PC-based measurement technology for Miele quality assurance

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PC Control 04 | 2010
Scientific Automation: High-precision temperature measurement and signal acquisition in climatic chambers
PC-based measurement technology
for Miele quality assurance
Quality, strength of innovation, tradition – these are values to which Miele & Cie. KG has been committed to since the business
was founded in 1899. True to the guiding principle of “Forever better,” the name Miele stands for the development and production of domestic appliances and commercial equipment that meet the highest quality standards. The quality assurance system and
the feedback of test and measurement data in all relevant processes are among the key factors in the success of the Miele brand.
“Scientific Automation” from Beckhoff has been a part of these sub-processes for about two years now.
For several generations, the family-owned Miele company, with headquarters in
Gütersloh, Germany, has produced premium household appliances and first-class
commercial equipment that are designed for high quality, long service life and
sustainable resource conservation. Each year, more than 7 % of the company’s sales
is invested in research and development. Miele has production facilities in eight
plants in Germany and one plant each in Austria, the Czech Republic, Romania
and China. The extensive product range includes washing machines, clothes dryers,
built-in cookers and ovens, dishwashers, fridges and freezers, fully automatic coffee
machines, microwave ovens and much more. The entire product range undergoes
constant, intensive quality tests and checks.
Automated test and measurement
The central division in Miele’s Gütersloh plant is also responsible for basic research
for the “Cooling & Coffee International” (CCI) business unit. “The CCI Test and
Development Center includes 11 climatic chambers with a total of 14 stand-alone
measuring systems, in which fridges and freezers, fully automatic coffee machines,
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PC Control 04 | 2010
A climatic chamber usually contains a measuring
system with four measuring stations; some climatic
chambers were equipped as dual chambers that
incorporate eight measuring stations. CCI’s Basic
Research has a total of 56 measuring stations at
its disposal for testing fridges and freezers, fully
automatic coffee machines, and microwave ovens.
Testing a built-in fully automatic coffee machine:
For each measuring system, 80 channels for
high-precision temperature measurement with
PT100 sensors are available.
In a total of 11 climatic chambers, fridges
and freezers, fully automatic coffee machines, and
microwave ovens are put through a rigorous testing
program to meet Miele’s exacting quality standards.
and microwave ovens are put through a rigorous testing program,” reports Benjamin Held, Development Engineer in CCI Basic Research. A climatic chamber usually
contains a measuring system with four measuring stations. Some climatic chambers
were equipped with double the number, however, so that a total of 56 measuring
stations are available for CCI’s basic research.
All measuring stations are equipped with automation and measurement technology from Beckhoff. For each measuring system, 80 measuring channels for highprecision temperature measurement with PT100 sensors are therefore available.
There are also connections for 80 thermocouples, 20 analog inputs and outputs,
and 20 digital inputs and outputs. All connection points take the form of Beckhoff
EtherCAT terminals. Each climatic chamber is equipped with a C6650 industrial
PC from Beckhoff that provides the central control functions for the chamber. A
Beckhoff CP6903 Control Panel is built into the control panel door and facilitates
visualization of the application and the dialog with the PLC.
The software-PLC used in the application is TwinCAT PLC. For programming, Miele
focuses on IEC 61131-3. On this subject, Benjamin Held commented that: “The
test applications are programmed in the Structured Text programming language.”
High-precision temperature measurement through certified accuracy
Climatic tests are accorded a very high priority at Miele, because refrigeration
technology is extremely sensitive to climatic conditions. Since the refrigerators
are used worldwide and have to run 24/7, year after year without interruption, all
operation conditions must be evaluated. As Benjamin Held continued: “Our fridges
and freezers are not just used in European households. Depending on the location,
and considering countries in Asia or the Middle East as examples, temperatures
up to 43 °C (109 °F) and air humidity up to 90 % RH can exist. We have therefore
defined ‘climate categories,’ and we test in conformance with standards that
category conditions are maintained. Other factors such as transportation condi-
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Michael Jost, Product Manager for EtherCAT and
Benjamin Held, Development Engineer in the
The stated aim is the highly accurate measurement
Beckhoff I/O systems: “The EL3201-0020 high-accu-
Basic Research department of Miele’s “Cooling
and control of the temperature in the appliances and
racy temperature terminals are metrology modules
& Coffee International” (CCI) business unit:
climatic chambers during the entire test period.
that undergo a special setup procedure by Beckhoff
“Painstaking environmental tests to evaluate
during manufacturing. If climate categories are to
all operating conditions guarantee that Miele
be tested very accurately, the control technology
refrigeration units can also be used in extremes
must always be one step better to ensure that the
of climates and will run around the clock, year
tolerances will be maintained.”
after year, and without interruption.”
tions are also simulated, where appliances are exposed to a temperature range of
-20 °C (-4 °F) to +60 °C (140 °F).”
As well as the adaptable test and measurement capabilities, precise temperature measurement is particularly important for Miele. The stated aims are highly
accurate temperature measurement in the appliances and precise control of
temperature in the climatic chambers. Fundamentally, temperature is a critical
process variable that is continuously acquired and evaluated throughout the one- to
three-week testing periods.
