model ca100 "compact cal" calibrator
MODEL CA100 “COMPACT CAL”
CALIBRATOR
OOMURA Hisahide *1
SAKAMAKI Yasuo *1
AIZAWA Shinji *2
TAJIMA Kazuaki *1
We have developed Model CA100 “Compact CAL,” a portable calibrator,
having 0.02% accuracy and the resistance generator function. The design
objectives were excellent accuracy, the support of essential functions, and easy
operation. The calibrator is ideal for maintaining the accuracy of such equipment
as converters at an optimum level. In order to calibrate such equipment in the field,
we have provided the compact, economical calibrator with the generator function,
measurement function and 24-V DC output function.
This paper introduces the functional overview of the CA100 calibrator.
INTRODUCTION
DESIGN CONCEPTS
I
As trends in the latest measuring instruments, we often notice
such products that are equipped with a variety of advanced
functions to highlight the superiority in cost-performance. In
contrast, users are hoping for a simple measuring instrument that
has only necessary functions with required accuracies and does
not mislead the operator during use. This requirement is all the
more strong for the excellently portable CA100 calibrator in order
to avoid accidents due to erroneous operation in the field. Users
who make a business of engineering tend to implement their own
know-how in the form of programs for a personal computer and
n all kinds of industrial plants, physical quantities such as
temperature, flowrate and pressure are changed to required
electrical signals through converters to use them for monitoring,
control or other purposes. The industry-standard accuracy of
these converters, which was 0.25%, has been changed to 0.1%
recently. Consequently, it has become a requirement for
measuring instruments used to inspect or calibrate converters,
receiving meters, and so on to also have higher accuracies.
The recently developed CA100 calibrator is a portable,
highly accurate measuring instrument having an accuracy high
enough to support the inspection and calibration of converters
with a 0.1%-order accuracy in the field. In addition, CA100 is
provided with the function for generating pseudo resistance
temperature detector (RTD) signals, the DMM function used to
measure the output signals of a converter, and the 24-V DC
output function used to supply power separately to a converter, all
of which were only available previously using a separate
instrument.
Figure 1 is the external view of the CA100 calibrator.
*1 Test & Measurement Business Division
*2 Industrial Measurement & Control Co.
Model CA100 “COMPACT CAL” Calibrator
Figure 1 External View of CA100
15
POWER SUPPLY/
COMMUNICATION BLOCK
SOURCE/CPU BLOCK DCV
Set
Display
CPU
15V DC
Charging
circuit
SOURCE
DCA
DCV
Lo
Current Overcurrent
detector
protector
Memory
Constant
current
source
MEASURE
BLOCK
DC/DC
converter
Hi
Overvoltage
protection
Current/voltage
converter
DC/DC
converter
OUT PUT
0
RS-232-C
driver
RS-232-C
FRQ
1
DCA
Voltage
reference
0
A/D
converter
TC
R.J.INPUT
Input
circuit
Hi
Power
control
FUSE
BATT.
Shunt
resistor
mA MEASURE
Lo
24V OUT
Reg.
+
−
24V OUT BLOCK 24V OUT
Figure 2 Block Diagram of CA100
Table 1 Typical Examples of Accuracies of the Basic Functions
Function Category
Generation
Measurement
Function
Range
Accuracy (%)
DC voltage
1V
±(0.02% of setting + 0.005% of range)
DC current
20mA
±(0.025% of setting + 0.015% of range)
Resistance
500Ω
±(0.02% of setting + 0.02% of range)
DC voltage
5V
±(0.02% of setting + 0.01% of range)
DC current
20mA
±(0.025% of setting + 0.02% of range)
connect a measuring instrument to the computer to efficiently
inspect equipment and make reports. Earlier advanced functions
are being absorbed in such computer programs. The result is a
demand that measuring instruments have the communication
function, in addition to their essential functions and high
accuracy, so they serve as I/O units for personal computers.
In the design of CA100, efforts were concentrated on high
accuracy and the enhancement of basic functions, as voiced by
users. For a human-machine interface, operability was
considered first; therefore, multiple definitions of keys and their
degree of multilayer structure were minimized and the number of
keys was reduced in order to achieve the principle of “one keyone-action” with the minimum number of required keys.
Advanced functions that would make operation complex were
removed by equipping the calibrator with the communication
interface to enable online control. All these efforts led to the
precision measuring instrument that can be used like a common,
commercially available hand-held tester.
