KR100327809B1 - Built-in programmable sensor compensation - Google Patents

Abstract

The present invention relates to an apparatus for calibrating a sensor quickly and at low cost. The apparatus includes an amplifier, a switch, a gain circuit, an offset circuit and a memory. It may be a serial write memory including the static RAM and the shift register, and store gain and offset coefficients for use by gain circuits, offset circuits, and amplifiers. The gain and offset circuits include a digital-to-analog converter that converts the stored gain and offset digital values into analog equivalent values, the gain and offset values being measured by first measuring the uncorrected sensor output at full scale output. Is calculated. These values are then used in the equations to calculate the offset and gain values.

Description

Calibration device with built-in programmable sensor

Background of the Invention

TECHNICAL FIELD The present invention relates to the field of sensors, and in particular, to calibration of sensors. This application is related to US patent application Ser. No. 08 / 175,908, filed on December 30, 1993 and assigned to Honeywell Incorporated.

Since the advent of electronic sensors, the calibration of sensors has entrusted the actual cost of the sensor costs to the end users of the sensors. In general, three calibrations were required, depending on the end use of the sensor.

Initial calibration was generally performed by the sensor manufacturer to meet a wide range of predefined tolerances. Currently this calibration involves laser fabrication of the bias resistor network for a given resistance. This calibration method generally requires a number of passes and a total elapsed time of 20 seconds or more when using an intermediate execution device.

The original device manufacturer (OEM) then performs a calibration to ensure that the sensor meets the requirements. In general, this has been done by including at least one multi-turn electrometer equipped with a sensor and adjusting the electrometer in a well known manner until the desired performance characteristics are achieved. Finally, it is possible to place an epoxy on the electrometer to fix the settings. This was an expensive way.

In the end, the performance characteristics of the sensors were affected by the conditions of the site in which they were used. Therefore, in-situ calibration was necessary to ensure that sensor implementation was acceptable when the sensor was installed in the intended application. In general, this calibration further included adjustments to the multi-turn electrometer.

One particular sensor of interest here was the flow sensor. Flow sensors have often been used in heating, ventilation and air conditioning equipment, for example, to measure the air flow through a conduit. This information could be used to control the controlled room temperature. Typically, such sensors have been placed in HVAC (heating, ventilation and air conditioning) controllers. This controller has typically operated in accordance with instructions stored by arithmetic arithmetic logic units (ALUs) stored in memory and clocked. The controller itself could be placed in the upper space of the room. This could make manual adjustment of the electrometer difficult after the controller was installed.

It is therefore an object of the present invention to substantially eliminate the need to make factory laser trimming of the resistor during calibration of the sensor. Another object of the present invention is to substantially reduce the cost of calibrating the sensor. Another object of the invention is to reduce the physical labor required for the calibration of the sensor.

Summary of the Invention

The present invention is an apparatus for calibrating sensors used in conjunction with clocked ALUs and memories. The present invention provides an amplifier means for amplifying a signal received from the sensor, a reference voltage source connected to the sensor and connected to the amplifier, and connected to the amplifier means and connected to the ALU and a memory to correct for known offsets. Offset means for adjusting the gain of the amplifier so that the gain means circuit is connected to the amplifier means and connected to the ALU and the memory to adjust the output of the amplifier means in accordance with a predetermined full scale output; Means, a gain circuit and a switch means connected to the ALU and multiplexing the preamplifier output and the programmable amplifier output.

In manufacturing, an uncorrected output occurs at the zero input level of the sensor, and the output voltage of that sensor is recorded. The sensor is then stimulated at the highest input level and the output voltage is recorded again. These are commonly known as the null and full scale output characteristics of the sensor. Then, a coefficient is calculated for use in the amplifier that is programmable to bring the null and full scale amplified output into a useful range for a given analog-to-digital converter.

1 is a block diagram of a controller incorporating the system of the present invention.

2 is a block diagram of a circuit according to the present invention.

3 is a schematic block diagram of an embodiment of a circuit of the present invention connected to a representative sensor.

Detailed description of the preferred embodiment

Referring to FIG. 1, a controller 10 is shown that includes the apparatus of the present invention. The controller 10 includes a sensor 15 connected to a correction circuit 20. The correction circuit 20 is connected to an analog-to-digital converter 22, an arithmetic logic unit (ALU), a memory, and an input / output unit 40. The ALU is connected to a clock 30.

