A Guide to Auto ID or Manual Calibration

The advent of low-cost EPROMs has opened the doors for an Auto-ID system, which is used to automatically calibrate sensors to signal conditioners. The Auto-ID system offers various benefits such as:

  • Calibration data is not easily lost anymore
  • Calibration can be rapid and automatic
  • Sensors do not require that matched outputs be interchangeable
  • System is flexible – use is not limited to strain gage sensors
  • Eliminates human errors in calibration
  • System is inexpensive

Discussion

The sensor is incorporated with an EPROM-based circuit programed with the sensors calibration information. Once the sensor is plugged into a compatible instrument, the instrument retrieves the calibration data from the EPROM and also calibrates itself to the sensor.

Circuit Hardware

Dallas Semiconductor’s single wire EPROM chip, part #DS2502, is the key component of the Auto-ID circuit. Transient Voltage Suppressors have been included for protection against static electricity. Figure 1 shows the schematic.

Circuit Hardware

The EPROM uses only two pins – one is for data and the other is for the ground reference. The EPROM, consisting of 128 bytes of programmable storage area, can be programed just once. Most of the microcontrollers can communicate with this chip via a programmable, bidirectional, open drain port bit. This open drain port bit must also be able to switch at sufficient speeds for timing purposes (<1 ms). The data line of the EPROM is also an open drain type, thus it is necessary to use a pull-up resistor.

By temporarily grounding the data line, commands and data are exchanged between the devices. The data encoding scheme necessitates that both pins be open drain, as occasionally both devices will be accessing the data line at the same time; for example, the microcontroller grounds the data line to read a bit from the EPROM. The EPROM will respond while the data line is grounded. If the data bit is 0, the EPROM will also ground the data line, but if the data bit is 1 it will not ground the data line.

On releasing the data line, the microcontroller will monitor the data line status to check if it immediately became high or remained low (More details on the encoding scheme are provided in the EPROM data sheet.). EPROM manufacturer specifies a range of pull-up resistors. Since a cable is likely to be present between the microcontroller and the EPROM, the Auto-ID uses a 2.7 K Ohms pull-up resistor. Consequently, the effects of cable capacitance are reduced. Moreover, the pull-up resistor provides power for the EPROM through the data pin. When the data line is released by both the microcontroller and EPROM, the pull-up resistor raises the data line to 5 volts, charging a power supply capacitor inside the EPROM. This charge is sufficient to keep the chip operational during communications.

Calibration

To calibrate an instrument to a strain gage sensor, the Auto-ID must provide the instrument with the data needed to derive a transfer function. The transfer function is a relationship between the input signal (force, torque, etc…) that is applied to the sensor and the displayed engineering value.

For linear sensors, an instrument requires the following information:

  • Engineering units
  • Capacity (Full Scale)
  • Output at Full Scale

For non-linear sensors, the apparatus will need curve fit constants for a more accurate, non-linear transfer function. See items 11 to 21 in Table 1. The equipment generates the transfer function by inserting the curve fit constants into the following generic equation: y = Ax2 + Bx + C where A , B, and C are the second order, first order, and the zero order curve fit constants, respectively. The force applied to the sensor in engineering units is y, and x is the output from the sensor in mV/V.

Data Format

SDI has defined the memory format for strain gage Wheatstone Bridge sensors, as illustrated in Table 1.

Table 1. Items through 1 and 5 are general descriptions and are used for record keeping. Every type of sensor will use them. The rest are specific to the sensor type, in this case, strain gage sensors. All data is stored MSB first. For IEEE data, this means the exponent is the lowest address followed by mantissa high, mid and low.

ITEM SIZE
(Bytes)
ADDRESS
(Hex)
DESCRIPTION FORMAT/EXAMPLE
(All codes hex)
1 2 00 Sensor type code (charted) 00 01 = Strain gage
Wheatstone Bridge
2 2 02 Mft. code (registered & charted) 00 01 = SDI
3 2 04 Revision number 00 02
4 4 06 Cal data (M/D/Y) OC 19 5E = 12/25/95
5 10 0A Serial number (ASCII) 31 32 ...41 36 = 12 ...A6
6 4 14 Capacity (full scale) IEEE floating pt.
7000 = 45 DA C0 00
7 2 18 Units (charted) 01 01 = LBS
8 4 1A Full scale output (mVN) IEEE SP floating point
9 4 1E Best fit through zero + coefficient IEEE SP floating point
10 4 22 Best fit through zero - coefficient IEEE SP floating point
11 4 26 0 order coefficient
(CW/Tension - ascending)
IEEE SP floating point
12 4 2A 1st order coefficient
(CW/Tension - ascending)
IEEE SP floating point
13 4 2E 2nd order coefficient
(CW/Tension - ascending)
IEEE SP floating point
14 4 32 0 order coefficient
(CWiTension - descending)
IEEE SP floating point
15 4 36 1st order coefficient
(OW/Tension - descending)
IEEE SP floating point
16 4 3A 2nd order coefficient
(CW/Tension - descending)
IEEE SP floating point
17 4 3E 0 order coefficient
(CCW/Compression - ascending)
IEEE SP floating point
18 4 42 1st order coefficient
(CCW/Compression - ascending)
IEEE SP floating point
19 4 46 2nd order coefficient
(CCW/Compression - ascending)
IEEE SP floating point
20 4 4A 0 order coefficient
(CCW/Compression - descending)
IEEE SP floating point
21 4 4E 1st order coefficient
(CCW/Compression - descending)
IEEE SP floating point
22 4 52 2nd order coefficient
(CCW/Compression - descending)
IEEE SP floating point
23 1 56 Shunt position - Which leg
of bridge (charted)
01 = +Exc to +Sig
02 = +Exc to -Sig
03 = -Exc to +Sig
04 = -Exc to - Sig
24 4 58 Shunt valve (Ohms) IEEE SP floating point
25 4 58-5B Shunt output (mV/V) IEEE SP floating point
26 4 5C Simulated shunt load
(units of item 7)
IEEE SP floating point
27 1 64 Option (charted) 00 01 = Encoder
28 31 66-7F Blank - Reserved for Options  

 

HITEC Sensor Developments, Inc

This information has been sourced, reviewed and adapted from materials provided by HITEC Sensor Developments, Inc.

For more information on this source, please visit HITEC Sensor Developments, Inc.

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