16
FUNCTIONAL OVERVIEW AND FEATURES
The generator function covers DC voltage, DC current,
resistance, thermocouple (TC) signals, resistance temperature
detector (RTD) signals, frequency and pulses to support different
types of input to a converter. The measurement function is
designed to support DC voltage, DC current and resistance with
the aim of measuring 1-5 V and 4-20 mA signals. The accuracies
have been made as high as possible, as shown in Table 1. In other
areas of functionality, the calibrator supports not only the source
mode but also the sink mode for DC current generation, enabling
inspection of receiving meters. For resistance generation, the
calibrator is provided with a 500Ω range so the single range
covers up to 332.66Ω which corresponds to 650°C for a Pt100
RTD. The response time has also been improved (10 ms),
considering signal generation for scan-mode input devices. For
TC signal generation, the function is made switchable taking into
consideration a case where equipment being measured is a
combination of earlier and later versions of JIS-standard
products. The calibrator is provided with the 24-V DC power
supply output function, in addition to the generator and
measurement functions. The display (as wide as 35% the area of
the operation panel) is located in the upper-middle of the
operation panel for better visibility. Integral, silicone-rubber flat
keys are employed to make the key panel dustproof. To protect
against a possible failure due to aging, mechanical switches are
placed on the circuit board so each rubber keytop pushes down
Yokogawa Technical Report English Edition, No. 24 (1997)
+1V
CH 1,000V
+0.505V
CH 0.5050V
Closeup
0V
CH -1,500V
-4.00sec
-2
0
+2
+4
+6
msec
+8
+10
+12
+0.5V
CH 0.4950V
-4.00sec
-2
0
+2
+4
+6
msec
+8
+10
+12
-16.00sec
+0.495V
+14
+16.00sec
-1V
+14
Figure 3 Response Waveform during Resistance Generation
the underneath mechanical switch. This arrangement gives the
operator a click feeling each time, ensuring that the key has been
pressed securely. As a main source of power to the calibrator, we
have selected an AA-size battery, among other batteries, which is
available almost anywhere and even when the calibrator is used
unexpectedly. In addition to the battery, dedicated Ni-Cd battery
packs and AC adapters are available. The Ni-Cd battery pack is
rechargeable through the calibrator using the AC adapter. For
communication, the calibrator is provided with the serial
communication function based on start-stop synchronization, as
standard, so generator-function settings and measured values can
be printed by connecting a printer externally. CA100 is compact
and lightweight, having an outline nearly the size of an A5
paper—237 (W) × 137 (H) × 63 (D) (mm)—and weighing only
1.2 kg (including batteries).
CONFIGURATION
Figure 2 illustrates the block diagram of the CA100
calibrator. The diagram can roughly be divided into four blocks:
the power supply/communication block, generator block,
measurement block and 24-V power supply block. To enable
these four blocks to be used separately, they are electrically
isolated from each other. The MPU, which controls the entire
calibrator, is located in the generator block. Control of the
respective blocks is done through photocouplers. The
communication section, which is connected to an external device,
is located in the power supply section and isolated from other
blocks. The 24-V power supply block for converters being
calibrated is isolated using a transformer and outputs power
through a three-terminal voltage regulator. Details on the power
supply, generator and measurement blocks are given in the
following paragraphs.
1. Power Supply Block
Since CA100 is a portable calibrator, the power supply block
needs to be highly efficient and emit less noise to support
prolonged operation on batteries and higher accuracy. The block
Model CA100 “COMPACT CAL” Calibrator
first regulates input voltage from such unstable, varying types of
power sources as an AC adapter, a Ni-Cd battery pack or dry cells
using a DC/DC converter; then using a transformer-based
switching regulator, the block generates and supplies electrically
isolated power to other blocks. The power is further processed
through a three-terminal voltage regulator to produce a stable
source of power. Each winding of the transformer is shielded to
prevent switching noise from mixing into the secondary stage.
Each switching regulator is as efficient as no less than 90% and
the three-terminal voltage regulator has a saturation voltage as
excellent as approximately 0.2 V. The generator block can
deliver an output voltage of up to 28 V (for generation of -22
mA); therefore, a switching regulator is used as the power source
to the output amplifier to control the power supply voltage by
means of the output voltage. This strategy achieves low power
consumption when the output voltage is low. All these features
have made it possible to produce a power supply that is extremely
immune to voltage variations in the power source, far less noisy
and highly efficient.
