Transcript Scanner - transmille.net

USING
SCANNERS
TO
AUTOMATE
CALIBRATION
APPLICATIONS
FOR SCANNERS IN
CALIBRATION
1: Removing lead changing when calibrating
Multi-product & Multi function Calibrators
2: Voltage Reference / Standard cell monitoring
3: Automating Standard Resistor Calibrations
4: Thermal transfer AC/DC switching
5: Measuring several PRT’s or thermocouples.
SCANNERS
NEED
SOFTWARE
TO AUTOMATE
MEASUREMENTS
• To take full advantage of a scanner the
calibration system needs to be automated using
PC to control the calibration process.
•This increases system complexity and cost.
ENGINEERS TIME
Vs
CALIBRATION TIME
A significant consideration for the use of scanners
is the saving in the engineers time as this can be
greatly reduced.
But this is not the only reason…………….
4 KEY
ADVANTAGE
CONSIDERATIONS
1: Time saving
Possible to save both engineers time and make full use of
the investment in the calibration system by calibrating
unattended, for example performing calibrations overnight
when equipment would otherwise be standing idle.
Often ‘closed loop’ procedures still need an engineer to
swap lead/connection in many cases negating the time
saving of automation.
Note the actual calibration time may not necessarily be
reduced. This is because there is generally very little cost
overhead in taking more measurements in a fully
automated system, but an important improvement in
calibration confidence.
4 KEY
ADVANTAGE
CONSIDERATIONS
2: Reducing Skill level
The calibration process can be made simpler by
avoiding lead changing, where errors, and even
damage to equipment can happed due to incorrect
connections, for example accidental
misconnection to the wrong shunt when changing
leads to measure high current.
4 KEY
ADVANTAGE
CONSIDERATIONS
3: Improving Uncertainties
Automated systems can easily repeat measurements and
reduce the ‘repeatability contribution in the uncertainty
calculation.
4 - KEY
ADVANTAGE
CONSIDERATIONS
4: Improved calibration quality and
repeatability
By removing the human variation effect from the measurements
The calibration becomes more repeatable, with more
consistence results giving greater confidence in the quality of
the calibration performed.
2 KEY
DISADVANTAGE
CONSIDERATIONS
1: Measurement degraded by switching
A Scanner adds additional errors into the
measurement by adding addition contacts in the
signal path.
The largest problem is thermally generated emf
voltages. The design of the scanner and its
operating environment can reduce these to less
than 150nV
This only effects low voltage DC measurements,
even resistance measurements are not necessarily
effected by emf if the ohms compensation
function on the DMM is used.
2 KEY
DISADVANTAGE
CONSIDERATIONS
2: More Complex system
The measurement system will need to be
controlled from a PC. Therefore there will be
software and procedures to write.
There will also be more cables and connection to
make.
If all that is required is a ‘one off’ measurement a
scanner is not the solution.
Calibration of Multi product Calibrators
A scanner can be used to reduce lead changing, between the
calibrator and the DMM - especially for current shunt selection.
With some DMMs it is necessary to use external shunt to obtain
the required uncertainties or range.
The 4 terminal switching arrangement below requires different
connection configuration than a normal scanner.
Current
Input
Voltage
output
Current
Shunt
Voltage Reference / Standard cell monitoring
A scanner can be used to measure differences between cells
in either a solid state reference or a standard cell enclosure.
PC control
Precision DMM
Solid State voltage standard
Scanner
STANDARD RESISTOR CALIBRATION
A scanner can be connected to a set of standard resistors in
an oil bath and used to ratio compare against an unknown
using a precision DMM. This technique can provide very low
uncertainties.
PC control
Standard
resistors
Precision DMM
4 wire
connection
Scanner
DESIGN OF A PRECISION SCANNER
The perfect scanner should switch signals without adding errors.
The most important being thermally generated emf voltages.
This is best achieved by minimizing the power dissipated in the
scanner by the use of low power processors, latching relays and
using external power supply.
Great care must also be taken in the connection to use gold plated
copper connections.
PC Interface
External Power
Brick
Processor
Inputs
Relay
Switching
matrix
Output
CONCULSIONS
The combination of a
Scanner
Digital Multi-meter
Software
Provides a cost effective measurement solution………