor network voltage divider circuits which include a switch device to select one circuit at a time and divide said second voltage signal.

6. A method for converting an input temperature signal into output voltage signals, the method comprising:

receiving an input temperature signal;

applying said input temperature signal to a temperature sensing probe to create a proportional resistance value in said probe;

applying said resistance value to a first operational amplifier stage that is a voltage-to-current converter to convert said resistance value into a proportional first voltage signal;

applying a first voltage source to two resistor network voltage divider circuits which include a first switch device to select one circuit at a time and divide said voltage source signal to create a second voltage signal;

applying said first voltage signal to a second operational amplifier that is a difference amplifier to convert said first voltage signal into a third voltage signal by comparing said first voltage signal to said second voltage signal;

processing said third voltage signal into a digital signal to control an oven heater device;

applying said first voltage signal to a resistor network to create a fourth voltage signal;

applying said first voltage source to a third operational amplifier stage that compares said first voltage source with said fourth voltage signal to create a fifth voltage signal;

processing said fifth voltage signal into a digital signal to control an oven door lock device;

applying said first voltage signal to dual resistor voltage divider circuits which include a second switch device to select one circuit at a time and divide said first voltage signal to create a sixth voltage signal;

applying said first voltage source to a fourth operational amplifier stage that compares said first voltage source with said sixth voltage signal to create a seventh voltage signal; and

processing said seventh voltage signal into a digital signal to control an oven heater over-temperature limit device.

7. A temperature sensing system, comprising:

a temperature sensor, wherein said temperature sensor means is a resistance temperature detector;

a first operational amplifier connected to said temperature sensor and a first voltage source, wherein said first operational amplifier includes a voltage-to-current converter with an output proportional to the temperature sensor output;

a second operational amplifier connected to said first operational amplifier, wherein said second operational amplifier converts said first operational amplifier output into a third voltage signal by comparing said first operational amplifier output to a second voltage signal;

a dual resistor network voltage divider circuit connected to said second operational amplifier, said divider circuit including a first switch for selecting one circuit at a time and dividing said first voltage source to create said second voltage signal;

a third operational amplifier connected to said first voltage source;

wherein said third operational amplifier compares said first voltage source with a fourth voltage signal to create a fifth voltage signal;

wherein said third operational amplifier is connected to a dual resistor network voltage divider circuit for processing said first voltage signal into said fourth voltage signal; and

a fourth operational amplifier connected to said first voltage source;

wherein said fourth operational amplifier compares said first voltage source with a sixth voltage signal to create a seventh voltage signal;

wherein said fourth operational amplifier is connected to a dual resistor voltage divider circuit which includes a second switch for selecting one circuit at a time and dividing said first voltage signal to create said sixth voltage signal; and

a processor connected to said second, third and fourth operational amplifier;

wherein said processor converts said third voltage signal into a first digital signal to control an oven heater and converts said fourth voltage signal into a second digital signal to control an oven door lock, and converts said seventh voltage signal into a third digital signal to control an oven heater over-temperature limit.