Solid state threshold detector with hysteresis

• US 4,065,721 A
• Filed: 11/15/1976
• Issued: 12/27/1977
• Est. Priority Date: 11/15/1976
• Status: Expired due to Term

• Current Assignee: General Motors Corporation (General Motors Company)
• Original Assignee: General Motors Corporation (General Motors Company)
• Inventors: Rabe, Paul R.
• Primary Examiner(s): Heyman, John S.
• Application Number:
  US05/741,729
• Time in Patent Office: 407 Days
• Field of Search: 328/3, 328/4, 328/138, 328/150, 328/151, 328/1, 328/127, 328/140, 328/141, 307/265, 307/271, 307/358
• US Class Current: 327/509
• CPC Class Codes: G01R 19/16585 for individual pulses, ripple or noise and other applications where timing or duration is of importance G01R19/16519, G01R19/16538 and G01R19/16595 take precedence; for pulse duration and rise time, see G01R29/02 and subgroups

Abstract

An apparatus for providing an output digital signal when the magnitude of a monitored parameter is within a critical range. An oscillator repeatedly charges each of first and second capacitors to a predetermined voltage level. A monitoring resistor is parallel coupled with the first capacitor and has a resistance progressively changing with the magnitude of the parameter to define a rate of discharge of the first capacitor, between repeated chargings, determined by the parameter. A reference resistor is parallel coupled with the second capacitor and has a resistance equal to the resistance of the monitoring resistor when the magnitude of the monitored parameter is at the threshold of the critical range and defines a first reference rate of discharge of the second capacitor between repeated chargings. The voltages across the first and second capacitors are monitored by respective threshold detectors, each of which provides an output when the voltage across the respective capacitor exceeds a specified level. The outputs of the threshold detectors are coupled to a logic circuit which compares the duration of the outputs of the threshold detectors and generates an output when the duration of the output of the level detector monitoring the first capacitor varies from the time duration of the output of the level detector monitoring the second capacitor in a sense representing a critical magnitude of the condition. A circuit is responsive to the output of the logic circuit to couple a third resistor in parallel with the reference resistor to define a second reference rate of discharge of the second capacitor between repeated chargings so as to provide a hysteresis in the output signal from the logic circuit in response to variations of the monitored parameter.

Claims

1. A threshold detector circuit having a setpoint with hysteresis for providing an output representing the critical magnitude of a condition, the circuit comprising:

a monitoring circuit having a first reactance element and a first resistor, at least one of which varies in accordance with the magnitude of the condition to define a monitoring circuit time constant determined by the magnitude of the condition, the monitoring circuit time constant varying in a predetermined sense from a critical time constant when the magnitude of the condition is critical;

a reference circuit having a second reactance element and a second resistor defining a first reference circuit time constant equal to the critical time constant;

means effective to repeatedly set a predetermined voltage across the first and second reactance elements in unison, the voltage across the first and second reactance elements varying after each repeated setting at rates determined by the respective time constants of the monitoring and reference circuits;

first level detector means responsive to the voltage across the first reactance element effective to produce a first series of digital pulses, each digital pulse in the first series having a duration related to the monitoring circuit time constant;

second level detector means responsive to the voltage across the second reactor effective to produce a second series of digital pulses, each digital pulse in the second series having a duration related to the reference circuit time constant and which represents the threshold detector setpoint;

output circuit means coupled to the first level detector means and the second level detector means effective to initiate a digital output signal representing a critical magnitude of the condition when the duration of the digital pulses in the first series differs from the duration of the digital pulses in the second series in a sense representing the monitoring circuit time constant varying in the predetermined sense from the critical time constant and terminating the digital output signal when the duration of the digital pulses in the first series again equals the duration of the digital pulses in the second series;

a third resistor; and

means responsive to the digital output signal effective to couple the third resistor in parallel with the second resistor for the duration of the digital output signal, the second reactance element and the second and third resistors defining a second reference circuit time constant during the period of the digital output signal that is different from the critical time constant in a sense opposite the predetermined sense, the duration of the digital pulses in the second series being varied upon the provision of the digital output signal so as to provide for hysteresis in the threshold detector setpoint.

