The goal is the development of a novel principle for the temperature
acquisition of refrigerants in CO2 air conditioning systems. The new approach
is based on measuring the temperature inside a pressure sensor, which is also
needed in the system. On the basis of simulative investigations of different
mounting conditions functional relations between measured and medium
temperature will be derived.Comment: Submitted on behalf of EDA Publishing Association
  (http://irevues.inist.fr/handle/2042/16838

Reitz, Sven

Schneider, Peter

Schroth, Andreas

English

arXiv

Abstract- The goal is the development of a novel principle for the temperature acquisition of refrigerants in CO2 air conditioning systems. The new approach is based on measuring the temperature inside a pressure sensor, which is also needed in the system. On the basis of simulative investigations of different mounting conditions functional relations between measured and medium temperature will be derived

Sven Reitz

Andreas Schroth

Peter Schneider

CiteSeerX

Model Based Sensor System for Temperature Measurement in R744 Air Conditioning Systems

The goal is the development of a novel principle for the temperature acquisition of refrigerants in CO2 air conditioning systems. The new approach is based on measuring the temperature inside a pressure sensor, which is also needed in the system. On the basis of simulative investigations of different mounting conditions functional relations between measured and medium temperature will be derived

I-Revues

9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
Model Based Sensor System for Temperature 
Measurement in R744 Air Conditioning Systems 
 
Sven Reitz1, Andreas Schroth2, Peter Schneider1 
1Fraunhofer Institute for Integrated Circuits, Division Design Automation, Dresden, Germany 
2Intelligente Sensorsysteme Dresden GmbH, Dresden, Germany 
Email: sven.reitz@eas.iis.fraunhofer.de, Tel: +49 (351) 4640 840 
 
Abstract - The goal is the development of a novel principle for 
the temperature acquisition of refrigerants in CO2 air 
conditioning systems. The new approach is based on measuring 
the temperature inside a pressure sensor, which is also needed 
in the system. On the basis of simulative investigations of 
different mounting conditions functional relations between 
measured and medium temperature will be derived. 
 
I. INTRODUCTION 
The number of sensor systems for monitoring and control 
tasks in the automotive industry, automation technology and 
medical technology has been highly increased by the 
ongoing technological development in the past years. 
Powerful sensors for the precise detection of process 
parameters such as pressure, temperature and flow play a 
key role in the realization of innovative system solutions.  
The goal is the development of a novel principle for the 
temperature acquisition of refrigerants in CO2 air 
conditioning systems. This type of air conditioners will be 
used in the automotive sector medium to long term, based on 
the current ongoing discussions on new EU regulations 
about banning on previously used refrigerants based on 
hydro fluorocarbons [1, 2].  
Essential requirements for the temperature measurement 
are a fast response behavior of the sensor and a preferable 
small impact to the flow of refrigerant. Both requirements 
are contradictory, since to reach a short response time a 
direct positioning of the temperature sensor in the refrigerant 
flow is useful. This is necessarily associated with a reduction 
of the cross-section, disturbing the flow of refrigerant. 
Another problem is that the sensor housing dissipates heat 
and this causes a distortion of the measured values. 
The new approach is based on measuring the temperature 
inside a pressure sensor, which is also needed in the system 
[3]. The temperature sensor will be outside the refrigerant 
flow and therefore the measured temperature will differ from 
the actual medium temperature. On the basis of simulative 
investigations of different mounting conditions correction 
maps will be determined. These maps, stored in the 
intelligent evaluation electronics of the system, are used for 
computation of the actual temperature of the medium.  
Fig. 1 shows the typical mounting condition of the 
pressure-temperature sensor. 
 
