Smart sensor module

• US 6,032,109 A
• Filed: 10/21/1997
• Issued: 02/29/2000
• Est. Priority Date: 10/21/1996
• Status: Expired due to Term

• Current Assignee: Telemonitor Incorporated
• Original Assignee: Telemonitor Incorporated
• Inventors: Ritmiller, III, George R.
• Primary Examiner(s): ASSOUAD, PATRICK J
• Application Number:
  US08/949,284
• Time in Patent Office: 861 Days
• Field of Search: 702/85, 702/87, 702/99, 702/100, 702/104, 702/107, 702/1.01
• US Class Current: 702/104
• CPC Class Codes: G01D 3/022 having an ideal characteristic, map or correction data stored in a digital memory
• CPC Class Codes: G01D 3/0365 the undesired influence being measured using a separate sensor, which produces an influence related signal

Abstract

A smart sensor module includes a microcontroller and a signal conditioning integrated circuit. In a preferred embodiment, the signal conditioning integrated circuit is a mixed signal application specific integrated circuit (ASIC) incorporating an instrumentation amplifier, offset correction, sensitivity correction, low pass filtering, an analog-to-digital converter (ADC), a temperature sensor, digital I/O interface, a crystal oscillator circuit, and power on reset open collector output. The ADC can be multiplexed to the transducer, the temperature sensor or an external input. All features of the ASIC can be dynamically reconfigured at any time by the microcontroller. Offset correction is implemented with a 4-bit coarse adjustment digital-to-analog converter (DAC) and a 7-bit fine adjustment DAC to control the instrumentation amplifier reference voltage. The coarse adjustment allows compensation of sensor-to-sensor offset variations, while the fine adjustment allows dynamic or real-time compensation of temperature-induced variations. The sensitivity correction is accomplished with a programmable system gain configured as a coarse gain adjustment and a fine gain adjustment. The coarse gain adjustment allows compensation of sensor-to-sensor sensitivity variations. The fine gain adjustment allows dynamic or real-time compensation of temperature-induced sensitivity variations. The smart sensor module allows very precise calibration of the sensor, signal processing capabilities, decision making capability and serial interface capability.

Claims

1. A smart sensor module for compensation of sensor-to-sensor and temperature-induced errors in the analog output of a sensor comprising

a temperature sensor providing an electrical output related to the temperature of the sensor;

fine gain and offset correction storage for storing fine gain and offset corrections for a plurality of different temperatures and, responsive to said temperature sensor output, for providing a fine gain correction output and a fine offset correction output;

coarse gain and offset correction storage for storing coarse gain and offset corrections and providing a coarse gain correction output and a coarse offset correction output;

an analog amplifier, having a DC offset responsive to said coarse and fine offset correction outputs, for amplifying the analog sensor output and providing an amplified analog sensor output;

a first low pass filter, having a filter gain responsive to said coarse gain correction output, for low pass filtering the amplified analog sensor output and providing a coarse gain corrected analog sensor output; and

a second low pass filter, having a filter gain responsive to said fine gain correction output, for low pass filtering the coarse gain corrected analog sensor output and providing a fully conditioned analog sensor output.

10. A smart sensor module as recited in claim 1 and further comprising a multiplexer having first and second input channels receiving said fully conditioned analog sensor output and said temperature sensor output, respectively, and an output channel providing an analog multiplexer output.

11. A smart sensor module as recited in claim 10 wherein said multiplexer further includes a third input channel receiving said coarse gain corrected analog sensor output.

12. A smart sensor module as recited in claim 11 wherein said multiplexer further includes a fourth input channel receiving an output from an external analog source.

13. A smart sensor module as recited in claim 10 and further comprising an analog output multiplexer with an input channel receiving at least one of said coarse and fine offset correction outputs, said coarse gain corrected analog sensor output, said fully conditioned analog sensor output, and said analog multiplexer output.

14. A smart sensor module as recited in claim 10 and further comprising an analog-to-digital converter receiving said analog multiplexer output and providing a digital output.

15. A smart sensor module as recited in claim 14 and further comprising a microcontroller for receiving said digital output from said analog-to-digital converter, retrieving said fine gain and offset corrections from storage based on said temperature sensor output, and transferring said fine gain and offset corrections to said second low pass filter and said analog amplifier, respectively.

16. A smart sensor module as recited in claim 15 wherein said microcontroller includes program memory and said fine gain and offset corrections and said coarse gain and offset corrections are stored in said program memory.

17. A smart sensor module as recited in claim 15 and further comprising an electrically erasable programmable read only memory, wherein said fine gain and offset corrections and said coarse gain and offset corrections are stored in said electrically erasable programmable read only memory.