In operation, the sensor 15 generates a sensor signal indicative of the condition to be sensed. The sensor is connected to a correction circuit 20 that uses an algorithm to correct for null input and full scale output (FSO) variations. The calibrated circuit in a digitalized manner is then sent to the ALU via analog digital converter 22. The coefficients used in the algorithm are stored in the memory 25. The memory 25 that may include both read only memory (ROM) and random access memory stores instructions for operating the ALU 35. The ALU 35 may be a microprocessor, a microcontroller, a microcomputer, or the like. The ALU 35 receives a modified signal and a command from a memory and is transmitted to an apparatus (not shown) controlled through the input / output unit 40. Generate a control signal. To ensure orderly operation and communication of the command, clock 30 generates timing signals used for all components of the controller.

Referring to FIG. 2, a block diagram of the correction circuit 20 of the present invention is shown. The correction circuit 20 comprises an amplifier means 205, a reference voltage generator 210, an offset means 215, a gain means 220, a switch means 225 and a memory 230. Amplifier 205 receives the sensor signal and amplifies the sensor signal to a level that can be used by the ALU. In addition, the amplifier 205 amplifies the signal in accordance with the signal received from the offset means 215 and the gain means 220 to adjust the output signal to have a predetermined span and offset characteristics. The reference voltage means 210 generates a reference voltage used by the sensor 15, the amplifier 205 and the offset means 215. By biasing the device using a common reference voltage, the circuit is subject to a common displacement in the reference voltage that can be induced by temperature changes. Thus, the temperature offset in such a device is canceled proportionally.

The switch means 225, the gain means 220 and the offset means 215 are controlled by the information stored in the memory. This information is supplied to the memory at each time the sensor is started from the memory 25. The memory 320 may have components of static RAM (SRAM), nonvolatile RAM (NOVRAM), and electrically erasable programmable ROM (EEPROM). The switch means 225 controls the corrected sensor signal or the amplified but not corrected sensor signal to be output from the amplifier means 205 based on the output received from the memory 230. The offset means 215 receives the stored coefficients from the memory 230 to correct the sensor signal for offset errors. Gain means 220 receives coefficients from memory 230 to correct the sensor signal for errors in the full scale output signal. A more detailed description thereof will be described with reference to FIG. 3.

Referring to FIG. 3, a schematic block diagram of the sensor 15 and the correction circuit 20 is shown. The correction circuit represents a preferred embodiment of the present invention and is shown in more detail. The amplifier means 205 comprises a preamplifier 305 and an amplifier 308. The preamplifier is used to scale the sensor output signal to a level up to the level available by the ALU. The amplifier 308 may be programmable to a certain gain level and is affected by the output of the offset means 215 and the gain means 220.

The offset means 215 comprises a digital to analog converter. The gain means 220 comprises a resistor R1 and a gain portion 221.

The memory 230 includes a digital-analog converter memory unit 316, a gain memory unit 321, and a latch memory unit 322. This memory section has the form of a serial recordable memory.

Switch means 225 comprises switches 325 and 326 and NOT gate 327. Analog-to-digital converter 350 is used to convert the output signal into a form that can be stored and used by the ALU.

The gain section 221 in the preferred embodiment is a ladder type digital-analog converter having a predetermined conversion function. This is the same as the digital-to-analog converter 215. The selection and structure of the conversion functions of these converters is a matter of design choice for a particular application and is well known to those skilled in the art. In the present invention, these converters consist of seven resistors that can be selectively selected to produce a predetermined resistance value.

The sensor 15 may be any sensor designed to generate different output signals. Here, the flow sensor is configured as a Wheatstone Bridge with resistors Rb1, Rb2, Rfs1 and Rfs2 as elements of the bridge. Resistor Rb3 level shifts the differential voltage generated at the positive and negative inputs of preamplifier 305.

The sensor may be a flow sensor such as a honeywell AWM series flow sensor. It is important to note that the sensor described herein is exemplary and, while the sensor is related to the present invention, it is never part of the present invention.

In correction, the present invention operates as follows. First, switch 325 is enabled, and switch 326 is disabled by writing a1 to the first bit of the serial write memory. All other bits do not matter at this point.

The sensor is then placed in a state where zero stimulus is applied to the sensor (zero input level), and the sensor output value is stored in memory Vlo. The sensor is then stimulated at the level at which the full scale output is generated, and the sensor output value is stored in the memory Vhi.