Convection Convection
Medium p, T
RadiationRadiation
TEnvironment
 
 
Fig. 1. Schematic representation of the pressure-temperature sensor 
 
II. METHODOLOGY 
The challenge is that the temperature of the medium 
directly in the tube has to be measured but the appropriate 
sensor can only be implemented in a certain distance from 
the desired measuring point (Fig. 2). Furthermore, it is 
characteristic that the temperature at the sensor point may 
deviate significantly from the temperature of the medium 
especially due to thermal convection and radiation (Fig. 1). 
An approach was developed, which is based on an 
approximation of the medium temperature at position x from 
measurements at one or more locations 1y , 2y , 3y , ... (Fig. 
2). In order to determine valid relations and characteristic 
diagrams 
 
          )),(),(),(()( 321 …yTyTyTfxT =  (1) 
 
for a specific application and design, simulation runs can 
replace complex or technically not feasible measurements [4, 
5]. Based on the characteristic diagrams determined by 
simulations, for example multidimensional regression can be 
used to derive the above mentioned functional relation (1). 
The correction calculation itself will be done in the sensor 
system (for example by means of integrated 
microcontrollers), or externally in a higher-level control unit. 
 
9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
some possible
sensor locations
(on PCB or
inside drill hole)
housing
PCB
pressure sensor
tube
medium
y1
y2
y3
y4 y5 y6
x
 
Fig. 2. Half-model of the sensor due to symmetry in ANSYS 
 
For the model-based acquisition of the relevant physical 
effects and the essential boundary conditions of the sensor 
system it was necessary to create ANSYS [6] finite-element 
models (FE models) of the existing pressure-temperature 
sensor including its connection to the medium and the 
environment. The models were created in such a manner that 
they include the heat conduction and capacity of the used 
materials as well as take the heat convection and radiation to 
the environment (air) into account. Furthermore, the 
symmetry of the assembly was exploited to reduce the 
problem to a half-model (Fig. 2). 
The detailed FE-models will be used to investigate 
− geometry variations of the sensor system 
− different positions of the temperature sensors and 
− the influence of mounting and ambient conditions, 
and thus they serve for the precise determination of the 
required characteristic diagrams. 
 
III. COMPREHENSIVE SIMULATION STUDIES 
As a first step static simulations were carried out. Medium 
and ambient temperature, convection and radiation were 
assumed to be constant. On the one hand, the resulting 
temperature distribution in the entire sensor system can be 
determined (Fig. 3 left), on the other hand, the temperatures 
at the selected 6 measurement points are a direct result of the 
simulation (Fig. 2). In the next step, by varying the medium 
and ambient temperatures characteristic diagrams, as shown 
in Fig. 3 right for the 3 sensors in the drill hole exemplary, 
were obtained. With the help of these maps the relationship 
between the temperatures at the sensor locations and the 
medium temperature had to be found. 
 
IV. RELATION BETWEEN SENSOR TEMPERATURES AND 
MEDIUM TEMPERATURE 
The optimal number and location of the at most 6 
temperature sensors (Fig. 2), necessary for an approximation 
of the medium temperature, had to be found. 
First, for different combinations of temperature sensors 
approximation functions for the medium temperature based 
on characteristic diagrams were derived. Afterwards, the 
resulting differences between calculated and given medium 
temperature were investigated. 
Essential for a most accurate approximation is the choice 
of suitable approach functions. From the multitude of 
approach functions and the use of 2 to 6 temperature sensors, 
the following approach 
 
5145535211110 ttbttbtbttbtbT ++++=  (2) 
 
should be mentioned. The medium temperature is calculated 
by this function using the lowest temperature sensor in the 
drill hole ( 1t ) and the middle sensor on the printed circuit 
board ( 5t ) (Fig. 2). These two sensors were selected because 
the lowest sensor in the hole is very close to the medium and 
the sensor on the printed circuit board is relatively close to 
the environment. Using this approach, even in practical 
temperature measurements the resulting differences between 
given and calculated medium temperatures are below 1.8 K. 
 
