December 2000
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Sensor Interface & Calibration Solutions

  via the Internet  

A digitally compensated sensor product can be put into production without the expense of developing specialized calibration software by using an Internet method that's fast, easy, and reliable.

By Rodger Reinhart, Atmos Engineering, Inc.

A new class of sensor interface solutions with calibration algorithms and production automation know-how is available online by combining standard low-cost integrated circuits with widespread Internet access. See the sidebar "Sensor Signal Processor."

Meeting the Challenges of Calibration
One of the greatest barriers for sensor manufacturers implementing digital calibration and compensation techniques has been the expense and complexity of developing a calibration test system. Calibration software has proven particularly challenging. Although most sensor development engineers concentrate on the hardware at the start of the design and assume the calibration software will be straightforward, a manufacturer can in fact expect to spend several hundred thousand dollars to achieve production-ready calibration-automation capability.

It's just a curve fit, right? Many engineers ask this question about the online calibration software. Because the software must include a mathematical model of the interface electronics and optimizes calibration on the basis of this model, calibration software involves much more than a curve fit. In addition, the calibration process must be completely automated to generate process feedback and alert the production engineer when the process has degraded. When a unit fails calibration, the calibration software is what enables the engineer to diagnose the problem. With the availability of online calibration, engineers no longer have to develop their own calibration software.

Sensor Signal Processor
The Atmos SSP14 Sensor Signal Processor is a customized version of the popular PIC microprocessor family from Microchip. The chip price includes a license to use the
Figure 1. SSP14 single-chip sensors signal processor combining analog-to-digital conversion, smart sensor operating system, and a license to use online calibration services into a SOIC14 package.
Atmos calibration and visualization software by means of online access. The chip is preprogrammed with a smart sensor operating system and a portion of the code memory is available for user-programmed custom features. The SSP receives serial commands on the RX pin and transmits results on the TX pin. Because the TX and RX pins are 5-V logic levels, connection to a host computer requires an RS232-level shifter. Commands to the SSP are two-letter ASCII commands in 2400 baud E,7,1 format. The SSP automatically detects baud rate, thereby eliminating the need for a precision time reference and allowing the SSP to talk at nonstandard baud rates. Current consumption of the SSP is <1 mA, which allows operation in loop-powered transmitter applications. A portion of the code memory is available for user-programmed custom features.

Digitally Trimmed Analog Output Sensor
The online calibration process for the smart pressure transducer shown in Figure 2 (below) involves taking readings from the sensor signal processor at various pressures and temperatures and recording the results in a data file. First, the sensor temperature is reduced to the low end of the operating range; span and offset are adjusted by changing digital-to-analog converter (DAC) values. DAC values and reference pressures and temperature are recorded in a calibration data file. To complete the data-taking portion of the calibration, span and offset adjustments are repeated at both room temperature and the upper end of the operating temperature range.

Digitally Trimmed Analog Output Sensor
The schematic in Figure 2 shows the SSP14 used in a digitally compensated analog throughpath pressure transducer. In addition to generating span and offset adjust voltages, the SSP digitizes temperature and output voltage signals. This allows for multivariable temperature compensations and DAC output of the calibrated pressure and temperature values. The SSP settings are stored in EEPROM memory. Pulse-density modulation (PDM) is used to generate the output voltages. A single-stage low-pass filter smoothes the PDM waveform to achieve a 12-bit D/A conversion. PDM modulation allows for a much lower modulation frequency than standard pulse-width modulation (PWM).
Figure 2. Analog throughpath pressure transducer using the SSP14 single-chip sensor signal processor. The span and offset are adjusted continuously as a function of both pressure and temperature.

The calibration data file is then e-mailed to a calibration server as a simple text file attachment. When the server receives the file, it is passed to the calibration visualization software for processing. After processing, an EEPROM file containing the calibration coefficients and calibration plots is returned by e-mail, and the EEPROM values are transferred into the sensor signal processor. The calibration is then complete.

The amount of calibration data taken depends on the application. Span and offset adjustments can consist of anywhere from 2 to 20 pressure points per temperature. Table 1 lists the tradeoffs between the number of pressure-temperature points sampled and the desired calibration accuracy. (A demonstration of the online calibration service can be found at the Atmos website.)

Calibration Accuracy vs.
Number of Pressure-Temperature Data Points
Pressure Points

Calibration Accuracy Total Error Band

Key Words Give Users Control
An example of a pressure sensor calibration data file is shown in Figure 3. The calibration data file is a text file with user-selected parameters passed to the visualization software as [BRACKETED KEY WORDS].

; SSP calibration data file example:





Analog Through Path





;Time	Tref	Pref	GDAC	ODAC	VP A/D	VB A/D

12:31:56	-40.0	29.321	2341	1234	3126	4732
12:33:22	-40.0	02.314	2341	1234	0537	4729
12:41:22	 20.0	29.330	2583	1126	3130	5220
12:43:38	 20.0	02.321	2583	1126	0541	5220
12:53:26	 85.0	29.328	2747	1202	3128	5725
12:57:06	 85.0	02.331	2748	1202	0542	5726

Figure 3. The calibration data file is sent by e-mail to the Atmos calibration server. The server processes the file and returns optimized calibration coefficients and calibration plots. Keywords control the calibration order and select which calibration plots are returned.

Keywords in the sample file tell the calibration visualization software to compute three sets of correction coefficients, one set for each third of the operating temperature range. Each set contains multinomial coefficients for a second-order pressure and third-order temperature correction. A programming file for the SSP EE-PROM is generated from correction coefficients. Programming files are returned to users by e-mail. Writing calculated EE-PROM values to the sensor signal processor completes the calibration.

In addition to EEPROM files, the calibration visualization software returns calibration plots. Users can use keywords to select from dozens of analysis and plot routine options, including standard 2D and 3D surface plots. Summary plots that are normally requested for production calibration combine text, statistics, and graphics on one page to show predicted sensor performance. These summary plots are used as production calibration travelers.

Keywords also pass along information such as calibration reference serial numbers. These ensure traceability of the calibration.

Figure 4 shows an engineering plot for a sixth-order calibration computation.

Figure 4. Sixth-order calibration error surface plot returned from the calibration server as a PDF file. The graph in the upper right corner of the plot is a calibration accuracy histogram for the 1650 pressure temperature points recorded during calibration.

Figure 5 shows an engineering plot of the temperature feedback signal and slope of the feedback signal vs. temperature.

Figure 5. Users can select from various 2D and 3D calibration plots. The temperature feedback plot displays the digitized temperature feedback voltage used by the sensor signal process to compensate the sensor.

Why E-Mail Instead of Racier, Newer Technologies?
Atmos picked e-mail to deliver calibration services for the simple reason that it is the most reliable and easily managed Internet technology available. Typical end-to-end e-mail transmission time is <1 min., which allows for the processing and return of calibration results and plots within 3 min. from the time the calibration data file is sent. The calibration server is implemented by using two identical computers, with each computer watching for incoming calibration files and processing them one at a time. If one of the computers fails, the remaining system will continue processing without interruption.

Internet delivery of calibration services so dramatically simplifies and reduces the cost of digital calibration that it's likely to be-come prevalent throughout the industry in a year or two. With low- to moderate-volume manufacturers gaining access to technology previously affordable only to high-volume manufacturers, we are anticipating a new generation of extremely high-accuracy sensor products soon.

Rodger Reinhart is president of Atmos Engineering, Inc., 443 Dearborn Park Rd., Pescadero, CA 94060; 650-879-1674, fax 650-879-1675;

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