2. Generator Block
The generator block consists of a voltage reference, a
multiplier, amplifiers, output selector contacts, a current/voltage
converter and protection circuits. For voltage generation, the
output of the voltage reference is varied by the multiplier to
produce a desired voltage through the output amplifier. For
current generation, the output amplifier works as a voltage/
current converter. In the case of resistance generation, the block
does not actually generate resistance. Rather, the block uses a
means known as the “active impedance method” that generates a
voltage consistent with the resistance being generated when a
current is applied externally. A current entering through the Lo
terminal is transduced to a voltage by the current/voltage
converter, the voltage is varied by the multiplier, and then a
desired voltage is outputted across the Hi and Lo terminals. This
strategy enables free generation of a desired voltage consistent
with the preset resistance for an incoming current simply by
changing the setpoint of the multiplier. This means the desired
17
PON
Initialization
task startup
PON
processing
RJC
Initialization
parameters
EEPROM
Key-driven
interrupt
handling
Initialization
task
RJC data
Key-driven
interruption
Generation
task
PWM data
Periodic
interruption
Generation task
startup event
Keyque
Key
task
Control parameters
ASCII data for display
Generation task
startup event
Listener
task
Periodic
task
(One-second
interval)
Input buffer
Output buffer
Measurement
task
A/D-conversion
data
Display
tasks
Communication
output data
Display Communication
data
data
flow of data
startup event
Communication
interrupt
handling
Communication
interrupt
interruption
Figure 4 Configuration of Software
resistance has been generated artificially. The method is useful
for a resistance meter using the voltage-drop measurement
method based on a constant current source. The multiplier, which
must be “varied” at high accuracy and with high stability, is
especially important for the generator block.
The recently developed CA100 calibrator employs a PWM
multiplier based on the interval integration method that multiplies
a pulse width given by the CPU by an input voltage. The
multiplier operates fast and has excellent linearity, enabling
highly accurate calculation. These features make it possible for
CA100 to calibrate even a scanned RTD thermometer that
requires fast response. The calibrator also features 0.02% high
accuracy for 5 1/2 -digit resolution. Figure 3 shows a response
waveform during resistance generation. There are two protection
circuits in this block: one for output overvoltage and one for
overcurrent that works only during voltage generation. If these
circuits detect a failure resulting from overload or erroneous
operation, they shut down the output using a relay to protect the
internal circuitry.
3. Measurement Block
The measurement block can measure the DC voltage, DC
current and resistance and gives 41/2 -digit readings. The block
uses auto-zeroing as its method of measurement in which it
measures voltages on both sides of the input terminal, finds the
18
voltage appearing in the absence of input, and corrects the offset
voltage.
The DC voltage measurement mode has three ranges: 500
mV, 5 V and 35 V. The 500-mV range has high input resistance
(equal to or greater than 1 GΩ) since the block directly receives
signals at a preamplifier. For the 5- and 35-V ranges, the block
receives signals at an approximately 1-MΩ voltage-dividing
resistor, divides the input voltage into a tenth and a hundredth
magnitude, respectively, and then feed the signal to the
preamplifier. The DC current measurement mode has two
ranges: 20 mA and 100 mA. A current being measured is
introduced to a resistor with known resistance (shunt resistor),
whereby the current’s value is determined from the voltage
developed across the resistor. The resistance measurement mode,
which is based on the two-wire method, has three ranges: 500 Ω,
5 kΩ and 50 kΩ. A known current from the constant current
source is introduced to the resistor being measured so the voltage
developed across the resistor is measured to determine the
resistance.
When in the TC output mode, the block also measures
temperature using an RJC sensor (thermistor) to support the
internal reference junction compensation of the instrument being
calibrated. Using the resistance measurement function noted
above, CA100 measures the resistance of the thermistor and
converts it to a temperature value internally to use it as the
Yokogawa Technical Report English Edition, No. 24 (1997)
compensation data for the TC output mode.
As the A/D converter, CA100 employs a delta-sigmaconversion CMOS LSI having 20-bit resolution. The block
performs averaging on measured data within an interval between
display updates to reduce effects due to output noise, thus
suppressing instability in readings.
MPU models in the SH series. This approach is a first step toward
the unification of development environments (use of µ ITRON,
an OS that supports the SH series and development tools, such as
a compiler, linker and debugger, from Greenhills) and the sharing
of resources for a reduction in development work processes.
CONCLUDING REMARKS
OVERVIEW OF FIRMWARE
The internal firmware consists of three types of interruption
and eight tasks (Figure 4). It undertakes different types of
hardware control such as processing for generation, processing
for measurement, measurement for RJC, processing for
communication, control of the keys and LCD display, power
monitoring and automatic power-on/off. Our conventional
precision instruments would use MPU’s of different types and
from different manufacturers in different ways. Beginning with
the newly developed products, however, we will use the 32 bit-
Model CA100 “COMPACT CAL” Calibrator
In this paper, we have discussed the functions, features and
internal circuits of CA100. Because of the instrument’s high
accuracy, wealth of basic functions and simple operation, we
expect it to win backing from a wide range of user classes in the
on-site inspection of various converters and receiving meters.
The resistance (RTD-signal) generator function, among other
functions, has been designed in pursuit of high speeds and,
therefore, is outstanding and unrivaled. We are confident the
function will be useful in a wide variety of applications.
19
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