V. IMPLEMENTATION OF APPROXIMATION FUNCTION 
For the implementation of the approximation functions a 
microcontroller board equipped with an 8-bit microcontroller 
from Atmel (ATmega128) was used as an experimental 
platform (Fig. 4). This controller is connected over two 
analog inputs to the temperature sensors and a serial 
Fig. 3. left: Temperature distribution of entire sensor system, 
right: Temperatures at sensor locations in drill hole as function of medium and environment temperatures 
9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
interface is used to transmit the calculated medium 
temperature to a PC.  
A block library for Matlab/Simulink, which was 
developed at the Institute, allows the configuration of the 
required peripherals on the test board and generates C-code 
automatically, e.g. for ATmega128. At first, the Simulink 
model was verified by simulation runs, and then 
automatically transferred to the board. Using this hardware 
solution the medium temperature was calculated 
successfully. 
The program code generated from the Simulink model 
will also be used for an estimation of the required memory 
resources of a microcontroller assembled in future sensor 
systems. For the setup described above, the controller uses 
10 kB of Flash, 320 Bytes of RAM and runs with sample 
rates up to 1 kHz. 
 
VI. CONCLUSIONS 
The given results show that simulative studies of the 
thermal behavior of the sensor system lead to a correlation 
between the temperatures at the possible sensor positions 
and the medium temperature. Furthermore, variations in the 
design of the sensor system and the thermal boundary 
conditions are much easier to investigate than in the real 
system. 
Comparisons between simulation and measurement results 
have shown a very good accordance of the finite element 
model with the real sensor system. 
The use of two temperature sensors in the sensor system is 
sufficient to compute the medium temperature in the case of 
known thermal convection (i.e. known mounting conditions 
in practice). The effect of changing thermal convection and 
radiation respectively is subject of further investigations. 
The derived approach and approximation functions can be 
described in behavioral description languages (e.g. Modelica 
[7, 8]) or Matlab/Simulink. 
Using system-level tools like Matlab/Simulink for design, 
simulation (Verification) and code generation has proven to 
be very time efficient and still allows the algorithm to be 
implemented in very small and cost effective 
microcontrollers. 
 
ACKNOWLEDGMENT 
This paper is based on the project MOBASENS which is 
supported by the Sächsische Aufbaubank - Förderbank - 
under support-no. 11939/1925. The authors of this paper are 
solely responsible for its content. 
 
REFERENCES 
[1] Verband der Automobilindustrie, Forschungsvereinigung Automobil-
technik, Arbeitskreis 30: „Simulation gemischter Systeme“: http://fat-
ak30.eas.iis.fraunhofer.de 
[2] Verband der Automobilindustrie, Annual report, 2006 
[3] P.T.J. Gennissen; C. RuizZwollo; R. Appelo; J. Stoll, S. Subramanyam: 
On the integration of a combined Pressure and Temperature sensor in a 
CO2 air conditioning system. VDA-Meeting Saalfelden, February 2005 
[4] P. Schneider, U. Eichler, K. Einwich, P. Schwarz: Simulation-based 
design of measurement systems. "Sensor und Test", Nürnberg, May 
2005 
[5] P. Schneider, S. Reitz, J. Bastian, P. Schwarz: Combination of analytical 
models and order reduction methods for system level modeling of 
gyroscopes. Nanotech 2005, MSM, Anaheim, Cal., May 2005 
[6] Product information ANSYS, ANSYS Inc., http://www.ansys.com 
[7] Product information DYMOLA, Dynasim AB, http://www.dynasim.se 
[8] D. Limperich, M. Braun, K. Prölß, G. Schmitz: System Simulation of 
Automotive Refrigeration Cycles. Proc. 4th International Modelica 
Conference 2005, Hamburg-Harburg, March 2005 
 
Fig. 4. Test platform using an AVR microcontroller (ATmega128) for data 
acquisition and medium temperature calculation 


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Model Based Sensor System for Temperature Measurement in R744 Air Conditioning Systems

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