Stable control system with positive feedback characteristic

• US 5,220,156 A
• Filed: 07/23/1991
• Issued: 06/15/1993
• Est. Priority Date: 07/23/1991
• Status: Expired due to Term

• Current Assignee: Cox USA
• Original Assignee: Cox USA
• Inventors: Schuyler, Martin
• Primary Examiner(s): Walberg, Teresa J.
• Application Number:
  US07/734,496
• Time in Patent Office: 693 Days
• Field of Search: 219/497, 219/491, 219/501, 219/492
• US Class Current: 219/497
• CPC Class Codes: G05D 23/1909 whose output amplitude can only take two discrete values
• CPC Class Codes: G05D 23/1913 delivering a series of pulses
• CPC Class Codes: G05D 23/24 the sensing element having a resistance varying with temperature, e.g. a thermistor

Abstract

A control system and circuit has a positive feedback characteristic but is nonetheless stable. Stability is achieved by superimposing a negative feedback loop with a short time constant and a positive feedback loop with a long time constant. The system and circuit are useful in situations where the sensor and the controlled variable (e.g. a heater) are well-coupled.

Claims

1. A stable control circuit, comprising:

an input;

an output;

a negative feedback loop connecting the input and output and having a first time constant; and

a positive feedback loop connecting the input and output and having a second time constant, the second time constant being longer than the first time constant.

2. A control system for regulating a load in accordance with the electrical characteristics of a sensor which is well-coupled to the load, comprising:

an input to which a sensor is connected;

an output to which a load is connected;

a negative feedback loop connecting the input and output and having a first time constant; and

a positive feedback loop connecting the input and output and having a second time constant, the second time constant being longer than the first time constant.

3. An improvement to a negative feedback control system which regulates a load in accordance with the electrical characteristics of a sensor using a negative feedback loop with a fixed time constant, the improvement compensating the system for coupling between the sensor and load and comprising a positive feedback loop having a time constant which is longer than the time constant of the negative feedback loop.

4. A proportional control circuit comprising:

a temperature-sensing network having an output voltage which varies with temperature;

an operational amplifier having an output and inverting and non-inverting inputs, said network being connected between said inverting and non-inverting inputs;

an electrical heater connected between said output and ground; and

a positive feedback loop connecting said output and said non-inverting input.

5. The circuit of claim 4, wherein the positive feedback loop directs, to the non-inverting input, a voltage which is proportional to average power through the heater.

6. The circuit of claim 5, wherein the positive feedback loop comprises:

a series RC circuit connected in parallel with the heater and having a first resistor which is connected to said output and a capacitor which is connected to ground, the first resistor being connected to the capacitor at a junction; and

a second resistor connected between said junction and said non-inverting input.