2. A signal conditioning circuit for use with a microcontroller as a smart interface for an analog output sensor comprising

coarse offset adjustment means, responsive to digital input from the microcontroller, for providing an analog coarse offset correction output to correct for offset errors due to sensor-to-sensor variations;

fine offset adjustment means, responsive to digital input from the microcontroller, for providing an analog fine offset correction output to correct for offset errors due to temperature-induced variations;

an amplifier stage, having a DC offset responsive to said fine and coarse offset correction outputs, for amplifying the analog sensor output and providing an amplified analog sensor output;

coarse gain adjustment means, responsive to digital input from the microcontroller, for providing an analog coarse gain correction output to correct for sensor-to-sensor variations;

a first low pass filter stage, having a filter gain responsive to said coarse gain correction output, for filtering the amplified analog sensor output and providing a coarse gain corrected analog sensor output;

fine gain adjustment means, responsive to digital input from the microcontroller, for providing an analog fine gain correction output to correct for temperature-induced variations; and

a second low pass filter stage, having a filter gain responsive to said fine gain correction output, for filtering the coarse gain corrected analog sensor output and providing a fully compensated analog sensor output.

18. A signal conditioning circuit as recited in claim 2 wherein said amplifier stage includes an input stage and an output stage, and wherein said coarse and fine offset corrections are used to define an adjustment voltage applied to an input of said amplifier output stage.

19. A signal conditioning circuit as recited in claim 2 wherein said first low pass filter stage includes a continuous time anti-aliasing filter having a selective corner frequency and a resistive feedback circuit for controlling filter gain.

20. A signal conditioning circuit as recited in claim 2 wherein said second low pass filter includes a switched capacitor filter having a capacitive array, said switched capacitor filter having a cutoff frequency controlled by varying a switched capacitor filter clock frequency and a filter gain controlled by adjusting relative values of capacitance in said capacitive array.

3. A method of compensating for sensor-to-sensor and temperature-induced errors in an analog sensor output using a microcontroller and signal conditioning analog circuitry comprising the steps of

taking a temperature sample;

retrieving fine gain and offset correction outputs from storage based upon the temperature sample;

retrieving coarse gain and offset correction outputs from storage;

amplifying the analog sensor output in an amplifier stage of the signal conditioning analog circuitry having a DC offset responsive to the coarse and fine offset outputs retrieved from storage and providing an amplified analog sensor output;

filtering the amplified analog sensor output in a first low pass filter stage of the signal conditioning analog circuitry having a filter gain responsive to the coarse gain correction output retrieved from storage and providing a coarse gain corrected analog sensor output; and

filtering the coarse gain corrected analog sensor output in a second low pass filter stage of the signal conditioning analog circuitry having a filter gain responsive to the fine gain correction output retrieved from storage and providing a fully compensated analog sensor output.

4. A method as recited in claim 3 and further comprising, prior to retrieving information from memory, the step of performing a calibration procedure including the steps of

connecting a sensor to the signal conditioning circuit;

applying known physical stimuli to the sensor;

choosing coarse gain and offset corrections for the sensor;

storing the coarse gain and offset corrections in non-volatile memory;

varying the temperature over the anticipated working range;

generating fine gain and offset corrections for different temperatures;

storing the fine gain and offset corrections in non-volatile memory;

wherein coarse gain and offset are retrieved from memory initially and fine gain and offset are retrieved dynamically during operation.

5. A method as recited in claim 3 and further comprising the steps of converting the fully compensated analog sensor output to a fully compensated digital sensor output and transferring the fully compensated digital sensor output from the signal conditioning analog circuitry to the microcontroller.

6. A method as recited in claim 5 wherein said step of taking a temperature sample includes using an analog temperature sensor to generate an analog temperature sensor output, and further comprising the steps of converting the analog temperature sensor output to a digital temperature sensor output and transferring the digital temperature sensor output to the microcontroller.

7. A method as recited in claim 6 and further comprising, prior to said converting steps, the step of multiplexing the fully compensated analog sensor output and the analog temperature sensor output.

8. A method as recited in claim 6 wherein the first low pass filter stage includes a continuous time anti-aliasing filter with a selectable corner frequency and a resistive feedback circuit for controlling filter gain, and wherein said step of filtering the amplified analog sensor output in the first low pass filter stage includes selecting a corner frequency and a feedback resistance to make coarse gain corrections.

9. A method as recited in claim 8 wherein the second low pass filter stage includes a switched capacitor filter having a cutoff frequency controlled by varying a switched capacitor filter clock frequency, and wherein said step of filtering the coarse gain corrected analog sensor output in the second low pass filter stage includes selecting a cutoff frequency and adjusting values of capacitance in the switched capacitor filter in order to make fine gain corrections.