Pressure transducer with integral digital temperature compensation

• US 4,765,188 A
• Filed: 11/24/1986
• Issued: 08/23/1988
• Est. Priority Date: 11/24/1986
• Status: Expired due to Term

• Current Assignee: Interia Optical Technology Applications, Inc.
• Original Assignee: Bourns Instruments, Inc.
• Inventors: Krechmery, Roger L., Finefrock, Mark D.
• Primary Examiner(s): WOODIEL, DONALD
• Application Number:
  US06/933,966
• Time in Patent Office: 638 Days
• Field of Search: 73/708, 73/720, 73/721, 73/726, 73/727, 73/4 R, 338/4, 338/6, 338/8, 338/10, 338/3, 374/143, 364/557, 364/558, 364/571
• US Class Current: 73/708
• CPC Class Codes: G01L 9/065 with temperature compensating means non electric temperature compensating means G01L19/04

Abstract

A pressure transducer of the type employing a pressure-responsive diaphragm (11) with silicon piezoresistive strain gauges epitaxially deposited thereon is provided with integral digital thermal compensation of span shift and zero shift. The strain gauges are connected in a bridge circuit (30), and a differential voltage across the bridge circuit is amplified by an instrumentation amplifier (46). A temperature sensitive resistor (36) is deposited on the pressure-responsive diaphragm. This resistor is connected in a bridge circuit (35) the output of which is connected to an analog-to-digital (A/D) converter (54) which generates a digital number corresponding to the measured temperature of the piezoresistive strain gauges. This digital number is used to address pre-programmed correction data stored in a programmable read only memory (PROM) (53). The numerical correction data from the PROM is converted by a digital-to-analog (D/A) converter (55) to an analog correction signal. The D/A converter has two outputs which are alternately selected by a timing and control circuit (56) to provide thermal span shift and thermal zero shift correction signals. The thermal span shift signal is used to modify the excitation voltage of the piezoresistive strain gauge bridge circuit, while the thermal zero shift signal is added to the amplified voltage output of the bridge circuit. A simple analog circuit (58) provides linearity compensation.

Claims

1. A pressure transducer with integral digital compensation comprising:

a pressure sensor assembly including a temperature sensitive element;

an analog signal conditioning circuit connected to said temperature sensitive element, said circuit providing an output proportional to the temperature of said pressure sensor;

an analog-to-digital converter connected to receive the output of said analog signal conditioning circuit and providing a digital output signal;

a read only memory having address lines and a data bus, said address lines being connected to receive said digital output signal from said analog-to-digital converter, and said read only memory being pre-programmed to selectively provide thermal span shift or thermal zero shift correction data on said data bus in response to said digital output signal;

a digital-to-analog converter having an input connected to the data bus of said read only memory and two selectable outputs, said digital-to-analog converter providing an analog signal output to a selected one of said two outputs corresponding to a correction number on said data bus of said read only memory, one of said two outputs being connected to said pressure sensor assembly to provide a thermal span shift compensation;

timing control means connected to said analog-to-digital converter, to said read only memory, and to said digital-to-analog converter for controlling the sampling of the output signal from said analog conditioning circuit by said analog-to-digital converter and for alternately selecting thermal span shift or zero shift data from said read only memory and one or the other of the two outputs of said digital-to-analog converter; and

amplifier means connected to said pressure sensor assembly for amplifying an output voltage from said assembly, the other of said two outputs of said digital-to-analog converter being connected to said amplifier means to modify the amplified voltage from said pressure sensor assembly to compensate for thermal zero shift.

2. The pressure transducer as recited in claim 1 wherein said read only memory is a programmable read only memory which is programmed with span shift and zero shift data generated at the time the transducer is calibrated.

3. The pressure transducer as recited in claim 1 wherein said pressure sensor assembly comprises:

a pressure-responsive diaphragm with silicon piezoresistive strain gauges deposited thereon, said temperature sensitive element being a temperature responsive resistive element deposited on said diaphragm and said pressure-sensitive strain gauges being electrically connected in a bridge circuit;

said temperature sensitive element providing an output proportional to the temperature of said piezoresistive strain gauges on said diaphragm;

one of said two outputs of said digital-to-analog converter being connected to said bridge circuit comprising said piezoresistive strain gauges to provide a thermal span shift compensation; and

said amplifier having an input connected to said bridge circuit comprising said piezoresistive strain gauges for amplifying a differential voltage across said bridge circuit.

4. The pressure transducer as recited in claim 3 wherein said pressure sensor assembly further comprises:

a base member supporting said pressure-sensitive diaphragm, said base member having a cavity covered by said pressure-sensitive diaphragm; and

a hermetically sealed housing enclosing said pressure-sensitive diaphragm and base member.

5. The pressure transducer recited in claim 4 wherein said pressure sensor assembly is an absolute pressure sensing device, the cavity in said base member containing a fluid at a predetermined pressure.

6. The pressure transducer as recited in claim 4 wherein said pressure sensor assembly is a differential pressure sensing device, said housing including a port communicating with said cavity in said base member.

7. The method of calibrating a pressure transducer of the type comprising a pressure sensor assembly including a pressure-responsive diaphragm with silicon piezoresistive strain gauges deposited thereon and electrically connected in a bridge circuit and a temperature sensitive element deposited thereon, an analog-to-digital converter responsive to said temperature sensitive element for providing a digital output signal, a programmable read only memory having address lines and a data bus, said address lines being connected to receive the digital output signal from said analog-to-digital converter to selectively provide thermal span shift or thermal zero shift correction data on said data bus in response to said digital output signal, a digital-to-analog converter having an input connected to the data bus of said programmable read only memory and two selectable outputs, one of said outputs being connected to said bridge circuit to provide thermal span shift compensation, timing control means connected to said analog-to-digital converter, said programmable read only memory, and said digital-to-analog converter for controlling the sampling of the output from said temperature sensitive element and the selecting data from said programmable read only memory and the selecting one or the other of the two outputs from said digital-to-analog converter, and amplifier means connected to said bridge circuit for amplifying a differential voltage across said bridge circuit, the other of said two outputs of said digital-to-analog converter being connected to said amplifier means to modify the amplified voltage from said bridge circuit to compensate for thermal zero shift, said method comprising the steps of:

replacing said programmable read only memory with a digital computer programmed to generate thermal span shift and thermal zero shift data;

connecting said transducer to a precision pressure source;

changing the temperature of said transducer over a range of temperatures;

for each digital output from said analog-to-digital converter for predetermined changes in temperature, generating thermal span shift and zero shift data with said digital computer so that ideal zero and span response are achieved as measured at the transducer analog output;

storing said generated thermal span shift and zero shift data with temperatures corresponding to said data; and

programming said programmable read only memory with said stored thermal span shift and zero shift data;

inserting the programmed programmable read only memory into said pressure transducer.

8. A pressure transducer having sensing gauges comprising:

means on a common substrate with said pressure sensing gauges for sensing the temperature of said pressure sensing gauges and producing an analog temperature signal;

means for digitizing said analog temperature signal to provide a digitized temperature signal;

means addressed by said digitized temperature signal for providing first and second temperature correction digital signals;

means for converting said first and second temperature correction digital signals to first and second analog temperature correction signals;

means for applying said first analog temperature correction signal to said pressure sensing gauges as a span correction signal; and

means for applying said second analog temperature correction signal to a pressure output signal produced by said pressure sensing gauges as a zero correction signal.