These two values can be obtained by the following equation.

Where

Vo2 is the predetermined voltage output (0.05 Vdc in the preferred embodiment) at the zero sensor input,

Vo1 is the predetermined voltage output (4.50 Vdc in the preferred embodiment) at the full scale output,

Vr is the required offset voltage.

The gain can then be calculated using Vr calculated above. In other words,

Rf = ((Vhi-Vo1) ^* R1) / (Vr-Vhi),

GAIN '= Rf / Rp.

In the above equation, the value of R1 is chosen to obtain the desired output and gain portion of the programmable amplifier. Methods of selecting such resistors are well known to those skilled in the art. That is, the applicant here selects R1 = 4990Ω.

It should be noted that the term "gain" refers to the ratio of Rf to R1, and the actual output voltage of the amplifier 308 is defined as Vo = (GAIN '+ 1) × Vin + Vr × (GAIN'), Vin. Is the output of the preamplifier.

It should be noted that the switch means 225 and latch memory portion are not necessary in the basic embodiment of the present invention. These functions can be performed by circuitry connected to the sensor output only during calibration. Once the calibration is completed, these functions are no longer needed and external circuits can be disconnected. The Vr and gain values may remain loaded as described herein.

Once the gain and Vr are determined, the decimel equivalent value of the gain and Vr is calculated. These values depend on the conversion function and analysis of the gain section and the analog-to-digital converter.

Dec (Offset) = Int ((Vref ^* 12.8 / Vr-44.8))

Dec (gain) = Int ((336 / Gain ')-32 + 0.1)

(Vref is = 5.45V in the preferred embodiment)

The decimel code obtained above is then arranged with an offset code as the first bit, a gain code as the second bit, and zero as the last bit to shut off the switch 325 and turn on the switch 326. The code is then input to the memory section and the corrected sensor output signal is checked for a predetermined null and full scale output level by re-stimulating the sensor at these levels. If the output levels at these output levels meet certain criteria, the offset and gain decimel codes are stored in the memory 25 and used during operation of the sensor.

At the start of the sensor, Vr and the digital value of the gain (Vr of the digital-analog converter memory portion 16 and the gain of the gain portion memory 321 and the digital value of the latch memory portion 322) are taken from the memory 25. Loaded into the appropriate memory of memory 230. Thus, the corrected output signal sent to analog-to-digital converter 350 is corrected by the gain and Vr value through the circuit shown in FIG.

The foregoing describes a novel and advanced apparatus for calibrating sensors for offset and null errors. Many simple variations within the spirit of the invention will be apparent to those skilled in the art. Applicants do not intend to limit the above description, but instead intend to define the scope of the invention through the appended claims.

Claims (7)

In the offset error and span error correction device in the sensor, A memory for receiving and storing an offset correction value for correcting a sensor output signal for the offset error and a span correction value for correcting a sensor output signal for the span error; Offset means connected to said memory and converting said offset correction value into an analog signal; Gain means connected to said memory, for converting a gain value into an analog signal of a predetermined level; The sensor output signal connected to the sensor, the gain means, the offset means and the memory, and amplified the sensor output signal to a predetermined first level, and by an analog equivalent signal substantially equal to the gain value and the offset value. The offset error and span error correction device in the sensor comprising an amplifier means for further amplifying. The method of claim 1, And said memory comprises a serial recording memory. The method of claim 2 The serial write memory consists of a static random access memory (SRAM) connected to a shift register; And the SRAM continuously provides a constant value to the shift register after initiation of the device. The method of claim 3, The serial write memory includes a first SRAM and a shift register for storing and shifting offset values, respectively, and a second SRAM and shift register for storing and shifting the gain values, respectively. Device. The method of claim 1, And a switch connected to the memory and the amplifier, wherein the memory transmits a value to the switch that causes the amplifier to output a sensor output signal at the predetermined first level during calibration of the sensor. And correction of span error. The method of claim 1, And said gain means and said offset means comprise a ladder-type digital-to-analog converter. The method of claim 1, The amplifier means, A preamplifier having an input terminal and an output terminal connected to the plurality of sensor output terminals and having a fixed gain; A corrected sensor signal having a first input terminal connected to said output terminal of said preamplifier and a second input terminal connected to said offset means and said gain means for receiving an analog equivalent signal, wherein the corrected sensor signal is a function of said analog equivalent value And a programmable amplifier having a generated output terminal.