Based on the “Scientific Automation” solution, the integration of automation technology with measuring tasks, Beckhoff has developed special fieldbus terminals for
highly accurate temperature measurement. The usual accuracy of PT100 terminals
in industrial applications is around ±1 °C. The Beckhoff EtherCAT temperature
measuring terminals used by Miele deliver an accuracy of ±0.1 °C, which is an
impressive improvement by a factor of 10. Michael Jost, Product Manager for
EtherCAT and Beckhoff I/O systems, describes the interrelationship as follows: “The
EL3201-0020 high-accuracy temperature terminals are measuring modules that
undergo a special setup procedure during manufacturing. The factory calibration
is recorded in an individual certificate, as required for high-precision measuring
equipment. Such dedicated measurement technology is the basis for the integration of high-performance and high-precision measurement technology without
special components. If climate categories are to be tested very accurately, then the
tolerances must be maintained as well. To ensure this, the control technology must
always be one step better. In our view, the measuring device must be better by a
factor of 10 than the equipment that it will be used with.”
High-accuracy signal acquisition with scalable precision
The second climatic parameter that is acquired in Miele’s application is air humidity.
“We use external measuring equipment for this purpose i.e. we use humidity sensors that likewise operate in the high-precision range. Their analog output signals
are connected to analog input terminals,” says Benjamin Held. For this application,
PC Control 04 | 2010
too, there is an ultra-accurate 2-channel analog input terminal in the form of the
EL3602 EtherCAT Terminal, as Michael Jost reports: “The EL3602 analog EtherCAT
input terminal processes signals in the ranges: ±10 V, ±5 V, ±2.5 V and ±1.25 V. The
voltage is digitized with a 24-bit resolution and transmitted as a measured value
to the parent controller in an electrically isolated manner. The input channels have
differential inputs and possess a common, internal ground potential.”
The capabilities of the EtherCAT terminals extend beyond high-precision signal
acquisition, because the terminals have a very high potential frequency for sampling measured values. “Our test procedures are mainly carried out at relatively
low clock rates ranging from 30 seconds to a minute,” explained Benjamin Held.
“Consequently, we mainly use the precision of the measuring terminals, less so for
their speed. On the other hand, we have to implement widely differing test conditions and requirements, especially for standards-regulated tests and for energy consumption measurements, and in the process guarantee an absolutely reliable and
precise results graph.” The experts at Miele therefore focus on basic functions and,
as and when required, on specialized functions. Basically, standard tests are carried
out with extensive preparation, with test-process verification, and with extensive
measurement technology. “In operations of this type, from 20 to 30 temperature
sensors are quickly distributed and positioned in the device to be tested. We also
carry out dynamic tests,” explains Benjamin Held. “For example, during commissioning, we also introduce disturbances into test procedures. We use warm or frozen
food, for example, or standardized test packets, or the fridge is opened and closed.
These are random variations that must be reproduced and tested.”
Automated test level
It is generally known that the sensor level is just one part of a test and control
task; the control of actuators or controllers is another part. “The actuators are not
standardized. This means that simple actuators, such as fans, can be directly controlled. Other actuators, such as amplifiers or controllers, are controlled via analog
output terminals,” says Benjamin Held, while pointing out that today’s refrigerators
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PC Control 04 | 2010
Fridge/freezer undergoing testing in the climatic
chamber. To ensure that the appliances can be
used worldwide, they are specified by ‘climate
categories,’ and Miele checks in conformance with
standards that category conditions are maintained.
are equipped not just with one compressor cycle or a chiller or compressor, but
sometimes have two combined compressors. Moreover, Miele also uses variablespeed compressors, which create extensive opportunities for work and research. In
addition - and depending on the climate category and appliance - arrangements
known as ‘winter circuits’ may also be implemented. Fans can also be incorporated
in an appliance to enable dynamic cooling with a precise temperature distribution. Benjamin Held assesses this interrelationship as follows: “We have a great
many parameters at our disposal for controlling the test and measurement tasks.
It is important to us that the automated test and measurement technology makes
us as flexible as possible, so that we can vary the control variables quickly and
accurately. The Beckhoff platform we have implemented means that we can
incorporate these requirements in programming and thus quasi-automate the
measurement procedures. This is particularly significant in the area of quality
checks, because the program procedures enable us to achieve a high throughput
of tested appliances.”
The sensor data from the test and measurement functions of the climatic chambers
are transmitted from TwinCAT via the standardized OPC interface to Miele’s parent
test-data acquisition system. “This way, we can connect other, external instruments,
such as our portable energy meters without any problems,” says Benjamin Held.
Miele & Cie. KG
Scientific Automation www.beckhoff.com/scientific-automation
Reproducible measurements through certified precision
Thanks to the high fundamental accuracy of the EL3201-0020 temperature measurement terminal,
the measuring error is reduced to ±0.1 K of the temperature measurement range. The EtherCAT
Terminal enables direct connection of a resistance sensor (PT100) using 4-wire technology. The
characteristic curves of the sensors are implemented over their complete measuring range and are
made available to the higher-level control system in a suitably linearized form.
Reliable reproducibility of results is becoming increasingly important in a wide range of automation applications, in order to optimize parameters and ensure verifiability, for example for online
quality monitoring in running processes. Beckhoff meets these requirements with a calibration
certificate that authenticates the measuring accuracy of the terminal and the exact measuring error. Online download of the certificate based on the serial number and terminal handling similar
to common analog terminals make the application of high-precision measurement technology
very convenient.