Manual Calibration or Auto ID – A Guide

The introduction of low-cost EPROMs has paved the way for an Auto-ID system, which is used for the automatic calibration of sensors to signal conditioners. The Auto-ID system provides various advantages such as:

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

Discussion

The sensor is integrated with an EPROM-based circuit, which is programed with the sensors calibration information. On plugging the sensor into a compatible instrument, the instrument not only retrieves the calibration data from the EPROM but also calibrates itself to the sensor.

Circuit Hardware

The single wire EPROM chip from Dallas Semiconductor, part #DS2502, is the most important component of the Auto-ID circuit. Transient Voltage Suppressors have been included to protect against static electricity. The schematic is shown in Figure 1.

Only two pins are used by the EPROM – one for data and the other for the ground reference. The EPROM, which consists of 128 bytes of programmable storage area, can only be programed once. Almost all microcontrollers can communicate with this chip through a programmable, bidirectional, open drain port bit. For timing purposes, this open drain port bit must also be able to switch at sufficient speeds (<1 ms). The EPROM’s data line is also an open drain type, making it necessary to use a pull-up resistor.

Data and commands are transferred between the devices by temporarily grounding the data line. The data encoding scheme demands that both pins be open drain, as sometimes both devices will be accessing the data line simultaneously; for instance, the microcontroller grounds the data line to read a bit from the EPROM. The EPROM will respond while the data line is grounded. The EPROM will also ground the data line if the data bit is 0, but it will not ground the data line if the data bit is 1.

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

Calibration

In order to calibrate an instrument to a strain gage sensor, the Auto-ID must provide the instrument with the data required to derive a transfer function. The transfer function is nothing but a relationship between the input signal (force, torque, etc…) applied to the sensor and the displayed engineering value.

For linear sensors, an instrument needs the following information:

  • Capacity (Full Scale)
  • Engineering units
  • 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 produces the transfer function by inserting the curve fit constants into the following generic equation: y = Ax2 + Bx + C where A is the second order curve fit constant, B the first order, and C the zero order. y is the force applied to the sensor in engineering units and x is the output from the sensor in mV/V.

Data Format

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

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.

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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