2. A threshold detector circuit having a setpoint with hysteresis for providing an output representing the critical magnitude of a condition, the circuit comprising:

a monitoring circuit having a first reactance element and a first resistor, at least one of which varies in accordance with the magnitude of the condition to define a monitoring circuit time constant determined by the magnitude of the condition, the monitoring circuit time constant being greater than a critical time constant when the magnitude of the condition is critical;

a reference circuit having a second reactance element and a second resistor defining a first reference circuit time constant equal to the critical time constant;

means effective to repeatedly set a predetermined voltage across the first and second reactance elements in unison, the voltage across the first and second reactance elements varying after each repeated setting at rates determined by the respective time constants of the monitoring and reference circuits;

first level detector means responsive to the voltage across the first reactance element effective to produce a first series of digital pulses, each digital pulse in the first series having a duration related to the monitoring circuit time constant;

second level detector means responsive to the voltage across the second reactance element effective to produce a second series of digital pulses, each digital pulse in the second series having a duration related to the reference circuit time constant and which represents the threshold detector setpoint;

output circuit means coupled to the first level detector means and the second level detector means effective to initiate a digital output signal representing a critical magnitude of the condition when the duration of the digital pulses in the first series becomes greater than the duration of the digital pulses in the second series representing the monitoring circuit time constant being greater than the critical time constant and terminating the digital output signal when the duration of the digital pulses in the first series again equals the duration of the digital pulses in the second series;

a third resistor; and

means responsive to the digital output signal effective to couple the third resistor in parallel with the second resistor for the duration of the digital output signal, the second reactance element and the second and third resistors defining a second reference circuit time constant during the period of the digital output signal that is less than the critical time constant, the duration of the digital pulses in the second series being decreased upon the provision of the digital output signal so as to provide for hysteresis in the threshold detector setpoint.

3. A threshold detector circuit having a setpoint with hysteresis for providing an output representing the critical magnitude of a condition, the circuit comprising:

a monitoring circuit having a first capacitor and a first resistor parallel coupled with the capacitor, at least one of which varies in accordance with a parameter to define a monitoring circuit time constant determined by the parameter, the monitoring circuit time constant increasing from a critical time constant when the magnitude of the condition is critical, one side of the parallel coupled first capacitor and first resistor being coupled to ground reference potential;

a reference circuit having a second capacitor and a second resistor parallel coupled with the second capacitor defining a first reference circuit time constant equal to the critical time constant, one side of the parallel coupled second capacitor and second resistor being coupled to ground reference potential;

means effective to repeatedly charge the first and second capacitors in unison to a predetermined voltage relative to ground reference potential, the voltages across the first and second capacitors decreasing after each repeated setting at rates determined by the respective time constants of the monitoring and reference circuits;

first level detector means responsive to the voltage across the first capacitor effective to produce a first series of digital pulses, each digital pulse in the first series having a duration related to the monitoring circuit time constant;

second level detector means responsive to the voltage across the second capacitor effective to produce a second series of digital pulses, each digital pulse in the second series having a duration related to the reference circuit time constant and which represents the threshold detector setpoint;

output circuit means coupled to the first level detector means and the second level detector means effective to normally provide a voltage signal at its output having a magnitude equal to the predetermined voltage and to initiate a ground reference potential signal at its output representing the critical magnitude of the condition when the duration of the digital pulses in the first series becomes greater than the duration of the digital pulses in the second series representing the monitoring circuit time constant being greater than the critical time constant and reinitiating the voltage signal at its output when the duration of the digital pulses in the first series again equals the duration of the digital pulses in the second series; and

a series circuit including a diode and a third resistor coupled between the ungrounded side of the parallel coupled second resistor and second capacitor and the output of the output circuit means, the diode being poled to discharge the second capacitor through the third resistor when the output circuit means provides the ground reference potential signal, the second and third resistors and the second capacitor defining a second reference circuit time constant less than the first reference circuit time constant during the period of the ground reference potential signal from the output circuit means, the duration of the digital pulses in the second series being varied upon the provision of the ground reference potential signal so as to provide for hysteresis in the threshold detector setpoint.